Title of Invention

A PROCESS OF MANUFACTURING HIGH STRENGTH LOW ALLOY CORROSION RESISTANT STRUCTURAL (HCRS) STEEL .

Abstract

There is disclosed a process for manufacturing high strength low alloy corrosion resistant structural (HCRS) Steel comprising, first making a low carbon steel by the basic open hearth process at a temperature in the range of 1650 ±15°C; said heat is tapped in a ladle in whicn weighed quantities of ferro-alloy and copper are added when the ladle is 1/3 of 2/3 full; homogenization of the molten steel wi:h nitrogen is carried out such that on chemical analysis of the final product exhibits a chemical composition on wt % basis as below: C Mn S P Cu N2 0.10- 0.25- 0.03- 0.08- 0.25 80 ppm(min) 0.14 0.45 Max 0.14 Min Al content up to 0.015 wt % is added to the steel; and rest is iron.

Full Text

Introduction to the Field of Invention: This invention relates to a method for the preparation of high strength Sow alloy corrosion resistant structurai steel, which is code named as HRCS, which have strength simiiar to that of some of the existing commercial grades but at lower cost. Prior Art and Drawbacks: It is already known to prepare low alloy corrosion resistant steels, which are hot rolled to provide necessary mechanical properties. As per the known practice, high strength superior corrosion resistant steel is prepared containing phosphorus, copper, nickel as well as chromium as alloying elements. Present commercial grades suffer from the three major drawbacks. a. These are very expensive and the cost of preparation is to be seriously monitored. b. Greater care for homogenization during production is necessary as the specification caiis for maintaining larger number of elements in specific range. c. These have lower bend formability due to the presence of chromium carbides. Other drawback, which can also be found in the present practice, is the presence of nickel and chromium in the alloy. This increases the carbon content/carbon equivalent, in turn affects the weidability characteristics of the steel. Moreover, the presence of high prosperous limits their section thickness to 12 mm. max. A further drawback in present practice is that the steels are generally of killed variety. Objects of the Invention: ft is therefore a primary object of the present invention to propose a method for the preparation of a cost effective high strength corrosion resistant structural steel with proper alloy compositions. It is another object of this invention to propose such a method by which it is possible to prepare satisfactory high strength tow alloy corrosion resistant structurai steel. It is a further object of this invention to propose such a method by which it is possible to obtain high strength low alloy corrosion resistant: structural steel, which we will have satisfactory bend formability and ductility as well better than the commercially available grades. It is a still further object of this invention to propose such a process, which will avoid the presence of or the use of nickel and chromium in the alloy composition. It is yet another object of this invention to propose such a process by which it is possible to make the steel either as semi killed or killed condition depending upon the facilities. This is possible by limiting the silicon content in the composition, which is specified in the presently available commercial grades in the range of 0.25 to 0.75% by weight. A still further object of this invention is to ensure that the proposed process is suitable for preparing semi-killed steel and killed steel. This is more important particularly because as steel melting shop having ingot casting facilities and yield of killed steel from liquid is inversely about 15% lower by weight compared to semi-killed steel. it is yet another object of this invention to propose such a process, which will be economical and significantly reduce the cost because it can be prepared by semi killed form than the killed form and the preparation of semi killed steel increases the product yield. Yet another object of the present invention is to propose such a process which will reduce a number of elements specified in the preparation of steel composition and more particularly, which will avoid nickel, chromium and silicon and concentrate mostly with respect to carbon, manganese, sulphur, phosphorus and copper. These and other objects of the invention will be more apparent from the following paragraphs. Brief