CLIAMS:We Claim:

1. Low carbon extra deep drawing(EDD) cold rolled steel sheets having composition comprising

C: 0.025-0.045 wt%,
Mn: 0.12-0.18 wt%,
N: 0 to 0.004 wt%,
P≤ 0.015 wt%,
Silicon≤0.015 wt%,
Cr: 0.01-0.02 wt%,
Al: 0.03-0.05 wt%,
and balance iron,
which is aluminium killed steel having specified Al/N atomic ratio between 8 to 12.

2. Low carbon extra deep drawing(EDD) steel sheet as claimed in claim 1 having enhanced plastic anisotropy ratio involving r-bar value of 1.9 min and yield strength of less than 160 MPa and free from stretcher strain.

3. Low carbon extra deep drawing(EDD) steel sheet as claimed in anyone of claims 1 or 2 comprising sheet having thickness 1.6 mm or less.

4. Low carbon extra deep drawing(EDD) steel sheet as claimed in anyone of claims 1 to 3 suitable for automobile body applications.

5. Low carbon extra deep drawing(EDD) steel sheet as claimed in anyone of claims 1 to 4 wherein carbon is maintained above 0.025wt% to avoid free carbon in steel matrix.
6. A process for producing low carbon extra deep drawing(EDD) steel sheets through batch annealing route as claimed in claims 1 to 5 comprising
providing molten steel having composition
C: 0.025-0.045 wt%,
Mn: 0.12-0.18 wt%,
N: up to 0.004 wt%,
P≤ 0.015 wt%,
Silicon≤0.015 wt%,
Cr: 0.01-0.02 wt%,
Al: 0.03-0.05 wt%,
and balance iron,
and processing for aluminum killed steel and with improving formability involving Al/N atomic ratio between 8 to 12 , HSM finishing temperature 880 to 910 0C and HSM coiling temperature of 5500 C or less preferably 5300 to 5500C .

7. A process as claimed in claim 6 carried out involving a combination of cold working deformation of 75% or more, Al/N ratio 12 or less , Cr addition 0.02wt% or less, HSM coiling Temperature 550 °C or less, cold spot temperature 680 °C or more to achieve said minimum r-bar value of 1.9.

8. A process as claimed in anyone of claims 6 to 7 carried out involving combination of HSM coiling temperature 530 °C or more and hot spot temperature 710°C or less to achieve strip surface with wrinkle free edges.

9. A process as claimed in anyone of claims 6 to 8 carried out involving a combination of Cr addition 0.01 % or more and batch annealing in 100% H2 atmosphere to achieve improved surface finish free from graphitization.

10. A process as claimed in anyone of claims 6 to 9 carried out involving a combination of batch annealing cold spot temperature 680 °C and skin pass elongation of 0.8±0.2% of colds rolled batch annealed steel sheets, to achieve yield strength less than 160 MPa.

11. A process as claimed in anyone of claims 6 to 10 carried out involving a combination of batch annealing cold spot temperature 680 °C and said selected composition of steel to achieve tensile strength of 310 MPa or less.

12. A process as claimed in anyone of claims 6 to 11 carried out in combination of Al/N ratio 8 or more, Cr 0.01% or more, HSM coiling temperature 530 °C or more such as to achieve yield point elongations zero even after 1 year of ageing.

13. A process as claimed in anyone of claims 6 to 12 comprising

(i) providing selective molten steel composition having
C: 0.025-0.045 wt%,
Mn: 0.12-0.18 wt%,
N: up to 0.004 wt%,
P≤ 0.015 wt%,
Silicon≤0.015 wt%,
Cr: 0.01-0.02 wt%,
Al: 0.03-0.05 wt%,
and balance iron,
prepared without vacuum degassing and killed in the conventional manner;

(ii) casting the steel into slabs through continuous casting route;
(iii) hot scarfing of slabs after casting to avoid any surface defects like slivers etc.
(iv) subjecting the slabs to hot rolling into hot rolled strip with slab reheating temperature kept between 1180 -1220 0C maintaining hot roll finishing temperature of at least 8800C and preferably 880-910 0C.
(v) subjecting the hot rolled steel strip to coiling while at a temperature exceeding >5300C, preferably 530-550 0C ;
(vi) subjecting the hot rolled strips to pickling in acid medium (HCl) having concentrations between 4-20%;
(vii) subjecting the hot rolled and acid pickeld strips to cold rolling with a minimum cold reduction of 75 percent, preferably 75-80 % to achieve higher plastic strain ratio post annealing;
(viii) Subjecting the cold rolled steel strip to batch annealing comprising
(a) heating a coil of cold rolled steel strip at a slow heating rate of about 40-45 0C /Hour in a batch annealing furnace in 100% Hydrogen atmosphere to maintain a uniform temperature throughout the coil from edge to core for achieving achieving excellent surface reflectivity 98% or more, avoiding graphitization;
(b) following annealing cycle of 710-680 °C maintaining the cold spot temperature of 6800C or high and hot spot temperature 710 °C or less to achieve a higher r –bar value of 1.9 and above;
(c) subjecting the batch annealed steel sheet to an optimum skin pass elongation of 0.6-1.0 % to get the yield strength less than 160 MPa, to avoid any yield point elongation.

Dated this the 27th day of March, 2015
Anjan Sen
Of Anjan Sen & Associates
(Applicants Agent)

,TagSPECI:FIELD OF THE INVENTION

The present invention relates to Extra Deep Drawing (EDD) Steel sheet having excellent formability, and method for manufacturing the same through batch annealing route. More particularly, the present invention is directed to provide extra deep drawing steel sheet having excellent formability with adequate surface finish suitable for outer panel for automobiles mainly passenger vehicles. Importantly, the EDD steel sheet according to the present invention comprises low Carbon (0.025-0.045weight %) aluminum killed steel wherein having specified Al/N atomic ratio between 8 to 12, Cr -0.01 to 0.02, P ≤0.015, Si- ≤0.015, Al -0.03 to 0.05 %, N- 0 to 0.004%, CCM reduction between 75 to 80%, HSM Finishing Temperature 880 to 910°C and HSM Coiling temperature 530 to 550°C to avoid AlN precipitation before batch annealing. Advantageously, availability of Al/N ratio more than 8 ensures AlN precipitation during batch annealing and Al/N ratio less than 12 ensures very less remaining Al in the solid solution which assist in achieving excellent plastic anisotropy ratio (r-bar- 1.9 min) value. Controlled Cr is used to stabilize free C and to get graphitization free surface finish. Hot scarfing after casting is used to avoid any surface defects like slivers. The optimum skin pass elongation (0.8±0.2%) used to get yield strength less than 160 MPa and free from stretcher strain. Thus, the EDD cold rolled steel developed through batch annealing route with excellent formability and surface finish which can be used for exposed panel of the automobile body and critical components which need excellent drawability.

