Claims:We Claim:

1. A high strength steel plate having 550-650 MPa yield strength which is obtained of low carbon steel composition in wt% comprising:

C: 0.04 - 0.11;
Mn: 1.20 – 1.80;
Cr: 0.40 max.;
Si: 0.10 – 0.70;
S: 0.01 max.;
Al: 0.01 – 0.10;
P: 0.05 max.;
Ti: 0.005-0.04;
Nb: 0.01-0.07; and rest is iron.

2. The high strength steel plate as claimed in claim 1 which is obtained through BOF-LHF-RHOB-CC route of steel making and continuous casting.

3. The high strength steel plate as claimed in anyone of claims 1 or 2 comprising of microstructure of high strength steel plate comprising of fine acicular ferrite with a grain size 2-6 µm along with dispersed fine martensite austenite M/A islands/constituents.

4. The high strength steel plate as claimed in anyone of claims 1 to 3 having ultimate tensile strength in the range of 670 to 750 MPa preferably in excess of 600MPa, Elongation in the range of 30 to 45% preferably in excess of 30% and Charpy impact energy at 0 oC in the range of 200 to 270 J preferably in excess of 200 J.

5. A process for manufacture of high strength steel plate having 550-650 MPa yield strength as claimed in anyone of claims 1 to 4 comprising :

method of rolling steel slabs in recrystallization region comprising of reheating of slabs in the reheating furnace at 1200-1250 0C, discharging at 1180-1220 0C and rolling starting at 1080-1150 0C and controlled hot rolling for maintaining a high initial temperature above no recrystallization temperature where recrystallization is completed before subsequent deformation steps.

6. The method as claimed in claim 5 comprising carrying out said rolling steel slabs in recrystallization region involving of reheating of slabs in the reheating furnace at 1200-1250 0C, discharging at 1180-1220 0C and rolling starting at 1080-1150 0C with one sizing pass, 2 - 3 broad sizing pass while roughing rolling consisted of 3 – 5 passes.

7. he method as claimed in anyone of claims 5 or 6 comprising carrying out reduction in each sizing, broad sizing of upto 20%, and in roughing passes upto 35 % such that at the end of this rolling operation, total reduction to be kept in the range of 70-80% with number of reduction steps upto 9 as per thickness.

8. The method as claimed in anyone of claims 5 to 7 wherein the intermediate thickness of plate after roughing rolling of the plate as 2.8 – 4.0 times of final thickness of the plate with the roughing exit temperature 25 0C above the calculated no recrystallization temperature (Tnr).

9. The method as claimed in anyone of claims 5 to 8 wherein method of rolling in non-recrystallization region of the steel plates comprises of air cooling the plate to 70 0C - 170 0C temperature below the calculated no recrystallization temperature (Tnr) followed by rolling the plate in 4 – 6 reduction passes with a finish rolling temperature at 810 - 8600C and wherein the cumulative reduction in the last three finishing passes was >50% with a total reduction of atleast 65% at the end of this finish rolling operation.

10. The method as claimed in anyone of claims 5 to 9 comprising of accelerated cooling of the plate after finish rolling with an entry temperature to ACC box 20 - 40 0C above the calculated Ar3 temperature, cooling rate at up to 20 0C/s and the cooling stop temperature at 500 – 580 0C followed by air cooling in cooling bed according to final plate thickness such as to produce the steel plate having microstructure of the high strength steel plates comprising of fine acicular ferrite with a grain size 2-6 µm along with dispersed fine martensite austenite M/A islands/constituents and mechanical properties of the high strength steel plates comprising of yield strength in excess of 500 MPa in the range of 550 to 650 MPa , having ultimate tensile strength in the range of 670 to 750 MPa preferably in excess of 600MPa, Elongation in the range of 30 to 45% preferably in excess of 30% and Charpy impact energy at 0 oC in the range of 200 to 270 J preferably in excess of 200 J.

Dated this the 13th day of March, 2021
Anjan Sen
Of Anjan Sen & Associates
(Applicants’ Agent)
IN/PA-199

, Description:FORM 2
THE PATENT ACT 1970
(39 OF 1970)
&
The Patent Rules, 2003
COMPLETE SPECIFICATION
(See Section 10 and Rule 13)

1 TITLE OF THE INVENTION :
STEEL PLATE HAVING 550-650 MPa YIELD STRENGTH AND ADEQUATE TOUGHNESS USING LEAN CHEMISTRY AND A PROCESS FOR ITS MANUFACTURE.



