FIELD OF THE INVENTION:
The present invention is directed to achieving improved cold reducibility, preferably consistently over 90%, of the hot rolled steel coil and decrease in the mill load during cold reduction. The present invention is directed to adopt control on the composition of hot rolled steel by limiting the constituent elements e.g. C, Mn and N restricted to low levels such as 0.06 %, 0.25% and 80 ppm respectively and selective addition of Boron. The resultant steel further exhibits low strain hardening coefficient, lower flow stress at given strain and reduced level of Strain Aging Index and higher elongation, all contributing to better cold reducibility.
BACKGROUND AND PRIOR ART:
The cold forming reducibility is carried out by various user industries of the hot rolled coils generated by steel plants. Cold reducibility of hot rolled steel is dependent primarily on the properties like yield strength and hardness and also on microstructure and cleanliness of steel. These properties are adversely affected by the amount of C, Mn, S, P and the amount of dissolved Nitrogen in steel. Low hardness value in as rolled condition for the hot rolled products results in less rolling load at the subsequent cold rolling operation. It is also desirable that pick-up of hardness of hot rolled coils during cold rolling should be low to accommodate higher reduction per pass.
Moreover, it requires secondary refining process e.g. hot metal desulphurisation, RH degasser, VAD etc. Higher rolling temperatures helps also in achieving these desired properties. The customers have achieved cold reduction maximum up to 88% with C:Q.06%max. and Mn:0.25max. in hot rolled steel. In order to achieve higher reduction (more than 90%) during cold rolling, the present method adopted the innovative way by adding Boron in present work.
It is well known in the art of steel making that addition of 10 to 30 ppm soluble boron to steel can produce an increased hardenability compared to obtain by Manganese, Chromium or Molybdenum. They possess hardenability equivalent to that of high carbon steels with much higher carbon content or alternatively more expensive low alloy steels. Boron in elemental form segregates at the prior austenite grain boundaries suppressing the ferrite reaction and thus improves hardenability. To be effective, B must not be in solid solution in Austenite. Boron contents above 30 ppm lead to loss in hardenability, and in excess of about 40 ppm causes loss in toughness through precipitation of Fe2B in austenite grain
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boundaries. Medium Carbon steels with addition of Boron gives excellent hardenability, and is best suited for applications like shovels, blades, agricultural knives/hand tools etc. However, effect of B in low carbon steel, when it is not in solution, is not well understood.
There was therefore the need for developing a variety of hot rolled steel that would increase the cold reducibility with low strain aging index and larger percentage elongation, for various cold reducing applications by the end users, by selective and effective amount of Boron addition in steels depending on the Nitrogen content of steel. The present invention is directed to a method of producing a low carbon (C:0.06%max, Mn:0.25max) hot strips for variety of Al killed steel, hot rolled into thicknesses from 2.5 to 3.1mm for cold reducing applications with the effective boron additions.
THE OBJECTS OF THE INVENTIONS:
The basic object of the present invention is thus directed to the invention of a variety of low carbon Al killed hot rolled steel strip coils with effective Boron addition for achieving the selective properties of high degree of cold reducibility of above 90%, consistently with decrease in the mill load during cold reduction.
A further object of the present invention is directed to find the effective proportion of Boron addition from the relation derived on the basis of Carbon, Manganese and Nitrogen content in the basic steel composition, the preferred variety being carbon 0.06% max. and Manganese 0.25% max. and the limit of Nitrogen maintained in the range of 40-80 ppm and preferably around or less than 40ppm.
A further object of the present invention is directed to find the effective proportion of Boron addition in Al killed steel are hot rolled into different thickness ranges by following standard practice of finishing and coiling .
A still further object of the present invention is directed to find the effective proportion of Boron addition in Al killed steel are hot rolled to strips with the starting material for the hot rolling having a selective chemical composition to achieve said improved cold formability over 90%.
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A further object of the present invention is directed to find the effective proportion of Boron addition in Al killed steel hot rolled to strips such as to result in softer hot rolled product with higher plasticity and not for increasing hardenability as conventionally used.
A yet further object of the present invention is directed to find the effective proportion of Boron addition in Al killed steel which are hot rolled to thin strips for improved cold reduction over 30 % per roll pass and a total reduction over 90% in seven passes, is associated with further property improvements e.g. low strain hardening coefficient along with high elongation value, lower flow stress per unit strain rate, no overloading of mill as less pick up of hardness, lower as rolled hardness and reduced level of Strain aging Index.
