Title of Invention

DEVELOPMENT OF MICROALLOYED ULTRA HIGH STRENGTH STEEL

Abstract

Conventional mode of production of high strength steel used lower carbon concentration, maintaining lower finish rolling temperatures. Reduction in carbon content during melting increased cost of melting operation of steel and lower finished rolling temperatures required high amount of rolling load and energy. The present invention aims at overcoming of above drawbacks by enhancing the carbon concentration of steel and using higher finish rolling temperatures. This invention provides a novel microalloyed ultra high strength steel having the composition - i) carbon 0.06 - 0.15% by wt.; ii) phosphorus - 0.01% max by wt.; iii) sulphur - 0.01% max by wt.; iv) silicon 0.2 - 0.7% by wt.; v) manganese 1.2 - 1.8% by wt.; vi) nickel 3.0 - 3.8% by wt.; vii) molybdenum 0.4 - l.0% by wt.; viii) copper - 1.5% max by wt.; ix) niobium 0.04 - 0.09% by wt.; x) titanium 0.02 - 0.08% by wt.; xi) aluminium 0.02 - 0.03% by wt.; xii) chromium - 1.5% max by wt. The subject invention also provides a process for preparing the aforesaid ultra high strength steel.

Full Text

-2- The present invention relates to development of ultra high strength steel and is also concerned with a process for preparing such steel. More particularly this invention pertains to the production of ultra high strength steel (identified herein as MICRO MSF-1700) using Mn, Ni, Mo, Cr and Cu along with Nb and Ti as microalloy components, thereby achieving a 160 ksi yield strength. Such low carbon ultra high strength steels using mieroalloying additions when subjected to radial forging followed by thermo-mechanical controlled processing (hereinafter referred to in this specification in its abbreviated form "TMCP") at various finish rolling temperatures (FRT) and accelerated cooling resulted in products found useful in making gun barrels, crank- shaft and for other structural purposes. During the last three decades considerable efforts were exerted towards development of micro-alloyed high strength steel using.No, Ti and V singly or in combination to achieve a good combination of mechanical properties. This Fe-C-Mn system alloyed with micro alloying elements undergoes thermo mechanical controlled processing (TMCP). New classes of high strength low alloy (HSLA) steel emerged to achieve better strength and toughness, better weldability and corrosion resistance. Two prime microstructures of th€!se new classes of steels are ferrite martensite (dual phase) and acicular ferrite. The basic objective of adding micro alloying elements is to hinder the recrystallisation and grain growth of austenite and by deforming the austenite, heavily pan- caked shaped grains can be produced. Ultimately this austenite when subjected to water quenching engenders fine acicular-ferrite along with some polygonal ferrite, to achieve attractive set of SRD/ns/FILE-1/PATENT —3— mechanical properties i.e, strength and toughness simultaneously. These fine ferrite grains are developed during controlled rolling technique through repeated recrystallisation. Microalloying elements not only help in developing fine ferrite grains, but also act as precipitation hardener where Mn increases strength properties through solid solution hardening. The strength properties of the acicular ferritic HSLA steel is achieved not only due to size of ferrite, but shape of ferrite also plays an important role and these steel posseses YS-500-600 Mpa, UTS-600- 750 Mpa along with toughness value 70-80 J at ambient temperature. One of the interesting properties of this kind of steel is low impact transition temperature value ~50°C, besides good weldability and corrosion resistance. These properties have led them for their use in several structural purposes. Microstructure and associated properties also vary depending on post cooling conditions. The major processing route for production of these categories of steels are thermomechanically controlled processing Controlled rolling though forging route is another viable technique to process these steels with better structural homogeneity and soundness. All these developments have opened a new era to produce strong and tough steel. These kinds of steels were produced earlier with lower carbon concentration and maintaining lower finish rolling temperatures. Reduction in carbon content during melting increases cost of melting operation of steel and maintaining lower finished rolling temperatures also required higher amount of rolling load and energy. SRD/ns/FILE-1/PATENT -4- The present invention aims at overcoming the drawbacks of the prior art and provides a solution by enhancing the carbon concentration of steel and using higher finish rolling temperature. The