Title of Invention

A PROCESS FOR PRODUCING LOW-ALLOY STEEL PLATES OF YIELD STRENGTH AT 450°C NOT LESS THAN TWO-THIRD OF THAT AT ROOM TEMPERATURE

Abstract

A process for producing low-alloy steel plates of yield strength at 450¯C not less than two-third of that at room temperature, suitable for fabrication of carrier tubes and heat exchanger shells for hot gases, characterised in that the process comprises the following steps in sequence: (a) melting predetermined quantities of pig iron, scrap iron, iron ores, aluminium bars and ferro-alloys such as petroleum coke, Si-Mn and Fe-Mo, in a basic oxygen furnace (BOF), and transferring to ladle as molten iron of chemical composition (by weight %): C-O.15 to 0.20, Mn-0.75 to 1,00, P-0.020 max., S-0.015 max., Si-0.25 to 0.50, Al-0.02 max., Mo-0.35 to 0.50, N-100 ppm max., H-2 ppm max. and Fe-the balance; (b) tapping the molten steel in a preheated ladle at 1630 Ý 20¯C; (c) transferring the molten steel from the ladle to a vacuum arc degassing (VAD) unit; (d) transferring the molten steel from the VAD unit to tundish of a continuous casting (CC) machine; (e) casting the molten steel at tundish temperature 1540-1550¯C into slabs of thickness 200 to 250 mm and of width 1500 mm in the CC machine at a speed 0.6-0.8 m/min and slab unbending temperature 900-920¯C; (f) cooling the slabs to room temperature in natural air; (g) reheating the slabs and soaking the reheated slabs in a furnace at 1240 Ý 20¯C for at least 3 hours; (h) rolling the slabs into plates of thickness 8-32 mm in a plate mill with 10-20% reduction per pass at a finishing temperature of 850-900¯C; (i) normalising the plates of thickness above 10 mm at 910 Ý 20¯C for 30-45 minutes; (j) cooling the plates to room temperature in natural air; and (k) straightening the plates which are found to be bent.

