Title of Invention

A PROCESS FOR MANUFACTURING A HIGH STRENGTH , FORMABLE AND FATIGUE RESISTANT STEEL .

Abstract

A process for manufacturing high strength, formable and fatigue resistant steel particularly useful in the production of automobile chassis, wheel disc and rims, and such other items. The process comprises the steps of heating steel slabs having a steel composition comprising 0.02 to 0 18 wt% carbon, 0. 01 to 0.14 wt% manganese, 0 to 0.07 wt% silicon, 0 to 0 .02 wt% sulfur, 0 to 0 .02 wt% phosphorous, 0 to 0.05 wt% aluminum, the rest being iron, at a temperature between 1100 to 1350 C, rolling of the slabs in 5 or 6 stands in a hot rolling mill wherein the finishing temperature is maintained between 850 to 950 oC and finally coiling of rolled steel strip at a temperature between 600 to 750 oC. The process controls pearlite/ferrite ratio in the range of 0.05 to 1 0 and is free from large size inclusions. The steel has desirably low ratio of yield strength to ultimate strength for arresting fatigue crack growth. The steel exhibits high percentage elongation and is weldable and free from harmful oxides

Full Text

1 PROCESS FOR MANUFACTURING A HIGH STRENGTH FORMABLE AND FATIGUE RESISTANT STEEL Field of the invention The present invention relates to a process for manufacturing a high strength, formable and fatigue resistant steel. The steel manufactured by the process of the present invention is particularly useful in the automobile sector, especially in the manufacture of automobile chassis, wheel disc and rims. Background of the invention Several high strength steels are known in the art which have applications in specific fields and are manufactured by various processes in order to optimize selected physical properties of steels so produced, such as the yield strength, fracture toughness and corrosion resistance. JP06081080 teaches steel sheet excellent in fatigue properties and deep drawing ability using IF steel having a specified composition and reducing its ferritic grain diameter into specified value or below. The steel comprises by weight, 2 at JP2000290745 teaches a high strength steel sheet for working, excellent in fatigue characteristic and safety against collision and capable of sufficiently meeting the demands from the viewpoint of stabilization of material quality and reduction of costs, and its manufacturing method. The steel sheet has, before forming, a microstructure which contains ferrite of >180 Vickers hardness as a principal phase and also contains retained austenito ( ) in an amount of ~3% by volume fraction and in which the volume fraction of the balance (secondary phase) of the structure is regulated to For the purpose of auto chassis production, it is required that the steel possess special characteristics such as a low ratio of Yield Strength to Ultimate Tensile Strength (YS/UTS ratio), good formability and high fatigue resistance. None of the aforesaid patents teach steel having optimum combination of properties desired for auto chassis production. Furthermore, the prior art specify a wide range of chemistry along with very wide range of phases achieved for the properties. Steel having specific characteristics for the purpose of auto chassis production is not known. Furthermore, the steels available in the market as well as the ones taught in the above prior art comprise costly alloying elements such as Niobium, Molybdenum, Nickel, Copper etc. to achieve the desired properties and are therefore, not economic. Moreover, the sulphur and phosphorous contents of these steels are not low enough for good elongation property and other mechanical properties. Steels in general having sulphur >0.015% and phosphorus >0.020% do not meet the requirement of high formability and fatigue resistance. Anisotropy of the steel is also worst affected by high sulphur and phosphorous. Therefore, there is a need of a process for manufacturing a high strength, formable and fatigue resistant steel which meets the requirements for production of automobile chassis, wheel disc and rims, and such other items. Objects of the invention The object of the present invention is to provide a process for manufacturing a steel which has high strength with good formability and high resistance to fatigue. 