Title of Invention

"A PROCESS FOR PRODUCING HIGH STRENGTH STEEL PLATES"

Abstract

A process, for producing high strength steel plates of thickness upto 32 mm, comprising the following steps in sequence: (1) preparing molten steel of chemical composition (by weight %): C-0.12 to 0.15, Mn-1.30 to 1.40, P-0.02 max, S-0.01 max, Si-0.30 to 0.40, Al-0.015 to 0.025, Nb-0.04 to 0.06, Ti-0.02 to 0.03, N-100 ppm max, Fe-balance, for plates of thickness less than 20 mm; and C-0.13 to 0.16, Mn-1.35 to 1.40, P-0.02 max, S-0.01 max, Si-0.30 to 0.40, Al-0.015 to 0.025, Nb-0.05 to 0.07, Ti-0.02 to 0.03, N-100 ppm max, Fe-balance, for plates of thickness 20-32 mm in a BOF converter; (ii) refining the chemical composition of the molten steel in a VAD plant; (iii) converting the molten steel into slabs of required cross-section by continuous casting characterised in that (a) slabs are continuously cast at tundish temperature of 1540-1550°C, casting speed of 0.6-0.8 m/min, cooling water consumption in secondary cooling stage of 0.45-0.50 litre/kg of steel produced and unbending point temperature of above 850°C; (b) the slabs are reheated to a temperature of 123O°C max; (c) the slabs are rolled at an initial temperature of 1100°C max in a roughing stand with reduction in slab thickness of 1596 min in each rolling pass; and in a finishing stand at an initial temperature of 900°C max and finish rolling temperature of 810 ± 10°C for plates of thickness 20-32 mm and of 830°C max for plates of thickness less than 20 mm with minimum cumulative reduction in plate thickness of 60% and minimum reduction in plate thickness of 10% in the last roll ing pass: (d) plates of thickness 20-32 mm are cooled by water-spraying to a temperature of 650 ± 20% at a cooling rate of 5-9°C/sec, followed by air cooling and (e) plates of thickness 8-20 mm are air-cooled to the ambient temperature.