SUMMARY OF THE INVENTION:
The basic aspect of the present invention is directed to a low carbon ( 0.06% max.) Al killed steel with Boron comprising high degree of cold reducibility > 90 % properties and low strain hardening coefficient in the range of 0.20 to 0.22 alongwith high elongation in the range of 43% to 45%.
A further aspect of the present invention is directed to a Low carbon (0.06% max.) Al killed steel with Boron comprising:
lower flow stress at given strain such as 310 Mpa at 10 % strain;
higher % cold reduction (> 30%) per pass; lower hardness in the range of 44 to 48 HRB;and reduced level of SAI 10 to 12%.
A further aspect of the present invention is directed to a low carbon (0.06% max.) Al killed steel with Boron having properties, comprising:
YS, MPa in the range of 243-250; UTS, Mpa in the range of 328-335;
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% Elongation in the range of 43-45;
Strain hardening Coefficient : 0.20-0.22;
Hardness,HRB in the range of 44-48;
Strain Ageing Index % in the range of 10-12; and
Grain Size in the range of 19-20.
Wherein said Strain Ageing Index in the range of 10-12% obtained is the property also responsible for achieving cold reducibility of the hot rolled steel coils exceeding 90% without mill overloading, at the customers'/users' end.
A still further aspect of the present invention is directed to a Low carbon (0.06% max.) Al
killed steel with Boron comprising a chemical composition of:
Cmax = 0.06;
Mn max = 0.25;
Si max = 0.04;
S max = 0.025
P max = 0.025
B in ppm = 15-30;
Al min = 0.02
N in ppm = 40-80.
A still further aspect of the present invention is directed to a low carbon (0.06% max.) Al killed steel with Boron adapted to achieve higher percentage of cold reducibility varying from 92 to 93.4% in 7 passes without mill overloading.
A still further aspect of the present invention is directed to a process for manufacture of said low carbon (0.06% max.) Al killed steel with Boron comprising,
following procedure for producing low carbon (C: 0.06% max, Mn :0.25 max) hot strips for variety of cold reducing applications incorporating effective proportion of Boron in Al killed steel whereby Boron was allowed to combine with oxygen and nitrogen in steel and thereby reduce dissolved C and N in ferrite and hot rolling to different thicknesses from 2.5 to 3.1
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mm following standard finishing and coiling conditions to thereby provide softer hot rolled product with higher plasticity.
A yet further aspect of the present invention is directed to a process for manufacture of Low carbon ( 0.06% max.) Al killed steel with Boron as claimed in claim 6 comprising adding boron depending on the nitrogen content of steel in amounts of 15-30 ppm. Effective proportion of Boron addition is calculated from the relation:
Beff = BTotal -[(N-0.002)-Ti/5]
Wherein the value of 0.002 is taken to be the part of the nitrogen contents that is always bound to the aluminium, silicon and vanadium.
A still further aspect of the present invention is directed to said hot rolled low carbon Al killed variety is obtained in the form of strip/coils with thickness range of 2.5mm to 3.1mm, by selective Boron addition in steel of selective chemistry, following standard practice of finishing roll at 880°C and coiling of the strip/sheet at 680°C
A further aspect of the present invention is directed to a process for achieving improved cold reducibility of more than 90%, by selectively using favoured composition of the starting material of said low carbon Al killed steel for boron addition and for subsequent hot rolling given as follows:

C max. Mn max Si max S max. P max. B in ppm Al min N in ppm
0.06 0.25 0.04 0.025 0.025 15-30 0.02 40-80
According to a further aspect of the present invention directed to a process for achieving higher cold reducibility of more than 90% in steel by selective boron addition, is further associated with improvement in the mechanical/microstructural properties such as, (a) low strain hardening coefficient('n' value of 0.20) alongwith high elongation ranging around 47%, (b) lower flow stress at given strain, (c) the percent cold reduction per pass is over 30%, (d)lower hardness range of 44-48HRB, (e) reduced level of Strain Aging Index in the range of 10-12%,(f) higher grain size of 19-20 micron.
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A further aspect of the present invention is directed to achieving improved cold reducibility of more than 90% in hot rolled low carbon Al killed steel, associated with said other allied properties of the hot rolled low carbon Al killed steel strip coils, eliminate overloading of the mill during cold reduction in successive passes.
