CLIAMS:We Claim :
1. Low carbon coil break free hot rolled steel sheets comprising a composition having
C: 0.04- 0.08wt%;
Mn: up to 0.4wt%;
Si: up to 0.03 wt%;
Al < 0.05wt%;
N: < 120 ppm;
Ca: 0-50 ppm;and
Balance is iron with
S & P in amounts of allowable impurities, alongwith selective alloying element Nb: 0.002-0.01wt% for fixing free N and C.

2. Low carbon coil break free hot rolled steel sheets as claimed in claim 1 having YS= 170 MPa, UTS = 290MPa and %El = 33%.
3. Low carbon coil break free hot rolled steel sheets as claimed in anyone of claims 1 or 2, in the form of sheets with thickness ranging from 1.5-8.0 mm.
4. Low carbon coil break free hot rolled steel sheets as claimed in anyone of claims 1 to 3 , comprising carbides and or carbo nitrides precipitates obtained by fixing said free N and C by said Nb.

5. A process for the manufacture of low carbon coil break free hot rolled steel sheets as claimed in anyone of claims 1 to 4 comprising:

(i) providing steel with composition having
C: 0.04- 0.08wt%;
Mn: up to 0.4wt%;
Si: up to 0.03 wt%;
Al < 0.05wt%;
N: < 120 ppm;
Ca: 0-50 ppm;and
Balance is iron with
S & P in amounts of allowable impurities, alongwith selective alloying element Nb: 0.002-0.01wt% for fixing free N and C.
(ii) subjecting the same to casting in thin slab caster with casting speed of 5.0 -6.5 m/min ;
(iii) rolling to sheet /strips in the temperature range of FRT 800-900Deg C; and
(iv) coiling in the temperature range of 550-650 Deg C.

6. A process as claimed in claim 5 comprising
(i) providing the selective low carbon steel having composition comprising C: 0.04- 0.08wt%; Mn: up to 0.4wt%;Si: up to 0.03 wt%;Al < 0.05wt%;Nb: 0.002-0.01wt%;N: < 120 ppm; Ca: 0-50 ppm; and balance is iron with S & P as impurities;
(ii) casting the steel slabs in a thin slab caster with slab thickness ranging from 50-65mm;
(iii) homogenizing said slabs in a tunnel furnace at temperature >10750C;
(iv) removing scales from slabs in a Descaler after tunnel furnace;
(v) controlling the desired thickness of the slab in hot rolling with finish rolling temperature in the range of 800-900 °C;
(vi) cooling the hot rolled strip so obtained in run out table by using laminar cooling and trim zone to get the desire coiling temperature of 550-650 °C followed by cooling of coils in coil yard.

7. A process as claimed in claim 6 wherein said controlled hot rolling comprising 6 stand reductions of slabs in 6 high tandem rolling mill to reduce the thickness to the required level for producing steel sheets of 1.5-8.0 mm thickness.

8. A process as claimed in anyone of claims 5 to 7, wherein said process carried out under controlled operating conditions comprising
(i) The Casting Speed 5.0- 6.5 m/min;
(ii) Slab Thickness 55-65 mm;
(iii) Slab Cutting Temp 980-1050 Deg C;
(iv) Homogenization Temp (Tunnel Furnace) 1080-1150 Deg C;
(v) Homogenization Time 8-15 min;
(vi) Finish Rolling Temp 800-900 Deg C;
(vii) Standwise reduction/time/temp comprising

(viii) Coiling Temp 550-650Deg C.

9. A process as claimed in anyone of claims 5 to 8, wherein said steel sheets are produced through Energy Efficient CSP process route comprising refining hot metal in EAF (Conarc)- adjusting chemistry in Ladle Furnace-Thin Slab Casting-Tunnel Furnace - Hot Strip Mill-Compact Cooing – coiling- slow cooling in coil yard.