BACKGROUND OF THE INVENTION

Generally low carbon cold rolled steel sheets suffers as far as drawability is concerned. Their poor drawability is attributed to low plastic anisotropy (r-bar) and high aging index. Due to their comparatively high yield strength, low carbon steel sheets do not match narrowly to stamping dies during forming or stamping operation, and consequently causes unwanted surface deflection in the formed parts. Thus, steel sheets with low yield strength and higher r-bar value are desired.

As a prior art regarding a low carbon steel sheet with good formability, patent application number 140/KOL/2008 discloses a method of manufacturing a low carbon steel having a carbon wt% ≤ 0.04, Mn wt% ≤ 0.15 , Si wt% ≤ 0.015, S wt% ≤ 0.01 , P wt% ≤ 0.015 with Titanium ≤0.008-0.012 wt%. In addition, Hot rolling the steel slab such that slab reheating temperature is 1250-12800C, finishing Temperature of 880-9200C and coiling temperature in a range of 610-6300C. A cold rolling reduction of about 50% with skin pass elongation of 2.0% has been described. The r-value of cold rolled batch annealed steel sheet in a range of 1.6-1.7, Yield strength of 170-210 MPa with ageing index of 2.0% - 2 .2% is claimed in prior art application number 140/KOL/2008.

However, batch annealed steel sheet obtained according to the process of patent application number 140/KOL/2008 has a major setback that wrinkling arises in the resultant steel sheet during the press forming operation due to low planar anisotropy value (r-bar) and yield point elongation due to ageing. Hence the resulting steel sheet cannot be drawn adequately. In, addition the resultant steel is prone to surface graphitization if C is 0.025 % or less when no cementite will form. Added Ti is used for fixing N where as adequate amount of free C remains unbound which accounts for graphitization and high ageing index.

The present invention relates in general to an extra deep drawing steel sheet having excellent formability with superior surface finish and a method for improving the drawing property of steel strip and more predominantly to a method for improving the drawing properties of low carbon Al killed cold rolled batch annealed steel strip containing between 0.025 and 0.045 wt % carbon.

The present invention is thus directed to achieve a significant improvement over the prior art as far as the drawability, good ageing property, high r-value and surface free from graphitization is concerned. In present invention, Cr has been used to stabilize C and to avoid graphitization. The mechanism for the same has been described herein after. N in present invention has been fixed only by Al as AlN by controlling the Al/N ratio precisely making such steel sheets suitable for outer panel for automobiles.

OBJECTS OF THE INVENTION

The basic object of the present invention is directed to provide Extra Deep Drawing (EDD) steel sheet having excellent formability and surface quality suitable for outer panel application in automobiles, and a method for manufacturing the same through batch annealing route.

A further object of the present invention is directed to provide Extra Deep Drawing (EDD) steel sheet having improved drawability, good ageing property, high r-value and surface free from graphitization making it suitable for exposed panel of the automobile body and critical components which need excellent drawability.

A still further object of the present invention is directed to provide Extra Deep Drawing (EDD) steel sheet having excellent formability and surface quality and a method of its production wherein low carbon Al killed cold rolled steel strip containing chromium as a carbide former is obtained having an r-value greater than 1.9 and yield strength of less than 160 MPa produced without a vacuum degassing operation or a decarburizing operation and with a carbon content between 0.025 and 0.045 wt%.

A still further object of the present invention is directed to provide Extra Deep Drawing (EDD) steel sheet having excellent formability and surface quality wherein Cr is used to stabilize C and to avoid graphitization.

A still further object of the present invention is directed to provide Extra Deep Drawing (EDD) steel sheet having excellent formability and surface quality wherein no wrinkling arises in the resultant steel sheet during the press forming operation due to improved planar anisotropy value (r-bar) and no yield point elongation due to ageing.

A still further object of the present invention is directed to provide Extra Deep Drawing (EDD) steel sheet having excellent formability and surface quality wherein N is fixed only by Al as AlN by controlling the Al/N ratio precisely.

A still further object of the present invention is directed to provide Extra Deep Drawing (EDD) steel sheet having excellent formability and surface quality wherein Al/N ratio range is kept between 8 to 12 to achieve a higher r-value of at least 1.9.

A still further object of the present invention is directed to provide Extra Deep Drawing (EDD) steel sheet having excellent formability and surface quality wherein coiling temperature is selectively maintained to avoid AlN precipitation before batch annealing and sufficient time is provided to precipitate out AlN during annealing.

A still further object of the present invention is directed to provide Extra Deep Drawing (EDD) steel sheet having excellent formability and surface quality wherein reheating temperature is kept high (~1200 0C) along with low coiling temperature to keep Al and N in solid solution.

SUMMARY OF THE INVENTION

The basic aspect of the present invention is directed to Low carbon extra deep drawing(EDD) cold rolled steel sheets having composition comprising
C: 0.025-0.045 wt%,
Mn: 0.12-0.18 wt%,
N: 0 to 0.004 wt%,
P≤ 0.015 wt%,
Silicon≤0.015 wt%,
Cr: 0.01-0.02 wt%,
Al: 0.03-0.05 wt%,
and balance iron,
which is aluminium killed steel having specified Al/N atomic ratio between 8 to 12.

