DESC:FIELD OF THE INVENTION

The present invention relates to high strength ultra-low Carbon steel having excellent bake hardenability, improved ageing resistance, excellent formability and surface finish, and method for manufacturing the same through continuous annealing route. More specifically, the advancement is directed to method of manufacturing two categories of ultra low carbon cold rolled continuous annealed steel sheet having excellent bake hardenability primarily proposed for outer panel and the like of an automobile body material where the strength of the drawn parts can be further increased by paint baking process and the method for manufacturing the same.

The bake hardening steels of the present advancement is characterized by its low yield strength and high ? before forming making it more drawable and also once drawn and coated with paint and subjected to paint a selective baking heat treatment favoring a considerable increase in yield strength imparting the material also good dent resistance.

More specifically, the advancement is targeted at producing two category of high strength BH steel having minimum Yield strength of 180 MPa and 220 MPa respectively before bake hardening. Both type of steel has Tensile strength less than 340 MPa. The high-strength cold rolled steel BH steel sheets of the advancement could be manufactured through continuous annealing route and appropriate for use in an outer panel and the like of an automobile body. Advantageously, the advancement favours achieving an average anisotropy ratio (? value) of higher than 1.8 and 1.6 for HC180B and HC220B grades (As per EN10268) respectively with both grads having Bake hardening (BH) value of more than 30 MPa at lower yield point. Additionally Cold rolled bake hardenable steel sheet described in present invention can be used for zinc coating using galvanizing process to produce Galvannealed and Galvanized steel and used as coated product for similar applications.

BACKGROUND ART

Automobile manufacturers are opting for manufacturing light and more fuel efficient vehicles to fulfill the norms of future legislation concerning emission and fuel consumption. This forces the development of materials having excellent drawability along with superior strength to fulfill the passenger safety norms.

As a prior art 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 present invention 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.

Specific properties are required for cold rolled sheet steels especially for automobile exposed panels. One of the strongest demands is for high strength sheet with thinner gauge to reduce weight, thereby reducing fuel consumption and CO2 gas emission. Automobile panels are press-formed from sheet and their integration requires high press formability. These two requirements made a way to bake hardened steel. When ultra-low carbon steel is alloyed with carbide or nitride forming elements show good formability, when heated to certain range of temperature the interstitial solute atoms create Cottrell atmosphere and increase yield strength. This effect is called Bake hardening effect.

Aim of the present advancement is targeted to provide a high strength continuous annealed cold-rolled steel sheet, which has excellent drawability, improved bake hardenability with superior aging resistance at room temperature, and a method of manufacturing the same.

The present invention is aimed to develop a steel grade which would provide significantly higher yield strength after paint baking as compared to conventional low carbon steel. Hence, it can be utilized for the automobile applications in those components, which are put for commercial paint baking operation. The dent resistance of BH grades is significantly improved after the paint baking making it excellent choice for panels that make up the car body parts and similar automobile body material. It can be proved excellent choice for improving the fuel efficiency as it is can replace conventional low carbon material having thicker gauge with the thinner BH grades ultimately reducing the vehicle weight.

OBJECTS OF THE INVENTION

It is thus the basic object of the present advancement to provide a high strength continuous annealed cold-rolled steel sheet, which would have excellent drawability, improved bake hardenability with superior aging resistance at room temperature, and also to develop a method of manufacturing the same.

Another object of the present invention is directed to development of high strength continuous annealed cold-rolled steel sheet with desired bake hardenability with superior aging resistance at room temperature which would be selectively favorable in that after paint baking the yield strength of steel could be significantly improved compared to conventional low carbon steel such as to suit its utilization for the automobile applications especially including those components, which are put for commercial paint baking operation.

Yet further object of the present advancement is directed to the development of sheet steel and process of manufacture thereof which would be beneficial in that the dent resistance of BH grades would be significantly improved after the paint baking making it excellent choice for panels that make up the car body parts and similar automobile body material.

A further object of the present advancement is directed to the development of sheet steel composition and its process for manufacture which would serve as an excellent choice to suit its utilization for the automobile applications for the purposes of improving the fuel efficiency as it is can replace conventional low carbon material having thicker gauge with the thinner BH grades ultimately reducing the vehicle weight.

SUMMARY OF THE INVENTION
The basic aspect of the present invention is directed to high strength ultra low carbon bake hardenable steel composition suitable for producing steel with BH value of more than 30 MPa and Yield point elongation after accelerated aging of <0.15% with desired formability including continuous annealing and skin pass elongation comprising
(In weight %) (In weight %)
C: 0.001-0.003 N: 0.004 or less
Mn: 0.35-0.55 Ti: 0.008-0.018
P: 0.035-0.06 B: 0.0007-0.0012
V: 0.005-0.02

such that [Ti+B] wt% = [2.64 N* +0.78N], and the balance being Fe and inevitable or associated impurities; wherein the constitutional elements additionally meet the following relations
120=6800 [C] - 14 [Mn] + 206 [P] + 8105 [N] - 2010 [Ti] + 212 [V] + 0.097 SS = 90
69.25[Ti] + 174.1[B] +16.9[V] + 0.0131 SPM = 1
(Where N* = N wt. % fixed by Ti, N=maximum N wt% which can be added, SPM= skin pass elongation %, SS- Soaking section temperature in ?C).
wherein said cold rolled continuously annealed steel has a mean value of X-ray random intensity ratio of group of {111} <110> to {001} <110> orientation more than 3.5 and mean value of X ray random intensity for {111} <112> orientation component is at least 2.5 multiple than that of {112} <110> orientation component.

A further aspect of the present invention is directed to high strength ultra low carbon bake hardening steel with improved ageing resistance and excellent formability having composition comprising
(In weight %) (In weight %)
C: 0.001-0.003 N: 0.004 or less
Mn: 0.35-0.55 Ti: 0.008-0.018
P: 0.035-0.06 B: 0.0007-0.0012
V: 0.005-0.02

such that [Ti+B] wt% = [2.64 N* +0.78N] and the balance being Fe and inevitable or associated impurities; wherein the constitutional elements additionally meet the following relations
120=6800 [C] - 14 [Mn] + 206 [P] + 8105 [N] - 2010 [Ti] + 212 [V] + 0.097 SS = 90
69.25[Ti] + 174.1[B] +16.9[V] + 0.0131 SPM = 1
(Where N* = N wt. % fixed by Ti, N=maximum N wt% which can be added, SPM= skin pass elongation %, SS- Soaking section temperature in ?C)
such as to achieve yield point elongation after accelerated aging of <0.15% and BH index of at least 30 MPa at lower yield point.
A still further aspect of the present invention is directed to said high strength ultra low carbon bake hardenable steel composition selectively workable to provide
(i) A first category having minimum yield strength of 180 MPa with ? value more than 1.8 before bake hardening and (ii) a second category has minimum yield strength value of 220 MPa coupled with ? value more than 1.6 before bake hardening and both category of steel has UTS less than 340MPa.
A still further aspect of the present invention is directed to high strength ultra low carbon bake hardenable steel composition suitable for producing steel with BH value of more than 30 MPa and ageing guarantee of 6 months with desired formability including continuous annealing and skin pass elongation obtained of steel composition comprising
(In weight %) (In weight %)
C: 0.001-0.003 N: 0.004 or less
Mn: 0.35-0.55 Ti: 0.008-0.018
P: 0.035-0.06 B: 0.0007-0.0012
V: 0.005-0.012

