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 :
FINE GRAINED COLD HEADING QUALITY STEEL WITH IMPROVED COLD FORMING ABILITY AND MANUFATURING METHOD THEREOF.



2 APPLICANT (S)

Name : JSW STEEL LIMITED.

Nationality : An Indian Company.

Address : Salem Works, Pottaneri P.O., Mecheri, Mettur Taluk, Salem District- 636453, Tamil Nadu, India;

Having the Regd. Office at:
JSW CENTRE, BANDRA KURLA COMPLEX, BANDRA(EAST), MUMBAI-400051, MAHARASHTRA, INDIA.



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 INVENTION
The present invention relates to uniform and fine grained carbon steel for cold heading application and a process for manufacturing the said steel. More particularly, the present invention is directed to the alloy design that restricts the growth of the grains during the rolling operation that gives very high cold forming/heading ability to the steel.
BACKGROUND OF THE INVENTION
Steels for cold heading applications range from low carbon rimming qualities up to heat treatable alloy grades where fasteners dominate the output of the cold heading and forging operations. The term fastener is usually restricted to bolts, nuts, screws and rivets but modern developments include special purpose fasteners such as self-tapping screws with integral drill ends and break bolts. The present strategy in the fastener manufacturing has the replacement of machining or traditional hot forging and machining operations by cold forging. It requires an optimization of the steel composition and processing route to give the required ductility during the forming operations.
The performance of the CHQ (Cold Heading Quality) steel is determined by the formability of the steel under the uniaxial compression load and generally, it is defined by means of upset ability or cold heading ability. The term upset ability defines that the ability of the material to cold deform under compression load without formation of any defect. Generally, the cold heading steel in the hot-rolled condition is expected to have minimum of 66% of upsetability. Further the steel is annealed to make the matrix softer to obtain the cold forming ability of more than 80% during the downstream manufacturing process and it increases the lead time as well as cost of manufacturing the same. The present invention directs to the development of cold heading steel having the cold forming efficiency of more than 80% in the as hot rolled condition itself.
PRIOR ART
1. The patent no. CN102560253B describes a method for producing uniform high-quality cold heading steel with specific structural properties. This method involves controlled rolling and cooling processes for Cold Heading Steel Wire. The process utilizes recrystallization during the final stage of hot rolling to refine the original austenite grains. By implementing precise cooling techniques, bainite becomes the primary structure, leading to uniform wire rods. This uniformity reduces issues like Cold Heading Crack during processing. The chemical composition of the steel includes percentages by weight: C (0.25% - 0.45%), Mn (0.45% - 0.90%), Si (0.05% - 0.40%), Cr (0.70% - 1.50%), Mo (0.03% - 0.50%), and Al (0.015% - 0.045%). The steel undergoes controlled rolling and cooling, with specific laying head temperature of 820 °C and cooling rates in conveyor 0.35 °C/s to 6.7 °C/s. The resulting steel exhibits enhanced mechanical properties of UTS 955 MPa and %Elongation 16% suitable for cold heading processes. This method improves the quality and efficiency of cold heading steel production, reducing processing issues and increasing economic benefits. The method is applicable to a range of steel specifications (Φ 5.5 - 26mm).
2. The patent CN101812644A presents a non-quenched cold heading steel and its manufacturing method for producing high-strength fasteners (e.g., 8.8 grade). Traditional fastener production involves energy-intensive processes like annealing and modification treatments. The proposed steel eliminates these steps, simplifying production to "wire drawing → cold heading → baking (or bluing) treatment." The steel's composition includes C (0.15-0.35%), Si (≤0.30%), Mn (0.80-1.80%), Cr (0.20-0.80%), Al (0.01-0.10%), P (≤0.035%), and S (≤0.035%). The steel exhibits moderate strength, good plasticity, and low cold-heading deformation resistance during fastener cold upsetting. This energy-efficient method reduces costs, environmental impact and production complexity
