WO2010049949A1

WO2010049949A1 - A method to process interstitial-free (if) steels by adapting multi-axial

Info

Publication number: WO2010049949A1
Application number: PCT/IN2009/000607
Authority: WO
Inventors: Satyam Suwas; Somjeet Biswas; Ayan Bhowmik; D. Satyaveer Singh; D. Bhattacharjee; R. K. Ray
Original assignee: Tata Steel Ltd
Current assignee: Tata Steel Ltd
Priority date: 2008-10-27
Filing date: 2009-10-27
Publication date: 2010-05-06
Anticipated expiration: 2011-04-27
Also published as: CN101889097A; IN2008KO01831A; CN101889097B

Abstract

The invention relates to a method to process Interstitial-free (IF) steels by adapting multi-axial forging (MAF) at room temperature to provide increased strength and ductility by producing ultrafine grain structure within the steel, the method comprising the steps of providing a multi-axial forging apparatus having a base-plate, at least two die-panels, at least one plunger with a plurality of adapters; placing a course grain IF steel billet in between the at least two lubricated die-panels with a first axis of the billet along the axis of the plunger; pressing the billet with the at least one plunger along the first axis to reduce the length of the billet to half-the-original size with a corresponding increase in the smaller sides lying along a second axis; imparting a first rotation to the billet and repeating the steps (b) and (c) along a third axis; imparting a second rotation to the billet and repeating the steps (b) and (c) such that an elongation takes place along the first axis; and repeating the cycle comprising steps (b) to (e) till such time the grain size is reduced by at least three order of magnitude.

Description

A METHOD TO PROCESS INTERSTITIAL-FREE fIH STEELS BY ADAPTING MULTI-AXIAL

FIELD OF THE INVENTION

The present invention relates in general to process Interstitial-free (IF) steels by Severe Plastic Deformation (SPD) method by Multi-axial forging (MAF) at room temperature to produce ultrafine grains of the order of few hundreds of nanometers and to increase the strength many folds than the initial material. More particularly, the present invention relates to a method to process IF-free steels by adapting multi-axial forging at room temperature to provide increased strength and ductility by producing ultrafine grain structure within the starting steel.

BACKGROUND OF THE INVENTION

Interstitial-free (IF) steels constitute an important class of industrial materials. In recent times it has been extensively employed in the automobile industries. These are a typical class of extra-low carbon steels, where the amounts of interstitial elements are present in the ppm. level. Typically the total interstitial content are in the region <0.0030 wt.% C and <0.0040 wt. %N. In IF steels either titanium or niobium or both are important alloying additions. These elements stabilize the carbon in the steel by forming carbide precipitates and hence, prevent the existence of any solute interstitial atoms, therefore, IF steels are also non-aging. IF steels provide very high levels of formability, as indicated by the ratio of width to thickness strains during forming (r>1.8) and are adopted to fabricate car body panels like rear floor pan, front and rear inners and spare wheel well. However, the future of these steels lies in the improving their strength along with restoration of appreciable amount of ductility and at the same instance improving the crack resistance of the fabricated automotive parts.

Grain size refinement of interstitial-free steels to submicron level leads to an obvious increase in strength along with an optimum amount of toughness in the material. It also improves the fatigue resistance and causes a significant drop in the superplastic temperature of the material.

In recent years, a number of innovative severe plastic deformation (SPD) techniques have been developed for deforming metals to a very high degree of plastic strains with the aim of producing greatly refined grain structures without entailing requirements of exotic alloy additions or costly thermomechanical treatments. These include severe plastic torsion straining (SPTS), multi-axial forging (MAF), accumulative roll bonding (ARB) and equal channel angular extrusions (ECAE). Severe plastic deformation processes can produce materials with a grain size of the order of 100-lOOOnm. A distinct advantage of these processes is that it can be scaled up to produce large billets in industry and is relatively simple and also a cheap process. The common novel feature of these processes is that the net shape of the final product remains essentially the same as the starting material after any given number of passes, so there is no constraint on the strain that is built up in the material In comparison to conventional metal working process, like rolling, extrusion, effective strains greater than 4 can only be obtained in filaments which have few structural applications.

MAF is basically a plane strain compression applied to all the three axes one after another for completion of one cycle. It involves abrupt changes in strain path. The process can be repeated for a number of cycles in order to obtain desired microstructure.