principal object of the present invention is to provide a novel microalloyed ultra high strength steel suited for structural purposes. A further object of this invention is to provide a process for preparing microalloyed ultra high strength steel for structural purposes. A still further object of this invention is to provide a microalloyed ultra high strength steel wherein acicular ferrite and/or Bainite in the microstructure is produceed by adopting controlled rolling operation followed by accelerated cooling. Another object of this invention is to provide a process for preparing microalloyed ultra high strength steel wherein products of desired grade and characteristics are obtained by radial forging and followed by controlled thermo mechanical processing. The foregoing objects are achieved by this invention according to which there is provided microalloyed ultra high strength steel having the composition - i) carbon - 0.06 - 0.15% by wt.; ii) phosphorus - 0.01% max by wt.; iii) sulphur - 0.01% max by wt.; iv) silicon - 0.2 - 0.7% by wt.; SRD/ns/FILE-1/PATENT v) manganese - 1.2 - 1.8% by wt.; vi) nickel - 3.0 - 3.8% by wt.; vii) molybdenum - 0.4 - 1.0% by wt.; viii) copper - 1.5% max by wt.; ix) niobium - 0.04 - 0.09% by wt.; x) titanium - 0.02 - 0.08% by wt.; xi) aluminium - 0.02 - 0.03% by wt.; xii) chromium - 1.5% max by wt.; The present invention further provides a process for preparing microalloyed ultra high strength steel as defined hereinabove, which comprises - a) introducing charge of raw materials in an electric arc furnace maintained at lower tap of around 2 for 15 minutes, using maximum tap voltage of 180 volts at tap 4 after subsidence of arc till the complete melting of the charge materials; b) lowering the furnace tap at 3 to avoid high temperature followed by addition of iron ore, mill scale and limestone along with Fe-Mn alloy and then carrying out slagging after around 40 minutes (slag out I); c) adding to the charge substances selected from lime stone, manganese and mill scale, followed by slag out (II) after 40 minutes of refining; d) adding after 20 minutes of slag out II in step(c), the following materials lime stone, Fe-Mn and mill scale SRD/ns/FILE-1/PATENT -6- and after 10 minutes of addition, and thereafter slagging out for the third time (III); e) after slag out III/ carrying out addition of lime stone, Fe-Si, manganese, Fe-Mo, pure nickel, metallic aluminium as per requirement and continuing refining; f) after 10 minutes of refining, adding Fe-Si and lime stone followed by slag out IV; g) carrying out complete slag out and making reducing slag by adding coke podwer, calcium carbide, spar and limestone, followed by addition of undernoted materials for higher requirement : Fe-Si, Fe-Mo, pure Ni-manganese, pure Cu, and covering the melt with reducing slag and tapping in the ladle preheated upto around 900°C; h) adding the following for formation of a right slag : lime - 10 kg/ton, spar - 3 kg/ton, pet coke - 0.3 kg/ton, Fe-Si - 3 kg/ton, Al - 1.2 Kg/ton, ramming mass - 50 kg and feeding 50 metres of Ca-Si and Al to the ladle as wire injection; i) adding to the ladle Fe-Nb and Fe-Ti for facilitating formation of micro-alloy, and j ) pouring the molten mass into moulds, followed by teeming, SRD/ns/FILE-1/PATENT -7- stripping of ingots, radial forging and thermo mechanical processing to obtain the desired products. The details of the experimental work are discussed below in which melting of the starting materials was carried out in an electric arc furnace of around 2.5 ton capacity. 10 inch square ingots were cast in metal moulds and thereafter forged in radial forging machine. Plants and equipments used in carrying out the production of microalloyed high strength steel are as given hereafter in numbered paragraphs : 1. Electric Arc Furnace : The present project work of "micro-alloyed steel MICRO MSF- 1700" has been planned to heat the charge and cast the ingots in 2T-EAF. It is a 2.5 ton capacity furnace and its details are given below. The furnace proper looks more like a saucepan covered from top with an inverted saucer. The electrodes are inserted through the cover from the top. The roof along with the electrodes swings clearly off the body to facilitate charging from top. SRD/ns/FILE-1/PATENT - 8 - The roof is lifted a little and the furnace body moves to one side clearly off the roof to facilitate charging. The furnace unit consists of following parts: i. Furnace body -i.e., the shell, the hearth, and the walls. The spout the door,etc. ii. Gears for furnace body movements, iii. Roof arrangements, iv. Electrodes, their holders and supports, v. Electrical equipment -i.e., the transformer, the cables the vi. Electrode control mechanism, etc. 2» The furnace body The furnace shell. The furnace shell is a welded or riveted steel plate construction and has a cylindrical saucepan like shape. It has spherical bottom. The spout for tapping the metal is welded to the bottom and the main door is situated directly opposite the spout. 