Statement of the invention: According to this invention, there is provided a process for the manufacture of alloy steel of the invention consists of first making a low carbon steel by the basic open hearth process at a temperature in the range of 1640±15° C, where after, the heat is tapped in a ladle in which weighed quantities of ferro-alloy and copper are added when the ladle is 1/3 of 2/a full, there after, homogenization of the steel with nitrogen and chemical analysis of the molten steel is carried out such that the final composition exhibits a chemical composition as below. Al content upto 0,015 may be added to the steel It is ensured that the addition of iron, silicon and aluminium are adjusted depending upon oxidation level of steel and the nitrogen alloying has been used to minimize segregation of phosphorus at grain boundaries keeping the minimum ratio of phosphorus to nitrogen to overcome this problem at 10. During ingot casting of 8 tonnes weight, a small amount of Al is preferably added in the mould depending upon the metal behaviour during teeming and the ingots are stripped with a track time of at least two hours followed by heating and soaking at a temperature of 1250° C, the soaking time being not less than 1 and 1/2 times the track time. in this process, excessive oxidation is avoided to reduce hot shortness and the rolled bioom/billet, after heating, is rolled in light or heavy structural milI depending upon the section thickness. We have found that atleast 80 parts per million of nitrogen is to be added to add about 0.1% of phosphorus to avoid segregation at the grain boundaries. Further Details of the Process of Manufacture: The process of manufacture consists of making of low carbon steel in the twin open hearth furnace followed by tapping the steel at temperature of 1640±15° C. The heat is tapped in the ladle where weighted quantity of ferro-alloys and copper are added when the ladle is 1/3 to 2/3 full. This follows homogenization with nitrogen and chemical analysis of the molten steel to meet the specific composition requirements. The presence of nitrogen in the range of 100 to 150 ppm (depending upon Phosphorus addition) is essential to avoid segregation of phosphorous at the grain boundaries in the finished product. The addition of Fe. Si and Al shall be adjusted depending upon the oxidation level of the final bath. During ingot casting of 8 tonnes weight, a small amount of Al may be added in the mould depending upon the metal behavior during teeming. The ingot are stripped with a track time of at least 2 hours followed by soaking at a temperature of 1250° C with the time duration not less than 1% times the track time. The oxygen content in the atmosphere is to be maintained to ≤0.5% to void excessive oxidation, which may lead to hot shortness due to excessive oxidation. It is found that in the other commerciai grade containing P, Cu, Ni, Cr and Si as additions, the scale is scky and requires extra efforts to remove but no such problem arises in this case. However a special emphasis should be given for removal of the scale. The rolled bioom/billet after heating is rolled in light/heavy structural miil depending upon the section size. After rolling, the product is tested for mechanical properties and metallographic features. Electron micro probe analysis shows that the phosphorous is uniformly distributed in the steel and no segregation was observed at the grain boundaries. Laboratory experience also shows that if bend formability is better than IT, then also this segregation is minimum. The micro structure consist of mainly ferrite with about 10-12% of pearlite. The grain size of the steel was ASTM 8 and finer. in the accompanying tables, we have stated the obvious properties of the known commercial products art as well as a properties of the invented steel. Table 1 clearly states the chemical composition of the low alloy corrosion resistant structural steel as per the invention. Table 2 clearly states the mechanical properties of hot rolled low alloy corrosion resistant steel of the commercial grades, which are as stated earlier, only killed variety. Table 3 clearly states the chemical compositions of the product of the invention with reference to heat 1 and heat 2 attention is invited to the extensive work details discussed herein below. Table 4 clearly states the mechanical properties of the rolled product of the invention. The Details of the Extensive Laboratory