Dated this the 29th day of January, 2014
Anjan Sen
Of Anjan Sen & Associates
(Applicants Agent)
,TagSPECI:FIELD OF THE INVENTION

The present invention relates to providing low carbon hot rolled steel grade/composition with minimum yield point phenomenon. More particularly, the present invention is directed to provide low carbon coil break free hot rolled (HR) steel sheet in the thickness range of 1.5-8.0 mm with minimum yield point phenomenon and a process for its production through thin slab caster route. Typical applications of the invented steel grade include direct sheet application in automotive and other general engineering application including machinery, white good application, BIW application etc.

BACKGROUND OF THE INVENTION

The steel grade conforming to the international specification - ISO 3573: 1999 & BIS specification - IS 1079 HR 1 to 3 is mainly used for direct sheet application in automotive and other general engineering purpose including machinery, white good application and BIW(Body in White)application. Such application requires good surface finish for aesthetic look. Plate / Sheets are produced by Cut to length (CTL) operation from coil. This includes unwinding of coil followed by cutting in desired length in CTL unit. There were regular problem of coil break/wrinkle formation on surface of sheets/plates during unwinding in CTL. As coil break is not acceptable for such application, there is high internal diversion as well as high customer complaint. The coil break lines are localized bands of plastic deformation (due to YPE) in metals experiencing tensile stresses and appear perpendicular to the direction of rolling. Conventionally this steel grade is made through only C-Mn chemistry. Various literature pointed out that free C or N in the steel are the main reason for coil break. These free C or N causes high yield point phenomenon and thus leading to coil break. This type of problem mostly occur in low strength material.

The previous research and developments in the field have resulted in several methods for producing coil break free hot rolled steel sheets or methods of eliminating coil break during CTL operation, many of which are discussed below.
It is known that flow lines does not occur in so-called dual-phase steels, such as steels having no distinct yield point (yield point elongation). It is also known that a skin-pass rolling can remove the distinct yield point as well, whereby the occurrence of flow lines is avoided. These measures are as indicated in Materials Science steel, volume 2, pages 97, 86, Springer Verlag, Berlin-Heidelberg-New York-Tokyo, 1985, Publisher steel iron mbH, Dusseldorf.
From a study of the aging behavior of soft steels, on the other hand, known that skin passing on the Elimination of pronounced maxima and minima of the yield strength may not be a permanent measure to prevent Coilbreaks because the definite yield point appears again with increasing storage time at room temperature (see archive for the iron and steel industry , 1955, page 72).
DE 4015248 A1(also published as WO 1990013672 A1 & EP 0435968 B1 ) disclosed a process for the preparation of coilbreak-free steel strip of 2-6 mm in thickness with a yield strength of less than 350 N/mm2 with a time constant yield point elongation without alternating minima and maxima, produced from steel of the following composition in percentage by weight: C: 0.02-0.08%, Si: 0.01-0.40%, Mn: 0.05-1.00%, P: 0.002-0.025% ,S: 0.001-0.015%, Al: 0.015 to 0.080%, N: 0.0080%, Ti: From 0.03 to 0.080%, and balance iron and unavoidable impurities, is cast in the strand, a separated slab of about room temperature, heated to at least 1100 ° C, then at temperatures above the Ar 3 point, finish rolled to hot strip and is finally coiled in the temperature range of 550 to 400 ° C. This prior art also claim that the titanium content of the steel is set to at least the stoichiometrically required to bind the nitrogen and less than the complete setting of the additional amount of carbon.
US 2059993 disclosed a mechanical device of bending and reverse bending by means of bends of relatively small radius, and thus stretch the sheets beyond their normal elastic limit and leave them in a state of what may be termed as cold plasticity. It is found that when sheets have been treated in this manner previous to die forming operations, the objectionable stretcher strain does not develop. Thus this prior art provide a device to preprocess sheet metal to avoid stretcher strain formation during cold forming of thin sheets for making different components. Such strain marks are not acceptable particularly for automotive and other high end application for aesthetic reason. The objective of this invention was to construct a machine adapted to cold-work sheet metal in such a manner that stretcher strains or wormy metal will not develop after forming or die drawing operations have been performed upon the sheets.
US 3079975 disclosed a process and apparatus to prevent coil breaks on thin sheets. To eliminate this defect a flexing machine such as a roller leveler has been employed wherein a sheet metal is made to pass between flexing rolls and elastic covered rotary metal drum. By providing the gauge of the treated strip not too light, the use of a roller leveler may be effective in the production of commercially acceptable products. As is well known, the leveler produces coil breaks but at such close intervals that the individual breaks cannot be detected by the eye, or will at least be so close together as not to interfere with the subsequent use of the product. However, for light gauges such as 0.0l5 inch and below known roller levellers are not capable of producing coil breaks close enough to one another in the product to make it commercially acceptable. Tension applied to the strip during leveling improves the operation of the device and yet the above named gauge is about the lower limit of permissible operation. The object of the invention to provide a method and apparatus which, while not limited for use on light gauge strip, will produce a strip which may be bent, formed or subjected to drawing operations without the formation of coil breaks or that appearance which is frequently termed “stretcher strains.”
The first mentioned patent disclosed the method of producing coil break free hot rolled steel strips by adding adequate alloying of Titanium to fix the free N and C responsible for Coil Break. Later two patents disclosed the apparatus/machine and method of preprocessing sheet metal by which stretcher mark/ coil break can be minimized during cold working/cold forming operation. Which is a preventive treatment of sheets to solve coil break problem.
There has been thus a need in the art to develop a steel grade/ sheets which would be resistant to coil break occurrence during cold forming/ die drawing for producing components particularly for special application including automobile components. The present invention is directed to providing coil break free steel composition and a process for producing steel strip thereof without any skin passing/cold rolling operation which would avoid coil break/wrinkle formation on surface of sheets/plates during unwinding of coil followed by cutting to desired length in CTL unit.