3. Refractory lining: - The furnace shell in lined from inside with suitable refractory to suit the refining operation. The details are given below. 4. The hearth: - Next to the shell is a layer of firebricks at the bottom and is followed by magnesite brickwork to form the brick sub-hearth. Necessary gaps are left in the brickwork for its expansion on heating. Ramming tarred dolomite or magnesite makes the working hearth. Newly rammed hearth is slowly dried and brought to the working temperature by placing coke on the bottom and striking an SRD/ns/FILE-1/PATENT — 9 — arc against it. A freshly rammed hearth is given a slag wash at the working temperature. 5 The side wall. Magnesite, dolomite or chrome-magnesite (preferably metal encased) is used to line the sidewalk 6. The tap hole. A round former is inserted and the space left around is rammed to make the tap hole. 7. The door: - Furnaces have only one door directly opposite the tap hole and any additions during refining are made through this door. This is the main door and is also used for slagging. 8. The qears for furnace body movements. The furnace body needs to be tilted nearly through 45° on the tapping side and 15° on the slagging side. The tilting gear is either hydraulic or electrical. In electrical a motor actuates the rear of the furnace. Guide rails are used to ensure true movements. Some locking arrangement is necessary for the horizontal positioning of the furnace. The furnace is tilted around its center of gravity so that the spout also changes its positions. The ladle needs to be either suitably located even for later position of the spout or it should be adjusted during tapping. - 10 - 9. The roof: - The roof is a domed construction and rises about one per cent in span towards the center. The roof has three holes located symmetrically to allow insertion of the electrodes. The holes are made from ring-type bricks. The roof in an electric arc furnace is subject to greater thermal fluctuations and hence basic roof cannot be used. The current trend is to use high alumina, 70-80% AI2O3 bricks to make the roof. 10. The electrode and its support: - The electrode. The electrodes are of graphite and are capable of carrying current at high density. Their sizes of Electrodes are 6 inches. These are circular in cross-section and each piece is about l-3m long. The electrode pieces are joined, one to the other, with the help of a nipple. Electrode control. For efficient melting the arc must be stabilized to supply uniformly a maximum amount of energy. This necessitates keeping the distance between the electrode and the charge constant. As melting occurs this distance increases and if any solid piece collapses in, it may decrease. The electrode is automatically adjusted to maintain a stable arc. The electrode control systems either tries to maintain a constant current or constant voltage across the gap. A third type of control tries to maintain constant impedance in the individual electrode circuit. Each electrode is provided with an independent Electro-mechanical or electrode is hydraulic system of control. The electrode support. The furnace is provided with a vertical column or mast to which horizontal arms are attached and the electrode is held by the arm in position. The mast itself rises - 11 - and falls; the electrode arm is rigidly attached to the mast. It allows easy titling of the furnace. Electro-mechanical or Electro-hydraulic drives are used for activation of the masts in effecting electrode control. The electrode arm supporting the electrode clamp is generally tubular and is water-cooled. It is insulated from the mast as well as the clamp. The electrode is held in a vertical position in a water-cooled copper clamp situated at the end of the electrode arm. It is vital to have the contact of the clamp with the electrode as clean as is possible, so as to keep the contact resistance to an absolute minimum. ii# The Transformer: - The primary voltage may be 6 KV and the secondary with series tappings from 1-4 i.e., from 90-180 volts. During meting more power is required than during refining. The transformer capacity is designed to suit melting requirements. The capacity is, therefore, under-utilized during refining. The capacity of the transformer is 3000 KVA. 12. Scrap yard: - The scrap yard in 2TON-EAFis having varieties of scraps with low sulphur and phosphor the following scraps are frequently used for the production of different grades of steels, i. Railway