3 Another object of the present invention is to provide a process for manufacturing of a steel which has a uniform polygonal ferrite-pearlite structure A further object of the present invention is to provide a process for manufacturing of a steel which exhibits higher percentage elongation. A further object of the present invention is to provide a process for manufacturing of a steel which is based on a lean chemistry and is thus economical. The applicants have now developed a thermo-mechanical process for manufacturing high strength, formable and fatigue resistant steel wherein the desired properties in steel plates are achieved by selection of steel chemistry, hot rolling parameters and roll crown. Summary of the invention According to the present invention there is provided a process for manufacturing of a high strength, formable and fatigue resistant steel comprising the steps of: i) heating steel slabs having a steel composition comprising 0 02 to 0 18 wt %carbon, 0.01 to 0 14 wt % manganese, 0 to 0.07 wt % silicon, 0 to 0 02 wt % sulfur, 0 to 0.02 wt %, phosphorous, 0 to 0.05 wt%, aluminum, the rest being iron, at a temperature between 1100 to 1350° C. ii) rolling of said slabs in five or six stands in a hot rolling mill wherein the finishing temperature is maintained between 850 to 950 ° C ; and iii) coiling said rolled steel strip at a temperature between 600 to 750 ° C. Detailed description of the invention The various physical properties of steel of the present invention are optimized by selecting a specific steel chemistry as well as selecting the heating temperature of steel slabs, finishing temperature, coiling temperature of rolled strip in the hot strip mill. The steel chemistry has been selected such that sulfur and phosphorous levels 4 are below 0.02 wt %. The process controls pearlite/ferrite ratio in steel in the range 0.05 -0.10 and the restriction on sulfur and phosphorous levels makes the steel free from large size inclusions. The steel is weldable and free from harmful oxides. The modified steel chemistry and the microstructure enhance the formability and fatigue resistance together. Although, the processing cost of the steel is marginally higher due to lower sulfur and phosphorous levels than in the conventional steels, the critical forming of components oversheds the cost of production. The process firstly comprises making steel of following preferred composition: 0.05 to 0.15 wt% carbon, 0.03 to 1.2 wt% manganese, 0 to 0.05 wt% silicon, 0 to 0.01 wt% sulfur, 0 to 0.02 wt% phosphorus, 0 to 0.03 wt% aluminum, the balance being iron. The desired composition may be achieved by partial addition of Ferro-alloys during steel refining. The slabs are heated at a temperature preferably between 1200 to 1250° C for a predetermined time, after which the heated slabs are rolled in at least 5 stands under strict control of finishing and coiling temperatures. The finishing temperature of the hot rolling is preferably maintained at temperature between 880 to 950°C. Roll crown and profile at different stands and temperature gradient across the work rolls are controlled by rolling speed and quantity of water sprayed on work rolls. Preferably 6 roll stands are used wherein the roll crowns have values of 0.5, 0.45, 0.35, 0.35 and 0.2 mm (+ 5% range). Such parameters reduce the variation in the width and thickness of the coil. Reduction in thickness at each roll pass is fed to computer, which maintains the final thickness of the plate. During the rolling the thickness of the steel is reduced substantially. After the steps of rolling, the steel strips so formed are coiled wherein the temperature are maintained such that the steel has 5 to 10% pearlite in ferritic structure. 5 Preferably, the coiling temperature is maintained between 650 and 700°C. The steel so formed has low value of the ratio of yield stress to ultimate tensile strength in order to arrest the fatigue crack growth. The process of the invention will now be described with reference to a non-limiting example. Example Steel slabs having chemical composition comprising 0.15 wt% carbon, 1.1 wt% manganese, 0.03 wt% silicon, 0.02 wt% sulfur, 0.01 wt% phosphorous, 0.02 wt% aluminum (the rest being iron), are heated in a reheating furnace at temperature of 1225 C for 3.5 - 4 hrs. The heated slabs are then rolled in a six-stand rolling mill wherein the finished temperature is maintained at 900 C. The roll crown values for the stands