3. The patent CN103966411B invention describes a method for manufacturing medium carbon cold heading steel Bar Wire Products. The method involves using processes like zerolling, immediate air cooling after rolling, heat pre-treatment, and spheroidizing heat treatment. The steps include: Heating and Rolling: Medium carbon cold heading steel billet is heated for 1-2 hours at 950-1050°C and then rolled multiple times to achieve a finishing temperature of 700-750°C with a total reduction ratio >=75%. Air Cooling: The rolled product is rapidly air-cooled to 450-550°C with a cooling rate >=0.5°C/s. Sensing Heat Pre-treatment: The cooled product is quickly heated to 730-750°C at a rate of 5-15°C/s, held for 2-3 minutes, using induction heating. Spheroidizing Heat Treatment: The product undergoes spheroidizing heat treatment by first heating to 760-780°C and holding for 60-80 minutes, then cooling to 680-710°C and holding for 80-120 minutes, followed by air cooling. The resulting medium carbon cold heading steel Bar Wire Product has a composition of C: 0.32-0.38%, Si: 0.10-0.35%, Mn: 0.60-0.90%, P≤0.030%, S≤0.035%, with the remainder being Fe and impurities. The process significantly reduces spheroidizing time, improves efficiency, and produces a product with similar properties to traditional long-term annealed steel.
4. The patent CN103710609B describes the method of producing high-quality high-strength cold forging steel by adding specific alloying elements and controlling chemical compositions during both steel-making and rolling processes. The steel is produced through a series of steps, including molten iron treatment, converter processing, LF refining, billet continuous casting, and rolling using a milling train unit and controlled cooling on a supply line. Key alloying elements added include Al (≤0.050%), RE (0.005% ~ 0.040%), and Ca (≤0.004%), along with controlled composition of C (0.30% ~ 0.50%), Si (≤0.30%), Mn (0.50% ~ 0.90%), P (≤0.030%), S (≤0.015%), Cr (≤1.20%), and trace elements. LF treatment is carefully managed to introduce alloy and promote inclusion removal, followed by continuous casting and controlled rolling to produce high-quality cold forging steel wire rods. The method results in stable cold heading performance, uniform spherical inclusions, and overall improved steel quality, making it a versatile and straightforward technique.
5. The invention of patent CN116288041A relates to a low-cost bolt steel composition and its production method. The steel composition includes: C (0.10% ~ 0.20%), Si (0.35% ~ 1.40%), Cu (0.10% ~ 0.20%), N (0.0060% ~ 0.0080%), Tb (0.0050% ~ 0.0080%), Ni (<0.10%), P (<0.015%), S (<0.015%), O (<0.0015%), and the remainder being Fe and impurities. To enhance corrosion resistance, a corrosion index WR is determined by percentages of various elements. The steel is processed without traditional annealing or quenching, relying on aging treatment, with controlled grain size and inclusion characteristics. The resulting steel has high strength and plasticity, suitable for bolt production. The method involves refining, casting, rolling, and cooling to achieve the desired properties. The finished bolts exhibit high fatigue life of about > 2 million times 100KN load Times and mechanical strength of 640MPa of Yield strength, 800 MPa of UTS, 12% of elongation and 50% of reduction in area.
6. The invention in the patent JPH0754041A aims to create steel with excellent cold forgeability by performing cold drawing after hot rolling and cooling a slab of specific composition under certain temperature and conditions. The composition includes 0.05-0.25% C, <0.30% Si, 0.50-2.00% Mn, <0.025% P, <0.025% S, <0.0060% N, 0.05-0.30% V, 0.010-0.100% Al, 0.10-0.50% Cu, and 0.005-0.050% Ti. The process involves heating the slab to 1000-1150°C, hot rolling, finishing at 800-900°C, controlled cooling to 500°C at 1-5°C/sec, and cold drawing. The resulting steel eliminates the need for annealing, quenching, and tempering in manufacturing small long-size parts, improving cold forgeability and die life.