OBJECTS OF THE INVENTION
The basic object of the present invention is thus directed to provide low carbon (HR) coil break free hot rolled steel grade/composition with minimum yield point phenomenon and a process for producing steel sheets thereof having thickness in the range of 1.5-8.0 mm.
A further object of the present invention is directed to producing low carbon coil break free hot rolled steel strips by adequate alloying of Nb to fix free N and C by forming carbides and or carbo nitrides precipitates.

A further object of the present invention is directed to producing low carbon coil break free hot rolled steel strips wherein the chemistry consists of low carbon along with Mn and Nb as strengthening elements.
A still further object of the present invention is directed to producing low carbon coil break free hot rolled steel strips by hot rolling only and without any further processing like skin pass etc.

A still further object of the present invention is directed to producing low carbon coil break free hot rolled steel strips through thin slab caster manufacturing route.
A still further object of the present invention is directed to producing low carbon coil break free hot rolled steel strips which can be cut to sheets / plates in a CTL machine even without anti coil break facilities.

A still further object of the present invention is directed to producing low carbon coil break free hot rolled steel strips which would be suitable for application in automotive and other general engineering application including machinery, white good application, BIW application etc.

SUMMARY OF THE INVENTION
The basic aspect of the present invention is thus directed to provide low carbon coil break free hot rolled steel sheets comprising a composition having
C: 0.04- 0.08wt%;
Mn: up to 0.4wt%;
Si: up to 0.03 wt%;
Al < 0.05wt%;
N: < 120 ppm;
Ca: 0-50 ppm; and
Balance is iron with
S & P in amounts of allowable impurities, alongwith selective alloying element Nb: 0.002-0.01wt% for fixing free N and C.

A further aspect of the present invention is directed to provide said low carbon coil break free hot rolled steel sheets having YS= 170 Mpa, UTS = 290MPa and %El = 33%.
A still further aspect of the present invention is directed to said low carbon coil break free hot rolled steel sheets in the form of sheets with thickness ranging from 1.5-8.0 mm.
A still further aspect of the present invention is directed to provide said low carbon coil break free hot rolled steel sheets comprising carbides and or carbo nitrides precipitates obtained by fixing said free N and C by said Nb.

