DESC:FIELD OF THE INVENTION

The present invention relates to high yield ratio high strength cold rolled continuously annealed bake hardenable steel sheet having yield strength 240MPa or more, excellent drawability with r-bar value =1.5, excellent phosphatability and surface property after zinc phosphate chemical conversion coating treatment on said steel sheet surface, superior room temperature aging resistance in combination with YPE less than 0.15% after accelerated aging and Ductile-Brittle Transition Temperature (DBTT) less than -50 0C. Steel is having composition in terms of weight %, C: 0.001-0.003 wt%, Mn: 0.4-0.7 wt% , P: 0.04-0.08 wt% , V: 0.005-0.02 wt%, Al: 0.01-0.05 wt%, N: 0.004 or less, Si: 0.03 or less, B: 0.0005-0.0015 and the balance being Fe and other unavoidable impurities, where the value of [Mn+Si+Al]/[P] must be in the range of 6 to 19 in order to achieve yield strength more than 240 MPa, excellent phosphatability and DBTT less than -50°C.

BACKGROUND OF THE INVENTION

Generally, Bake hardenable steel are designed for outer panel automobile application for good drawability and dent resistance for light weighing and low fuel consumption. Ultra low carbon bake hardenable steel with UTS more than 340MPa with low yield ratio less than 0.65 are very common in bake hardening category of steel sheet for automotive application. These bake hardening steel sheet having compete ferrite matrix with precipitates contained therein are manufactured by adding solid solution strengthening elements such as Mn and P and controlling the C and N weight percent to get steel containing plenty of carbon in solid solutions which will impart subsequent bake hardening to the steel. However, a large amount of carbon in solid solutions results in yield point elongation at the stage of drawing of the steel sheet, which results in a poor outer panel appearance. Also, these steel sheets have initial yield strength less than 240 MPa in order to achieve low yield ratio and drawability before bake hardening. Usually in the attempt to achieve a higher yield ratio bake hardenable steel sheet having YS/UTS ratio =0.65 with YS=240 MPa the drawability deteriorates due to higher alloying and solid solution strengthening. In addition, room temperature aging, surface property and low temperature brittleness property of the steel sheet is rather poor.

As a prior art application number US 7540928 B2 discloses a bake hardenable steel sheet comprising in terms of weight percent: 0.03 -0.06 %of C 0.5-1.1 % of Mn, 0.08-0.20 % of Si, 0.015-0.070% of Al, 0.007 % or less N, 0.04 % or less of Ni, 0.04 %or less of Cu, 0.035% or less P, 0.015 or less S, 0.010 % or less Mo, 0.005% or less Ti; with specified B/N ratio. Specified invention is able to achieve a bake hardenable steel with YS > 240 MPa however the r-bar value and room temperature aging properly of the steel achieved by US 7540928 B2 will be rather poor due to very high solute carbon and low Ti weight %.

European Patent No. EP1704261B1 discloses bake hardenable cold rolled steel sheet having excellent formability, which Comprises: 0.003 - 0.005 wt% of C; 0.02 wt% or less of N; 0.03-0.2 wt% Mn, 0.2 wt% or less P optionally containing 0.2-1.2 wt% of Cu , 0.1-0.8 wt% Si, 0.2-1.2 wt% Cr ,0.01-0.2 wt% Mo .In this invention along with claimed chemistry range few relationships based on compositions have been claimed emphasizing on amount of Mn ,Cu required to fix S and Al/N ratio to fix N. Major Drawback in this prior art is that it doesn’t specify any alloying element to fix added 30 to 50 ppm of carbon which significantly hinders the drawability and r-value . To achieve bake hardenability of 30-60 MPa around 15-20 ppm of unfixed carbon would be sufficient. Invention specified in European Patent No. EP1704261B1 will suffer from low drawability and poor aging resistance as a result of 30 to 50ppm unfixed carbon. Also European Patent No. EP1704261B1 discloses MnS-precipitated steel with AlN precipitation strengthening with N level up 0.02 wt%. By adding such amount of N drawability and room temperature aging deteriorates badly. Also to fix such amount of N, amount of Al to be added will be very high which ultimately hinders the surface property. Also an additional amount of added Mo in present invention increases the cost of production.

US Patent number 4,410,372 discloses A process for producing non-ageing, deep-drawing steel strip having excellent paint bake-hardening property by continuous annealing having 0.001 -0.01 C , less than 1.5 Mn ,0.005-0.2 A , less than 0.007 wt% N along with B/N ratio 0.5 to 2.5 .The present invention emphasizing on fixing N only by B and Al . Major problem in this inventive grade is that room temperature aging cannot be guaranteed. As BN is not as stable as TiN and also more than 30ppm of boron results in edge cracking during hot rolling. No alloying addition has been done to fix excess C which deteriorates the aging as a consequence of uncontrolled BH Index.

There has been thus a need in the related filed to developing high strength with high yield ratio bake hardenable steel sheets in combination with good aging resistance and surface property with minimized yield point elongation to suit automobile application. Present invention thus attempts to solve problems of prior arts and provide a cold rolled continuously annealed bake hardening cold-rolled steel sheet having yield strength of 240MPa or more provided steel sheet has superior baking hardenability, excellent aging resistance, and good surface and Phosphating properties with adequate resistance to low temperature brittleness, high yield ratio which impart good dent resistance of the steel sheet. The present invention also relates to a process for manufacturing the said steel sheet.

