FIELD OF INVENTION
The present invention relates to high strength steel alloy composition capable of developing yield strengths in the range of 700 to lOOOMPa and having excellent impact toughness properties (Charpy V-Notch impact energy 80 to lOOJ) at low temperatiires -20 to -40°C. The steel alloy of present invention has potential applications for naval structures such as hull and other structural applications.
BACKGROUND OF THE INVENTION
Structural steels for naval applications, especially those used in external hulls of surface ships and submarines, encounter extreme conditions of operation that include constant exposure to sea water (with high salinity), repeated static and dynamic loading, and thermal excursions ranging from very low (< -40°C) to high ambient (~ 45-50°C) temperatures. To withstand such severe conditions, the steel alloys must have an outstanding combination of high yield and ultimate tensile strength, excellent sub-zero impact toughness, reasonable ductility and adequate corrosion resistance. In addition, from a processing and fabrication point of view, the steel alloy should (a) be amenable for rolling into large plates, (b) possess good weldability and (c) be capable of being bent to the required shape, for example during hull fabrication. Until recently, all the steel required for shipbuilding were being imported from various foreign sources in USA, Russia, France, Germany etc. However, beginning in 2004-05, two specialty steels for naval applications, with minimum yield strengths of 390MPa and 588MPa respectively, are being produced in the country for use in the construction of warships. Increasing the strength of steels to higher levels, without unduly compromising the other properties can provide significant strategic advantages in service offering the possibility of weight reduction. It is the main object of the present invention to develop and offer such steel (with yield strength ~
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750 MPa, for example) that can be used in naval structural applications. It is worth noting that such steels are not being produced in the country presently. There are few steels that have strength levels above 700 MPa that are fit for naval construction, in which one of the main requirements of the material is a reasonably good impact toughness at sub-zero temperatures. The U.S. Navy has been extensively using HSLA-80 and HSLA-100 steels, with minimum specified yield strength (YS) of 80ksi (~ 552MPa) and lOOksi (~ 690MPa) respectively, in naval hull construction. These steels have been produced to U.S. military specifications, MIL-S-24645A (SH), which has been later superceded by NAVSEA document T9074-BD-GTB-010/300. These steels replaced the 'high yield' HY80 and HYIOO steels which had relatively poor weldability (Zone III, 'high susceptibility to heat affected zone cracking' of the Graville diagram). By reducing the maximum carbon content from 0.20 weight percent (for the HY 80/100 steels) to 0.06 weight percent in HSLA 80/100 steels, it was possible to obtain much better weldability (Zone I: 'safe, not susceptible to HAZ cracking, of the Graville diagram) and also excellent low temperature impact toughness. More recently, there are reports of HSLA 115, with YS of 115ksi (~ 790 MPa), being used in naval construction. It has been reported that HSLA 115 has composition identical to HSLA 100, and the increase in yield strength has been achieved through modifications to thermo mechanical processing. The current invention does not rely on advanced thermo mechanical processing to achieve the target properties of yield strength greater than 700MPa, typically above 800MPa. U.S. Patent Application 20110041965 discloses High strength thick gauge steel plate superior in weldability and having tensile strength of 780MPa or more and method of production of same. The prior art describes a steel able to achieve a tensile strength of 780MPa or more, in plates of thickness 12-40mm. The patent further discloses that the steel has high weldability and does not require pre-heating prior to welding. This steel uses a high amount of manganese, to the extent of 2.4 to 3.5 wt. percent, and contains only small amounts (less than 0.5 weight percent) of nickel and molybdenum. The steel is designed to have yield strength of 685MPa or more, and at
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-20°C Charpy absorption energy of 100J or more. In contrast, the present invention is designed to have yield strength (YS) of 700MPa or more, an ultimate tensile strength (UTS) varying from 850 to 1150 MPa, and Charpy absorption energy of 78J or more at -40°C.The initial studies indicate that the present invention steel exhibits good weldability. It is possible to weld up to a certain thickness with minimal or no preheating, but thicker plates may require moderate preheating. Indian Patent No. 235705, 'Development of microalloyed ultra high strength steel', discloses a steel produced using microalloying with yield strength of 160ksi (~ 1100 MPa), a much higher yield strength than that of concern in the present disclosure. The intended application is for gun barrels and crank shafts and this steel is not suitable for naval hull construction. The steel reported in patent 235705 is, as quoted in the invention, "MICRO-MSF-1700 ultra high strength steel", and has UTS of 1680-1750 MPa, but more importantly has a room temperature impact toughness of only 60-70J, and a -40°C impact