Title of Invention

A PROCESS FOR MANUFACTURING OF BOILER GRADE HOT ROLLED STEEL COILS

Abstract

The present invention relates to a process for the manufacture of boiler grade Hot Rolled steel Coils of 2.0 mm to 5.0 mm thickness for manufacture of tube for boilers having impact strength of 50-70 Joules, which comprises: (i) taking liquid steel composition having the following alloy chemistry- (ii) subjecting the same to calcium treatment by adding calcium silicate followed by; (iii) casting into a slab; (iv) avoiding re-oxidation during casting using argon injection as necessary; (v) re-heating the cast slab at temperature of 1200-1250°C in a re-heating furnace to make it suitable for rolling; (vi) rolling the reheated slab into suitable HR strip and (vii) finally, subjecting the strip to finishing treatment, where the Hot Rolled Coil comes out of the finishing mills at a temperature of 860-890°C and it is cooled by water jets on run-out table by Laminar Cooling System to a temperature of 650-680°C.

Full Text

A PROCESS FOR MANUFACTURE OF BOILER GRADE HOT ROLLED STEEL COILS INTRODUCTION TO THE FIELD OF INVENTION The present invention relates to a process for the manufacture of boiler grade Hot Rolled Steel Coils of 2.0 mm to 5.0 mm thickness, for manufacture of Boiler Tubes, having improved toughness, fracture strength, weldability and creep strength and meeting the basic requirements of boiler quality steels. PRIOR ART AND DRAWBACKS There is a Specification IS-2002-1992, which is made for steel plates for pressure vessels for intermediate and high temperature service including boilers. The specification is applicable for material thickness of 5mm and above. There are also some International Specifications like ASTM and BS for boiler tubes and plates. The document US4,067,215 discloses hot rolled steel plate having an absolute warp value of less than 20mm for a 3m length of cut plate and not subjected to deterioration of mechanical properties, is formed by subjecting steel strip from a hot rolled coil to a strong cold leveling to provide a maximum surface strain epsilon max within the range of 0.60% Itoreq epsilon max Itoreq. 3.0%. The steel strip may then be subjected to a light cold leveling for slowly diminishing the surface strain. Further, after the straightening or leveling operation the plates are inspected, coated with a rust preventative, marked with identifying indicia and directed to a piling device. The inventive and novel method and apparatus of the present invention for manufacturing thick steel plate from hot rolled coil, which has not been previously successful, is extraordinarily excellent in amelioration of productivity compared with any of the conventional thick steel plate producing methods. The document US4,463,061 discloses a boiler tube having improved high temperature strength, improved high temperature corrosion resistance, and resistivity to embrittlement during the service thereof, comprising an outer surface layer and an inner part, the outer surface layer consisting essentially, by weight, of 0.03-0.20% carbon, 1.5-4.0% silicon, 0.1-3.0% manganese, 13-25% chromium, 13- 40% nickel, 0.5-3.0% at least one kind selected from the group consisting of molybdenum and tungsten, 0.05-0.5% at least one kind selected from the group consisting of titanium, niobium and vanadium, and the balance iron and inevitable impurities, the inner part consisting essentially, by weight, of 0.03-0.20% carbon, 0.3-1.0% silicon, 0.1-3.0% manganese, 13-25% chromium, 13-40% nickel, 0.5-3.0% at least one kind selected from the group consisting of molybdenum and tungsten, 0.05-0.5% at least one kind selected from the group consisting of titanium, niobium and vanadium, and the balance iron and inevitable impurities. Boron of 0.001-0.01% may be further added to the alloy to improve creep strength while keeping improved high temperature corrosion resistance. The document JP 2002155315 discloses a method capable of stably manufacturing a high strength hot-rolled steel sheet which has high uniformity in the whole longitudinal and transverse directions of a coil, and to provide the high strength hot rolled steel sheet manufactured by the method. However, for Indian conditions, boiler tubes of less than 5mm thickness are in great demand and the above Specifications do not meet the requirements. BACKGROUND OF INVENTION Specification IS-2002-1992 covers three grades corresponding to different strength levels. However, in the chemistry specified for these grades, Carbon, sulphur and Phosphorous levels are high. These specified grade of steels with % S and % P contents of 0.040 and .035 respectively, have impact strength around 30 Joules whereas the boiler grade steel of the present invention having % S and % P content less than 0.025 have impact strength of 50-70 Joules. Similarly BS 3059 steel for boiler and super heater tubes stipulates Carbon as 0.16% maximum and sulphur and phosphorous limits have been kept at 0.050% maximum. Because of high Carbon, Sulphur and Phosphorous limits, this steel has poor weldability and lower toughness. Carbon increases short-term tensile strength but does not add appreciably to creep resistance at temperatures above 540°C because carbides get spherodised. Higher Carbon results into a larger amount of undissolved carbide which increases the total amount of carbide in the product resulting in reduction in the weldability and ductility. When steel with higher carbon content is exposed to higher temperatures for long duration during use, the pearlite gets transformed thus increasing the grain boundary carbides. Thermal conductivity, thermal diffusivity and thermal expansions are critical properties for boiler steels. Higher Carbon adversely affects all these properties. In view of the above, it is always preferred to have lower Carbon provided long-term high temperature strength can be maintained in steels. Manganese prevents hot shortness and enhances effectiveness of Nitrogen in increasing strength of steel and Silicon increases elevated temperature strength of steel. Silicon also increases the resistance of steel towards oxidation at elevated temperatures. Phosphorous and Sulphur are considered undesirable because they reduce elevated temperature ductility and hence affect the stress rapture strength and thermal fatigue life. Sulphur forms MnS/ FeS in steel which during rolling from stringers. These have low melting point and result into under-bead cracks during welding. Higher content of Phosphorous increases the strain aging tendency of the steel. Based on the above considerations objective was to evolve a chemistry, which meets the basic requirements of boiler steel along with improved weldability, toughness, fracture strength and creep strength. OBJECT OF THE INVENTION It is therefore an object of the invention to propose a new chemistry of steel and a process to produce boiler grade Hot Rolled Steel Coils of 2.0 mm to 5.0 mm thickness for manufacture of tubes for boilers, having %S and %P content less than 0.025, which can provide impact strength of around 50-70 Joules. It is another object of the invention to propose such a process, which will ensure the production of boiler grade Hot Rolled Steel Coils of 2.0 mm to 5.0 mm thickness for manufacture of tubes for boilers , having satisfactory yield-structure (YS), ultimate tensile strength (UTS) and necessary elongation. It is further object of the invention to propose a process for manufacture of boiler grade Hot Rolled Steel Coils of 2.0 mm to 5.0 mm thickness for manufacture of tubes for boilers, having the Sulphur content of the steel less than 0.025% so that the chances of under-bead cracks get reduced by 50%. It is further object of the invention to propose a process for manufacture of boiler grade Hot Rolled Steel Coils of 2.0 mm to 5.0 mm thickness for manufacture of tubes for boilers, having lower Carbon content of the steel, as in general, weldability and toughness improve as Carbon content of the steel goes down. Hence, as compared to the Carbon content of 0.18 - 0.22% present in IS 2002, Carbon content in SAIL BR is less than 0.12% which is much lower. BRIEF STATEMENT OF THE INVENTION According to this invention, there is provided a process for the manufacture of boiler grade Hot Rolled Steel Coils of 2.0 mm to 5.0 mm thickness for manufacture of tubes for boilers which comprises: (ii) subjecting the same to calcium treatment by adding calcium silicate followed by; (iii) casting into a slab; (iv) avoiding re-oxidation during casting using argon injection as necessary; (v) re-heating the cast slab at temperature of about 1200-1250°C in a re-heating furnace to make it suitable to rolling; (vi) rolling the reheated slab into suitable HR strip and (vii) finally, subjecting the HR strip to finishing treatment, where the Hot Rolled Coil comes out of the Finishing Mill at a temperature of 860-890°C and it is cooled by water jets on run-out table by Laminar Cooling System to a temperature of 650-680°C. The detail of each steps are as follows: Calcium treatment has been envisages in steel making to have modification in inclusion morphology and rolling temperatures have been optimized to get grain size of ASTM 9/10. Calcium treatment is done to modify the inclusion population in steels with propensity for blockage and to give low melting point species, which will not clog the caster nozzles in the casting machines and float out of the steel. The Re-heating Furnaces serve as heating and buffer zone where the cast slabs are heated upto predetermined rolling temperatures to obtain desirable austenitic grain structure suitable for rolling. After Roughing Stands the austenite has an ASTM Grain size of 2-3 which gets converted into a Ferrite-Pearlite matrix of ASTM Grain size of 9-10 after rolling in Finishing Stands and Laminar Cooling on Run-Out-Table. In most melt shops the cored wire containing Ca-Si injection system is used in the calcium treatment of steel. The melting and boiling points of calcium are 839°C and 1500°C respectively. During calcium treatment, the alumina and silica inclusions are converted to molten calcium aluminates and silicate which are globular in shape because of the surface tension effect. The change in inclusion composition and shape is known as the inclusion morphology control. The calcium aluminates inclusions retained in liquid steel suppress the formation of MnS stringers during solidification of steel. This change in the composition and mode of precipitation of sulphide inclusion during solidification of steel is known as sulphide morphology or sulphide shape control. Several metallurgical advantages are brought about with the modification of composition and morphology of oxide and sulphide inclusions by calcium treatment of steel, as for instance: • To improve steel castability in continuous casting, i.e. minimize nozzle blockage; • To minimize inclusion related surface defects in billet, bloom and slab castings; • To prevent lamellar tearing in large restrained welded structures; • To minimize the susceptibility of high-strength low alloy (HSLA) linepipe steels to hydrogen-induced cracking (HIC) in sour gas or sour oil environments. The Ca content in the final product can be controlled within the range of 15 to 20 ppm. PROCESSING DETAILS Steel making: The steel is made through the Basic Oxygen Process, where pure Oxygen is blown into a bath of molten blast-furnace iron and scrap. The Oxygen initiates a series of intensively exothermic reactions, including the oxidation of such impurities as Carbon, Silicon, Phosphorous and Manganese. Most of the alloying additions and deoxidation carried out at Converter shop. The crude steel tapped from the converter contains impurities, which are not desired in the final steel chemistry, hence, the steel is processed through Steel Refining Unit where final additions are made to have desired chemistry. Steel is purged by argon to facilitate floatation of inclusions and uniformity of chemistry. Argon bubbling is applied to homogenize the steel composition and for avoiding re-oxidation during casting. Calcium Silicate is added for inclusion modification. Calcium forms compounds with AI2O3, which are liquid and float to the top. Sulphur forms Calcium Sulphide, which are of globular, shape. The adverse affect of Sulphur is taken care by this. HOT ROLLING The slabs are heated to1200- 1250°C and rolled in continuous mill with finishing temperature 860-880°C, the aim is to get grain size of ASTM 9/10 which is optimum with respect to strength, creep strength, toughness and ductility. ADVANTAGES It can be seen that though the Carbon content of SAIL BR is lower than other two specifications, its mechanical properties are superior. WE CLAIM: 1. A process for the manufacture of boiler grade Hot Rolled steel Coils of 2.0 mm to 5.0 mm thickness for manufacture of tube for boilers having impact strength of 50-70 Joules, which comprises: (ii) subjecting the same to calcium treatment by adding calcium silicate followed by; (iii) casting into a slab; (iv) avoiding re-oxidation during casting using argon injection as necessary; (v) re-heating the cast slab at temperature of 1200-1250°C in a re- heating furnace to make it suitable for rolling; (vi) rolling the reheated slab into suitable HR strip and (viii) finally, subjecting the strip to finishing treatment, where the Hot Rolled Coil comes out of the finishing mills at a temperature of 860- 890°C and it is cooled by water jets on run-out table by Laminar Cooling System to a temperature of 650-680°C. 2. A process for the manufacture of boiler grade Hot Rolled steel Coils of 2.0 mm to 5.0 mm thickness for manufacture of tube for use in boilers substantially as herein described. The present invention relates to a process for the manufacture of boiler grade Hot Rolled steel Coils of 2.0 mm to 5.0 mm thickness for manufacture of tube for boilers having impact strength of 50-70 Joules, which comprises: (i) taking liquid steel composition having the following alloy chemistry- (ii) subjecting the same to calcium treatment by adding calcium silicate followed by; (iii) casting into a slab; (iv) avoiding re-oxidation during casting using argon injection as necessary; (v) re-heating the cast slab at temperature of 1200-1250°C in a re-heating furnace to make it suitable for rolling; (vi) rolling the reheated slab into suitable HR strip and (vii) finally, subjecting the strip to finishing treatment, where the Hot Rolled Coil comes out of the finishing mills at a temperature of 860-890°C and it is cooled by water jets on run-out table by Laminar Cooling System to a temperature of 650-680°C.

