CLIAMS:1. Low carbon high strength hot rolled dual phase steel sheets comprising
C: 0.04- 0.08wt%
Mn: 1.0-2.0 wt%
Si: 0.2-1.0 wt%
Al < 0.05 wt%
Cr: 0.2-1.0 wt%
B: 20-60 ppm
N: < 100 ppm
Ca: 0-50 ppm
and
rest is iron, having tensile strength atleast =780MPa and hole expansion ratio atleast >70%.

2. Low carbon high strength hot rolled dual phase steel sheets as claimed in claim 1, wherein said steel sheets comprise steel slabs of thickness less than 65mm.

3. Low carbon high strength hot rolled dual phase steel sheets as claimed in anyone of claims 1 or 2 comprising S <0.008 wt % and P <0.018 wt %.

4. Low carbon high strength hot rolled dual phase steel sheets as claimed in anyone of claims 1 to 3 , having micro structure comprising a combination of Ferrite 50 to 70% by volume, and Martensite + Bainite 30 to 50% by volume.

5. A process for the production of Low carbon high strength hot rolled dual phase steel sheets as claimed in anyone of claims 1 to 4 comprising :

(i) providing the selective low carbon composition comprising of C: 0.04- 0.08wt%; Mn: 1.0-2.0wt%;Si: 0.2 to 1.0wt%;Al< 0.05wt%;Cr:0.2 to 1.0 wt%;B: 20-60ppm; N<100 ppm;Ca:0-50ppm; and rest is iron and treating in a furnace;
(ii) casting in a thin slab caster with casting speed of 4.5-6.5 m/min;
(iii) charging the slab in tunnel furnace and rolling to sheet/strip at FRT 7500 to 8500 C and at coiling temperature < 2000C.

6. A process for the production of Low carbon high strength hot rolled dual phase steel sheets as claimed in claim 5 wherein said treating of the hot metal comprises treating said hot metal at EAF (CONARC) furnace and followed by ladle furnace and

wherein the slabs after hot rolling are subjected to two stage cooling in the run out table to bring down the coiling temperature below the desired level of <200 Deg C for facilitating coiling.

7. A process as claimed in claim 6, wherein said two stage ultra fast cooling comprising a first ultrafast cooling Just after the finishing stand used to cool the strip fast and then allow the strip to air cool and after certain distance a second ultrafast cooling along with some laminar cooling and trim zone cooling is used to bring down the coiling temperature below the desired level of <200 Deg C.

8. A process as claimed in anyone of claims 6 or 7, wherein the temperature before said second ultrafast cooling is in the range of 650-750°C.

9. A process as claimed in anyone of claims 6 to 8 wherein said first ultra fast cooling ensures no pearlite formation in the final Micro structure and after first ultrafst cooling, air cooling is adopted for ferrite transformation and wherein time duration of air cooling decides the amount of ferrite form in the final Micro structure and said second ultrafast cooling along with laminar and trim zone ensures transformation of remaining austenite into martensite and or bainite.

10. A process as claimed in anyone of claims 5 to 9 wherein said process is carried out under controlled operating conditions comprising
I. Casting Speed 4.5- 5.5 m/min;
II. Slab Thickness 55-65 mm;
III. Slab Cutting Temp 980-1050 0C;
IV. Homogenization Temp (Tunnel Furnace) 1080-1150 0C;
V. Homogenization Time 8-15 min;
VI. Finish Rolling Temp 750-850 0C;
VII. Standwise reduction/time/temp comprising:

Parameters F1 F2 F3 F4 F5 F6
Relative Reduction 40-50 40-50 30-40 20-30 20-30 15-20
Inter stand time 5-15 sec 4-15 sec 3-10 sec 3-10 sec 2-10 sec 2-10 sec
Stand Entry Temp(°C) 1050-1080 1000-1040 970-1000 900-950 870-900 800-850

VIII. Temp before 2nd Ultrafast cooling- 650-700 0C;
IX. Coiling Temp <200 0C;
X. 1st Ultrafast Cooling Rate 60-80 0C;
XI. 2nd ultrafast cooling rate Cooling rate 150-200 0C.

Dated this the 20th day of December, 2013
Anjan Sen
Of Anjan Sen & Associates
(Applicants Agent)
,TagSPECI:FIELD OF THE INVENTION

The present invention relates to providing hot rolled dual phase steel sheet having tensile strength minimum 780 MPa with hole expansion ratio of more than 70% through thin slab caster processing route. The invented steel grade is having a selective cost effective composition comprising low carbon along with Mn, Cr, Si and Boron as strengthening elements. The steel sheets produced through controlled hot rolling and subsequent controlled two stage ultrafast cooling result in a microstructure comprising a combination of ferrite (50-70) % and Martensite + Bainite(30-50)% ensure achieving the desired strength properties.Typical applications are automotive structural and wheel rim and disk application and any other application aimed at weight reduction of existing component by replacement of existing low strength higher thickness steel grades with this newly invented high strength steel sheets of lower thickness.

