DESC:FIELD OF THE INVENTION

The present invention relates to 600 MPa Tensile strength level dual phase cold rolled steel sheet and its method of manufacture with desired characteristics including excellent spot weldability and yield ratio less than 0.65 for excellent drawability. The development is further directed to dual phase steel having excellent Phosphatability and surface properties. The advancement is specifically directed for automotive structural parts, pillars and rails, body structures, reinforcements and brackets, door-intrusion beams, bumper-reinforcement beams, seating components with ability for crash energy absorption.

BACKGROUND OF THE INVENTION
With utilization of dual phase high strength steel automobile manufacturers are incorporating more high strength materials with UTS 590 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 susceptible to rather poor drawability or press formability when yield ratio increases. Also the continuously annealed dual phase steels are susceptible to surface oxidation also called as temper color which hampers the coating and surface treatment properties such phosphatability. With an attempt to increase the strength by addition of more C, Mn and Si the weldability and phosphatability deteriorates due to high carbon equivalent.
For instance patent application number 3148/MUM/2012 teaches a method of manufacturing a cold rolled trip steel with very low planner anisotropy with having retained austenite in microstructure. However, steel sheet disclosed in patent application number 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 European patent application EP1674586A1 will be rather poor along with high yield ratio.
European patent application number EP1808505A1 discloses a high strength thin gauge steel with excellent hole expansibility with tensile strength of 500 MPa minimum. However, steel sheet as disclosed in EP1808505A1 has rather high Al weight percent in order to achieve trip effect as listed in example table. Higher Al wt% hampers the surface property and cleanliness of steel during casting and casting become very difficult due to excessive oxide formation. Also phosphatability of European patent application EP1808505A1 will be rather poor along with poor bake hardenability.

OBJECTS OF THE INVENTION
The basic object of the present invention is directed to provide low yield ratio dual phase cold rolled steel sheet with excellent weldability, phosphatability bake hardenability a method for manufacturing the same.

A further object of the present invention is directed to provide low yield ratio dual phase cold rolled steel sheet having ability for crash energy absorption to suit application automobile structural parts.

A still further object of the present invention is directed to low yield ratio dual phase cold rolled steel sheet having yield ratio selectively maintained below 0.65 for excellent drawability.

A still further object of the present invention is directed to low yield ratio dual phase cold rolled steel sheet wherein the ratio of (Mn+Cr+Si)/C is selectively maintained in a range of 20 to 40 for excellent combination of weldability and strength.

A still further object of the present invention is directed to low yield ratio dual phase cold rolled steel sheet that demonstrate 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.

SUMMARY OF THE INVENTION

The basic aspect of the present invention is directed to a steel composition suitable for producing cold rolled steel sheet having YS/UTS ratio of said steel sheet is less than 0.65, bake hardening index is atleast 30 MPa, Yield strength of 430 MPa or less and tensile strength is atleast 600MPa. comprising:

(In weight %) (In weight %)
C: 0.07-0.1 % Mn: 1.4-1.9 %
Nb: 0.01-0.04 % Cr: 0.51-0.65 %
Si- 0.3 % or less N-0.006 % or less

and the balance being Fe and other unavoidable impurities; and (Mn+Cr+Si)/C in a range of 20 to 40, and wherein the steel composition further satisfying the following relation,
2*log10(%Fea-%Ms)= {(Mn+Si)/ (C+Cr+Nb)}
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 all compositional elements are in weight %.

A further aspect of the present invention is directed to said steel composition wherein the constitutional elements of said steel sheet meet the following relation with respect to SCS Temperature, and RCS temperature:
log10 [(SCS-RCS)/40] = 4.6 - 2.3*[%Cr] -1.7* [%Mn],
where SCS- Slow cooling section Temperature in 0C, RCS- Rapid cooling section Temperature in 0C, [M] = Elemental composition in wt%.

A still further aspect of the present invention is directed to a cold rolled steel sheet comprising steel composition having
(In weight %) (In weight %)
C: 0.07-0.1 % Mn: 1.4-1.9 %
Nb: 0.01-0.04 % Cr: 0.51-0.65 %
Si- 0.3 % or less N-0.006 % or less

and the balance being Fe and other unavoidable impurities; and (Mn+Cr+Si)/C in a range of 20 to 40, and wherein the steel composition further satisfying the following relation;
2*log10(%Fea-%Ms)= {(Mn+Si)/ (C+Cr+Nb)}
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 all compositional elements are in weight %.

