Title of Invention

A PROCESS OF MANUFACTURING OF SUPERIOR CORROSION RESISTANT COPPER-MOLYBDENUM TMT REBARS.

Abstract

TITLE: PROCESS FOR THE MANUFACTURE OF TMT REBARS HAVING IMPROVED MECHANICAL AND CORROSION PROPERTIES. Process for manufacture of improved corrosion resistant TMT rebars, in particular, copper-molybdenum TMT rebars having improved strength and corrosion resistant properties suitable for applications in chloride containing environment. The process comprises providing steel ingots having compositions comprising 0.14 to 0.18 wt% carbon 1.0 to 1.2 wt% manganese, 0.2 to 0.3 wt% silicon, up to 0.03 wt% sulphur, up to 0.03 wt% phosphorous, 0.3 to 0.4 wt% copper and 0.15 to 0.2 wt% molybdenum, the balance being iron, soaking said steel ingots and processing to billets, cooling the surface of said billets in Thermix Unit with water pressure between 15 to 22 kg/mm2 and equalization temperature between 600 to 680 degree C and finish rolling said surface cooled steel bars between 950 to 1060 degree C.

Full Text

Field of the invention The present invention relates to a process for manufacturing of improved corrosion resistant TMT rebars. In particular, the present invention relates to a process for manufacturing copper-molybdenum TMT rebars having improved strength and corrosion resistant properties suitable for applications in chloride containing environment. Background of the invention Effect of alloying elements such chromium, copper, molybdenum, vanadium etc. are known for improving mechanical strength and corrosion properties of steel. These properties are found to vary in different processing parameters. Several processes for manufacturing of steel are known in the art where the properties of steel have been improved by varying the steel chemistry and processing parameters. TMT rebars or thermo-mechanically treated rebars are extra high strength reinforcing which eliminate any form of cold twisting. TMT rebars have wide applications in the construction sector such as general concrete reinforcement in buildings, bridges and various other concrete structures and especially high rise buildings. For its various applications, specially in chloride containing environments, it is desirable that TMT rebars have enhanced corrosion resistance along with necessary mechanical strength. It is further desired that the rim of TMT rebars have greater hardness as compared to hardness of the core. The rebars should be weldable as well. Among the known TMT rebars, copper-phosphorous TMT rebars are widely used. However, such known TMT rebars do not have the sufficient mechanical strength required for specific applications. More importantly, the corrosion properties of known copper-phosphorous TMT rebars (CRI of around 1.6) are not suitable for use in chloride containing environment in which cause quick degradation of such known TMT rebars occurs. Therefore, in view of the growing need of TMT rebars attempts have been made in the past to provide TMR rebars with improved strength and corrosion resistant properties suitable for use in chloride containing environments. Objects of the Invention Therefore the basic object of the present invention is to provide a process for manufacture of TMT rebars having improved strength and corrosion properties by proper selection of steel chemistry and processing parameters. Another object of the present invention is to provide a process for manufacture of TMT rebars having higher hardness of rim compared to hardness of core. Further object of the present invention is to provide for a process for manufacture of TMT rebars having enhanced weldability. After extensive investigation the applicants have now found that by suitably selecting the steel chemistry, in particular use of copper and molybdenum as alloying elements and by proper selection of process parameters, TMT rebars with improved mechanical strength and corrosion resistant properties is achieved. Summary of the Invention Therefore according to the present invention there is provided a process for the manufacture of thermo-mechanically treated (TMT) Cu-Mo rebars having improved mechanical and corrosion properties comprising: (i) providing steel ingots having composition