Title of Invention

CARBON-MANGANESE STRAP STEEL WITH HIGH STRENGTH-DUCTILITY.

Abstract

Carbon-Manganese strap steel with high strength and ductility is for intended use in packaging of coils and sheet piles. A selective alloy chemistry of the steel in BOF after treatment for Sulphur control before continuous casting into slab with specified degree of superheat, help achieve desired properties after hot and cold rolling with controlled parameters and subjected to a austempering process at subsequent stage. The steel strap obtained having microstructure showing 100% bainite with non-lamellar carbide in it, demonstrating high strength-ductility properties is suitable for the application for packaging. The grade of steel obtained in the form of narrow strap, usually 1.27 mm thick and 31.75 mm wide having high strength (UTS) in the range from 1040 MPa to 1100 MPa coupled with a ductility value (% elongation) in the range of 8-10% is capable to withstand the load during transportation. The present invention is directed to producing a grade of steel strap having far superior strength without any loss of ductility. Figure 2 a & b.

Full Text

FIELD OF THE INVENTION: The present invention relates to production of Carbon-Manganese strap steel with high strength and ductility. In particular, the invention is directed to an improved quality strap steel through development of suitable microstructure. Advantageously, the invention is directed to achieving much desired maximum strength-ductility combination, which would further enable reducing the width of the strap without compromising on strength. This would further favour saving of material and cost of strap steel. In particular the invention would favour obtaining a grade of steel in the form of narrow strap, usually 1.27 mm thick and 31.75 mm wide having high strength (UTS) in the range from 1040 MPa to 1100 MPa coupled with a ductility value (% elongation) in the range of 8-10%(150mm gauge length), such as to withstand the nature of load they are expected to experience during transportation being used as a packing material for steel coils or bundle of sheets. BACKGROUND ART: Strap steel of 1.27 mm thickness is produced in some steel plants for packaging of steel coils and sheets. Such steel straps are produced for use as a packing material for binding the fully processed coils and packets of sheets after they are covered with HDPE cloth and galvanized sheet. Additionally, a number of coils are utilized together with the help of 1.27mm strap in the railway wagons for the supply of the coils to the customers at distant locations from the steel plants. Hence, such straps need to have high strength as binding/packing of material for steel coils/sheets of heavy weight, to withstand the load including sudden jerks, during transportation and also should be reasonably ductile so that it is easily bent without failure during its use/application as packing material. US 6814817 states about a heat treated steel strap usable in a strapping machine, having a tensile strength of at least about 170 KSI and an elongation of at least about 6.5%. the steel strap is fabricated from a coiled steel reduced by cold rolling, the steel strap composition comprising 0.3-0.36% Carbon, 0.90-1.25% Manganese, and 0.75-1.10% Silicon; the strap being heated to a temperature of about 850°C to about 900°C; and quenched to a temperature of about 370°C to about 510°C, the strap having a seal joint break strength of about 4350 pounds when the strap has a width of about one inch and a thickness of 0.03 inch.Preferably, the strap is preheated to a temperature of about 370°C to about 540°C prior to heating, heated and subsequently quenched. This prior art further states that that the addition of silicon in an amount of about 0.75 to about 1.10 percent by weight of the material tends to reduce the embrittlement that the material would otherwise exhibit. Thus this US prior art makes use of selective weight percent of Silicon to reduce embrittlement and have attained a ductility of 6.5%. In our co-pending patent application no. 708 CAL/2001 dated 24-12-2001, there is disclosed a process for producing steel strap involving austempering after casting and rolling to achieve improved strength properties. However, there has been a continuous need in the art to provide for possible further improvement in quality of strap steel by achieving suitable microstructure, which would favour increase in strength without loss of ductility (% elongation). The present invention is thus specifically directed to providing the C-Mn steel strap with enhanced strength- ductility properties achieved through selective microstructure of the grade of steel obtained through selective use of changed process parameters. OBJECTS OF THE INVENTION: Thus the basic object of the invention is directed to providing the C-Mn steel strap with enhanced properties achieved through selective microstructure of the grade of steel which would on one hand achieve increased strength characteristics and on the other hand also favours maintaining desired ductility (% elongation). Another object of the present invention is directed to obtaining a grade of carbon- manganese steel strap having high strength and reasonable ductility/percent elongation such as to ensure the use of this steel strap as reliable packaging materials without failure for steel coils and bundles of steel sheets for transportation to distant places from the manufacturing plants, by rail/road transport. A further object of the present invention is to obtain said high strength and ductility in steel strap for packaging, usually in the size of 1.27mm thick and 31.75mm wide. A further object of the present invention is to obtain said high strength and ductility in steel strap for packaging wherein the improved strength and ductility properties are achieved through development of selective all bainite microstructure with non-lamellar carbides in said grade of steel by controlled parameters for selective alloy chemistry in the furnace, slab casting and austempering processes. A further object of the present invention is to obtain said high strength and ductility