7. The patent US3278345A describes a method for producing ultrafine grain size in heat-treatable steels. By rapidly heating the steel just above its AC3 temperature and then quickly deforming it, the austenite grain size is refined. The steel is rapidly cooled to create a martensitic or bainitic microstructure, repeating the process multiple times for desired results. Comparatively tested specimens demonstrated increased strength and ductility, showing the method's effectiveness. For instance, using AISI 4340 steel, specimens processed according to the method achieved higher tensile strength (e.g., 298.7 ksi vs. 268.4 ksi) and elongation (6% vs. 6%) compared to conventionally treated ones. The technique optimizes mechanical properties through controlled heating, deformation, and cooling cycles.
8. The patent EP2199422A1 pertains to a low-carbon bainitic steel suitable for cold headed products without heat treatment. The steel's composition by weight is: 0.04% ≤ C ≤ 0.1%, 1.8% ≤ Mn ≤ 2.0%, 0.15% ≤ Si ≤ 0.30%, S ≤ 0.025%, P ≤ 0.025%, Cr ≤ 0.50%, Mo ≤ 0.08%, Ni+Cu ≥ 0.30%, 0.01% ≤ Al ≤ 0.05%, V ≤ 0.05%, 10 ppm ≤ B ≤ 30 ppm, N ≤ 100 ppm, Ti ≥ 0.06%. The steel possesses a granular bainite microstructure with retained austenite, bainitic ferrite, and TiC nano-particles. It maintains good workability and high strength after cold forming. A conventional hot rolling or thermo-mechanically controlled process can be used to produce the steel, with the latter showing improved impact toughness. Cold drawing increases strength without significant loss in ductility.
9. The patent JPS5591935A pertains to the wire rod or steel bar, is manufactured by carrying out hot rolling of steel material added C: 0.01-0.20%, Si: 0.03-1.20%, Mn: 0.30-2.50%, Al< 0.10%, Ti: 0.005-0.30%, B: 0.0003-0.0050%, Cu: 0.1-0.5% or moreover, Cr: 0.1-0.8% and contained Ni: 0.10-1.0%, if necessary. Finishing temperature is made 1050-850 °C and cooling is carried out at the cooling speed of 1-60 °C/sec. from a temperature of Ar1 transformation point +50 °C or more to 500 °C. Ferrite and pearlite or bainite structure, are formed minutely and strength-ductility balance is improved remarkably. The above material is suitable for the material of high strength bolt for motor car, electrical machinery and appliances, architecture, engineering works or machine parts.
10. This invention CN105296865A pertains to a production method for manufacturing medium carbon chromium-containing cold heading steel. It addresses the need for high-strength cold forging steels, such as 40Cr, used in automotive fastening pieces. The method involves specific chemical composition, casting, heating, rolling, and cooling processes. The steel composition includes C (0.38-0.43%), Si (0.20-0.35%), Mn (0.55-0.75%), P (≤0.025%), S (≤0.015%), Cr (0.80-1.0%), B (0.0010-0.0020%), Ti (0.0060-0.010%), and Fe. The controlled cooling model ensures the desired ferrite+pearlitic structure. The resulting cold heading steel exhibits strengths of 760-850 MPa, elongation of 18-28%, and reduction of area of 60-76%, improving cold-heading distortion and substantially reducing fastening piece cracking rates
References:
1. Li Hengkun, Wu Nianchun, Yin Yuqun, Wang Xiaowen. Manufacturing method of high-quality cold heading steel with uniform structure property, Patent no: CN102560253B, 2014
2. He Yi, Zhou Lei, Chen Shuming, Yang Hao, Gao Lirong, Qi Zhen, Yu Gan, Wang Yifeng, Gongxiong. Non-quenched cold heading steel for high-strength fasteners and manufacturing method thereof, Patent no: CN101812644A, 2010
3. Zhang Aiwen. Manufacturing method for medium-carbon cold forging steel rods and wires, Patent no: CN103966411A, 2016
4. Liu Zhilong, Zhang Xuchang, Zheng Anmin, Wang Chongqing, HaoYuanbin, Li Junqian. High-quality high-strength cold heading steel and production method thereof, Patent no: CN103710609A, 2016
5. Jiang Ting, Wang Kaizhong, Hu Fangzhong, Zhou Dayuan, HaoJie, Tang Peng Zhang, XiaoruiMouZumao, Wang Zhongle. A kind of steel for low-cost bolt, production method and a kind of low cost bolt, Patent no: CN116288041A, 2023
6. Shuichi Fukushi and HeijiHagita. Manufacture of steel for cold forging, Patent no: JPH0754041A, 1995
7. Raymond A Grange and Ronald S Mulhauser. Method of producing fine grained steel, Patent no: US3278345A, 1983
8. Swiss steel AG, Low-carbon precipitation-strengthened steel for cold heading applications, Patent no: EP2199422A1, 2010
9. Naoki Eguchi, Masaki Araki. Preparation of high tension, high ductility wire rod and steel bar for high strength bolt, Patent no: JPS5591935A, 1983
10. Ma Liguo, Guo Xiaobo, GuoDayong, Wang Bingxi, Luo Jianhua, Gao Hang, Zhang Bo, CheAn, Yuan Ye. Production method for medium-carbon chromium-bearing cold heading steel wire rod, Patent no: CN105296865A, 2016