toughness of 55-65J. Many naval applications require a minimum impact toughness of 78J at -40°C, and for this reason, the steel reported in Patent 235705 is not likely to be suitable. Further the Patent 235705 is related to a steel belonging to the 'ultra high strength' category, with YS and UTS 400-450 MPa higher than HSLA 100. It is well known that UHS steels have a 'different' physical metallurgy compared to steels usually designed for naval applications, and usually do not possess adequate sub-zero impact toughness. The most recent steel used by US Navy has a yield strength of 115ksi (~800MPa), and the 'blast-resistant steel' under development in the USA has a target yield strength of about 160ksi (~1100MPa).In contrast, the UHS steel category has minimum yield strength of 1200MPa. Indian Patent No. 232515, 'A process for producing high strength and high toughness plates fi-om lean alloy HSLA steel', discloses a steel having YS of 750MPa (min.) and Charpy impact toughness of 82J (min.) at -85°C. In this steel, the total alloy content (copper, chromium, nickel, molybdenum and niobium) is kept below 2.93 weight percent as against ~ 6.40 weight percent for HSLA-100 steel. The present invention, which has carbon content higher than that specified for HSLA-100 steel
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(0.06 weight percent max), has significant amount of alloy content, varying between 4.5 and 11.0 weight percent. The higher alloy content is required for corrosion resistance, fatigue and extreme temperature stability. The relative amounts of nickel, manganese and copper are higher in present invention steel alloy which enables to achieve desired yield strength. More importantly, the higher alloy content ensures the availability of a wide processing (particularly tempering) window over which target properties can be obtained. This is a clear advantage when dealing with production facilities which may not have tight controls over furnace temperatures, especially when heat treating large and heavy gauge plates.
The present invention possesses all the desired properties in terms of having high yield and ultimate tensile strengths, excellent impact toughness at sub-zero temperatures and adequate ductility as measured by percentage elongation in tensile tests. The steel alloy of the present invention produced by air induction melting, and subsequently processed into plates by hot rolling, exhibits the desired combination of high yield strength and excellent low temperature impact toughness upon suitable heat treatment. In the said condition, the alloy exhibits a tempered bainitic-martensitic microstructure, containing a distribution of carbides, copper-rich and other precipitates.
OBJECT OF INVENTION
It is an object of the invention to provide high strength structural steel for naval applications, preferably for hulls of surface ships and submarines.
It is another object of the present invention to provide high strength level with yield strength ~ 750MPa while maintaining a Charpy V Notch impact toughness of 80 to lOOJ at low temperatures (-20 to -40°C).
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It is yet another object of the present invention to provide a process for manufacture of said steel at 30kg scale by air induction melting and subsequent conversion to plates by hot rolling and suitable heat treatment.
SUMMARY OF INVENTION
According to an aspect of the present invention there is provided a high strength steel alloy with composition (by weight):
i. 0.05 to 0.12% carbon;
ii. 0.50 to 1.75% manganese (Mn);
iii. 0.20 to 0.40% silicon (Si);
iv. 3.0 to 6.0% nickel (Ni);
V. 0.20 to 0.40% chromium (Cr);
vi. 0.5 to 2.0% copper (Cu);
vii. 0.30 to 0.80% molybdenum (Mo);
viii. 0.01 to 0.05% aluminium (Al);
ix. 0.01 to 0.05% titanium (Ti);
X. The balance essentially being iron (Fe).
BRIEF DESCRIPTION OF DRAWINGS
Figure 1: Effect of austentizing temperature on as-quenched hardness in alloys 1 and
2, compositions of which are given in Table 2.
Figure 2: Yield strength (YS) and ultimate tensile strength (UTS) as a function of
tempering temperature
Figure 3: Charpy impact toughness at -40°C as a function of tempering temperature
for samples austentized at 900°C.
Figure 4: Optical (left) and scanning electron (right) micrograph of steel sample tempered at 600°C showing tempered bainitic/martensitic structure.
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Figure 5: Images (microstructures) that show the tempered bainitic/martensitic microstructure, and the distribution of copper rich precipitates.
DETAILED DESCRIPTION OF THE PRESENT INVENTION
The present invention relates to development of high strength steel with yield strength ~ 750MPa while maintaining a Charpy V Notch impact toughness of 80 to lOOJ at low temperatures -20 to -40''C. Amongst all alloying elements in iron, carbon is one of the most potent strengthening elements; however, it is well known that increasing the carbon can also lead to problems with weldability. Thus, it was decided to keep the carbon content of the steel below about 0.12 weight percent, preferably below 0.10 weight percent. The present inventors have established that in high strength low alloy steels, it is possible to achieve a range of yield strength and sub-zero impact toughness values by altering the relative amounts of nickel, manganese and copper. Other alloying elements such as Si, Cr and Mo have been retained at typical levels found in other HSLA steels.