Full Text

The present invention relates to a process for producing low-alloy steel plates of yield strength at 450°C not less than two-third of that at room temperature. The invention relates more particularly to a process for producing steel plates suitable for fabrication of carrier tubes and shells for heat exchargers and hot gases and various other applications where conventional mild steel is not found suitable because of reduction of its yield strength at an elevated temperature of 450°C to be below two-third of its yield strength at room temperature. The demand for low-alloy steel plates of yield strength at 45O°C of at least two-third of its yield strength at room temperature and superior weldability with low carbon-content steel electrodes has been increasing rapidly in the recent past for fabrication of carrier tubes and shells for heat exchangers, hot gases and other like applications. There is no process for producing such steel plates in the prior art. The object of the present invention is to provide a process for producing low alloy-content steel plates of yield strength at 450°C not less than two-third of its yield strength at room temperature and of superior weldability with low-carbon steel electrodes to meet the abovementioned growing demand. At present, there is no known process for producing such steel. The steel is produced in the invented process following the basic oxygen furnace (BOP)-continuous casting (CC) route, and converted into plates of thickness 8-32 mm. The chemical composition of the steel produced has been optimised by carrying out extensive investigations on the pilot plant scale. The microstructure, yield Strength, Charpy impact toughness and weldability of the steel plates produced have been studied at different temperatures. - 2 - Thus the invention provides a process for producing low-alloy steel plates of yield strength at 450°C not less than two-third of that at room temperature, suitable for fabrication of carrier tubes and heat exchanger shells for hot gases, characterised in that the process comprises the following steps in sequence: (a) melting predetermined quantities of pig iron, scrap iron, iron ores, aluminium bars and ferro-alloys such as petroleum coke, Si-Mn, and Fe-Mo, in a "basic oxygen furnace (BOF), and transferring to ladle &s molten iron of chemical composition (by weight 90: C-0.15 to 0.20, Mn-0.75 to 1,00, P-0.020 max., S-0.015 max., Si-0.25 to 0.50, Al-0.02 max., Mo-0.35 to 0.50, N-100 ppm max., H-2 ppra max. and Fe-the balance; (b) tapping the molten steel in a preheated ladle at 1630 ± 20°C; (c) transferring the molten steel from the ladle to a vacuum arc degassing (VAD) unit; (d) transferring the molten steel from the VAD unit to tundish of a continuous casting (CC) machine; (e) casting the molten steel at tundish temperature 1540-1550°C into slabs of thickness 200 to 250 mm and of width 1500 mm in the CC machine at a speed 0.6-0.8 m/min and slab unbending temperature 900-920°C; (f) cooling the slabs to room temperature in natural air; (g) reheating the slabs and soaking the reheated slabs in a furnace at 1240 £ 20°C for at least 3 hours; (h) rolling the slabs into plates of thickness 8-32 mm in a plate mill with 10-20& reduction per pass at a finishing temperature of 850-900°C; (i) normalising the plates of thickness above 10 mm at 910 ± 20°C for 30-45 minutes; (j) cooling the plates to room temperature in natural air; and (k) straightening the plates which are found to be bent. - 3 - and (vi) the plates of thickness above 10 mm are normalised by soaking at 910 + 20°C for 30-45 minutes. 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 showing the main steps followed in the invented process. Referring to Fig. 1, in the invented process predetermined quantities of pig iron, iron scrap, iron ores, aluminium bars and ferro-alloys such as petroleum coke, Si-Mn, Fe-Mo and Fe-Si are melted in a basic oxygen furnace (BOF); the molten steel is tapped in a preheated ladle at a temperature of 1630 £ 20°C; ifa(r) molten steel is transferred from the ladle to a vacuum arc degassing (VAD) unit for lowering sulphur content of the liquid steel to a level not exceeding 0.015^ by weight and dissolved hydrogen content to a level 2 ppm max.; chemical composition of the liquid steel is optimised to be (by weight %) C-0.15 to 0.20, Mn-0.75 to 1.00, P-0.020maX., S-0.015 max., Si-0.25 to 0.50, Al-0.02 max., M0-0.35-0.50, N-100 ppm max., H-2 ppm max. and Fe-the balance, and more preferably to be (by weight %) C-0.15 to 0.17, Mn-0.75 to 0.80, P-0.020 max., S-0.015 max., Si-0.25 to 0.35, Al-0.02 max., Mo-0.45 to 0.50, N-100 ppm max. and Fe-the balance; the liquid steel is transferred from the VAD unit to a continuous casting (CC) machine for casting the liquid steel at tundish temperature of 1540 to 155O°C into slabs of preferred thickness 200 to 250mm, width 1500mm and 'unbending point' temperature above 850°C; - 4 - the slabs are cooled to room temperature in natural air, and are reheated and soaked at a temperature of 1240 + 20°C for at least 3 hours; the slabs are rolled in a plate mill into plates of thickness 8 to 32 mm in 17 to 19 rolling passes at a finishing temperature of 850-900°C; the plates of "ttiickness above 10 mm are reheated and normalised by soaking at a temperature of 910 + 20°C for 30-45 minutes; the plates are cooled to room temperature in natural air and inspected ultrasonic ally for detecting flows like cracks, fissures and voids present therein; flaw-free plates of thickness 8-32 mm are straightened, if found necessary, and made ready for storage/despatch* Micrographic studies conducted on the plates reveal a polygonal ferrite pearlite microstructure. The mechanical properties of the plates are found to be! yield strength (YS) 280 MlPa The plates are found to be of satisfactory weldability with low-carbon steel electrodes containing Mo-0.20 to 0.50%. Field trials have been conducted on the plates produced on a large scale in the invented process by fabricating carrier tubes and shells for hot gases from the steel plates. The performance of the steel plates has been found to be satisfactory. - 5 - We Claim :- 1. A process for producing low-alloy steel plates of yield strength at 450°C not less than two-third of that at room temperature, suitable for fabrication of carrier tubes and heat exchanger shells for hot gases, characterised in that the process comprises the following steps in sequence: (a) melting predetermined quantities of pig iron, scrap iron, iron ores, aluminium bars and ferro-alloys such as petroleum coke, Si-Mn and Fe-Mo, in a basic oxygen furnace (BOF), and transferring to ladle as molten iron of chemical composition (by weight %)i C-0.15 to 0.20, Mn-0.75 to 1.00, P-0.020 max., S-0.015 max., Si-0.25 to 0,50, Al-0.02 max., Mo-0.35 to 0.50, N-100 ppm max., H-2 ppm max. and Fe-the balance; (b) tapping the molten steel in a preheated ladle at 1630 ± 20°C; (c) transferring the molten steel from the ladle to a vacuum arc degassing (VAD) unit; (d) transferring the molten steel from the VAD unit to tundish of a continuous casting (CC) machine; (e) casting the molten steel at tundish temperature 1540-1550°C into slabs of thickness 200 to 250 mm and of width 1500 mm in the CC machine at a speed 0. 6-0.8 m/min and slab unbending temperature 900-920°C; (f) cooling the slabs to room temperature in natural air; (g) reheating the slabs and soaking the reheated slabs in a furnace at 1240 ± 20°C for at least 3 hours; (h) rolling the slabs into plates of thickness 8-32 mm in a plate mill with 10-20% reduction per pass at a finishing temperature of 850-900°C; (i) normalising the plates of thickness above 10 mm at 910 ± 20°C for 30-45 minutes; (j) cooling the plates to room temperature in natural air; and (k) straightening the plates which are found to be bent. - 6 - 2. The process as claimed in claim 1, wherein the chemical composition of the steel plates produced is (by weight 90 : C-0.15 to 0.17, Mn-0.75 to 0.80; P-0.020 max., S-0.015 max., Si-0.25 to 0.35, Al-0.02 max., Mo-0.45-0.50, N-100 ppm max., and Be- the balance. 3. A process for producing low-alloy steel plates of yield strength at 450°C not less than two-third of that at room temperature, substantially as herein described and illustrated in the accompanying drawings. - 7 - A process for producing low-alloy steel plates of yield strength at 450¯C not less than two-third of that at room temperature, suitable for fabrication of carrier tubes and heat exchanger shells for hot gases, characterised in that the process comprises the following steps in sequence: (a) melting predetermined quantities of pig iron, scrap iron, iron ores, aluminium bars and ferro-alloys such as petroleum coke, Si-Mn and Fe-Mo, in a basic oxygen furnace (BOF), and transferring to ladle as molten iron of chemical composition (by weight %): C-O.15 to 0.20, Mn-0.75 to 1,00, P-0.020 max., S-0.015 max., Si-0.25 to 0.50, Al-0.02 max., Mo-0.35 to 0.50, N-100 ppm max., H-2 ppm max. and Fe-the balance; (b) tapping the molten steel in a preheated ladle at 1630 Ý 20¯C; (c) transferring the molten steel from the ladle to a vacuum arc degassing (VAD) unit; (d) transferring the molten steel from the VAD unit to tundish of a continuous casting (CC) machine; (e) casting the molten steel at tundish temperature 1540-1550¯C into slabs of thickness 200 to 250 mm and of width 1500 mm in the CC machine at a speed 0.6-0.8 m/min and slab unbending temperature 900-920¯C; (f) cooling the slabs to room temperature in natural air; (g) reheating the slabs and soaking the reheated slabs in a furnace at 1240 Ý 20¯C for at least 3 hours; (h) rolling the slabs into plates of thickness 8-32 mm in a plate mill with 10-20% reduction per pass at a finishing temperature of 850-900¯C; (i) normalising the plates of thickness above 10 mm at 910 Ý 20¯C for 30-45 minutes; (j) cooling the plates to room temperature in natural air; and (k) straightening the plates which are found to be bent.

