Description:Technical Field
The present invention relates to steel for marine and shipbuilding application. More particularly it relates to steel for marine application with appropriate physical properties.
Background
Indigenous development of steel for ship building operation was started in the late 90’s. This was inline with the gathering momentum of the Indian Navy’s ship building program. Indian shipyards were spending several crores of rupees on importing these grades. The construction of the first Air Defence Ship could not take off as it was facing major delays. The reason being difficulties in procuring high quality marine grade steel. It was at this juncture that DRDO was requested to take up a research program for developing this grade of steels. Efforts were on since then at the different DRDO’s laboratories. It had become a ‘national strategic project’ which needed to be done and preferably at the lowest possible cost.
Afterwards, few years of research in this area revealed that the low alloy steels are an attractive option for high performance structural application.
Plate rolling are the usual processing route for these steels. After the finish rolling they are quenched and tempered to achieve higher failure–ductility values (UTS × %Elongation). Achieving higher failure–ductility values is a very tedious task and involves a lot of complexities. Moreover, such steel with failure–ductility values are achieved using process that involves quenching and tempering (Q&T) and through the existing hot strip mill facility. Such processes are economically expensive.
Few prior arts are available with Publication nos. CN110158022A, CN101984119B.
The art CN110158022A discloses the method to improve the corrosion resistance property of the marine steel. But it does not disclose to achieve higher failure-ductility values or toughness values.
The art CN101984119B discloses the method to manufacturing ships and offshore platforms. But it discloses the making of steel plates rather not the sheet. Also, the it does not seem to be disclosing more on elongation and toughness.

Objects
An object of the invention is to develop steels with higher failure–ductility values without quenching and tempering (Q&T).
Another object of the invention is develop steels through existing hot strip mill facility.
Another object of the invention is to propose low alloy steel grades with requisite hardness, strength, ductility and bend properties, which can be produced directly from hot rolling mill.
Disclosure of the Invention
The present invention provides a process for producing a low alloy steel, the process comprising:
casting a steel with composition 0.10 - 0.15 carbon, 1.0 – 1.2 Manganese, 0.10 – 0.20 Silicon, 0.05 – 0.15 Vanadium, 0.01 – 0.05 Nb, 0.55 – 0.70 Nickel, 0.01-0.04 Ti, 0.2-0.6 Cu, balance being Iron and residual impurities (all in wt.%) at casting unit;
homogenizing the steel by austenitizing at 1150-1200°C for 120-130 mins at furnace;
hot rolling the steel at rolling mill; and
quenching or air cooling the steel
The said process with composition is helpful in achieving the appropriate strength, ductility and failure–ductility value (UTS × %Elongation).
In an embodiment, the medium for quenching is water or oil.
In a preferred embodiment, number of rolling pass is 6-10.
In a still another embodiment FRT is 890-900.
The present invention alos provides the low alloy steel, comprising :
failure–ductility value (UTS × %Elongation) of 18000 – 20000 MPa%.
In an embodiment composition of the low alloy steel is of 0.10 - 0.15 carbon, 1.0 – 1.2 Manganese, 0.10 – 0.20 Silicon, 0.05 – 0.15 Vanadium, 0.01 – 0.05 Nb, 0.55 – 0.70 Nickel, 0.01-0.04 Ti, 0.2-0.6 Cu, balance being Iron and residual impurities (al in wt.%).
In another embodiment the low alloy steel comprises 100% martensite in case of water quenching.
In a preferred embodiment, the low alloy steel comprises mixture of martensite, bainite and Widmanstätten ferrite in case of oil quenching.
In a preferred embodiment, the low alloy steel comprises ferrite pearlite structure in case of air cooling.
In a preferred embodiment, the low alloy steel Hardness (at 1 kg load) is 390 – 410 Hv, 280 – 295 Hv, 210 – 220 Hv for water quenching, oil quenching and air cooled respectively.
In a still preferred embodiment, the low alloy steel has Yield Strength of 970¬+15 MPa for water quenching, 650+15 MPa for oil quenched and 588+15 MPa for air cooled.
In a still preferred embodiment, the low alloy steel has Ultimate Tensile Strength of 1090+20 MPa for water quenching, 780+20 MPa for oil quenched and 627+20 MPa for air cooled.
In a still preferred embodiment, the low alloy steel Total elongation is 18%+5 for water quenching, 24%+5 for oil quenching and 31%+5 for air cooling.