A still further aspect of the present invention is directed to said cold rolled steel sheet further comprising by mass % any one or both from Ti: 0.005% to 0.03 % and B: 0.0005% to 0.002%; and atleast 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 %.

Another aspect of the present invention is directed to said cold rolled steel sheet wherein the ratio of Ti to N is less than 15 to achieve a bake hardening index of 35 MPa or more.

Yet another aspect of the present invention is directed to said cold rolled steel sheet wherein the constitutional elements of said steel sheet meet the following relation with respect to SCS Temperature, and RCS temperature:
log10 [(SCS-RCS)/40] = 4.6 - 2.3*[%Cr] -1.7* [%Mn]
where SCS- Slow cooling section Temperature in 0C, RCS- Rapid cooling section Temperature in 0C, [M] = Elemental composition in wt%.

A still further aspect of the present invention is directed to said steel sheet wherein YS/UTS ratio of said steel sheet is less than 0.65, bake hardening index is atleast 30 MPa, Yield strength of 430 MPa or less and tensile strength is atleast 600MPa.

A still further aspect of the present invention is directed to said steel sheet 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.

A further aspect of the present invention is directed to a process for manufacturing cold rolled steel sheet, comprising the steps of:
a) providing steel slab involving a selective steel composition having
(In weight %) (In weight %)
C: 0.07-0.1 % Mn: 1.4-1.9 %
Nb: 0.01-0.04 % Cr: 0.51-0.65 %
Si- 0.3 % or less N-0.006 % or less

and the balance being Fe and other unavoidable impurities; and (Mn+Cr+Si)/C in a range of 20 to 40, and wherein the steel composition further satisfying the following relation;
2*log10(%Fea-%Ms)= {(Mn+Si)/ (C+Cr+Nb)}
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 all compositional elements are in weight %.

b) Reheating said slab having said composition to reheating temperature of 1220 °C or less;
c)Said Reheated slab next roughing rolled in roughing mill with roughing mill delivery temperature of 1060°C or less;
d)Said rough rolled steel next subjected to finish rolling after at temperature range of 850°C to 910°C;
e)Coiling the finish rolled steel at with run out table cooling rate of 8 °C/second or more; and
f)Cold rolling the said hot rolled steel sheet with cold reduction of 40% or more.

A still further aspect of the present invention is directed to said process 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 of360°C or less with cooling rate of 10°C / second to 30°C / second;
d) Overaging said steel at temperature range of 280°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 %.

Another aspect of the present invention is directed to said process carried out such that the constitutional elements of said steel sheet meet the following relation with respect to SCS Temperature, and RCS temperature:
log10 [(SCS-RCS)/40] = 4.6 - 2.3*[%Cr] -1.7* [%Mn],
where 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 carried out to produce steel sheet wherein (i) YS/UTS ratio of said steel sheet is less than 0.65, bake hardening index is atleast 30 MPa, Yield strength of 430 MPa or less and tensile strength is atleast 600MPa and/or (ii) 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 are described hereunder in greater details with reference to following accompanying illustrative figures and examples.

BRIEF DESCRIPTION OF THE ACOMPNAYING DRAWINGS

Figure 1: SEM micrograph of steel sample according to the invention at different magnifications showing Ferrite + low temperature transformed (Martensite+Bainite) phase.

Figure 2(a): is phosphatability test result of steel sheet “G” showing excellent phosphatability with uniform phosphate crystal size of 3µm and phosphate coating weight of 2.3 g/m2..

figure 2(b): shows the phosphatability test result of steel sheet “E” which does not satisfy the equation “ 2*log10(%Fea-%Ms) = {(Mn+Si)/ (C+Cr+Nb)}“ resulted in poor phosphatability with non uniform phosphate crystals having average crystal size of 8µm and coating weight of 3.1 g/m2 .

