DESC:FIELD OF THE INVENTION
The present invention relates to high strength dual phase cold rolled steel sheet having Tensile strength of at least 800 MPa with low yield ratio comprising chemical elements in terms of mass percent: 0.121% to 0.15% of C, Si: 0.3% or less, Mn: 1.5% to 1.95 ,N: 0.006% or less, Cr: 0.4% to 0.6%, Ti: 0.005% to 0.03%, and the balance being Fe and other inevitable impurities, whereas (Mn+Si)/C is in a range of 10 to 18 for excellent combination of phosphatability and hole expansion ratio. Cold rolled steel described in present invention has an excellent surface property comprising 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 along with good hole expansion ration of =40 % and BH index of =35 MPa suitable for automobile body applications.

BACKGROUND OF THE INVENTION

With utilization of dual phase high strength steel automobile manufacturers are incorporating more high strength materials with UTS 800 MPa or more in their reinforcement, structural components and pillars for light weighing, improving fuel efficiency and to satisfy the norms of future legislation concerning emission and fuel consumption.

However high strength dual phase steels are prone to rather poor drawability or press formability when yield ratio increases. Also the phosphatability and surface coating properties are rather poor.

As a prior art United states patent application number US 7780799 B2 teaches a method of manufacturing a cold rolled high strength steel sheet 780 MPa strength level with excellent local formability and weldability. Local formability and weldability has been achieved by keeping a lower Carbon weight % of 0.09 % max and incorporating very high Mn and Si weight % to achieve the desired strength level of 780 MPa. However the surface property and Phosphatability of the said steel is rather poor due to higher Mn and Si weight %.

Also Indian patent application number 3148/MUM/2012 teaches a method of manufacturing cold rolled trip steel with very low planner anisotropy with having retained austenite in microstructure. However, steel sheet disclosed in 3148/MUM/2012 rather has higher yield ratio and no Bake hardenability. Also the steel sheet is susceptible to poor surface property and oxidation due to high Mn and Si weight % range and higher annealing temperature as listed in example tables. Inclusion of higher Al results casting defects such as inclusion which deteriorates drawability.

European patent application number EP1674586A1 discloses a 590 MPa TRIP type steel having 0.5 to 2% of Al + Si in its composition to achieve retained austenite in steel microstructure. However, steel sheet as disclosed in EP1674586A1 has rather poor surface property due to higher Si and Al weight percent as these are strong oxide formers and seriously damages the phosphatability. Also bake hardenability of EP1674586A1 is rather poor along with high yield ratio.

The present inventions aims at advantageously solving the problems of the prior art described above and an object thereof is to provide a cold rolled dual phase steel sheet having Tensile strength of at least 800 MPa with low yield ratio and excellent combination of phosphatability and hole expansion ratio and a process for producing the same. The problem of Non Uniform phosphate grain size solved by proper selection of Mn and Si wt % and low yield ratio achieved by proper selection of Micro-alloying like Ti and Nb.

OBJECTS OF THE INVENTION

The basic object of the present invention is directed to provide a cold rolled dual phase steel sheet having Tensile strength of at least 800 MPa with low yield ratio and excellent combination of phosphatability and hole expansion ratio and a process for producing the same.

A further object of the present invention is directed to provide a cold rolled dual phase steel sheet having Tensile strength of at least 800 MPa which is capable of ensuring good surface property by stably exhibiting uniform phosphate grain size with less coating weight, i.e. having excellent phosphatability.

A still further object of the present invention is directed to provide a cold rolled dual phase steel sheet having Tensile strength of at least 800 MPa involving selective composition and process parameters to ensure good hole expansion ration of =40 % and BH index of =35 MPa of the resulting steel sheets.

A still further object of the present invention is directed to provide a cold rolled dual phase steel sheet having Tensile strength of at least 800 MPa having a composition satisfying the ratio of (Mn+Si)/C in a range of 10 to 18 for achieving said excellent combination of phosphatability and hole expansion ratio.
A still further object of the present invention is directed to provide a cold rolled dual phase steel sheet having Tensile strength of at least 800 MPa wherein since the hole expansion ratio deteriorates due to higher difference in the hardness between ferrite and martensite phase, the difference between volume fraction of Ferrite and Martensite is optimized to higher value which is controlled by selective chemical composition and RCS temperature.

