Title of Invention

A PROCESS FOR PRODUCING HIGH STRENGTH AND HIGH TOUGHNESS PLATES FROMLEAN ALLOY HSLA STEEL

Abstract

There is disclosed A process for producing high strength of YS: 750 MPa Min. and high toughness of 82J at 85°C min. steel products such as steel plates from lean composition alloy steel which comprises i. pre-fixing the selective various alloying components and their percentages for a steel composition pre-obtained by experiments and test results obtained thereof, such as to adjust the various alloying components to ensure that the amount of alloying element Cu does not exceed 0.90 weight %, that of Cr does not exceed 0.75 weight %, that of Ni does not exceed 1.00 weight %, that of Mo does not exceed 0.25 weight % and the amount of alloying element No does not exceed 0.03 weight %, such that the modified steel contains upto 2.93 weight % of the concerned alloying elements taken together. ii. preparing a normal steel alloy composition in the usual manner; iii. carrying out a chemical analysis of the steel composition prepared in step ii, iv. comparing with the pre-selected amounts and percentages of the various components as prefixed, v. making necessary additions and adjustments to the alloy steel prepared in step (ii) to obtain a modified alloy steel with the desired chemistry, vi. thereafter, casting the modified alloy steel into ingots in the usual manner, vii. soaking the ingots at 1200° C ±20° C, viii. followed by hot-rolling the soaked ingots to the desired form of steel plates such that the final rolled plate has a chemical composition of lean alloy Cu Bearing HSLA steel in weight % as follow.