OBJECTS OF THE INVENTION

The basic object of the present invention is directed to provide a cold rolled continuously annealed steel sheet having yield strength of 240MPa or more with superior baking hardenability, excellent aging resistance, and good surface and Phosphating properties with adequate resistance to low temperature brittleness, high yield ratio which impart good dent resistance and a method for producing the same.

A further object of the present invention is directed to provide a high yield ratio high strength cold rolled steel sheet wherein the properties of bake hardenability in combination with good drawability and aging resistance is achieved by limiting the unfixed carbon and/or nitrogen by judicious selection of weight percent of alloying elements in the composition and its processing.

A further object of the present invention is directed to provide a high yield ratio high strength cold rolled steel sheet wherein the composition is selectively controlled so that [Mn+Si+Al]/[P] must be in the range of 6 to 19 in order to ensure desired strength level YS 240MPa or more alongwith desired drawability(r-bar value) =1.5.

A still further object of the present invention is directed to provide a high yield ratio high strength cold rolled bake hardenable steel sheet wherein excellent phosphatability and surface property is achieved with phosphate crystal size of 4 µm or less and phosphate coating weight of 1.5-2.5 g/m2 after zinc phosphate chemical conversion coating treatment on said steel sheet surface.

A still further object of the present invention is directed to provide a high yield ratio high strength cold rolled bake hardenable steel sheet wherein vanadium added in composition at a level that ensure formation of vanadium carbide (VC) which is stable at room temperature may release the carbon at moderate temperature during bake hardening supplying excess free carbon to pin down the dislocations so that during baking process adding up to BH index without affecting the ageing resistance at room temperature.

SUMMARY OF THE INVENTION
The basic aspect of the present invention is directed a continuous annealed ultra low carbon high strength bake hardening steel sheet for automobile outer panel and the like of an automobile body comprising:
(In weight %) (In weight %)
C: 0.001-0.003 Al: 0.01-0.05
Mn: 0.4-0.7 N: 0.004 or less
P: 0.04-0.08 Si: 0.03 or less
V: 0.005-0.02 B: 0.0005-0.0015

and the balance being Fe and other unavoidable impurities, where as the value of [Mn+Si+Al]/[P] must be in the range of 6 to 19,
having yield strength of atleast 240MPa as bake hardenable sheet having excellent drawability with r-bar value =1.5.

A further aspect of the present invention is directed to a continuous annealed ultra low carbon high strength bake hardening steel sheet wherein the steel sheet contains Ti in an amount such that
7 = [Al +Ti]/ [N] = 30, whereas [M] = weight % of element M.

A still further aspect of the present invention is directed to said continuous annealed ultra low carbon high strength bake hardening steel sheet wherein the elemental composition satisfy following relation to achieve BH index more than 30 MPa and aging resistance of atleast 6 months,
[Ti+B] wt% = [2.6 N* +0.785 N],
Where N* = (14/48) x [Ti] wt%, N=Total N wt%, [M] = weight % of element M.

A still further aspect of the present invention is directed to a Continuous annealed ultra low carbon high strength bake hardening steel sheet, comprising by mass % atleast one type of element selected from the group comprising Nb, Zr, Mg, Cr, Mo, W, Hf, Co, Ni, Cu, Zn, Sc, Ca, Pb and Sn such that each element by content in the range of 0.002 to 0.025 %.

Another aspect of the present invention is directed to said continuous annealed ultra low carbon high strength bake hardening steel sheet wherein Ti, B, V are In relation to SPM elongation such that:
2.5 = 70[Ti] + 175[B] +27[V] +0.013[SPM] = 1
[M] = weight % of element M and SPM= skin pass elongation % after continuous annealing.

Yet another aspect of the present invention is directed to a continuous annealed ultra low carbon high strength bake hardening steel sheet, having YS/UTS ratio =0.65, mean planer anisotropy ratio (r-bar) of =1.4, bake hardening index of atleast 30MPa, and yield point elongation =0.15 after accelerated aging for atleast 6 months.

A further aspect of the present invention is directed to a continuous annealed ultra low carbon high strength bake hardening steel sheet having DBTT temperature less than -50 ºC and a phosphate crystal size of 4 µm or less and phosphate coating weight of 1.5-2.5 g/m2 after zinc phosphate chemical conversion coating treatment on said steel sheet surface.

A still further aspect of the present invention is directed to a continuous annealed ultra low carbon high strength bake hardening steel sheet having complete ferritic microstructure with precipitates of nitride and carbide forming elements where as ferrite grains having an average ferrite grain sizes are in the 11 to 19 micron.

A still further aspect of the present invention is directed to a process for manufacturing cold rolled steel sheet as described above, comprising the steps of:
a.) Reheating the slab having said composition to reheating temperature of 1160 °C -1220 °C;
b.) Said Reheated slab being roughing rolled in roughing mill with roughing mill delivery temperature of 1060°C or less ;
c.) Said rough rolled steel being subjected to finish rolling after at temperature range of 860°C to 920°C;
d.) Coiling the finish rolled steel at with run out table cooling rate of 8 °C/second or more; and
e.) Cold rolling the said hot rolled steel sheet with cold reduction of 60% or more.

Another aspect of the present invention is directed to a process for manufacturing cold rolled steel sheet comprising:
a) Annealing at soaking section temperature range of 770 °C to 830°C with residence time of for 45 to 110 seconds;
b) Slow cooling the steel up to a temperature range of 660°C to 720°C after soaking ;
c) Rapid cooling the steel up to a temperature range of 420 °C to 500 °C with cooling rate of 10°C / second to 30°C / second;
d) Overaging said steel at temperature range of 320°C to 400°C with residence time of 100 seconds to 300 seconds; and
e) Subjecting the overaged steel to skin pass elongation of 1.4% to 2%.

