BACKGROUND OF THE INVENTION
FIELD OF INVENTION
This invention relates to Ultra - high strength low alloy (USLA) steels which are presently available as Ultra - high Strength Low Alloy Martensitic (USLAM) steels which are generally used in 'Hardened and Tempered' condition. These steels have low carbon content and having good combination of mechanical properties, i e. tensile strength, ductility and toughness, particularly with respect to ductility and toughness. The present invention is also related to high strength low alloy steels adapted to use for the material for, such as, space research e g. Rocketry, satellites, Defence ammunitions like missiles, aviation industry for aircraft under carriages, pressure vessels, automobiles, various steel half-products, such as, round steels, profiles, slabs and plates for products of manufacture of hooks, suspensions, tensile members, support members, and so on due to their extremely high strength / density ratio in the quenched and tampered condition.
The demand for steel products of the nature described above rests to a considerable extent on increasing need for high strength in steel strip, sheet structural and the like, with a minimum of weight and, understandably, at as little cost as possible For example, steels of this nature have many uses in vehicle constructions, particularly in the automotive area and construction where for fuel economy, strength is required, and it is desirable to reduce the weight of the structure, yet without impairing strength.
The present improvements are notably designed to afford a steel of very low carbon content, with excellent properties in both longitudinal and transverse directions in reference to cold shaping operations viz. bending machining cutting, shearing etc
DESCRIPTION OF THE RELATED ART
The Ultra - high Strength Low Alloy Martensitic (USLAM) steels are known in the prior art. More specifically, this type of steel is used in hardened and tempered condition. The USLA-steel heretofore devised and utilized are known to consist basically of familiar, expected and obvious structural configurations, notwithstanding the myriad of designs encompassed by the crowded prior art, which have been developed for the fulfillment of countless objectives and requirements
KATAGIRI TADAO, of KAWASAKI STEEL CORP in a Japanese Patent document JP000062120430 discloses a process of ultra-high strength steel pipe as under.
PURPOSE To manufacture an ultra-high-strength steel pipe by properly regulating the chemical composition of steel and simultaneously by setting up
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appropriately heat treatment conditions, particularly tempering conditions, in quench-and- temper treatment.
CONSTITUTION: A low-alloy steel stock has a composition consisting of by weight, 0 3W0.5% C, 0.15W3% Si, 0.6W1% Ma <0.015% P <0 01% S 1 25W5.5% Cr, 0.1W2% Mo, <0.1% Al, and the balance Fe with inevitable impurities This low-alloy steel stock is hot-worked into a seamless steel pipe, which is cooled from a temp, of Ar 3 point or above at a cooling rate of 0.5°C/ secW10°C/sec, followed by tempering under the conditions where, with regard to tempering temp T(°C) and tempering time (t)(min), parameter TP defined by an equation is in the range of 15,000W17,000 and tempering temp, is 500°C and above
TSUMURA TERUTAKA of SUMITOMO METAL IND LTD in a Japanese Patent document JP000061270355AA discloses a method of manufacture of high strength steel excelling in resistance to delayed fracture as under :-
PURPOSE; To develop high strength steel having superior yield strength and resistance to delayed fracture by subjecting Ni-Cr-Mo low alloy steel containing trace constituents of various kinds to heat treatment under specific conditions.
CONSTITUTION: The low alloy steel contains, by weight, 0.15W0.45% C, <1.50% Si, 0 OlWt.50% Mn, 0.10W4.00% (not including 0.10%) Ni, 0.50W2.00% Cr, either or both of Mo and W in the amount satisfying Mo+1/2W=0.30W1 50%, 0.01W0.20% V, 0 005W0.20% Nb, 0.01W0.15% Zr and 0.01W0.10% Al, to which specific small amounts of Cu, Ca, Ti and B are further added independently or in combination. This steel is subjected to hardening from the temp of AC3 point or above and then to tempering from the temp, between 580° and ACi point under the condition that PLM represented by expression (1) is 16.8x1 o3 or more. In this way, the steel stock having austenite grains of ASTM No.8.5 or above, excelling in resistance to delayed fracture and suitable for ultra-high strength oil well pipes can be obtained
YAM ABA RYOTA of NIPPON STEEL CORP in a Japanese Patent Document JP000059129724 discloses a method of production of production of thick walled ultra high-tension steel
PURPOSE: To obtain a thick-walled steel plate having high strength o£90kg/ mm2 with>40mm thickness and good and high toughness at a low temp, by setting the content of costly alloy elements such as Cr, Mo, Ni or the like at lower levels by using a high carbon steel and subjecting the steel to hardening and tempering.