OBJECTS OF THE INVENTION
The basic object of the invention is directed to fine grained cold heading quality(CHQ) steel with utilization of Niobium as a micro-alloying element to enhance the properties of cold heading steels. This includes achieving greater strength and ductility through micro-alloying and improving the upsetability (the material's ability to undergo deformation) in cold heading steels. The invention also encompasses the optimization of the steel production process including continuous casting and hot rolling.
A further object of the present invention is directed to improved upsetability in Cold Heading Boron Steels which is crucial for processes like cold heading where the material is deformed without heating.
The further object of the invention involves the effects of Niobium micro-alloying on grain refinement at various stages of production, including the cast stage, intermediate rolling stage, and the final rolled wire rod stage.
The further object of the invention is exploring the impact of using higher input section billets, specifically up to 220*220 mm², to influence grain refinement. This technique is aimed at reducing the input austenitic grain size before rolling and ultimately affecting the final grain structure in the rolled product.
The further objective of the invention is the addition of Niobium to the higher input billet section to result in a substantial refinement of the grains in the rolled product. This is expected to lead to grain sizes as small as 15µm or less.
The invention also focuses on assessing the impact of Niobium micro-alloying on the yield strength and impact strength. A significant increase in this could indicate improved mechanical properties and material behavior under load.

SUMMARY OF THE INVENTION
The basic aspect of the present invention is directed to a fine grained cold heading quality(CHQ) steel with improved cold forming or upsetting ability having composition by wt.% comprising
carbon :0.19 to 0.21%,
manganese : 0.92 to 0.96%,
silicon : 0.13-0.15%,
sulphur: 0.001 to 0.007%,
phosphorous:0.008 to 0.019%,
aluminium:0.029to 0.039%
niobium: 0.01 to 0.025%,
titanium: 0.030 to 0.036%,
boron: 0.0018 to 0.0022%,
chromium:0.12 to 0.19%,
nitrogen: 0.005 to 0.007%,
and the balance iron;
with said Niobium sourced Niobium carbide pin grain boundaries enabling fine grain structure for enhanced upset ability of wire rod at greater than 80% cold deformation and resistant to cracking during cold forming.

A further aspect of the present invention is directed to said fine grained cold heading quality(CHQ) steel as wire rod coils with the final rolling diameter ranging from 6.5 mm to 28mm having microstructure with fine grain size of 15 µm uniform throughout the cross section of the wire rod coils favouring cold heading free of any crack or burst opening defect.

A still further aspect of the present invention is directed to said fine grained cold heading quality(CHQ) steel souring fine carbides, nitrides and carbo-nitrides of Ti, B and Nb in the steel matrix enabled finer grain size in the range of 10 to 25µmfor an excellent cold forming capability of >80% and the maximum of 88% under the uniaxial compression load.

A still further aspect of the present invention is directed to said fine grained cold heading quality(CHQ) steel having yield strength of 362 to 370MPa, UTS of 520 to 535 MPa, %EL of 33 to 40% and %RA of 63 to 69%, impact strength CVN 15J at25deg C.

A still further aspect of the present invention is directed to said a process for the production of fine grained cold heading quality steel with improved cold forming as described above comprising
i)involving selectively cold heading quality steel composition comprising
carbon : 0.19 to 0.21%,
manganese : 0.92 to 0.96%,
silicon : 0.13 to 0.15%,
sulphur: 0.001 to 0.007%,
phosphorous: 0.008 to 0.019%,
aluminium: 0.029 to 0.039%
niobium : 0.01 to 0.025%,
titanium : 0.030 to 0.036%,
boron : 0.0018 to 0.0022%,
chromium:0.12 to 0.19%,
nitrogen :0.005-0.007%, and the balance iron;
and following cold heading quality steel making;
wherein the said Niobium addition in the cold heading quality steel alongwith the presence of boron-titatnium provided for favouring generation of fine carbides, nitrides and carbo-nitrides of Ti,B and Nb in the steel matrix and control the growth of austenite grains during hot rolling with resultant finer austenite grain size and formation of fines ferrite and pearlite in microstructure and homogeneous precipitation of titanium, niobium and boron with homogeneous grain size throughout cross section of the steel.