scraps i.e., railway wheels, railway type, axle etc. ii. Rejected ingot scraps, iii. Shell scraps, iv. Strips v. Return scraps i.e., dead head cutting tail and cuttings etc. - 12 - Here scraps have been divided into 3 categories i. HNCM i.e., high Nickel-chrome-molybdenum, ii. LNCM i.e., low Nickel-chrome-molybdenum, iii. Plain carbon steel scraps. Characteristic need for scraps: It is preferred to get the moisture free and grease/oil free scraps. But unfortunately the same is not available at normal conditions. M&S factory, Ichapur has no scraps-preheating facility for moisture and oil-grease removal and also no facility of shot blasting to remove the rust 13. Ladle heating system: - Ladle in the EAF-2 Ton is being heated by hard coke while in case of eaf-15ton ladle is heated by oil fired burner upto a temperature of 900°- 1000°c. 14. Wire-feeding system: - This consists of CaSi and Al wires. These both are fed to liquid metal in automatic manner. 15• Teeming bay/ pit side with pouring mould facility: - There are different sizes of cast iron moulds ranges in sizes from 10", 14" & 16" and also 23". The moulds are arranged in the pit and liquid metal poured in moulds from the ladle with over head crane. Radial forging: - The Technical Data of Radial Forging Section is given below. - 13 - SPECIFICATIONS INPUT DIMENSIONS :- 650 mm Diameter (ESR Ingot) 23"- Octagon (LFVD Ingot) Max. Wt. = 7.05 Ton OUT PUT DIMENSIONS: - Min. Dia.= 75mm Square = 80mm Length Max. = 11000 mm TUBE DIMENSIONS: - Min. Bore = 60mm Max. Bore = 250mm Starting Length = 4300 mm Max. FORGING FORCE / DIE = 12000KN FORGING POWER = 1260KW NO. OF STROKES / DIE = 200/min ADJUSTMENT RANGE IN DIA = Upto 330mm 17. Furnaces: - Radial forging section have the following furnaces, all made by Stewart & Lloyds Ltd. 18. Batch type furnace: - 3 nos. each having 2 zones Size 4.5x4.5x2 misters Oil fired / and LPG fired Capacity = 50 tons/ heat Max. Temp=1250°C Electrode load = 120 Kw Rate of heating =150°C/hr. - 14 - 19. Working hearth fumes: - it is having 2 zones. Size 13x2.5x2metres Oil fired/and LPG fired Capacity 36tons/heat Electrode load = 110 Kw Temp=1150°C max. 20. Zones of bogie hearth furnace: - Size: - 11.00x2.00x2.00 meters Zones = 7 Capacity = 50tons Max temperature =1000°C Electrically, hearted -load: 1800kw Heating element: - 84nos/fce H.E material = Nichronthal- 40 i.e., 80-20nicr Size 1586x45x2.5 mm Wt/ element = 15kg Kw/element=21.6 Rate of heating = 200°C/hr 21. Pit furnace: - Have one zone Size: 2.0x3.0metres Temp = 600°C Electrically heated-load = 72kw Heating element: Ni-Cr=80-20. -15- The invention will be further illustrated by means of the following Examples which is given by way of illustration and not by way of limitation. (a) Charge planned : It is found the furnace capacity is 2280 kg. Basing on the furnace capacity the following charge materials were planned to load/charge. MSF CLASSIFICATION FORM OF SCRAP QTY (Kg) IV/9 Strip 1600 kg III/9 Ingot 570 kg Mild steel Pressed Bundles 120 kg(l no) Pure Nickel 75 kg Fe-Mo 20 kg Broken Electrodes 15 kg Total 2390 kg Considering the oxidation losses of alloy additions, higher than the required level was added in the charge. The chemical compositions of charges materials are given below : MSFCLAN C S P Si Mn Ni Cr Cu Al IV/9 0.09 .025 .025 0.13 .30- 0.15 0.02 0.20 0.03 SRD/ns/FILE-1/PATENT - 16 - 0.13 MAX MAX MAX .50 MAX MAX MAX 0.09 III/9 0.150.25 .06max .06max .35max 1.00max All the charge materials were placed in the furnace at a time. (b) Fettling: - After tapping the previous heat, lining is inspected. The slag-metal line, doorsill, tap hole, spout and damaged area of hearth are all repaired manually with the ramming mass i.e., magnesite. (c) Charging: - Roof is opened and furnace is charged with bucket charger. Initially 80Kg of the carbon steel pressed bundles are charged. Upon, III/9 ingot was placed. Over that, IV/9 strips are charged. Finally the remaining 40Kg of pressed bundles (C.S.) were put. (d) Power on: - After completion of charge, roof is placed and power is ON. Electrodes are lowered down and arc is struck. Furnace is kept at lower tap i.e., 2 for 15 mins. And when the arc subsides inside the scrap, maximum tap voltage is used i.e., 180 volts at tap 4. It took 2 hours for complete melting of the charge materials. (e) Refining: - - 17 - After ensuring the charge is melted, furnace tap is lowered i.e., at 3 in order to avoid higher temperature. For maintaining dephosphorisation favorable condition i.e., high oxidizing atmosphere and high basicity (and low temperature also) 50kg of Iron ore and 15kg mill scale is added along with 50kg of limestone. As the Mn requirement is above 1.5%, Fe-Mn of 10kg is also added. After 40mins slagging