SUMMARY OF THE INVENTION

The basic aspect of the present invention is directed to a cold rolled steel sheet comprising:
(In weight %) (In weight %)
C: 0.121-0.15 % Mn: 1.5-1.95 %
Ti: 0.005-0.03 % Cr: 0.4-0.6 %
Si- 0.3 % or less N-0.006 % or less

and the balance being Fe and other unavoidable impurities; wherein (Mn+Si)/C is in a range of 10 to 18, and steel microstructure in relation with compositional elements satisfying;
2.7log10 (%Fea-%Ms) = {(Mn+Si)/ (C+Cr+Ti)}
Wherein %Fea is total area percent of ferrite and %Ms is total area percent of martensite relative to a whole microstructure of said steel and having Yield strength is 530 MPa or less and tensile strength is atleast 800MPa.

A further aspect of the present invention is directed to a cold rolled steel sheet further comprising any one or more Nb: from0.01% to 0.04 % and B: 0.0005% to 0.002%; and at least one type of element selected from the group comprising V, Zr, Mg, Mo, W, Hf, Co, Ni, Cu, Zn, Ca, Pb and Sn such that each element by content in the range of 0.002 to 0.03 %by wt.

A still further aspect of the present invention is directed to a cold rolled steel sheet wherein Ti to N ratio is in the range of 1 to 10 for BH index of 35 MPa or more.

A still further aspect of the present invention is directed to a cold rolled steel sheet which is obtained of continuous annealing and wherein the steel composition is based on 0.9 = log10 [(SCS-RCS)/40] = 4.6 – 1.7*[%Mn] -2.3* [%Cr]- 2*[C%],
Wherein SCS is slow cooling section temperature and RCS is rapid cooling section temperature in continuous annealing.

Another aspect of the present invention is directed to said cold rolled steel sheet having excellent surface property comprising 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 along with good hole expansion ration of =40 % and BH index of =35 MPa.

Yet another aspect of the present invention is directed to said cold rolled steel sheet comprising YS/UTS ratio of not more than 0.65, bake hardening index is atleast 35 MPa, Hole Expansion Ratio (HER%)= 40 %.

A further aspect of the present invention is directed to a process for manufacturing cold rolled steel sheet, comprising the steps of:
a.) Reheating the slab having said composition to reheating temperature in the range of 1180°C -1250 °C or less;
b.) Said Reheated slab being roughing rolled in roughing mill with roughing mill delivery temperature of 1080°C or less;
c.) Said rough rolled steel being subjected to finish rolling after at temperature range of 840°C to 910°C;
d.) Coiling the finish rolled steel at with average 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 35% or more.

A still further aspect of the present invention is directed to said process for manufacturing cold rolled steel sheet comprising:
a) Annealing at soaking section temperature range of 770 °C to 820°C with residence time of for 70 to 140 seconds;
b) Slow cooling the steel up to a temperature range of 670°C to 730°C after soaking;
c) Rapid cooling the steel up to a temperature range of350°C or less with cooling rate of 10°C / second to 25°C / second;
d) Overaging said steel at temperature range of 260°C to 360°Cwith residence time of 100 to 300 seconds; and
e) Subjecting the overaged steel to skin pass elongation of 0.2% to 1 %.

A still further aspect of the present invention is directed to process for manufacturing cold rolled steel sheet which is carried out maintaining the following relation with respect to SCS Temperature, and RCS temperature:
0.9=log10 [(SCS-RCS)/40] = 4.6 –1.7*[%Mn] -2.3* [%Cr]- 2*[C%]
Wherein SCS- Slow cooling section Temperature in 0C, RCS- Rapid cooling section Temperature in 0C, [M] = Elemental composition in wt%.

Yet another aspect of the present invention is directed to said process for manufacturing cold rolled steel sheet which is carried out such as to achieve YS/UTS ratio of said steel sheet is not more than 0.65, bake hardening index is atleast 35 MPa, Hole Expansion Ratio (HER%)= 40 %, Yield strength is 530 MPa or less and tensile strength is atleast 800MPa and having 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.

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

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1: shows the SEM micrograph of inventive 800MPa level steel sample (steel B from table 1) at different magnifications showing Ferrite + low temperature transformed (Martensite) phase.
Figure 2: shows graphically the Relation between the Eq1 = (Mn+ Si)/C vs the phosphatability crystal size in µm after zinc phosphate chemical conversion coating.
Figure 3: shows the graphical plot of Ti/N ratio vs the Bake hardening index of inventive and comparative steel sheets.
Figure 4: shows graphically the variation of log10 [(SCS-RCS)/40] vs YS/UTS ratio of inventive and comparative steel sheets.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO ACCOMPANYING DRAWINGS AND EXAMPLES INCLUDING A PREFERRED EMBODIMENT
The present invention is directed to provide high strength cold rolled dual phase steel sheets having low yield Ratio, excellent surface property, hole expansion ratio and bake hardening index suitable for automotive components and a process for manufacturing the same.