in steel strap for packaging wherein provision for further reduction in the width of the steel strap having higher strength (UTS) in the range of UTS: 1040-1100 MPa and ductility in the range of 8 to 10% (150 mm gauge length). SUMMARY OF THE INVENTION: Thus according to the basic aspect of the present invention there is provided a Carbon- manganese strap steel with high strength-ductility combination comprising: a hundred % bainite microstructure with non-lamellar carbides providing for a high level strength UTS 1040-1100 MPa and ductility in the range of 8-10%. A-further aspect of the present invention directed to said Carbon-manganese strap steel with high strength-ductility combination, comprising the weight % of: C in the range of 0.40 to 0.45 ; Mn in the range of 1.6 to 1.65; P in the range of upto 0.025 (max.); S in the range of upto 0.015 (max.); Si in the range of 0.45 to 0.50; and AI in the range of 0.025 to 0.040. A still further aspect of the present invention directed to said Carbon-manganese strap steel with high strength-ductility combination wherein the steel strap is of 1.27mm thickness. A still further aspect of the present invention directed to said Carbon-manganese strap steel with high strength-ductility combination wherein the optical and SEM micrographs are as illustrated in Figures 2a and 2b respectively. Another important aspect of the present invention directed to said process for the manufacture of carbon-manganese strap steel with high strength-ductility combination comprising: (i) preparing the steel in basic oxygen furnace (BOF) having the composition(wt. %) C in the range of 0.40 to 0.45 ; Mn in the range of 1.6 to 1.65; P in the range of upto 0.025 (max.); S in the range of upto 0.015 (max.); Si in the range of 0.45 to 0.50; and Al in the range of 0.025 to 0.040. ii) processing for sulfur control before continuous casting; iii) subjecting the slabs to continuous casting maintaining a superheat of upto 30°C max; iv) soaking and hot rolling the trips maintaining finishing and coiling temperatures of 880 ± 10°C and 690 ± 10°C respectively, further processing and cold rolling to obtain cold rolled coils which are slit into straps of suitable width; v) austempering the straps thus obtained in a furnace maintained at 930°C-950°C to get it austenitised; passing the austenitised straps through lead bath maintained at 430 to 460°C to thereby achieve selective bainitic transformation for desired strength and ductility combination. A still further aspect of the present invention directed to said process for the manufacture of carbon-manganese strap stee%with high strength-ductility wherein the casting speed is about 0.9 to 1.1 meter/min and preferably 1 meter/min; slabs are soaked at a temperature range of 1240°C to 1260°C preferably at 1250°C and hot rolled to 2.75 mm to 2.85 mm, preferably 2.8mm strips. A further aspect of the present invention directed to said process for the manufacture of carbon-manganese strap steel with high strength-ductility combination wherein the hot rolled strips were pickled in HCL before cold rolling to coils of 1.25 to 1.3 mm preferably 1.27 mm thickness and the cold rolled coils are slit into straps of 31.5 mm to 32 mm width straps preferably 31.75 mm width straps and subsequently austempered. According to yet another aspect of the present invention directed to said process for the manufacture of carbon-manganese strap steel with high strength-ductility combination as claimed in anyone of claims 5 to 7 wherein the lead bath is maintained at a distance of 1.1 to 1.3 meter, preferably 1 meter and after completion of bainitic transformation in lead bath , the strap are passed through cooling tower and finally coiled. BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES: Figure 1: is the illustration of the schematic diagram of austempering unit showing the movement of cold rolled slitted strip through different sections of the unit during processing. Figure 2(a): is the optical micrograph showing bainitic microstructure according to the present invention ; Figure 2(b): is the SEM micrograph showing non-lamellar carbide in bainite, according to the present invention; Figure 3 : is the illustration of the flow chart for the process used for production of C-Mn strapping steel with high strength-ductility combination, according to the present invention. DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE ACCOMPANYING FIGURES: To obtain the combination of improved high strength and ductility properties in the particular grade of steel used for producing the strap for packaging according to the present invention, a number of laboratory trials were conducted to study, observe and record the influence of alloying elements like C, Mn and Si on the hardenability of the resulting steel. Different corresponding microstrutures of steel grades were obtained. The 100% bainitic micro-structure with non-lamellar carbides resulted in the maximum strength-ductility combination. Such microstructure was achieved during carrying out experimental trial production of this product after austempering stage. According to the present invention the alloy chemistry on Wt. % basis of the steel obtained after melting in the basic oxygen furnaceand subsequent treatment in secondary refining unit(SRU) for Sulphur control, as starting material was as follows: C in the range of 0.40 to 0.45, Mn in the range of 1.6 to 1.65, P in the range of 0.025(max), S in the range of 0.015(max), Si in the range of 0.45 to 0.50, Al in the range of 0.025 to 0.040. This grade of steel was further processed through continuous casting. The slabs were cast maintaining the superheat of 30°C max. and at a casting speed of about 1m/min. The slabs were soaked at 1250°C and hot rolled to 2.8mm strips, maintaining finishing and coiling temperature 880±10°C and 690±10°C respectively. Hot rolled strips were pickled in Hydrochloric acid (HCL) media before cold rolling to further reduction in thickness to 1.27mm thick sheet wound in coils. Cold rolled coils were slit in 31.75mm width straps and subsequently austempered. Attention is first invited to Figure 1 that shows the schematic diagram