Brief Description of the Drawings
Fig. 1 shows a process for producing a low alloy steel in accordance with an embodiment of the invention.
Fig. 2 shows relationship between carbon content and carbon equivalent in the low alloy steel.
Fig. 3 shows a schematic diagram of the hot rolling schedule in one of the embodiment.
Fig. 4a-4c shows water quenched microstructure of the low alloy steel (at different magnifications) in one of the embodiment.
Fig. 5a-5c shows oil quenched microstructure of the low alloy steel (at different magnifications) in one of the embodiment.
Fig. 6a-6b shows air cooled microstructure of the low alloy steel (at different magnifications) in one of the embodiment.
Fig. 7 shows Failure-Ductility-Value (MPa%) for the steel processed through different routes of water quenching, oil quenching and air cooling in each embodiments.
Detailed description of the Invention
In accordance with an embodiment of the invention a process (100) is shown in Fig. 1 depicting production of a low alloy steel. The process (100) comprises steps of
Step (104) where a steel is casted with composition 0.10 - 0.15 carbon, 1.0 – 1.2 Manganese, 0.10 – 0.20 Silicon, 0.05 – 0.15 Vanadium, 0.01 – 0.05 Nb, 0.55 – 0.70 Nickel, 0.01-0.04 Ti, 0.2-0.6 Cu, balance being Iron and residual impurities (in wt.%) at casting unit.
At step (108) homogenizing the of the steel is done by austenitizing at 1150-1200°C for 120 -130 mins at furnace.
At step (112) hot rolling of the steel is performed at rolling mill.
At step (116) the steel is quenched or air cooled. Quenching is performed at quenching unit and air cooling is done at open atmosphere.
The rolling mill in an embodiment is hot strip mill.
The number of rolling pass is in an embodiment is 6-10.
The quenching medium in an embodiment can be water or oil.
The Finish Rolling Tempertaure in an embodiment is 890-900 deg. C.
The failure–ductility value (UTS × %Elongation) of the obtained low alloy steel is 18000 – 20000 MPa%.
The failure ductility value signifies the capacity of the steel to absorb large amount of energy before failure. Such properties are essential for the steels to be used in ship building to increase the shock loading capacity. At the time of desiging such low alloy steel, this property was kept in mind. If steel possess very high UTS and less ductility, it will become extremely brittle. In contrary, if it has a very high ductility but low UTS, the steel will suffer from the strength point of view. Therefore one has to strike a balance between these two properties, namely UTS and %Elongation in order to use the material for any practical purpose.
In an embodiment, the steel can be hot forged at forging unit to break the cast structure and it becomes easier to homogenize during the homogenization step (108). The forging unit may comprise pneumatic open-die forging hammer.
The constituents and process steps are so chosen to provide appropriate failure–ductility value (UTS × %Elongation).
Carbon provides strength, therefore it has been added appropriately. The low alloy steel is so chosen as it comprises low carbon equivalent configured to eliminate the problem of weldability. The weldability being an important feature for marine application. This is also shown in Fig 2. Fig. 2 depicts relationship between carbon content and carbon equivalent in the low alloy steel. The designed alloy is placed in zone 2 i.e. weldable zone. Though an alloy placed in zone 1 is easily weldable but one has to slightly compromise with strength and the alloy has reduced susceptibility to hydrogen cracking during welding. Similarly, zone 3 is quite difficult to weld and has reduced susceptibility to produce crack-sensitive microstructures during fusion welding.
Also, the basis for choosing the low alloying elements makes more free carbon available for partitioning. Also, the optimum amount of Si addition has been chosen to prevent carbide formation during carbon partitioning.
Vanadium and Manganese has been added to provide appropriate strength to the steel.
Nickel and Titanium has been added to provide appropriate toughness to the steel.
Copper has a role in mainly precipitation in steel. Therefore, it has been added appropriately to provide hardness, strength and corrosion resistivity.
Also, the micro alloys have been added appropriately so that the composition does not enriches itself so that elongation is undermined.
Niobium: The nioubium is added to increase the strength by precipitation. It is one of the important microalloys for grain refinement and added in small quantities.
Three steels were casted at 30 Kg (each) scale with composition in Table 1
Table 1
C Mn Si V Nb Ni Ti Cu
Steel 0.13 1.08 0.16 0.09 0.02 0.60 0.01 0.4
The steel were processed as per schematic diagram shown in FIG. 3. The steels were heated at rate 10 deg/C till it reaches 1200 deg. C. The steel were austenized at 1200 deg. C for 120 mins. The steels were passed through rolling mill with 6 passes. The entry temperature is at 1150 deg. C with FRT at 900 deg. C. The final thickness obtained is 6 mm. The steels were quenched in water, oil and air cooled.
Water Quenched