CONSTITUTION. A steel contg, by weight, 0 20W0.35% C, <1 0% Si, 0.30W1.50% Mn, 0.3W2.5% Cr, 2.0W4.5% Ni, 0.2W1 0% Mo, 0.002W01% Al and <1.5 Mn/Cr is hot rolled. The P in the steel is preferably incorporated at
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<0.15% and S at <0.010%. The steel hot-rolled to a prescribed thickness is subjected to a hardening heat treatment for either cooling quickly online the steel from the temp. Above Ar 3 or cooling once the same by allowing the steel to cool then reheating the steel to the Ac3 point or above and quickly cooling the steel from the Ac3 point or above. The steel subjected to the hardening heat treatment is subjected to a tempering heat treatment in a ferrite region, whereby the intended thick- walled ultra-high tension steel is obtained.
SUMMARY OF THE INVENTION
For the above and other purposes, and notably for attainment of high strength, low alloy steel products having superior properties and yet characterized by economy of cost and ease of processing, the invention, in an important aspect, consists of steel characterized by additions of the microalloying elements like vanadium, aluminium, niobium and titanium in low to moderate amounts, with low content of carbon and a relatively high manganese, silicon and chromium
With such proportions of elements, the balance of the steel being iron and incidental substances, and actual numerical ranges for the above elements and also nominal values for normal minor elements such as sulphur and phosphorus being as given herein below, a paramount feature of the invention is the attainment of desired strength and toughness in an unusually lean alloy, with respect both to the so-called microalloying elements and to elements such as manganese, silicon and chromium.
The process for manufacturing of a cost effective Ultra-high Strength Low Alloy (USLA) steel as disclosed in the present specification comprising the steps of: Melting and Casting of the different compositions of raw materials; Hot working of the Cast alloy; Heat treatment cycle of the Cast Alloy;Final shaping of the Cast alloy; wherein the said Heat treatment cycle is characterized by a single step of normalizing of the cast alloy.
The cost effective Ultra-high Strength Low Alloy (USLA) steel consists essentially of, in weight percent of about 0.18% to 0.22% Carbon, 1.00% to 1.30% Manganese, 0.90% to 1.10% silicon, 1.10% to 170% Chromium, 0 65% to 0.75% Nickel, 0.25% to 0.35% Molybdenum, 0.08% to 0.12% Vanadium, 0 04% to 0.07% Aluminium, 0.018% (max) Sulphur, 0.018% (max) Phosphorus, 0.06% (max) Niobium and 0.02% (max) Titanium and balance essentially iron.
A more specific finding is that in the new compositions, the yield strength is directly related to the percentages of these two elements Niobium and Titanium. Yield strengths in the range of 775 MPa to 1050 Mpa can be achieved. The suitable hot working temperature was found to be 760-950 degrees Celsius.
A second important aspect of the present invention is that with the stated microalloyed compositions, especially having the prescribed or preferred levels of
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Carbon, Nickel, Molybdenum, Manganese, Silicon and Chromium, the rolled products are found to exhibit superior transverse formability and good toughness without special additions or processing, u
The heat treatment processes involved in imparting ultra-high strength to the steel or its components are normalizing followed by hardening and tempering. The last phase of the heat treatment i.e hardening and tempering makes the steel / component quite expensive as these involves huge investments in terms of facilities, high operating cost on energy, time and skilled human contributions. In spite of these heat treatment operations, these steel generally suffer from low toughness and poor ductility. Moreover, these steels are highly susceptible to Temper-embnttlement. Hence special caution is required in selection of tempering temperature and processes.
Accordingly, the third aspect of the present invention is to eliminate the expensive hardening and tempering treatment process.
The fourth objective of the present invention is to provide a cost effectiveness by judicious usage of the alloying elements like Nickel and Molybdenum.
The fifth objective of the present invention is to provide a good combination of strength, ductility and toughness at room temperature
The sixth objective of the present invention is to provide flexibility in terms of impurity contents i.e. both S & P to the tune of 0.018% max.
The seventh objective of the present invention is to provide thermo-mechanically controlled processing, keeping finish rolling temperature (FRT) in the range of 760-900 degree Celsius coupled with >15% deformation.