Full Text

The present invention relates to a process for producing high strength steel plates of minimum yield strength 450 MPa and thickness upto 32 mm. The invention relates more particularly to a process for producing 8 to 32 mm thick high strength steel plates which are microalloyed with Nb, Ti and V, and of minimum yield strength (YS) 450 MPa, tensile strength (TS) 570 MPa, elongation % (EL) 19, Charpy Impact Energy (CIE) 30 J at 0°C and 20 J at -20°C, and bend 3 T, and are meant for use in the construction of bridges, dams, converters, plant machinery, mining machinery, impellers, excavators etc. In the existing processes, two grades of high tensile strength steel plates, namely grades I and II, having specified minimum mechanical properties YS 350 MPa, TS 490 MPa, EL% 21, CIE 40 J at 0°C and 30 J at -20°C, bend 3 T; and YS 410 MPa, TS 540 MPa, EL% 20, CIE 35 J at 0°C and 25 J at -20°C, bend 3 T; respectively are produced. The drawbacks of the existing processes are: (a) the mechanical properties are not of such high values as to allow use of plates of reduced thickness for lowering consumption of steel in the construction of bridges, plants and machineries; and (b) an additional cost for applying the normalising treatment of the plates of thickness >12 mm is required to meet the specified mechanical properties thereof. The objective of the invention is to provide a process for producing steel plates of thickness 8-32 mm having the - 2 - specified improved mechanical properties along with satisfactory weldability thereof. The other objective is to reduce the consumption of steel by lowering the thickness of steel plates for a given tensile strength thereof and reduce thereby the effective cost of the plates for given end uses. The existing process comprises the following steps in sequence :- (i) preparing molten steel of chemical composition, as given in Table I for Grades I and II steel in a BOF converter in the following method. Hot metal from blast furnace is used as the input metal along with Fe-Mn, Fe-Si, Al, lime, fluorspar etc. Oxygen is blown, deslagged and the liquid metal is poured into the ladle. Ferroalloy additions are done to arrive at the desired chemistry. The liquid steel is then subjected to secondary refining in a ladle furnace (LF) or vacuum arc degassing (VAD) unit to reduce the S level and the inclusion content in the steel. Ti is added in the final stages of steelmaking, after the liquid steel is completely dexoidised with Al and Si. This prevents loss of Ti in the form of TiO and facilitates formation of TiN and TiCN during the hoi rolling process which adds to the strength; (ii) converting the molten steel into slabs by ingot/ continuous casting; (iii) reheating the slabs to a temperature of 125O°C max in a furnace: - 3 - (iv) rolling the slabs at an initial temperature of about 1150°C in a roughening stand; (v) finish rolling the slabs in a finishing stand at an initial temperature of 1000°C max and finish rolling temperature of 850-900°C into plates of thickness 8-40 mm for grade I and 8-32 mm for grade II steel; (vi) cooling the plates of thickness 20-40 mm by water spraying to a temperature of 720 ± 20°C; (vii) subjecting the plates of thickness 12-40 mm and 12-32 mm respectively of the two grades to a normalising treatment in a furnace at a temperature of 910 ± 10°C for attaining the specified mechanical properties thereof through uniform distribution of the fine grains in the microstructure of the steel. The invented process comprises the following steps in sequence :- (i) preparing molten steel of chemical composition as given for grade III steel in Table I, in a BOF converter; (ii) secondary refining the chemical composition of the molten steel in a ladle furnace (LF) or in a VAD (vacuum are degassing) unit to reduce the S level and the inclusion content; (iii) converting the molten steel into slabs of preferred cross-section 200 x 1500 mm or 250 x 1500 mm by continuous casting at tundish temperature of 154O-155OOC, casting speed of 0.6-0.8 m/min, cooling water consumption in secondary cooling stage of 0.45-0.50 litre/kg of steel produced, and unbending point slab temperature of above 850°C; - 4 - (iv) reheating the slabs to a temperature of 123O°C max in a furnace; (v) rolling the slabs at an initial temperature of 1100°C max in a roughing stand, with reduction in slab thickness of 15% min in each rolling pass; (vi) rolling the slabs in a finishing stand at an initial temperature of 900°C max and finish rolling temperature of 810 ± 10°C for plates of thickness 20-32 mm and of 830°C max for plates of thickness less than 20 mm with minimum cumulative reduction in plate thickness of 60% and minimum reduction in plate thickness in the last rolling pass of 10%; (vii) cooling the plates of thickness 20-32 mm by water-spraying at a cooling rate of 5-9°C/sec to a temperature of 650 + 20°C; followed by air cooling to ambient temperature; (viii) air-cooling the plates of thickness 8-20 mm. The improved mechanical properties of grade III steel plates produced in the invented process as given in Table II has been attained v/ithout applying the normalising treatment on the finished plates, as followed in the existing processes for producing grades I and II steel. This has been achieved partly through the modification of the chemical composition of the steel and partly through the optimisation of the casting and rolling parameters in the invented process, such as herein described. Certain special features of