Following abbreviations of related terminologies are used to describe the present invention:
SS- Soaking Section
SCS – Slow Cooling Section
RCS -Rapid Cooling Section
OAS - Over-ageing section
UTS-Ultimate Tensile Strength in MPa
YS-Yield Strength in MPa
El% – Total Elongation
SPM % -Skin Pass Elongation in %
FT-Finishing Temperature
CT- Coiling Temperature
%Fea- Ferrite %
% Ms – Martensite %
HER – Hole expansion ratio

With the aim of achieving low yield ratio Dual phase 800 MPa strength level cold rolled steel sheet, through continuous annealing route, effect of Metallurgical factors affecting the mechanical properties and microstructure are described in detail as follows:

Carbon (C: 0.121-0.15 wt %) - Carbon being the main alloying element improves hardenability significantly. All transformations are noticeably affected and by which the final microstructure and the mechanical properties are controlled. Carbon stabilizes the austenite which leads to the formation of martensite in the case of dual phase steels. However, other requirements such as spot weldability, formability and hole expansion ratio limits the use of carbon to round about 0.16 mass %. To further improve the spot weldability by reduced carbon equivalent, carbon has been limited to 0.15 wt% or less in present inventive dual phase steel grade.

Manganese (Mn: 1.5-1.95 wt %) -Mn significantly improves the hardenability, hence, DP steel can be produced easily even by a simple air cooling. Also it assists fine dispersion of martensite phase which leads to higher tensile strength and good ductility. The addition of small amount of Si (<0.3 wt %) gives beneficial effects, the tensile strength further increases without a significant loss of ductility.

However Higher Mn tends to form micro segregations during the steel casting process, i.e. the distribution of Mn in the slab will not be homogeneous. Since Mn lowers the AC1-temperature, the Mn-rich areas will start to transform to austenite prior to the surrounding areas with lower Mn-content. The consequence will be a structure of ferrite with the martensite phase to some extent distributed in bands, a so called banded structure. Also higher Mn wt% increases C equivalent value and deteriorates spot weldability and hole expansion. In addition, Higher Mn wt% results in oxidized surface after continuous annealing having somewhat Yellow and Blue in color seriously damaging the surface and coating properties. To avoid the above inadequacy upper limit of Mn has been restricted to 1.95 wt% for present inventive grade.

Silicon (Si: 0.3 wt % or less) –Si being a ferrite stabiliser increases the strength of Ferrite phase and assists to increase the overall strength. However higher silicon content causes problems during hot rolling and coating due to formation of oxidized surface commonly known as scale. For that reason upper limit of Si has been restricted to 0.3 wt% or less.

Chromium (Cr: 0.4-0.6 wt %) – Cr assists Mn in improving strength by improving Mn equivalent. Chromium is Ferrite stabiliser and in present invention is used to reduce and replace silicon, which may cause problems during hot rolling and coating. Chromium also reduces the annealing time in order to achieve dual phase structure and to do so minimum amount of Cr must be >0.4 wt%. However Higher Cr content reduces the workability, Therefore upper limit should be 0.6 wt% or less.

Niobium (Nb: 0.01-0.04 wt %) - Niobium has a notable role on grain size development in conjunction with carbon enrichment, transformation mechanism of the austenite followed by nucleation of martensite which makes controlling the process parameter much easier, which further improve the mechanical property. To attain the explained benefits minimum amount of Nb which should be added is 0.01 wt%. Nb more than 0.04 wt% unnecessarily adds up to the cost of production and increases YS/UTS ratio. Hence, upper limit for present inventive DP grade in 0.04wt%.

Titanium (Ti: 0.005-0.03 wt %)- Ti acts as a nitrides forming element to fix solute N in steel thus helps in getting aging resistance. Formability of steel sheet improves by reducing solute N in solution with Ti instead of Al. And so, Amount of Ti preferably added should be 0.005 wt% or more. However, when Ti contents exceeds 0.03 wt%, the effects are saturated, therefore the amount of Ti is made to be 0.03% or less. In addition, when Ti is added in excess of the amount required for reducing solid solution N, excessive TiC and TiN may form, which inhibits the bake hardening properties and stable formation of second phase, which is not preferable .

Boron (B: 0.0005% - 0.002% wt%) – Boron is a nitride former which also strengthen the grain boundary, however Boron content exceeding more than 0.003 wt% cause defects during casting and hot rolling such as edge crack. Hence it is preferable to keep boron below 0.002 wt%.

