CLIAMS:We Claim:
1. High tensile strength steel plates of a steel composition comprising

C: 0.20 to 0.30 wt%,
Mn: 1.20 to 1.60 wt%,
Si: 1.20 to 1.60 wt%,
Cr. 0.10 to 0.25 wt%,
Ni: 0.15 to 0.45 wt%,
Mo: 0.15 to 0.30 wt%,
P: upto 0.015 wt%,
S: upto 0.010 wt%,
H: upto 2 ppm , and
the balance is Fe and having a minimum tensile strength of 1500 MPa.

2. High tensile strength steel plates as claimed in claim 1 having properties in quenched and tempered state comprising
Yield Strength (YS): 1310 to 1450 MPa;
Ultimate Tensile strength (UTS) of 1507 to 1650MPa;
Elongation (EL) at failure 8.3 to 9.4 %;
Reduction in area (RA) at failure of 25 to 30 %;
Hardness of 470 to 488 BHN.

3. High tensile strength steel plates as claimed in anyone of claims 1 or 2 having microstructure in quenched and tempered state comprising about 95% tempered martensite and some amount of bainite.

4. High tensile strength steel plates as claimed in anyone of claims 1 to 3 which is suitable for fabricating components of armoured vehicles, firing range baffles, bank ATM panels, mining equipment and other high tensile applications.

5. A process for the production of high strength steel plates having a minimum tensile strength of 1500 MPa comprising the steps of:
(i) making of C-Mn-Si steel with microalloying through EAF-VAD route, having involving a selective composition (in weight %) within the range of C:0.20-0.30, Mn:1.20-1.60, Si:1.20-1.60, Cr.0.10-0.25, Ni:0.15-0.45, Mo:0.15-0.30, P:0.015max, S:0.010max and H:2ppm max, with balance of Fe.
(ii) continuous casting of slabs through Continuous Casting Machine (CCM); and
(iii) hot rolling of CCM slabs to hot rolled (HR) coils and shearing the same to required size of plates.

6. A process as claimed in claim 5 comprising the steps of
(i) providing said C-Mn-Si steel produced through electric arc furnace and vacuum treated in VAD unit involving said selective composition comprising
C: 0.20 to 0.30 wt%,
Mn: 1.20 to 1.60 wt%,
Si: 1.20 to 1.60 wt%,
Cr. 0.10 to 0.25 wt%,
Ni: 0.15 to 0.45 wt%,
Mo: 0.15 to 0.30 wt%,
P: upto 0.015 wt%,
S: upto 0.010 wt%,
H: upto 2 ppm , and
the balance is Fe.
(ii) Continuous casting of slab from above steel grade into 170 x 1160 mm slabs of desired length;
(iii) Cooling the slabs with slow rate, using hood cooling system;
(iv) Reheating of slabs in reheating furnace;
(v) Hot rolling of reheated slabs to 5 mm thick gauge in coil form;
(vi) Shearing hot rolled coils of width 1135 mm, on cooling, into 4600 mm long sheets/plates for subsequent processing;
(vii) Examining as-hot-rolled plates ultrasonically to identify internal/surface defect, if any, before carrying out hardening and tempering operations;
(viii) heating the plates to the specified temperature and time in the hardening furnace and quenching in a well circulated oil bath;
(ix) carrying out tempering of hardened plates by heating in the tempering furnace, wherein uniformity of temperature is maintained to its greater extent and cooling to ambient temperature.

7. A process as claimed in anyone of claims 5 or 6 wherein said continuous slab casting comprises a casting speed of 0.75 meter/minute, mould oscillations of 120/ minute and use of automatic mould level control, said slow cooling of slabs comprising a time period of >50 hours, said reheating of slab comprising 4½ hours soaking at temperature between 1290-1300°C, said hot rolling of slabs to sheet and coiling comprising rolling within the temperature range of 1047-1056°C, finishing between 910-915°C and coiling at 810-812°C, said hardening involving heating and quenching of plates comprising 920?10?C heating for 25 minutes followed by quenching in oil bath, respectively.