Yet another aspect of the present invention is directed to said process for manufacturing cold rolled steel sheet wherein Ti, B, V are selectively In relation to SPM elongation such that:
2.5 = 70[Ti] + 175[B] +27[V] +0.013[SPM] = 1
[M] = weight % of element M and SPM= skin pass elongation % after continuous annealing.

A further aspect of the present invention is directed to said process for manufacturing cold rolled steel sheet carried out such as to provide for steel sheet having selectively:
YS=240 MPa, YS/UTS ratio =0.65, mean planer anisotropy ratio (r-bar) of =1.4, bake hardening index of atleast 30MPa, and yield point elongation =0.15 after accelerated aging for atleast 6 months;

DBTT temperature less than -50 ºC and aphosphate crystal size of 4 µm or less and phosphate coating weight of 1.5-2.5 g/m2 after zinc phosphate chemical conversion coating treatment on said steel sheet surface.; and

complete ferritic microstructure with precipitates of nitride and carbide forming elements where as ferrite grains having an average ferrite grain sizes are in the 11 to 19 micron.

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

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1: shows the plot of {70[Ti] + 175[B] +27[V] +0.013[SPM]} versus YPE after 6 hours of artificial aging in oil bath maintained at 100 0C (Shown as Triangle) to simulate 6months of aging, wherein data labeled above the triangle shows the respective BH index values.

Figure 2: shows optical micrograph of inventive 240MPa Yield strength level steel sheet (Steel number 2) according to present invention having complete ferrite gains with precipitates wherein the average ferrite grain size is 16.2 micron.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO ACCOMPANYING DRAWINGS AND EXAMPLES INCLUDING A PREFERRED EMBODIMENT

The present invention is directed to provide a cold rolled continuously annealed 240MPa yield strength level bake hardenable steel sheet having excellent drawability with r-bar value =1.5, excellent phosphatability and surface property with phosphate crystal size of 4 µm or less and phosphate coating weight of 1.5-2.5 g/m2 after zinc phosphate chemical conversion coating treatment on said steel sheet surface, superior room temperature aging resistance in combination with YPE less than 0.15% after accelerated aging and DBTT less than -50 0C.

Following abbreviations have been used to describe the present invention:

SS – Soaking section in continuous annealing furnace
SCS – Slow cooling section section in continuous annealing furnace
RCS- Rapid cooling section in continuous annealing furnace
OAS- Over aging section in continuous annealing furnace
YS – Yield strength in MPa
UTS- Ultimate tensile strength in MPa
BH – Bake hardening index in MPa
SPM %– Skin passes elongation in %
YP- Yield Point
YPE- Yield Point Elongation %
El%-Total Elongation at 50mm gauge length sample
CCM%- Cold rolling reduction %
DBTT- Ductile-Brittle Transition Temperature

The steel sheet according to present invention is having the composition in terms of weight %, C: 0.001-0.003 wt% , Mn: 0.4-0.7 wt% , P: 0.04-0.08 wt% , V: 0.005-0.02 wt%, Al: 0.01-0.05 wt%, N: 0.004 or less, Si: 0.03 or less, B: 0.0005-0.0015 and the balance being Fe and other unavoidable impurities, where as the value of [Mn+Si+Al]/[P] must be in the range of 6 to 19 in order to achieve excellent phosphatability , Yield strength =240 MPa and average ferrite grain diameter in the range of 11 to 19 micron and very low DBTT.

Furthermore, the cold rolled steel as described above contains Ti in an amount in relation to Al and N such that 7 = [Al +Ti]/ [N] = 30.

Also various compositional elements as described above have in relation to skin pass elongation provided after annealing such that the following relation must be satisfied in order to achieve good room temperature aging with desired strength level of 240 MPa,
70[Ti] + 175[B] +27[V] +0.013[SPM] = 1.
Furthermore, the B and Ti weight % are in specified relation with total allowable N in steel so as to achieve minimum room temperature aging resistance of atleast 6 months,
[Ti+B] wt% = [2.6 N* +0.785 N]
Where N* = (14/48) x [Ti] wt%, N=Total N wt%, [M] = weight % of element M.

The bake-hardenable cold rolled continuously annealed steel sheet of the present invention, and a method of manufacturing the same is described in detail as follows,

Carbon (0.001-0.003wt %) – Carbon is the principal element determining the Bake hardenability. As the C wt% decreases it reduces the strength and increases drawability (r-bar value). For getting a Bake hardening value of 30 MPa or more at lower yield point after paint baking it is required that around 5-10 ppm of free C should remain in solution. In this regard, controlled amount of Ti and V are added to get the desired amount of free carbon.
Increasing the amount of unfixed solute C (>15ppm) drastically reduces the formability (r-bar value) of cold –rolled steel sheet. Additionally, higher C content in solution triggers room temperature aging resulting in YPE in material. Hence the upper limit is fixed to 0.003wt%.

Mn (0.4-0.7) wt. % -For present Invention, Mn ranges from 0.4 to 0.7wt% to achieve the desired strength level without compromising with the drawability. Mn also fixes S to form MnS and prevent hot shortness and edge cracking. Higher wt% of Mn unnecessarily increase the strength above the specified strength level as claimed in this invention. In addition, drawability is reduced drastically. Higher Mn level also deteriorates the phosphatability and zinc coating properties due to surface oxide formation. Therefore the upper limit is set to 0.7 wt%.