such that [Ti+B] wt% = [2.64 N* +0.78N] and the balance being Fe and inevitable or associated impurities; wherein the constitutional elements additionally meet the following relations
120=6800 [C] - 14 [Mn] + 206 [P] + 8105 [N] - 2010 [Ti] + 212 [V] + 0.097 SS = 90
69.25[Ti] + 174.1[B] +16.9[V] + 0.0131 SPM = 1
(Where N* = N wt.% fixed by Ti, N=maximum N wt% which can be added, SPM= skin pass elongation %, SS- Soaking section temperature in ?C),
and having minimum yield strength of 180 MPa with ? value more than 1.8 and UTS <340 MPa before bake hardening.
Another aspect of the present invention is directed to high strength ultra low carbon bake hardenable steel composition suitable for producing steel with BH value of more than 30 MPa and Yield point elongation after accelerated aging of <0.15% with desired formability including continuous annealing and skin pass elongation obtained of steel composition comprising
(In weight %) (In weight %)
C: 0.001-0.003 N: 0.004 or less
Mn: 0.35-0.55 Ti: 0.008-0.018
P: 0.035-0.06 B: 0.0007-0.0012
V: 0.005-0.02

such that [Ti+B] wt% = [2.64 N* +0.78N] and the balance being Fe and inevitable or associated impurities; wherein the constitutional element additionally meets the following relations
120=6800 [C] - 14 [Mn] + 206 [P] + 8105 [N] - 2010 [Ti] + 212 [V] + 0.097 SS = 90
69.25[Ti] + 174.1[B] +16.9[V] + 0.0131 SPM = 1
(Where N* = N wt.% fixed by Ti, N=maximum N wt% which can be added, SPM= skin pass elongation %, SS- Soaking section temperature in ?C)
and having minimum yield strength value of 220 MPa coupled with ? value more than 1.6 before bake hardening and both having UTS less than 340 MPa.
Yet another aspect of the present invention is directed to high strength ultra low carbon bake hardening steel sheet with improved ageing resistance and excellent formability having complete polygonal ferrite microstructure with ASTM grain size number of 9.5 or less along with Nitride and Carbide precipitates of alloying elements.
A further aspect of the present invention is directed to high strength ultra low carbon bake hardening steel sheet with improved ageing resistance and excellent formability having a mean value of X-ray random intensity ratio of group of {111} <110> to {001} <110> orientation more than 3.5 and mean value of X ray random intensity for {111} <112> orientation component is at least 2.5 multiple than that of {112} <110> orientation component.

A further aspect of the present invention is directed to a method of producing high strength ultra low carbon bake hardening steel sheet with improved ageing resistance and excellent formability comprising
(i) Providing selective steel composition comprising
(In weight %) (In weight %)
C: 0.001-0.003 N: 0.004 or less
Mn: 0.35-0.55 Ti: 0.008-0.018
P: 0.035-0.06 B: 0.0007-0.0012
V: 0.005-0.02

such that [Ti+B] wt% = [2.64 N* +0.78N] and the balance being Fe and inevitable or associated impurities; wherein the constitutional elements additionally meet the following relations
120=6800 [C] - 14 [Mn] + 206 [P] + 8105 [N] - 2010 [Ti] + 212 [V] + 0.097 SS = 90
69.25[Ti] + 174.1[B] +16.9[V] + 0.0131 SPM = 1
Where N* = N wt. % fixed by Ti, N=maximum N wt% which can be added, SPM= skin pass elongation %, SS- Soaking section temperature in ?C.
ii) carrying out steel manufacture involving continuous annealing and including Soaking Temperature in the temperature range of 800°C to 850°C to achieve UTS of 340 MPa or less , Slow Cooling Temperature in the temperature range of 690°C to 730°C, and Skin pass elongation in the range of 1.4 to 1.8 preferably 1.4 % or more to achieve the minimum YS 180 MPa or more and skin pass elongation (SPM %) in the range of 1.8 to 2.2 preferably 1.8% or more to achieve the minimum YS 220 MPa or more.
A further aspect of the present invention is directed to said method including a combination of Cold Reduction 75 % or more, Soaking Temperature 800°C or more, Slow Cooling Temperature 690 °C or more, C addition 0.003 wt% or less, Ti addition 0.018 wt% or less, B addition 0.0012 wt% or less, Mn addition 0.55 wt% or less, V addition 0.02 wt% or less, to achieve mean planner anisotropy ratio(r-bar) 1.7 or more and BH index 30 MPa or more at lower yield point.

A still further aspect of the present invention is directed to said method carried out such as to achieve complete polygonal ferrite microstructure with ASTM grain size number of 9.5 or less together with Nitride and Carbide precipitates of alloying elements.

A still further aspect of the present invention is directed to said method comprising continuously casting , hot rolling keeping slab reheating temperature below 1200°C to achieve roughing mill delivery temperature under 1050°C and finishing mill entry temperature under 1020°C to control surface defects including rolled in scale. Hot mill finishing temperature of 880°C to 920°C and coiling temperature of 600°C to 660°C ,Hot rolled coiled later processed through pickling coupled with tandem cold rolling mill, to remove the oxide surface present in the surface and to provide cold reduction of 70% or more ;
following pickling and cold rolling to desired thickness, cold rolled steel strip 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 800 °C or more, annealing time 40-140 seconds gives desired results for present inventive grades, at soaking section complete recrystallization results softer steel to get desired ductility, after soaking section steel strip passes through slow cooling section at cooling rate of 0.5- 5 °C/sec , slow cooling section temperature of 690 °C or more was maintained, following slow cooling section annealed strip sheet been rapid cooled at 8- 30 °C/sec up to 440 °C or more , after rapid cooling section annealed strip was over aged keeping the over aging section temperature of 340 °C or more , after over aging Skin-pass elongation (Temper rolling) was carried out based on different yield strength requirement and to remove any stretcher strain.

Another aspect of the present invention is directed to a method as claimed in anyone of claims 7 to 10 comprising additionally Cold rolled sheet subjected to zinc coating involving galvanizing process to produce GA/GI steel and used as coated product for similar applications.

The above and other objects and advantages of the present advancement are discussed hereunder in greater detail in relation to the following non-limiting exemplary illustrations as per the following non-limiting accompanying figures:

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1: is an illustration of the mechanism and evaluating method of bake hardening in a tensile test;
Figure 2: shows Effect of skin pass elongation % on yield strength value of inventive steel;
Figure 3a: optical microstructure of inventive steel grades with YS>180MPa showing complete polygonal ferrite grains with average ASTM grain size 8;
Figure 3b: optical microstructure of inventive steel grades with YS>220MPa showing complete polygonal ferrite grains with average ASTM grain size 9;
Figure 4: describes relation between Equation 2 (6800 [C] - 14 [Mn] + 206 [P] + 8105 [N] - 2010 [Ti] + 212 [V] + 0.097 SS) for different steel Numbers vs. the resultant Bake hardening index (BHI);
Figure 5: shows Orientation Distribution Function (ODF) at ?2 450 showing strong ?-fibre intensity for present inventive steel;

Figure 6: Shows relation between Equation 1 (69.25Ti+174.1B+16.9V+ 0.0131SPM) for different steel Numbers vs. the resultant Yield Point elongation % (YPE %) after 3 Months of Aging.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO ACCOMPANYING FIGURES AND EXAMPLES:
The present invention provides two category of cold-rolled bake hardenable steel sheet having excellent drawability along with superior bake hardenability and the method for manufacturing the same.
The description includes references to the following abbreviations/terms:
SS – Soaking section in continuous annealing furnace
SCS – Slow cooling 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 %

Description of the same is given below
Type 1 – Steel of first category has minimum yield strength of 180 MPa with ? value more than 1.8 before bake hardening.
Type 2 - Steel of second category has minimum yield strength value of 220 MPa coupled with ? value more than 1.6 before bake hardening.
Both categories of cold rolled steels have UTS value of less than 340 MPa, YPE after accelerated aging of less than 0.15% and a bake hardening value of 30 MPa or more at lower yield point.
Both the cold rolled bake hardenable steels having composition comprising in terms of weight % :- C: 0.001-0.003 wt% , Mn: 0.35-0.55 wt% , P: 0.035-0.06 wt% ,N: 0.004 wt% or less, Ti: 0.008-0.018 wt% , B: 0.0007-0.0012 wt%, V: 0.005-0.02 wt% and the balance being Fe and other inevitable or associated impurities. Both type of steels are satisfying the following two achieve YPE after accelerated aging of <0.15% -
(Ti+B) wt% = (2.64 N* +0.785 N)
69.25Ti + 174.1B +16.9V+0.0131 SPM = 1
Where N* = N wt% fixed by Ti, N=maximum N wt% which can be added, SPM= skin pass elongation % after continuous annealing.