V, Zr, Mg, Mo, W, Hf, Co, Ni, Cu, Zn, Ca, Pb and Sn 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 act as carbide former and/or nitride former and/or solid solution strengthening elements, however adding each of these elements in an amount more than 0.03 wt% unnecessarily adds up to the cost of the steel.

(Mn+Si)/C in a range of 10 to 18: The claimed ratio of (Mn+Si)/C are significant for high strength dual phase steel with 800 MPa level UTS. To achieve a combination of excellent phosphatability along with good hole expansion ratio (Mn+Si)/C ration must be in the range of 10 to 18. (Mn+Si)/C ratio more than 18 results in poor phosphatability whereas ratio less than 18 results in poor hole expansion ratio due to very high hard martensite area fraction.

2.7log10 (%Fea-%Ms) = {(Mn+Si)/ (C+Cr+Ti)} –For given 800 MPa strength level steel sheet the value of 2.7log10 (%Fea-%Ms) must be more than (Mn+Si)/ (C+Cr+Ti)} in order to achieve the good hole expansion ratio =35% along with excellent surface property. As the % and hardness of martensite increases the hole expansion ratio deteriorates due to higher difference in the hardens between ferrite and martensite phase. In order to avoid that the difference between volume fraction of Ferrite and Martensite must be optimized to higher value which is controlled by selective chemical composition and RCS temperature.

Complete description of Process:
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 1180°C to 1240°C intended to control roughing mill delivery temperature under 1080°C and finishing mill entry temperature under 1050°C to check surface defects like rolled in scale. During hot rolling finishing mill temperature range of 850°C to 910°C and run out table cooling rate from finishing mill to coiler of more than 80C/sec was maintained to achieve coiling temperature range of 540 °C to 600 °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 35% to 65%.

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 770 °C -820 °C. Annealing time of 70-140 seconds gives desired results for present dual phase grade. At soaking section temperature intercritical annealing results in ferrite and austenite microstructure which later transforms to ferrite + martensite or Ferrite+ Martensite + Bainite microstructure based on the cooling rate from slow cooling section to rapid cooling section inside continuous annealing line. After soaking section steel strip passes through slow cooling section at cooling rate of less than 3°C/sec .Slow cooling section temperature of 670 °C - 730°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 of 350 °C or less to avoid pearlite formation and attain the desired strength of 800 MPa or more. After rapid cooling section annealed strip was over aged keeping the over aging section temperature of 280°C -360 °C to temper the transformed strengthening phase (Martensite and/or Bainite). After over aging Skin-pass elongation (Temper rolling) in the range of 0.2 % to 1% was applied to avoid yield point elongation. In addition following relation must be fulfilled in favor of chemical composition and during annealing in order to achieve Tensile Strength 800 MPa and yield ratio of 0.65 or less –

log10 [(SCS-RCS) /40] = 4.6 -1.7* [%Mn] - 2.3*[%Cr] -2*[%C],
where SCS- Slow cooling section Temperature in 0C, RCS- Rapid cooling section Temperature in 0C, [M] = Elemental composition of element M in wt%.

Furthermore, Cold rolled dual phase 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 phosphatability –

Phosphating process can be defined as the treatment of a metal surface so as to give a 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 topochemical reaction, which causes the surface of the base metal to integrate itself as a part of the corrosion resistant film. [1]
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.

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 hole expansion ratio: The hole expansion ratio (HER %) is significant to assess the stretch flangeability of steel sheets. It is acquired by the hole expansion test utilizing conical or cylindrical punch in forming test machine. Whole expansion tests were performed as per ISO 16630-2009 utilizing forming test machine. Samples having a pouched hole of 10mm diameter were used for the test. Conical punch having an angle of 600 and cylinder diameter 50 mm was used. The punching speed of the conical punch during hole expansion was 0.3 mm/s. The conical punch was moved up against the sample with 10mm hole until the small crack appeared at the edge of hole and detected by optical instrument. The final average diameter of the hole after the small crack appeared was determined by measuring in two directions. Test were repeated for four to five times for each steel numbers and average HER% was taken with the following standard equation -
HER% = [(Df - Do) / D0] X 100
Where Do = Initial hole diameter, Df = final hole diameter before crack.