FIGs 4a-4c show the water quenched microstructure of the steel. The structure is predominantly martensitic in nature. A distinct fragmentation of the martensitic structure and its gradual alignment along the rolling direction, can be seen in these micrographs. Obviously, the martensite becomes finer as the amount of rolling reduction increases. Since the rolling is performed close to the no-recrystallization temperature, typical pancaking was not evident in the microstructure. Here the martensite reveals a lath shaped structure. In few places some amount of carbide precipitation can also be observed. It is interesting to note that some white patches in the microstructure are also visible which may correspond to retained austenite. However, the volume fraction appears to be quite low in this case.
The hardness of the water quenched sample varies in the range of 390 – 410 Hv.
Oil Quenched
FIGs. 5a-5c is the oil quenched microstructure of the steel. Some interconnected white regions are present in the structure. They are mostly retained austenite. In some places Widmanstätten like rooftop features are also visible. This is formed by the transformation of austenite below the Ae3 temperature. It often forms simultaneously and competitively with allotriomorphic ferrite and pearlite. Although the microstructure appears to be mostly martensite, however the volume fraction and appearance of this phase is little different than water quenched steel shown in FIGS 4a-4c. Since during oil quenching the cooling rate is lower than the water quenching, there exists a possibility that it might intersects the bainitic nose. Due to this reason, careful observation reveals that some regions of the microstructure also contain bainite. Sometimes when pearlite and bainite grow competitively in the same specimen, it becomes difficult to distinguish the two structures. In summary, the oil quenched microstructure appears to be little complex and interesting than the normal water quenched structure. There is an even mix of different phases that is illustrated in the microstructure.
The hardness for oil quenched varies in the range of 280 – 295 Hv.
Air Quenching
FIGS. 6a-6b shows the air-cooled microstructure of the steel. This is a typical ferrite-pearlite structure which usually appears due to slow cooling from the austenitizing temperature. Here the microstructure is almost fully recrystallized. In a way, it also depicts that the finish rolling was performed near to the no-recrystallization temperature. The ferrite grain size varies in the range of ~15 - 20 µm. The fully resolved pearlites are visible in few places. In some other places, they are not completely resolved. This in turn shows that the pearlite spacing is not uniform across the structure. Different pearlite colonies have different lamellae orientations.
The hardness for air-cooled samples varies in the range of 210 – 220 Hv.
Comparison of physical properties
Table 2: Different mechanical properties of the steel processed through water quenching, oil quenching and air cooling.
Physical Properties Water Quenched Oil Quenched Air Cooled
Hardness (Hv) 390-410 280-295 210-220
YS (MPa) 970+15 650+15 588+15
UTS (MPa) 1090+20 780+20 627+20
% Uniform Elongation 10 - 15 13 - 18 16 - 20
% Total Elongation 18+5 24+5 31+5
Bend Test (IS 1599) 2T, 180 deg 2T, 180 deg 2T, 180 deg
The YS and UTS values were highest in case of water quenched sample as opposed to that of oil quenched sample and air cooled sample.
Therefore, the hardness, YS and UTS values decrease progressively with the change in cooling conditions i.e. water quenching, oil quenching and air cooling.
Severity of the quenching is the most in case of water quenching. This is also reflected in the microstructural constituents. In case of water quenched sample, the structure is almost fully martensitic in nature. Since martensite is the hardest phase in the steel microstructure, this particular steel is exhibiting the highest hardness, YS and UTS values.
However, due to less sever quenching in case of oil quenched sample, the microstructure is a mixture of different phases, namely, martensite, bainite and Widmanstätten ferrite. Thus, the hardness, YS and UTS values dropped a bit from the water quenched condition.
In case of air cooling, the rate of cooling is the slowest. This is also manifested in the microstructural phase distribution which is mainly ferrite-pearlite in nature. In this case the Hardness, YS and UTS are the least when these are compared with their water and oil quenched counter-part. However, in case of %uniform and total elongation the trend is just the reverse. That is to say, the air-cooled sample displayed the highest values followed by oil quenched and water quenched samples.
All the steel samples were investigated in 2T, 180° bend test. Despite of differential strength (depending on different microstructure), all the different steels passed the test without any edge cracking.
FIG. 7 shows a plot of failure–ductility values (UTS × %Elongation) for the steels, at different conditions, i.e. water quenching, oil quenching and air cooling. It is clear from this figure that there is not much difference in the failure-ductility-values for the steels quenched under different conditions. In other words, it also specifies that the composition and processing of the steels are such that the final mechanical properties especially in terms of failure-ductility-values are somewhat independent to different cooling conditions, although the microstructures are fairly different. This seems to be a unique property of the investigated steel.