Furthermore, slabs with previous mentioned chemistry is hot rolled keeping the Hot strip mill Finishing Temperature 880 to 920 °C and HSM coiling temperature of 600°C - 660°C. After acid pickling and cold rolling both the cold rolled bake hardenable steels are annealed in continuous annealing furnace keeping the Annealing temperature (soaking temperature) 800°C or more slow cooling section (SCS) temperature 690°C or more . After annealing, Skin pass elongation (Temper rolling) of 1.4 % or above was applied for achieving the desired Yield strength value of 180 MPa more. Skin pass elongation of 1.8 or more is claimed for achieving Yield strength value of 220 MPa or more.

The present invention relates to high-strength cold rolled steel sheet manufactured through continuous annealing route appropriate for use in an outer panel and the like of an automobile body and having a yield strength YS of not less than 180 MPa coupled with average r-value of higher than 1.8 before bake hardening .A second category of High strength BH grade is claimed having minimum yield strength of 220 MPa and Average r-value of higher than 1.6. Bake hardening (BH) value of more than 30 MPa at lower yield point is claimed for both the categories.

The mechanism and evaluating method of bake hardening in a tensile test is illustrated in accompanying Figure 1.
Bake hardening -IF steel having carbon content of 0.003 wt% or less has very low yield strength initially. Small amount of carbon and/or Nitrogen (approx 6 to 15ppm wt %) is intentionally left unfixed in steel solution by controlled Ti addition. Following press forming via plastic deformation the induced dislocations results in work hardening .The press formed components are heated at approx 1700C for 20mins during bake hardening process. Through the paint baking process the solute carbon and/or Nitrogen stabilizes the dislocations which were induced during work hardening by diffusing next to the core of a dislocation. An additional stress is now required to promote the slip movement once mobile dislocations are pinned down by solute carbon after paint baking. Therefore, a bake hardenable steel sheet exhibits a low yield strength value before press forming and a high yield strength value in a finished car component after paint baking. The increase in yield strength due to bake hardening generally is generally 30 MPa or more with 6-15 ppm of unfixed solute carbon and/or Nitrogen.

The unfixed carbon and/or Nitrogen in solution is altogether responsible for bake hardening effect after paint baking as it acts as a barrier to dislocation movement and increases the strength significantly by pinning down the dislocations.

Since a dent resistance is required in an outer panel of the automobile body, it is desirable that the strength after paint baking is high, and therefore it is also required that the bake hardenability (BH) property is excellent.

However, the conventional steel sheet having an improved BH property has a tendency that the formability and deep drawability is poor as compared with the usual mild steel sheet because a greater amount of solute C and/or N is contained. In order to establish both the weight reduction and the safeness of the automobile body, therefore, the steel sheet used in the automobile body is required to have further excellent bake hardenability in addition to the high strength and excellent deep drawability.

The present invention describes a method of manufacturing a cold rolled continuous annealed bake hardening steel, where the molten hot metal is processed through LD converter route followed by RH-Degassing treatment of molten steel for getting the desired level of C and other alloying elements as claimed in present invention . Then, continuously casting the molten steel into slabs, hot rolling the slabs and finishing the resulted strip at required temperature ranges and coiling according to the specification cold rolling the hot rolled coils with > 70% deformation, the resulted cold rolled coils being annealed in continuous annealing furnace with optimum annealing parameters and the annealed coils being given skin pass rolling with deformation of 1.4 % or more. The resulted cold rolled annealed steel being evaluated for Bake Hardening as per the method described in Figure 1. Method consists straining the tensile test steel specimen to 2%, baking the same at 170°C for 20 minutes in a furnace and then air cooled. After baking, tensile test being carried out for baked sample again and calculating the difference between lower yield point (Point B in fig 1) after baking and strength after 2% elongation (Point A in fig 1). Additionally Cold rolled bake hardenable steel sheet described in present invention can be used for zinc coating using galvanizing process to produce GA/GI steel and used as coated product for similar applications.
With the aim of developing a Non-aging, High strength cold rolled BH grades by continuous annealing route, Effect of Metallurgical factors affecting the mechanical properties and drawability are described in detail.
Steel of present invention consist of C: 0.001-0.003 wt% , Mn: 0.35-0.55 wt% , P: 0.035-0.06 wt% , N: 0.004 wt% or less, Ti: 0.008-0.018 wt% , B: 0.0007-0.0012 wt%, V: 0.005-0.02 wt% having [Ti+B] wt% = [2.64 N* +0.78N] and the balance being Fe and other inevitable or associated impurities.

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 8-14 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. Our prime aim is to fix all the Nitrogen while keeping small fraction of C (8 to 14ppm) in solution since diffusivity of C is very low as compared to Nitrogen. By doing so, it is possible to achieve Improve Bake hardenability along with room temperature aging property.

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%.

Ti (0.01-0.018) wt%- For inventive BH grade pre-requisite is to fix all the Solute N and keep fraction of Solute carbon in steel matrix to get improved Bake hardenability along with good room temperature aging resistance. Titanium is very efficient in fixing nitrogen, Sulphur, and carbon. TiN forms during casting and TiS during slab reheating. Remaining Ti after fixing N and S is utilized to fix C during coiling stage. The minimum amount of titanium required for complete stabilization is given by
Tistab = 4C+3.42N+1.5S
Equation 1.Gives the minimum amount of Ti as 0.021 wt% which is required to fix N and S along with C for the minimum specified chemistry range. As a pre-requisite for achieving bake hardenability of 30 MPa or more at least 8ppm of solute carbon in solution is required. Therefore, maximum Ti level should be kept 0.02 wt% for achieving bake hardening effect. More preferably upper limit of Ti level should be kept less than 0.018wt% for present inventive steel to have fraction of solute C in solution.

To improve the room temperature aging property it is compulsory to fix all the solute N as TiN since diffusivity of N is very high compared to carbon. Also, Free N is mobile enough at room temperature to diffuse to core of the dislocation and pinning it down resulting in YPE (Yield point elongation).To completely fix 30ppm of N minimum amount of Ti to be added can be calculated with following equation –
Ti Nitrogen = (48/14)*(0.003 wt %)

Based on this we have fixed the minimum amount Ti as 0.01wt% to avoid any free N.

For the maximum N content 0f 0.004wt% as claimed in present invention, rest 10ppm can be fixed by B, V and Al as BN, VN, and AlN respectively.
With rigorous studies and trials we have come in to conclusion that following 2 regressions must be satisfied to achieve the aging guarantee of more than 6 months along with improved BH index of more than 30 MPa.
i) 69.25 [Ti] + 174.1 [B] +16.9 [V] +0.0131 SPM = 1 ………… (Equation 1)
ii) 120 = 6800 [C] - 14 [Mn] + 206 [P] + 8105 [N] - 2010 [Ti] + 212 [V] + 0.097 SS = 90 ………… (Equation 2)
Where [X] is weight % of chemical element X, SS – Is soaking section temperature and SPM – skin pass elongation

(N - 0.004wt% or less) –Nitrogen in solution deteriorates the drawability (r-value) and room temperature aging property therefore it must be completely 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 30ppm or less to achieve improved room temperature aging resistance. The amount of Ti required to completely fix Solute N can be calculated using the following relation-
Tistab = (48/14)*N
This gives a minimum 0.01 wt% of Ti to be added to completely fix solute N .Hence to avoid any yield point elongation (YPE ) before bake Hardening due to free N, minimum 0.01wt% of Ti must be added. If the N level is 40 ppm as specified in present steel, then rest 10ppm of N will be fixed by B and V.