A still further aspect of the present invention is directed to said process comprising
(i) providing liquid steel produced through steel making route including Energy Optimizing FurnaceLadle Refining FurnaceVacuum Degassing, having composition comprising
carbon : 0.19 to 0.21%,
manganese : 0.92 to 0.96%,
silicon : 0.13 to 0.15%,
sulphur: 0.001 to 0.007%,
phosphorous: 0.008 to 0.019%,
aluminium:0.029to 0.039%
niobium : 0.01 to 0.025%,
titanium : 0.030 to 0.036%,
boron : 0.0018 to 0.0022%,
chromium:0.12 to 0.19%,
nitrogen : 0.005 to 0.007%,
and the balance iron;
(ii) casting said steel into billets of size 220x220mm2 in continuous caster;
(iii) reheating said billets in reheating furnace with Reheating Furnace Output Temperature of 1150 to 1170°C;
(iv) subjecting the reheated billets to hot rolling following Roughing MillContinuous Mill (Horizontal -Vertical(HV) Mills) Finishing Mill;
(v) passing the rolled wire rod through water boxes where the growth of the refined grains is controlled;
(vi) subjecting the hot rolled wire rod to cooling on cooling conveyor followed by air cooling and coiling.

A still further aspect of the present invention is directed to said process wherein said hot rolling and coiling comprising
Roughing Mill entry temperature: 1100-1140°C;
Continuous mill entry temperature: 950-970 °C;
Finishing mill entry temperature: 930-950 °C;
Coiling Temperature: 870-900 °C.

Another aspect of the present invention is directed to said process wherein said cooling and coiling comprising rolled wire rod is coiled in Laying head in stelmore conveyor at the temperature of 870 °C to 900 °C followed by controlled cooling with the cooling rate of 0.2-0.5 °C/s up to 600 °C, followed by further cooling the wire rod-coil to room temperature in the atmospheric air.

Yet another aspect of the present invention is directed to said process wherein said finer austenite grain size promoting finer ferrite and pearlite in the final microstructure is selectively controlled in the range of 50-55% of ferrite and 45-50% of pearlite and said homogeneous grain size maintained is in the range of 10 to 25µm.