8. A process as claimed in anyone of claims 5 to 7 wherein said tempering of hardened plates comprising heating at 240?C for 90 minutes.

9. A process as claimed in anyone of claims 5 to 8 wherein said quenching and tempering is carried out involving microstructural modifications such that the microstructure of quenched and tempered steel plates comprises about 95% tempered martensite and some amount of bainite.

Dated this the 28th day of February, 2014
Anjan Sen
Of Anjan Sen & Associates
(Applicants Agent)

,TagSPECI:FIELD OF THE INVENTION

The present invention is related to high tensile strength steel plates and a cost effective process of manufacturing such steel plates suitable for use in very high tensile applications. More particularly, the present invention is directed to providing quenched and tempered steel plates with minimum tensile strength of 1500 MPa. The plates having high tensile strength are produced according to the present invention using C-Mn-Si chemistry with microalloying, through EAF-VAD-CC route, coupled with microstructural modifications attained through thermomechanical controlled processing and heat treatment involving hardening, quenching and tempering. The defect free plates having a minimum tensile strength of 1500 MPa may suitably be utilized for fabricating components of armoured vehicles, firing range baffles, bank ATM panels, mining equipment and other high tensile applications.

BACKGROUND OF THE INVENTION

The steels used for general purpose applications are processed and produced through the 'carbon-manganese chemistry' route. This is based on restricting carbon to a low level and alloying with manganese. However, based on the strength requirements, relatively higher amounts of carbon and manganese contents may be selected for producing steels for high tensile applications. Medium carbon steels with carbon in the range of 0.25-0.55% have wide ranging engineering applications because mechanical properties in these steels can be varied by varying the carbon content. Their higher carbon contents make them amenable to processing through the quench and temper heat treatment. For special applications such as high temperature etc. alloying with elements such as Ni, Cr, Mo, etc is employed.

In existing steel grades, achieving the tensile strength was limited to about 1200 MPa and the major problems faced in developing such steel grades of higher tensile strength were due to the reasons of achieving higher hardness levels through grain refinement, hardenability control, etc. There has been thus a need in the field of steel making to developing a cost effective composition and process for producing high tensile strength plates/sheets suitable for a variety of engineering applications.

The present invention thus attempts to solve the problem of the prior art by adopting suitable manufacturing technology at different stages viz– steel making, casting, rolling and heat treatment. In order to have a low cost steel product with very high tensile strength, the present invention explore the possibility of developing steel plates that are manufactured using C-Mn-Si chemistry with higher levels of Si (1.20-1.60wt%) as compared to the prior available grades along with microalloying. Silicon is the cheapest alloying element as it is inherently present from the steel melting stage when used as a deoxidiser. In larger amounts, it increases resistance to scaling at elevated temperatures and also raises 500?F embrittlement range for ultra high strength applications. It also introduces processing difficulties due to its adverse effect on machinability and increases susceptibility to decarburisation. By adopting the proposed technology these impediments are eliminated by suitably designed processing steps according to the present invention.

In order to obtain higher level of strength properties in steels, especially tensile strength in excess of 1500 MPa, suitably designed heat treatment schedules are also important as they allow various alloying elements to contribute through solid solution strengthening and/or precipitation hardening effects. As far as the usability of quenched steels with martensite as the microconstituent is concerned they are not suitable for structural applications unless tempered within specified range of temperature and time. Tempering therefore is necessary as it eliminates stresses generated due to athermal transformation.

OBJECTS OF THE INVENTION

The basic object of the present invention is directed to providing high tensile strength steel plates and a cost effective process of manufacturing such steel plates with minimum tensile strength of 1500 MPa, suitable for use in very high tensile applications.