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.

Mn (0.35-0.55) wt. % -For present invention, Mn ranges from 0.35 to 0.55wt% 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. Therefore the upper limit is set to 0.55 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 0.06 wt% to achieve the desired strength level without negotiating much with drawability. Reducing P wt% to 0.03 % or less would not give any strengthening effect hence the claimed lower limit has been set as 0.035wt%.

B (0.0007-0.0012) 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. Amount of B to be added to fix excess 10ppm of nitrogen can be calculated by –
B (wt %) = (11/14)*N wt% ………………………….. (4)

As described earlier, 30ppm of N wt% can be fixed by minimum claimed Ti addition. Above equation gives minimum 8ppm of B wt% to be added to fix excess nitrogen in case N content reaches to maximum level of 40ppm as claimed in present invention. To avoid any excess B the safe limit has been fixed to 7 to 12 ppm in present invention.

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 (0.007-0.02 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 %) ………………………….. (5)
Equation (5) gives minimum 16ppm of C which can be fixed with minimum 0.007 wt% of V. Excess 8-15 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.

Relation for Desired Bake hardening and aging resistance – A relation has also been claimed here to achieve a BH value of more than 30 MPa at lower yield point (point B in figure 1) along with aging guarantee of 6 Months for present inventive grades having the composition range as described above.
120 = 6800 [C] - 14 [Mn] + 206 [P] + 8105 [N] - 2010 [Ti] + 212 [V] + 0.097 SS =90
Where [X] is weight % of chemical element X, SS – Is soaking section temperature.

Details of the Process according to present invention -

Slabs with the chemistry range as described in scope of invention are continuously casted, hot rolled keeping slab reheating temperature below 1200°C intended to achieve roughing mill delivery temperature under 1050°C and finishing mill entry temperature under 1020°C to check surface defects like rolled in scale. Hot mill finishing temperature of 880°C to 920°C and coiling temperature of 600°C to 660°C being employed. Hot rolled coiled later processed through pickling coupled with tandem cold rolling mill, to remove the oxide surface present in the surface and to provide cold reduction of 70% or more .

Following pickling and cold rolling to desired thickness, cold rolled steel strip 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 800 °C or more. Annealing time 40-140 seconds gives desired results for present inventive grades. At soaking section complete recrystallization results softer steel to get desired ductility. After soaking section steel strip passes through slow cooling section at cooling rate of 0.5-5°C/sec .Slow cooling section temperature of 690 °C or more was maintained. Following slow cooling section annealed strip sheet been rapid cooled at 8- 30 °C/sec up to 440 °C or more .After rapid cooling section annealed strip was over aged keeping the over aging section temperature of 340 °C or more. After over aging Skin-pass elongation (Temper rolling) was applied based on different yield strength requirement and to remove any stretcher strain. Additionally Cold rolled sheet described in present invention can be utilized for zinc coating using galvanizing process to produce GA/GI steel and used as coated product for similar applications.

Complete description of Inventive steel and comparative steel grades are illustrated by way of following table I and V:
Table I- Compositions of the steel sheets of the present invention along with some comparative examples.
Table II- Hot rolling, cold rolling, annealing parameters along with the mechanical properties of respective steel sheets.
Table III- Room temperature ageing properties of invented and comparative steels.
TABLE IV – Justification of claimed Equations for minimum BH index and room temperature Aging guarantee of >6 months
Table V– ASTM grain size and Mean value of X-ray random intensity ratio of group of {111} <110> to {001} <110> orientation along with mean value of X ray random intensity for {111} <112> and {112} <110> orientation component for Inventive and comparative steel.

As can be witnesses from Table 1 and 2, Casting and processing of Steels remarked as Inventive are carried out by strictly controlling the content of C, Mn ,P, Ti, B, sol Al, N and V to satisfy the condition of 0.001-0.003 weight% C , 0.35-0.55 weight% Mn, 0.035-0.06 weight% P, N less than 0.004 %, 0.008-0.018 weight% Ti, 0.0007-0.0012 weight % B, 0.005-0.02 weight% V and the remainder being Fe and inevitable impurities. For Inventive steels, Keeping annealing temperature =800 0C, SCS Temperature =690 0C, SPM elongation of 1.4% or more to achieve the desired YS value >180MPa and 1.8% or more to achieve desired YS value of >220 MPa without yield point elongation at room temperature.

TABLE I
Steel
No. C
wt% Mn
wt% S
wt% P
wt% Si
wt% Al
wt% N
wt% Ti
wt% B
wt% V
wt% HSM FT,0C HSM CT,0C Remarks
1 0.002 0.37 0.007 0.035 0.005 0.04 0.0035 0.014 0.0011 0.007 904 615 Inventive
2 0.0025 0.37 0.007 0.055 0.005 0.04 0.0035 0.01 0.0011 0.007 904 615 Inventive
3 0.002 0.37 0.007 0.04 0.005 0.04 0.0035 0.016 0.0011 0.007 899 626 Inventive
4 0.002 0.37 0.007 0.04 0.005 0.04 0.0035 0.014 0.0011 0.007 899 626 Inventive
5 0.002 0.37 0.007 0.04 0.005 0.04 0.0035 0.017 0.0011 0.007 903 622 Inventive
6 0.0035 0.37 0.007 0.04 0.005 0.04 0.0035 0.005 0.0011 - 903 622 Comparative
7 0.002 0.4 0.007 0.045 0.005 0.04 0.0035 0.016 0.0011 0.007 896 619 Inventive
8 0.002 0.37 0.007 0.04 0.005 0.04 0.0035 0.017 0.0011 0.01 899 626 Inventive
9 0.002 0.37 0.007 0.04 0.005 0.04 0.0035 0.015 0.0011 0.007 899 626 Inventive
10 0.002 0.37 0.007 0.04 0.005 0.04 0.0035 0.017 0.0011 0.007 899 626 Inventive
11 0.002 0.37 0.007 0.04 0.005 0.04 0.0035 0.02 0.001 - 903 625 Comparative
12 0.002 0.37 0.007 0.04 0.005 0.04 0.0035 0.014 0.0011 0.007 903 622 Inventive
13 0.002 0.37 0.007 0.04 0.005 0.04 0.0035 0.016 0.0011 0.007 903 622 Inventive
14 0.002 0.37 0.007 0.04 0.005 0.04 0.0035 0.014 0.0011 0.007 899 626 Inventive
15 0.003 0.37 0.007 0.04 0.005 0.04 0.0035 0.006 0.0008 - 904 615 Comparative
16 0.002 0.37 0.007 0.04 0.005 0.04 0.0035 0.017 0.0011 0.007 904 615 Inventive
17 0.002 0.37 0.007 0.035 0.005 0.04 0.0035 0.024 - 0.008 903 621 Comparative
18 0.002 0.37 0.007 0.04 0.005 0.04 0.0035 0.015 0.0011 0.007 903 622 Inventive
19 0.0025 0.35 0.008 0.04 0.002 0.036 0.003 0.006 0.0007 - 899 622 Comparative
20 0.001 0.38 0.008 0.032 0.007 0.038 0.0032 0.011 0.0008 0.016 899 605 Inventive
21 0.001 0.38 0.008 0.032 0.007 0.038 0.0032 0.011 0.0008 0.014 899 605 Inventive
22 0.001 0.3 0.008 0.025 0.007 0.038 0.0032 0.011 0.0008 0.01 903 613 Comparative
23 0.001 0.38 0.008 0.032 0.007 0.038 0.0032 0.011 0.0008 0.009 908 620 Inventive
24 0.002 0.36 0.006 0.03 0.007 0.04 0.0033 0.013 0.0009 0.009 908 620 Inventive
25 0.005 0.36 0.006 0.03 0.007 0.04 0.005 0.013 0.0007 0.01 909 622 Comparative
26 0.002 0.36 0.006 0.03 0.007 0.04 0.0027 0.013 0.0007 0.008 909 622 Inventive
27 0.002 0.36 0.006 0.03 0.007 0.04 0.0027 0.013 0.0007 0.008 900 650 Inventive
28 0.001 0.38 0.006 0.033 0.007 0.035 0.0033 0.012 0.0009 0.008 898 630 Inventive
29 0.0015 0.38 0.008 0.09 0.007 0.038 0.0032 0.022 0.0008 - 908 620 Comparative
30 0.002 0.38 0.006 0.031 0.007 0.035 0.0029 0.011 0.0008 0.01 904 618 Inventive
31 0.001 0.38 0.006 0.01 0.007 0.035 0.0029 0.01 0.0008 - 904 623 Comparative
32 0.002 0.35 0.006 0.02 0.007 0.04 0.0027 0.013 0.0007 - 904 618 Comparative
33 0.001 0.38 0.006 0.015 0.007 0.035 0.0034 0.013 0.001 0.002 905 619 Comparative
34 0.002 0.45 0.008 0.032 0.007 0.038 0.0032 0.011 0.0008 0.008 895 621 Inventive
35 0.002 0.36 0.007 0.045 0.006 0.036 0.0039 0.014 0.0011 0.008 906 620 Inventive
36 0.002 0.36 0.007 0.045 0.006 0.036 0.0039 0.016 0.0011 0.009 906 620 Inventive
37 0.002 1 0.007 0.045 0.006 0.036 0.0039 0.02 0.0011 - 904 623 Comparative
38 0.003 0.85 0.007 0.065 0.006 0.036 0.0039 0.002 0.0011 - 898 616 Comparative
39 0.003 0.8 0.007 0.045 0.006 0.036 0.0039 0.003 0.0011 - 906 617 Comparative
40 0.001 0.96 0.007 0.055 0.006 0.036 0.0039 0.025 0.0011 - 905 625 Comparative
41 0.0025 0.75 0.007 0.08 0.006 0.036 0.0039 0.002 0.0011 - 906 619 Comparative
42 0.0028 0.35 0.008 0.04 0.002 0.036 0.003 0.005 0.0008 - 899 629 Comparative
43 0.001 0.6 0.007 0.066 0.005 0.04 0.0035 0.01 0.0011 0.005 904 615 Comparative
44 0.002 0.37 0.007 0.035 0.005 0.04 0.0035 0.014 0.0011 0.007 903 635 Inventive
45 0.003 0.6 0.008 0.06 0.011 0.037 0.004 0.006 0.0009 0.007 906 610 Comparative
46 0.0035 0.6 0.008 0.06 0.011 0.037 0.004 0.005 0.0009 - 910 613 Comparative