Documents:

02015-cal-1998 abstract.pdf

02015-cal-1998 assignment.pdf

02015-cal-1998 claims.pdf

02015-cal-1998 correspondence.pdf

02015-cal-1998 description(complete).pdf

02015-cal-1998 drawings.pdf

02015-cal-1998 form-1.pdf

02015-cal-1998 form-2.pdf

02015-cal-1998 form-3.pdf

02015-cal-1998 form-5.pdf

02015-cal-1998 pa.pdf

2015-cal-1998-granted-abstract.pdf

2015-cal-1998-granted-acceptance publication.pdf

2015-cal-1998-granted-assignment.pdf

2015-cal-1998-granted-claims.pdf

2015-cal-1998-granted-correspondence.pdf

2015-cal-1998-granted-description (complete).pdf

2015-cal-1998-granted-drawings.pdf

2015-cal-1998-granted-examination report.pdf

2015-cal-1998-granted-form 1.pdf

2015-cal-1998-granted-form 2.pdf

2015-cal-1998-granted-form 3.pdf

2015-cal-1998-granted-form 6.pdf

2015-cal-1998-granted-letter patent.pdf

2015-cal-1998-granted-pa.pdf

2015-cal-1998-granted-reply to examination report.pdf

2015-cal-1998-granted-specification.pdf