A further object of the present invention is directed to providing high tensile strength steel plates with minimum tensile strength of 1500 MPa using C-Mn-Si chemistry with microalloying, coupled with microstructural modifications attained through thermomechanical controlled processing and heat treatment comprising hardening, quenching and tempering.

A still further object of the present invention is directed to providing high tensile strength steel plates with minimum tensile strength of 1500 MPa and a process for its production, wherein carbon and manganese are so adjusted that optimum benefits accrue in terms of properties.

A still further object of the present invention is directed to providing high tensile strength steel plates with minimum tensile strength of 1500 MPa and a process for its production, wherein microalloying with Ni, Cr, Mo is done to achieve maximum benefits from a typical low carbon steel microstructure in terms of high strength without sacrificing good ductility and toughness.

A still further object of the present invention is directed to providing high tensile strength steel plates with minimum tensile strength of 1500 MPa and a process for its production, wherein suitably designed heat treatment schedules are adopted to allow various alloying elements to contribute through solid solution strengthening and/or precipitation hardening effects.

A still further object of the present invention is directed to providing high tensile strength steel plates with minimum tensile strength of 1500 MPa and a process for its production, wherein quenched and tempered steel plates produced would have smooth surface finish free from defects such as holes, scabs, rolled-in scale, scratch marks, etc.

SUMMARY OF THE INVENTION

The basic aspect of the present invention is thus directed to high tensile strength steel plates of a steel composition comprising

C: 0.20 to 0.30 wt%,
Mn: 1.20 to 1.60 wt%,
Si: 1.20 to 1.60 wt%,
Cr. 0.10 to 0.25 wt%,
Ni: 0.15 to 0.45 wt%,
Mo: 0.15 to 0.30 wt%,
P: upto 0.015 wt%,
S: upto 0.010 wt%,
H: upto 2 ppm , and
the balance is Fe and having a minimum tensile strength of 1500 MPa.

A further aspect of the present invention is directed to said high tensile strength steel plates having properties in quenched and tempered state comprising
Yield Strength (YS): 1310 to 1450 MPa;
Ultimate Tensile strength (UTS) of 1507 to 1650MPa;
Elongation (EL) at fracture 8.3 to 9.4 %;
Reduction in area (RA) at fracture of 25 to 30 %;
Hardness of 470 to 488 BHN.

A still further aspect of the present invention is directed to said high tensile strength steel plates having microstructure in quenched and tempered state comprising about 95% tempered martensite and some amount of bainite.

A still further aspect of the present invention is directed to said high tensile strength steel plates which is suitable for fabricating components of armoured vehicles, firing range baffles, bank ATM panels, mining equipment and other high tensile applications.

Yet another aspect of the present invention is directed to a process for the production of high strength steel plates having a minimum tensile strength of 1500 MPa comprising the steps of:
(i) making of C-Mn-Si steel with microalloying through EAF-VAD route, having involving a selective composition (in weight %) within the range of C:0.20-0.30, Mn:1.20-1.60, Si:1.20-1.60, Cr.0.10-0.25, Ni:0.15-0.45, Mo:0.15-0.30, P:0.015max, S:0.010max and H:2ppm max, with balance of Fe and obtaining C-Mn-Si steel ;
(ii) continuous casting of slabs through Continuous Casting Machine (CCM); and
(iii) hot rolling of CCM slabs to hot rolled (HR) coils and shearing the same to required size of plates.

