Title of Invention

"AN IMPROVED PROCESS FOR PRODUCING STEEL REINFORCEMENT BARS"

Abstract

The invention provides an improved process for producing steel reinforcement bars of increased tensile strength, elongation at break and corrosion resistance, comprising the following steps in sequences (i) preparing molten steel in open hearth/LD furnace with addition of the required alloys mostly in ladle and of copper,nickel, ferro alloys in furnace to produce reinforcement bars debars) of compositions: C-0.16 to 0.20, Mn-0.80 to 1.10, S-0.03 (max), P-0.05 (max), Si-0.05 to 0.08, Fe-the balance, for the plain-carbon grade rebars, with addition of Cu-0.20 to 0.30 for each of the Cu bearing and Cu-Cr bearing grades of the rebars, and with addition of Cr-0.40 to 0.80 and Ni-0.20 to 0.30 for the Cu-Cr bearing grade of the rebars produced; (ii) casting of semi-killed/killed steel into ingots (bottom/top poured) each of weight 8 ton and size-top 770 x 700 mm, bottom 700 x 630 mm, height 2.4 m, or into slabs/ billets (continuous cast) (each billet of size - 100 mm x 100 mm x 9 m); (iii) soaking of the ingots in soaking pits at 1250-1300°C for 4-6 hours; (iv) rolling of the ingots into billets; (v) reheating of the folled/concast billets at 1225-1250°C in furnace; (vi) hot rolling of the billets in Merchant mill into rebars of 12 to 25 mm diameter by means of 14 strand groove rollers at a finishing temperature of 925 to 960°C; (vii) partial cooling of the rebars at a controlled rate of cooling varying from 125 to 310°C/Sec in at least one cooling water pipe; (viii) further cooling of the rebars in a cooling bed at an equalisation temperature of 560 to 680°C and at a cooling rate of 200 to 300°C/hour.