was done and sample 1 was taken. Keeping in view the reversible nature of phosphor and still high requirement of Mn, the following additions done. Lime stone-5okg-For basicity Mn-metal-10kg-For higher requirement Mill scale-10kg-For o2 potential After 40 mins of refining, slag out - II was done and the following additions were made after 20 mins of slag out- II. Lime stone-50kg Fe-Mn-5kg Mill scale-5kg After 10mins of additions slag out-Ill was done and again the following additions were made Lime stone-50kg-For basicity Fe-si-12 kg-For still higher requirement Mn-metal-35 kg-For still higher requirement Fe-mo-6 kg-For still higher requirement Pure nickel-5 kg-For still higher requirement AI-5 kg-For the requirement After lOmins of refining, sample -II was taken and To achieve the final requirement, the following additions were made after sample-II. Fe-si-15kg-for still higher requirement - 18 - Lime stone-50kg Again slag out-IV was done and sample-Ill was taken (f) Slag-off & reducing slag: - After comparing the above analysis with the required one, complete slag out (oxide slag) was done and the reducing slag was made by adding Coke powder-50kg CaC2 -5kg} for the removal of 02 in slag Spar & lime stone-for basicity The following alloy additions also made for still higher requirement Fe-Si-6kg Fe-Mo-6kg pure Ni-8kg Mn-Metal-8kg pure Cu-10kg. Higher tap position made for higher temperature, which is a favorable condition for desulphurization along with high basicity and low 02 potential. Finishing and tapping the heat: - Temperature measured and found as 1672°c which is sufficient for tapping. The melt is covered with the reducing slag and tapped in the already preheated (upto 900°c temperature) ladle. The following additions are made into the ladle for keeping a right slag. Lime - 10Kg/ton Spar - 3Kg/ton Pet coke -0.3Kg/ton Fe-Si-3Kg/ton - 19 - AI-1.2Kg/ton Ramming mass-50Kg Wire injection is also fed to the ladle as follows CaSi -50mtrs. AI-50mtrs. The following important micro-allay addition are made to the ladle Fe-Nb-21/2Kg Fe-Ti-21/2Kg During refining the total additions made in the furnace are as follows: - Additions Qty (kg) Lime stone 500 Iron ore 50 Flour spar 50 Coke power 50 Ca-carbide 50 Mill scale 30(shovels) Fe-Si 33 Fe-Mn 15 Fe-Mo 12 Pure Ni 13 Pure Al 5 Mn metal 53 P Cu 10 (h) Pit practice & teeming: - Ingot moulds were thoroughly cleaned by wire brush and acetylene sheet is applied to gins smooth coating for smooth removal of ingot. Corrugated card board or anti splashing disc or clear steel borings were kept at the bottom position of moulds to over come the problem of splashing during top pouring. Dead heads - 20 - were placed properly. The contact between mould and dead was sealed with asbestos threads for leak proof of hot metal as well as Casey removal of dead head. (i) Teeming: - Liquid steel tapped in ladle was taken to pitside with the help of overhead crane for teeming in moulds of size 10". During this top pouring ladle nozzle was centered over each hot top of each mould and was cast one by one in sequence. Total 5nos of wide-end-up 10" square moulds are used. After completion of teeming in every mould, the hot top was covered with exothermic powder and oxygen lancing . The total moulds filled are 41/2, and the total weight of metal cast was 570x4=2280Kg + 200Kg(approx) = 2480Kg (approx). (j) Sampling: - After completion of Teeming, 2No.s of Samples were collected. These were sent for chemical analysis and found as follows: (k) Stripping of ingots: - Ingots cast in the ingot moulds were stripped off when solidified. Solidification time is generally as follows : - {ingot size in inches (cross section)/2} 2- x mins. The Tapping temperature calculated as per the Appendix- C (1) RADIAL FORGING -21- Heating was carried out to one of the 4 ingots for subsequent forging as per the following HT cycle in the batch type furnace No-1, and the Time- Temperature curve given above shows the result. The loading and unloading of ingots into the furnace is done by the loader. Radial forging is a fully automated plant. The 10" ingot has been reduced to 95 mm Round Comer Square with the single heat. The finish forging Temperature was foundto be 952°C. After forging, the ingot was water cooled to room temperature. After breaking of scale, the temperature of ingot was found as 1111°C Forging started at 10-38 hrs Temperature found at 10-40 hrs = 1054ºC Forging finished at 10-45 hrs was at the temperature of 952°C. Time taken for completion of Forging =■ 07 mins. Thanao Mechanical Controlled Prooassing The ingots of 95 mm X 95 mm were reheated at various temperatures label (1150 to 1300°C) and controlled rolled to 14 mm thick plates. Finished rolling temperatures were 880°C, 850°C, 800°C and 750°C. Finally the rolled plates were subjected to either air cooling or water quenching. Table 1 : Chemical composition (WTPCT of the Steels) E1e- C Si S P Mn Ni Mo Cu Cr Nb Ti Al nents 0 0.06^ 0.2- 0.01 0.01 1 1.2- 3 .0- 0 .4- 2.1 1.5 3.04- 0. 