Full Text

Introduction to the Field of the Invention: This invention relates to a method for producing high strength and high toughness steel plates. Prior Art and Drawbacks It is well known to manufacture steel plates in a rolling mill from steel slabs cast from alloy steel of desired composition. It is known to adjust the alloy composition before forming the steel slabs so to give the desired properties to the end product such as steel sheets. We are aware of presently available steel plates called (High Strength Low Alloy) HSLA grades HY-100, HSLA-100 and others. These steel plates have exhibited the properties desired and are currently in use and the country's needs are met by importation. However, there is always room for improvement and it is possible to realize better and products like steel plates than possible so far. Objects of the Invention : It is therefore a primary object of this invention to propose an improved method for the production of improved alloy steel products such a plates exhibiting better strength than the known similar products. It is a second object of this invention to propose such an improved method, which, in addition to producing better strength steel, will also impart better impact toughness to the end product than so far realized. It is a third object of this invention to propose such an improved method, which will give, in addition to the above properties, also the property of better weldability characteristics than possible so far. A still further object of this invention is to propose a method by which the new alloy steel will have leaner chemistry with respect to certain alloying elements and still have the above better property characteristics. These and other objects of the invention will be apparent from the following paragraphs. Brief Statement of the Invention : Thus according to this invention there is provided A process for producing high strength of YS: 750 MPa Min. and high toughness of 823 at 85°C min. steel products such as steel plates from lean composition alloy steel which comprises i. pre-fixing the selective various alloying components and their percentages for a steel composition pre-obtained by experiments and test results obtained thereof, such as to adjust the various alloying components to ensure that the amount of alloying element Cu does not exceed 0.90 weight %, that of Cr does not exceed 0.75 weight %, that of Ni does not exceed 1.00 weight %, that of Mo does not exceed 0.25 weight % and the amount of alloying element Nb does not exceed 0.03 weight %, such that the modified steel contains upto 2.93 weight % of the concerned alloying elements taken together. ii. preparing a normal steel alloy composition in the usual manner; iii. carrying out a chemical analysis of the steel composition prepared in step ii, iv. comparing with the pre-selected amounts and percentages of the various components as prefixed, v. making necessary additions and adjustments to the alloy steel prepared in step (ii) to obtain a modified alloy steel with the desired composition, vi. thereafter, casting the modified alloy steel into ingots in the usual manner, vii. soaking the ingots at 1200° C ± 20° C, viii. followed by hot-rolling the soaked ingots to the desired form of steel plates such that the final rolled plate has a chemical composition of lean alloy Cu Bearing HSLA steel in weight % as follow. In this process, the normal steel is made in an air induction furnace and the molten steel is cast into suitable size ingots preferably into 100 x 100 x 300 mm size ingots. Thereafter, the cast ingots are soaked in the temperature range of 1200° C ± 20° C for preferably 3 hours and the soaked ingots are subjected to hot rolling in several passes. The finish rolling temperature for such plates is maintained in the region of 8501° C-1000°C in the usual manner and subsequently, the finish rolled plates are subjected to a step of quenching and tempering at temperatures in the range of 50001 C to 700° C. In adjusting the various alloying components, care is taken to ensure that the amount of costly alloying element Cu does not exceed 0.90 weight %, that of Cr does not exceed 0.75 weight %, that of Ni does not exceed 1.00 weight %, that of Mo does not exceed 0.25 weight % and the amount of alloying element Nb does not exceed 0.03 weight %, such that the modified steel contains around 2.93 weight % of the concerned alloying elements taken together. Detailed Example of Manufacture: The invention will now be more fully explained with reference to the following example. Composition of End Products: The activities were oriented in developing a steel alloy having the chemical composition shown in Table 1 below :- Design of Steel Composition and Processing Details : In order to optimize the alloy composition of steel, 100 Kg. heats were made in air-induction furnace in laboratory of Research and Development Centre For Iron & Steel(RDCIS). The steels were cast into 100 x 100 x 300 mm size ingots. The cast ingots were soaked at 1200° C ± 20° C for 3 hours and hot rolled to 12-25 mm plate thickness in experimental rolling mill at RDCIS. Rolling was done in 5-7 passes and finish rolling temperature was maintained in between 850° C to 1000° C. After finish rolling, the plates were quenched and tempered in the range of 500° C to 700° C. Charpy and tensile test blocks of suitable dimensions - were cut from the heat-treated plates for