A further aspect of the present invention is directed to Low carbon extra deep drawing(EDD) steel sheet having enhanced plastic anisotropy ratio involving r-bar value of 1.9 min and yield strength of less than 160 MPa and free from stretcher strain.
A still further aspect of the present invention is directed to low carbon extra deep drawing(EDD) steel sheet comprising sheet having thickness 1.6 mm or less.

Importantly, said low carbon extra deep drawing(EDD) steel sheet is suitable for automobile body applications.

A still further aspect of the present invention is directed to low carbon extra deep drawing(EDD) steel sheet wherein carbon is maintained above 0.025wt% to avoid free carbon in steel matrix.

Yet another aspect of the present invention is directed to a process for producing low carbon extra deep drawing(EDD) steel sheets as stated above comprising
providing molten steel having composition
C: 0.025-0.045 wt%,
Mn: 0.12-0.18 wt%,
N: up to 0.004 wt%,
P≤ 0.015 wt%,
Silicon≤0.015 wt%,
Cr: 0.01-0.02 wt%,
Al: 0.03-0.05 wt%,
and balance iron,

and processing for aluminum killed steel and with improving formability involving Al/N atomic ratio between 8 to 12 , HSM finishing temperature 880 to 910 0C and HSM coiling temperature of 5500 C or less preferably 5300 to 5500C .;

A further aspect of the present invention is directed to a process carried out involving a combination of cold working deformation of 75% or more , Al/N ratio 12 or less , Cr addition 0.02wt% or less, HSM coiling Temperature 550 °C or less, cold spot temperature 680 °C or more to achieve said minimum r-bar value of 1.9.

A still further aspect of the present invention is directed to a process carried out involving combination of HSM coiling temperature 530 °C or more and hot spot temperature 710°C or less to achieve strip surface with wrinkle free edges.

A still further aspect of the present invention is directed to a process carried out involving a combination of Cr addition 0.01 % or more and batch annealing in 100% H2 atmosphere to achieve improved surface finish free from graphitization.

Another aspect of the present invention is directed to a process carried out involving a combination of batch annealing cold spot temperature 680 °C and skin pass elongation of 0.8±0.2% of colds rolled batch annealed steel sheets, to achieve yield strength less than 160 MPa.

Yet another aspect of the present invention is directed to a process carried out involving a combination of batch annealing cold spot temperature 680 °C and said selected composition of steel to achieve tensile strength of 310 MPa or less.

A further aspect of the present invention is directed to a process carried out in combination of Al/N ratio 8 or more, Cr 0.01% or more, HSM coiling temperature 530 °C or more such as to achieve yield point elongations zero even after 1 year of ageing.
A still further aspect of the present invention is directed to a process comprising
(i) providing selective molten steel composition having
C: 0.025-0.045 wt%,
Mn: 0.12-0.18 wt%,
N: up to 0.004 wt%,
P≤ 0.015 wt%,
Silicon≤0.015 wt%,
Cr: 0.01-0.02 wt%,
Al: 0.03-0.05 wt%,
and balance iron,
prepared without vacuum degassing and killed in the conventional manner;
(ii) casting the steel into slabs through continuous casting route;
(iii) hot scarfing of slabs after casting to avoid any surface defects like slivers etc.
(iv) subjecting the slabs to hot rolling into hot rolled strip with slab reheating temperature kept between 1180 -1220 0C maintaining hot roll finishing temperature of at least 8800C and preferably 880-910 0C.
(v) subjecting the hot rolled steel strip to coiling while at a temperature exceeding >5300C, preferably 530-550 0C ;
(vi) subjecting the hot rolled strips to pickling in acid medium (HCl) having concentrations between 4-20%;
(vii) subjecting the hot rolled and acid pickeld strips to cold rolling with a minimum cold reduction of 75 percent, preferably 75-80 % to achieve higher plastic strain ratio post annealing;
(viii) Subjecting the cold rolled steel strip to batch annealing comprising
(a) heating a coil of cold rolled steel strip at a slow heating rate of about 40-45 0C /Hour in a batch annealing furnace in 100% Hydrogen atmosphere to maintain a uniform temperature throughout the coil from edge to core for achieving achieving excellent surface reflectivity 98% or more, avoiding graphitization;
(b) following annealing cycle of 710-680 °C maintaining the cold spot temperature of 6800C or high and hot spot temperature 710 °C or less to achieve a higher r –bar value of 1.9 and above;
(c) subjecting the batch annealed steel sheet to an optimum skin pass elongation of 0.6-1.0 % to get the yield strength less than 160 MPa, to avoid any yield point elongation.

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

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

Figure 1: is the schematic diagram showing {111} <112> and {111} <110> components of Y-fiber considered as ideal crystallographic textures for deep drawing steel to maximize r bar–value having the strength in the thickness direction greater than that in the plane of the sheet.

Figure 2 & Figure 3: show the Orientation distribution function (ODF) Ø2 = 45° for the EDD steel according to present invention which clearly shows the uniform Y-fiber [111] texture with negligible orientation density of rotated cube orientation {001} <110> attributing mainly to a very high r-bar value after annealing.

Figure 4: shows the Optical Microstructure of cold rolled batch annealed EDD steel at 500X magnification.

Figure 5(a): shows the SEM Microstructure of cold rolled batch annealed EDD steel at 500X magnification showing elongated grains.

Figure 5(b): shows the SEM Microstructure of cold rolled batch annealed EDD steel at 1000X showing elongated grains.

Figure 6: shows the graph of Al wt% vs respective r-bar.

Figure 7: shows the graph of relationship between the Al/N ratio and respective r-values.

Figure 8: shows the graph of relationship between the Cr wt% and respective r-values.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE ACCOMPANYING DRAWINGS

The present invention is directed to provide extra deep drawing(EDD) steel sheet having excellent formability with adequate surface finish suitable for outer panel for automobiles.

A method in accordance with the present invention produces low carbon Al killed cold rolled steel strip containing chromium as a carbide former in place of titanium and having an r-value greater than 1.9 and yield strength of less than 160 MPa produced without a vacuum degassing operation or a decarburizing operation and with a carbon content between 0.025 and 0.045 wt%. The method comprises controlling the rolling and annealing parameters as described more fully below.