BACKGROUND OF THE INVENTION:

Vehicles for personal travel as well as goods transport are the most important need for human civilization. Usage of vehicles is increasing day by day leading to burning of more fuels and more air pollution. Requirement of lower weight of vehicle is getting attention for reducing fuel consumption. This combines with demand of more capacity and higher safety requirements of vehicles a subject of attention for the automobile designers. There has been thus a need of very high strength steel with toughness to fulfil these requirements. As a result steel manufacturers are facing continuous challenges to develop advanced high strength steels (AHSS). Dual Phase steel possess micro structures of martensite islands embedded in a ferrite matrix. Dual Phase steel is the family of AHSS steel having characteristic of low YS, high UTS and thus low YS/UTS ratio, continuous yielding and high strain hardening exponent. Because of these properties, this steel can improve vehicle crash worthiness and durability, and also can be made thin to help to reduce vehicle weight as well.

The previous research and developments in the field of dual phase steel sheets have resulted in several methods for producing dual phase steel sheets, some of which are discussed below.

US2003/0084966A1 discloses a dual phase steel sheet having low yield ratio, good combination of strength-elongation and bake hardening properties. The steel contains 0.01-0.20 mass % C, 0.5 or less mass % Si, 0.5-3.0 mass % Mn, 0.06 or less mass % Al, 0.15 or less mass % P, and 0.02 or less mass % S. The method of producing this steel sheet includes hot rolling and continuous annealing or galvanization steps. The hot rolling step includes a step of completing finish rolling at a temperature of (Ar3—50)° C., or higher, and a step of cooling at an average cooling rate of 20° C. per second or more, down to the Martensite Start temperature. The continuous annealing step includes a step of heating to a temperature of the A1 point or higher and the A3 point or lower, and a step of cooling at an average cooling rate of 3° C/ s or higher down to the Martensite Start point or lower, and, optionally, a step of further applying averaging at a temperature from 100 to 600° C.

US 6440584 is directed to provide a hot dip galvanized steel sheet containing 0.04 to 0.12 wt% of C, 0.5 or less wt% of Si, 1.0 to 2.0 wt% of Mn, 0.05wt% or less of P,0.005wt% of S, 0.05 to 1.0wt% of Cr, 0.005 to 0.2 wt% of V,0.1wt% or less of sol. Al, and 0.01 wt% or less of N. The hot dip galvanized steel sheet is produced by rough rolling a steel, finish rolling the rough rolled steel at a temperature of Ar3 point or more, coiling the finish rolled steel at a temperature of 700° C or less, and hot dip galvanizing the coiled steel at a pre-plating heating temperature of AC1 to AC3. A continuous hot dip galvanizing operation is performed by soaking a pickled strip at a temperature of 750 to 850° C, cooling the soaked strip to a temperature range of 600° C or less at a cooling rate of 1 to 50° C/s, hot dip galvanizing the cooled strip, and cooling the galvanized strip so that the residence time at 400 to 600° C is within 200 seconds.

US 6423426 relates to a high tensile hot dip zinc coated steel plate having a composition comprising 0.05-0.20 mass % carbon, 0.3-1.8 mass % silicon, 1.0-3 .0 mass % manganese, and iron as the balance. The steel is subjected to a primary step of primary heat treatment and subsequent rapid cooling to the martensite transition temperature point or lower, a secondary step of secondary heat treatment and subsequent rapid cooling, and a tertiary step of galvanizing treatment and rapid cooling, so as to obtain 20% or more by volume of tempered martensite in the steel structure.

US 4708748 divisional application to US 4615749 (Parent), disclosed a cold rolled dual phase structure steel sheet, which consists of 0.001 - 0.008 weight % C, not more than 1.0 Weight % Si, 0.05-1.8 Weight % Mn, not more than 0.15 weight % P, 0.01-0.10 weight % Al, 0.002-0.050 Weight % Nb and 0.0005-0.0050 Weight % B. The steel sheet is manufactured by hot and cold rolling a steel slab with the above chemical composition and continuously annealing the resulting steel sheet in such a manner that the steel sheet is heated and soaked at a temperature from a to ? transformation point to 1000° C and then cooled at an average rate of not less than 05° C/s but less than 20° C/s in a temperature range of from the soaking temperature to 750° C, and subsequently at an average cooling rate of not less than 20° C/ s in a temperature range of from 750° C to not more than 300 °C .

US 8337643 B2, disclosed a hot rolled steel sheets having dual phase micro structure with a martensite phase of less than 35% by volume and ferrite phase more than 50 % by volume. The steel contains by weight percent 0.01-0.2 % C, 0.3 – 3.0 % Mn, 0.2-2.0 % Si, 0.2-2.0% (Cr+Ni), 0.01-0.1% Al, 0-0.2 % Mo, 0.0005-0.01 % Ca and rest is Fe and incidental ingredients. Hot rolled steel sheets has a tensile strength of 500 Mpa minimum, hole expansion ratio more than 50 % and Yield to Tensile strength ratio less than 70%

The above referred first four patents disclose the method of producing dual phase steel by heat treatment after rolling. The fifth and last mentioned patent discloses the method of producing hot rolled dual phase steel but of much lower tensile strength (minimum 500 Mpa) without using Boron.