Yet another aspect of the present invention is directed to a process for the manufacture of low carbon coil break free hot rolled steel sheets as described above comprising:

(i) providing steel with composition having
C: 0.04- 0.08wt%;
Mn: up to 0.4wt%;
Si: up to 0.03 wt%;
Al < 0.05wt%;
N: < 120 ppm;
Ca: 0-50 ppm;and
Balance is iron with
S & P in amounts of allowable impurities, alongwith selective alloying element Nb: 0.002-0.01wt% for fixing free N and C.
(ii) subjecting the same to casting in thin slab cater with casting speed of 5.0 -6.5 m/min ;
(iii) rolling to sheet /strips in the temperature range of FRT 800-900Deg C; and
(iv) coiling in the temperature range of 550-650 Deg C.

A still further aspect of the present invention is directed to said process as comprising
(i) providing the selective low carbon steel having composition comprising C: 0.04- 0.08wt%; Mn: up to 0.4wt%;Si: up to 0.03 wt%;Al < 0.05wt%;Nb: 0.002-0.01wt%;N: < 120 ppm; Ca: 0-50 ppm;and balance is iron with S & P as impurities;
(ii) casting the steel slabs in a thin slab caster with slab thickness ranging from 50-65mm;
(iii) homogenizing said slabs in a tunnel furnace at temperature >10750C;
(iv) removing scales from slabs in a Descaler after tunnel furnace;
(v) controlling the desired thickness of the slab in hot rolling with finish rolling temperature in the range of 800-900 °C;
(vi) cooling the hot rolled strip so obtained in run out table by using laminar cooling and trim zone to get the desire coiling temperature of 550-650 °C followed by cooling of coils in coil yard.

A still further aspect of the present invention is directed to said process wherein said controlled hot rolling comprising 6 stand reductions of slabs in 6 high tandem rolling mill to reduce the thickness to the required level for producing steel sheets of 1.5-8.0 mm thickness.

A still further aspect of the present invention is directed to said process wherein said process carried out under controlled operating conditions comprising
(i) The Casting Speed 5.0-6.0 m/min;
(ii) Slab Thickness 55-65 mm;
(iii) Slab Cutting Temp 980-1050 Deg C;
(iv) Homogenization Temp (Tunnel Furnace) 1080-1150 Deg C;
(v) Homogenization Time 8-15 min;
(vi) Finish Rolling Temp 800-900 Deg C;
(vii) Standwise reduction/time/temp comprising

(viii) Coiling Temp 550-650Deg C.

A still further aspect of the present invention is directed to said process wherein said steel sheets are produced through Energy Efficient CSP process route comprising refining hot metal in EAF (Conarc)- adjusting chemistry in Ladle Furnace-Thin Slab Casting-Tunnel Furnace - Hot Strip Mill-Compact Cooing – coiling- slow cooling in coil yard.

The objects and advantages of the present invention are described hereunder in greater details with reference to the following accompanying non limiting illustrative drawing and example.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Figure 1: shows the flow chart illustrating the different steps involved in producing the low carbon coil break free hot rolled steel grade/sheets with minimum yield point phenomenon according to the present invention.
Figure 2(a): shows Stress- Displacement Curve of conventional normal C-Mn grade showing yield point phenomenon.
Figure 2(b): shows Stress-Displacement Curve of coil break free steel according to the present invention showing no yield point phenomenon.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE ACCOMPANYING DRAWING
The present invention is thus directed to provide coil break free hot rolled (HR) steel grade/ composition and a process for producing steel sheets of 1.5-8.0 mm thickness using such steel with Nb as alloying element to fix the free N and C to ensure minimum yield point phenomenon through thin slab caster processing route and without needing any further processing like skin pass etc.