The EDD steel according to the present invention is having excellent drawability, good ageing property, high r-value and surface free from graphitization is concerned. In present invention, Cr has been used to stabilize C and to avoid graphitization. The mechanism for the same has been described later. N in present invention has been fixed only by Al as AlN by controlling the Al/N ratio precisely.

In the present invention, a “steel sheet” stands for a cold-rolled steel sheet of which sheet thickness is 1.6 mm or less and all the steel component composition percentage is in weight %.

EDD Steel of present invention having composition comprising carbon :0.025-0.045 wt% , manganese :0.12- 0.18 wt% , nitrogen up to 0.004 wt% , Phosphorus ≤ 0.015 wt% , Silicon ≤0.015 wt%, Chromium :0.01-0.02 wt%, aluminum :0.03 -0.05 wt% and balance iron. The chromium is added as carbide forming agent. Chromium addition also adds up in getting excellent surface properties which is free from graphitization. Following are the justification for keeping the compositions as given above:

CARBON: ( 0.025-0.045 wt.%) – While carbon increases the steel strength, it reduces the cold workability , r value and the deep drawability of the cold rolled steel sheet remarkably and thus the higher limit of the cold rolled sheet is set at 0.1 wt%. For improving the r-value i.e. drawability it desirable to reduce the C level less than 0.05 wt%. Lowering the carbon content below 0.025 wt% results in poor ageing property since below 0.025 wt% steel is in complete α-ferrite region in Iron-Cementite phase diagram resulting no cementite formation. Consequently more free carbon will be available in steel matrix which deteriorates the ageing property. Keeping low carbon <0.025 % also requires vacuum degassing the molten steel which add more cost of production. To avoid that the C level is maintained between 0.025-0.045 wt%.

MANGANESE: (0.12-0.18) wt% - Manganese is essential for avoiding the red shortness caused by Sulphur. However, it hardens the steel and lowers the drawability. The r-value of a cold rolled steel sheet drops in addition. Thus its upper limit is kept at 0.3%. It is desirable to control the manganese content below 0.2% for the present grade where strength is not particularly required. Keeping the Mn content below 0.1 wt % could make steel susceptible to hot shortness due to unbound Sulphur.

ALUMINIUM: (0.03-0.05) wt% - Aluminum is present by default as a result of killing of steel. Major advantage in low carbon grade is that it scavenges off the N by forming AlN precipitate which improves drawability by reducing interstitial N and by promoting the formation of {111} type of texture during batch annealing hence facilitating the drawability and high r-values. Availability of Al/N ratio more than 8 ensures sufficient AlN precipitation during batch annealing to improve ageing properties and Al/N ratio less than 12 ensure very less remaining Al in the solid solution which deteriorate the drawability. In addition, higher Al content of more than 0.06 % may deteriorate the cleanliness of the steel during casting due to formation of unavoidable oxide Al2O3 inclusions which deteriorates the surface properties. Thus the upper value is limited to 0.06%. When better drawability is required it is desirable to restrict the acid soluble aluminum content to less than 0.05%. The following relationship holds satisfactory for r-value vs Al content: ṝ =2.471-12.92Al wt%. Al wt% vs respective r-bar is shown in graph of Figure 6.

NITROGEN: (less than 0.004 wt %) - Normally, nitrogen is limited in an amount between 0.0015% and 0.008% in Al-killed steel. N as an interstitial element deteriorates the ageing property and is damaging to the cold workability. Further, an increased quantity of nitrogen increases the amount of Al required for non-ageing and as a result, lowers the cold workability and cleanliness. Thus it is advantageous to keep the Nitrogen level below 0.004 wt%. Moreover, the following relationship holds satisfactory for r-value vs. Al/N ratio, ṝ =2.09- 0.02 (Al/N). Hence Al/N ratio range is kept between 8 to 12 to achieve a higher r-value of more than 1.9. Relationship between the Al/N ratio and respective r-values are shown in the Graph of Figure 7.

CHROMIUM: (0.01-0.02 wt%)- Advantage of adding chromium is twofold, firstly it’s a strong carbide former so it helps in scavenging off Carbon and stabilizes it by forming chromium carbide (Cr23C6). Secondly it improves the surface property of batch annealed steel by eliminating the graphitization which is a major surface defect. The upper limit is 0.02 wt% as higher than that will restrict the grain growth and achieving high r-value will not be feasible. The following relationship holds true for r-value and Cr wt% - ṝ= 2.188-17.08 Cr wt%. Hence for achieving the higher r-value more than 1.9, it’s required to keep Cr content less than 0.02. Keeping the chromium content more than 0.01 wt% is desired to avoid graphitization. Relationship between the Cr wt% and respective r-values are shown in the Graph of Figure 8.

Effect of Chromium in avoiding graphitization:
Low carbon steel (~0.04weight% C) suffers from surface graphitization in batch annealing furnace where the atmosphere is reducing to Iron and carbon concentration is higher than the solubility limit of graphite in α-iron. Graphitization can be identified as a black stain on the surface of the batch annealed steel. It cannot be removed by pickling or cleaning operation which needs to be done prior to coating. Consequently it deteriorates the coating property. The supply of carbon for surface graphitization is from inside the steel and not due to residual oil.

Graphite is thermodynamically stable phase of carbon in Fe-C alloys at equilibrium state but in steel C is commonly present as cementite (Fe3C) which is a metastable phase. Cementite to graphite transformation is associated with large volume expansion which kinetically slow process. However, on surface of the steel free energy of iron is larger than the bulk. For this reason the resistance to volume expansion for the formation of graphite is lower on surface than that in the bulk.

Moreover, the partial pressure of carbon monoxide plays a major role in graphite formation. It decreases with decarburization and increases with graphite precipitation. The amount of graphitization depends on whether the steel is in tight contact or not. Evidently, Batch annealed steel are vulnerable to surface graphitization because sheets are tightly coiled which maintains the carbon monoxide gas pressure.