Advantages:
1. One can understand that the process claimed eliminate the need for any extra facility to perform tempering heat treatment, i.e. to obtain low alloy steel product directly from the hot strip mill.
2. Also the low alloy steel has got requisite strength ductility combination with low alloy composition.

Claims:
1. A process for producing a low alloy steel, the process comprising:
casting a steel with composition 0.10 - 0.15 carbon, 1.0 – 1.2 Manganese, 0.10 – 0.20 Silicon, 0.05 – 0.15 Vanadium, 0.01 – 0.05 Nb, 0.55 – 0.70 Nickel, 0.01-0.04 Ti, 0.2-0.6 Cu, balance being Iron and residual impurities (all in wt.%) at casting unit;
homogenizing the steel by austenitizing at 1150-1200°C for 120-130 mins at furnace;
hot rolling the steel at rolling mill; and
quenching or air cooling the steel.

2. The process as claimed in claim 1, wherein the medium for quenching is water or oil.

3. The process as claimed in claim 1, wherein number of rolling pass is 6-10.

4. The process as claimed in claim 1, wherein FRT is 890-900.

5. A low alloy steel, comprising:
failure–ductility value (UTS × %Elongation) of 18000 – 20000 MPa%.

6. The low alloy steel as claimed in claim 5, wherein composition is of 0.10 - 0.15 carbon, 1.0 – 1.2 Manganese, 0.10 – 0.20 Silicon, 0.05 – 0.15 Vanadium, 0.01 – 0.05 Nb, 0.55 – 0.70 Nickel, 0.01-0.04 Ti, 0.2-0.6 Cu, balance being Iron and residual impurities (all in wt.%).

7. The low alloy steel as claimed in claim 5 comprises 100% martensite in case of water quenching.

8. The low alloy steel as claimed in claim 5 comprises mixture of bainite and Widmanstätten ferrite in case of oil quenching.

9. The low alloy steel as claimed in claim 5 comprises ferrite pearlite structure in case of air cooling.

10. The low alloy steel as claimed in claim 5, wherein Hardness (at 1 kg load) is 390 – 410 Hv, 280 – 295 Hv, 210 – 220 Hv for water quenching, oil quenching and air cooled respectively.
11. The low alloy steel as claimed in claim 5, wherein Yield Strength is 970¬+15 MPa for water quenching, 650+15 MPa for oil quenched and 588+15 MPa for air cooled.

12. The low alloy steel as claimed in claim 5, wherein Ultimate Tensile Strength of low alloy steel is 1090+20 MPa for water quenching, 780+20 MPa for oil quenched and 627+20 MPa for air cooled.

13. The low alloy steel as claimed in claim 5, wherein Total elongation of low alloy steel is 18%+5 for water quenching, 24%+5 for oil quenching and 31%+5 for air cooling.