comprising 0.14 to 0.18 wt% carbon, 1.0 to 1.2 wt% manganese, 0.2 to 0.3 wt% silicon, up to 0.03 wt% sulphur, up to 0.03 wt% phosphorous, 0.3 to 0.4 wt% copper and 0.15 to 0.2 wt% molybdenum, the balance being iron; (ii) soaking said steel ingots and processing to billets; (iii) cooling the surface of said billets in Thermix Unit with water pressure between 15 to 22 kg/mm2 and equalization temperature between 600 to 680°C; and (iv) finish rolling said surface cooled steel bars between 950 to 1060°C. Detailed description of the Invention The alloying elements, in particular copper and molybdenum, are found to reduce corrosion particularly the effect of chloride ions in chloride containing environment. The alloying elements are found to form a layer on the surface of the rebars and develop resistance to the diffusion of chloride ions to steel surface, thus preventing the interaction between corrosive media and steel surface. These also improve the corrosion resistance under non-stress and stress conditions. In addition, Molybdenum induces resistance to chloride ions. Molybdenum also improves stress corrosion properties. Following the process of the present invention improved TMT rebars are manufactured by selecting alloy chemistry formulation and adjustment of finishing rolling temperature, water pressure and equalization temperature in the TMT line. The steel ingots are prepared by conventional methods such as melting of steel in a furnace and tapping it in a preheated ladle at around 1600 to 1650°C, preferably between 1620 to 1640°C. The required quantity of preheated ferroalloys and copper cathode plate are kept in the ladle prior to tapping of steel to meet the aimed chemistry. The liquid steel is preferably homogenized with purging of argon and cast as ingots. The ingots are soaked at temperature between 1250 to 1350°C for around for six hours and are processed to form billets. The billets are then surface cooled in Thermix unit preferably consisting of four tubes through which pressurized water is circulated to cool the surface of bars to lower temperature for specific metallurgical changes. The finishing rolling temperature is between 950 to 1060°C. The water pressure selected between 15 to 22 kg/mm2 for 22-40 mm diameter bars. The equalization temperature is selected between 600 to 680°C. Several tests were conducted to assess different properties of the copper- molybdenum TMT rebars of the present invention. The result revealed improved mechanical properties over conventional copper-phosphorous TMT rebars. The hardness of core and rim of the copper- molybdenum TMT rebars were also found to be higher as compared to the known phosphorus TMT rebars. Improved corrosion properties and good weldability were also revealed by the tests. The present invention will now be described with reference to non-limiting examples and following figures in which : Fig. 1 shows the flow chart of the process of the present invention ; Fig. 2 shows optical microstructure of 16mm diameter Cu-Mo TMT Bar and reveals the presence of tempered martensite at rim and ferrite/bainite at core. Example Example 1 - Manufacture of TMT rebars of 16mm diameter. Steel ingots having composition comprising 0.15 wt% carbon, 0.75 wt% manganese, 0.21 wt% silicon, 0.03 wt% sulphur, 0.024 wt% phosphorous, 0.3 wt% copper and 0.17 wt% molybdenum, the balance being iron were prepared by melting of steel in a furnace and tapping it in a preheated ladle at 1630ºC. The required quantity of preheated ferroalloys and copper cathode plate were kept in the ladle prior to tapping of steel to meet the aimed chemistry. The Liquid steel was homogenized with purging of argon and cast as ingots of weight 9 tones each or continuously cast as 100 x 100 mm billets. The ingots were soaked at 1300°C Tor six hours and then processed to form billets. The billets were then surface cooled in Thermix unit consisting of four tubes through which pressurized water is circulated to cool the surface of bars to lower temperature for specific metallurgical changes. The finishing rolling temperature was maintained at 1000°C. The water pressure was maintained at 15 w 22 kg/mm2 for 22-40 mm diameter bars, the equalization