*HSM FT- Hot Strip Mill finishing temperature in 0C
*HSM CT- Hot Strip Mill Coiling temperature in 0C
Wt% - Composition in weight percent

Table II
Steel No. SS TEMP,0C SCS TEMP, 0C RCS TEMP
,0C OAS TEMP, 0C SPM ELONG,% CCM% YS, MPa TS, MPa EL% r-BAR n BH INDEX (at Lower YP) Remarks
1 810 715 480 405 1.5 82.6 190 328 45.9 1.94 0.223 33 Inventive
2 840 723 480 390 1.8 82.6 240 335 48 1.91 0.215 46 Inventive
3 821 719 480 396 1.5 82.6 211 327 48.7 2.04 0.223 36 Inventive
4 834 723 481 404 1.52 82.6 219 327 48.2 2.06 0.214 41 Inventive
5 837 719 478 398 1.4 82.6 200 324 47.9 2.07 0.225 35 Inventive
6 780 710 480 382 1.6 82.6 227 348 45 1.7 0.21 64 Comparative
7 828 702 479 391 1.8 82.6 242 337 46.4 2.1 0.217 37 Inventive
8 821 719 479 396 1.52 82.6 204 326 46.99 2.08 0.22 38 Inventive
9 834 723 481 404 1.53 82.6 207 329 47.61 2.17 0.23 39 Inventive
10 821 719 480 404 1.45 82.6 213 329 46.25 1.96 0.22 34 Inventive
11 775 720 479 398 1.43 82.6 202 322 46.76 1.9 0.223 22 Comparative
12 841 723 481 398 1.23 82.6 187 321 48.55 2.34 0.228 42 Inventive
13 825 724 480 405 1.21 82.6 234 321 46.69 2.15 0.223 36 Inventive
14 819 723 479 396 1.32 82.6 218 326 46.25 2.15 0.222 40 Inventive
15 810 702 479 405 1 82.6 200 343 43.82 1.7 0.208 62 Comparative
16 810 725 480 390 1.41 82.6 208 329 46.39 1.9 0.217 33 Inventive
17 780¬ 660 480 382 1.2 82.6 191 322 47.53 1.93 0.21 16 Comparative
18 837 719 480 382 1.59 82.6 202 322 48.23 2.19 0.221 35 Inventive
19 791 717 479 389 2 82.6 226 327 45.2 1.41 0.21 52 Comparative
20 810 704 480 388 1.5 76.5 184 309 51.2 2.1 0.232 34 Inventive
21 810 711 480 377 1.55 76.5 206 309 49.5 2.05 0.227 39 Inventive
22 813 694 481 394 1.4 76.5 175 305 50.8 1.74 0.233 35 Comparative
23 821 717 480 388 1.54 76.5 204 306 50.4 2.04 0.228 35 Inventive
24 829 730 482 391 1.4 76.5 201 305 49.7 2.1 0.231 40 Inventive
25 810 681 480 389 1.6 76.5 222 312 44.7 1.3 0.21 70 Comparative
26 818 691 480 389 1.55 76.5 199 301 51.2 1.99 0.230 37 Inventive
27 822 721 482 391 1.57 76.5 199 303 51.7 2.07 0.229 33 Inventive
28 810 711 480 379 1.54 76.5 206 309 49.9 1.88 0.227 33 Inventive
29 808 718 481 377 1.3 76.5 241 355 40.3 1.31 0.2 23 Comparative
30 831 723 480 379 1.5 76.5 207 311 48.9 1.80 0.228 45 Inventive
31 781 710 480 379 1.2 76.5 165 310 50.7 2.0 0.228 26 Comparative
32 810 717 480 388 1.54 76.5 170 300 50.9 1.8 0.242 27 Comparative
33 841 720 481 377 1.4 76.5 168 308 48.6 1.82 0.229 26 Comparative
34 810 696 480 370 1.49 76.5 205 311 50.3 1.90 0.226 39 Inventive
35 820 716 481 374 1.71 76.5 240 334 47.6 1.85 0.219 45 Inventive
36 822 711 480 387 1.65 76.5 229 335 49.3 1.91 0.221 41 Inventive
37 790 700 478 392 1.3 76.5 269 381 36.1 1.2 0.173 18 Comparative
38 784 710 480 404 1.4 76.5 257 349 37.9 1.3 0.193 68 Comparative
39 810 710 489 407 1.61 76.5 253 357 38.4 1.3 0.193 65 Comparative
40 785 703 480 390 1.5 76.5 256 367 39.3 1.27 0.196 4 Comparative
41 820 727 484 407 1.2 76.5 269 378 42.1 1.2 0.19 71 Comparative
42 820 727 481 387 1.51 76.5 222 326 48.8 1.90 0.223 55 Comparative
43 760 710 480 390 1.9 76.5 251 385 44.9 1.6 0.21 35 Comparative
44 849 725 481 398 1.5 76.5 205 321 48.55 2.33 0.23 33 Inventive
45 780 722 478 391 1.85 76.5 258 363 40.65 1.46 0.206 62 Comparative
46 779 720 489 400 2 76.5 276 379 40.56 1.33 0.207 68 Comparative

Table III illustrates the room temperature aging property of inventive and comparative cold rolled bake hardenable steel. To simulate the room temperature aging tensile test specimen of 50mm gauge length was immersed in oil bath which was homogeneously maintained at 1000C for 3 hours and 6 hours to simulate 3 months and 6 months aging at room temperature (30°C) respectively.