of the austempering unit, according to the present invention. The steel strap is passed with the help of an uncoiler through a austenitising furnace maintained at 930-950°C at its different zones where it gets austenitised over duration of about 2 min. Subsequently, it passes through a lead bath at 1.2 meter distance and the temperature of said bath is maintained at 430- 460°C. After completion of bainitic transformation while passed through the lead bath, with residence time for about 1/2 min, the strap passes through the cooling tower and finally coiled as finished product. The optical and SEM micrograph of the corresponding processed grade of strap material are shown in the accompanying figures 2(a) and 2(b) respectively. The micrograph of the steel strap according to the present invention shows that a 100% bainitic structure and the carbide in the bainite is non-lamellar which is the characteristic of bainitic transformation.The resulting steel garde was further tested for tensile strength and percent elongation carried out in the shop itself using WNC Tensile Testing Machine. The test results achieved are UTS: 1040-1100 MPa and % elongation 8-10 (150 mm gauge length) which are substantially higher than those specified in IRS: M-41 specification. Such microstructure is responsible for a high level of strength such as UTS in the range of 1040 MPa to 1100 MPa, without any sacrifice in the ductility, which is maintained in the range of 8-10% to favour the use of the steel strap for bending during packaging without failure. The flow chart for the entire process and its sequences to produce the C-Mn steel strap having high strength-ductility combination properties, according to the present invention is illustrated in the accompanying Figure 3. Because of very high strength of the product, the width of the strap are capable to be reduced further from the existing 31.75mm in order to effect material saving. We Claim: 1. Carbon-manganese strap steel with high strength -ductility combination comprising: C in the range of 0.40 to 0.45 ; Mn in the range of 1.6 to 1.65; P in the range of upto 0.025 (max.); S in the range of upto 0.015 (max.); Si in the range of 0.45 to 0.50; and Al in the range of 0.025 to 0.040. having a hundred % bainite microstructure with non-lamellar carbides providing for a high level strength (UTS) 1040-1100 MPa and ductility in the range of 8-10%. 2. Carbon-manganese strap steel with high strength -ductility combination as claimed in claim 1 wherein the steel strap is of 1.27mm thickness. 3. Carbon-manganese strap steel with high strength -ductility combination as claimed in anyone of claims 1 or 2wherein the optical and SEM micrographs are as illustrated in Figures 2a and 2b respectively. 4. A process for the manufacture of carbon-manganese strap steel with high strength- ductility combination as claimed in anyone of claims 1 to 3 comprising: (i) preparing the steel in basic oxygen furnace (BOF) having the ladle chemistry (wt. %) after SRU treatment: C in the range of 0.40 to 0.45 ; Mn in the range of 1.6 to 1.65; P in the range of upto 0.025 (max.); S in the range of upto 0.015 (max.); Si in the range of 0.45 to 0.50; and Al in the range of 0.025 to 0.040. ii) processing for sulfur control in SRU before continuous casting; iii) subjecting the slabs to continuous casting maintaining a superheat of upto 30°C max; iv) soaking and hot rolling the strips maintaining finishing and coiling temperatures of 880 ± 10°C and 690 ± 10°C respectively, further processing and cold rolling to obtain cold rolled coils which are slit into straps of suitable width; v) austempering the straps thus obtained in a furnace maintained at 930- 950°C to get it austenitised; passing the austenitised straps through a lead bath maintained at 430 to 460°C to thereby achieve selective bainitic transformation for desired strength and ductility combination. 5. A process for the manufacture of carbon-manganese strap steel with high strength- ductility combination as claimed in claim 4 wherein the casting speed is about 0.9 to 1,1 meter/min preferably 1 meter/min; slabs are soaked at a temperature range of 1240°C to 1260°C preferably at 1250°C and hot rolled to 2.75 mm to 2.85 mm preferably 2.8mm strips. 6. A process for the manufacture of carbon-manganese strap steel with high strength- ductility combination as claimed in anyone of claims 4 or 5 wherein the hot rolled strips were pickled in HCL before cold rolling to coils of 1.25 mm to 1.30 mm preferably 1.27 mm thickness and the cold rolled coils are slit into straps of 31.5 mm to 32 mm width straps preferably 31.75 mm width straps and subsequently austempered. 7. A process for the manufacture of carbon-manganese strap steel with high strength- ductility combination as claimed in anyone of claims 4 to 6 wherein the lead bath is maintained at a distance of 1.1 meter to 1.3 meter preferably 1.2 meter and after completion of bainitic transformation in lead bath , the straps are passed through cooling tower and finally coiled. ABSTRACT TITLE: CARBON-MANGANESE STRAP STEEL WITH HIGH STRENGTH -DUCTILITY COMBINATION. Carbon-Manganese strap steel with high strength and ductility is for intended use in packaging of coils and sheet piles. A selective alloy chemistry of the steel in BOF after treatment for Sulphur control before continuous casting into slab with specified degree of superheat, help achieve desired properties after hot and cold rolling with controlled parameters and subjected to a austempering process at subsequent stage. The steel strap obtained having microstructure showing 100% bainite with non-lamellar carbide in it, demonstrating high strength-ductility properties is suitable for the application for packaging. The grade of steel obtained in the form of narrow strap, usually 1.27 mm thick and 31.75 mm wide having high strength (UTS) in the range from 1040 MPa to 1100 MPa coupled with a ductility value (% elongation) in the range of 8-10% is capable to withstand the load during transportation. The present invention is directed to producing a grade of steel strap having far superior strength without any loss of ductility. Figure 2 a & b.

