Title of Invention

A PROCESS FOR PREPARATION OF HIGH STRENGTH STRUCTURAL STEEL

Abstract

A process for the preparation of high strength structural steel such as herein described, melting the mixture of 0.02-0.04 wt% of Nickel, 0.5-0.7% wt of Chromium, 0.35-0.45 wt% of Molybdenum, 1.0-1.2 wt% of Copper done at a vacuum level of 5.Omicrons then casting the melt into a preheated mould at around 1450-1475° C, cutting the defective portion of ingot, homogenisation the ingot by soaking for 2 hrs, and forging into a slab, controlled rolling at around 800 ° C, air cooling the ingot.

Full Text

This invention relates to a process for preparation of high strength structural steel. More specifically but without implying any limitation thereto the invention relates to an advanced controlled rolling process leading to high strength structural steel of improved toughness and weldability. Conventional high strength structural steels particularly refer to HY (High Yield) class of steels having yield strength of about 550 Mpa and sufficiently high impact toughness depending on its application. For example HY-80 steel exhibits toughness of more than 80 J at-18 degree Celsius. A good combination of strength and toughness makes the steel resistant to high pressures, impact loading or sub-ambient catastrophic brittle failures. Such high strength steel find applications in pressure vessels, ship-building, line pipe, earth moving equipment, cranes, bridges, road and rail tankers, wagons and other engineering and structural applications where loading is high and weight saving is critical. The mechanical properties of HY class of steel is achieved by heat treatment (e.g. quenching and tempering) of the steel and for this purpose the steel is designed to respond to heat treatment by keeping the carbon content and other allying elements sufficiently high. This imposes a limitation in the steel as due to its high carbon equivalent (carbon + other alloying elements) the steel becomes difficult to weld unless preheating is carried out. Development of newer grades of stronger and tougher High Strength Low Alloying (HSLA) steels involves not only the achievement of improved steel chemistry and manufacturing process but also various advancements in Thermo-Mechanical processing (TMP). The use of advanced processing techniques like controlled rolling (CR) and controlled cooling (CC) have been observed to be particularly suited to HSLA steels containing microallaying elements like Neabium (Mb), Titanium (Ti ) , Venadium (V) and Do ran ( B ) . The sole purpose of controlling the hot-deformation in •a controlled rolling process is to refine the final austenite grains., which, in turn, transform into fine grained ferritic and/or bainitic microstructure. This fine grained microstructure not only cause enhancement in strength but also improve toughness. Grain refining is the only strengthening mechanism which can cause improvement in both strength and toughness unlike strengthening mechanisms like solid solution strengthening, precipitation hardening,' dislocation hardening and the likes. In the known controlled rolling process of any microalloyed steel, the Slab Reheat Temperature (SRT) is chosen to be sufficiently high enough (1150 -1200 degree celsius) to bring the microalloyed carbides and carbonitrides in solid solution. Deformation of austenite in the recrysta 11isat ion region is carried out to obtain uniform distribution of fine equiaxed austenite grains by repeatedly breaking with minimum interpass time to minimise grain growth after static recrystal1isation. Deformation in any pass is carefully controlled to avoid either partial recrysta 11isat ion which causes mixed grain structure, or to avoid dynamic recrystalisation where there is no further grain .refining with increased deformation. The resulting » structure is required to have uniform distribution of fine nrained micros t r.yc:,turc? . Defarmatin of austenite? in the non-'"uc ryst al 1 i sat ion region is carried out to cause further breaking of austenite grains so as to result in further refinement of transformed ferritic microstructure due to additional nucleation sites from deformation bands. In this kind of deformation, aus'tenite gets elongated and pancaked * ' i containing 3 numbnr of deformation bands. Qccurence of nucleation at rlongated gamma-grain boundaries as well as at interiors (i.e. deformation bands) further causes a Cosiderable refinement in the transformed microstructure. The conventional processes known in the art have the drawback that there are chances of coarse grain/mixed grain structure with less control over uniform distribution of fine-equiaxed grain structure. This result in loss of ductility and also less impact toughness. This is particularly so in microalloyed steels where fine precipitates control the recrystallisation and grain growth behaviour. Another drawback, of the steel obtained by the process of the present invention is that it is