Title of Invention

"A PROCESS FOR PRODUCING/COLD-TWISTED REBARS WITH INCREASED CORROSION RESISTANCE"

Abstract

The invention provides a process for producing cold-twisted-deformed steel reinforcement bars of increased yield strength and corrosion resistance, meant primarily for applications in the construction work undertaken in the coastal regions, comprising the steps in sequence: (a) making molten steel in a 250t twin-hearth furnace by feeding hot metal, scrap iron, lime and coke into the furnace as raw ingredients with addition of copper to the molten steel in the bath of the furnace and ferro-alloys to the molten steel in ladle in quantities of each said ingredient required to produce steel of grade (A) of composition (by weight %) : C-0.25 max., Mn-1.40 max., S-0.04 max., P-0.04 max., Si-0.08 max., Cr-0,40 min., Cu-0.25 min. and Fe-balance; and of grade (B) of composition (by weight %) : C-0.25 max., Mn-1.40 max., S-0.04 max., P-0.04 max., Si-0.08 max., Cr-0.40 min., Cu-0.25 min.; V-0.10 max. and Fe-balance; (b) casting the molten steel at a tapping temperature of 1640°C into ingots of weight 8t each; (c) reheating the ingots at a temperature 1340-1350°C for 2 hours; (d) rolling the reheated ingots into blooms of 325 mm x 325 mm x 4 m size each; (e) rolling the blooms into billets of 100 mm x 100 mm x 6 m size each; (f) reheating the billets at a temperature of 1200°C; (g) rolling the reheated billets into reinforcement bars of required diameters at a finish-rolling temperature of 960-980°C; (h) cooling the hot-rolled reinforcement bars to room temperature in natural air; and (i) cold twisting the cooled reinforcement bars with the cold deformation of 13D-11D for grade (A) and 11D-9D for grade (B) steel bars.