2 APPLICANT (S)

Name : STEEL AUTHORITY OF INDIA LIMITED.

Nationality : Indian.

Address : Research & Development Centre for Iron & Steel,
Doranda, Ranchi, Jharkhand, India. PIN-834002.






3 PREAMBLE TO THE DESCRIPTION

COMPLETE

The following specification particularly describes the invention and the manner in which it is to be performed.

FIELD OF THE INVENTION
The present invention relates to a high strength steel plate having 550-650 MPa yield strength which is obtained of low carbon steel composition and a process for its manufacture through basic oxygen furnace (BOF) - Ladle Heating Furnace (LHF) – Ruhrstahl Heraeus Oxygen Blowing (RHOB) – Continuous Casting (CC) route of steel making.

BACKGROUND OF THE INVENTION
High strength plates with high low temperature toughness find wide range of applications including naval ship, various structural, oil and gas storage tank and line pipe etc. Large naval ship shell structures are manufactured using ultra-high strength, sufficient cryogenic toughness and superior weldability to service in extreme environments. Similarly, in the modern line pipe technology, much effort has been devoted towards improving strength, toughness and corrosion resistance of line pipe steel used for the transportation of crude oil or natural gas. Generally, the high strength low alloy (HSLA) steels contain small amount of alloying elements, such as Nb, V, Ti, B, Cu, etc.. The principal goal of adding microalloying elements is to achieve grain-refined strengthening and precipitation hardening. The lower carbon equivalent in these steels leads to excellent weldability and toughness. It will thus result in the sacrifice of strength and hardness of steel. Therefore, it is of great concern for developing high-strength low-alloy (HSLA) steels with the combination of high strength-toughness and low carbon equivalent. Great improvements in steelmaking practices have allowed improved alloy composition through production of steels with low interstitial element contents (C, and N), low sulfur contents, which is required to reduce hydrogen-induced cracking, and low phosphorus levels, which is required to lower the hardening tendency. The thermo-mechanical controlled processing is most important part of processing this high strength steel for obtaining high strength as well as high toughness with a low carbon lean chemistry.

The optimization of hot rolling schedules is critical in maintaining a high initial temperature during roughing pass above no recrystallization temperature where recrystallization is complete before subsequent deformation steps. Below this temperature and above Ar3, finish rolling is carried out where recrystallization can be suppressed, so that grain flattening and strain accumulation occur. As the temperature of steel decreases in austenite, nucleation of precipitate occurs followed by particle growth. Accelerated cooling after finish rolling results in formation of ferrite bainite or acicular ferrite or a combination of these phases leading to high strength and toughness.

Patent No.: CN104213019A, Patent Granted on: 06-07-2016.
Title: 600 MPa level automobile axle housing steel and production method thereof.
This invention discloses 600 MPa level automobile axle housing steel and a production method of the 600 MPa level automobile axle housing steel. Compositions of the steel comprise, by weight, 0.21 percent to 0.26 percent of C, 0.51 percent to 0.6 percent of Si, 1.1 percent to 1.5 percent of Mn, 0.01 percent to 0.06 percent of Al, P smaller than or equal to 0.02 percent, S smaller than or equal to 0.01 percent, 0.05 percent to 0.06 percent of V, 0.012 percent to 0.016 percent of N, and the balance Fe and inevitable impurities. The ratio of V to N is smaller than or equal to 5:1. According to the method, the accurate V content, the accurate N content and the controlled rolling and controlled cooling process window are designed, hot rolling strip steel for a 600 MPa level automobile axle housing is produced, the strip steel has the good strength and toughness, and meanwhile various mechanical performance indexes of the axle housing after hot forming at the temperature more than 800 DEG C can be ensured. The method can be applied to automobile axle housing production, the thickness of materials can be effectively reduced, and on the premise that the safety of an automobile is ensured, the lightweight aim can be effectively achieved.
In contrast to this, the chemistry of present invention does not contain V and hence, the ration of V to N is not considered. Further, thermo-mechanical processing (TMCP) and Accelerated Cooling (ACC) is used in the present invention to achieve the properties.

Patent No.: EP1954847B1, Patent Granted on: 23-07-2014.

Title: High-strength steel for seamless, weldable steel pipes.