very difficult to weld HY grade of steel obtained by these processes as it requires preheating for successful welding without any occurance of Hydrogen induced cracking (HIC). The primary and the main object of the present invention is to propose a process for the preparation of high strength structural steel. Another object of the present invention is to propose a process for the preparation of high strength structural steel with improved mechanical properties like improved impact toughness and enhanced strength. Still another object of the present invention is to propose a process for the preparation of high strength structural steel with enhanced low—temperaturi?. toughness at sub—zero temperatures of the order of minus (-) 40 degree celsius. Yet another object of the present invention is to propose a process for the preparation of high strength structural steel wherein both strength and ductility can be simultaneously enhanced unlike the conventional processes where one is achieved at the cost of other. A further object of the present invention is to propose a ^process for the preparation of high strength structural steel wherein the proposed process altogether dispenses off the elaborate heat-treatment processes like quenching/ sol ut ionising / tempering or ageing which are required in conventional processes, thereby making the process more cost-effective and energy-efI icieni . i A still further object of the present invention is to propose a process for the preparation of high strength structural steel which obviates the need for special facilities like "roll quenching to obtain desired package of strength, ductility and toughness and provides steel with most of these properties in "as rolled" condition. An- even further object of the present invention is to propose a process for the preparation of high strength structural steel which provides steel with low carbon content and low carbon equivalent (carbon and alloying elements) thereby making the steel easily weldable. A still further object of the present invention is to propose a process for preparation of high strength structural steel which 1 ea'ds to extremely fine ferrite grain size of about 3-4 microns as compared to grain size of about 18-20 microns produced by conventional processes. Yet further object of the present invention is to propose a process for preparation of^ high strength structural steels which involves addition of microalloying elements like boron and niobium and keeping the carbon "content much leaner which besides significantly improving weldability leads to cost-reduction. Yet further object of the present invention is to propose a process for the preparation of high strength structural steel which eliminates intermediate holding adopted in conventional processes to extend finish rolling pass in non-recrystallisation region of austenite. This intermediate holding is difficult to be controlled under shop floor conditions and may also result in static recrystallisation followed by grain growth thereby adversely affecting the microstructure refinement. The process of the present invention provides a new generation of high strength steel with enhanced mechanical propertion, . The advanced controlled rolling process proposed. under the present invention involves optimisation of various processing parameters which include slab t'cheat ing temperature, deformation in the gamma-recrysta 11isotion and non-recrystal1isation temperature regions, finish rolling temperature and subsequent post deformation cooling. The process involves addition of micros 11oving elements like boron and niobium and keeping the carbon content much lower. This, in addition to cost, reduction, improves weldability of steel to a significant f"'tent. Since number of welding operations are required to be performed during manufacture, improvement in welding is UK! significant feature of the steel obtained by the I • process. The combined addition of the allaying elements results in a fine microstrueture having lath morphology and availability of a large population of mobile dislocations in the as rolled condition itself. Thus, this steel exhibVta superior mechanical properties in the as rolled state as compared to the conventional steels which otherwise1 rnqui-re elaborate heat treatment. In this process both strength and ductility can be simultaneously enhanced lUniike the conventional processes, where one is achieved at the cost of other. A louier slab reheating temperature (SRT),in the range oi 10OO-1050 degree Celsius, improves the sub-zero impact tauqhnr'-sT. to a considerable extent as compared to higher slab reheating temperature (SRT) in the range of 1100-1150 degree cellus in the conventional process. This also enables to start the hot deformation process from the fine grained gamma region. This not only results in finer final microstrueture but also leads to improved low temperature toughness. The deformation in each pass is kept -sufficient