temperature being 650°C. Example 2 - Manufacture of TMT rebars of 32 mm diameter TMT rebars of 32 mm diameter were manufactured following the process of Example 1 and using steel chemistry of 0.17 wt% carbon, 1.18 wt% manganese, 0.034 wt% silicon, 0.021 wt% sulphur, 0.034 wt% phosphorous, 0.33 wt% copper and 0.15 wt% molybdenum, the balance being iron. Example 3 - Manufacture of TMT rebars of 36 mm diameter TMT rebars of 36 mm diameter were manufactured following the process of Example 1 and using a steel chemistry of 0.13 wt% carbon, 0.7 wt% manganese, 0.2 wt% silicon, 0.027 wt% sulphur, 0.026 wt% phosphorous, 0,32 wt% copper and 0.11 wt% molybdenum, the balance being iron. Mechanical properties of the TMT rebars of 16, 32 and 36 mm diameter manufactured as described in Examples 1 to 3 were determined as per IS- 1786:1985. Table-1 shows the test results of TMT rebars of the present invention as also mechanical properties of known copper phosphorous TMT rebars. Table - 2 shows the hardness of core and rim, Table - 3 shows the corrosion index and Table - 4 shows the carbon equivalent for weldability. From the above tables, it is clear that copper-molybdenum TMT rebars of the present invention have improved mechanical properties, corrosion resistance, hardness and carbon equivalent compared to known copper phosphorous TMT rebars. We Claim: 1. Process for the manufacture of thermo-mechanically treated (TMT) Cu-Mo rebars having improved mechanical and corrosion properties comprising: (i) providing steel ingots having composition comprising 0.14 to 0.18 wt% carbon, 1.0 to 1.2 wt% manganese, 0.2 to 0.3 wt% silicon, up to 0.03 wt% sulphur, up to 0.03 wt% phosphorous, 0.3 to 0.4 wt% copper and 0.15 to 0.2 wt% molybdenum, the balance being iron; (ii) soaking said steel ingots and processing to billets; (iii) cooling the surface of said billets in Thermix Unit with water pressure between 15 to 22 kg/mm2 and equalization temperature between 600 to 680°C; and (iv) finish rolling said surface cooled steel bars between 950 to 1060°C. 2. Process as claimed in claim 1, wherein said steel ingots have composition comprising 0.17 wt% carbon, 1.18 wt% manganese, 0.30 wt% silicon, 0.021 wt% sulphur, 0.03 wt% phosphorous, 0.33 wt% copper and 0.15 wt% molybdenum, the balance being iron. 3. Process as claimed in claim 1, wherein said ingots are soaked at temperature between 1250 to 1350°C. 4. Process as claimed in claim 3, wherein said ingots are soaked at 1300°C. 5. Process as claimed in any preceding claim, wherein said soaking of ingots is carried out for 6 hours. 6. Process as claimed in claim 1 wherein said Thermix Unit consists of four tubes. 7. Process as claimed in claim 1 wherein said equalization temperature is 650°C. Process for the manufacture of TMT rebars having improved mechanical and corrosion properties Process for manufacture of improved corrosion resistant TMT rebars, in particular, copper-molybdenum TMT rebars having improved strength and corrosion resistant properties suitable for applications in chloride containing environment. The process comprises providing steel ingots having composition comprising 0.14 to 0.18 wt% carbon 1.0 to 1.2 wt% manganese, 0.2 to 0.3 wt% silicon, up to 0.03 wt% sulphur, up to 0.03 wt% phosphorous, 0.3 to 0.4 wt% copper and 0.15 to 0.2 wt% molybdenum, the balance being iron, soaking said steel ingots and processing to billets, cooling the surface of said billets in Thermix Unit with water pressure between 15 to 22 kg/mm2 and equalization temperature between 600 to 680°C and finish rolling said surface cooled steel bars between 950 to 1060°C.

Documents:

47-KOL-2004-(01-02-2012)-FORM-27.pdf

47-KOL-2004-FORM 27.pdf

47-kol-2004-granted-abstract.pdf

47-kol-2004-granted-claims.pdf

47-kol-2004-granted-correspondence.pdf

47-kol-2004-granted-description (complete).pdf

47-kol-2004-granted-drawings.pdf

47-kol-2004-granted-examination report.pdf

47-kol-2004-granted-form 1.pdf

47-kol-2004-granted-form 18.pdf

47-kol-2004-granted-form 2.pdf

47-kol-2004-granted-form 3.pdf

47-kol-2004-granted-letter patent.pdf

47-kol-2004-granted-pa.pdf

47-kol-2004-granted-reply to examination report.pdf

47-kol-2004-granted-specification.pdf