According to a preferred aspect of the present invention, the properties achieved in the steel following the process of the present invention, the cold reducibility obtained varied from 92 to 93.4%, in seven passes without overloading the mill, as compared to 85-88% obtained in conventional process without Boron addition.
According to a further aspect of the present invention directed to achieving improved cold reducibility of more than 90% in low carbon Al killed steels, the process followed with starting materials having higher 'N' of around 75 ppm and 'C' content of 0.06%, also could undergo cold reduction in excess of 90%.
According to a still further aspect of the present invention is directed to achieving improved cold reducibility of more than 90%, the plot of stress versus strain revealed that a strain rate of 10%, the stress is 310 Mpa for WTCR with boron, while that of conventional steel is 330Mpa, favoring said enhanced cold reducibility. Also at any given strain, rate of increase in stress per unit strain is more in steel without boron, and thus facilitating further cold formation by the end users of the variety of steel produced.
According to a still further aspect of the present invention is directed to achieving improved cold reducibility/formability of more than 90%, obtained by hot rolling of low carbon Al killed steel having selective composition with boron addition, the mechanical properties and microstructure obtained to favour said desired cold formability is given as follows: YS: 243-250 Mpa, UTS: 328-335Mpa,% Elongation: 43-45, Strain Hardening Co-efficient: 0.20, Hardness,HRB:44-48, Strain Aging Index(SAI): 10-12%, Grain Size(#m): 19-20.
The said properties achieved in conventional hot rolling of low carbon steel strip without boron addition is however at a higher range as follows, providing a maximum cold formability of 85-88 % conventionally, as given below:
YS: 260-265 Mpa, UTS: 357-362 Mpa, % Elongation: 40-42, Strain Hardening Co-efficient: 0.24, Hardness(HRB): 47-54, Strain Aging Index(SAI): 16-20%, Grain Size (urn): 16-18
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The present invention and its objectives and advantages are described in greater details with reference to accompanying figures and non-limiting illustrative examples.
BRIEF DESCRIPTION OF FIGURES:
Figure 1: Stress Vs Strain plot up to necking for WTCR, with and without 'B' addition that highlight the achievement mechanical properties, particularly the lower flow stress (310 Mpa) for a given strain for the low carbon Al killed hot rolled steel with boron addition, according to the present invention, favoring higher cold formability exceeding 90%.
DETAILED DESCRIPTION WITH REFERENCE TO ACCOMPANYING FIGURE:
Cold reducibility of hot rolled steel Coils is primarily dependent on yield strength, hardness, microstructure and cleanliness of steel. These properties are adversely affected by amount present in wt. percent of C, Mn, S, P and dissolved nitrogen in steel in ppm. Low hardness value in as rolled condition results in lesser load during cold rolling. It is also desirable that pick up of hardness of hot rolled coils, during cold rolling should be low to accommodate higher reduction(>30%) per pass. For achieving these properties in the hot rolled steel and consistently getting cold reducibility of more than 90%, C, Mn, and nitrogen are restricted to low levels such as preferably less than 0.02%, 0.15% and 40ppm, respectively. Available S and P should also be maintained as low as possible. Moreover, it requires secondary refining process e.g. hot metal desulphurisation, RH degasser, VAD etc. Higher rolling temperatures helps also in achieving these desired properties. The customers/end users have achieved cold reduction maximum up to 88% with C:0.06%max. and Mn:0.25max. in hot rolled steel. In order to achieve higher reduction (more than 90%) during cold rolling, the present method adopted the innovative way by adding Boron in the present invention.
It is well known in the art of steel making that addition of 10 to 30 ppm soluble boron to steel can produce an increased hardenability compared to obtain by Manganese, Chromium or Molybdenum. They possess hardenability equivalent to that of high carbon steels with much higher carbon content or alternatively more expensive low alloy steels. Boron in elemental form segregates at the prior austenite grain boundaries suppressing the ferrite
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reaction and thus improves hardenability. To be effective, B must not be in solid solution in Austenite. Boron contents above 30 ppm lead to loss in hardenability, and in excess of about 40 ppm causes loss in toughness through precipitation of Fe2B in austenite grain boundaries. Medium Carbon steels with addition of Boron gives excellent hardenability, and is best suited for applications like shovels, blades, agricultural knives/hand tools etc.