This steel alloy has highest possible yield strength that can be produced in Indian steel plants for naval applications in the form of large plates. The steel with such high yield strength are not being produced in the country presently. Thus this steel can be used as an economic alternative for imported high strength steel. The present invention steel alloy is more compatible with existing lower strength grades especially pertaining to welding and corrosion related aspects. The developed steel is likely to be quite amenable to cross-welding with lower strength steel grades currently being used, especially those of Russian origin such as ABA and AB2, and their indigenized versions designated DMR-249A and DMR-249B respectively. Computations using ThermoCalc™* software were performed by varying the relative amounts of nickel, manganese and copper in order to investigate their effects on the microstructure. Based on these computations, several different trial compositions were chosen with varying amounts of nickel, manganese and copper. The
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investigations on the processing, microstructure and the properties of these trial alloys in different heat treated conditions form the basis of the disclosed invention. Alloys are largely dependent on the composition especially the combination or proportions of the different alloying elements for their properties. Any slight variation in the composition will have implication on the properties which may even be completely unexpected and large. Because of aforementioned reasons, i.e. of small composition changes potentially having large unintended or unexpected effects, we can confirm that this is a novel steel alloy composition developed more particularly for naval structures such as hull and other structural applications The composition (in weight percent) of the high strength steel alloy is disclosed in table 1. Table 1:
C Mn Si Ni Cr Cu Mo Al Ti S P Fe
Min. 0.05 0.50 0.20 3.0 0.20 1.0 0.30 0.01 0.01 ^^^4 QQ20
. ' ' Bal.
Max. 0.12 1.75 0.40 6.0 0.40 2.0 0.80 0.05 0.05 ™^- "^^•
Table 2 discloses composition (in weight percent) of alloys 1 and 2, both of which conform to the range given in Table 1. Table 2:
C Mn Si Ni Cr Cu Mo Al Ti S P Fe
Alloyl 0.086 0.59 0.38 4.44 0.28 1.48 0.46 0.006 0.010 0.014 0.015 Bal. Alloy2 0.097 0.58 0.29 4.20 0.35 1.77 0.57 0.023 0.014 0.008 0.020 Bal.
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It has been identified that, when sufficiently pure raw materials are used, the air induction melting route explained below is sufficient to achieve the target properties. However, on an industrial scale, the properties can be ensured or even enhanced further by adoption of vacuum or inert atmosphere wherever possible, for example during the secondary (or ladle) refining and casting processes. In the air induction melting route, the melting process and the charging sequence has to be optimized to minimize losses and to achieve the desired alloy composition. Upon completion of melting under slag cover, the hot metal has to be poured into ingot moulds. The stripped ingots have to be homogenized at temperatures ranging fi-om 1100 to I300°C for adequate time. Unsound portions of the ingots have to be discarded. The remnant sound portions of the ingots are to be forged at temperatures of 1100 to 1300°C into feedstock fi-om which plates of thickness up to 10-30mm can produced by hot rolling. Due to the nature of the process, lower thicknesses are not expected to be a problem.. A minimum reduction ratio of 1:3 between the feedstock and the final product is to be maintained. It is also essential to maintain finish rolling temperatures of between 850 and 950°C.
Plates thus produced need to be austenitized in the temperature range of 850°C to 1000°C and tempered in the temperature range of 500°C to 700°C. Figure 1 shows the range of hardness that can be obtained in two steel alloys conforming to the composition range specified above, in the as-quenched condition, by varying the austenitizing temperature in the range of 850°C to 1000°C. Firstly, it can be seen that by varying composition within the specified range, it is possible to tune the hardness, and by extension, other mechanical properties such as strength and impact toughness, over a wide range. In general, as the austenitizing temperature is increased, the hardness initially increases and then subsequently decreases. This non-monotonic behaviour is due to the conflicting influence of two main factors: with increase in austenitizing temperature, more of the carbides and other precipitates go into solution in austenite thereby contributing to increased as-quenched hardness; however, there is a concomitant increase in prior austenite grain size which leads to a decrease in as-
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quenched hardness. At lower temperatures, the former dominates leading to an overall increase in hardness, while at much higher temperatures the latter dominates leading to an overall decrease in hardness. As increased grain size also adversely affects the impact toughness, it is prudent to avoid austenitizing temperatures higher than the temperature at which the as-quenched hardness starts to decrease. The strength and toughness properties as a function of tempering temperature are shown in figures 2 and 3 respectively. All the samples were austenitized at 900°C followed by water quenching prior to tempering. It is demonstrated that the yield strength (YS) can be varied in the range of 650MPa to lOOOMPa, and the ultimate tensile strength (UTS) can be varied in the range of 850MPa to 1 ISOMPa, by suitable heat treatment. High YSAJTS ratio (in the range of 0.85 to 0.95) is obtained for lower tempering temperatures and low YS/UTS ratio (in the range of 0.55 to 0.75) is obtained for higher tempering temperatures. From figure 3, it is demonstrated that samples have an impact toughness of not less than 80J at -40°C for tempering temperatures in the range of 550°C to 675°C. The tempered bainitic/martensitic nature of the microstructure is shown in figure 4 and figure 5. It has been confirmed through transmission electron microscopy that the tempered samples contain several copper-rich and carbide precipitates; their size and spatial distribution varies significantly as a function of tempering temperature.