In the paper entitled "Formation of sub-micron and nanocrystalline grain structure by severe plastic deformation' by P.B. Pragneel, et al. submitted during the 22^nd Risø International Symposium on Materials Science, pp. 05-126 (2001), the definition of submicron or nanocrystalline grain size has been proposed as a structure where (a) average spacing of the high angle grain boundaries (HAGBs), having misorientation angle greater than 15°, must be less than 1 micron in all orientations, and (b) the proportions of

HAGB area with respect to total boundary area in the material must be greater than 70%.

Multiaxial forging, multiple forging or ^λabc' deformation process, as it has been variously called, was originally developed by G.A. Salishchev. This method is very effective in producing sub-micron grain size in metals and alloys and the processing temperature lies typically between ~0.1-Q.5T_m, where T_m represent the melting temperature. The principle of multiaxial assumes multiple repeats of a free forging operation with a change of the axis of applied load after every forging operation. Although the heterogeneity of strain developed in the material is much more in multiaxial forging than that developed during ECAE or HPT, compared to other SPD processes, the essence of this process lies in its simplicity both in terms of its principle and tooling associated with it. This technique has tremendous potential to produce billets at large industrial scale.

Until now multiaxial forging has been conducted on titanium, magnesium and nickel based alloys. In the papers entitled "Submicrocrystalline and Nanocrystalline Structure Formation in Material and Search for Outstanding Superplastic Properties" by Gennady A. Salishchev, Oleg Valiakhmetoy, V.A. Valitov, S.K. Mukhtarov published in Materials Science Forum, vol.17-172, 1994m ppl21 and "Diffusion and Related Phenomenon in bulk

Nanostructured Materials" by G.A. Salishchdev, O.R. Valiahmetov, R.M.. Galeev and S.P. Malysheva published in Russian Metally, vol. 4, 1996, pp86, nanostructured pure titanium were fabricated using multiple forging. In the US patent entitled "Method of processing titanium alloys" PCT/US97/18642, O 9817836 Al, 1998 by O.A. Kalibyshev, G.A. Salishchev, G.A. Salishchev, R.M. Galeyev, R. Ya. Lutfullin, O.R. Valiakhmetov and paper "Production of submicrocrystalline structure in large-scale Ti-641-4V billet by warm severe deformation processing" by S.V. Zherebtsov, G.A. Salishchev, R.M. Galeyey, IN2009/000607

O.R. Valiakhmetov, S. Yu. Mironov, S.L. Semiatin published in Scripta materialia, vol.51, 2004, ppll47, a two-phase Ti-6A1-4V alloy has been successfully experimented to produce bulk submicron grain size. Nanostructured magnesium alloy (Mg-6%Zr) and high strength nickel base alloys has been fabricated and reported respectively in the papers entitled "Formation of Submicrocrystaline Structure in Materials During Dynamic Recrystallization" by O. Kaibyshev, R. Kaibyshev and G. Salishchev published in Materials Sceince Forum, vol.113-115, pp.423 and "Submicrocrystalline and nanocrystalline structure formation in materials and search for outstanding superplastic" by G.A. Salishchev, O.R.

Valiakhmetov, V.A. Valitov, S.K. Mukhtarov published in Materials Science Forum, vol. 170-172, 1994, PP.121. In all the cases, the forging operations were carried out at elevated temperatures so that the process is associated with dynamic recrystallization. The average grain size obtained after the deformation process is of the order less than 500 nm.

Researchers had been trying to generate materials of submicron grain size of IF steel by different SPD processes. ECAE is one of the attractive methods of imparting severe straining in one pass without distorting the material geometry. In this process, the die comprises at least two interconnected channels-entry and exit channels, intersecting at an angle of 90°, 120° or 135°. The material is fed through the entry channel and forced out through the exit channel. The processing conditions and -the properties derived henceforth has been reported in some of the papers entitled "Effect of processing route on microstructure and texture development in equal channel angular extrusion of interstitial-free steel" by Saivi Ii, Azdiar A. Gazder, IJ. Beyerlein, E.V. Pereloma, C. H. J. Davies published in Acta Materialia, vol. 54, 2007, pp.1087-1100 and "On the strength of boundaries in submicron IF steel" by J. De Messemaeker, B. Verlinden, J. Van Humbeeck published in Materials Letters, vol.58, 2004, pp.3782-3786.