Thus according to present invention, a composition and method of making a coil break free hot rolled steel sheet is provided. Chemistry consists of low carbon along with Mn and Nb as strengthening elements. Steel is processed through a thin slab caster and 6 high tandem rolling mill. Mechanical properties achieved are YS= 170 Mpa, UTS = 290MPa and %El = 33%.

In order to produce the a coil break free hot rolled steel grade/ steel sheet with above stated properties and with minimum yield point phenomenon, the selective steel composition used for processing through thin slab caster is as follows:
C: 0.04- 0.08wt%;
Mn: up to 0.4wt%;
Si: up to 0.03 wt%;
Al < 0.05wt%;
Nb: 0.002-0.01wt%;
N: < 120 ppm;
Ca: 0-50 ppm;and
Balance is iron with
S & P as impurities.

The detailed consideration for selecting the above chemical composition are as below:

C: 0.04- 0.08%: Carbon is essential for solute strengthening and formation of carbide and carbonitrides of Nb but upper limit is restricted because of poor effect on weldability and surface quality. Higher ‘C’ at peritectic range increases crack sensitivity, which is more prevalent in thin slab caster due to very high casting speed. Carbon level above 0.08% is particularly prone to surface defect in thin slab caster.
Mn: 0.4 % max: Mn is an important element for solid solution strengthening and also increases hardenability inexpensively. Higher ‘Mn/S’ ratio reduces central segregation and improves castability. Higher ‘Mn/Si’ improves surface finish of HR coil by reducing stickiness of scale. Upper values are restricted because higher load requirement in thinner gauge apart from cost implication. Also the minimum yield strength and tensile strength requirement is low for this grade.
Si: 0.03% max: It acts as a de oxidiser and also increases strength by substitution solid solution hardening effect in low carbon steel. Upper value is restricted because it has detrimental effect on surface quality during galvanizing operation.
Al: < 0.05 %: It is used predominantly as a de oxidizer. But it also restrict the grain growth by pinning grain boundaries with ‘AlN’ precipitation as well as improve formability by fixing free ‘N’. Upper limit is restricted to reduce ‘alumina’ inclusion & improve castability particularly in thin slab caster.
Nb: 0.002-0.01%: Nb is essential for fixing the free N and C by formation of nitrides and carbonitrides precipitates. It is one of the main sources of strengthening. Upper limit is restricted because of its effect on rolling load during hot rolling in thinner gauge, cost implication and low mechanical properties requirement of this grade.
N< 120 ppm: Nitrogen contributes to some extent in tensile strength by formation of nitride and carbonitride precipitates; however upper limit is restricted because of its poor effect on formability of steel and strain aging and also free nitrogen is responsible for stretcher strain formation.
Ca: 0-50 ppm: Steel has to be Calcium treated to improve castability particularly in thin slab casting by converting remaining inclusions in steel at LF to lower melting eutectic Ca-compounds and scavenging upto slag. Remnant Ca-bearing oxides / suphides are also round shaped , which are much less harmful in finished steel.
The process route followed was: Electric Arc Furnace?Ladle Furnace? thin slab Caster? 6 stand hot rolling mill?coiling . However, any other combination of processes (before caster) which gives steel of same chemistry can also be used.
Accompanying Figure 1 shows the flow chart illustrating the different steps involved in producing the coil break free hot rolled steel grade/sheets with minimum yield point phenomenon according to the present invention.