This invention was aimed to provide a high-strength, weldable steel seamless pipe, comprising an alloy steel containing, by weight percent, C 0.03-0.13%, Mn 0.90-1.80%, Si = 0.40%, P = 0.020%, S = 0.005%, Ni 0.10-1.00%, Cr 0.20-1.20%, Mo 0.15-0.80%, Ca = 0.040%, V < 0.10%, Nb 0.02-0.04%, Ti 0.0070-0.020%, N = 0.011%, the balance being Fe and incidental impurities also characterized in that the microstructure of the alloy steel is more than 60% martensite and the yield stress is greater than 750 MPa for subgrains smaller than 1.1 µm and the packets with size smaller than 3 µm reach very low FATT values (< -80°C).

In contrast to this, there is no Mo in the present invention. The YS in present invention is in lower range, i.e. 550-650 MPa and the microstructure is mainly consists of ferrite and bainite.

Patent No.: US2012247605A1, Patent Granted on: 04-10-2012.

Title: Molybdenum-Free, High-Strength, Low-Alloy X80 Steel Plates Formed by Temperature-Controlled Rolling Without Accelerated Cooling.

In this invention, steel alloy, plate, and longitudinally welded pipe formed from a molybdenum-free, high-strength, low-alloy steel, said steel alloy consisting essentially of, in wt. %: C: 0.05-0.09; Mn: 1.70-1.95; Ti: 0.01-0.02; Al: 0.02-0.055; Nb: 0.075-0.1; P: =0.015; S: =0.003; V 0.01-0.03; Mo: =0.003; and the remainder Fe and inevitable impurities. The plate is produced by rolling from a slab without the use of accelerated cooling.

In contrast to this, the present invention does not contain V and it contains sufficiently Nb low content. Most importantly. thermo-mechanical processing (TMCP) and Accelerated Cooling (ACC) is used in the present invention to achieve the properties with lean chemistry thereby making it cost effective.

Patent No.: CN1265711A, Patent Granted on: 10-07-2002

Title: Ultra-high strength, weldable, essentially boron-free steels with superior toughness.
An ultra-high strength essentially boron-free steel having a tensile strength of at least about 900 MPa (130 Ksi), a toughness as measured by Charpy V-notch impact test at -40 DEG C (-40 DEG F) of at least about 120 joules (90ft-lbs), and a microstructure comprising predominantly fine-grained lower bainite, fine-grained lath martensite, or mixtures thereof, transformed from substantially unrecrystallized austenite grains and comprising iron and specified weight percentages of the additives: carbon, silicon, manganese, copper, nickel, niobium, vanadium, molybdenum, chromium, titanium, aluminum, calcium, rare earth metals, and magnesium, is prepared by heating a steel slab to a suitable temperature; reducing the slab to form plate in one or more hot rolling (10) passes in a first temperature range in which austenite recrystallizes; further reducing said plate in one or more hot rolling (10) passes in a second temperature range below said first temperature range and above the temperature at which austenite begins to transform to ferrite during cooling; quenching (12) said plate to a suitable quench stop temperature (16); and stopping said quenching and allowing said plate to air cool (18) to ambient temperature. Another purpose of the present invention is to produce as the low-alloy of line pipe mother metal, the steel plate of super strength, the yield strength of described steel plate is at least about 690MPa (100Ksi), and tensile strength is at least about 900MPa (130Ksi), and has low temperature, promptly be low to moderate the still sufficient toughness in application scenario of-40 ? (40) approximately, simultaneously, quality product remains unchanged, and, during the thermal cycling that welding causes, the loss of strength minimum at place, HAZ district. Chemistry: About 0.03-0.10%C, About 1.6-2.1%Mn, About 0.01-0.10%Nb, About 0.01-0.10%V, About 0.3-0.6%Mo, and About 0.005-0.03%Ti, And it is further characterized in that: 0.5=Ceq=about 0.7, Pcm=about 0.35.
In contrast to this, the present invention does not contain V, does not contain Mo and the YS is marginally lower. The present invention is cost effective

The innovation of present patent application is directed to designing of process parameters with such % of reduction in each pass during roughing and finishing, especially during last three finish pass along with other parameters such as cooling rate, cooling stop temperature etc resulting in high strength as well as high toughness of the steel plate.

OBJECTS OF THE INVENTION

The basic object of the present invention is directed to provide a high strength steel plate having 550-650 MPa yield strength which is obtained of low carbon steel composition and a process for its manufacture through BOF-LHF-RHOB-CC route.