A further aspect of the present invention is directed to said process comprising the steps of
(i) providing said C-Mn-Si steel produced through electric arc furnace and vacuum treated in VAD unit involving said selective composition comprising
C: 0.20 to 0.30 wt%,
Mn: 1.20 to 1.60 wt%,
Si: 1.20 to 1.60 wt%,
Cr. 0.10 to 0.25 wt%,
Ni: 0.15 to 0.45 wt%,
Mo: 0.15 to 0.30 wt%,
P: upto 0.015 wt%,
S: upto 0.010 wt%,
H: upto 2 ppm , and
the balance is Fe.
(ii) Continuous casting of slab from above steel grade into 170 x 1160 mm slabs of desired length;
(iii) Cooling the slabs with slow rate, using hood cooling system;
(iv) Reheating of slabs in reheating furnace;
(v) Hot rolling of reheated slabs to 5 mm thick gauge in coil form;
(vi) Shearing hot rolled coils of width 1135 mm, on cooling, into 4600 mm long sheets/plates for subsequent processing;
(vii) Examining as-hot-rolled plates ultrasonically to identify internal/surface defect, if any, before carrying out hardening and tempering operations;
(viii) heating the plates to the specified temperature and time in the hardening furnace and quenching in a well circulated oil bath;
(ix) carrying out tempering of hardened plates by heating in the tempering furnace, wherein uniformity of temperature is maintained to its greater extent and cooling to ambient temperature.

A still further aspect of the present invention is directed to said process wherein said continuous slab casting comprises a casting speed of 0.75 meter/minute, mould oscillations of 120/ minute and use of automatic mould level control, said slow cooling of slabs comprising a time period of >50 hours, said reheating of slab comprising 4½ hours soaking at temperature between 1290-1300°C, said hot rolling of slabs to sheet and coiling comprising rolling within the temperature range of 1047-1056°C, finishing between 910-915°C and coiling at 810-812°C, said hardening involving heating and quenching of plates comprising 920?10?C heating for 25 minutes followed by quenching in oil bath, respectively.
A still further aspect of the present invention is directed to said process wherein said tempering of hardened plates comprising heating at 240?C for 90 minutes.

A still further aspect of the present invention is directed to said process wherein said quenching and tempering is carried out involving heat treatment such that the microstructure of quenched and tempered steel plates comprise about 95% tempered martensite and some amount of bainite.

The objects and advantages of the present invention are described hereunder in greater details with reference to accompanying example.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to providing high tensile strength steel plates with minimum tensile strength of 1500 MPa using C-Mn-Si chemistry with microalloying, coupled with microstructural modifications attained through thermomechanical controlled processing and heat treatment comprising hardening, quenching and tempering.

Strengthening in steels is produced through the combined effect of alloying elements such as C, Mn, Si, Ni, Cr, Mo, etc. The effects are further enhanced by microstructural modifications attained via heat treatment processes such as controlled cooling and/or quenching from temperatures above Tnr. The alloying elements affect mechanical properties through solid solution strengthening and precipitation hardening, while heat treatment produces harder microconstituents like bainite and martensite in the steel. In case of martensitic transformation the microstructure is not useful for structural applications due to presence of internal stresses. Therefore, it is tempered for relieving the stresses.

The present invention thus provides a cost effective process for producing high tensile strength steel plates by adopting suitable manufacturing technology at different stages – steel making, casting, rolling and heat treatment. The steel plates are manufactured using C-Mn-Si chemistry with microalloying. The C-Mn-Si steel comprises higher levels of Si (1.20-1.60 wt %) as compared to the prior available grades. Silicon is the cheapest alloying element as it is inherently present from the steel melting stage when used as a deoxidiser. It is not a carbide forming element, but enters into solid solution in ferrite thereby contributing to strengthening/ hardening of steels. It increases the strength of ferrite without loss of ductility. In larger amounts, it increases resistance to scaling at elevated temperatures and also raises 500?F embrittlement range for ultra high strength applications. It also introduces processing difficulties due to its adverse effect on machinability and increases susceptibility to decarburisation. By adopting the proposed technology these impediments are eliminated by suitably designed processing steps. Silicon, in combination with other alloying elements, therefore could help in achieving strengthening in steels plates >1500 MPa by controlling: cleanliness in liquid steel, continuous casting parameters, use of TMCP during rolling, quenching and tempering parameters during heat treatment, etc.