Surface graphite inhibition effect of Chromium can be explained as follows -
1. Cr is a carbide forming element which decreases the solubility of carbon in iron.
2. Cr stabilizes cementite.
3. Cr prohibits the diffusion of carbon to the surface of steel inhibiting the formation of surface graphite.
4. Cr has higher affinity for oxygen below 1200 0C, hence it will take away the oxygen from the steel surface inhibiting CO formation. This adds up in reducing graphite precipitation.

SILICON (less than 0.015 wt %) – Silicon is an element used for increasing the strength of steel. As the silicon content increases the ductility and r-value noticeably deteriorated. Since silicon deteriorates plating /surface properties as well by forming SiO2 type of oxide (Scale).It is advantageous to add as low an amount of silicon in the steel as is possible. The following relation holds true for r-value and Si wt% - ṝ=2-12.6 Si wt%. For r-value >1.9, the added amount of silicon is preferably 0.015wt% or less.

COILING TEMPERATURE (530 to 550°C) - Coiling temperature is kept between 530°C to 550°C to avoid AlN precipitation before batch annealing as sufficient time is provided to precipitate out AlN during annealing hence no prior precipitation is needed. Also reheating temperature is kept high (~12000C) along with low coiling temperature to keep Al and N in solid solution.

Advantage of AlN precipitation During Batch Annealing:-

1. AlN precipitation during batch annealing reduces number of nuclei so that the recrystallized structure exhibit larger pancake-shaped grains with stronger {111} texture.
2. Zener pinning from AlN particles on subgrain stops nucleation unfavorably oriented grains.

Manufacturing Method
Molten steel of the above noted composition is prepared without vacuum degassing, killed in the conventional manner and cast into slabs. Hot scarfing after casting is used to avoid any surface defects like slivers etc. The slabs are then hot rolled into hot rolled strip.

Molten steel with composition range described above is casted through continuous casting route in to slabs and hot rolled in strip form. Slab reheating temperature is kept between 1180 -1220 0C.

The hot roll finishing temperature is kept at least 8800C (880-910 0C preferably).

The hot rolled steel strip is then coiled while at a temperature exceeding >5300C (530-550 0C preferably). After hot rolling strips are pickled in acid medium (HCl) having concentrations between 4-20% and cold rolled with a minimum cold reduction of 75 percent (preferably 75-80 %) to achieve higher plastic strain ratio post annealing.

Following cold rolling, the steel strip is batch annealed. In batch annealing, a coil of cold rolled steel strip is heated at a slow heating rate of about 40-45 0C /Hour in a batch annealing furnace in 100% Hydrogen atmosphere to maintain a uniform temperature throughout the coil from edge to core as H2 has very high heat conductivity .In addition, 100% H2 atmosphere is beneficial in avoiding graphitization and achieving excellent surface reflectivity 98% or more. Annealing cycle of 710-680 °C was implied maintaining the cold spot temperature of 6800C or high and hot spot temperature 710 °C or less to achieve a higher r –bar value of 1.9 and above. Cold spot temperature is so chosen as the higher temperature than 680 0C would cause sticker formation during annealing process which is a major surface defect in batch annealing route, where as a low cold spot temperature reduces the drawing properties along with high yield strength. Hot Spot temperature is so chosen as the higher temperature than 710 °C causes the wrinkling on the edges and less temperature cause the production loss in batch annealing.

After the batch annealing an optimum skin pass elongation of 0.6-1.0 % is used mainly to get the yield strength less than 160 MPa. The occurrence of any yield point elongation is avoided by keeping skin pass elongation more than 0.6 %.

Complete characteristics and advantageous features of extra deep drawing steel sheet according to the present invention having excellent formability with superior surface finish and comparative steel grades are illustrated in the accompanying table I to III and also through following illustrative examples 1-6.

Table I- Compositions of the invented steel sheets along with some comparative examples.
Chemical Composition (Wt %)
Sample No C Mn S P Si Al N Al/N Ratio Cr Remarks
1 0.039 0.18 0.007 0.013 0.004 0.035 0.0031 11.3 0.013 Invention
2 0.039 0.18 0.007 0.013 0.004 0.035 0.0031 11.3 0.013 Invention
3 0.039 0.18 0.007 0.013 0.004 0.035 0.0031 11.3 0.013 Invention
4 0.039 0.18 0.007 0.013 0.004 0.035 0.0031 11.3 0.013 Invention
5 0.036 0.18 0.008 0.011 0.009 0.043 0.0043 10.0 0.014 Invention
6 0.036 0.18 0.008 0.011 0.009 0.043 0.0043 10.0 0.014 Invention
7 0.036 0.18 0.008 0.011 0.009 0.043 0.0043 10.0 0.014 Invention
8 0.036 0.18 0.008 0.011 0.009 0.043 0.0043 10.0 0.014 Invention
9 0.042 0.16 0.006 0.013 0.02 0.055 0.0037 15.0 0.02 Comparative Steel
10 0.035 0.17 0.007 0.005 0.019 0.055 0.0034 16.0 0.024 Comparative Steel
11 0.05 0.18 0.004 0.019 0.02 0.045 0.0042 11.0 0.022 Comparative Steel
12 0.048 0.17 0.007 0.012 0.009 0.05 0.0037 14.0 0.02 Comparative Steel
13 0.035 0.16 0.006 0.011 0.017 0.053 0.0036 15.0 0.03 Comparative Steel
14 0.035 0.16 0.006 0.011 0.017 0.053 0.0036 15.0 0.03 Comparative Steel
15 0.038 0.18 0.004 0.01 0.02 0.045 0.0038 12.0 0.025 Comparative Steel
16 0.039 0.18 0.006 0.02 0.014 0.044 0.0037 12.0 0.020 Comparative Steel
17 0.03 0.17 0.008 0.01 0.007 0.041 0.0041 10.0 0.019 Invention
18 0.033 0.15 0.005 0.011 0.007 0.047 0.0044 10.7 0.023 Invention
19 0.04 0.17 0.007 0.015 0.005 0.046 0.0037 12.4 0.021 Invention
20 0.034 0.16 0.007 0.007 0.008 0.04 0.0036 11.1 0.013 Invention
21 0.032 0.15 0.007 0.009 0.01 0.038 0.0034 11.2 0.015 Invention
22 0.036 0.17 0.008 0.014 0.005 0.04 0.0042 9.5 0.019 Invention
23 0.038 0.17 0.008 0.01 0.002 0.032 0.004 8.0 0.016 Invention
24 0.038 0.18 0.008 0.013 0.011 0.045 0.004 11.3 0.017 Invention
25 0.035 0.17 0.007 0.006 0.007 0.044 0.0041 10.7 0.013 Invention
26 0.023 0.18 0.008 0.009 0.006 0.041 0.0033 12.4 0.011 Invention
27 0.034 0.16 0.008 0.01 0.009 0.05 0.0042 11.9 0.014 Invention
28 0.042 0.18 0.008 0.014 0.003 0.039 0.0033 11.8 0.013 Comparative Steel
29 0.035 0.16 0.008 0.01 0.004 0.049 0.0039 12.6 0.015 Comparative Steel
30 0.035 0.16 0.008 0.01 0.004 0.046 0.004 11.5 0.016 Invention
31 0.035 0.16 0.008 0.01 0.004 0.046 0.004 11.5 0.016 Invention
32 0.035 0.16 0.008 0.01 0.004 0.049 0.0039 12.6 0.015 Invention
33 0.041 0.18 0.008 0.01 0.005 0.038 0.005 7.6 0.013 Comparative Steel
34 0.021 0.016 0.008 0.012 0.008 0.04 0.004 10 0.015 Comparative Steel
35 0.04 0.018 0.007 0.011 0.006 0.045 0.004 11.25 0.002 Comparative Steel