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO ACCOMPNAYING DRAWING AND EXAMPLES INCLUDING A PREFERRED EMBODIMENT

The present invention is directed to provide low yield ratio dual phase cold rolled steel sheet with excellent weldability and phosphatability and a method for manufacturing the same. The invented steel grade comprises chemical elements in terms of mass percent: 0.07% to 0.1% of C, Si: 0.3% or less, Mn: 1.4% to 1.9% ,N: 0.006% or less, Cr: 0.51% to 0.65%, Nb: 0.01% to 0.04%, and the balance being Fe and other inevitable impurities, whereas (Mn+Cr+Si)/C is in a range of 20 to 40 for excellent combination of weldability and strength, and the steel further satisfying the following relation;

2*log10(%Fea-%Ms) = {(Mn+Si)/ (C+Cr+Nb)}

(Where %Fea is total area percent of ferrite and %Ms is total area percent of martensite relative to a whole microstructure of said steel, and all compositional elements are in weight %). 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.

Following abbreviations are used to describe the present invention:
Abbreviations
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- Hot rolling Finishing Temperature
CT- Hot rolling Coiling Temperature
%Fea- Ferrite %
% Ms – Martensite %

With the aim of obtaining low yield ratio Dual phase 600 MPa strength level Non-aging steel sheet, through continuous annealing route, Effect of Metallurgical factors affecting the mechanical properties and microstructure are described hereunder in details.

Carbon (C: 0.07-0.1 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 and formability limit the use of carbon to round about 0.12 mass %. To further improve the spot weldability by reduced carbon equivalent, carbon has been limited to 0.1 wt% or less for present dual phase steel grade.

Manganese (Mn: 1.4-1.9 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.
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. In addition, Higher Mn wt% results in oxidized surface after continuous annealing having somewhat yellowish and bluish in color seriously damaging the surface and coating properties. To avoid the above inadequacy upper limit of Mn has been restricted to 1.9 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. Si also inhibits the formation of cementite hence is useful in retaining austenite at lower temperature .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: 0.51-0.65 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% more preferably >0.51 wt%. However Higher Cr content reduces the workability, Therefore upper limit should be 0.65 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 0.025 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. when Ti is added in excess of the amount required for reducing solid solution N, excessive TiC may form, which inhibits the 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 sulphide 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.

Process of manufacturing
To achieve steel slab chemistry as described above, 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 below 1220°C intended to control roughing mill delivery temperature under 1060°C and finishing mill entry temperature under 1020°C to check surface defects like rolled in scale. During hot rolling finishing mill temperature range of 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 40% to 75%.

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 in the range of 770 °C -820 °C. Annealing time of 80-140 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 in the range of 660 °C - 720°C was maintained. Following slow cooling section annealed strip sheet been rapid cooled at 10 °C/sec or more up to rapid cooling section temperature of 360 °C or less to avoid pearlite formation and attain the desired strength of 600 MPa or more. After rapid cooling section annealed strip was over aged keeping the over aging section temperature in the range of 280°C -360 °C to temper the transformed strengthening phase (Martensite and/or Bainite). Accompanying Figure 1 is the SEM micrograph of steel sample according to the invention at different magnifications showing Ferrite + low temperature transformed (Martensite+Bainite) phase.
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 600 MPa or and yield ratio of 0.65 or less –

log10 [(SCS-RCS) /40] = 4.6 - 2.3*[%Cr] -1.7* [%Mn],

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.

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.

Accompanying Figure 2(a) is phosphatability test result of steel sheet “G” showing excellent phosphatability with uniform phosphate crystal size of 3µm and phosphate coating weight of 2.3 g/m2 . Figure 2(b) shows the phosphatability test result of steel sheet “E” which does not satisfy the equation “ 2*log10(%Fea-%Ms) = {(Mn+Si)/ (C+Cr+Nb)}“ resulted in poor phosphatability with non uniform phosphate crystals having average crystal size of 8µm and coating weight of 3.1 g/m2 .

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 and tensile test.

Complete description of Inventive steel and comparative steel grades are illustrated in the following table 1 to table 4:
Table 1- Elemental Compositions of the inventive steel sheets along with comparative example and their respective values of Eq1 = (Mn+Cr+Si)/C.
Table 2- Hot rolling, cold rolling, annealing parameters of inventive and comparative steel sheets having chemical compositions as per table 1.
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.
Table 4-Mechanical properties surface phosphatability properties and bake hardening index of inventive and comparative steels having chemical composition as per table 1 and being processed as per table 2.