Based on the strength requirements, relatively higher amounts of carbon and manganese contents may be selected for producing steels for high tensile applications. Medium carbon steels with carbon in the range of 0.25-0.55% have wide ranging engineering applications because mechanical properties in these steels can be varied by varying the carbon content. Their higher carbon contents make them amenable to processing through the quench and temper heat treatment. For special applications such as high temperature etc. alloying with elements such as Ni, Cr, Mo, etc is employed.

Carbon addition is by far the most effective and economical way of increasing the strength of steels. Although strength could be significantly increased, the toughness decreases monotonically. From a fracture mechanical viewpoint, increasing the strength without a corresponding improvement in toughness (or its remaining unaltered) is tantamount to not utilizing the improved load bearing capacity. Keeping this in mind, a compromise is generally arrived at between the strength and the toughness to be attained. There are many engineering applications where toughness is less important than the strength.

Manganese is added in the range of 0.20 to 1.60 % for increasing strength at a minimum cost. In plain carbon steels it affects the kinetics of austenite to ferrite/pearlite transformation by reducing diffusion rate which in turn retards the transformation of austenite. Manganese additions may influence properties either through solid solution strengthening, grain size refinement and/or due to an increase in proportion of pearlite. Manganese strengthens the steel by refining the grains and by increasing the volume fraction of pearlite based on its austenite stabilising effect with relatively smaller contribution through solution hardening. Due to the carbon content being higher, another possibility could be the formation of a large volume fraction of twinned sub-structure in martensite, especially in thinner sections or on quenching. Equally significantly, as the martensite start temperature (Ms) is lowered, it suppresses auto-tempering and may lead to an increase in the incidence of quench cracking. Thus precaution is exercised while adding Mn.

In view of the above effects carbon and manganese are so adjusted that optimum benefits accrue in terms of properties. In more than 50% of the commercially produced steels, different combinations of carbon and manganese are used to arrive at the requirements of a specific end application.

Silicon is another element which contributes to strengthening in carbon steels. This element is inherently present from the steel melting stage as it is used as a deoxidiser. Silicon is not a carbide forming element, but enters into solution in ferrite thereby hardening it. It increases the strength of ferrite without loss of ductility when added in amounts up to 2.50 wt.%. In large quantities, however, it introduces processing difficulties due to its adverse effect on machinability and increases susceptibility to decarburisation. Silicon, in general, increases strength of low alloy structural steels when present in amounts up to 0.35 wt.%. In larger amounts, it increases resistance to scaling at elevated temperatures and also raises 500?F (290?C) embrittlement range/limit for ultra high strength applications. Thus, silicon serves as a useful and cost effective element if added in controlled amounts.

Ni, Cr and Mo produce strengthening through solid solution strengthening and precipitation hardening at elevated temperatures. In case the steel is liable to be used for bullet proof applications, high temperature strength is highly desirable. The alloying elements therefore, have been included in the specification. As it is well accepted, alloy steels have an advantage over plain carbon structural steels since the former exhibit a better combination of ductility, strength, toughness and weldability. They are also useful in other application areas such as corrosion resistance, heat resistance, wear resistance and where good toughness at low temperatures is required. While it is customary to think in terms of conventional alloying, a specific category of elements classified as microalloying is used to obtain customized strength properties in steels. The main advantage associated with their use is that they are invariably added in amounts varying from 0.10-0.50 % and when added to a low carbon steel base offer the best possibility of extending substantially the level of strength that can be achieved without sacrificing toughness. Thus in a way, maximum benefits are being extracted from a typical low carbon steel microstructure in terms of strength without sacrificing its main attribute, namely good ductility and toughness. The mechanism through which improvement in properties are induced through microalloying are grain refinement and precipitation hardening.