The process used for making the product according to an embodiment of the present invention is described in details with the help of following example :

EXAMPLE :

(i) Hot metal from blast furnace was refined with the help of CONARC ( combined facilities of Electric Arc Furnace & LD converter) furnace. Final chemistry adjustments were done in a ladle refining furnace. The steel composition so obtained is as given above and the composition(wt%) of a sample grade having Chemistry consisting of low carbon along with Mn and Nb as strengthening elements obtained on trial as compared to normal C-Mn steel is as follows:

Chemistry
GRADE C Si S P Mn Nb Al N
Normal C-Mn Grade 0.054 0.014 0.004 0.012 0.233 -- 0.035 57
Newly Developed Grade 0.053 0.023 0.003 0.009 0.117 0.003 0.035 60

(ii) Steel was cast using a thin slab caster with slab thickness ranging from 50-65 mm.
(iii) Slab was further homogenized in a Tunnel furnace at temperature >1075 DegC. Descaler was used after Tunnel Furnace to remove scales.
(iv) 6 stand reductions of slabs was carried out in 6 high tandem rolling mill to reduce the thickness to the required level for producing steel sheets of 1.5-8.0 mm thickness with finish rolling temperature in the range of 800-900 DegC.
(v) The hot rolled strip then cooled in run out table by using laminar cooling and trim zone to get the desire coiling temperature of 550-650 Deg C. The coil was then cooled in coil yard.
The Critical Process Parameters maintained during the above stated process steps are as follows:
I. The Casting Speed 5.0- 6.0 m/min;
II. Slab Thickness 55-65 mm;
III. Slab Cutting Temp 980-1050 Deg C;
IV. Homogenization Temp (Tunnel Furnace) 1080-1150 Deg C;
V. Homogenization Time 8-15 min;
VI. Finish Rolling Temp 800-900 Deg C;
VII. Standwise reduction/time/temp:
Parameters F1 F2 F3 F4 F5 F6
Relative Reduction 40-65 40-65 30-45 35-40 25-35 20-30
Inter stand time 5-15 sec 4-15 sec 3-10 sec 3-10 sec 2-10 sec 2-10 sec
Stand Entry Temp(°C) 1050-1080 1000-1040 970-1000 900-950 870-900 820-900

VIII. Coiling Temp 550-650Deg C.

The resulting steel grade was subjected to testing and inspection to ascertain attainment of desired mechanical properties and minimum yield point phenomenon of a sample steel grade of low carbon coil break free hot rolled produced according to the present invention as compared to normal C-Mn steel are as follows:

Mechanical properties:
GRADE YS UTS %El HRD
Normal C-Mn Grade 270 350 40 58
Newly Developed Grade 320 390 37 67

Accompanying Figure 2(a) shows Stress-Displacement Curve of conventional normal C-Mn grade showing yield point phenomenon and accompanying Figure 2(b) shows Stress-Displacement Curve of coil break free steel according to the present invention showing no/minimum yield point phenomenon.

It is thus possible by way of the present invention to providing low carbon coil break free hot rolled steel sheets with minimum yield point phenomenon having the following advantages :
1. Suitable for direct sheet application in automotive and other general engineering application including machinery, white good application, BIW application. It will reduce internal diversion as well as customer complaint. It will also increase customer satisfaction.
2. Product can further be used directly without any skin pass or similar treatment and thus reduce cost.
3. Product can be cut to sheets/plates in a CTL machine even without anti coil break facilities, which will enhance customer satisfaction and increase market share.

We Claim :
1. Low carbon coil break free hot rolled steel sheets comprising a composition having
C: 0.04- 0.08wt%;
Mn: up to 0.4wt%;
Si: up to 0.03 wt%;
Al < 0.05wt%;
N: < 120 ppm;
Ca: 0-50 ppm;and
Balance is iron with
S & P in amounts of allowable impurities, alongwith selective alloying element Nb: 0.002-0.01wt% for fixing free N and C.

2. Low carbon coil break free hot rolled steel sheets as claimed in claim 1 having YS= 170 MPa, UTS = 290MPa and %El = 33%.
3. Low carbon coil break free hot rolled steel sheets as claimed in anyone of claims 1 or 2, in the form of sheets with thickness ranging from 1.5-8.0 mm.
4. Low carbon coil break free hot rolled steel sheets as claimed in anyone of claims 1 to 3 , comprising carbides and or carbo nitrides precipitates obtained by fixing said free N and C by said Nb.