P (0.035-0.06) wt% - Addition of phosphorus serves following purpose
1. Among all the alloying additions in present invention phosphorus has the highest solid solution strengthening effect hence it helps to achieve the desired yield strength and Ultimate tensile strength values through grain refinement and solid solution strengthening .
2. When added in controlled amount it improves the anisotropy and strength without compromising with drawability and r-bar value.
3. P has site competition with C as P segregates at the grain boundary which is favourable precipitation site for C. As less carbon segregates at the grain boundary higher solute C will be available inside the gains, hence bake hardenability is improved.

In contrast, Increase in phosphorus content more than 0.1 wt% drastically reduces the secondary work Embrittlement resistance (Increasing ductile to brittle transition temperature) by deteriorating the bonding force between grain boundaries. High P content also causes welding problems .Therefore the upper limit has been set as 0.08 wt% to achieve the desired strength level without negotiating much with drawability. Reducing P wt% to 0.035 % or less would not give desired strengthening effect hence the claimed lower limit has been set as 0.035wt%.

V (0.005-0.02) wt% - As compared to Ti and Nb, V is a weak carbide former and tends to free carbon in to solution even at moderate temperature. This tendency has been used as an advantage to get improved BH index in present invention by adding small amount (0.005-0.02 wt %) of V. Vanadium carbide (VC) which is stable at room temperature may release the carbon at moderate temperature during bake hardening supplying excess free carbon to pin down the dislocations. Amount of solute carbon which can be fixed with adding V can be calculated as –
C (wt %) = (12/51)*V (wt %)

Above relation gives minimum 12 ppm of C which can be fixed with 0.005 wt% of V. Excess 5-10 ppm of solute carbon is intentionally left free to achieve the BH Index of 30 MPa or more at lower yield point. The fraction of C which was earlier fixed by V, released during baking process adding up to BH index without affecting the ageing resistance at room temperature.

B (0.0005-0.002) wt% -Controlled Boron addition improves the secondary work embrittlement resistance and BH Index. B can either reside as interstitial solid solution element at grain boundaries or form Boron Nitride (BN) to fix solute Nitrogen. Boron has major effect in steel properties compared with the quantity added; hence the amount should be precisely controlled. Excess free B in interstitial space hinders the drawability and also cause edge cracking during hot rolling hence the upper limit is restricted to 0.002 wt%

7= [Al+Ti]/N =30 - Amount of Ti along with Al and N weight % mainly governs the value of BH index achievable. It’s the combination of these three elements in relation which results in best combination of Bake hardenability and aging resistance. To achieve a bake hardening index of atleast 30MPa and room temperature aging resistance of minimum 6 months relation 7= [Al+Ti]/N =30 must be satisfied. [Al+Ti]/N value less than 7 results in room temperature aging where as value >30 results in lower BH index.

(N - 0.004wt% or less) –Nitrogen in solution deteriorates the drawability (r-value) and room temperature aging property therefore it must be fixed. The upper limit for N is 0.004 wt%. As diffusivity of N is very high, steel having Higher N content are vulnerable to room temperature aging. To encounter that, a sufficient amount of Ti and B must be added to fix the mobile N as TiN and BN. It is advisable to keep N to 40ppm or less to achieve improved room temperature aging resistance
Additionally following equation must be fulfilled to achieve the aging guarantee of more than 6 months –
(Ti+B) wt% = (2.64 N* +0.785 N)
Where N* = N wt% fixed by Ti, N=maximum N wt% which can be added.

Si (0.03 wt% or less) – Si is present in the present steel as a result of steel making process and not added intentionally. The amount of Si must be kept to its minimum as it deteriorates the surface and coating property.

[Mn+Si+Al]/[P] wt% in the range of 6 to 19 – In present invention the amount of solid solution strengthening elements added must be controlled as per the relation so that [Mn+Si+Al]/[P] must be in the range of 6 to 19. Failing to do so will result either in lower strength if [Mn+Si+Al]/[P] is less than 6 or poor drawability(r-bar value) and phosphatability if the [Mn+Si+Al]/[P] value is above 19 .

Nb, Zr, Mg, Cr, Mo, W, Hf, Co, Ni, Cu, Zn, Sc, Ca, Pb and Sn (atleast one in the range of 0.002 to 0.03 %) - each of from V, Zr, Mg, Mo, W, Hf, Co, Ni, Cu, Zn, Ca, Pb and Sn acts either as carbide former and/or nitride former and/or solid solution strengthening elements, however adding each of these elements in an amount more than 0.025 wt% unnecessarily adds up to the cost of the steel and deteriorate the BH index and r-bar value.

Complete Description of process of manufacturing:
To achieve Slab chemistry as described in scope of the invention Heat from basic oxygen furnace (BOF) is processed through RH degasser and subsequently continuously casted. Special measure have been taken to hot roll resulted slabs by keeping slab reheating temperature in the range of 1160°C to 1220°C intended to control roughing mill delivery temperature under 1060°C and finishing mill entry temperature under 1020°C to check surface defects like rolled in scale .During hot rolling finishing mill temperature range of 860°C to 920°C and run out table cooling rate from finishing mill to coiler of more than 8 0C/sec was maintained to achieve coiling temperature range of 620 °C to 670 °C .Hot rolled coils were subsequently processed through pickling coupled with tandem cold rolling mill to remove the oxide surface present in the surface and to provide cold reduction of 60% or more .