In order to obtain higher level of strength properties in steels, especially tensile strength in excess of 1500 MPa, suitably designed heat treatment schedules are also important as they allow various alloying elements to contribute through solid solution strengthening and/or precipitation hardening effects. When steels are rapidly cooled from austenitising temperatures to room temperature they undergo martensitic phase transformation which provides high tensile strength and hardness to steels. The austenite to martensite transformation in steels is of great importance because through it the engineering materials are hardened. The transformation in a majority of instances is athermal in nature and takes place within a fraction of a second.

As far as the usability of quenched steels with martensite as the microconstituent is concerned they are not suitable for structural applications unless tempered within specified range of temperature and time. Tempering therefore is necessary as it eliminates stresses generated due to athermal transformation.

Based on the above consideration, a selective composition of steel was developed through experimental trial as illustrated in the following example.

EXAMPLE:
In order to obtain chemical composition within the range as specified in following Table-1, the steel was made in 50 ton electric arc furnace and vacuum treated in VAD unit.

Table-1 Range of chemistry (weight %)
C Mn Si Mo Cr Ni S P H
0.20-0.30 1.20-1.60 1.20-1.60 0.15-0.30 0.10- 0.25 0.15- 0.45 0.010 Max 0.015 Max 2 ppm Max

During steel making clean and sized scrap was charged into the furnace. In the pit side for taping the heat, skull and slag free preheated ladle with slag line life of 1-2 heats was used. On maintaining a tap time of about 14 minutes the steel was poured into the ladle. About 80 kg of Al, 200-250 kg of Silico-Manganese and required amount of pet coke were added to the 50 ton steel capacity ladle. Based on the mean chemistry, carbon equivalent (CE) of 0.1282 and ferrite potential of 0.9295 were calculated, which are useful in deciding the continuous casting (CC) parameters mentioned at the later stage. After taping the steel the ladle was brought to VAD unit, wherein a VAD temperature of 1559?C and degassing time of 45 minutes were maintained. Predetermined amounts of various ferroalloys (Fe-Mn: 90kg, Fe-Cr: 65kg, Fe-Si: 950kg, Si-Mn: 20kg, Ni: 100kg, Al: 20kg and pet coke: 35kg) were added into the ladle during treatment.

After completion of VAD treatment and achieving chemical composition as shown in Table-2 the vacuum was released and ladle was taken to continuous casting shop (CCS) at about 1590?C temperature.

Table-2 Chemical composition of experimental steel (wt.%)
C Mn Si Cr Ni Mo S P Al H
0.24 1.32 1.32 0.125 0.21 0.16 0.008 0.014 0.014 1.72 ppm

The steel was then continuous cast with a casting speed of 0.75 meter/minute and mould oscillations of 120/ minute into 170 x 1160 mm slab of total length 32.6 meters, using automatic mould level control in the tundish. The cast slab was cut in hot condition into 4 pieces having length about 8 meters each. All the slabs were cooled with slow rate (> 50 hours), using hood cooling system.

Prior to rolling, the slabs were charged to the reheating furnace and allowed to soak for about 4½ hours at temperature between 1290-1300°C for homogeneity and dissolution of alloy compounds in the solid solution. The material was rolled within the temperature range of 1047-1056°C, finished between 910-915°C and coiled at 810-812°C. As per the requirement of the application, slabs were rolled to 5 mm thick and 1135 mm wide sheets in coil form. The coils after cooling to ambient temperature were sheared to 4600 mm long plates for further processing at the heat treatment shop.

The as-hot-rolled plates were examined ultrasonically to identify internal/surface defect, if any, before carrying out hardening and tempering operations.

After heating to the specified temperature of 920?10?C for 25 minutes in the hardening furnace the plates were quenched in a well circulated oil bath to ambient temperature. Subsequently, tempering was performed by heating in the tempering furnace at 240?C for 90 minutes, wherein uniformity of temperature is maintained to its greater extent. On cooling to ambient temperature the specimens from representative plates were subjected to hardness, tensile test and microstructural evaluation. The results are presented in Table-3 as follows.