This is thus possible to obtain a variety of hot rolled steel that would increase the cold reducibility with low strain aging index and larger percentage elongation, for various cold reducing applications by the end users, by selective and effective amount of Boron addition in steels depending on the Nitrogen content of steel. The present invention is directed to a method of producing a low carbon (C:0.06%max, Mn:0.25max) hot strips for variety of Al killed steel, hot rolled into thicknesses from 2.5 to 3.1mm for cold reducing applications with the effective boron additions. Said selective addition of Boron preferably in the amount decided by the presence of Carbon, Manganese and in particular the presence of Nitrogen;
effective amount of said boron addition is given by the relation Beff = BTotal - [(N-0.002)-Ti/5]
Wherein the value of 0.002 is taken to be the part of the nitrogen contents that is always bound to the aluminium, silicon and vanadium.
Boron reacts with Oxygen to form B2O3, with Nitrogen to form boron nitride BN and with Carbon to form iron borocarbide[Fe23(CB)6] and iron borocementite[Fe3(CB)]. The basic idea behind this work was that boron was allowed just to combine with Oxygen and Nitrogen in steel, thereby reducing dissolved 'C' and 'N' in ferrite and results in softer hot rolled product with higher plasticity, but not for the conventional purpose of increasing hardenability.
The present invention is directed to a process for steel making and hot rolling with selective Boron addition for achieving higher cold reducibility and related mechanical properties and microstructure, wherein boron was added to the common variety of Al killed low carbon variety of hot rolled strip production in effective proportion such that it was reduced in successive rolling passes to a range of thicknesses 2.5mm to 3.1mm, by adhering to standard practice of finishing at 880°C and coiling at a temperature of 680°C.
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According to a preferred example of the manner of accomplishment of the object property of cold formability of more than 90%, the basic steel composition of the starting steel section prior to hot rolling has been selectively obtained with boron addition, as follows:

C max. Mn max Si max S max. P max. B in ppm Al min N in ppm
0.06 0.25 0.04 0.025 0.025 15-30 0.02 40-80
The addition of Boron was decided on the basis of presence of Nitrogen in the steel and calculated by the relation for effective Boron as already described.
Reference is now invited to the Figure 1 that illustrates the stress vs strain plot up to necking for the desired properties achieved in low carbon Al killed steel with selective Boron addition according to present invention so that cold formability of over 90% could be consistently ensured. The mechanical and microstructural properties of the steel with boron addition achieved according to the present invention is presented in the following Table 1,

Properties Low carbon Al killed steel with Born addition. Low carbon Al killed Steel without Boron addition.
YS, Mpa 243-250 260-265
UTS, Mpa 328-335 357-362
% Elongation 43-45 40-42
Strain Hardening Coefficient 0.20-0.22 0.24-0.26
Hardness, HRB 44-48 47-54
Strain Aging Index(SAI),% 10-12 16-20
Grain Size, mm 19-20 16-18
Properties of newly developed steel showed improvement with respect to lower YS, UTS, strain hardening coefficient, hardness, strain aging index (SAI), larger grain size and higher percent elongation as distinctive contribution due to selective boron addition. Such a combination of properties, particularly with reference to lower work hardening exponent ('n' Value) (preferably show the complete relation where the exponent 'n' appear) and higher % elongation of steel treated with 'B' is considered to be unique in features reported for the first time. This has been a prime responsible factor in obtaining higher percentage of cold reducibility, varying from 92% to 93.4%, in seven roll passes without overloading the mill. In comparison the low carbon steel without boron, could undergo cold reducibility to the
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extent of 85 to 88%. Further trials conducted even with higher Nitrogen content in steel up to 75ppm and Carbon content of 0.06% also could undergo cold formability in excess of 90%. The flow stress for corresponding to 10% strain for Al killed low carbon steel without boron addition, as illustrated in said figure 1 is found to be at higher level, though both the steel varieties follow the Hollomon equation (please refer/mention the equation if relevant, at proper place.) The said flow stress is 310Mpa for WTCR for steel with boron, while that of conventional steel is 330 Mpa. Also at a given strain, rate of increase in stress per unit strain is more in steel without boron. This is the reason that the load on mill during cold reduction at the customers'/users' end remained within acceptable limits in spite of higher reduction of over 30% per pass in successive hot rolling passes.