Following pickling and cold rolling to desired thickness, cold rolled steel sheet being processed through continuous annealing line, where electrolytic cleaning removes rolling emulsion present on the surface. Cleaned surface passes through the preheating and heating section where the strip is heated at the rate of 1-10 0C/sec to soaking section temperature maintained at 770 °C -820 °C. Annealing time of 45-110 seconds gives desired results for present bake hardenable steel. After soaking steel strip passes through slow cooling section at cooling rate of less than 3°C/sec .Slow cooling section temperature of 660 °C - 720°C was maintained. Following slow cooling section annealed strip sheet been rapid cooled at 10 °C/sec or more up to rapid cooling section temperature maintained in the range of 420-500 0C. After rapid cooling section annealed strip was over aged keeping the over aging section temperature of 320°C -400 °C for 100 to 300 seconds. Subsequent to over aging steel strip is given a skin-pass elongation (temper rolling) in the range of 1.4 % to 2% to avoid yield point elongation and to improve flatness of steel strip. In addition in order to achieve improved aging resistance and BH index >30 MPa following relation must be fulfilled –

2.5 = 70[Ti] + 175[B] +27[V] +0.013[SPM] = 1

Where, [M] = weight % of element M and SPM= skin pass elongation % after continuous annealing. The relationship of 70[Ti] + 175[B] +27[V] +0.013[SPM] and their respective BH index values vs YPE after 6 hours of artificial aging in oil bath maintained at 100 0C to simulate 6months of aging and is shown in Figure 1.

Furthermore, Cold rolled bake hardenable steel sheet described in present invention can be processed through continuous galvanizing route for zinc coating to produce GA/GI steel sheets and used as coated product for similar applications.

Method of evaluating bake hardening in a tensile test:
Tensile test specimen as per JIS Z2241 No.5 with 50mm gauge length 25mm width was and prepared across the rolling direction of steel sheet. Tensile test specimen was then strained to 2% at strain rate of about 0.008/second and then heated at 1700C for 20 minutes. Heated sample was then subjected to tensile test. Bake hardening index was then evaluated by measuring the difference between the initial strength at 2% strain before bake hardening and final yield strength (at lower yield point) after heating at 1700C for 20 minutes.

Method of evaluating phosphatability :
Phosphating process is a chemical the treatment of a metal surface which gives reasonably hard, electrically non-conducting surface coating of insoluble phosphate which is contiguous and highly adherent to the underlying metal and is considerably more absorptive than the metal which provides excellent corrosion resistance and paint ability to steel surface .The coating is formed as a result of a top chemical reaction, which causes the surface of the base metal to integrate itself as a part of the corrosion resistant film. [1]
[1]- T.S.N. Sankara Narayanan, SURFACE PRETREATMENT BY PHOSPHATE CONVERSION COATINGS, Rev.Adv.Mater.Sci. 9 (2005) 130-177
To evaluate phosphatability firstly alkali degreasing was performed on steel sheet at 400 C for 120 sec using FC-E2032 chemical manufactured by NIHON PARKERIZING India Pvt Ltd to the obtained cold rolled steel sheet without any oil/grease on surface. Degreasing was followed by water rinsing and then surface conditioning at room temperature for 30 seconds using PL-Z chemical manufactured by NIHON PARKERIZING India Pvt Ltd. Phosphate treatment using PB-L3020 chemical, manufactured by NIHON PARKERIZING India Pvt was done at 400 C for 90 seconds. Subsequently, the surface after phosphate treatment was observed under a Scanning electron microscope using Secondary Electron image mode. Average grain size was measured assuming circular phosphate crystals. Crystal size < 4µm is considered as excellent for phosphatability. The phosphate coating weight was measured using the XRF method and steel sheet with average coating weight after zinc phosphate chemical conversion coating of 1.5-2.5 g/m2 is considered having excellent phosphatability.

Complete description of Inventive steel and comparative steel grades are illustrated from table 1 to 3
Table-1- Table 1 listing chemical compositions of example and comparative steel sheets.
Table-2- Hot rolling, cold rolling and annealing parameters of inventive and comparative steel sheets having chemical compositions as per table 1.
Table-3- Mechanical properties, surface phosphatability property, DBTT temperatures, bake hardening index and artificial aging property of inventive and comparative steels having chemical composition as per table 1 and being processed as per table 2.

As can be witnessed from Table 1 and 2, Casting and processing of Steels remarked as “Ex.” are carried out by strictly controlling the content of C, Mn ,P, B, Al, N, V and Ti to satisfy the condition of 0.001-0.003 weight% C , Si:<0.03 weight% , 0.4-0.7 weight% Mn, 0.01-0.05 weight% Al , 0.04-0.08 weight% P, N less than 0.004 % , 0.0005-0.002 weight % B, 0.005-0.02 weight% V , 7= [Al+Ti]/N =30 and the remainder being Fe and inevitable impurities . All the relation specified in the scope of the invention has been satisfied by the steels remarked as “Ex.” .Whereas steel numbers remarked as “Comp.” doesn’t comply with atleast one of the scope of present invention.