As evident from table 3, comparative steel number 15 and 42 does not fulfill the room temperature aging for 3 months as YPE observed was more than 0.15%. Whereas inventive steels are comfortably comply with room temperature aging resistance of more than 3 and 6 months as YPE observed after 6 hours of aging in oil bath are less than 0.15%.

Following conclusion are made from above observation –
1. For present inventive steel Ti level must be =0.008 to fulfill aging guarantee of more than 6 months by achieving YPE Less than 0.015 % after accelerated aging.
2. Steel having value of equation 2 (6800 [C] - 14 [Mn] + 206 [P] + 8105 [N] - 2010 [Ti] + 212 [V] + 0.097 SS > 120) in table IV does not comply with the aging resistance of even 3 months as listed in Table III.
3. Present inventive steel comfortably meets the room temperature aging guarantee of 6 months as well minimum Bake hardening requirement of =30MPa at lower yield point.

Table III
Steel
Number Initial YS
(MPa) After 3 Hours, at 100deg C YPE,
<0.15% After 6 Hours, at 100deg C YPE, <0.15% Remarks
LYP
(MPa) UYP
(MPa) YS LYP
(MPa) UYP
(MPa) YS
1 190 195 0 196 200 200 0.1 Inventive steel- Room temperature Aging pass
3 211 210 213 213 0.08 211 220 220 0.14 Inventive steel -Room temperature Aging pass
5 200 203 0 202 211 211 0.03 Inventive steel -Room temperature Aging pass
10 213 219 0 216 227 227 0.06 Inventive steel- Room temperature Aging pass
16 208 210 0 208 215 215 0.03 Inventive steel -Room temperature Aging pass
15 200 215 231 231 0.7 219 237 237 0.77 Comparative, Room temperature aging fail, YPE 0.7 after 3 months of aging
42 222 227 243 243 0.68 234 251 251 0.92 Comparative, Room temperature aging Fail, YPE 0.68 after 3 months of aging
6 220 231 237 237 0.73 249 264 1.2 Comparative, YPE > 0.76 % and 1.2 % after 3 and 6 Months of Aging respectively

38 257 249 271 271 1.1 259 281 281 1.5 Comparative, YPE > 1.1 % and 1.5 % after 3 and 6 Months of Aging respectively
19 226 223 235 235 0.55 237 248 248 0.87 Comparative, YPE 0.55 %After 3 Months of aging

46 276 295 312 312 1.2 311 327 327 1.43 Comparative,, YPE 1.2 % After 3 Months aging
38 257 242 269 269 1.21 251 275 275 1.6 Comparative, YPE 1.21 % After 3 Months of aging
39 253 248 265 265 1.17 253 268 268 1.74 Comparative, YPE 1.17 % After 3 Months aging

* To simulate the room temperature aging tensile test specimen of 50mm gauge length was immersed in oil bath homogeneously maintained at 1000C for 3 hours and 6 hours to simulate 3months and 6months aging respectively. LYP–Lower yield Point, UYP–Upper Yield Point. Steels having YPE <0.15 % after three months of accelerated artificial aging fulfills the scope of the invention and comply with the three month aging guarantee.

TABLE IV
Steel C Mn P N Ti B V SPM % SS, 0C BH Index, MPa Equation 1 Equation 2 Remarks
1 0.002 0.37 0.035 0.0035 0.014 0.0011 0.007 1.5 810 33 1.3 95.9 I
2 0.0025 0.37 0.055 0.0035 0.01 0.0011 0.007 1.8 811 46 1.0 111.6 I
3 0.002 0.37 0.04 0.0035 0.016 0.0011 0.007 1.5 821 36 1.4 94.0 I
4 0.002 0.37 0.04 0.0035 0.014 0.0011 0.007 1.52 834 41 1.3 99.3 I
5 0.002 0.37 0.04 0.0035 0.017 0.0011 0.007 1.4 837 35 1.5 93.5 I
6 0.0035 0.37 0.04 0.0035 0.005 0.0011 0 1.6 780 64 0.6 121 C
7 0.002 0.4 0.045 0.0035 0.016 0.0011 0.007 1.8 828 37 1.4 95.3 I
8 0.002 0.37 0.04 0.0035 0.017 0.0011 0.01 1.52 821 38 1.6 92.6 I
9 0.002 0.37 0.04 0.0035 0.015 0.0011 0.007 1.53 834 39 1.4 97.3 I
10 0.002 0.37 0.04 0.0035 0.017 0.0011 0.007 1.45 821 34 1.5 92.0 I
11 0.002 0.37 0.04 0.0035 0.02 0.001 0 1.43 775 22 1.6 80.0 C
12 0.002 0.37 0.04 0.0035 0.014 0.0011 0.007 1.23 841 42 1.3 99.9 I
13 0.002 0.37 0.04 0.0035 0.016 0.0011 0.007 1.21 825 36 1.4 94.4 I
14 0.002 0.37 0.04 0.0035 0.014 0.0011 0.007 1.32 819 40 1.3 97.8 I
15 0.003 0.37 0.04 0.0035 0.006 0.0008 0 1 810 62 0.6 118.3 C
16 0.002 0.37 0.04 0.0035 0.017 0.0011 0.007 1.41 810 33 1.5 90.9 I
17 0.002 0.37 0.035 0.0035 0.024 0 0.008 1.2 800 16 1.8 75.1 C
18 0.002 0.37 0.04 0.0035 0.015 0.0011 0.007 1.59 837 35 1.4 97.6 I
19 0.0025 0.35 0.04 0.003 0.006 0.0007 0 2 791 52 0.6 109.3 C
20 0.001 0.38 0.032 0.0032 0.011 0.0008 0.008 1.5 810 34 1.1 92.2 I
21 0.001 0.38 0.032 0.0032 0.011 0.0008 0.005 1.55 810 39 1.0 91.5 I
23 0.001 0.38 0.032 0.0032 0.011 0.0008 0.009 1.54 821 35 1.1 93.4 I
24 0.002 0.36 0.03 0.0033 0.013 0.0009 0.009 1.4 829 40 1.2 97.7 I
25 0.005 0.36 0.03 0.005 0.013 0.0007 0.01 1.6 810 70 1.2 130.2 C
26 0.002 0.36 0.03 0.0027 0.013 0.0007 0.008 1.55 818 37 1.2 91.5 I
27 0.002 0.36 0.03 0.0027 0.013 0.0007 0.008 1.57 822 33 1.2 91.9 I
28 0.001 0.38 0.033 0.0033 0.012 0.0009 0.008 1.54 810 33 1.1 91.2 I
29 0.0015 0.38 0.09 0.0032 0.022 0.0008 0 1.3 808 23 1.7 83.5 C
30 0.002 0.38 0.031 0.0029 0.011 0.0008 0.01 1.5 831 45 1.1 98.8 I
31 0.001 0.38 0.01 0.0029 0.01 0.0008 0 1.2 801 26 0.8 84.6 C
32 0.002 0.35 0.02 0.0027 0.013 0.0007 0 1.54 810 27 1.0 87.1 C
33 0.001 0.38 0.015 0.0034 0.013 0.001 0.002 1.4 841 26 1.1 88.0 C
34 0.002 0.45 0.032 0.0032 0.011 0.0008 0.008 1.49 810 39 1.1 98.0 I
35 0.002 0.36 0.045 0.0039 0.014 0.0011 0.008 1.71 820 45 1.3 102.5 I
36 0.002 0.36 0.045 0.0039 0.016 0.0011 0.009 1.65 822 41 1.5 98.9 I
37 0.002 1 0.045 0.0039 0.02 0.0011 0 1.3 800 18 1.6 77.9 C
38 0.003 0.85 0.065 0.0039 0.002 0.0011 0 1.4 801 68 0.3 127.2 C
39 0.003 0.8 0.045 0.0039 0.003 0.0011 0 1.61 810 65 0.4 122.6 C
40 0.001 0.96 0.055 0.0039 0.025 0.0011 0 1.5 785 4 1.9 62.2 C
41 0.0025 0.75 0.08 0.0039 0.002 0.0011 0 1.2 820 71 0.3 130.1 C
42 0.0028 0.35 0.04 0.003 0.005 0.0008 0 1.51 820 55 0.5 116.2 C
44 0.002 0.37 0.035 0.0035 0.014 0.0011 0.007 1.5 849 33 1.3 99.7 I
45 0.003 0.6 0.06 0.004 0.006 0.0009 0.007 1.85 780 62 0.7 121.9 C
46 0.0035 0.6 0.06 0.004 0.005 0.0009 0 2 779 68 0.5 125.7 C
I =Inventive, C=Comparative
Table IV explains the calculated values of equation 1 = (69.25Ti+174.1B+16.9V+ 0.0131SPM) and Equation 2= (6800 [C] - 14 [Mn] + 206 [P] + 8105 [N] - 2010 [Ti] + 212 [V] + 0.097 SS) respectively. Steels having calculated value of 69.25[Ti]+174.1[B]+16.9[V]+0.0131SPM less than 1 in TABLE IV does not comply with ageing guarantee of 3 months itself whereas steel with having calculated value =1 complying with 6 months of ageing guarantee with YPE after aging less than 0.15% as shown in table III .