Table II- Hot rolling, cold rolling, annealing parameters along with the mechanical properties of respective steel sheets.
HSM Rolling Cold Rolling CR Mechanical Properties
Sample No HSM FT HSM CT CR Thick Reduction BAF Cycle SPM Elongation YS UTS Elongation r -bar n value
1 890 545 0.8 75 700-680 0.8 155 294 45.8 2.11 0.23
2 890 545 0.8 75 700-680 1.2 160 298 46.7 1.988 0.228
3 890 545 0.8 75 700-680 0.8 159 295 47.8 2.05 0.226
4 890 545 0.8 75 700-680 0.8 148 293 47.9 2.021 0.234
5 890 545 0.7 75 700-680 0.8 160 304 46.3 2.13 0.23
6 890 545 0.7 75 700-680 0.8 164 293 45.4 2.15 0.223
7 890 545 0.7 75 700-680 0.8 151 299 41.6 2.25 0.165
8 890 545 0.7 75 700-680 0.8 161 296 46.1 1.995 0.227
9 890 555 0.75 71.15 700-680 0.8 178 318 45.6 1.601 0.204
10 890 555 0.8 69.23 700-660 0.8 169 313 46.5 1.6 0.239
11 890 555 0.75 71.15 700-680 0.8 180 320 44.2 1.68 0.24
12 890 555 0.7 70.83 700-680 0.8 163 304 47 1.65 0.241
13 890 555 0.76 70.77 700-680 0.8 166 306 47.7 1.61 0.242
14 890 555 0.8 69.23 700-680 0.8 162 305 46.5 1.70 0.241
15 890 555 0.8 69.23 700-680 0.8 162 306 45.9 1.64 0.237
16 890 555 0.8 69.23 700-680 1.0 170 312 45.4 1.76 0.239
17 890 545 0.7 70.83 700-680 0.8 154 295 47.3 2.01 0.244
18 890 545 0.8 69.23 700-680 0.8 150 290 48.5 2.16 0.242
19 890 545 0.8 69.23 700-680 0.8 156 286 46.5 2.166 0.238
20 890 545 0.7 73.08 700-680 0.8 155 283 46.9 2.179 0.237
21 890 545 0.7 73.08 700-680 0.8 157 287 46.3 2.12 0.236
22 890 545 0.8 69.23 700-680 0.8 156 285 47.7 2.041 0.238
23 890 545 0.8 69.23 700-680 0.8 155 296 46.7 2.182 0.226
24 890 545 0.8 69.23 700-680 0.9 151 289 46.1 2.103 0.226
25 890 545 0.8 69.23 700-680 0.8 154 286 47.7 2.276 0.227
26 890 545 0.8 69.23 700-680 0.8 156 297 47.1 2.228 0.228
27 890 545 0.8 69.23 700-680 0.8 157 288 45.3 2.10 0.221
28 890 560 1.6 68 700-690 0.9 168 296 47.1 1.75 0.224
29 890 550 1.6 68 700-690 0.9 163 284 48.7 1.75 0.224
30 890 545 1.4 68 700-690 0.9 156 285 47.9 2.317 0.221
31 890 545 1.6 68 700-690 1.0 151 281 48.6 2.09 0.225
32 890 545 1.6 68 700-690 0.9 149 280 49.2 2.1 0.227
33 890 545 1.6 68 700-690 0.9 163 293 47.8 2.13 0.222
34 890 555 0.8 69.23 700-680 0.8 154 295 47.2 2.01 0.224
35 890 555 0.8 69.23 700-680 0.8 175 310 44.1 1.63 0.24

Table III – Ageing properties of respective steels.