Full Text

The present invention relates to an improved process for producing steel reinforcement bars (rebars). The invention relates more particularly to a process for producing thermo- mechanic ally treated (TMT) plain-carbon and alloy (Cu/Cu-Cr ) steel rebars of improved quality as required for specific areas of applications. The torsteel rebars produced in the conventional process of manufacture are found to be deficient in tensile strength under cold and hot (600°C) conditions, elongations at break, toughness, bend and corrosion resistance as required for applications in the construction of bridges, flyovers, multistoried buildings, basement of chemical plants, off-shore structures, and storage for chemicals and industrial by-products. The object of the present invention is to provide an improved process for producing plain carbon as well as alloy steel rebars having higher tensile strength under cold and hot (600°C) conditions, elongation, toughness, bend and corrosion resistance compared with the torsteel rebars produced in the conventional process. The invented process is distinguished from the conventional process for producing torsteel rebars mainly in respect of the optimisation of (a) chemical composition of the alloy steel, (b) flow rate and pressure of cooling water used for cooling of the rebars, hot rolled in Merchant mills, and (c) the equalisation temperature of the bars produced. The invention is described fully and Particularly in an unrestricted manner with reference to the accomoanying drawings, in which - Figure 1 is a schematic diagram of the water cooling pipe used for controlling the cooling rate of hot rolled rebars; Figure 2 shows microstructure of torsteel rebars; Figure 3 shows microstructure of TMT plain carbon rebars; Figure A shows microstructure of TMT Cu bearing rebars; Figure 5 shows microstructure of TMT Cu-Cr bearing rebars. In Fig. 2, (a) shows the microstructure at the core and (b) shows the same at the rim region of the rebars. In each of Figs. 3 to 5, the microstructures shown in (a), (c) and (e) are for the core region and those shown in (b), (d) and (f) are for rim region of the rebars of nominal yield strength 415, 500 and 550 MPa respectively. The invented process comprises the following steps in sequence :- i) preparing molten steel in open hearth/ID furnace with addition of alloying elements mostly in ladle and of copoer in furnace to produce reinforcement bars (rebars) of chemical composition (by weight %) : C-0.16 to 0.20, Mn-0.80 to 1.10, S-0.03 (max), P-0.05 (max), Si-0.05 to 0.08, Fe-the balance for the plain-carbon grade rebars, with addition of Cu-0.20 to 0.30 for each of Cu bearing and Cu-Cr bearing grades of rebars, and further addition of Cr-0.40 to 0.80 and Ni-0.20 to 0.30 for the Cu-Cr bearing grade of rebars produced; ii) casting of semi-killed/killed steel into ingots (bottom/too poured) each of weight 8 ton and size - top 770 x 700 mm, bottom 700 x 630 mm, height 2.4 m; or into billets (continuous cast) (each billet, of size 100 mm x 100 mm x 9 m); iii) soaking of ingots in soaking pits at 1250-1300°C for 4-6 hours; iv) rolling of ingots into billets; v) reheating of rolled/concast billets at 1225-1250°C in furnace; vi) hot rolling of billets in Merchant mill into rebars of 12 to 25 mm diameter by means of 14 strand groove rollers at a finishing temperature of 925 to 960°C; vii) partial cooling of the rebars carried out in at least one water cooling Pipe, such as herein described, at cooling rate varying from 135 to 310°C/Sec by varying the pressure of pressurised cooling, water from 0.25 to 6.00 MPa, more preferably from 1.5 to 6.0 MPa and also the flow rate of compressed air as required; viii) further cooling of the rebars in a cooling bed at an equalisation temperature of 560 to 680°C and at a cooling rate of 200 to 300°C/hour with air blast to Prevent formation of elongated copper/Cr-carbide precipitates, which enhances embrittlement of the rebars; The nominal and optimum chemical compositions of the three grades of TMT rebars produced in the invented process are presented in Table-I from which it is noted :- (a) the nominal content (by weight %) of C, Mn, S, P and Si is the same in each of the three grades of TMT rebars, namely, plain-carbon, Cu bearing and Cu-Cr bearing i.e. C-0.16 to 0.20, Mn-0.80 to 1-10, S-0.03 (max), P-0.05 (max) and Si-0.05 to 0.08; (b) nominal content of Cu in Cu bearing and Cu-Cr bearing grades of TMT rebars is 0.20 to 0.30; (c) actual content of Cu in Cu bearing and Cu-Cr bearing grades of TMT rebars is 0.20 and 0.30 respectively; (d)nominal and actual content of Cr in Cu-Cr bearing grade of TMT rebars is 0.40 to 0.80 and 0.80 respectively; (e) nominal and actual content of Ni in Cu-Cr bearing grade of TMT rebars is 0.20 to 0.30 and 0.30 respectively; and (f) optimum composition of Cu-Cr bearing grade of TMT rebars is : C - 0.14 to 0.18, Mn-O.yO to 0.80, S-0.02 to 0.025, P-0.02 to 0.03, Si-0.02 to 0.03, Cu-0.30 (max), Cr-0.50 to 0.60, and Ni-0.08 to 0.10, the balance being Fe. Referring to Fig. 1, the cooling oioe comprises an inner pipe (4) through which the hot rolled rebars are passed from the entrance end (11) to the delivery end (12) for treating the same thermo-mechanically. The inner pipe is disposed co-axially in outer oipe (4a) of internal diameter 30/40/50 mm and length 7.0 m. In the outer pipe are provided inlet funnel (1), prechamber (2), return pipe (9) for cooling water, inlet (8a) for pressurised water, inlet (8b) for pressurised water, supports (13, 14), back water chamber (6) with return pipe (9), drain pipe (5) for free water outlet, deflection chamber (7), inlet (10) for compressed air. The inner Pipe/ has got 1st and 2nd nozzle heads (3a, 3b). The hot rolled rebars are treated thermo-mechanically by controlling the cooling rate of TMT rebars at 135 to 310°C/ sec by varying the flow rate of cooling water through the cooling pipe (4a) and also inlet water pressure at 0.25 to 6.0 MPa, more preferably at 1.5 to 6.0 MPa, using one or two cooling water pipes (as per finished bar diameter). The equalisation temperature for the rebars which are Partially cooled in the cooling pipe(s) is carried out at a temperature varying from 560 to 680°C in the cooling bed, more preferably from 570 to 