In some papers entitled "Nanoscale crystallographic analysis of ultrafine grained IF steel fabricated by ARB process" by N. Tsuji, R. Ueji, Y. Minamino published in scripta materialia. Vol.47, 2002, pp.69-76 and "Ultra-fine grained bulk steel produced by accumulative roll-bonding (ARB) process" by N. Tsuji, Y. Saito, H. Utsunomiya and S. Tanigawa published in Scripta materialia, vol.40, 1999, pp.795-800, IF steel has been processed via a different SPD technique called ARB. In ARB, strips of metal sheets are rolled by stacking them on top of each other, which get adhered after being rolled at a fairly high rolling speed. The process is generally carried out at higher temperature and results in the formation of sub-micron grain sizes.

SPD of IF steel is possible by ECAE at room temperature and by ARB at higher temperature. These methods only help in understanding the scientific benefits achieved by grain refinement to submicron levels and development of preferred texture. However, none of these publications provides any teaching or suggestion so as to develop a technical method feasible enough to implement at industrial scale.

OBJECTS OF THE INVENTION

It is therefore an object of this invention to propose a method to process interstitial free (IF) steels with coarse-grain microstructure by adapting multi-axial forging at room temperature to provide increased strength by producing ultra-fine grain structure Within the starting steel.

A further object of this invention is to propose a method to process interstitial free (IF) steels with coarse-grain microstructure by adapting multi-axial forging at room temperature to provide increased strength by producing ultra-fine grain structure within the starting steel, which is cost- effective.

Another objection of this invention is to propose a method to process interstitial free (IF) steels with coarse-grain microstructure by adapting multi-axial forging at room temperature to provide increased strength.by producing ultra-fine grain structure within the starting steel, which provides IF-steel having sufficient strength to meet the improved demands of the automobile industry.

Yet another of this invention is to propose a method to process interstitial free (IF) steels with coarse-grain microstructure by adapting multi-axial forging at room temperature to provide increased strength by producing ultra-fine grain structure within the starting steel, which can be scaled up to meet the need of mass-scale production.

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 INVENTION

According to the invention, the Deformation processes such as severe plastic deformation (SPD) methods are used in order to generate specific texture and possible grain refinement in order to enhance the properties of the IF-steel, such as strength and ductility. Multi-axial forging (MAF) is one of the SPD methods in which IF steel billets are compressed along the three axes one after another to complete one cycle. The process is repeated for number of cycles in order to obtain grain size of submicron level.

A combination of severe plastic deformation at room temperature will definitely generate grains of the order of few hundreds of nanometers. Multi-axial forging is one of the severe plastic deformation techniques, which is used to deform IF steel at room temperature. This is done by plane strain compression of the billets along the three axes one after another to complete one cycle. The^* process is repeated for a total number of four cycles in order to obtain grain size of 22 nanometers. The yield strength increases six times after four cycles to -600 MPa.

BRIEF DESCRIPTIONS OF THE ACCOMPANYING DRAWINGS

The accompanying figures, which are incorporated in and form a part of the specification, illustrate embodiments of the present invention and, together with the description, serve to explain the principles^* of the invention.

Figure 1 - Schematic diagram of a multi-axial forging apparatus.

Figures 2A, 2B, 2C and 2D - show schematic representation of the IF-steel billet and the reference directions, along which the billet is compressed along its three axes with reference to the die axes.

Figures 3A and 3B - each shows a pictorial view of the billet in its initial condition and after undergoing forging up to four cycles.

Figure 4 - Optical Micrograph of the starting material.

Figures 5A and 5B - shows the inverse pole figure (IPF) maps and pattern quality maps of the IF steel billet superimposed after 1 cycle and 4 cycles. N2009/000607

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Figure 6 - shows a stress strain curve of the starting material, material after 1^st cycle, 2^nd cycle and 4^th cycle.

DETAILED DESCRIPTION OF THE INVENTION

Interstitial Free (IF) steel with submicron grains size has been produced for the first time using the technique of multiaxial forging (MAF). The invention adapted an indigenously designed MAF die (3), which resulted in an effective true strain of -0.7 per compression along an axis i.e., a true strain of -2.1 per cycle. An MAF apparatus (1) has been configured (Fig.