The eighth objective of the present invention is to provide elimination of the possibility to cause Temper Martensite Embrittlement (TME) & Temper Embrittlement (T E)
The ninth objective of the present invention is to provide improved weldability because of lower carbon equivalent.
The tenth objective of the present invention is to make the process of manufacturing the process of the alloy having extremely good processing related properties like, hot workability, post hot working, cooling etc.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig 1 illustrates the chemistry and mechanical properties of some existing USLAM chemicals.
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FIG. 2 is a flowchart for the manufacture of the USLA
FIG 3 is a chart showing the weight of different constituents of the alloy in percentage
DETAILED DESCRIPTION OF THE INVENTION
The ultra-high strength low alloy steels (USLA) of the invention having compositions within the ranges stated above, or indeed within more specific ranges related to particular and notably advantageous aspects of the invention, are prepared in an essentially conventional way, e.g. for making a low carbon, low alloy steel, following known practices for producing a clean ingot product, with good control of desired contents of small percentages of alloying elements. Thus the basic melt is achieved in a customary manner, as in a standard electric or basic oxygen furnace, appropriate attention being paid to the desired carbon content. It is understood that carbon levels as low as 0.18 to 0.22% are effectively obtainable without special treatment of the melt after tapping, and indeed the carbon ranges contemplated as preferred for the present steels appear to pose no special problem in melting practice.
For the processing of the said alloy the first operation, which is to be carried out, is melting of the alloy and Casting it to a shape and size as per the requirement. The raw material additions to the basic charge of steel scrap and the like are made in the manner appropriate for such materials Complete deoxidation of the melt with Si, Mn and Al is done. The checking and analysis of the melt and making final additions of the allowing elements is done in order to achieve the desired chemical composition. In this connection the ladleA/AD treatment is preferred. Moreover ingot casting by bottom pouring practice is preferred.
It is greatly preferred that the steel compositions of the invention are fully deoxidized; although other de-oxidation practice may be used, satisfactory results are achieved by the usual killing with aluminum silicon and manganese Thus constituents can be added to the ladle for de-oxidizing so that oxygen is reduced to values, for example, less than 0.005%.
To the extent that the desired moderate level of manganese is not inherently present in the charge, this element may be added in the furnace and/or ladle, e.g as ferromanganese. Very preferably the minor, i.e. microalloy additions, Niobium, Titanium, Vanadium and Aluminium are effected by adding appropriate material, for example as ferroalloys, to the melt in the ladle after tapping.
The preferred process technology for manufacturing the ultra high strength low alloy (USLA) steel having superior combination of strength, consists essentially of, in weight percent, about 0.18 to 0.22% carbon, from 1 00% to 1.30% manganese, from 0 90% to 1.10% silicon, from 1.10% to 1.70 chromium, 0.65 to about 0 75% Nickel, 0.25% to 0.35 molybdenum and 0.08 to 0.12%Vanadium,
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0.04% to 0.07% Aluminium, about 0.018% maximum sulphur and phosphorus each and 0.06% niobium and 0.02% titanium and balance essentially iron.
After pouring the steel of the melt, which has been suitably controlled as to content of the several required elements, the resulting ingots are handled in conventional way, being reduced to slab or the like for final reduction by hot deformation For most purposes, this is effected by hot working, i.e. Preheating, Forging or roiling Initial slow heating of the cast stock is resorted to wherein the input temperature is below 500 deg Celsius.
For the forging / rolling operation, the primary rolling of the ingots are done at a temperature range more than 1000 degree Celsius. The forging / rolling of the blooms / billets / slabs are done at more than 900 degree Celsius.
The final shaping of the forged / rolled steels if required, is done by the method of cold shaping of the stock. The cold shaping operation viz, bending machining, cutting shearing are operated upon as per the requirements etc
The next treatment is normalizing for the hot rolled/ forged or cold shaped alloy by heating the said alloy to a temperature of 880 to 950 degree Celsius The temperature chosen depends upon the composition, shape and size of the alloy stock. The soaking time in the furnace is computed on the basis of stock shape, size, heating rate etc. Thereafter the heated stock is air cooled to room temperature. The controlled furnace temperature, although is not a must, but is always preferable in order to eliminate the phenomena like scaling, decarbunzation, and penetration of oxygen in the surface of the stock.
