Title of Invention

A PROCESS FOR PRODUCTION OF HIGH STRENGTH WELDABLE STEEL PLATES WITH SUPERIOR LOW TEMPERATURE IMPACT TOUGHNESS

Abstract

There is suggested an improved method for the production of high strength weldable steel plates with superior low temperature impact toughness comprising subjecting a steel slab having the following chemical composition, based on the weight of the slab: c Mn S P At Nb V 0.07 1.1 0.005 0.02 0.02 0.03 0.03 to to max. max. to to to 0.10 1.3 0.04 0.05 0.06 to a two stage rolling with an intermediate delay or cooling stage, the first stage being a high temperature rolling stage carried out at temperatures in the region of 1200°C to 1050°C, the delay or cooling stage being provided to cool the rolled and deformed plate of the first stage to a temperature of around 950°C and the second low temperature rolling stage being carried out at temperatures in the region of 950°C to 800°C. The final rolled plate produced confirmed to the following mechanical properties of 12mm thick plates obtained were as follows: YS:440-500 MPa, TS:520-560 MPa, EI.:32-38%.

Full Text

Introduction to the Field of Invention: This invention relates to a method for the production of high strength (YS: 430 MPa min.), weldable plates with superior low temperature impact toughness (200 J min. at -20° C) and the plates so produced. Prior Art and Drawbacks: It is already known to produce weldable plates of high strength in conventional rolling mills. The major limitations with the plate mills are lower load bearing capacity (-3000 T) and absence of accelerated cooling facility. With these limitations, it is very difficult and is not possible to produce high strength weldable plates with superior low temperature impact toughness using a low C steel without addition of alloy elements (Cu, Ni, Mo, Cr). Moreover, in the known art, we found a number of Patents, which deal with controlled rolling of plates and hot strips using low alloy (containing Cu and Ni) low C steel micro-alloyed with Nb, V and Ti, resulting in products with excellent strength and impact toughness combination. Various types of techniques/technologies involving control of the cooling rate (between 800° C to 600° C and 600° C to 400° C), accelerated cooling at rates of 25° C/S, two stages controlled rolling has been employed. o None of the Patents provides a strength guarantee of 430 MPa min. and a concurrent impact toughness guarantee of 200 J min. at -20° C. o No accelerated cooling or controlled cooling has been resorted to get the desired strength toughness combination. o The present proposal imposes a limit on the inclusion level, which none of the other Patent does. And we propose to avoid alloying elements like Cu, Ni and Cr. o The deformation schedule used is different from others. Objects of the Invention: It is therefore an object of this invention to produce high strength low temperature impact toughness steel plates in the existing plate mills so that cost can be minimized. It is another object to propose an improved method of rolling of steel plates in an existing-platemill without overloading the mill. It is yet another object of this invention to adopt a two stage rolling, the first one at a higher temperature " than the second stage. It is a further object to use a specially designed chemistry of steel for the rolling purposes. It is a still further object to propose an improved method of rolling plates in a plate milt by which it is possible to achieve API X-60 grade properties in plates up to 20mm plate thickness without overloading the mill. Yet another object is to propose such a method whereby the plates produced will exhibit excellent low temperature impact toughness properties in excess of 200 J at -20 °C. These and other objects of the invention will be apparent from the following paragraphs. Brief Statement of the Invention: According to one aspect of the invention, there is provided an improved method for the production of high strength weldable steel plates with superior low temperature impact toughness comprising subjecting a steel slab having the following chemical composition(wt %), C Mn S P Al Nb V 0.07 1.1 0.005 0.02 0.02 0.03 0.03 to to max, max. to to to 0.10 1.3 0.04 0.05 0.06 to a two stage rolling with an intermediate delay or cooling stage, the first stage being a high temperature rolling stage carried out at temperatures in thje region of 1200 °C to 1050°C, 3 the delay or cooling stage being provided to cool the rolled and deformed plate of the first stage to a temperature of around 950° C and the second low temperature rolling stage being carried out at temperatures in the region of 950° C to 800° C. In this method, the preferred chemical composition of the slab used was as follows: C: 0.08, Mn: 1.2, Si: 0.24, Al: 0.036, S: 0.005, P: 0.02, Nb: 0.03, V: 0.04 and before subjecting to the high temperature rolling, the slabs were austenitised at 1250±10°C to ensure complete solid solution of Nb-V precipitates. The high temperature rolling (deformation) was carried out in the y-re-crystallization zone where the austenite was subjected to repeat re-crystallization and the total deformation in the first stage was about 80%, reducing the plate thickness from about 210 mm to 40 mm. In this