evaluation of microstructure and mechanical properties. Based on the results, the steel composition was optimized to achieve the desired microstructure and targeted requisite mechanical properties of YS (750 Mpa, Min.) and Charpy impact toughness (82 J at -85° C, Min). The addition of alloying elements is made in this steel for achieving the desired combination of strength and toughness in heat-treated plates. Compared to conventional HSLA-100 steel where total alloy content is approximately 6.40 weight %, the present lean alloy composition of steel contains 2.93 weight % alloying elements. Besides, costly alloying elements such as Cu, Ni and Mo are significantly reduced in this lean alloy composition of steel to make it more cost effective. We have carried out necessary extension studies on the various property characteristics of the improved lean alloy composition Cu bearing HSLA steel and the following results were observed. Non-metallic inclusion Characteristics: The steel was very clean, inclusions were mostly thin sulphides, stringers of alumina and sulphide enveloped oxides as revealed by optical microscopy. Microstructure: The optical microstructure of the quenched and tempered steel plates, polished and etched in 2% natal solution, consisted of tempered martensite. The prior austenite grain size was found to be extremely fine. Transmission electron microscopy of the tempered steel revealed martensite laths. Fine globular precipitates of epsilon Cu could also be seen in the tempered steel. Tensile Properties: Tensile testing was carried out on round transverse-orientation specimens in accordance with ASTM A 370 specification and details of the results obtained are given below in Table 2. Shouldered tensile specimens of 25 mm gauge length and 6.25 mm gauge diameter were used. During testing, the cross head speed was maintained at 1 mm/min and a 258 mm extensometer was used for determining elongation percent. Table 2: Tensile Properties of the Quenched and Tempered Steel Plate: YS (0.2% Proof Stress) = 780-900 Mpa UTS = 850-980 Mpa %EI = 20-25 %RA = 65-75 Charpy Impact Toughness : The Charpy impact toughness of steel plates was evaluated using standard 55 x 10 x 10 (LXWXT) mm transverse specimens with 2 mm 'V notch in transverse through thickness (TT) direction as per ASTM E 23 standard. Tests were conducted at various temperatures viz., room temperature 25° C, -20° C, -60° C and -85° C and the results are provided in Table 3.Typical CVN energy versus temperature plot of the steel is depicted in Figure 1. Fractoqraphy The fracture surfaces of impact tested specimens were studied in a JEOL-make, JSM-840 model scanning electron microscope. A typical secondary electron image of a fractured charpy impact specimen was tested at -85° C at 3000X magnification. The fracture surfaces mostly revealed dimples indicating ductile mode of failure. This is in accordance with the good impact toughness obtained in this steel at this testing temperature. A typical non-metallic inclusion was found in the fractured specimen at 5000X magnification. The same inclusion was analyzed by Energy Dispersive Spectrometer (EDS) and found to be essentially a mixed inclusion of MnS, Al2O3 and SioO2 A EDS spectrum of the inclusion is depicted in Figure 2. As the atomic number of oxygen is lower (atomic No. 8) than the detection limit of the EDS (Minimum atomic No. 11), it is not indicated in the above spectrum. It is thus evident from the above study that despite leaner composition (with respect to Ni, Cu and Mo), the tensile strength and Charpy V Notch (CVN) energies of the proposed steel are comparable to that of HSLA-100 steel grade (minimum specified YS-692 Mpa and CGVN energy-81 Joules at -85° C) and HY-100 steel grade (minimum specified YS-692 Mpa and CVN energy-54 joules at -85° C). Advantages of the Invention: i. This steel despite having a leaner composition, has higher yield strength than similar HSLA grades (Hy-100, hsla-100 etc.) of steel. ii. The low temperature Charpy impact toughness of this lean alloy composition steel is comparable to that of the similar HSLA steel grades mentioned above. iii. The proposed lean alloy composition steel is easily weldable because of its lower carbon content (0.07 weight %) and carbon equivalent (0.58). This is confirmed from Graville's diagram shown in Figure 3. Moreover, the fabrication cost of the steel is lower as it requires no pre or post welding heat treatments. iv. High yield strength (750 MPa, Min), good low-temperature Charpy impact toughness (82 Joules at -85° C, Min.) indicates the suitability of this steel for construction of naval ships and submarines. It can also be used for making components of engineering bodies such as mining and dredging equipment, heavy-duty trucks, bridges, off-shore drilling platforms etc. Usefulness of the Innovation: Despite leaner composition, the proposed steel shows better strength, good impact toughness and weldability compared to existing HSLA grades (e.g. HY-100, HSLA-100 etc.) of steel. Thus this steel can be used as a more economic alternative to the above mentioned steel grades for making naval and engineering structures. Moreover, since the above mentioned steel grades are currently being imported into India, the present steel can be used for import substitution and save considerable foreign exchange. We claim: 1. A process for producing (high strength of YS: 750 MPa Min. and