The improved high strength, low alloy steels can be produced, as hot rolled product, in a usefully wide variety of gauges. The products are readily hot rolled, without excessive rolling loads and with exhibition of relatively low or moderate hardness factors.
The final shaping is done after the heat treatment, which consists the usual process on grinding, precision machining, bending shaping etc as per the
requirement
The improved high strength, low alloy steels can be produced, in a usefully wide variety of gauges, for instance to about 0.20 mm and above. The product properties as observed from the analysis of the samples are CE < 0.8, Y S. in the range of 775 MPa to 1050 MPa, U.T.S. was found to be in the range of 1275 MPa to 1450 MPa, the impact toughness (CVN) at room temperature was observed in the range of 54 J to 60 J or more. Herein there is an increase in impact toughness to the tune of 10 J to 15 J (20-25%) compared to the existing / conventional grades of steels. The percentage elongation was found in the range of 10% to 12 5%. Rolling trials were conducted at various temperatures in order to access the hot workability. Apart from the working on the at-cast structure the
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suitable hot - working temperature was found to be in the range of 760 - 950 degree Celsius.
Four different alloy combinations were designed keeping the following ratio's under consideration: -
1. Carbon content = 0.20%
2 Nickel content = 0.70%
3 Molybdenum content = 0.30%
4 Vanadium = 0 10%
5. Aluminium content = 0.06%
6. S & P content 0.018% max ; each 8 Niobium 0.06% max
9. Titanium 0 02% max
Optimum utilization of costly alloying elements i.e. Ni and Mo is done in the preparation of the said high strength and low alloy steel
The new steel products have been tested through a significant range of compositions, with experimental results fully supporting the properties and characteristics described herein. The experimentation was conducted for all heat treatment operations viz. Normalization, Hardening and tempering with various sets of parameters like temperatures, period of soaking etc for the experimental heats.
The innovation in the process for designing of the alloy with the following superior qualities and difficult to achieve combinations of the physical properties:-
Good impact toughness (CVN - 55J to 60J at RT) and strength (1275 MPa to 1450Mpa)
Adequate flexibility on the residual elements (S & P each 0.018 max)
Suitable combination of micro alloying elements like Nb, Ti, V and Al may impart adequate impact toughness and can work as partials substitutes for costly alloying elements (Ni & Mo) primarily attributable for imparting impact toughness in this class of steel.
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Thermo mechanically controlled processing and control finish rolling temperature (FRT) and final reduction.
Only normalizing at the pre-determined temperature in order to accomplish the above-mentioned properties of USLA Steels.
The followings are the prospects of the utilization of the USLA steels manufactured by the aforesaid process are as under :-
TECHNICAL -
The benefit of addition of Nb, Ti, V and Al as micro alloying elements on strength, ductility and toughness could be accured even after the Normalizing heat treatment operation.
TMCP is beneficial with respect to strength, ductility and toughness
Reduction in susceptibility to both Temper Martensite Embrittlement (TME) & Temper Embrittlement (T.E).
Feasibility of elimination of the expensive Hardening and Tempering processes with some compromise on YS and UTS
Economical ;-
The steel manufactured by the disclosed process and the composition then the same will be cheaper compared to similar existing grades due to -
Lower Nickel and Molybdenum components.
Lower Hot working temperature.
Elimination of expensive hardening and tempering processes.
However the actual quantum of benefits in terns of cost of the production of USLA Steel will depend on many factors, some of the factors are enumerated below; -
a) Which particular USLAM (or any other steel) will be replaced by the present steel?
b) What are the absolute costs of ingredients like Ni, Mo, Si, Cr, Win vis-avis their ferroalloys at the plant of production?
c) Yield / recovery of the above mentioned alloying elements
d) The facilities available at the production unit or the process route
followed compared to existing similar commercial grades of USLAM
steel e.g Secondary refining, Heat treatment, furnace type and size etc.
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e) The benefit will depend upon whether secondary-refining techniques like VAD, Ladle, degrassing etc. is resorted to or by-passed.
f) Benefit at the heat treatment operations will depend on the type of furnace, their sizes and capabilities.
g) The desired final properties (application specific), which will ultimately decide all the processing parameters.
h) The plant specific cost of the expensive hardening and tempering processes.
Although the steels are conveniently defined by their properties as produced by the hot rolling, coiling and cooling procedure, it will be understood that an ultimate product embodying a steel of the invention may have had further processing that affects the value of a property, for example, decrease in yield strength upon cold rolling and annealing.