650°C. Rebars of 20 mm diameter for each of the three grades, namely plain-carbon, Cu bearing and Cu-Cr bearing are treated thermo-mechanically at varying cooling rate, using one or two water cooling pipes, and subjected to equalisation treatment at varying temperature of the cooling bed. The yield strength of thermo-mechanically treated (TMT) rebars is determined. The results obtained are presented in Table-II from which it is noted :- (a) yield strength increases with increase in the cooling rate and inlet pressure of cooling water, and with decrease in the equalisation temperature; and (b) optimum conditions for Producing TMT rebars of the three grades are as given in Table III. The tensile strength, elongation, drop in yield strength at 600°C compared with yield strength at room temperature, impact strength, bending property and corrosion resistance of the three grades of TMT rebars produced in the invented process have been studied and compared with those of torsteel rebars produced in the conventional process. The results are presented in Table IV from which it is noted :- (a) tensile strength, elongation and impact value of TMT rebars are appreciably higher in comparison with those of torsteel rebars; (b) corrosion rate, particularly in salt fog, of TMT rebars is appreciably lower in comparison with that of torsteel rebars; (c) tensile strength of TMT rebars increases from plain- carbon to Cu bearing and from Cu bearing to Cu-Cr bearing TMT rebars; (d) drop in yield strength at 600°C from that at room temperature is lower in Cu and Cu-Cr bearing TMT rebars compared with plain-carbon TMT rebars; (e) corrosion rate of TMT rebars decreases from plain- carbon to Cu bearing and from Cu bearing to Cu-Cr bearing TMT rebars. From a comparison of the micro-structure of the torsteel and TMT rebars as illustrated in Figs. 2 to 5, it is noted :- (a) microstructure of torsteel rebars (Fig. 2) is Ferrite-Fearlite both at the core and rim regions thereof; (b) microstructure of TMT plain-carbon (Fig. 3), Cu bearing (Fig. 4) and Cu-Cr bearing (Fig. 5) TMT rebars are generally each Ferrite-Pearlite at the core region and tempered Martensite at the rim region thereof, and more particularly, Martensite near the surface and Bainite, Ferrite, Acicular Ferrite and pearlite towards/at the centre of the rebars. Table-I : Nominal, Actual and Optimum Chemical Composition of Different Grades of Steel Produced. (Table Removed) Table-II : Yield Strength of Rebars Thermo-Mechanically Treated and Equalised under Varying Condition. (Table Removed) Table-III : Optimum conditions for thermo-mechanical treatment (TMT) and equalisation treatment (ET) for the three grades of rebars. (Table Removed) Table IV: Comparative properties of TMT Rebars produced in the invented process and Torsteel Rebars produced in the conventional Drocess. (Table Removed) * No transverse crack. ** Standard 2 mm V-notch charpy specimen (10x10x55 mm) from the central region of the bars. We Claim :- 1. An improved process for producing steel reinforcement bars of increased tensile strength, elongation at break and corrosion resistance, comprising the following steps in sequence; (i) preparing molten steel in open hearth/ID furnace with addition of the required alloys mostly in ladle and of copper alloy in furnace to produce reinforcement bars (rebars) of specified compositions, such as herein described; (ii) casting of semi-killed/killed steel into ingots (bottom/top poured) each of weight 8 ton and size-top 770 x 700 mm, bottom 700 x 630 mm, height 2.4 m, or into slabs/billets (continuous cast) (each billet of size - 100 mm x 100 mm x 9 m); (iii) soaking of the ingots in soaking pits at 1250-1300°C for 4-6 hours; (iv) rolling of the ingots into billets; (v) reheating of the rolled/concast billets at 1225-1250°C in furnace; (vi) hot rolling of the billets in Merchant mill into rebars of 12 to 25 mm diameter by means of 14 strand groove rollers at a finishing temperature of 925 to 960°C; (vii) partial cooling of the rebars at a controlled rate of cooling in at least one cooling water pipe, such as herein described; (viii) further cooling of the rebars in a cooling bed at an equalisation temperature and at a cooling rate, as herein stated; characterised in that (a) the chemical compositions of the rebars (by weight %) are: C-0.16 to 0.20, Mn-0.80 to 1.10, S-0.03 (max), P-0.05 (max), Si-0.05 to 0.08, Fe-the balance,for the plain-carbon grade rebars, with addition of Cu-0.20 to 0.30 for each of the Cu bearing and Cu-Cr bearing grades of the rebars, and with addition of Cr-0.40 to 0.80 and Ni-0.20 to 0.30 for the Cu-Cr bearing grade of the rebars produced; (b) partial cooling of the rebars in at least one water cooling pipe is performed at a cooling rate varying from 135 to 310°C/sec; and (c) further cooling the rebars in a cooling bed is carried out at an equalisation temperature of 560 to 680°C and at a cooling rate of 200 to 300°C/hour. 2. The process as claimed in claim 1, wherein the compositions of the rebars (by weight %) are : C-0.17, Mn-0.88, S-0.038, P=0.023, Si-0.075 for the plain-carbon bearing grade; C-0.18, Mn-0.90, S-O.O4, P-0.035, Si-0.065, Cu-0.20 for the Cu bearing grade and S-0.18, Mh-0.82, S-0.036, P-0.02, Si-0.07, Cu-0.30, Cr-0.80, Ni-0.30 for the Cu-Cr bearing grade, the balance in each said grade being Fe. 3. The process as claimed in claim 1, wherein the composition (by weight %) of the Cu-Cr bearing grade of rebars is : C-0.14 to 0.18, Mn-0.70 to 0.80, S-0.02 to 0.025, P-0.02 to 0.03, Si-0.05 to 0.0? Cu-0.30 (max), Cr-0.50 to 0.60, Ni-0.08 to 0.10 and Fe-the balance. 4. The process as claimed in any of the preceding claims, wherein pressure of the pressurised cooling water used in at least one water cooling pipe is varied from 0.25 to 6.00 MPa, more preferably from 1.5 to 6.0 MPa. 5. The process as claimed in any of the preceding claims, wherein the rebars are partially cooled in two water cooling pipes.

Documents:

210-del-1998-abstract.pdf

210-del-1998-claims.pdf

210-del-1998-correspondence-others.pdf

210-del-1998-correspondence-po.pdf

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

210-del-1998-drawings.pdf

210-del-1998-form-1.pdf

210-del-1998-form-19.pdf

210-del-1998-form-2.pdf

210-del-1998-form-4.pdf

210-del-1998-gpa.pdf