Documents:

149-kol-2003-abstract.pdf

149-kol-2003-claims.pdf

149-KOL-2003-CORRESPONDENCE 1.1.pdf

149-kol-2003-correspondence.pdf

149-kol-2003-description (complete).pdf

149-KOL-2003-EXAMINATION REPORT 1.1.pdf

149-kol-2003-examination report.pdf

149-kol-2003-form 1.pdf

149-KOL-2003-FORM 13 1.1.pdf

149-kol-2003-form 13.pdf

149-KOL-2003-FORM 18 1.1.pdf

149-kol-2003-form 18.pdf

149-kol-2003-form 2.pdf

149-KOL-2003-FORM 26 1.1.pdf

149-kol-2003-form 26.pdf

149-KOL-2003-FORM 3 1.1.pdf

149-kol-2003-form 3.pdf

149-KOL-2003-FORM-27.pdf

149-KOL-2003-GRANTED-ABSTRACT.pdf

149-KOL-2003-GRANTED-CLAIMS.pdf

149-KOL-2003-GRANTED-DESCRIPTION (COMPLETE).pdf

149-KOL-2003-GRANTED-FORM 1.pdf

149-KOL-2003-GRANTED-FORM 2.pdf

149-KOL-2003-GRANTED-SPECIFICATION.pdf

149-kol-2003-others.pdf

149-KOL-2003-PA 1.1.pdf

149-kol-2003-pa.pdf

149-KOL-2003-REPLY TO EXAMINATION REPORT.pdf

149-kol-2003-specification.pdf