Similarly Steel Numbers from table IV satisfying relation 120 = (6800 [C] - 14 [Mn] + 206 [P] + 8105 [N] - 2010 [Ti] + 212 [V] + 0.097 SS) = 90 comply with the requirement of BH index more than 30 MPa at lower YPE and aging resistance of 6 Months as listed in Table III. Value less than 90 does not comply with the minimum BH requirement of 30 MPa, whereas value more than 120 does not comply with aging guarantee.

In conclusion, following regression satisfies aging requirement of 6 months and minimum BH index of 30MPa –
i) 69.25Ti+174.1B+16.9V+0.0131SPM =1 …………… (Equation 1)
ii) 120 = (6800 [C] - 14 [Mn] + 206 [P] + 8105 [N] - 2010 [Ti] + 212 [V] + 0.097 SS) = 90………………………………………………. (Equation 2)

TABLE -V
Steel Number Ratio of {111} <110> to {001} <110> component group X-ray intensity X ray random intensity for {111} <112> component group X ray random intensity for {112} <110> component group ASTM Grain Size Remarks
1 5.1 16.81 4.09 8 Inventive
4 4.6 18.12 4.31 8.3 Inventive
5 5.3 17.3 3.94 8.5 Inventive
9 5.8 16.35 3.71 8.5 Inventive
13 6.1 19.46 4.35 9 Inventive
37 2.4 7.11 4.81 10.9 Comparative
38 2.1 7.3 5.24 10.8 Comparative
39 2.5 4.93 3.87 11 Comparative
40 1.9 7.36 5.65 11.3 Comparative
41 1.97 8.24 5.22 11.2 Comparative
45 2.9 9.31 6.37 10.7 Comparative
46 2.4 9.12 6.45 10.5 Comparative

Table 5 listing a mean value of X-ray random intensity ratio of group of {111} <110> to {001} <110> orientation components. Steels having X–ray random intensity ratio for {111} <110> to {001} <110> texture component more than 4 are mentioned as inventive example as these steel comply with the r-bar and BH requirements whereas comparative steels with ratio lower than 3.5 does not comply with r-bar (Drawability) requirement . Also inventive steels with mean value of X ray random intensity for {111} <112> orientation component atleast 2.5 multiple than that of {112} <110> orientation components are listed as inventive examples owing to their higher r-bar values.

In addition steels having ASTM grain size number >9.5 are listed as comparative examples as comparative steels listed in Table V shows poor drawability (r-bar) value due to very High YS or BH index or both .

Example 1 - Effect of SPM% on Yield strength of present invention and minimum SPM% required to suppress YPE is shown in figure 2. Figure 2 shows declining trend of YS with SPM up to <1.4%. It is attributable to observed YPE at SPM<1.4 %. An increasing trend of YS with SPM % is observed after SPM= 1.4 as YPE is completely suppressed at 1.4% SPM elongation. For this reason minimum skin pass elongation for present invention has been fixed as 1.4% to suppress any yield point elongation (YPE).

Example 2 - Effect of annealing temperature and alloying content on final microstructure of interstitial free cold rolled high strength bake hardening steel can be distinguished in figure 3(a) and figure 3(b).

Figure 3(a) optical microstructure of inventive steel grades with YS>180MPa showing complete polygonal ferrite grains with average ASTM grain size 8.

Figure 3(b) optical microstructure of inventive steel grades with YS>220MPa showing complete polygonal ferrite grains with average ASTM grain size 9.

Figure 3(a) corresponds to steel sample number 44 with annealing temperature 8490C and P wt% 0.035% whereas figure 3(b) corresponds to steel number 2 with annealing temperature 8110C and P wt% of 0.055 %.

Figure (4) describes relation between Equation 2 (6800 [C] - 14 [Mn] + 206 [P] + 8105 [N] - 2010 [Ti] + 212 [V] + 0.097 SS) for different steel Numbers vs. the resultant Bake hardening index (BHI). As evident from the figure, Steel numbers having value of Equation 2 between 90 to 120 in table IV comply with the BH requirement and Aging guarantee of 3 Months (YPE<0.15).
Figure 5 shows Orientation Distribution Function (ODF) at ?2 450 showing strong ?-fibre intensity for present inventive steel.

Figure 5 showing ratio of mean value of X-ray random intensity ratio of group of {111} <110> to {001} <110> orientation is > 4 for inventive steel number 7. Also mean value of X ray random intensity for {111} <112> orientation is larger than that of {112} <110> orientations.

Figure 6 Shows relation between Equation 1 (69.25Ti+174.1B+16.9V+ 0.0131SPM) for different steel Numbers vs. the resultant Yield Point elongation % (YPE %) after 3 Months of Aging. As evident from the graph, Steel numbers having value of (Equation1=69.25Ti+174.1B+16.9V+ 0.0131SPM) < 1 in table IV does not comply with the Aging guarantee of 3 Months (YPE > 0.15 %), whereas steel with Value of Eq1 >1 fulfills the requirement of 3 Months aging (YPE<0.15 %).

It is thus possible by way of the present advancement to provide high strength ultra-low Carbon steel having excellent bake hardenability, improved ageing resistance, excellent formability and surface finish, and method for manufacturing the same through continuous annealing route. More specifically, the advancement is directed to method of manufacturing two categories of ultra low carbon cold rolled continuous annealed steel sheet having excellent bake hardenability primarily proposed for outer panel and the like of an automobile body material where the strength of the drawn parts can be further increased by paint baking process.