The above and other aspects of the present invention are described hereunder in greater details with reference to accompanying drawings and example.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1: show the Flowchart of the cold heading quality steel manufacturing process.
Figure 2: show the Microstructures at the surface during the processing of 220*220mm2 bloom to 16mm wire rod coil with the different niobium level.
Figure 3: show the Microstructures at the core during the processing of 220*220mm2 bloom to 16mm wire rod coil with the different niobium level.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO ACCOMPANYING DRAWINGS AND EXAMPLES
The present invention relates to uniform and fine grained carbon steel for cold heading application and a manufacturing method thereof. During the steel processing stages, the phenomenon of grain refinement plays a significant role in influencing the microstructure and mechanical properties of the final product. Grain refinement refers to the reduction in the average size of the individual grains in the microstructure. This has the considerable impact on the material's properties such as strength, ductility, toughness and hardness.
In the conventional cold heading quality steel making route consists of Energy Optimization furnace as primary steel making process and composition is achieved in secondary steel making process in Ladle refining furnace of capacity 65MT. The gaseous contents are controlled in vacuum degassing stage followed by casting in continuous casting machine of size 220*220 mm2as shown in Figure 1. In general, cold heading steel have the composition consists of 0.15-0.35% carbon, 0.3-1.80% manganese, < 0.35% silicon, <0.40% chromium, 0.0005-0.0030% boron, < 0.008% nitrogen, 0.03-0.04% titanium, <0.0020% oxygen, <2ppm hydrogen,<0.035% Sulphur,<0.025% phosphorous and the remaining with iron. These solute elements present in the steel provides better strength, ductility, hardenability along with the forge ability.
The cast blooms are reheated in the blast furnace gas (CO/CO2) fired furnace up to 1170 °C. Upon heated above the recrystallization temperature, the blooms are descaled with high pressure water jet of 220 bar pressure. The descaled blooms are rough rolled at the temperature of 1100 °C – 1140 °C. In the rough rolling process, columnar structures are broken down and more equi-axed grains are produced and there is a grain refinement during this stage. The austenitic grain size formed at this stage decides the final microstructure and here in the conventional process the formed austenitic grain size is coarser and non-uniform. Further the rolling is continued in horizontal – vertical mill and the final size of the wire rod coil is achieved in no twist mill (NTM) at the temperature of 930 °C-950 °C. The wire rod passes through the water boxes where the growth of the refined grains is controlled. The rolled wire rod is coiled in Laying head in stelmore conveyor as shown in Figure 1 at the temperature of 870 °C to 900 °C followed by controlled cooling with the cooling rate of 0.2-0.5 °C/s up to 600 °C. Further the wire rod coil is cooled to room temperature in the atmospheric air.
The resultant microstructure has the average grain size of 30µm with few larger grains closer to the surface at random locations. The coarse grain size is observed up to 100 microns in certain regions towards the surface. This results in surface crack when it is subjected to cold heading or upsetting limiting the upset ability to<70%.
Niobium addition in the cold heading quality steels, along with the presence of boron-titanium promotes the fine carbides, nitrides and carbo-nitrides of Ti, B and Nb in the steel matrix and it restricts the growth of the austenite grains during the hot rolling. The resultant austenite grain size is finer and it promotes the formation of finer ferrite and pearlite in the final microstructure. Because of the homogenous precipitation of titanium, niobium and boron, there is a homogenous grain size throughout the cross section of the steel when compared with conventional cold heading quality steels. It improves the cold heading ability of the steel.
Table 1 Processing details of producing cold heading quality steels
Steel Making Route Energy Optimizing FurnaceLadle Refining Furnace& Vacuum DegassingContinuous Casting Machine
Heat Size 65metric tonnes
As cast cross section (mm*mm) 220*220
Rolling Route Reheating FurnaceRoughing MillContinuous Mill (Horizontal Vertical(HV) Mills) Finishing Mill Cooling conveyor
Reheating Furnace Output Temperature (°C) 1150-1170
Roughing Mill entry temperature (°C) 1100-1140
Continuous mill entry temperature (°C) 950-970
Finishing mill entry temperature (°C) 930-950
Coiling Temperature (°C) 870-900

Table 2 Chemical composition of the experimental heats
Heat No. %C %Si %Mn %P %S %Al %Cr %B %N %Ti %Nb
E86305 0.190 0.144 0.960 0.019 0.002 0.036 0.120 0.0018 0.0050 0.030 0.000
A41833 0.205 0.140 0.949 0.012 0.002 0.039 0.162 0.0019 0.0061 0.032 0.012
F03072 0.200 0.130 0.950 0.012 0.003 0.029 0.186 0.0015 0.0060 0.034 0.018
E87575 0.195 0.133 0.925 0.014 0.005 0.038 0.174 0.0022 0.0070 0.036 0.025
A29934 0.206 0.129 0.930 0.008 0.001 0.036 0.160 0.0023 0.0068 0.037 0.000
A30333 0.200 0.137 0.920 0.012 0.007 0.032 0.183 0.0020 0.0070 0.032 0.015

Table 3 Properties of as rolled-wire rod coils of 16mm diameter with the as cast size-220*220mm2
Heat No. %Nb Average
Grain Size (µm) YS
(MPa) UTS (MPa) %RA %El CVN
(J) Upset ability (%)
E86305 0.000 29.17 332 524 57.8 34.0 8.5 68
A41833 0.012 15.50 402 521 62.0 33.0 15.0 80
F03072 0.018 14.30 419 555 63.0 36.0 15.5 88
E87575 0.025 15.20 420 553 62.0 35.6 15.0 85