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WE CLAIM:
1. Low carbon ( 0.06% max.) Al killed steel with Boron comprising high degree of cold
reducibility > 90 % properties and low strain hardening coefficient in the range of 0.20 to
0.22 alongwith high elongation in the range of 0.43% to 0.45%.
2. Low carbon ( 0.06% max.) Al killed steel with Boron as claimed in claim 1 comprising:
lower flow stress at given strain such as 310 Mpa at 10% strain; higher % cold reduction (> 30%) per pass; lower hardness in the range of 44 to 48 HRB;and reduced level of SAI 10 to 12%.
3. Low carbon ( 0.06% max.) Al killed steel with Boron comprising:
YS, MPa in the range of 243-250;
UTS, Mpa in the range of 328-335;
% Elongation in the range of 43-45;
Strain hardening Coefficient : 0.20-0.22;
Hardness,HRB in the range of 44-48;
Strain Ageing Index % in the range of 10-12; and
Grain Size in the range of 19-20 micron.
4. Low carbon ( 0.06% max.) Al killed steel with Boron as claimed in anyone of claims 1 to 3
comprising a chemical composition of :
Cmax = 0.06; P max = 0.025
Mn max = 0.25; B in ppm = 15-30;
Si max = 0.04; Al min = 0.02
S max = 0.025 N in ppm = 40-80.
5. Low carbon (0.06% max.) Al killed steel with Boron as claimed in anyone of claims 1 to 4 adapted
to achieve higher percentage of cold reducibility varying from 92 to 93.4% in 7 passes without mill
overloading.
6. A process for manufacture of Low carbon (0.06% max.) Al killed steel with Boron as claimed in
anyone of claims 1 to 5 comprising:
following procedure for producing low carbon (C: 0.06% max, Mn :0.25 max) hot strips for variety of cold reducing applications incorporating Boron in Al killed steel whereby Boron was allowed to combine with oxygen and nitrogen in steel and thereby reduce dissolved C and N in ferrite and hot rolling to different thicknesses from 2.5 to 3.1 mm following
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standard finishing and coiling conditions to thereby provide softer hot rolled product with higher plasticity.
7. A process for manufacture of Low carbon ( 0.06% max.) Al killed steel with Boron as
claimed in claim 6 comprising adding boron depending on the nitrogen content of steel in
amounts of 15-30 ppm. Effective proportion of Boron addition is calculated from the
relation:
Beff = BTotal - [(N-0.002)-Ti/5]
Wherein the value of 0.002 is taken to be the part of the nitrogen contents that is always bound to the aluminium, silicon and vanadium.
8. A process for manufacture of Low carbon ( 0.06% max.) Al killed steel with Boron as
claimed in anyone of claims 6 or 7 wherein the chemical composition of the steel comprises
C max., 0.06 Mn max. 0.25, Si max. 0.04, S max 0.025, Pmax 0.025, B in PPM 15-30, Al
min 0.02 and N in ppm 40 -80.
9. A process for manufacture of Low carbon ( 0.06% max.) Al killed steel with Boron as
claimed in anyone of claims 6 to 8 adapted to favour cold reducibility varying from 92 to
93.4% in 7 passes without overloading the mill.
10. Low carbon ( 0.06% max.) Al killed steel with Boron and its process of manufacture
substantially as hereindescribed and illustrated with reference to the accompanying figures
and examples.

Dated this day 8th December, 2006. Anjan Sen
Of Anjan Sen & Associates (Applicant's agent)
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A low carbon (0.06%max) Al killed steel with Boron for achieving improved cold reducibility, preferably consistently over 90%,with reduced mill load during cold reduction. The composition of hot rolled steel comprise of the constituent elements e.g. C, Mn and N restricted to low levels such as 0.06 %, 0.25% and 80 ppm respectively and also maximum limit on S and P to 0.025%.Al killed low carbon steel is obtained with effective boron around 15-30ppm, added just to combine with Oxygen and Nitrogen in steel,and thereby reducing dissolved 'C' and 'N' in ferrite and results in softer hot rolled sheet products with higher plasticity, but not to increase hardenability, during successive roll passes. Effective Boron must not be in solid solution in Austenite ,calculated based on Nitrogen content. The resultant steel also exhibit low strain hardening coefficient,lower flow stress of around 310 Mpa at given strain of about 10% and reduced level of Strain Aging Index(10-12%)and higher elongation, contributing to better cold reducibility.