02- 0.02 0 0.18 0.7 Max Max 1 1.8 3. 8 1. 0 Max Max 0.09 0. 06 Max SRD/ns/FILE-1/PATENT -22- Determation Ac1 and Ac3 tempereture of the steels Specimens for dilatometric study were machined from the forged bars. For low rates of cooling, specimens were 90 mm long and 6 mm in diameter. Specimens, for high cooling rates were 85 mm long and 6.3 mm in diameter with a 3 mm diameter hole through the sample centerline. Dilation curves were obtained by heating the sample (in an argon atmosphere) at a rate of 10°C/s from room temperature to 1200°C, austinitising at 1200°C for 240 seconds and then cooling at various controlled rates from 0.1°C/s in a Gleeble 1500 thermo mechanical simulator. Phase transformation temperatures were ontained by radial contraction measurements as a function of temperatures and samples formetallography and hardness were removed from the sections at switch temperature and dimensional changes were measured. Microatructural Characterisation Optical, SEM with EDS and TEM with EDS were used for microstructural characterization and X-ray diffractometric study was performed to determine the volume fraction of retained austenite in the TMCP steel. Evolution of Mechanical Properties The mechanical properties of TMCP steels were determined by hardness, Tensile, and Charpy Impact testing. Results Table below shows Ac1 and Ac3 temperatures of the steels from the dilametric Study. Table : Ac1 and Ac3 temperatures of the steel Steel Ac1 Ac3 642-685 840-855 SRD/ns/FILE-1/PATENT -23- Microstructure The microstructure at various finished rolling temperatures show predominantly lath martensite within pancaked grains and comparatively finer lath size has been obtained for the steel processed below 800°C finished rolling temperature (FRT). Transmission Electron Microscopic (TEM) study shows predominantly lath martensite structure at various FRTs. Formation of fine twins in the lath is also observed. Selected Area Diffraction Pattern shows reflection from TiN/TiCN and NbCN along with matrix. TEM study also reveals fine precipitates interacting with dislocation and copper particles distributed in the matrix. Mechanical Properties Tensile and impact values of the steels are given below : Tensile properties Impact toughness(CVN) Hardness UTS YS % Room Temp ~40°C (VHN) (Mpa) (Mpa) Elongation (Joules) (Joules) 398-452 1680-1750 1375-1450 12 - 17.8 60 - 70 55 - 65 It is a notable feature of this invention that the mechanical properties achieved in industrial heats in the course of production of "MICRO MSF-1700" ultra high strength steel are closely similar to those obtained in laboratory scale heats. In other words, translation of laboratory scale production into industrial scale production is quite successful and poses no procedural difficulties. -24- The microstructure of steel produceed by following the procedure described above has been found to contain a mixture of bainite and martensite, and the volume fraction of martensite varies with the rate of cooling. Air-cooled steel shows less amount of mertensite. The steel thus produced contains higher amount of carbon in comparison to HSLA-100 steel, whereby the yield strength and ultimate tensile strength are increased by ~400 to 450 Mpa attended with moderate toughness. This product possesses strength equivalent to ultra high strength steel. The steel possesses yield strength to the ~ of 1400 Mpa and UTS to the ~ of 1700 Mpa along with 15 to 17% elongation and toughness of 60 to 70 Joules. The strength values are much higher than those of HSLA-100 steel. In thermo mechanical controlled processing finish rolling temperature can also be maintained at a higher level. The microalloyed high strength steel produced in accordance with the present invention can be suitably used for making crank- shaft, hull, structural purposes, and also for making barrels such as gun barrels and small arms and ammunitions. As the present invention may be embodied in several forms without departing from the spirit or essential characteristics thereof, it should be understood that the above-described example is not limited by any of the details of the foregoing description, unless otherwise specified, but rather should be construed broadly within its spirit and scope as defined in the appended claims and, therefore, all changes and modifications that fall within the meets and bounds of the claims, or equivalents of such meets and bounds, are therefore intended to be embraced by the claims appended hereafter. -25- We claim : 1. Microalloyed ultra high strength steel having the composition- i) carbon 0.06 - 0.15% by wt.; ii) phosphorus - 0.01% max by wt.; iii) sulphur - 0.01% max by wt.; iv) silicon 0.2 - 0.7% by wt.; v) manganese 1.2 - 1.8% by wt.; vi) nickel 3.0 - 3.8% by wt.; vii) molybdenum 0.4 - 1.0% by wt.; viii) copper - 1.5% max by wt.; ix) niobium 0.04 - 0.09% by wt.; x) titanium 0.02 - 0.08% by wt.; XX) aluminium 0.02 - 0.03% by wt.; xii) chromium — 1.5% max by wt.; . 