A further object of the present invention is directed to provide said high strength steel plate which is selectively processed with selected reduction in each pass of roughing and finishing stages in combination with controlled cooling steps to favour high strength as well as high toughness of the steel plate.

A still further object of the present invention is directed to provide said high strength steel plate wherein accelerated cooling after finish rolling results in formation of ferrite bainite or acicular ferrite or a combination of these phases leading to high strength and toughness.

A still further object of the present invention is directed to provide said high strength steel plate wherein method of rolling in non-recrystallization region of the steel plates comprises of air cooling the plate to 70 0C - 170 0C temperature below the calculated no recrystallization temperature (Tnr) followed by rolling the plate in 4 – 6 reduction passes with a finish rolling temperature at 810 - 8600C to obtain a microstructure favouring high strength with adequate toughness.

SUMMARY OF THE INVENTION

The basic aspect of the present invention is directed to a high strength steel plate having 550-650 MPa yield strength which is obtained of low carbon steel composition in wt% comprising:
C: 0.04 - 0.11;
Mn: 1.20 – 1.80;
Cr: 0.40 max.;
Si: 0.10 – 0.70;
S: 0.01 max.;
Al: 0.01 – 0.10;
P: 0.05 max.;
Ti: 0.005-0.04;
Nb: 0.01-0.07; and rest is iron,
A further aspect of the present invention is directed to said high strength steel plate which is obtained through BOF-LHF-RHOB-CC route of steel making and continuous casting.

A still further aspect of the present invention is directed to said high strength steel plate comprising of microstructure of high strength steel plate comprising of fine acicular ferrite with a grain size 2-6 µm along with dispersed fine martensite austenite M/A islands/constituents.

A still further aspect of the present invention is directed to said high strength steel plate having ultimate tensile strength in the range of 670 to 750 MPa preferably in excess of 600MPa, Elongation in the range of 30 to 45% preferably in excess of 30% and Charpy impact energy at 0 oC in the range of 200 to 270 J preferably in excess of 200J.

Another aspect of the present invention is directed to a process for manufacture of high strength steel plate having 550-650 MPa yield strength as described above comprising :
method of rolling steel slabs in recrystallization region comprising of reheating of slabs in the reheating furnace at 1200-1250 0C, discharging at 1180-1220 0C and rolling starting at 1080-1150 0C and controlled hot rolling for maintaining a high initial temperature during roughing pass above no recrystallization temperature where recrystallization is completed before subsequent deformation steps.

A further aspect of the present invention is directed to said method comprising carrying out said rolling steel slabs in recrystallization region involving of reheating of slabs in the reheating furnace at 1200-1250 0C, discharging at 1180-1220 0C and rolling starting at 1080-1150 0C with one sizing pass, 2 - 3 broad sizing pass while roughing rolling consisted of 3 – 5 passes.

A still further aspect of the present invention is directed to said method comprising carrying out reduction in each sizing, broad sizing of upto 20%, and in roughing passes upto 35 % such that at the end of this rolling operation, total reduction to be kept in the range of 70-80% with number of reduction steps upto 9 as per thickness.

A still further aspect of the present invention is directed to said method wherein the intermediate thickness of plate after roughing rolling of the plate as 2.8 – 4.0 times of final thickness of the plate with the roughing exit temperature 25 0C above the calculated no recrystallization temperature (Tnr).
Another aspect of the present invention is directed to said method wherein method of rolling in non-recrystallization region of the steel plates comprises of air cooling the plate to 70 0C - 170 0C temperature below the calculated no recrystallization temperature (Tnr) followed by rolling the plate in 4 – 6 reduction passes with a finish rolling temperature at 810 - 8600C and wherein the cumulative reduction in the last three finishing passes was >50% with a total reduction of atleast 65% at the end of this finish rolling operation.

Yet another aspect of the present invention is directed to said method comprising of accelerated cooling of the plate after finish rolling with an entry temperature to ACC box 20 - 40 0C above the calculated Ar3 temperature, cooling rate at up to 20 0C/s and the cooling stop temperature at 500 – 580 0C followed by air cooling in cooling bed according to final plate thickness such as to produce the steel plate having microstructure of the high strength steel plates comprising of fine acicular ferrite with a grain size 2-6 µm along with dispersed fine martensite austenite M/A islands/constituents and mechanical properties of the high strength steel plates comprising of yield strength in excess of 500 MPa in the range of 550 to 650 MPa , having ultimate tensile strength in the range of 670 to 750 MPa preferably in excess of 600MPa , Elongation in the range of 30 to 45% preferably in excess of 30% and Charpy impact energy at 0 oC in the range of 200 to 270 J preferably in excess of 200 J.