Complete description of Inventive steel and comparative steel grades are illustrated from table 1 to table 4:

Table 1 -Elemental Compositions in weight % of the inventive steel sheets along with comparative example and their respective values of Eq1 = (Mn+Si)/C:

Steel No. C Mn Si N Ti Cr Other Elements Eq1=
(Mn+Si)/C Ti/N Remarks
A 0.13 1.7 0.15 0.005 0.028 0.5 V:0.02,Mo:0.02,Zr:0.003 ,Nb-0.01 14.2 5.6 Ex.
B 0.125 1.8 0.1 0.004 0.015 0.45 Nb:0.02, Ni:0.01,Cu:0.02,Ca:0.003,W:0.003 15.2 3.8 Ex.
C 0.141 1.6 0.25 0.005 0.025 0.58 Hf:0.003,B:0.0015,Sc:0.002 ,Nb:0.025,Co:0.003 13.1 5 Ex.
D 0.127 1.71 0.23 0.005 0.02 0.45 Nb:0.02, B:0.001,Ti:0.02 ,W:0.01,Co:0.002 15.3 4.0 Ex.
E 0.08 2.5 0.8 0.003 0.05 0.1 Mo:0.2, Nb:0.07 41.3 16.7 Comp.
F 0.121 1 0.02 0.002 0.03 0.2 Nb:0.04,B:0.003 8.4 15 Comp.
G 0.08 3.2 0.3 0.002 0.035 0.2 Nb:0.05 43.8 17.5 Comp.
H 0.09 1.7 0.55 0.004 0.002 0.02 V:0.05 25.0 0.5 Comp.
I 0.05 2.6 1.3 0.003 0.04 0.02 Mo:0.2 78 13.3 Comp.

* Ex. - Present inventive example, Comp.- Comparative Examples
** shaded boxes indicates “outside the appropriate range”
*** Eq1 = (Mn+Si)/C

Table 2- Hot rolling, cold rolling, annealing parameters of inventive and comparative steel sheets having chemical compositions as per table 1:

S.No Steel No. FT,0C Cooling rate after Finish Rolling ,0C/sec CT,0C Cold reduction ,% SS Temperature,0C Soaking Time, Sec SCS Temperature,0C Cooling rate, 0C/sec RCS Temperature,0C OAS Temperature,0C SPM Elongation,%
1 A 885 10.5 555 50 800 100 700 17.8 330 290 0.3
2 890 9.6 560 50 805 95 695 18.5 325 285 0.25
3 890 9.8 560 50 790 110 685 16.2 330 300 0.4
4 880 10.3 570 50 800 115 670 11.2 400 360 0.3
5 B 890 10.7 563 45 790 110 700 16.3 320 280 0.25
6 C 895 11.2 550 45 810 120 695 14.5 330 301 0.3
7 D 870 9.3 570 50 780 125 705 14.3 325 305 0.4
8 E 870 11.2 560 45 830 103 710 18.8 300 230 0.2
9 F 890 12 570 50 810 85 680 17.9 350 290 0.4
10 G 890 11.5 570 40 780 110 700 12.9 400 230 0.1
11 H 890 11.5 570 50 780 120 690 11.5 420 340 0.5
12 I 890 11.5 570 40 780 110 700 12.9 400 230 0.1
* FT- hot finish rolling temperature , CT- Hot coiling temperature , SS- soaking section ,SCS- Slow cooling section , RCS- Rapid cooling section , OAS- Overaging section , SPM- Skin pass elongation
** Shaded boxes indicates “outside the appropriate range”

Table 3 –Micro structural phase fractions and value of different equations of respective steel sheets listed in table 1 and processed as per table 2 for comparative ad Inventive examples:

S.No Steel No. Ferrite area fraction ,% Martensite area Fraction , % Other Phases area (B+P),% Eq2 Eq3 Eq4 Eq5 Remarks
1 A 71 27 2 4.44 2.8 0.97 0.3 Ex.
2 68 29 3 4.40 2.8 0.97 0.3 Ex.
3 68 30 2 4.40 2.8 0.95 0.3 Ex.
4 69 28 3 4.40 2.8 0.83 0.3 Comp.
5 B 71 29 0 4.38 3.2 0.98 0.3 Ex.
6 C 68 30 2 4.27 2.5 0.96 0.3 Ex.
7 D 71 29 0 4.38 3.2 0.98 0.4 Ex.
8 E 61 39 0 3.62 14.3 1.01 0.0 Comp.
9 F 78 19 3 4.78 2.9 0.92 2.2 Comp.
10 G 55 30 15 3.77 11.3 0.88 -1.5 Comp.
11 H 76 5 19 5.00 20.1 0.83 1.5 Comp.
12 I 55 11 34 4.44 35.5 0.88 0.0 Comp.
* Where, Fea = Ferrite Phase, Ms= Martensite Phase, P= pearlite Phase, B= Bainite Phase,
** Eq2 = 2.7log10 (%Fea-%Ms) , Eq3= {(Mn+Si)/ (C+Cr+Ti)}, Eq4= log10[(SCS-RCS) /40] , Eq5= 4.6 – 1.7*[%Mn] -2.3* [%Cr]- 2*[C%]
*** shaded boxes indicates “outside the appropriate range”
*Steels having the value of Eq20.65 and poor drawability.
Table 4- Mechanical properties, surface phosphatability properties and Hole expansion ratios of inventive and comparative steels having chemical composition as per table 1 and being processed as per table 2:
S.No Steel No. YS,
MPa UTS,
MPa YS/UTS BH Index Total EL% Phosphate Crystal Size, µm Phosphate Coating weight , g/m2 Phosphatability Remark Hole Expansion Ratio ,(HER)% Remarks
1 A 482 822 0.59 41 24.2 3.5 2.1 Good 42 Ex.
2 468 809 0.58 39 23.7 3 2.1 Good 48 Ex.
3 481 810 0.59 38 22.7 3 2.3 Good 47 Ex.
4 501 764 0.66 35 16.7 3.5 2 Good 24 Comp.
5 B 468 835 0.56 44 22.3 4 2.2 Good 48 Ex.
6 C 475 826 0.58 39 23.5 3 2.3 Good 46 Ex.
7 D 478 819 0.58 40 22.3 3.5 2.4 Good 42 Ex.
8 E 630 1010 0.62 2 13.1 6 3.2 Poor 21 Comp.
9 F 410 570 0.72 4 24.9 3.5 2.3 Good 50 Comp.
10 G 753 1057 0.71 2 11.1 8 3.1 Poor 17 Comp.
11 H 422 623 0.68 55 27 7.5 3.1 Poor 49 Comp.
12 I 781 1017 0.77 8 12.1 8.5 3.3 Poor 19 Comp.

* Shaded boxes indicates “outside the appropriate range”
** Steel sheets having YS/UTS ratio =0.65 does not conform with the scope of the present invention. also ,steels with phosphatability remark poor does not fulfill the phosphatability requirement as the phosphate crystal size after zinc phosphate chemical conversion coating is >4µm and phosphate coating weight is >2.5 g/mm2 for these steel sheets which is harmful for coating and painting on steel surface.

It can be appreciated from Table 1 to Table 4 that steel sheets remarked as “Ex.” are satisfying all the scopes of present invention and exhibits excellent phosphatability having phosphate crystal size =4µm and phosphate coats weight 1.5-2.5 g/m2 post zinc phosphate chemical conversion coating along with yield ratio of =0.65, BH index =35MPa, UTS =800 MPa and YS=530 MPa, Hole expansion ratio =40%. Whereas, Steel remarked as “Comp.” from Table 1 to Table 4 does not comply with atleast one of the scope of the present invention and does not conform with minimum one or more of the end product attributes as mentioned in the scope of the invention.

Example 1: Steel sheet “A” as listed in table 1 has chemical composition as per the scope of invention. However Steel “A” was processed through different hot rolling and annealing conditions as listed in table 2. Steel A with a rapid cooling section temperature of 330 0C is confirming to the relation of log10 [(SCS-RCS)/40] = 4.6 – 1.7*[%Mn] -2.3* [%Cr]- 2*[%C] satisfy the scope of the invention with UTS of 822 MPa and YS/UTS ratio of 0.59 as described in table 4. However steel sheet “A” when processed with higher RCS temperature of 4000C (as listed in Table 2, S.No 4) and the value of log10 [(SCS-RCS)/40] < 0.9 (table 3 S. No 3) resulted in lower UTS value of 764 MPa with yield ratio of 0.66 which does not conform to the scope of present invention.

Example 2- Steel B, C and D having chemical composition as per the scope of the invention and being processed as described in the scope of the invention showing excellent phosphatability with phosphate crystal size =4µm and phosphate coats weight 1.5-2.5 g/m2 post zinc phosphate chemical conversion coating along with yield ratio of =0.65, BH index =35MPa, UTS =800 MPa, YS=530 MPa and Hole expansion ratio =40%. Accompanying Figure 1 shows the SEM micrograph of inventive 800MPa level steel sample (steel B from table 1) at different magnifications showing Ferrite + low temperature transformed (Martensite) phase.