1) that consists of a base plate (l),.two die panels (3), at least one plunger and adapter (2) to fit with the hydraulic press machine. The plunger and the adapters (2) are made up of H-13 tool steel. The grain structure of the coarse grained IF steel billet (B) is refined to an ultra-fine grain size by repeated forging along the three axes one after another a number of times. The resulting ultra-fine grained IF steel has strength exceeding that of HSLA steel with an appreciable amount of ductility.

By way of a preferred embodiment and without implying any limitation on the scope of the invention the material used for the process was a titanium-stabilized IF steel, the composition of which is given below in Table 1, Table 1

Fe C MN S P Si Al Ti N

BaI. 0. 0027 0.07 0 .007 0.007 0 .006 0.05 0 .056 0 .003

A detailed description of the MAF process has been shown through the schematic representation in Figs 2A, 2B, 2C. In the figure 2A, a, b, and c- axes represent the external or the die coordinate system while x, y and z- axes represent the initial reference system of the billet sample. The billet (B) used has a square cross-section of 20 mm x 20 mm and a height of 40 mm. The billet (B) is first kept with its longest dimension, i.e., the z-axis, parallel to the load axis in between the two die panels (3). The load is then applied by means of the plunger (2), having a crosshead speed of ~1 mm/s. The load is applied till the height of the billet (B) reduces to half of its original height. As the second axis, y, is constrained by the die walls, (as shown in Figure 2B), there occurs an equivalent flow of the material along the third axis i.e. the x-axis. The billet is then given a clockwise rotation, first about the a-axis of the die reference system as shown by rotation W and then a second rotation ^λB' again in a clockwise sense, as depicted in Figure 2. These subsequent rotations again bring the longest dimensions of the billet (B), which in this case is the prior y-axis, parallel to the direction of the applied load. After the load is applied and the billet strained to half its width, the same sequence of rotations, as described earlier, is followed. This time the third axis i.e. the x-axis of the initial billet (B) has the longest dimension and final deformation is given to the sample along the axis (Figure 2C). Thus, the billet (B) is compressed subsequently along all the (x, y, z), three axes which constitutes one cycle of operation. Since there is no significant change in the dimension of the Billet (B) after each pressing, this process in principle can be repeated for any number of pressing or cycles. In accordance with a preferred embodiment, the given billet is forged up to four cycles. During pressing, in order to reduce frictional effects, a lubricant of molybdenum disulphide (M0S₂) powder mixed with grease, is applied at the interface of the tool and the billet. It is to be mentioned here that unlike the prior art where isothermal forgings are carried out at higher temperatures, the pressings in the disclosed invention is conducted at room temperature.

A pictorial view of the billet in its initial condition (Figure 3A) and after undergoing forging till four cycles is shown in Figure 3B. Figure 4 shows the optical micrograph of the initial material having an average grain size of -225 μm. After the first cycle only, there is a drastic reduction in the grain size to submicron level (~260 nm). A further refinement of the grains to -220 nm takes place at the end of four cycles. Figures 5A and 5B respectively shows a representative microstructure in the form of Inverse pole figure (IPF) maps obtained after the first and fourth cycle measured by electron back scattered diffraction (EBSD) using field emission gun-scanning electron microscope (FEG-SEM). Thus after the four cycles of MAF, the grain size gets reduced by three orders of magnitude. 009/000607

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In order to test the mechanical properties of the multi-axially forged samples, corresponding miniature tensile samples, as per ASTM specification, were extracted from the billets. The tensils tests were carried out on an universal testing machine (not shown). The stess-strain curves for the starting material and after one, two and four cycles are given in Figure 6. Table 2 shows the values of the yield stress and ultimate tensile stress for all the samples. The yield strength of the initial material was ~105 MPa. There occurred a dramatic increase in the yield strength after first pass by almost about five-fold. After four cycles yield strength increases to -60 Mpa. An acceptable range of tensile ductility of nearly 5% was also observed in all the cases. Thus, a reasonably good combination of strength and ductility resulted after the process of multi-axial forgoing.