Table 1

Steel No. C Mn Si N Nb Cr Other Elements Eq1 Ti/N Remarks
A 0.08 1.5 0.22 0.005 0.024 0.52 Mo:0.02,V:0.01,Zr:0.01 ,Ti:0.01 28 2 EX.
B 0.13 2.4 0.6 0.003 0.002 0.1 Mo:0.3 ,Ti:0.05 23.8 16.7 Comp.
C 0.09 1.8 0.1 0.005 0.03 0.53 Hf:0.003,Ni:0.01,Cu:0.02 ,Ti:0.01 27.0 2 Ex.
D 0.095 1.7 0.12 0.003 0.025 0.55 B:0.0009,Ti:0.02 ,W:0.01,Co:0.002 24.95 6.67 Ex.
E 0.2 2.1 0.8 0.001 0.0015 0.2 Mo:0.3,Ti:0.03 15.5 20 Comp.
F 0.05 0.9 0.02 0.002 0.03 0.02 B:0.003,V:0.08 18.8 - Comp.
G 0.07 1.8 0.1 0.004 0.034 0.6 Ti:0.02 ,Pb:0.002 35.7 5.8 Ex.
H 0.09 1.5 0.12 0.0015 0.03 0.61 B:0.0015,Ni:0.004, Zn:0.003,Ti:0.006 24.8 4 Ex.
I 0.06 2.2 1.1 0.002 0.02 0.5 Ti-0.05 63.3 25 Comp.

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

Table 2
S.No Steel No. FT,0C Cooling rate after Finish Rolling ,0C/sec CT,0C Cold reduction ,% SS Temperature,0C SCS Temperature,0C Soaking Time, Seconds RCS Temperature,0C OAS Temperature,0C SPM Elongation,% Remarks
1 A 910 12.4 558 50 810 712 108 320 305 0.3 Ex.
2 902 11.1 561 50 791 706 105 326 283 0.25 Ex.
3 900 9.7 550 50 805 660 150 400 381 0.45 Comp.
4 895 6.2 720 50 800 720 130 350 342 0.5 Comp.
5 B 902 10.5 560 50 810 710 95 300 271 0.7 Comp.
6 C 880 11.2 550 55 790 706 90 317 289 0.3 Ex
7 890 11.6 565 55 802 714 110 328 296 0.3 Ex.
8 920 6.1 710 55 820 705 110 360 338 0.3 Comp.
9 D 900 12.2 543 60 810 710 95 330 289 0.25 Ex
10 905 11.5 552 60 815 710 95 327 280 0.25 Ex.
11 E 910 11.3 560 55 780 695 100 320 276 0.2 Comp.
12 F 905 10.1 580 60 810 700 130 340 312 0.4 Comp.
13 G 897 11.5 560 60 810 710 120 324 289 0.25 Ex
14 879 11.9 545 55 798 702 108 304 283 0.025 Ex.
15 H 900 12.6 561 50 804 710 108 320 300 0.3 Ex.
16 I 896 10.7 570 55 770 671 150 405 357 0.25 Comp.
* 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
** Underlined and shaded boxes indicates “outside the appropriate range”