Example 3: Steel “E” as listed in table 1 has chemical composition out of the scope of the present invention with higher Mn, Si, and Mo weight % and lower Cr weight %. Also Ti/N ratio is more than 10 resulting in poor bake hardening index of 7 MPa. Steel “E” being processed keeping hot and cold rolling condition as per scope of the invention with hot rolling finishing temperature of 870 0C, coiling temperature of 5600C, annealed at 830 0C for 103 seconds with rapid cooling section temperature of 300 0C (table 2 S.No 8). However steel “E” does not satisfy the relation 2.7*log10(%Fea-%Ms) = {(Mn+Si)/ (C+Cr+Ti)} resulting in very poor phosphatability with phosphate crystal size of 6 µm and phosphate coating weight of 3.2 g/m2 ( as shown in Table 4 S.No 8). As weight % of Mn and Si increases tendency of surface oxide formation during hot rolling and annealing increases, resulting in poor phosphatability. It can also be seen from table 4 that steel “E” has higher YS and UTS value of 630 MPa and 1010 MPa respectively due to excessive Manganese (2.5 wt%) and Silicon (0.8%) % .

Example 4: Steel sheet “F” having lower Mn and Cr weight % to achieve the desired UTS value shows lower UTS value of 570 MPa with higher YS/UTS ratio of 0.72 against the desire ratio of =0.65. Also steel “F” has lower Martensite area fraction of 19 % which is not adequate to achieve desired strength level .Also, steel “F” does not fulfill the relation log10 [(SCS-RCS)/40] = 4.6 -1.7* [%Mn]- 2.3*[%Cr]-2[%C] as described in Table 3 S.No 9 therefore the UTS is lower than the desired value of 800 MPa (Table 4 S.No 9).

Example 5: Steel “G” as listed in table 1 has chemical composition out of the scope of the present invention with higher Mn weight % and lower Cr weight % with (Mn+Si)/C ratio of 43.8 which is outside the desired range of 10 to 18. Steel “G” being processed keeping hot and cold rolling condition as per scope of the invention with hot rolling finishing temperature of 890 0C, coiling temperature of 5700C, annealed at 780 0C for 110 seconds. However rapid cooling section temperature was 400 0C higher than the scope of invention (table 2 S.No 10). As a result, steel “G” does not satisfy the relation 2.7*log10(%Fea-%Ms) = {(Mn+Si)/ (C+Cr+Ti)} resulting in very poor phosphatability with phosphate crystal size of 8 µm and phosphate coating weight of 3.1 g/m2 ( as shown in Table 4 S.No 10). It can be attributed to higher % of Mn and Si resulting in poor surface property during hot rolling and annealing with excess oxidation. It can also be seen from table 4 that steel “G” has higher YS and UTS value of 753 MPa and 1057 MPa respectively with poor elongation of 11.3 % and poor HER of 17% due to excessive Manganese content (3.2 wt%).

Similar observation can be made for steel “I” having Higher Mn and Si weight % and the value of (Mn+Cr+Si)/C being 78 which is outside the range of 10 to 18. The same is attributed to poor phosphatability of steel “I” with phosphate crystal size of 8.5 µm and phosphate coating weight of 3.3 g/m2.Furthermore, poor bake hardening index was observed for steel “I” as Ti/N ratio is 13.3. Higher YS/UTS of 0.77 for steel “I” resulted due to higher RCS temperature of 400 0C (table 2 S.No 12) resulting 34% of pearlitic + bainitic phase (table 3 S.No 12).

Example 6: Figure 2 shows the relation between (Mn+ Si)/C vs the Phosphate crystal size after zinc phosphate chemical conversion coating. It can be observed that inventive steel sheet with (Mn+ Si)/C ratio in the range of 10 to18 meets the phosphatability requirement with phosphate crystal size =2.5 g/m2.

Example 7: Figure 3 illustrates the plot between Ti/N ratio vs the Bake hardening Index of inventive and comparative examples. Ti/N ratio in the range of 1 to 10 meets the Bake hardening requirement of atleast 35 MPa however steel sheets with Ti/N ratio > 10 doesn’t comply with the Bake hardening requirement.

Example 8: Figure 4 describes the relation between log10 [(SCS-RCS)/40] vs the Yield ratio. As shown in figure 4, steel sheets with log10 [(SCS-RCS)/40] value more than 0.9 complying with YS/UTS ratio requirement of = 0.65 .