TABLE -1
Steel
No C,
wt% Mn,
wt% P,
wt% Si,
wt% Al,
wt% N,
wt% Ti,
wt% B,
wt% V,
wt% [Mn+Si+Al]
/[P] [Al+Ti]/ [N] Other Elements Remarks
1 0.0025 0.55 0.035 0.005 0.02 0.003 0.015 0.001 0.01 16.4 11.7 Zr:0.003, Mg:0.003, Cr:0.01, Mo:0.004 Ex.
2 0.002 0.51 0.052 0.004 0.04 0.0036 0.014 0.001 0.009 10.7 15.0 W:0.002, Hf:0.002, Co:0.003, Ni:0.005 Ex.
3 0.0025 0.55 0.06 0.002 0.04 0.0037 0.018 0.001 0.01 9.9 15.7 Cu: 0.005, Zn:0.004 Ex.
4 0.002 0.2 0.045 0.005 0.02 0.004 0.002 0.003 5.0 5.5 Nb:0.015 Comp.
5 0.015 1.5 0.08 0.2 0.034 0.004 0.002 0 21.7 9.0 Nb: 0.03 Comp.
6 0.002 0.6 0.054 0.005 0.032 0.004 0.013 0.001 0.009 11.8 11.3 Ca:0.003, Pb:0.003 Ex.
7 0.003 0.67 0.065 0.005 0.032 0.004 0.014 0.002 0.015 10.9 11.5 Cr:0.01, Mo:0.005,Ni:0.003 Ex.
8 0.0025 0.15 0.014 0.007 0.02 0.004 0.005 0.005 12.6 6.3 Nb: 0.015 Comp.
9 0.005 0.22 0.12 0.3 0.01 0.004 0.006 0.003 4.4 4.0 - Comp.
10 0.004 0.45 0.051 0.004 0.05 0.002 0.05 0 9.9 50.0 Ni:0.022,Cu:0.025,
Mo:0.003 , Comp.
11 0.002 0.45 0.05 0.02 0.02 0.0035 0.01 0.002 0.013 9.8 8.6 Sc: 0.002 , Hf:0.004 Ex.

* Ex. - Present inventive example, Comp.- Comparative Examples
* All compositional elements are in terms of weight %.
* Underlined and shaded boxes indicates “outside the appropriate range”
* Steels having value of [Al+Ti]/ [N] less than 7 do not comply with room temperature aging resistance. Aging test results for the same have been described in table number 3.
* Steel having [Mn+Si+Al]/[P] ratio in the range of 6 to 19 comply with the scope of the invention satisfying the strength and phosphatability requirement .

TABLE -2
Steel
No FT,
0C ROT cooling rate ,
0C/sec SS,
0C SCS,
0C RCS,
0C Rapid Cooling rate,
0C/sec
OAS,
0C
SPM,
% CR %,
AT,
0C [Ti+B] [2.6 N* +0.785 N] 70[Ti] + 175[B] +27[V] +0.013[SPM] Remarks
1 910 10.5 810 710 475 13.7 387 1.7 74 75 0.016 0.014 1.52 Ex.
2 920 11.2 800 710 480 16.4 400 2 74 63 0.015 0.013 1.46 Ex.
3 914 12.8 760 711 479 16.2 384 1.85 70 67 0.019 0.017 1.73 Ex.
4 918 10.3 800 720 480 10.5 389 1 78 105 0.002 0.005 0.23 Comp.
5 900 9.4 797 695 474 20 381 1.5 65 49 0.002 0.005 0.16 Comp.
6 914 11.5 775 710 480 19.8 380 1.8 75 48 0.014 0.013 1.39 Ex.
7 914 9.3 770 701 470 19.5 395 1.9 75 48 0.016 0.014 1.67 Ex.
8 910 9.7 830 690 457 10.8 349 0.5 78 102 0.005 0.007 0.49 Comp.
9 894 12.7 786 684 427 15.5 361 0.7 71 58 0.006 0.008 0.51 Comp.
10 890 13.1 780 691 448 13.1 379 1.2 71 73 0.050 0.039 3.52 Comp.
11 919 12.6 812 715 471 19.9 372 1.9 75 49 0.012 0.01 1.43 Ex.
* Ex. - Present inventive example, Comp.- Comparative Examples
* FT- hot finish rolling temperature , ROT- run out table in hot strip mill , SS- soaking section ,SCS- Slow cooling section , RCS- Rapid cooling section , OAS- Overaging section , SPM- Skin pass elongation ,CR – Cold rolling reduction %,AT- Annealing Time
** Underlined and shaded boxes indicates “outside the appropriate range”
*** Steel having value of 70[Ti] + 175[B] +27[V] +0.013[SPM] in the range of 2.5 to 1 comply with the scope of the invention.
*** Steel sheets which does not satisfy the relation [Ti+B] wt% = [2.6 N* +0.785 N], Where N* = (14/48) x [Ti] wt% and N=Total N wt%, does not comply with the scope of present invention.

TABLE -3
S.No YS,
MPa UTS,
MPa YS /
UTS El % r-bar n -value Ferrite Grain Size,
µm BH Index,
MPa Phosphat-
-ability
Remark YPE ,(After 6 hours of aging at 100 0C) DBTT,
0C Remarks
1 245 361 0.68 45.9 1.7 0.21 16.2 37 O 0.1 -60 Ex.
2 249 352 0.71 46.8 1.69 0.2 14.3 41 O 0.05 -60 Ex.
3 258 361 0.71 45.2 1.73 0.202 13.6 42 O 0.15 -70 Ex.
4 183 321 0.57 47.2 1.6 0.215 21.2 63 O 1.22 -60 Comp.
5 345 451 0.76 32.1 1.02 0.13 7.5 55 ? 1.4 -15 Comp.
6 261 363 0.72 43.8 1.59 0.201 14.8 42 O 0.15 -60 Ex.
7 258 371 0.70 44.3 1.63 0.21 13.9 38 O 0 -70 Ex.
8 169 303 0.56 49.3 1.7 0.22 22.8 61 O 1.35 -60 Comp.
9 227 371 0.61 41.1 1.3 0.175 18.1 57 ? 1.17 -10 Comp.
10 220 351 0.63 42.5 1.5 0.19 20.5 2 O 0 -60 Comp.
11 248 359 0.7 47.2 1.8 0.212 14.5 40 O 0 -70 Ex.