case, the degree of deformation per pass ranged from 10% to 22% increasing with decreasing temperature of deformation. In this method, there was introduced an intermediate delay or cooling stage, after the first rolling stage, wherein, the deformed slabs of the first stage were oscillated in the run-out table to bring down the temperature frwiffiOSO0 C to&£50° C. After the delay or cooling stage, in the second stage, the rolled slabs of the first stage were subjected to a cumulative deformation of about 70%. The rolled slabs from the first stage were deformed, from about 40 mm to the final thickness (about 12 mm), in the second stage in the y-non re-crystallization zone. The finish rolling temperature was maintained in the range of 790° C to 830° C and the deformation in the last pass is 15% min. It was noticed that the mechanical properties of the 12 mm thick rolled plates conrormea to API X-60 Specification (YS: 430 MPa min. TS: 520 MPa, min, El. 30% min.) and even was found to be slightly better. 4 The mechanical properties of the 12 mm thick plates obtained were more or less as follows: YS: 440-500 MPa, TS: 520-560 MPa, El.: 32-38% and thus, the final plate showed excellent impact toughness in excess of 300 J at -20° C. Detailed Description of the Invention: Example No. 1: For experimental purposes, we preferred a low C steel micro-alloyed with Nb and V. In this example, the preferred chemical composition of the steel used was as follows: C: 0.08, Win: 1.2, Si: 0.24, Al: 0.036, S: 0.005, P: 0.02, Nb: 0.03, V: 0.04 The above amounts are in percentages and based on the weight percent of the steel plate. The slabs having of the above specially micro-alloyed composition were austenitised at 1250±10°C to ensure complete solid solution of Nb-V precipitates. The slabs were then subjected to a two stage controlled rolling. First Stage Rolling: High temperature rolling at 1200°C to 1050°C: This high temperature rolling ensures deformation in the y-re-crystallization zone where the austenite is subjected to repeated re-crystallization. The total deformation in this zone was around 80% (The thickness of the plate was reduced from about 210 mm to about 40 mm), the degree of deformation per pass ranged from 10% to 22%, increasing with decreasing temperature of deformation. An Intermediary Stage: The stage called "a delay stage" was introduced before proceeding to the second stage low temperature rolling. In this stage, the deformed slabs of the first stage were oscillated in the run table to bring down the temperature froM 10500C to950°C. 5 Once the temperature of the slabs deformed in the first stage reached a temperature of about 950° C, they were subjected to a second stage rolling which was a low temperature rolling carried out at a temperature in the range of 950° C to 800° C. The Second Stage Low Temperature Rolling at 950° C to 800° C: Here, the rolled slabs of the first stage were subjected to a cumulative deformation of about 70% (The thickness of the slab was reduced from about 40 mm to 12 mm) in the y-non re-crystallization zone. The finish rolling temperature was maintained in the range of 790° C-850°C. The deformation in the last pass was 15% min. Verification of the Properties of the End Product: The micro-structure of the steel plates, after the second stage, revealed a ferrite-pearlite structure. The ferrite grain size varied from surface to centre within a narrow band, being slightly coarser at the centre (6.8-7.2 um) as compared to the surface (5-6 urn). The pearlite volume fraction was found similar for all the plates samples, irrespective of FRT, varying between 12-15%. The mechanical properties of the 12 mm thick plates conformed to the standard API X-60 Specification (YS: 430 MPa min, TS: 520 MPa, El. 30% min.) and even was slightly better. The typical range of mechanical properties obtained were as follows: YS: 440-500 MPa, TS: 520-560 MPa, El.: 32-38%. Confirmation of the Advantages Obtained: Impact tests were carried out on standard charpy samples of 10 x 10 x 55 mm at RT to -60° C. Ail the plates showed excellent impact toughness properties in excess of 300 J at -20° C. The impact energy values obtained at different temperature are given below: Plate No. CIE, J RT 0 -20 -40 -60 856 336 336 336 288 240 857 336 336 336 336 336 858 336 336 336 336 192 859 336 336 336 336 153 6 The preferred controlling parameters of the two rolling stages were confined to the following details. # Control Parameters of the rolling stages: First stage: o High temperature rolling (210 mm to 40 mm) A. $JJ&% No. of passes : 9-11 Reduction per pass : 10% to 22% (to increase with decreasing temperature of rolling). Minimum 20 mm drafts to be given in the first 5 passes. Rolling start temperature : 1160uC-1200uC. Rolling finish temperature^.:^. i ' - o Delay stage : Oscillation of the plates in the run-out table to bring down the temperature to 950±10°C. o Second Stage Low temperature rolling: (40 mm to 12 mm) To be carried out in 4-6 passes. FRT of 810u±20uC with 15% min. reduction in the final pass. Leveling passes to be given after the final pass to ensure desired flatness. # Inclusive Level: The steel to be secondary refined to achieve an inclusion volume fraction of 0.10 max. # Re-heating: Soaking