Experimental trials:
The chemical composition of the experimental heats with and without niobium added cold heading quality steels are given in Table 2. To identify the microstructure, samples are taken in different stages such as in the as cast stage, post roughing mill stage and the final rolling stage and the corresponding microstructures are given in Figure 2 (surface) and Figure 3 (core). The grain size in the intermediate stage of rolling (post roughing mill) is coarser and non-uniform of 45-100 µm in the conventional steel whereas the niobium added steels have finer and uniform grain size of 35-50 µm as shown in Figure 2 and Figure 3. Since the initial grain size observed during rolling is finer and uniform in niobium added steel, the grain size of the final hot rolled steel is also finer when compared with the conventional steel without niobium. The average grain size of the material with different niobium levels are shown in Table 3 and the addition of niobium resulted in decrease of the average grain size from 30µm to 15µm. As shown in Figure 2 and Figure 3, the grain size range of the niobium heats are from 10-25µm whereas the heats which do not have the niobium addition have the grain size ranges from 25-50 µm. The final microstructure of the non-niobium added heats have 55-62% of ferrite and 38-45% of pearlite whereas niobium added heats have 50-55% of ferrite and 45-50% of pearlite.
It resulted in improvement in the upset ability of the wire rod coil from 68% to 88%(i.e., max). Also, there is an improvement in the yield strength (YS) without affecting ductility parameters such as %RA (percentage of reduction in area) and %El (percentage of elongation in length) and there are no significant changes observed in ultimate tensile strength(UTS). But the impact strength of the material is increased from 8.5J to 15J with the addition of niobium.As shown in Table 3, upset ability of greater than 80% can be achieved by the addition of 0.012% in the conventional cold heading quality steel and the maximum upset ability can be achieved with the niobium addition of 0.018%. When adding 0.025%Nb into the conventional cold heading quality steel, there is no significant improvements in the upset ability when compared to 0.018%Nb added steel.
Effect of varying the final rolled product dimension:
Similar experiments were repeated for the different final hot rolling sizes of 28mm and 6.5 mm with the same cast size of 220*220 mm2. The table 4 shows the properties of the 28mm wire rod coil. There is an improvement of Upset (%) in the wire rod coil from 50% to 80%. Also there is an improvement in the yield strength of from 312 MPa to 362 MPa without affecting the ductile property of %RA (reduction in area) and %El (Elongation). Similarly, as shown in the table 5, the upset ability of 6.5mm wire rod coil is improved to 89% from 70% and Yield strength is improved from 345 MPa to 370 MPa with the same %RA and %El.
The improved upset ability is achieved through the refinement in the average grain size of 32 µm to 18 µm and 28µm to 12µm in 28mm and 6.5mm wire rod coils respectively.
Table 4 Properties of as rolled coils of 28mm diameter with the as cast size-220*220mm2
Heat No. %Nb Average Grain Size (µm) YS
(MPa) UTS (MPa) %RA %El Upset ability(%)
E86305 0.000 32.67 312 508 60.3 37.0 50
F03072 0.018 18.45 362 520 63.5 33.0 80

Table 5 Properties of as rolled coils of 6.5mm diameter with the as cast size-220*220mm2
Heat No %Nb Average Grain Size(µm) YS
(MPa) UTS (MPa) %RA %El Upsetability (%)
A29934 0.000 28.45 345 513 70 39 70
A30333 0.015 12.34 370 535 69 40 89

, Claims:We Claim:

1. A fine grained cold heading quality(CHQ) steel with improved cold forming or upsetting ability having composition by wt% comprising
carbon :0.19-0.21%,
manganese : 0.92-0.96%,
silicon : 0.13-0.15%,
sulphur: 0.001 to 0.007%,
phosphorous: 0.008 to 0.019%,
aluminium:0.029to 0.039%
niobium : 0.01-0.025%,
titanium : 0.030-0.036%,
boron : 0.0018-0.0022%,
chromium:0.12 to 0.19%,
nitrogen : 0.005-0.007%,
and the balance iron;
with said Niobium sourced Niobium carbide pin grain boundaries enabling fine grain structure for enhanced upsetability of wire rod at greater than 80% cold deformation and resistant to cracking during cold forming.
2. The fine grained cold heading quality(CHQ) steel as claimed in claim 1 as wire rod coils with the final rolling diameter ranging from 6.5 mm to 28mm having microstructure with fine grain size of 15 µm uniform throughout the cross section of the wire rod coils favouring cold heading free of any crack or burst opening defect.
3.The fine grained cold heading quality(CHQ) steel as claimed in anyone of claims 1 or 2 souring fine carbides, nitrides and carbo-nitrides of Ti, B and Nb in the steel matrix enabled finer grain size in the range of 10 to 25µm for excellent cold forming capability of >80% and the maximum of 88% under the uniaxial compression load.
4. The fine grained cold heading quality(CHQ) steel as claimed in anyone of claims 1 to 3 having yield strength of 362 to 370MPa, UTS of 520 to 535 MPa, %EL of 33 to 40% and %RA of 63 to 69%, impact strength CVN 15J at 25deg C.