enough to avoid partial recrystal1isation. This low slab-reheating temperature alongwith repeated deformation (16-17V. per pass) uiith minimum inter pass time (5-6 seconds) makes it passible to extend the finish rolling down into the nan-recryst al 1. isat ion region but above Austenite to Ferrite transformation (Ar 3) temperature without any intermediate holding. Rolling in two-phase region is carefully avoided, as this deteriorates the low temperature toughness of this grade of steel, through the occurence of splitting. This requires a heavy deformaiton (20-30%) preferably 22-25% in the finishing pass when the finish rolling temperature (FRT) is above Ar3. Conventional high strength steels used for critical ship building application envolve elaborate heat tratment processes such as quenching, solutionising tempering or ageing. Special facility such as "roll quenching" is required to obtain a desired package of strength, ductility and toughness. Whereas, most of these properties are obtained in the "as rolled" condition in the current steel prepared by process of the present invention through control of the rolling paramters. The conventional steel making is more time consuming and is less energy-efficient as it involve the use of higher Slab Reheating Temperature (SRT) ('1150-1200 dgree ceslius) and complicated post deformation heat treatment schedules. Since the steel is reheated at 1150-1200 degree Celsius, the benefit of rolling in the nan rcrystal1ised region of austenite cannot be fully exploited. This is because, by the time last rolling pass is carrid out, the finish rolling temperature may not necessarily come down to the non-recrystal1isat ion region. Attempts often made to extend the finish rolling pass in non-recrystallisation region of austenite, by suitable intermediate holding, may not result in improvement in i mechanical properties. This is because the process of intermediate holding is difficult to be controlled under shop floor conditions. Moreover, the intermediate holding may also result in static recrystal1isat ion followed by grain growth and thereby hindering microstructural refinement. On the other hand SRT for the current rolling schedule being lower at 1000-1050 degree celsius, maximum benefit of rolling in the austenite non-recrystal1isation range is ensured. This results in obtaining desired combirlation of properties in the "as rolled" condition itsel f . According to this invention there is provided a process for the preparation of high strength structural steel comprising of the steps of: a) melting and casting; b) hot top cutting; c) homogenisation and forging; d) controlled rolling; e) air cooling, characterised in that the step of melting is done at a vacuum level better than 5.0 microns, then casting the melt into a preheated mould at around 1450-1457°C, then cutting the defective portion of ingot then homogensing the ingot by soaking for about 2 hrs and then forging into a slab, then controlled rolled at around 800°C and then the ingot is air cooled. Further in accordance with the present invention acid picked armco iron is taken in a crucible and to which are added micro-alloying elements nickel 0.02-0.04 wt% preferably 0.03 wt%, Chromium 0.5-0.7 wt% preferably 0.6 wt% Molybdenum 0.35-0.45 wt% preferably 0.4wt% and copper 1.0-1.2 wt% preferably 1.1 wt%. The mixture is then subjected to vacuum induction melting at a vacuum level better than 5.0 microns. The crucible prior to melting is properly dried to drive out any entrapped moisture. Carbon 0,02-0.04 wt7. preferably 0.03 wt0/0 Manganese 1.4 - 1.6 wt% preferably 1.5 wt0/0, Silicon 0.25 - 0.35 wt7. preferably 0.3 wt7., aluminium 0.02 - 0.04 wt0/0 preferably 0.03 wt%, Titanium 0.02 - 0.03 wt0/0 preferably 0.02 wt0/0, Niobium 0.03 - 0.05 wt0/0. preferably 0.04 wt% and Boron 0.015 - 0.02 wt7. preferably 0.002 wt% are added to the melt. The melt is then tapped under vacuum at around 1450-1475 degree Celsius into a pre-heated cyclindrical mold positioned beneath the crucible. Radiography of the cast ingot is carried out to observe the presence of piping and other defects. The portion of the ingot having defects is cut off so as to obtain the remaining sound ingot for further processing. The sound ingot obtained is soaked at 1100 — 1150 degree celsius for about 2 hours for dehydrogenation and chemical homogenisation and then forging into a slab. The slab is reheated to a temperature of 1000—1050 degree celsius for about 2 hours. The first pass temperature is then kept in the range of 1000-980 degree celsius, the slab is then rough rolled to 20-25% deformation upto recrystallisation stop temperature of the order of 950—970 degree Celsius. The ingot is then subjected to repeated deformation of about 16-17'/. per pass in the austenite recrystallisation region to avoid partial recrystallisation, keeping the interpass time between two successive passes as minimum to the order of 5—6 seconds. The ingot is further subjected