* O - Steel shows good phosphatability after zinc phosphate chemical conversion coating having phosphate crystal size less than 4µm and phosphate coating weight in the range of 1.5-2.5 g/m2 ,
? - Steel shows poor phosphatability after zinc phosphate chemical conversion coating having phosphate crystal size more than 4µm and phosphate coating weight more than 2.5 g/m2 ,
* Steel sheets having DBTT more than -50 0C does not comply with the scope of the invention susceptible to low temperature embrittlement.
* To simulate the room temperature aging tensile test specimen of 50mm gauge length was immersed in oil bath homogeneously maintained at 1000C for 6 hours to simulate 6 months aging .Aged sample were subjected to tensile test. Steels having YPE <0.15 % after three months of artificial aging and subsequent tensile test fulfills the scope of the invention and comply with six months of aging guarantee.
* “Ex.” Steel sheets have the average ferrite grain size in the range of 11 to 19 micron.

The present invention and its scope are further illustrated with the help of following examples:

Example 1: It can be appreciated from Table-1 to Table-3 that sheet samples 1,2,3,6,7 and 11 marked as “Ex.” are satisfying all the scope of the invention and shows good drawability (r-bar>1.5), Excellent phosphatability with phosphate crystal size <4µm post zinc phosphate chemical conversion coating, good resistance to low temperature embrittlement with DBTT value less than -50 0C, improved aging resistance to artificial aging with YPE less than 0.15 % post artificial aging in oil bath for 6 hours maintained at 100 0C , ferrite grain size in the range of 11 to 19 micron , minimum yield strength of 240 MPa with yield ratio of =0.65 .

Example 2: Steel number 4 (Tbale-1) has Mn weight % lower than the specified range. Also the value of [Mn+Si+Al]/[P] is 5.0 (as shown in Table-1)which is out of the specified range of 6 to 19 . In addition, steel sheet number 4 has [Al+Ti]/[N] value of 5.5 which is outside the specified range of 7 to 30. As a result steel number 4 shows lower YS of 183 MPa, poor room temperature aging property with YPE 1.22% after artificial aging due to very high BH index of 63 MPa (as shown in Table-3). In steel number 9 having lower Mn and higher P weight % than the specified range. The value of [Mn+Si+Al]/[P] is 4.4 which is out of the specified range of 6 to 19 and it resulted in poor phosphatability and poor DBTT -10°C. Similarly in steel number 5 having high Mn and higher Si than the specified range. The value of [Mn+Si+Al]/[P] is 21.7 which is out of the specified range of 6 to 19 which resulted in poor phosphatability and poor DBTT -15°C.

In steel sheet number 8 has [Al+Ti]/[N] value of 6.3 which is outside the specified range of 7 to 30 . Steel number 8 also violate the relation 2.5 = 70[Ti] + 175[B] +27[V] +0.013[SPM] = 1 with 70[Ti] + 175[B] +27[V] +0.013[SPM] value 0.49 (Table-2). Ferrite grain size of steel number 8 is 22.8 micron due to lower solid solution strengthening. As a result steel number 8 doesn’t comply with scope of the present specification. Steel number 8 has lower YS of 169 MPa, lower YS/UTS ratio of 0.56, Higher YPE of 1.35 after artificial aging and very high BH index of 61 MPa.(as shown in Table-3 )

Example 3: Steel number 5 (Table-1) having Mn and C weight higher than the specified range of present inventive steel doesn’t comply with the r-bar as well as the phosphatability requirement. Higher Mn of 1.5 weight resulted in excessive surface oxidation where as higher C of 0.015 weight with higher Mn %resulted in poor r-bar value due to excessive solid solution strengthening. In addition steel number 5 doesn’t comply with the relation 6= [Mn+Si+Al]/[P] =19 . Also, the relation [Ti+B] wt% = [2.6 N* +0.785 N] has been violated for steel number 5. As a result steel number 8 shows higher YPE 0f 1.4 % after artificial aging. (as shown in Table -3 )

Example 4 – Steel number 9 (Table-1) with high phosphorus weight % of 0.12 shows increased DBTT value of -10 0C because of higher P higher phosphorus segregation at the gain boundary resulting in weakening the ferrite grain boundary at lower temperature. In addition steel number has 70[Ti] + 175[B] +27[V] +0.013[SPM] value 0.51 (Table-2) which is outside the minimum specified value of 1. As a result steel number 9 doesn’t comply with room temperature aging requirement.

Example 5- Steel number 10 has [Al +Ti]/ [N] value of 50 which is outside the upper specified range of 30. As a result steel number 10 has BH index of 2 MPa and doesn’t comply with the minimum BH index of 30 MPa.