Table-3 Range of hardness and tensile properties of quenched and tempered steel plates
Hardness
(BHN) Yield Strength
(MPa) Tensile Strength
(MPa) Elongation at Fracture
(%) Reduction in Area at Fracture
(%)
470-488 1310-1450 1507-1650 8.3–9.4 25-30

The microstructure of plates as examined under optical microscope comprised about 95 % tempered martensite with some amount of bainite. In addition to the strength properties as shown above, the steel plates were observed to have smooth surface finish free from defects such as holes, scabs, rolled-in scale, scratch marks, etc.

The salient aspects of the present invention that is derived from the above experimental trial are as follows:

(i) The C-Mn-Si steel comprising various elements within the following range could be made through EAF-VAD-CC route.
C:0.20-0.30, Mn:1.20-1.60, Si:1.20-1.60, Cr.0.10-0.25, Ni:0.15-0.45, Mo:0.15-0.30, P:0.015 max, S:0.010 max and H:2 ppm max, with balance of Fe and inevitable impurities.
(ii) Continuous cast slabs of size 170mm x 1160mm x 8000mm could be produced with no surface defects and internal cracks in the material.
(iii) The CC slabs were hot rolled to 5 mm thick sheets/plates having adequately smooth surface finish and free from defects.
(iv) The 5 mm thick plates having 1135 mm width and 4600 mm length were hardened, oil quenched and tempered to obtain desired strength properties.
(v) On quenching and subsequent tempering, the plates attained strength properties in the following range :
Hardness 470-488 BHN, yield strength 1310–1450 MPa, tensile strength 1507-1650 MPa, elongation at failure 8.3–9.4 % and reduction in area 25-30 %.
(vi) The quenched and tempered steel plates were observed to have smooth surface finish free from defects such as holes, scabs, rolled-in scale, scratch marks, etc.
It is thus possible by way of the present invention based on the above trials and properties evaluation of product, to commercially produce high strength steel plates having minimum tensile strength of 1500 MPa using C-Mn-Si chemistry with microalloying through EAF-VAD-CC route, followed by hardening and tempering of the hot rolled sheets using selective parameters. The steel with such strength levels is amenable to be used advantageously for fabricating components of armoured vehicles, firing range baffles, bank ATM panels, mining equipment and other high tensile applications. In this regard we claim as follows.

We Claim:
1. High tensile strength steel plates of a steel composition comprising

C: 0.20 to 0.30 wt%,
Mn: 1.20 to 1.60 wt%,
Si: 1.20 to 1.60 wt%,
Cr. 0.10 to 0.25 wt%,
Ni: 0.15 to 0.45 wt%,
Mo: 0.15 to 0.30 wt%,
P: upto 0.015 wt%,
S: upto 0.010 wt%,
H: upto 2 ppm , and
the balance is Fe and having a minimum tensile strength of 1500 MPa.

2. High tensile strength steel plates as claimed in claim 1 having properties in quenched and tempered state comprising
Yield Strength (YS): 1310 to 1450 MPa;
Ultimate Tensile strength (UTS) of 1507 to 1650MPa;
Elongation (EL) at failure 8.3 to 9.4 %;
Reduction in area (RA) at failure of 25 to 30 %;
Hardness of 470 to 488 BHN.

3. High tensile strength steel plates as claimed in anyone of claims 1 or 2 having microstructure in quenched and tempered state comprising about 95% tempered martensite and some amount of bainite.

4. High tensile strength steel plates as claimed in anyone of claims 1 to 3 which is suitable for fabricating components of armoured vehicles, firing range baffles, bank ATM panels, mining equipment and other high tensile applications.