5. A process for the manufacture of low carbon coil break free hot rolled steel sheets as claimed in anyone of claims 1 to 4 comprising:

(i) providing steel with composition having
C: 0.04- 0.08wt%;
Mn: up to 0.4wt%;
Si: up to 0.03 wt%;
Al < 0.05wt%;
N: < 120 ppm;
Ca: 0-50 ppm;and
Balance is iron with
S & P in amounts of allowable impurities, alongwith selective alloying element Nb: 0.002-0.01wt% for fixing free N and C.
(ii) subjecting the same to casting in thin slab caster with casting speed of 5.0 -6.5 m/min ;
(iii) rolling to sheet /strips in the temperature range of FRT 800-900Deg C; and
(iv) coiling in the temperature range of 550-650 Deg C.

6. A process as claimed in claim 5 comprising
(i) providing the selective low carbon steel having composition comprising C: 0.04- 0.08wt%; Mn: up to 0.4wt%;Si: up to 0.03 wt%;Al < 0.05wt%;Nb: 0.002-0.01wt%;N: < 120 ppm; Ca: 0-50 ppm; and balance is iron with S & P as impurities;
(ii) casting the steel slabs in a thin slab caster with slab thickness ranging from 50-65mm;
(iii) homogenizing said slabs in a tunnel furnace at temperature >10750C;
(iv) removing scales from slabs in a Descaler after tunnel furnace;
(v) controlling the desired thickness of the slab in hot rolling with finish rolling temperature in the range of 800-900 °C;
(vi) cooling the hot rolled strip so obtained in run out table by using laminar cooling and trim zone to get the desire coiling temperature of 550-650 °C followed by cooling of coils in coil yard.

7. A process as claimed in claim 6 wherein said controlled hot rolling comprising 6 stand reductions of slabs in 6 high tandem rolling mill to reduce the thickness to the required level for producing steel sheets of 1.5-8.0 mm thickness.

8. A process as claimed in anyone of claims 5 to 7, wherein said process carried out under controlled operating conditions comprising
(i) The Casting Speed 5.0- 6.5 m/min;
(ii) Slab Thickness 55-65 mm;
(iii) Slab Cutting Temp 980-1050 Deg C;
(iv) Homogenization Temp (Tunnel Furnace) 1080-1150 Deg C;
(v) Homogenization Time 8-15 min;
(vi) Finish Rolling Temp 800-900 Deg C;
(vii) Standwise reduction/time/temp comprising

(viii) Coiling Temp 550-650Deg C.

9. A process as claimed in anyone of claims 5 to 8, wherein said steel sheets are produced through Energy Efficient CSP process route comprising refining hot metal in EAF (Conarc)- adjusting chemistry in Ladle Furnace-Thin Slab Casting-Tunnel Furnace - Hot Strip Mill-Compact Cooing – coiling- slow cooling in coil yard.

Dated this the 29th day of January, 2014
Anjan Sen
Of Anjan Sen & Associates
(Applicants Agent)

ABSTRACT

TITLE: LOW CARBON COIL BREAK FREE HOT ROLLED STEEL SHEETS AND A PROCEES FOR ITS PRODUCTION.

The present invention relates to providing low carbon coil break free hot rolled (HR) steel sheet in the thickness range of 1.5-8.0 mm with minimum yield point phenomenon and a process for its production through thin slab caster route followed by controlled hot rolling, having composition comprising C: 0.04-0.08wt%; Mn: up to 0.4wt%; Si: up to 0.03 wt%;Al < 0.05wt%;Nb: 0.002-0.01wt%;N: < 120 ppm; Ca: 0-50 ppm; and balance is iron with S & P in amounts of allowable impurities alongwith selective alloying element Nb: 0.002-0.01wt% for fixing free N and C. Typical applications of the invented steel grade include direct sheet application in automotive and other general engineering application including machinery, white good application, BIW application etc.