high) toughness of 823 at 85°C min. steel products such as steel plates from lean composition alloy steel which comprises i. pre-fixing the selective various alloying components and their percentages for a steel composition pre-obtained by experiments and test results obtained thereof, such as to adjust the various alloying components to ensure that the amount of alloying element Cu does not exceed 0.90 weight %, that of Cr does not exceed 0.75 weight %, that of Ni does not exceed 1.00 weight %, that of Mo does not exceed 0.25 weight % and the amount of alloying element Nb does not exceed 0.03 weight %, such that the modified steel contains upto 2.93 weight % of the concerned alloying elements taken together. ii. preparing a normal steel alloy composition in the usual manner; iii. carrying out a chemical analysis of the steel composition prepared in step ii, iv. comparing with the pre-selected amounts and percentages of the various components as prefixed, v. making necessary additions and adjustments to the alloy steel prepared in step (ii) to obtain a modified alloy steel with the desired composition, vi. thereafter, casting the modified alloy steel into ingots in the usual manner, vii. soaking the ingots at 1200° C ± 20° C, viii. followed by hot-rolling the soaked ingots to the desired form of steel plates such that the final rolled plate has a chemical composition of lean alloy Cu Bearing HSLA steel in weight % as follow. 2. A process as claimed in Claim 1, wherein, the normal steel is made in an air induction furnace. 3. A process as claimed in claims 1 and 2, wherein, the molten steel is cast into suitable size ingots preferably into 100 x 100 x 300 mm size ingots. 4. A process as claimed in claims 1 to 3, wherein, the cast ingots are soaked at 1200° C±20° C for preferably 3 hours. 5. A process as claimed in claim 4, wherein, the soaked ingot is subjected to hot rolling in several passes. 6. A process as claimed in claim 5, wherein, the hot-rolled plate is subjected to finish rolling at temperature in the region of 850° C to 1000° C. 7. A process as claimed in claim 6, wherein, the finish rolled plates are subjected to a step of quenching and tempering at temperatures in the range of 500° C to 700° C. 8. A process as claimed in claims 1 to 7, wherein, the amount of costly alloying element Cu does not exceed 0.90 weight %. 9. A process as claimed in claims 1 to 8, wherein, the amount of costly alloying element Cr does not exceed 0.75 weight %. 10. A process as claimed in claims 1 to 9, wherein, the amount of costly alloying element Ni does not exceed 1.00 weight %. 11. A process as claimed in claims 1 to 10, wherein, the amount of costly alloying element Mo does not exceed 0.25 weight % 12. . A process as claimed in claims 1 to 11, wherein, the amount of alloying element Nb does not exceed 0.03 weight % 13. A process as claimed in claims 1 to 12, wherein, the modified steel contains around 2.93 weight % of the concerned alloying elements taken together. 14. A process for preparing high strength and high toughness steel plates exhibiting strength of YS: 750 MPa Min. and toughness of 82J at 85° C min substantially as herein described with reference to the examples. 15. A high strength, high toughness rolled steel plate having strength of YS: 750 MPa min and toughness of 82J at 85° C Min. There is disclosed A process for producing high strength of YS: 750 MPa Min. and high toughness of 82J at 85°C min. steel products such as steel plates from lean composition alloy steel which comprises i. pre-fixing the selective various alloying components and their percentages for a steel composition pre-obtained by experiments and test results obtained thereof, such as to adjust the various alloying components to ensure that the amount of alloying element Cu does not exceed 0.90 weight %, that of Cr does not exceed 0.75 weight %, that of Ni does not exceed 1.00 weight %, that of Mo does not exceed 0.25 weight % and the amount of alloying element No does not exceed 0.03 weight %, such that the modified steel contains upto 2.93 weight % of the concerned alloying elements taken together. ii. preparing a normal steel alloy composition in the usual manner; iii. carrying out a chemical analysis of the steel composition prepared in step ii, iv. comparing with the pre-selected amounts and percentages of the various components as prefixed, v. making necessary additions and adjustments to the alloy steel prepared in step (ii) to obtain a modified alloy steel with the desired chemistry, vi. thereafter, casting the modified alloy steel into ingots in the usual manner, vii. soaking the ingots at 1200° C ±20° C, viii. followed by hot-rolling the soaked ingots to the desired form of steel plates such that the final rolled plate has a chemical composition of lean alloy Cu Bearing HSLA steel in weight % as follow.

Documents:

468-cal-2001-granted-abstract.pdf

468-cal-2001-granted-claims.pdf

468-cal-2001-granted-correspondence.pdf

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

468-cal-2001-granted-drawings.pdf

468-cal-2001-granted-examination report.pdf

468-cal-2001-granted-form 1.pdf

468-cal-2001-granted-form 13.pdf

468-cal-2001-granted-form 18.pdf

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

468-cal-2001-granted-form 3.pdf

468-cal-2001-granted-pa.pdf

468-cal-2001-granted-reply to examination report.pdf

468-cal-2001-granted-specification.pdf