sample no Angle After 8% strain After Heating 6 hrs 1000C (equivalent to 6-months ageing ) After Heating 12 hrs 1000C (equivalent to 1-year ageing ) Surface Remarks
YS YPE TS YS YPE TS Aging YS YPE TS Aging
MPa % MPa MPa % MPa
21 90° 157 0 264 274 0 310 OK 274 0 310 OK Free from graphitization
0° 151 0 266 277 0 317 OK 276 0 316 OK
5 90° 160 0 267 278 0 312 OK 272 0 306 OK Free from graphitization
0° 158 0 270 281 0 319 OK 279 0 317 OK
6 90° 164 0 267 275 0 307 OK 273 0 304 OK Free from graphitization
0° 160 0 272 280 0 318 OK 280 0 317 OK
25 90° 153 0 264 269 0 304 OK 278 0 312 OK Free from graphitization
0° 148 0 265 275 0 314 OK 275 0 314 OK
27 90° 157 0 265 277 0 312 OK 277 0 311 OK Free from graphitization
0° 154 0 266 279 0 318 OK 278 0 316 OK
33 90° 163 0 293 283 0.9 320 YPE Observed 300 2.1 323 YPE Observed Free from graphitization
0° 160 0 294 286 0.8 322 YPE Observed 296 1.9 325 YPE Observed
34 90° 154 0 295 282 1 310 YPE Observed 302 2.4 310 YPE Observed Free from graphitization
0° 153 0 296 281 0.9 308 YPE Observed 299 2.1 308 YPE Observed
35 90° 175 0 310 277 0 324 OK 278 0 325 OK Graphitization Observed
0° 173 0 312 280 0 327 OK 282 0 330 OK

*The yield point elongation was measured after the test specimen was artificially aged at temperature of 100 °C for 12 hours which is equivalent to 1 year ageing.

* YPE observed in steel sample with Al/N ratio of <8 and in steel sample with C wt% <0.025, Graphitization observed in sample with Cr wt% <0.01

Example 1- from table I and II above it can be found that steel sample number 1 to 8 with chromium wt% ranges between 0.013-0.014 and cold reduction of 75% shows improved r-value of >1.95 with low yield strength (<160MPa) where as steel sample number 13 to 14 with higher chromium content of 0.03 wt% and lower cold reduction of about 70 % shows reduced r-value of about 1.6 along with high yield strength.

Example 2- The impact of Al/N ratio can be observed from steel sample number 1 to 8 where the ratio is between 10 -11.the outcome is high r-value with low yield strength due to less soluble Al content in steel matrix. Steel sample number 9 and 10 with high Al/N ratio of about 15 shows less r-value with higher yield strength as compared to sample 1 to 8. This can be concluded from the fact that higher Al/N ratio leads to higher solute Al content inside the steel matrix hence higher strength due to solid solution hardening effect, which reduces the drawability.
Example 3 – keeping the carbon level below 0.045wt% is justified by taking the examples of steel sample number 11 in table I and II. For sample number 11 with Carbon wt% of 0.05 wt%. A remarkable drop in r-value is observed along with increase in yield strength for sample 19. Hence it is concluded to keep the carbon level between 0.025-0.045 as keeping the level below 0.025 deteriorates the ageing property.

Example 4- Impact of carbon wt% of less than 0.025 wt% on ageing property of cold rolled batch annealed steel has been illustrated in table I and table III. Yield point elongation is observed in steel sample no 21 with carbon wt% of 0.021 while keeping the other parameters the same. Hence it is claimed to keep the carbon wt% more than 0.025 to achieve ageing guarantee of 1 year.

Example 5- Effect of Cr for avoiding graphitization is explained by taking example of sample 35 in table I and Table III. Reappearance of graphitization as a consequence of reducing the chromium wt% to 0.002 is observed for steel sample no 35 while keeping the other elements and process parameters unaltered.

To maximize r bar -value {111} <112> and {111} <110> components of Y-fiber (Figure 1) are the ideal crystallographic textures for deep drawing steel, because these texture gives the proper orientation of slip system for low carbon steel (BCC structure) so that the strength in the thickness direction is greater than that in the plane of the sheet. For example, if {100} plane parallels rolling plane, the strength is lowest in the thickness direction of sheet. This, in turn, adversely influences the formability of the sheet.

Example 6 - Figure 2 and Figure 3 showing Orientation distribution function (ODF) Phi2 = 45° for the present inventive steel which clearly shows the uniform Y-fiber [111] texture with negligible orientation density of rotated cube orientation {001} <110>. This attributes mainly for a very high r-bar value of 1.9 and above after annealing. The uniform [111] texture is an outcome of keeping a low coiling temperature of 530-550 0C to avoid Al/N precipitation during coiling ,high cold rolling reduction (75-80 % ) prior to annealing to get elongated grain structure [as shown in Figure 4, Figure 5(a) and Figure 5(b)] and maintaining selective Al/N ratio of 8-12 favoring sufficient AlN precipitation during annealing to facilitate {111} texture.

It is thus possible by way of the present invention to provide extra deep drawing(EDD) steel sheet having excellent formability with adequate surface finish suitable for outer panel for automobile. The EDD steel sheets according to the invention is low Carbon (0.025-0.045weight %) aluminum killed steel having specified composition comprising Al/N atomic ratio between 8 to 12, Cr :0.01 to 0.02, P≤0.015, Si≤0.015, Al:0.03 to 0.05 %, N:0 to 0.004%, CCM reduction between 75 to 80%, HSM Finishing Temperature 880 to 910°C and HSM Coiling temperature 530 to 550°C to avoid AlN precipitation before batch annealing . Al/N ratio of more than 8 ensures AlN precipitation during batch annealing and Al/N ratio less than 12 ensures very less remaining Al in the solid solution which assist in achieving excellent plastic anisotropy ratio (r-bar- 1.9 min) value. Controlled Cr is used to stabilize free C and to get graphitization free surface finish. Hot scarfing after casting used to avoid any surface defects like slivers. The optimum skin pass elongation (0.8±0.2%) used to get yield strength less than 160 MPa and free from stretcher strain. Thus, the EDD cold rolled steel developed through batch annealing route with excellent formability and surface finish which can be used for exposed panel of the automobile body and critical component which need excellent drawability.
We Claim:

1. Low carbon extra deep drawing(EDD) cold rolled steel sheets having composition comprising

C: 0.025-0.045 wt%,
Mn: 0.12-0.18 wt%,
N: 0 to 0.004 wt%,
P≤ 0.015 wt%,
Silicon≤0.015 wt%,
Cr: 0.01-0.02 wt%,
Al: 0.03-0.05 wt%,
and balance iron,
which is aluminium killed steel having specified Al/N atomic ratio between 8 to 12.