2. Microalloyed steel as claimed in Claim l, wherein aluminium and chromium are present in amounts of 0.02% and 1.0% by weight, respectively. 3. Microalloyed steel as claimed in Claims 1 and 2, wherein last traces of oxygen in the molten mass in the ladle are removed by wire injection of CaSi and/or Al in amounts as described hereinbefore. 4. Microalloyed ultra high strength steel, substantially as hereinbefore described with particular reference to the illustrative examples and graphical representations. SRD/ns/FILE-1/PATENT -26- 5. A process for preparing micro alloyed ultra high strength steel as claimed in Claims 1-4, which comprises - a) introducing charge of raw materials in an electric arc furnace maintained at lower tap of around 2 for 15 minutes, using maximum tap voltage of 180 volts at tap 4 after subsidence of arc till the complete melting of the charge materials; b) lowering the furnace tap at 3 to avoid high temperature followed by addition of iron ore, mill scale and limestone along with Fe-Mn alloy and then carrying out slagging after around 40 minutes (slag out I); c) adding to the charge substances selected from lime stone, manganese and mill scale, followed by slag out (II) after 40 minutes of refining; d) adding after 20 minutes of slag out II in step(c), the following materials lime stone, Fe-Mn and mill scale are added and after 10 minutes of addition, and thereafter slagging out for the third time (III); e) after slag out III, carrying out addition of lime stone, Fe-Si, manganese, Fe-Mo, pure nickel, metallic aluminium as per requirement and continuing refining; f) after 10 minutes of refining, adding Fe-Si and lime stone followed by slag out IV; -27- g) carrying out complete slag out and making reducing slag by adding coke podwer, calcium carbide, spar and limestone, followed by addition of undernoted materials for higher requirement : Fe-Si, Fe-Mo, pure Ni-manganese, pure Cu, and covering the melt with reducing slag and tapping in the ladle preheated upto around 900°C; h) adding the following for formation of a right slag : lime - 10 kg/ton, spar - 3 kg/ton, pet coke - 0.3 kg/ton, Fe-Si - 3 kg/ton, Al - 1.2 Kg/ton, ramming mass - 50 kg and feeding 50 metres of Ca-Si and Al to the ladle as wire injection; i) adding to the ladle Fe-Nb and Fe-Ti for facilitating formation of micro-alloy, and j) pouring the molten mass into moulds, followed by teeming, stripping of ingots, radial forging and thermo mechanical processing to obtain the desired products. 6. A process as claimed in Claim 5, wherein the ingots are released from moulds, subjected to radial forging, cooled to room temperature, reheated to various temperature levels varying between 1150° and 1300°C, rolled into 14 mm thick plates which were finally subjected to either air cooling or water quenching. -28- 7. A process as claimed in Claims 5 and 6, wherein initial forging temperature employed was around 1054°C, finishing temperature was around 952°C and time taken for completion of forging was 7 minutes. 8. A process for preparing microalloyed ultra high strength steel, substantially as hereinbefore described with particulars reference to the illustrative Example given hereinbefore. SHVRS/FJLE-J/JWOtT Conventional mode of production of high strength steel used lower carbon concentration, maintaining lower finish rolling temperatures. Reduction in carbon content during melting increased cost of melting operation of steel and lower finished rolling temperatures required high amount of rolling load and energy. The present invention aims at overcoming of above drawbacks by enhancing the carbon concentration of steel and using higher finish rolling temperatures. This invention provides a novel microalloyed ultra high strength steel having the composition - i) carbon 0.06 - 0.15% by wt.; ii) phosphorus - 0.01% max by wt.; iii) sulphur - 0.01% max by wt.; iv) silicon 0.2 - 0.7% by wt.; v) manganese 1.2 - 1.8% by wt.; vi) nickel 3.0 - 3.8% by wt.; vii) molybdenum 0.4 - l.0% by wt.; viii) copper - 1.5% max by wt.; ix) niobium 0.04 - 0.09% by wt.; x) titanium 0.02 - 0.08% by wt.; xi) aluminium 0.02 - 0.03% by wt.; xii) chromium - 1.5% max by wt. The subject invention also provides a process for preparing the aforesaid ultra high strength steel.