Table 3
S.No Steel No. Ferrite area fraction , Fea % Martensite area Fraction , Ms% Other Phases, (100-F-M)% Eq2 Eq3 Eq4 Eq5 Remarks
1 A 81 17 B 3.61 2.75 0.99 0.9 Ex.
2 78 20 B 3.53 2.75 0.98 0.9 Ex.
3 78 5 P+B 3.73 2.75 0.81 0.9 Comp.
4 82 15 B 3.65 2.75 0.97 0.9 Comp.
5 B 64 36 _ 2.89 10.35 1.01 0.3 Comp.
6 C 78 22 _ 3.50 2.81 0.99 0.3 Ex
7 79.3 20.7 _ 3.54 2.81 0.98 0.3 Ex.
8 80 16 B 3.61 2.81 0.94 0.3 Comp.
9 D 80 20 - 3.56 2.8 0.98 0.4 Ex
10 81 19 - 3.58 2.8 0.98 0.4 Ex.
11 E 51 49 - 0.60 7.25 0.97 0.6 Comp.
12 F 88 3 P+B 3.86 9.2 0.95 3.0 Comp.
13 G 82 18 - 3.61 2.7 0.98 0.2 Ex
14 80 20 - 3.56 2.7 1.00 0.2 Ex.
15 H 80 20 - 3.56 2.22 0.99 0.6 Ex.
16 I 75 5 P+B 3.69 5.84 0.82 -0.3 Comp.
* Where, Fea = Ferrite Phase, Ms= Martensite Phase, P= pearlite Phase, B= Bainite Phase,
** Eq2 = 2*log10(%Fea-%Ms), Eq3= {(Mn+Si)/ (C+Cr+Nb)}, Eq4= log10[(SCS-RCS) /40] , Eq5= 4.6 - 2.3*[%Cr] -1.7* [%Mn]
*** Underlined and shaded boxes indicates “outside the appropriate range”
*Steels having the value of Eq20.65 and poor drawability.

Table 4

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 Remarks
1 A 381 634 0.60 40 29.5 3.5 2.2 Good Ex.
2 375 642 0.58 43 28.7 3 2.3 Good Ex.
3 450 571 0.79 30 18 4 2.5 Good Comp.
4 420 560 0.75 30 24.5 6 3.3 Poor Comp.
5 B 540 854 0.63 2 15 7 3.3 Poor Comp.
6 C 390 660 0.59 35 28.2 3 2.3 Good Ex
7 385 649 0.59 43 28.5 3 2.2 Good Ex.
8 411 572 0.72 30 25.1 6 3.2 Poor Comp.
9 D 389 638 0.61 35 28.5 3 2.1 Good Ex
10 374 642 0.58 35 29.1 3 2.2 Good Ex.
11 E 695 1105 0.63 3 10.1 8 3.1 Poor Comp.
12 F 370 480 0.77 6 23.8 3 2.1 Good Comp.
13 G 390 639 0.61 40 28.7 3 2.3 Good Ex
14 385 647 0.60 37 28.9 3 2.1 Good Ex.
15 H 379 629 0.60 45 29.4 2.5 2.1 Good Ex.
16 I 580 750 0.77 0 13.2 6.5 3.3 Poor Comp.

* Underlined and 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 >3g/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 =30MPa, UTS =600 MPa and YS=430 MPa. Whereas, Steel remarked as “Comp.” from Table 1 to Table 4 doesn’t 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 320 0C and confirming to the relation of log10 [(SCS-RCS)/40] = 4.6 - 2.3*[%Cr] -1.7* [%Mn] satisfy the scope of the invention with UTS of 634 MPa and YS/UTS ratio of 0.6 as described in table 4. However steel sheet “A” when processed with RCS temperature of 4000C (as listed in Table 2) and relation log10 [(SCS-RCS)/40] = 4.6 - 2.3*[%Cr] -1.7* [%Mn] was not satisfied (table 3 S.No 3 ) resulted in lower UTS value of 571 MPa with yield ratio of 0.75 which does not conform to the scope of present invention .

Similarly, Steel “A” with lower cooling rate of 6.2 0C/sec and very high coiling temperature of 7200C (Table 2 S.No 4) after finish rolling resulted in poor phosphatability with phosphate crystal size of 6 µm and phosphate coating weight of 3.3 g/m2 (shown in Table 4 S.No 4) as higher coiling temperature leads to sever surface oxidation which deteriorates the surface property.
Similar observation can be made with steel “C” as listed in table 1. Steel “C” when processed at higher coiling temperature of 710 0C (Table 2 S.No 8) resulted in poor phosphatability (crystal size of 6 µm and phosphate coating weight of 3.2 g/m2) due to excessive surface oxidation. However, lower coiling temperature of 550 0C for steel “C” resulted in good phosphatability (crystal size of 3 µm and phosphate coating weight of 2.3 g/m2).