,CLAIMS:We Claim:
1. High strength ultra low carbon bake hardenable steel composition suitable for producing steel with BH value of more than 30 MPa and ageing guarantee with desired formability including continuous annealing and skin pass elongation comprising
(In weight %) (In weight %)
C: 0.001-0.003 N: 0.004 or less
Mn: 0.35-0.55 Ti: 0.008-0.018
P: 0.035-0.06 B: 0.0007-0.0012
V: 0.005-0.02

such that [Ti+B] wt% = [2.64 N* +0.78N] and the balance being Fe and other inevitable or associated impurities; wherein the constitutional elements essentially meet the following relations
120=6800 [C] - 14 [Mn] + 206 [P] + 8105 [N] - 2010 [Ti] + 212 [V] + 0.097 SS = 90
69.25[Ti] + 174.1[B] +16.9[V] + 0.0131 SPM = 1
(Where N* = N wt. % fixed by Ti, N=maximum N wt% which can be added, SPM= skin pass elongation %, SS- Soaking section temperature in ?C)
wherein said cold rolled continuously annealed steel has a mean value of X-ray random intensity ratio of group of {111} <110> to {001} <110> orientation more than 3.5 and mean value of X ray random intensity for {111} <112> orientation component is at least 2.5 multiple than that of {112} <110> orientation component.

2. High strength ultra low carbon bake hardening steel with improved ageing resistance and excellent formability having composition comprising
(In weight %) (In weight %)
C: 0.001-0.003 N: 0.004 or less
Mn: 0.35-0.55 Ti: 0.008-0.018
P: 0.035-0.06 B: 0.0007-0.0012
V: 0.005-0.02

such that [Ti+B] wt% = [2.64 N* +0.78N] and the balance being Fe and other inevitable or associated impurity; wherein the constitutional elements essentially meets the following relations
120=6800 [C] - 14 [Mn] + 206 [P] + 8105 [N] - 2010 [Ti] + 212 [V] + 0.097 SS = 90
69.25[Ti] + 174.1[B] +16.9[V] + 0.0131 SPM = 1
(Where N* = N wt. % fixed by Ti, N=maximum N wt% which can be added, SPM= skin pass elongation %, SS- Soaking section temperature in ?C),
such as to achieve Yield Point Elongation (YPE) less than 0.15 % after accelerated aging for 6 Hours and BH index of at least 30 MPa at lower yield point.
3. High strength ultra low carbon bake hardenable steel composition as claimed in claim 1 selectively workable to provide
(i) A first category having minimum yield strength of 180 MPa with ? value more than 1.8 before bake hardening and (ii) a second category has minimum yield strength value of 220 MPa coupled with ? value more than 1.6 before bake hardening and both having UTS less than 340MPa and Bake Hardening index of 30MPa or more at Lower Yield Point.
4. High strength ultra low carbon bake hardenable steel composition suitable for producing steel with BH value of more than 30 MPa at lower yield point and ageing guarantee with desired formability including continuous annealing and skin pass elongation obtained of steel composition comprising
(In weight %) (In weight %)
C: 0.001-0.003 N: 0.004 or less
Mn: 0.35-0.55 Ti: 0.008-0.018
P: 0.035-0.06 B: 0.0007-0.0012
V: 0.005-0.02

such that [Ti+B] wt% = [2.64 N* +0.78N] and the balance being Fe and other inevitable or associated impurity; wherein the constitutional elements essentially meets the following relations
120=6800 [C] - 14 [Mn] + 206 [P] + 8105 [N] - 2010 [Ti] + 212 [V] + 0.097 SS = 90
69.25[Ti] + 174.1[B] +16.9[V] + 0.0131 SPM = 1
(Where N* = N wt.% fixed by Ti, N=maximum N wt% which can be added, SPM= skin pass elongation %, SS- Soaking section temperature in ?C),
having minimum yield strength of 180 MPa with ? value more than 1.8 before bake hardening.
5. High strength ultra low carbon bake hardenable steel composition suitable for producing steel with BH value of more than 30 MPa and ageing guarantee of 6months with desired formability including continuous annealing and skin pass elongation obtained of steel composition comprising
(In weight %) (In weight %)
C: 0.001-0.003 N: 0.004 or less
Mn: 0.35-0.55 Ti: 0.008-0.018
P: 0.035-0.06 B: 0.0007-0.0012
V: 0.005-0.012

Such that [Ti+B] wt% = [2.64 N* +0.78N] and the balance being Fe and other inevitable or associated impurities; wherein the constitutional elements essentially meet the following relations
120=6800 [C] - 14 [Mn] + 206 [P] + 8105 [N] - 2010 [Ti] + 212 [V] + 0.097 SS = 90
69.25[Ti] + 174.1[B] +16.9[V] + 0.0131 SPM = 1
(Where N* = N wt.% fixed by Ti, N=maximum N wt% which can be added, SPM= skin pass elongation %, SS- Soaking section temperature in ?C)
And having minimum yield strength value of 220 MPa coupled with ? value more than 1.6 before bake hardening and having UTS less than 340MPa.
6. High strength ultra low carbon bake hardening steel sheet with improved ageing resistance and excellent formability as claimed in anyone of claims 3 or 4 having complete polygonal ferrite microstructure with ASTM grain size number of 9.5 or less along with Nitride and Carbide precipitates of alloying elements.
7. A method of producing high strength ultra low carbon bake hardening steel sheet with improved ageing resistance and excellent formability comprising
(i) Providing selective steel composition comprising
(In weight %) (In weight %)
C: 0.001-0.003 N: 0.004 or less
Mn: 0.35-0.55 Ti: 0.008-0.018
P: 0.035-0.06 B: 0.0007-0.0012
V: 0.005-0.012

Such that [Ti+B] wt% = [2.64 N* +0.78N] and the balance being Fe and other inevitable or associated impurities; wherein the constitutional elements essentially meet the following relations
120=6800 [C] - 14 [Mn] + 206 [P] + 8105 [N] - 2010 [Ti] + 212 [V] + 0.097 SS = 90
69.25[Ti] + 174.1[B] +16.9[V] + 0.0131 SPM = 1
Where N* = N wt. % fixed by Ti, N=maximum N wt% which can be added, SPM= skin pass elongation %, SS- Soaking section temperature in ?C.
ii) carrying out steel manufacture involving continuous annealing and including Soaking Temperature in the temperature range of 800°C to 850°C, Slow Cooling Temperature in the temperature range of 690°C to 730 °C, and Skin pass elongation in the range of 1.4% to 1.8% preferably 1.4 % or more to achieve the minimum YS 180 MPa or more and skin pass elongation (SPM %) in the range of 1.8 to 2.2 preferably 1.8 or more to achieve the minimum YS 220 MPa or more.

8. A method as claimed in anyone of claims 5 to 7 comprising
continuously casting , hot rolling keeping slab reheating temperature below 1200°C to achieve roughing mill delivery temperature under 1050°C and finishing mill entry temperature under 1020°C to control surface defects including rolled in scale. Hot mill finishing temperature of 880°C to 920°C and coiling temperature of 600°C to 660°C, Hot rolled coiled later processed through pickling coupled with tandem cold rolling mill, to remove the oxide surface present in the surface and to provide cold reduction of 70% or more ;
Following pickling and cold rolling to desired thickness, cold rolled steel strip 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 800 °C or more, annealing time 40-140 seconds gives desired results for present inventive grades, at soaking section complete recrystallization results softer steel to get desired ductility, after soaking section steel strip passes through slow cooling section at cooling rate of 0.5- 5 °C/sec , slow cooling section temperature of 690 °C or more was maintained, following slow cooling section annealed strip sheet been rapid cooled at 8- 30 °C/sec up to 440 °C or more , after rapid cooling section annealed strip was over aged keeping the over aging section temperature of 340 °C or more, after over aging Skin-pass elongation (Temper rolling) was carried out based on different yield strength requirement and to remove any stretcher strain.

Dated this the 2nd day of August, 2016 Anjan Sen
Of Anjan Sen and Associates
(Applicants Agent)