5. A process for the production of high strength steel plates having a minimum tensile strength of 1500 MPa comprising the steps of:
(i) making of C-Mn-Si steel with microalloying through EAF-VAD route, having involving a selective composition (in weight %) within the range of C:0.20-0.30, Mn:1.20-1.60, Si:1.20-1.60, Cr.0.10-0.25, Ni:0.15-0.45, Mo:0.15-0.30, P:0.015max, S:0.010max and H:2ppm max, with balance of Fe.
(ii) continuous casting of slabs through Continuous Casting Machine (CCM); and
(iii) hot rolling of CCM slabs to hot rolled (HR) coils and shearing the same to required size of plates.

6. A process as claimed in claim 5 comprising the steps of
(i) providing said C-Mn-Si steel produced through electric arc furnace and vacuum treated in VAD unit involving said selective composition comprising
C: 0.20 to 0.30 wt%,
Mn: 1.20 to 1.60 wt%,
Si: 1.20 to 1.60 wt%,
Cr. 0.10 to 0.25 wt%,
Ni: 0.15 to 0.45 wt%,
Mo: 0.15 to 0.30 wt%,
P: upto 0.015 wt%,
S: upto 0.010 wt%,
H: upto 2 ppm , and
the balance is Fe.
(ii) Continuous casting of slab from above steel grade into 170 x 1160 mm slabs of desired length;
(iii) Cooling the slabs with slow rate, using hood cooling system;
(iv) Reheating of slabs in reheating furnace;
(v) Hot rolling of reheated slabs to 5 mm thick gauge in coil form;
(vi) Shearing hot rolled coils of width 1135 mm, on cooling, into 4600 mm long sheets/plates for subsequent processing;
(vii) Examining as-hot-rolled plates ultrasonically to identify internal/surface defect, if any, before carrying out hardening and tempering operations;
(viii) heating the plates to the specified temperature and time in the hardening furnace and quenching in a well circulated oil bath;
(ix) carrying out tempering of hardened plates by heating in the tempering furnace, wherein uniformity of temperature is maintained to its greater extent and cooling to ambient temperature.

7. A process as claimed in anyone of claims 5 or 6 wherein said continuous slab casting comprises a casting speed of 0.75 meter/minute, mould oscillations of 120/ minute and use of automatic mould level control, said slow cooling of slabs comprising a time period of >50 hours, said reheating of slab comprising 4½ hours soaking at temperature between 1290-1300°C, said hot rolling of slabs to sheet and coiling comprising rolling within the temperature range of 1047-1056°C, finishing between 910-915°C and coiling at 810-812°C, said hardening involving heating and quenching of plates comprising 920?10?C heating for 25 minutes followed by quenching in oil bath, respectively.

8. A process as claimed in anyone of claims 5 to 7 wherein said tempering of hardened plates comprising heating at 240?C for 90 minutes.

9. A process as claimed in anyone of claims 5 to 8 wherein said quenching and tempering is carried out involving microstructural modifications such that the microstructure of quenched and tempered steel plates comprises about 95% tempered martensite and some amount of bainite.

Dated this the 28th day of February, 2014
Anjan Sen
Of Anjan Sen & Associates
(Applicants Agent)

ABSTRACT

TITLE: HIGH TENSILE STEEL PLATES HAVING A MINIMUM TENSILE STRENGTH OF 1500 MPa AND A PROCESS FOR ITS MANUFACTURE.

The present invention is related to high tensile strength steel plates and a cost effective process of manufacturing such steel plates obtained as quenched and tempered steel plates with minimum tensile strength of 1500 MPa. The high tensile strength steel plates are produced according to the present invention using C-Mn-Si chemistry with microalloying, through EAF-VAD-CC route, coupled with microstructural modifications attained through thermomechanical controlled processing and heat treatment involving hardening, quenching and tempering ensuring achieving high tensile strength with reasonably good ductility and toughness. The defect free plates having a minimum tensile strength of 1500 MPa may suitably be utilized for fabricating components of armoured vehicles, firing range baffles, bank ATM panels, mining equipment and other high tensile applications.