temperature Residence time 1240±10°C. 5 hrs. We are avoiding alloying elements like Cu, Ni and Cr. o None of the Patents provides a strength guarantee of 430 MPa min. and a impact toughness guarantee of 200 J min. at -20° C. 7 o No accelerated cooling or controlled cooling has been restored to get the desired strength toughness combination. o The present proposal imposes a limit on the inclusion level, which none of the other Patent does. o The deformation schedule used is different from others. We claim : 1. An improved method for the production of high strength weldable steel plates with superior low temperature impact toughness comprising subjecting a steel slab having the following chemical composition, based on the weight of the slab: c Mn S P Al Nb V 0.07 1.1 0.005 0.02 0.02 0.03 0.03 to to max. max. to to to 0.10 1.3 0.04 0.05 0.06 to a two stage rolling with an intermediate delay or cooling stage, the first stage being a high temperature rolling stage carried out at temperatures in the region of 1200°C to 1050°C, the delay or cooling stage being provided to cool the rolled and deformed plate of the first stage to a temperature of around 950°C and the second low temperature rolling stage being carried out at temperatures in the region of 950°C to 800°C. 2. A method as claimed in claim 1, wherein, the preferred chemical composition of the slab used was as follows: C:0.08, Mn:1.2, Si: 0.24, Al: 0.036, S: 0.005, P: 0.02, Nb:0.03, V:0.04. 3. A method as claimed in claims 1 and 2, wherein, before subjecting to the high temperature rolling, the slabs were austenitised at 1250 ±10°C to ensure complete solid solution of Nb-V precipitates. 4. A method as claimed in claims 1 to 3, wherein, the high temperature rolling(deformation) was carried out in the y-re-crystallization zone where the austenite was subjected to repeat re- crystallization. 5. A method as claimed in claim 4, wherein, the total deformation in the first stage was about 80%, reducing the plate thickness from about 210mm to 40mm. 9 6. A method as claimed in claim 5, wherein, the degree of deformation per pass ranged from 10% to 20 %, increasing with decreasing temperature of deformation. 7. A method as claimed in claims 1 to 6, wherein, in the intermediate delay or cooling stage, the deformed slabs of the first stage were oscillated in the run-out table to bring down the temperature from 1050°C to 950°C. 8. A method as claimed in claims 1 to 7, wherein, in the second stage, the rolled slabs of the first stage were subjected to a cumulative deformation of 70%. 9. A method as claimed in claim 8, wherein, the rolled slabs from the first stage were deformed~from about 40mm to about 12mm in the second stage in the y-non re-crystallization zone. 10. A method as claimed in claims 1 to 9, wherein, the finish rolling temperature was maintained in the range of 790°C to 830°C and the deformation in the last pass was 15% min. 11. A method as claimed in claims 1 to 10, wherein, the mechanical properties of 12mm thick plates conformed to API X-60 Specification(YS:430 MPa min.,TS:520 MPa min.,EI.30% min.). 12. A method as claimed in claim 11, wherein, the mechanical properties of 12 mm thick plates obtained were more or less, as follows: YS: 440-550 MPa, TS:520-560 MPa, EI.:32-38%. 13. A method as claimed in claims 1 to 12, wherein, the final plate showed excellent impact toughness in excess of 200 J at -20°C. 14. An improved method for the production of high strength weldable steel plates with superior low temperature impact toughness substantially as herein described with reference to the examples. (ANJAN SEN) Of Anjan Sen and Associates Agent for Applicant 10 15. Rolled steel plates whenever produced by the method as claimed in claims 1 to 14. There is suggested an improved method for the production of high strength weldable steel plates with superior low temperature impact toughness comprising subjecting a steel slab having the following chemical composition, based on the weight of the slab: c Mn S P At Nb V 0.07 1.1 0.005 0.02 0.02 0.03 0.03 to to max. max. to to to 0.10 1.3 0.04 0.05 0.06 to a two stage rolling with an intermediate delay or cooling stage, the first stage being a high temperature rolling stage carried out at temperatures in the region of 1200°C to 1050°C, the delay or cooling stage being provided to cool the rolled and deformed plate of the first stage to a temperature of around 950°C and the second low temperature rolling stage being carried out at temperatures in the region of 950°C to 800°C. The final rolled plate produced confirmed to the following mechanical properties of 12mm thick plates obtained were as follows: YS:440-500 MPa, TS:520-560 MPa, EI.:32-38%.

Documents:

00692-cal-2001 abstract.pdf

00692-cal-2001 claims.pdf

00692-cal-2001 correspondence.pdf

00692-cal-2001 description(complete).pdf

00692-cal-2001 form-1.pdf

00692-cal-2001 form-13.pdf

00692-cal-2001 form-18.pdf

00692-cal-2001 form-2.pdf

00692-cal-2001 form-3.pdf

00692-cal-2001 letters patent.pdf

00692-cal-2001 p.a.pdf

00692-cal-2001 reply f.e.r.pdf

692-cal-2001-granted-abstract.pdf

692-cal-2001-granted-claims.pdf

692-cal-2001-granted-description (complete).pdf

692-cal-2001-granted-form 2.pdf

692-cal-2001-granted-specification.pdf