The present invention was made based on the composition as given in table 1, which, when processed and heat treated as outlined above, can develop claimed strength and impact toughness properties. The gist of the invention is as follows: A structural, high strength steel alloy comprising, in combination, by weight: 0.05 to 0.12% carbon, 0.50 to 1.75% manganese (Mn), 0.20 to 0.40% silicon (Si), 3.0 to 6.0% nickel (Ni), 0.20 to 0.40% chromium (Cr), 0.5 to 2.0% copper (Cu), 0.30 to 0.80% molybdenum (Mo), 0.01 to 0.05% aluminium (Al), and 0.01 to 0.05% titanium (Ti), the balance essentially being iron (Fe) along with other incidental elements and impurities.
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The alloy having deleterious elements, sulphur no more than 0.015 weight percent (preferably no more than 0.010 weight percent) and phosphorus no more than 0.020 weight percent (preferably no more than 0.015 weight percent), The high strength steel alloy having yield strength (YS) varying from 700 to 1000 MPa, ultimate tensile strength (UTS) varying from 850 to 1150 MPa and YS to UTS ratio vary between 0.6 and 0.95.
The high strength steel alloy having Charpy V- notch impact energy of not less than 80J at -40°C and not less than lOOJ at ambient temperature (25°C). The alloy exhibits a tempered bainitic-martensitic microstructure, containing a distribution of carbides, copper-rich and other precipitates. This alloy has higher yield strength than similar high strength grades of steel. The low temperature charpy impact toughness of this alloy steel is also comparable to that of the similar high strength grades of steel mentioned in prior art. The present invention steel alloy is more compatible with existing lower strength grades, especially pertaining to welding and corrosion related aspects. The high yield strength (700 to 1000 MPa), ultimate tensile strength (850 to 1150 MPa) and excellent impact toughness properties (Charpy V-Notch impact energy 80 to lOOJ) at low temperatures (-20 to -40°C) make this steel suitable for naval structural application.
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We claim:
1. A high strength steel alloy for naval structural application comprising
composition by weight:
i. 0.05 to 0.12% carbon;
ii. 0.50 to 1.75% manganese (Mn);
iii. 0.20 to 0.40% silicon (Si);
iv. 3.0 to 6.0% nickel (Ni);
V. 0.20 to 0.40% chromium (Cr);
vi. 0.5 to 2.0% copper (Cu);
vii. 0.30 to 0.80% molybdenum (Mo);
viii. 0.01 to 0.05% aluminium (Al);
ix. 0.01 to 0.05% titanium (Ti);
X. The balance essentially being iron (Fe).
2. The high strength steel alloy as claimed in claim 1, wherein amount of deleterious element sulphur no more than 0.015 weight percent preferably no more than 0.010 weight percent.
3. The high strength steel alloy as claimed in claim 1, wherein amount of deleterious element phosphorus no more than 0.020 weight percent preferably no more than 0.015 weight percent.
4. The high strength steel alloy as claimed in any one of the preceding claims, wherein amount of total alloy content is in the range between 4.5 and 11.0 weight percent.
5. The high strength steel alloy as claimed in any one of the preceding claims,, wherein yield strength of said alloy is in the range of 700 to 1000 Mpa.
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6. The high strength steel alloy as claimed in any one of the preceding claims,,
wherein impact toughness of said alloy is 80 to 100J at low temperatures -20
to -40°C.
7. The high strength steel alloy as claimed in any one of the preceding claims,, wherein ultimate tensile strength (UTS) of said alloy is in the range of 850 to 1150 Mpa, with Yield strength to ultimate tensile strength ratio in the range of 0.6 and 0.95.
8. The high strength steel alloy as claimed in any one of the preceding claims,, wherein alloy has potential applications for naval structures such as hull and other structural applications.
Dated this the 20* day of March 2012
Mythili Venkatesh OfS.Majumdar&Co. Applicant Agent
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