to 75-807. deformation in the austenite nan-recrystallisation region. The ingot is subjected to deformation of 20-307. preferably 22-257. in the finish rolling pass when the Finish Rolling Temperature (FRT) is in the range of 800-780 degree Celsius. The ingot is then subjected to the step of aircooling. Comparative features of conventional controlled rolling process as compared to the proposed advanced controlled process are tabulated below: Comparative Features of Conventional Vs Proposed Rol1 ing Process Slab Reheating Temperature (SRT) (degree Celsius) Finish Rolling Temperature(FRT) (degree Celsius) Intermediate Holding Conventional 1100-1150 Control led Rol1 ing Process 800 Yes Advanced Control led Rolling Process 1000-1-50 800 No A schematic of the new advanced controlled rolling schedules proposed in the present process vis-a-vis the conventional rolling schedules, is shown in Fig. 1. Far molt size of 100 Kg, the approximate quantities (in I.IIMS ) o'l thp different constituents to be taken are: Carbon (C)-50g, Ferro-Manganese (Fe-Mn ) -2000g , Si 1 icon (S1 )-300g , Nicke 1 (Ni)-1200g , Copper (Cu)-l lOOg , Chromium (Cr)-600g , Molyodenumnum (Mo ) -400g , Niobium (Nb ) -4Qg , Alum in lum ( Al ) -30g , T i t an i urn ( T i ) -20g , Ferro-Baron (Fe-B)-8g and RestArmco Iron. Out of these constituents, iron nickel, chromium, molybedenum and copper are first taken in a dried crucible. The mixture is subjected to vacuum level of 5.0 microns. Thereafter remaining constituents carbon, Fe-Mn , Silicon, Alununimum, titanium, niobium and Fe-B are added from the top, int.o tho crucible? in the given sequential order. Immediately after this, when the melt temperature was about 1-16C decree1 Celsius, the melt was tapped under vacuum into a nre-heat^d cylindrical mold positioned beneath the crucible. The radiography of the cast ingot was carried out to observe the presence of piping and other defects. The portion of the having offsets was then cut off to obtain a sound ingot for- further processing. The sound ingot was then soaked at 1130 degree Celsius for about 2 hours for dehydrdgenat ion , chemical homoggn isat ion and was then forged into a slab. Thy slab K-JS reheated to 1020 cegree celsius for 2 hours and then control ••• rolled. Dur.ng the first pass, the temperature was kppt about 1000 degree Celsius. The slab ui-Ts subjectid to rough railing of about 20'/. deformation uptrecystallisation stop temperature of the order of 950 degree celsius. The slab was subject.ed to sufficient deformation of about 16-17'/. per piss in the austenite r ec rys .; a 1 1 1 sa t ion reg ion to avoid partial recryst al 1 isat ion , keeping the interpass time of 5-6 seconds between two successive passes. About 80V. deformation was carried out in the austenite non-recryst a 1 1 isat ion region and about 22-25'X. deformation was carried qut in the finish rolling pass. The finish rolling temperature was kept around 800 degree cf?l The table below illustrates the improvement . in mechanical properties of the steel obtained by the proposed process using advanced controlled rolling schedules as t * compared to the known processes using conventional rolling schedu1es : TABLE:- Comparative features of the steel obtained by conventional and by the proposed process (Table Removed) It is to be understood that the proposed process is subject to" modifications, adaptations and changes by the persons skilled in the art. Such modifications are intended to be covered within the scope of the ps-esent invention which is set forth by the following claims:- CLAIM: 1. A process for the preparation of high strength structural steel such as herein described. (a) Melting the mixture of 0.02-0.04 wt% of Nickel, 0.5-0.7% wt of Chromium, 0.35-0.45 wt% of Molybdonum, 1.0-1.2 wt% of Copper done at a vacuum level of 5.Omicrons then casting the melt into a preheated mould at around 1450-1475° C; (b) Cutting the defective portion of ingot; (c) homogenisation the ingot by soaking for 2 hrs, and forging into a slab; (d) controlled rolling at around 800 ° C; (e) air cooling the ingot, 2. A process as claimed in claim 1 wherein said mixture preferably comprises 0.03 wt% of Nickel, 0.6 wt% of Chromium, 0.4 wt% of Molybdonum, 1.1 wt% of Copper are mixed. 3. A process as claimed in claim 1 wherein the sound ingot is soaked at a temperature of 1100-1150°C for dehydrogenation and chemical homogenisation and then forging into a slab. 4. A process as claimed in claim 3 wherein said slab is reheated to a temperature of 1000-1050°C. A process for preparation of high strength structural steel substantially as herein described.

Documents:

1561-del-1997-abstract.pdf

1561-del-1997-claims.pdf

1561-del-1997-correspondence-others.pdf

1561-del-1997-correspondence-po.pdf

1561-DEL-1997-description (complate).pdf

1561-del-1997-drawings.pdf

1561-del-1997-form-1.pdf

1561-del-1997-form-19.pdf

1561-del-1997-form-2.pdf

1561-del-1997-form-26.pdf

1561-del-1997-form-3.pdf