are selected as 0.5, 0.45, 0.4, 0.35, 0.35 and 0.2 mm. Initial thickness of slab is 180 mm which is reduced to 32 mm before finish rolling. The thickness of steel strips after rolling was 5 to 10 mm. The rolled steel strips were then coiled at temperature of 675 C. The strip is coiled on a coiler which is made for coiling the long strips. The steel composition made by the above process was found to have yield strength of 310 MPa/mm2 and ultimate tensile strength of 470 MPa/mm2. The percentage elongation was found to be 35%. The steel was found to possess 5 to 10% pearlite structure in the ferrite matrix. This steel is found to have sufficient fatigue resistance for use in making vehicles, particularly automobile chassis. 6 We claim: 1. A process for manufacturing a high strength, formable and fatigue resistant steel comprising the steps of: (i) heating steel slabs having a steel composition comprising 0.02 to 0.18 wt% carbon, 0.01 to 0.14 wt% manganese, 0 to 0.07 wt% silicon, 0 to 0.02 wt% sulfur, 0 to 0.02 wt% phosphorous, 0 to 0.05 wt% aluminum, the rest being iron, at a temperature between 1100 to 1350 C for a predetermined time; (ii) rolling of said slabs in five or six stands in a hot rolling mill wherein the finishing temperature is maintained between 850 to 950 C by maintaining the temperature of feed plate from 1020 to 1050°C; and (iii) coiling of rolled steel strip at a temperature between 600 to 750 C. 2. A process according to claim 1, wherein said steel composition comprises 0.05 to 0.15 wt% carbon, 0.03 to 1.2 wt % manganese, 0 to 0.05 wt% silicon, 0 to 0.01 wt % sulfur, 0 to 0.02 wt% phosphorus, 0 to 0.03 wt% aluminum, the balance being iron. 3. A process according to any of claims 1 or 2, wherein said steel composition comprises 0.15 wt% carbon, 1.1 wt% manganese, 0.03 wt% silicon, 0.01 wt% sulfur, 0.01 wt% phosphorous, 0.02 wt% aluminum, the balance being iron. 4. A process according to claim, wherein said heating of steel slabs is carried out at temperature between 1200 to 1250°C for 3.5-4hrs. 5. A process according to claim 4, wherein said temperature is maintained at 1225°C. 6. A process according to claim 1, wherein said rolling is carried out in at least five stands. 7 7. A process according to claim 1, wherein said rolling is carried out in a six-stand rolling mill. 8. A process according to claim 7, wherein roll crowns of said six roll stands have values of 0 5, 0 45, 0.40, 0 35; 0.35 and 0.20 mm (- 5% range) 9. A process according to claim 1, wherein said finishing temperature is maintained between 880 to 950 C 10. A process according to claim 8, wherein said temperature is maintained at 900°C. 11. A process according to claim 1, wherein said coiling is carried out at temperature between 650 to 700 C. 12. A process according to claim 11, wherein said temperature is maintained at 675°C A process for manufacturing high strength, formable and fatigue resistant steel particularly useful in the production of automobile chassis, wheel disc and rims, and such other items. The process comprises the steps of heating steel slabs having a steel composition comprising 0.02 to 0 18 wt% carbon, 0. 01 to 0.14 wt% manganese, 0 to 0.07 wt% silicon, 0 to 0 .02 wt% sulfur, 0 to 0 .02 wt% phosphorous, 0 to 0.05 wt% aluminum, the rest being iron, at a temperature between 1100 to 1350 C, rolling of the slabs in 5 or 6 stands in a hot rolling mill wherein the finishing temperature is maintained between 850 to 950 oC and finally coiling of rolled steel strip at a temperature between 600 to 750 oC. The process controls pearlite/ferrite ratio in the range of 0.05 to 1 0 and is free from large size inclusions. The steel has desirably low ratio of yield strength to ultimate strength for arresting fatigue crack growth. The steel exhibits high percentage elongation and is weldable and free from harmful oxides

Documents:

00345-kol-2003-abstract.pdf

00345-kol-2003-claims.pdf

00345-kol-2003-correspondence.pdf

00345-kol-2003-description(complete).pdf

00345-kol-2003-form-1.pdf

00345-kol-2003-form-18.pdf

00345-kol-2003-form-2.pdf

00345-kol-2003-form-3.pdf

00345-kol-2003-letters patent.pdf

00345-kol-2003-p.a.pdf

345-kol-2003-granted-abstract.pdf

345-kol-2003-granted-claims.pdf

345-kol-2003-granted-description (complete).pdf

345-kol-2003-granted-form 2.pdf

345-kol-2003-granted-specification.pdf