References

Formation of sub-micron and nanocrystalline grain structure by severe plastic deformation P.B. Pragnell, J. R. Bowen and A. Gholina, The

Proceedings of 22^nd Risø International Symposium of Materials Science, pp. 105-126 (2001) Submicrocrystajline and Nanocrystalline Structure Formation in Materials and Search for Oustanding Superplastic Properties G. A. Salishchev, O. Valiakhmetov, V.A. Valitov, S.K. Mukhtarov, Materials Sceince Forum, vol.170-172, 1994, ppl21 Diffusion and Related Phenomenon in bulk Nanostructured^" Materials G.A. Salishchev, O.R. Valiahmetov, R.M.. and S.P. Malysheva, Russian Metally, vol.4, 1996, pp86,

US patent No.: PCT/US97/18642, WO 9817836 A1 Method of processing titanium alloys 1998

O.A. Kaibyshev, G.A. Salishchev, R.M. Galeyev, R.Ya. Lutfullin, O.R. Valiakhmetov

Production of submicrocrystalline structure in large-scale Ti-6A1-4V billet by warm severe deformation processing S.V. Zherebstsov, G.A. Salishchev, R.M. Galeyev, O.R. Valiakhmetov, S. Yu. Micronov, S.L. Semiatin, Scripta materialia, vol.51, 2004, ppll47.

Formation of Submicrocrystaline Structure in Materials During Dynamic Recrystallization

O.Kaibyshev, R. Kaibyshev and G. Salishchev, Materials Science Forum, vol.113-115, pp.423

Submicrocrystalline and nanocrystalline structure formation in materials and search for outstanding superplastic properties G.A. Salishchev, O.R. Valiakhmetov, V.A. Valitov, S.K. Mukhtarov, Materials Science Forum, vol. 170-172, 1994, ppl21.

Effect of processing route on microstructure and texture development in equal channel angular extrusion of interstitial-free steel S.Li, A.A. Gazder, IJ. Beyerlein, E.V. Pereloma, CHJ. Davies, Acta Materialia, vol.54, 2007, pp. 1087-1100

On the strength of boundaries in submicron IF steel J. De Messemaeker, B. Verlinden, J. Van Humbeeck, Materials Letters, vol.58, 2004, pp.3782-3786.

Nanoscale crystallographic analysis of ultrafine grained IF steel fabricated

ARB process

N. Tsuji, R. Ueji, Y. Minamino, Scripta maerialia, vol.47,2002, pp.69-76

Ultra-fine grained bulk steel produced by accumulative roll-bonding (ARB) process N. Tsuji, Y. Saito, H. Utsunomiya and S. Tanigawa, Scripta materialia, vol.4, 1999, pp.795-800

Claims

WE CLAIM

1. A method to process Interstitial-free (IF) steels by adapting multi-axial forging (MAF) at room temperature to provide increased strength and ductility by producing ultrafine grain structure within the steel, the method comprising the steps of:-

(a) providing a multi-axial forging apparatus having a base-plate, at least two die-panels, at least one plunger with a plurality of adapters;

(b) placing a course grain IF steel billet in between the at least two lubricated die-panels with a first axis of the billet along the axis of the plunger;

(c) pressing the billet with the at least one plunger along the first axis to reduce the length of the billet to half-the-original size with a corresponding increase in the smaller sides lying along a second axis;

(d) imparting a first rotation to the billet and repeating the steps (b) and (c) along a third axis;

(e) imparting a second rotation to the billet and repeating the steps (b) and (c) such that an elongation takes place along the first axis; and

(f) repeating the cycle comprising steps (b) to (e) till such time the grain size is reduced by at least three order of magnitude.

2. The method as claimed in claim 1, wherein the step (c) provides a plane strain compression of the billet at room temperature.

3. The method as claimed in claim 1, wherein the steps (b),. (c), (d) and (e) are repeated for a total of four cycles.

4. The method as claimed in claim 1, wherein ultra-fine grains^' of 220 nanometers without any fracture in the billet are generated.

5. The method as claimed in claim 1, wherein the yield strength of the IF- steel billet increases to 600 MPa, which represents six times of that of the initial material.

6. A method to process Interstitial-free (IF) steels by adapting multi-axial forging (MAF) at room temperature to provide increased strength and ductility by producing ultrafine grain structure within the steel, as substantially described and illustrated herein with reference to the accompanying drawings.