The above and other objects and advantages of the present invention are described hereunder in greater details with reference to the accompanying non limiting illustrative drawings.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1: Optical micrographs of plate nos. (a) A71, (b) B72, and (c) C74.
Figure 2: Scanning electron micrographs of plate nos. (a) A71, (b) B72, and (c) C74.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE ACCOMPANYING DRAWINGS

The present invention is directed to provide a high strength steel plate having 550-650 MPa yield strength combined with adequate toughness obtained using lean chemistry and a process for its manufacture through BOF-LHF-RHOB-CC route.

The composition, processing and product characteristics achieved by way of present invention are illustrated by the following example:

EXAMPLE: I
Heat making was carried out through BOF-LHF-RHOB-CC route. The chemical composition is provided in Table 1.
Table1: Chemical composition of experimental steels (in wt. %)
C Mn S P Si Al Nb Cr Ti
0.06 1.47 0.004 0.014 0.287 0.034 0.042 0.17 0.015

No recrystallization temperature calculated for this steel composition was 976 0C using Borrato equation.
The thermo-mechanical processing conditions followed during rolling in a high power single stand 4-high rolling mill are provided in Table 2. The 250 mm thick slab was hot rolled into 17.8 mm thick plates. Discharge temperature was in the range of 1192-1210 oC while rolling start temperature was in the range of 1093 – 1105 0C. The roughing exit temperature was in the range of 976 - 983 0C and finish entry temperature was in the range of 804-827 0C. The steel was cooled below non-recrystallization temperature and subsequently rolled in non-recrystallization region. The reduction in last finishing pass was kept > 18% and cumulative reduction >50% was ensured in the last three finishing passes in all cases with finish rolling temperature was in the range of 826-8600C. After that, the steel was cooled with water-jet inside accelerated cooling (ACC) box. The cooling rate during accelerated cooling (ACC) was varied between 14.0 to 16.0 0C/s and cooled up to 500-5600C followed by air cooling. The important processing parameters are provided in Table 2. The room temperature tensile properties and Charpy impact energy at 00C were evaluated using standard tensile and Charpy specimens. Samples were also examined using optical and scanning electron microscopy.

Table 2: Thermo-chemical processing condition
Plate No. % Reduction in last Roughing Intermediate Thickness (mm) Holding Time (s) % Reduction in last finishing FRT
(°C) Cooling rate (°C/) Temp after ACC (°C)
A71 29.8 57.44 172 21.1 826 14.6 560
B72 30.1 57.53 186 22.2 832 15.8 560
C74 32.4 51.40 166 18.8 828 14.0 554

RESULTS:
The optical and scanning electron micrographs of all the three steels are provided in Figure 1 and 2, respectively. Both optical and SEM micrographs of all three steels in Figure 1 reveal non equiaxed grains of irregular shape and chaotic arrangement, called as acicular ferrite (AF). In addition to this, the fine martensite/retained austenite (M/A) constituents are also found within the matrix, which are dark under optical microscope while white under SEM observation. The prior austenite grain boundary was eliminated as the nucleation of AF was mainly on dislocations, inclusions such as complex oxides or sulphides and the growing austenite/ferrite interface. The M/A islands which were formed mostly at the grain boundaries of the acicular ferrite. The achieved grain size was 2-6 µm.
The mechanical properties all three steels are provided in Table 3. It shows high yield strength (YS) and high ultimate tensile strength of all steels. The % elongation is more than 30% in each case and also Charpy impact energy at 00C is more than 236-250 J.
Table 3: Mechanical Properties of thermo-mechanically process plates

Plate No. YS
(MPa) UTS
(MPa) YS/UTS % El Charpy Impact Value
(0°C)
A71 594 726 0.81 36 248
B72 599 685 0.87 32 250
C74 589 738 0.79 40 236

It is thus possible by way of the present invention to provide 550-650 MPa yield strength steel plate having adequate toughness using lean chemistry and a process for its production through BOF-LHF-RHOB-CC route of steel making that shows high yield strength (YS) and high ultimate tensile strength of all steels. The % elongation is more than 30% and also Charpy impact energy at 00C is in the range of 236-250J.