the chemical composition of the steel are further discussed below. (i) Carbon content is restricted to 0.13-0.15% for plates of thickness below 20 mm and to 0.14-0.16% for plates of thickness 20-32 mm. - 5 - (ii) Titanium is added in a small proportion of 0.02-0.03% to reduce the Austenite grain size during the reheating treatment of the slabs and thereby produce a relatively fine Perrite-Pearlite grain size in the finish rolled plates with consequent improvement in the mechanical properties of the plates. The presence of Titanium in the steel prevents also grain coarsening during welding of the steel in the heat affected zones thereof and improves thereby the toughness and integrity of the waldded joints of the plates. (iii) The content of sulphur which has a deleterious effect on the mechanical properties of steel, particularly on the ductility and toughness thereof is restricted to 0.01% through the secondary refining treatment applied on the molten steel using a VAD plant. The cost effectiveness of the process is increased by elimination of the "normalising process" and by replacement of "V with Ti". Elimination of the normalising treatment leads to a saving of Rs. 500/- per ton and replacement of V with Ti results in an additional saving of Rs. 250/- per ton. The cost effectiveness is further increased by the use of plates of reduced thickness for the required mechanical properties thereof. - 6 - Table I Chemical Composition (by wt.%) - 6A - We Claim :- 1. A process for producing high strength steel plates of thickness upto 32 mm, comprising the following steps in sequence : (i) preparing molten steel of chemical composition (by weight %): C-0.12 to 0.15, Ma-1.30 to 1.40, P-0.02 max, S-0.01 max, S1-0.30 to 0.40, Al-0.015 to 0.025, Hb-0.04 to 0.06, Ti-0.02 to 0.03, N-100 ppm max, Fe-balance, for plates of thickness less than 20 mm; and C-0.13 to 0.16, Mn-1.35 to 1.40, P-0.02 max, S-0.01 max, Si-0.30 to 0.40, Al-0.015 to 0.025, Nb-0.05 to 0.07, Ti-0.02 to 0.03, N-100 ppm max, Fe-balance, for plates of thickness 20-32 mm in a BOF converter; (ii) refining the chemical composition of the molten steel in a VAD plant; (iii) converting the molten steel into slabs of required cross-section by continuous casting,Characterised in that (a) slabs are continuously cast at tundish temperature of 1540-1550°C, casting speed of 0.6-0.8 m/min, cooling water consumption in secondary cooling stage of 0.45-0.50 litre/kg of steel produced and unbending point temperature of above 850°C; (b) the slabs are reheated to a temperature of 1230°C max; (c) the slabs are rolled at an initial temperature of 1100°C max in a roughing stand with reduction in slab thickness of 15% min in each rolling pass; and in a finishing stand at an initial temperature of 900°C max and finish rolling temperature of 810 ,± 10°C for plates of thickness 20-32 mm and of 830°C max for plates of thickness less than 20 mm with minimum cumulative reduction in plate thickness of 60% and minimum reduction in plate thickness of 10# in the last rolling pass; (d) plates of thickness 20-32 mm are cooled by water-spraying to a temperature of 650 + 20% at a cooling rate of 5-9°C/sec, followed by air cooling and (e) plates of thickness 8-20 mm are air-cooled to the ambient temperature. _ 8 - 2. The process as claimed in claim 1, wherein the slabs are of cross-section 200 x 1500 mm or 250 x 1500 mm. A process, for producing high strength steel plates of thickness upto 32 mm, comprising the following steps in sequence: (1) preparing molten steel of chemical composition (by weight %): C-0.12 to 0.15, Mn-1.30 to 1.40, P-0.02 max, S-0.01 max, Si-0.30 to 0.40, Al-0.015 to 0.025, Nb-0.04 to 0.06, Ti-0.02 to 0.03, N-100 ppm max, Fe-balance, for plates of thickness less than 20 mm; and C-0.13 to 0.16, Mn-1.35 to 1.40, P-0.02 max, S-0.01 max, Si-0.30 to 0.40, Al-0.015 to 0.025, Nb-0.05 to 0.07, Ti-0.02 to 0.03, N-100 ppm max, Fe-balance, for plates of thickness 20-32 mm in a BOF converter; (ii) refining the chemical composition of the molten steel in a VAD plant; (iii) converting the molten steel into slabs of required cross-section by continuous casting characterised in that (a) slabs are continuously cast at tundish temperature of 1540-1550°C, casting speed of 0.6-0.8 m/min, cooling water consumption in secondary cooling stage of 0.45-0.50 litre/kg of steel produced and unbending point temperature of above 850°C; (b) the slabs are reheated to a temperature of 123O°C max; (c) the slabs are rolled at an initial temperature of 1100°C max in a roughing stand with reduction in slab thickness of 1596 min in each rolling pass; and in a finishing stand at an initial temperature of 900°C max and finish rolling temperature of 810 ± 10°C for plates of thickness 20-32 mm and of 830°C max for plates of thickness less than 20 mm with minimum cumulative reduction in plate thickness of 60% and minimum reduction in plate thickness of 10% in the last roll ing pass: (d) plates of thickness 20-32 mm are cooled by water-spraying to a temperature of 650 ± 20% at a cooling rate of 5-9°C/sec, followed by air cooling and (e) plates of thickness 8-20 mm are air-cooled to the ambient temperature.

Documents:

00172-cal-1999-abstract.pdf

00172-cal-1999-claims.pdf

00172-cal-1999-correspondence.pdf

00172-cal-1999-description(complete).pdf

00172-cal-1999-form-1.pdf

00172-cal-1999-form-18.pdf

00172-cal-1999-form-2.pdf

00172-cal-1999-form-3.pdf

00172-cal-1999-form-5.pdf

00172-cal-1999-letters patent.pdf

00172-cal-1999-p.a.pdf