Example 2: Steel “B” as listed in table 1 has chemical composition out of the scope of the present invention with higher C, Mn, Si, and Mo content. Also Ti/N ratio is more than 14 resulting in poor bake hardening index of 7 MPa . Steel “B” being processed with similar hot and cold rolling condition as steel “A” with hot rolling finishing temperature of 902 0C, coiling temperature of 5600C, annealed at 800 0C for 95 seconds with rapid cooling section temperature of 300 0C (table 2 S.No 5). However steel “B” does not satisfy the relation 2*log10(%Fea-%Ms) = {(Mn+Si)/ (C+Cr+Nb)} resulting in very poor phosphatability with phosphate crystal size of 7 µm and phosphate coating weight of 3.3 g/m2 ( as shown in Table 4 S.No 5). 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 “B” has higher YS and UTS value of 540 MPa and 854 MPa respectively due to excessive carbon and martensite % .

Example 3: The value of (Mn+Cr+Si)/C for Steel “E” is 15.5 which is outside the range of 20 to 40 required to achieve the desired strength and phosphatability combination. Similar to steel “B” steel “E” has excessive Carbon, Manganese and Silicon resulting in higher YS and UTS value of 695 MPa and 1105 MPa respectively due to excessive hard martensite % (table 4 S.No 11). Excessive Mn and Si weight % also impaired the phosphatability of steel sheet “E” due to severe surface oxide formation .Poor phosphatability with phosphate crystal size of 8 µm and phosphate coating weight of 3.1 g/m2 (figure 2B) for steel “E” is attributable to excessive oxidized surface also called “temper color”. In addition the value of BH index is 3 MPa for steel “E” as Ti/N ratio is higher than 15.

Similar observation was made for steel “I” having Higher Mn and Si weight % and the value of (Mn+Cr+Si)/C being 63.3 which is outside the range of 20 to 40. The same is attributed to poor phosphatability of steel “I” with phosphate crystal size of 6.5 µm and phosphate coating weight of 3.3 g/m2.Furthermore, no bake hardening index was observed for steel “E” as Ti/N ratio is 25. Higher YS/UTS of 0.75 for steel “I” resulted due to delayed annealing time of 150 seconds and higher RCS temperature of 4000C (table 2 S.No 16) resulting 20% of pearlitic + bainitic phase (table 3 S.No 16).

Example 4 : Steel sheet “F” having lower C, Mn and Cr weight % to achieve the desired UTS value shows properties similar to HSLA type of steel with YS/UTS ratio of 0.77 which is higher than the desire ratio of =0.65. Also steel “F” does not satisfy the relation log10 [(SCS-RCS)/40] = 4.6 - 2.3*[%Cr] -1.7* [%Mn], therefore the UTS is lower than 600 MPa (Table 4 S.No12).

It is thus possible by way of the present invention to provide low yield ratio dual phase cold rolled steel sheet with excellent weldability and phosphatability having YS/UTS ratio less than 0.65, bake hardening index is atleast 30 MPa, Yield strength of 430 MPa or less and tensile strength is atleast 600 MPa as well as excellent surface property with 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, making it suitable for application in automobile structural parts.

,CLAIMS:We Claim:
1. Steel composition suitable for producing cold rolled steel sheet having YS/UTS ratio of said steel sheet is less than 0.65, bake hardening index is atleast 30 MPa, Yield strength of 430 MPa or less and tensile strength is atleast 600MPa. comprising:

(In weight %) (In weight %)
C: 0.07-0.1 % Mn: 1.4-1.9 %
Nb: 0.01-0.04 % Cr: 0.51-0.65 %
Si- 0.3 % or less N-0.006 % or less

and the balance being Fe and other unavoidable impurities; and (Mn+Cr+Si)/C in a range of 20 to 40, and wherein the steel composition further satisfying the following relation,
2*log10(%Fea-%Ms)= {(Mn+Si)/ (C+Cr+Nb)}
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 all compositional elements are in weight %.

2. Steel composition as claimed in claim 1 wherein the constitutional elements of said steel sheet meet the following relation with respect to SCS Temperature, and RCS temperature:
log10 [(SCS-RCS)/40] = 4.6 - 2.3*[%Cr] -1.7* [%Mn],
where SCS- Slow cooling section Temperature in 0C, RCS- Rapid cooling section Temperature in 0C, [M] = Elemental composition in wt%.