Wort: Extensive laboratory work was done to achieve high strength with minimum corrosion index of 6 (as per ASTM G 101), subsequently two semi killed 250 tonne heats were made at commercial Plant. The heats were made in twin hearth and cast to ingots of 8 tons. The hot ingots were initially rolled to blooms and then to channels of 300 and 600 mm as well as angles of 65 x 65 x 6 mm. Similarly, four killed heats were made in the laboratory and rolled to 6 mm plates in two stages of rolling. In this investigation, a ratio of P to N has been found to be 10 (min) to avoid P segregation. Based upon the mechanical tests and metallurgical investigations, a Cu -P steei is recommended for use by railways for manufacture of rolling stocks. We also state below the chemical composition and mechanical properties in a brief manner. Further the product range of the steel of the invention can be used joists channel, angles and other shapes. We also state below the application ranges of the product of the invention. We Claim: 1. A process for manufacturing high strength low alloy corrosion resistant structural (HCRS) Steel comprising, first making a low carbon steel by the basic open hearth process at a temperature in the range of 1650 ±15°C; said heat is tapped in a ladle in which weighed quantities of ferro-alloy and copper are added when the ladle is 1/3 o 2/3 full; homogenization of the molten steel with nitrogen is carried out such that on chemical analysis of the final product exhibits a chemical composition on wt % basis as below: C Mn S P Cu N2 0.10- 0.25- 0.03- 0.08- 0.25 80 ppm(min) 0.14 0.45 Max 0.14 Min Al content up to 0.015 wt % is added to the steel; and rest is iron. 2. A process as claimed in claim 1 wherein, the addition of iron, silicon and aluminium are adjusted depending upon oxidation level of steel. 3. A process as claims 1 and 2, wherein, nitrogen alloying has been used to minimize segregation of phosphorus at grain boundaries keeping the minimum ratio of phosphorus to nitrogen to overcome this problem at 10. 4. A process as claimed in claims 2 and 3, wherein, the during ingot casting of 8 tonnes weight, a small amount of Al s preferably added in the mould depending upon the metal behaviour during teeming. 5. A process as claimed in claims 1, 2 and 3,wherein, the ingots are stripped with a track time of at least two hours followed by heating and soaking at a temperature of 1250°C. 6. A process as claimed in claims 1 to 4, wherein, a soaking time is not less than 1 and 1/2 times the track time. 7. A process as claimed in claims 1 to 5, wherein, excessive oxidation is avoided to reduce hot shortness. 8. A process as claimed in claims 1 to 6, wherein, the rolled bloom/billet, after heating, is rolled in light or heavy structural mill depending upon the section thickness. 9. A process as claimed in claims 1 to 7, wherein, atleast 80 parts per million of nitrogen is added to add 0.1 wt.% of phosphorus to avoid segregation at the grain boundaries. 10. A process as claimed in claims 1 to 8, wherein, the oxygen content in the atmosphere is maintained at 0.5 wt.% to avoid excessive oxidation. 11. A process for manufacturing high strength low alloy corrosion resistant structural (HCRS) Steel substantially as hereindescribed. There is disclosed a process for manufacturing high strength low alloy corrosion resistant structural (HCRS) Steel comprising, first making a low carbon steel by the basic open hearth process at a temperature in the range of 1650 ±15°C; said heat is tapped in a ladle in whicn weighed quantities of ferro-alloy and copper are added when the ladle is 1/3 of 2/3 full; homogenization of the molten steel wi:h nitrogen is carried out such that on chemical analysis of the final product exhibits a chemical composition on wt % basis as below: C Mn S P Cu N2 0.10- 0.25- 0.03- 0.08- 0.25 80 ppm(min) 0.14 0.45 Max 0.14 Min Al content up to 0.015 wt % is added to the steel; and rest is iron.

Documents:

186-cal-2002-granted-abstract.pdf

186-cal-2002-granted-claims.pdf

186-cal-2002-granted-correspondence.pdf

186-cal-2002-granted-description (complete).pdf

186-cal-2002-granted-examination report.pdf

186-cal-2002-granted-form 1.pdf

186-cal-2002-granted-form 13.pdf

186-cal-2002-granted-form 18.pdf

186-cal-2002-granted-form 2.pdf

186-cal-2002-granted-form 3.pdf

186-cal-2002-granted-pa.pdf

186-cal-2002-granted-reply to examination report.pdf

186-cal-2002-granted-specification.pdf