It is thus possible by way of the present invention to provide high strength dual phase cold rolled steel sheet having low yield Ratio, excellent surface property, hole expansion ratio and bake hardening index suitable for automotive components and a process for manufacturing the same. The steel sheet according to the invention having selective composition and processing steps with parameters to ensure not more than 0.65, bake hardening index is atleast 35 MPa, Hole Expansion Ratio (HER%)= 40 %, Yield strength is 530 MPa or less and tensile strength is atleast 800 MPa along with desired formability and spot weldability to suit automobile application.

,CLAIMS:We Claim:

1. A cold rolled steel sheet comprising:
(In weight %) (In weight %)
C: 0.121-0.15 % Mn: 1.5-1.95 %
Ti: 0.005-0.03 % Cr: 0.4-0.6 %
Si- 0.3 % or less N-0.006 % or less

and the balance being Fe and other unavoidable impurities; wherein (Mn+Si)/C is in a range of 10 to 18, and steel microstructure in relation with compositional elements satisfying;
2.7log10 (%Fea-%Ms) = {(Mn+Si)/ (C+Cr+Ti)},

Wherein %Fea is total area percent of ferrite and %Ms is total area percent of martensite relative to a whole microstructure of said steel and having Yield strength is 530 MPa or less and tensile strength is atleast 800MPa.

2. A cold rolled steel sheet according to claim 1, further comprising any one or more Nb: from0.01% to 0.04 % and B: 0.0005% to 0.002%; and at least one type of element selected from the group comprising V, Zr, Mg, Mo, W, Hf, Co, Ni, Cu, Zn, Ca, Pb and Sn such that each element by content in the range of 0.002 to 0.03 %by wt.

3. A cold rolled steel sheet according to anyone of claim 1 and 2 where as Ti to N ratio is in the range of 1 to 10 for BH index of 35 MPa or more.

4. A cold rolled steel sheet according to anyone of claims 1 to 3 which is obtained of continuous annealing and wherein the steel composition is based on 0.9 = log10 [(SCS-RCS)/40] = 4.6 – 1.7*[%Mn] -2.3* [%Cr]- 2*[C%],
wherein SCS is slow cooling section temperature and RCS is rapid cooling section temperature in continuous annealing.

5. Cold rolled steel sheet according to anyone of claims 1 to 4 having excellent surface property comprising 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 along with good hole expansion ration of =40 % and BH index of =35 MPa.

6. Cold rolled steel sheet as claimed in anyone of claims 1 to 4 comprising YS/UTS ratio of not more than 0.65, bake hardening index is atleast 35 MPa, Hole Expansion Ratio (HER%)= 40 %,.

7. A process for manufacturing cold rolled steel sheet as claimed in anyone of claims 1 to 6, comprising the steps of:
a) Reheating the slab having said composition to reheating temperature in the range of 1180°C -1250 °C or less;
b) Said Reheated slab being roughing rolled in roughing mill with roughing mill delivery temperature of 1080°C or less;
c) Said rough rolled steel being subjected to finish rolling after at temperature range of 840°C to 910°C;
d) Coiling the finish rolled steel at with average 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 35% or more.

8. A process for manufacturing cold rolled steel sheet as claimed in claim 7, comprising:
a) Annealing at soaking section temperature range of 770 °C to 820°C with residence time of for 70 to 140 seconds;
b) Slow cooling the steel up to a temperature range of 670°C to 730°C after soaking;
c) Rapid cooling the steel up to a temperature range of350°C or less with cooling rate of 10°C / second to 25°C / second;
d) Overaging said steel at temperature range of 260°C to 360°Cwith residence time of 100 to 300 seconds; and
e) Subjecting the overaged steel to skin pass elongation of 0.2% to 1 %.

9. A process for manufacturing cold rolled steel sheet as claimed in anyone of claims 7 or 8which is carried out maintaining the following relation with respect to SCS Temperature, and RCS temperature:
0.9=log10 [(SCS-RCS)/40] = 4.6 –1.7*[%Mn] -2.3* [%Cr]- 2*[C%]
Wherein SCS- Slow cooling section Temperature in 0C, RCS- Rapid cooling section Temperature in 0C, [M] = Elemental composition in wt%.

10. A process for manufacturing cold rolled steel sheet as claimed in anyone of claims 7 to 9 which is carried out such as to achieve YS/UTS ratio of said steel sheet is not more than 0.65, bake hardening index is atleast 35 MPa, Hole Expansion Ratio (HER%)= 40 %, Yield strength is 530 MPa or less and tensile strength is atleast 800MPa and having 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.

Dated this the 14th day of September, 2016
Anjan Sen
Of Anjan Sen & Associates
(Applicants Agent)