Documents:

00272-kol-2007 correspondence-1.1.pdf

00272-kol-2007 p.a.pdf

00272-kol-2007-correspondence-1.2.pdf

00272-kol-2007-form-1-1.1.pdf

0272-kol-2007 abstract.pdf

0272-kol-2007 claims.pdf

0272-kol-2007 correspondence others.pdf

0272-kol-2007 description(complete).pdf

0272-kol-2007 drawings.pdf

0272-kol-2007 form-1.pdf

0272-kol-2007 form-2.pdf

0272-kol-2007 form-3.pdf

272-KOL-2007-(05-09-2013)-CORRESPONDENCE.pdf

272-KOL-2007-(16-09-2013)-ABSTRACT.pdf

272-KOL-2007-(16-09-2013)-CLAIMS.pdf

272-KOL-2007-(16-09-2013)-CORRESPONDENCE.pdf

272-KOL-2007-(16-09-2013)-DESCRIPTION (COMPLETE).pdf

272-KOL-2007-(16-09-2013)-FORM-2.pdf

272-KOL-2007-(22-04-2013)-CLAIMS.pdf

272-KOL-2007-(22-04-2013)-CORRESPONDENCE.pdf

272-KOL-2007-(22-04-2013)-OTHERS.pdf

272-KOL-2007-CANCELLED PAGES.pdf

272-KOL-2007-CORRESPONDENCE OTHERS-1.3.pdf

272-KOL-2007-CORRESPONDENCE.pdf

272-KOL-2007-EXAMINATION REPORT.pdf

272-kol-2007-form 18.pdf

272-KOL-2007-GRANTED-ABSTRACT.pdf

272-KOL-2007-GRANTED-CLAIMS.pdf

272-KOL-2007-GRANTED-DESCRIPTION (COMPLETE).pdf

272-KOL-2007-GRANTED-DRAWINGS.pdf

272-KOL-2007-GRANTED-FORM 1.pdf

272-KOL-2007-GRANTED-FORM 2.pdf

272-KOL-2007-GRANTED-FORM 3.pdf

272-KOL-2007-GRANTED-SPECIFICATION-COMPLETE.pdf

272-KOL-2007-PA.pdf

272-KOL-2007-REPLY TO EXAMINATION REPORT.pdf

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