It is thus possible by way of the present invention to provide high strength cold rolled continuous annealed bake hardenable steel sheet having YS =240 MPa, YS/UTS ratio = 0.65, mean planer anisotropy ratio (r-bar) of =1.4, bake hardening index of atleast 30 MPa, and yield point elongation =0.15 after accelerated aging for atleast 6 months, DBTT temperature less than -50 ºC and a phosphate crystal size of 4 µm or less and phosphate coating weight of 1.5-2.5 g/m2 after zinc phosphate chemical conversion coating treatment on said steel sheet surface and a ferritic microstructure having an average ferrite grain sizes in the range of 11 to 19 micron, suitable for automobile outer panel and the like of an automobile body applications

,CLAIMS:We Claim:
1. A Continuous annealed ultra low carbon high strength bake hardening steel sheet for automobile outer panel and the like of an automobile body comprising:
(In weight %) (In weight %)
C: 0.001-0.003 Al: 0.01-0.05
Mn: 0.4-0.7 N: 0.004 or less
P: 0.04-0.08 Si: 0.03 or less
V: 0.005-0.02 B: 0.0005-0.0015

and the balance being Fe and other unavoidable impurities, where as the value of [Mn+Si+Al]/[P] must be in the range of 6 to 19,
having yield strength of atleast 240MPa as bake hardenable sheet having excellent drawability with r-bar value =1.5.

2. A Continuous annealed ultra low carbon high strength bake hardening steel sheet as per claim 1 wherein the steel sheet contains Ti in an amount such that
7 = [Al +Ti]/ [N] = 30, whereas [M] = weight % of element M.

3. A Continuous annealed ultra low carbon high strength bake hardening steel sheet as per anyone of claim 1 and 2 wherein the elemental composition satisfy following relation to achieve BH index more than 30 MPa and aging resistance of atleast 6 months,
[Ti+B] wt% = [2.6 N* +0.785 N]
, Where N* = (14/48) x [Ti] wt%, N=Total N wt%, [M] = weight % of element M.
4. A Continuous annealed ultra low carbon high strength bake hardening steel sheet as per anyone of claim 1 to 3, comprising by mass % atleast one type of element selected from the group comprising Nb, Zr, Mg, Cr, Mo, W, Hf, Co, Ni, Cu, Zn, Sc, Ca, Pb and Sn such that each element by content in the range of 0.002 to 0.025 %.

5. A Continuous annealed ultra low carbon high strength bake hardening steel sheet as per anyone of claim 1 to 4
whereinTi, B, V are In relation to SPM elongation such that:
2.5 = 70[Ti] + 175[B] +27[V] +0.013[SPM] = 1
[M] = weight % of element M and SPM= skin pass elongation % after continuous annealing.

6. A Continuous annealed ultra low carbon high strength bake hardening steel sheet as per anyone of claims1 to 5, having YS/UTS ratio =0.65, mean planer anisotropy ratio (r-bar) of =1.4, bake hardening index of atleast 30MPa, and yield point elongation =0.15 after accelerated aging for atleast 6 months.

7. A Continuous annealed ultra low carbon high strength bake hardening steel sheet as per anyone of claims 1 to 6 having DBTT temperature less than -50 ºC and a phosphate crystal size of 4 µm or less and phosphate coating weight of 1.5-2.5 g/m2 after zinc phosphate chemical conversion coating treatment on said steel sheet surface.

8. A Continuous annealed ultra low carbon high strength bake hardening steel sheet as per anyone of claim 1 to 7 having complete ferritic microstructure with precipitates of nitride and carbide forming elements where as ferrite grains having an average ferrite grain sizes are in the 11 to 19 micron.
9. A process for manufacturing cold rolled steel sheet as per anyone of claim 1 to 8, comprising the steps of:
a) Reheating the slab having said composition to reheating temperature of 1160 °C -1220 °C;
b) Said Reheated slab being roughing rolled in roughing mill with roughing mill delivery temperature of 1060°C or less ;
c) Said rough rolled steel being subjected to finish rolling after at temperature range of 860°C to 920°C;
d) Coiling the finish rolled steel at with run out table cooling rate of 8 °C/second or more; and
e) Cold rolling the said hot rolled steel sheet with cold reduction of 60% or more.

10. Process for manufacturing cold rolled steel sheet according to claim 9, comprising:
a) Annealing at soaking section temperature range of 770 °C to 830°C with residence time of for 45 to 110 seconds;
b) Slow cooling the steel up to a temperature range of 660°C to 720°C after soaking ;
c) Rapid cooling the steel up to a temperature range of 420 °C to 500 °C with cooling rate of 10°C / second to 30°C / second;
d) Overaging said steel at temperature range of 320°C to 400°C with residence time of 100 seconds to 300 seconds; and
e) Subjecting the overaged steel to skin pass elongation of 1.4% to 2%.

11. Process for manufacturing cold rolled steel sheet according to anyone of claims 9 or 10 wherein Ti, B, V are selectively In relation to SPM elongation such that:
2.5 = 70[Ti] + 175[B] +27[V] +0.013[SPM] = 1
[M] = weight % of element M and SPM= skin pass elongation % after continuous annealing.

12. Process for manufacturing cold rolled steel sheet according to anyone of claims 9 to 11carried out such as to provide for steel sheet having selectively:
YS=240 MPa, YS/UTS ratio =0.65, mean planer anisotropy ratio (r-bar) of =1.4, bake hardening index of atleast 30MPa, and yield point elongation =0.15 after accelerated aging for atleast 6 months;

DBTT temperature less than -50 ºC and aphosphate crystal size of 4 µm or less and phosphate coating weight of 1.5-2.5 g/m2 after zinc phosphate chemical conversion coating treatment on said steel sheet surface.; and

complete ferritic microstructure with precipitates of nitride and carbide forming elements where as ferrite grains having an average ferrite grain sizes are in the 11 to 19 micron.

Dated this the 4th day of October, 2016
Anjan Sen
Of Anjan Sen & Associates
(Applicants Agent)