3. A cold rolled steel sheet comprising steel composition having
(In weight %) (In weight %)
C: 0.07-0.1 % Mn: 1.4-1.9 %
Nb: 0.01-0.04 % Cr: 0.51-0.65 %
Si- 0.3 % or less N-0.006 % or less

and the balance being Fe and other unavoidable impurities; and (Mn+Cr+Si)/C in a range of 20 to 40, and wherein the steel composition further satisfying the following relation;
2*log10(%Fea-%Ms)= {(Mn+Si)/ (C+Cr+Nb)}
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 all compositional elements are in weight %.

4. A cold rolled steel sheet according to claim 3, further comprising by mass % any one or both from Ti: 0.005% to 0.03 % and B: 0.0005% to 0.002%; and atleast 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 %.

5. A cold rolled steel sheet according to claim 3and 4, wherein the ratio of Ti to N is less than 15 to achieve a bake hardening index of 35 MPa or more.

6. A cold rolled steel sheet as claimed in anyone of clams 3 to 5 wherein the constitutional elements of said steel sheet meet the following relation with respect to SCS Temperature, and RCS temperature:
log10 [(SCS-RCS)/40] = 4.6 - 2.3*[%Cr] -1.7* [%Mn]
where SCS- Slow cooling section Temperature in 0C, RCS- Rapid cooling section Temperature in 0C, [M] = Elemental composition in wt%.

7. Steel sheet as per claim 3 to 6, wherein YS/UTS ratio of said steel sheet is less than 0.65, bake hardening index is atleast 30 MPa, Yield strength of 430 MPa or less and tensile strength is atleast 600MPa.

8. Steel sheet as per claim 3 to 7 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.

9. A process for manufacturing cold rolled steel sheet, comprising the steps of:

a) providing steel slab involving a selective steel composition having

(In weight %) (In weight %)
C: 0.07-0.1 % Mn: 1.4-1.9 %
Nb: 0.01-0.04 % Cr: 0.51-0.65 %
Si- 0.3 % or less N-0.006 % or less

and the balance being Fe and other unavoidable impurities; and (Mn+Cr+Si)/C in a range of 20 to 40, and wherein the steel composition further satisfying the following relation;
2*log10(%Fea-%Ms)= {(Mn+Si)/ (C+Cr+Nb)}
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 all compositional elements are in weight %.

b) Reheating said slab having said composition to reheating temperature of 1220 °C or less;
c)Said Reheated slab next roughing rolled in roughing mill with roughing mill delivery temperature of 1060°C or less;
d)Said rough rolled steel next subjected to finish rolling after at temperature range of 850°C to 910°C;
e)Coiling the finish rolled steel at with run out table cooling rate of 8 °C/second or more; and
f)Cold rolling the said hot rolled steel sheet with cold reduction of 40% or more.

10. A process as per claim 9 comprising:
f) Annealing at soaking section temperature range of 770 °C to 820°C with residence time of for 70 to 140 seconds;
g) Slow cooling the steel up to a temperature range of 670°C to 730°C after soaking ;
h) Rapid cooling the steel up to a temperature range of360°C or less with cooling rate of 10°C / second to 30°C / second;
i) Overaging said steel at temperature range of 280°C to 360°Cwith residence time of 100 to 300 seconds; and
j) Subjecting the overaged steel to skin pass elongation of 0.2% to 1 %.

11. A process as per anyone of claims 9 or 10 carried out such that the constitutional elements of said steel sheet meet the following relation with respect to SCS Temperature, and RCS temperature:
log10 [(SCS-RCS)/40] = 4.6 - 2.3*[%Cr] -1.7* [%Mn],
where SCS- Slow cooling section Temperature in 0C, RCS- Rapid cooling section Temperature in 0C, [M] = Elemental composition in wt%.

12. A process as per anyone of claims 9 to 11 carried out to produce steel sheet wherein (i) YS/UTS ratio of said steel sheet is less than 0.65, bake hardening index is atleast 30 MPa, Yield strength of 430 MPa or less and tensile strength is atleast 600MPa and/or (ii) 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 24th day of August, 2016
Anjan Sen
Of Anjan Sen & Associates
(Applicants Agent)