It is to be understood that the invention is not limited to the specific features herein set forth for example but may be carried out in other ways without departing from its spirit.
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We claim;
1 The process for manufacturing of a cost effective Ultra High Strength Low
Alloy (USLA) steel comprising the steps of: -
melting and casting of the different compositions of raw materials, carried out by
ingot casting method by a preferred bottom pouring procedure or through
continuous casting machine,
hot working of the Cast alloy;
cold shaping of the hot worked alloy, if required, is carried out by cold shaping
operations such as machining, bending, cutting, shearing etc.;
heat treatment of the shaped Alloy;
final shaping of the heat treated alloy, is carried out by the process of precision
machining, grinding, bending, shaping, shearing etc.;
wherein the said heat treatment cycle is characterized by a single step of
normalizing of the shaped alloy.
2 The process as claimed in claim 1, wherein the said melt consists of in
weight percent of about 0.18% to 0.22% Carbon, 1.00% to 1.30% Manganese,
0.90% to 1.10% silicon, 1.10% to 1.70% Chromium, 0 65% to 0 75% Nickel,
0.25% to 0.35% Molybdenum, 0.08% to 0.12% Vanadium, 0.04% to 0 07%
Aluminium, 0 018% (max) Sulphur, 0.018% (max) Phosphorus, 0.06% (max)
Niobium and 0 02% (max) Titanium and balance essentially iron and the said
melt is completely deoxidized with silicon, manganese and aluminium
3. The process as claimed in claim 2, wherein the said melt is analyzed for making final addition of alloying elements by a preferred Ladle / VAD treatment.
4. The process as claimed in claim 1, wherein the hot working of the cast alloy is carried out by pre heating i.e. initial slow heating of the cast alloy, on condition that the input temperature is below 500 degree celsius.
5. The process as claimed in claim 4, wherein the pre heated cast alloy is subjected to primary rolling of ingots to blooms / billets / slabs above 1000 degree Celsius and the rolled ingots are subjected to further Forging / Rolling above 900 degree Celsius.
6. The process as claimed in claim 5, wherein the Forged / Rolled alloy is finished (FRT) in the range of temperature 760 to 900 degree Celsius following Thermo-mechanically controlled process (TMCP) with >15% deformation
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7. The process as claimed in claim 1, wherein the normalizing operation is carried out by air-cooling from a range of temperature between 880 to 950 degrees Celsius and also the said operation is dependent upon the composition, shape and size of the stock.
8 The process as claimed in claim 1, wherein the said heat treatment cycle does not consist of any expensive hardening or tampering processes
9 The cost effective Ultra High Strength Low Alloy (USLA) steel consist essentially of, in weight percent, weight percent of about 0.18% to 0.22% Carbon, 1.00% to 1 30% Manganese, 0 90% to 1.10% silicon, 1.10% to 1.70% Chromium, 0.65% to 0.75% Nickel, 0.25% to 0.35% Molybdenum, 0.08% to 0.12% Vanadium, 0.04% to 0.07% Aluminium, 0.018% (max) Sulphur, 0.018% (max) Phosphorus, 0.06% (max) Niobium and 0.02% (max) Titanium and balance essentially iron
10 The alloy as claimed in claim 15, wherein the said alloy lower percentage of costly alloying elements such as nickel and molybdenum and also the said alloy provides adequate flexibility on residual elements such as sulphur and phosphorus (0 018 % max) the alloy also provides good impact toughness (CVN 55J to 60J at RT) and strength (1275 Mpa to 1450 Mpa)

To,
The Controller of Patents,
The Patent Office,
Kolkata.
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The process for manufacturing of a cost effective Ultra High Strength Low Alloy
(USLA) steel comprising the steps of: -
melting and casting of the different compositions of raw materials, carried out by
ingot casting method by a preferred bottom pouring procedure or through
continuous casting machine;
hot working of the Cast alloy;
cold shaping of the hot worked alloy, if required, is carried out by cold shaping
operations such as machining, bending, cutting, shearing etc;
heat treatment of the shaped Alloy;
final shaping of the heat treated alloy, is carried out by the process of precision
machining, grinding, bending, shaping, shearing etc.;
wherein the said heat treatment cycle is characterized by a single step of
normalizing of the shaped alloy.