Full Text

The present invention relates to a process for producing high strength cold twisted steel reinforcement bars with increased corrosion resistance. The invention relates more particularly to a process for producing reinforcement bars of two grades, termed (A) (Cr-Cu) Fe-500/550 and (B) (Cr-Cu-V) Fe-600/650, having minimum yield strength of 500/550 MPa and 600/650 MPa, micro alloyed respectively with Cr-Cu and Cr-Cu-V, and higher corrosion resistance in comparison with the conventional plain (C-Mn) steel reinforcement bars. The construction of fly-overs, bridges, and buildings in the coastal regions requires the use of reinforcement steel bars of high corrosion resistance along with high yield strength. The conventional plain (C-Mn) steel reinforcement bars do not meet these requirements satisfactorily and consequently are made relatively heavy, and hence are expensive not only for increased material consumption but also for increased frieght, handling and fabrication cost and reduced service life in end uses. The object of the present invention is to provide a process for producing (C-Mn) steel reinforcing bars containing (Cr-Cu) and (Cr-Cu-V), having increased yield strength and corrosion resistance to make the same suitable for use in the construction work undertaken in the coastal regions. The other object is to provide a process for producing reinforcement bars of lighter weight for a given yield strength, and hence of reduced cost to the end-users thereof. The invented process comprises making molten steel in a furnace with minimum slag carry over during tapping of the same, adding copper in required quantity in the furnace bath containing molten steel, adding other ferro-alloys such as ferro-aanganese, ferro-vanadium, ferro-chrome and ferro-silicon in required quantities to molten steel in ladle, casting molten steel into ingots, reheating ingots and rolling ingots into blooms, rolling blooms into billets, reheating billets and rolling billets into reinforcement bars at a specified finishing temperature, cooling the hot rolled bars to room temperature in natural air, and cold twisting bars with varying degree of deformation. The invention is described fully and particularly in an unrestricted manner with reference to the accompanying drawings in which Figure 1 is a flow diagram illustrating the main steps followed in sequence in the invented process. Referring to Fig. 1, the process according to the present invention comprises the following steps in sequence :- (a) making molten steel in a 250t twin-hearth furnace by feeding hot metal - 230 to 240t, scrap iron - 50 to 60t, lime stone - 12 to I6t and coke as required to attain C level of 0.2596 mox (by weight), into the furnace as raw ingredients; (b) adding copper - 0.70 to 0.75t to the molten steel in the bath of the furnace; (c) adding ferro-alloys such as ferro-manganese - 6.0 to 6.5t, ferro-vanadium - 0.5t (if required), ferro-chrome - 2.0 to 2.5t and ferro-silicon - 0.5t to the molten steel in ladle; (d) adjusting the quantities of the ingredients added in steps (a) to (c) to produce steel of two grades, namely (A) (Cr-Cu) Fe-500/550 and (B) (Cr-Cu-V) Fe-600/650 of chemical composition as presented in Table 1 herein; (e) casting the molten steel at a tapping temperature of 1640°C into ingots of preferred weight 8t each; (f) reheating the ingots at a preferred temperature 1340-1350°C for 2 hours; (g) rolling the reheated ingots into blooms of preferred size 325 mm x 325 mm x 4 m each; (h) rolling the blooms into billets each of preferred size 100 mm x 100 mm x 6 m and weighing about 0.438t; (i) reheating the billets in a pusher type reheating furnace at a preferred temperature of 1200°C; (j) rolling the preheated billets into reinforcement bars of required diameters at a finish-rolling temperature preferably in the range of 960-980°C; (k) cooling the hot-rolled reinforcement bars in natural air to room temperature; and (l) cold twisting the cooled bars with the degree of cold deformation required for attaining the desired yield strength thereof. The tensile and bending properties of the two grades of reinforcement bars produced by varying the degree of deformation in cold twisting thereof are given in Table 2 from which it is noted that the minimum yield strength (YS), ultimate tensile strength (UTS) and % elongation at break, and maximum bend determined according to the method provided in IS : 1786-1985, meet the requirements desired for the two grades of bars by applying cold twisting to the extent of 13D to 11D for grade A and 11D to 9D for grade B. The corrosion resistance of the two grades of bar produced has been evaluated in the known Potentiodynamic method by applying an external potential in the range of -900 to -400 mV in steps of 10 mV through the Tafel region from the cathode to the anode side. From a plot of the applied potential and the corresponding current flow in the Tafel region, the corrosion rate of the bars was determined. The comparative test results obtained for the two grades (A and B) of the bars produced in the invented process and of the conventional plain C-Mn steel bars are presented in Table 3 from which it is noted that the corrosion resistance of the two grades (A and B) of bars is higher than the conventional plain C-Mn steel bars by 1.5 to 2.0 times. Field trials conducted on the application of the two grades of bars have shown satisfactory results. The process has been found to be cost effective in view of reduction in the weight of bars for a given yield strength with consequent savings in the cost of transportation, welding and fabrication of structures, and extended service life of the bars because of their increased corrosion resistance. Table 1 Chemical Composition of the Bars Produced. (Table Removed) We Claim :- 1. A process for producing high strength cold twisted steel reinforcement bars with increased corrosion resistance, characterised in that the process comprises the following steps : (a) making molten steel in a 250t twin-hearth furnace by feeding a charge containing hot metal - 230 to 240t, scrap iron - 50 to 60t, lime stone - 12 to l6t and coke - as required to attain C level of 0.25% mox (by weight), into the furnace, with addition of copper - 0.70 to 0.75t to the molten steel in the bath of the furnace, and ferro-alloys such as ferro- manganese - 6.0 to 6.5t, ferro-vanadium - 0«5t (if required); ferro-chrome - 2.0 to 2.5t and ferro-silicon - 0.5t, to the molten steel in ladle, to make the composition of the steel to be (by weight %): C-0.25 max. Mn-1.40 max., S-0.04 max., P-0.04 max., Si-0.08 max., Cr-0.40 min, Cu-0.25 min, and Fe-balance for Grade A bars or C-0.25 max., Mn-1.40 max., S-0.04 max., P-0.04 max., Si-0.08 max., Cr-0.40 min. Cu-0.25 min.; V-0.10 max. and Fe-balance for Grade B bars; (b) casting the molten steel at a temperature of 1640°C into ingots; (c) reheating the ingots at a temperature of 1340-1350°C for 2 hours; (d) rolling the reheated ingots into blooms of size 325 mm x 325 mm x 4 m each; (e) rolling the blooms into billets of size 100 mm x 100 mm x 6 m each; (f) reheating the billets at a temperature of 1200°C; (g) rolling the reheated billets into bars of required diameters at a finish-rolling temperature of 960-980°C; (h) cooling the hot-rolled bars to room temperature in natural air; and (i) cold-twisting the bars of grade A to the extent of 13D-11D; and the bars of grade B to the extent of 11D-9D. 2. The process as claimed in claim 1, wherein the ingots are of weight 8t each. 3. The process as claimed in claim 1 or 2, wherein the billets are reheated in a push bar type reheating furnace.

Documents:

673-del-1998-abstract.pdf

673-del-1998-claims.pdf

673-del-1998-correspondence-others.pdf

673-del-1998-correspondence-po.pdf

673-del-1998-description (complete).pdf

673-del-1998-drawings.pdf

673-del-1998-form-1.pdf

673-del-1998-form-19.pdf

673-del-1998-form-2.pdf

673-del-1998-form-4.pdf

673-del-1998-gpa.pdf