Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D7/00—Modifying the physical properties of iron or steel by deformation
  • C21D7/02—Modifying the physical properties of iron or steel by deformation by cold working
  • C21D7/10—Modifying the physical properties of iron or steel by deformation by cold working of the whole cross-section, e.g. of concrete reinforcing bars
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21J—FORGING; HAMMERING; PRESSING METAL; RIVETING; FORGE FURNACES
  • B21J1/00—Preparing metal stock or similar ancillary operations prior, during or post forging, e.g. heating or cooling
  • B21J1/02—Preliminary treatment of metal stock without particular shaping, e.g. salvaging segregated zones, forging or pressing in the rough
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21J—FORGING; HAMMERING; PRESSING METAL; RIVETING; FORGE FURNACES
  • B21J1/00—Preparing metal stock or similar ancillary operations prior, during or post forging, e.g. heating or cooling
  • B21J1/04—Shaping in the rough solely by forging or pressing
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2201/00—Treatment for obtaining particular effects
  • C21D2201/03—Amorphous or microcrystalline structure

Definitions

• 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.
• SPD Severe Plastic Deformation
• MAF Multi-axial forging
• 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.
• r>1.8 the ratio of width to thickness strains during forming
• 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.
• SPD severe plastic deformation
• SPTS severe plastic torsion straining
• MAF multi-axial forging
• ARB accumulative roll bonding
• ECAE equal channel angular extrusions
• 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.
• HAGB area with respect to total boundary area in the material must be greater than 70%.
• Multiaxial forging, multiple forging or ⁇ abc' deformation process 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.
• 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.
• 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.
• 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
• 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.
• 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.
• IF interstitial free
• 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.
• IF interstitial free
• 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.
• IF interstitial free
• 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.
• IF interstitial free
• 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.
• SPD severe plastic deformation
• MAF Multi-axial forging
• 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.
• 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.
• 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.
• 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.
• IPF inverse pole figure
• Figure 6 - shows a stress strain curve of the starting material, material after 1 st cycle, 2 nd cycle and 4 th cycle.
• 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.
• 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.
• the material used for the process was a titanium-stabilized IF steel, the composition of which is given below in Table 1, Table 1
• 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.
• 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 ⁇ B' 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.
• the given billet is forged up to four cycles.
• a lubricant of molybdenum disulphide (M0S 2 ) 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.
• FIG. 3A 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 ⁇ m. 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).
• IPF Inverse pole figure

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• Crystallography & Structural Chemistry (AREA)
• Materials Engineering (AREA)
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• Organic Chemistry (AREA)
• Forging (AREA)

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Cited By (7)

Citations (2)

• 2009
  • 2009-10-27 IN IN1831KO2008 patent/IN2008KO01831A/en unknown
  • 2009-10-27 CN CN2009801009725A patent/CN101889097B/zh active Active
  • 2009-10-27 WO PCT/IN2009/000607 patent/WO2010049949A1/en active Application Filing

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