2. Low carbon extra deep drawing(EDD) steel sheet as claimed in claim 1 having enhanced plastic anisotropy ratio involving r-bar value of 1.9 min and yield strength of less than 160 MPa and free from stretcher strain.

3. Low carbon extra deep drawing(EDD) steel sheet as claimed in anyone of claims 1 or 2 comprising sheet having thickness 1.6 mm or less.

4. Low carbon extra deep drawing(EDD) steel sheet as claimed in anyone of claims 1 to 3 suitable for automobile body applications.

5. Low carbon extra deep drawing(EDD) steel sheet as claimed in anyone of claims 1 to 4 wherein carbon is maintained above 0.025wt% to avoid free carbon in steel matrix.
6. A process for producing low carbon extra deep drawing(EDD) steel sheets through batch annealing route as claimed in claims 1 to 5 comprising
providing molten steel having composition
C: 0.025-0.045 wt%,
Mn: 0.12-0.18 wt%,
N: up to 0.004 wt%,
P≤ 0.015 wt%,
Silicon≤0.015 wt%,
Cr: 0.01-0.02 wt%,
Al: 0.03-0.05 wt%,
and balance iron,
and processing for aluminum killed steel and with improving formability involving Al/N atomic ratio between 8 to 12 , HSM finishing temperature 880 to 910 0C and HSM coiling temperature of 5500 C or less preferably 5300 to 5500C .

7. A process as claimed in claim 6 carried out involving a combination of cold working deformation of 75% or more, Al/N ratio 12 or less , Cr addition 0.02wt% or less, HSM coiling Temperature 550 °C or less, cold spot temperature 680 °C or more to achieve said minimum r-bar value of 1.9.

8. A process as claimed in anyone of claims 6 to 7 carried out involving combination of HSM coiling temperature 530 °C or more and hot spot temperature 710°C or less to achieve strip surface with wrinkle free edges.

9. A process as claimed in anyone of claims 6 to 8 carried out involving a combination of Cr addition 0.01 % or more and batch annealing in 100% H2 atmosphere to achieve improved surface finish free from graphitization.

10. A process as claimed in anyone of claims 6 to 9 carried out involving a combination of batch annealing cold spot temperature 680 °C and skin pass elongation of 0.8±0.2% of colds rolled batch annealed steel sheets, to achieve yield strength less than 160 MPa.

11. A process as claimed in anyone of claims 6 to 10 carried out involving a combination of batch annealing cold spot temperature 680 °C and said selected composition of steel to achieve tensile strength of 310 MPa or less.

12. A process as claimed in anyone of claims 6 to 11 carried out in combination of Al/N ratio 8 or more, Cr 0.01% or more, HSM coiling temperature 530 °C or more such as to achieve yield point elongations zero even after 1 year of ageing.

13. A process as claimed in anyone of claims 6 to 12 comprising

(i) providing selective molten steel composition having
C: 0.025-0.045 wt%,
Mn: 0.12-0.18 wt%,
N: up to 0.004 wt%,
P≤ 0.015 wt%,
Silicon≤0.015 wt%,
Cr: 0.01-0.02 wt%,
Al: 0.03-0.05 wt%,
and balance iron,
prepared without vacuum degassing and killed in the conventional manner;

(ii) casting the steel into slabs through continuous casting route;
(iii) hot scarfing of slabs after casting to avoid any surface defects like slivers etc.
(iv) subjecting the slabs to hot rolling into hot rolled strip with slab reheating temperature kept between 1180 -1220 0C maintaining hot roll finishing temperature of at least 8800C and preferably 880-910 0C.
(v) subjecting the hot rolled steel strip to coiling while at a temperature exceeding >5300C, preferably 530-550 0C ;
(vi) subjecting the hot rolled strips to pickling in acid medium (HCl) having concentrations between 4-20%;
(vii) subjecting the hot rolled and acid pickeld strips to cold rolling with a minimum cold reduction of 75 percent, preferably 75-80 % to achieve higher plastic strain ratio post annealing;
(viii) Subjecting the cold rolled steel strip to batch annealing comprising
(a) heating a coil of cold rolled steel strip at a slow heating rate of about 40-45 0C /Hour in a batch annealing furnace in 100% Hydrogen atmosphere to maintain a uniform temperature throughout the coil from edge to core for achieving achieving excellent surface reflectivity 98% or more, avoiding graphitization;
(b) following annealing cycle of 710-680 °C maintaining the cold spot temperature of 6800C or high and hot spot temperature 710 °C or less to achieve a higher r –bar value of 1.9 and above;
(c) subjecting the batch annealed steel sheet to an optimum skin pass elongation of 0.6-1.0 % to get the yield strength less than 160 MPa, to avoid any yield point elongation.

Dated this the 27th day of March, 2015
Anjan Sen
Of Anjan Sen & Associates
(Applicants Agent)

ABSTRACT

TITLE: LOW CARBON EXTRA DEEP DRAWING(EDD) COLD ROLLED STEEL SHEETS AND A PROCESS FOR ITS PRODUCTION.

The present invention relates to Extra Deep Drawing (EDD) Steel sheet having excellent formability, and method for manufacturing the same through batch annealing route. The extra deep drawing steel sheet is having excellent formability with adequate surface finish suitable for outer panel for automobiles. Importantly, the EDD steel having selective composition comprising low Carbon (0.025-0.045weight %) aluminum killed steel having specified Al/N atomic ratio between 8 to 12, sheets produced through CCM reduction between 75 to 80%, HSM Finishing Temperature 880 to 910°C and HSM Coiling temperature 530 to 550°C to avoid AlN precipitation before batch annealing. Advantageously, selective Al/N ratio ensures very less remaining Al in the solid solution which assist in achieving excellent plastic anisotropy ratio (r-bar- 1.9 min) value. Controlled Cr is used to stabilize free C and to get graphitization free surface finish making the steel suitable for exposed panel of the automobile body and critical components which need excellent drawability.