Documents:

00551-kol-2007-abstract.pdf

00551-kol-2007-claims.pdf

00551-kol-2007-correspondence others 1.1.pdf

00551-kol-2007-correspondence others 1.2.pdf

00551-kol-2007-correspondence others.pdf

00551-kol-2007-correspondence.pdf

00551-kol-2007-description complete.pdf

00551-kol-2007-form 1.pdf

00551-kol-2007-form 18.pdf

00551-kol-2007-form 2.pdf

00551-kol-2007-form 3.pdf

00551-kol-2007-form 9 1.1.pdf

00551-kol-2007-form-9.pdf

00551-kol-2007-gpa.pdf

551-KOL-2007-(26-03-2012)-CORRESPONDENCE.pdf

551-KOL-2007-(26-03-2012)-FORM-27.pdf

551-KOL-2007-CANCELLED DOCOMENT-1.1.pdf

551-KOL-2007-CANCELLED DOCUMENTS.pdf

551-KOL-2007-CLAIMS 1.1.pdf

551-KOL-2007-CLAIMS-1.2.pdf

551-KOL-2007-CORRESPONDENCE 1.3.pdf

551-KOL-2007-CORRESPONDENCE-1.4.pdf

551-KOL-2007-DESCRIPTION COMPLETE.pdf

551-KOL-2007-FORM 1.pdf

551-KOL-2007-FORM 2.pdf

551-KOL-2007-FORM 27.pdf

551-KOL-2007-OTHERS 1.1.pdf

551-KOL-2007-OTHERS-1.2.pdf

551-KOL-2007-REPLY TO EXAMINATION REPORT.pdf