5.A process for the production of fine grained cold heading quality steel with improved cold forming as claimed in anyone of claims 1 to 4 comprising
i)involving selectively cold heading quality steel composition comprising
carbon :0.19-0.21%,
manganese : 0.92-0.96%,
silicon : 0.13-0.15%,
sulphur: 0.001 to 0.007%,
phosphorous: 0.008 to 0.019%,
aluminium:0.029to 0.039%
niobium : 0.01-0.025%,
titanium : 0.030-0.036%,
boron : 0.0018-0.0022%,
chromium:0.12 to 0.19%,
nitrogen : 0.005-0.007%,
and the balance iron;
and following cold heading quality steel making;
wherein the said Niobium addition in the cold heading quality steel along with the presence of boron-titatnium provided for favouring generation of fine carbides, nitrides and carbo-nitrides of Ti,B and Nb in the steel matrix and control the growth of austenite grains during hot rolling with resultant finer austenite grain size and formation of fines ferrite and perlite in microstructure and homogeneous precipitation of titanium, niobium and boron with homogeneous grain size throughout cross section of the steel.
6. The process as claimed in claim5 comprising
(i)providing liquid steel produced through steel making route including Energy Optimizing FurnaceLadle Refining FurnaceVacuum Degassing, having composition comprising
carbon : 0.19-0.21%,
manganese : 0.92-0.96%,
silicon : 0.13-0.15%,
sulphur: 0.001 to 0.007%,
phosphorous: 0.008 to 0.019%,
aluminium:0.029to 0.039%
niobium : 0.01 to 0.025%,
titanium : 0.030-0.036%,
boron : 0.0018-0.0022%,
chromium:0.12 to 0.19%,
nitrogen : 0.005 to 0.007%,and the balance iron;

(ii) casting said steel into billets of size 220x220mm2 in continuous caster;
(iii) reheating said billets in reheating furnace with Reheating Furnace Output Temperature of 1150 to 1170°C;
(iv) subjecting the reheated billets to hot rolling following Roughing MillContinuous Mill (Horizontal -Vertical(HV) Mills) Finishing Mill;
(v) passing the rolled wire rod through water boxes where the growth of the refined grains is controlled;
(vi) subjecting the hot rolled wire rod to cooling on cooling conveyor followed by air cooling and coiling.
7. The process as claimed in anyone of claims 5 or 6 wherein said hot rolling and coiling comprising
Roughing Mill entry temperature: 1100-1140°C;
Continuous mill entry temperature: 950-970 °C;
Finishing mill entry temperature: 930-950 °C;
Coiling temperature: 870-900 °C.
8. The process as claimed in anyone of claims 5 to 7 wherein said cooling and coiling comprising rolled wire rod is coiled in Laying head in stelmore conveyor at the temperature of 870 °C to 900 °C followed by controlled cooling with the cooling rate of 0.2-0.5 °C/s up to 600 °C, followed by further cooling the wire rod-coil to room temperature in the atmospheric air.
9. The process as claimed in anyone of claims 5 to 8 wherein said finer austenite grain size promoting fines ferrite and pearlite in the final microstructure is selectively controlled in the range of 50-55% of ferrite and 45-50% of pearlite and said homogeneous grain size maintained is in the range of 10 to 25µm.

Dated this the 27th day of September, 2023
Anjan Sen
Of Anjan Sen & Associates
(Applicant’s Agent)
IN/PA-199