The present invention is an advancement over the above stated and other known patents in the related field as it produces high strength dual phase steel ( TS = 780 MPa) by thermo mechanical hot rolling without any further heat treatment using Boron as a strengthening element in addition to Mn, Si and Cr in the composition of said steel, processed through thin slab caster route.

OBJECTS OF THE INVENTION

The basic object of the present invention is directed to provide hot rolled dual phase steel sheets having tensile strength minimum 780 Mpa with hole expansion ratio of more than 70% suitable for automobile and the like application.

A further object of the present invention is directed to provide said hot rolled dual phase steel sheets by thermo mechanical hot rolling only and without any further heat treatment, making the process and the product cost effective.

A still further object of the present invention is directed to provide hot rolled dual phase steel sheets through a processing route involving thin slab caster wherein cost effective composition of said steel comprises low carbon along with Mn, Cr, Si and Boron as strengthening elements.

Yet another object of the present invention is directed to provide said hot rolled steel sheets wherein the microstructure comprises of ferrite (50-70) % and Martensite + Bainite(30-50)% obtained through controlled process parameters, favouring attaining desired strength-elongation properties in steel.

A further object of the present invention is directed to provide hot rolled dual phase steel with very high strength and high hole expansion ratio for application in automotive structural components which will improve crash worthiness as well as passenger safety and will also help reducing the overall weight of the vehicle by using thinner gauge.

SUMMARY OF THE INVENTION

The basic aspect of the present invention is directed to provide low carbon high strength hot rolled dual phase steel sheets comprising
C: 0.04- 0.08wt%
Mn: 1.0-2.0 wt%
Si: 0.2-1.0 wt%
Al < 0.05 wt%
Cr: 0.2-1.0 wt%
B: 20-60 ppm
N: < 100 ppm
Ca: 0-50 ppm
and
rest is iron, having tensile strength atleast =780MPa and hole expansion ratio atleast >70%.

A further aspect of the present invention is directed to said low carbon high strength hot rolled dual phase steel sheets, wherein said steel sheets comprise steel slabs of thickness less than 65mm.

A still further aspect of the present invention is directed to said low carbon high strength hot rolled dual phase steel sheets comprising S <0.008 wt % and P <0.018 wt %.

Yet another aspect of the present invention is directed to said low carbon high strength hot rolled dual phase steel sheets having micro structure comprising a combination of Ferrite 50 to 70% by volume, and Martensite + Bainite 30 to 50% by volume.

A further aspect of the present invention is directed to a process for the production of Low carbon high strength hot rolled dual phase steel sheets as stated above comprising:

(i) providing the selective low carbon composition comprising of C: 0.04- 0.08wt%; Mn: 1.0-2.0wt%;Si: 0.2 to 1.0wt%;Al< 0.05wt%;Cr:0.2 to 1.0 wt%;B: 20-60ppm; N<100 ppm;Ca:0-50ppm; and rest is iron and treating in a furnace;
(ii) casting in a thin slab caster with casting speed of 4.5-6.5 m/min;
(iii) charging the slab in tunnel furnace and rolling to sheet/strip at FRT 7500 to 8500 C and at coiling temperature < 2000C.

A still further aspect of the present invention is directed to a process for the production of low carbon high strength hot rolled dual phase steel sheets wherein said treating of the hot metal comprises treating said hot metal at EAF (CONARC) furnace and followed by ladle furnace and

wherein the slabs after hot rolling are subjected to two stage cooling in the run out table to bring down the coiling temperature below the desired level of <200 Deg C for facilitating coiling.

Yet another aspect of the present invention is directed to said process wherein said two stage ultra fast cooling comprising a first ultrafast cooling Just after the finishing stand used to cool the strip fast and then allow the strip to air cool and after certain distance a second ultrafast cooling along with some laminar cooling and trim zone cooling is used to bring down the coiling temperature below the desired level of <200 Deg C.

According to a further aspect of the present invention, the same is directed to said process wherein the temperature before said second ultrafast cooling is in the range of 650-750°C.

A still further aspect of the present invention is directed to said process wherein said first ultra fast cooling ensures no pearlite formation in the final Micro structure and after first ultrafst cooling, air cooling is adopted for ferrite transformation and wherein time duration of air cooling decides the amount of ferrite form in the final Micro structure and said second ultrafast cooling along with laminar and trim zone ensures transformation of remaining austenite into martensite and or bainite.

Yet another aspect of the present invention is directed to said process which is carried out under controlled operating conditions comprising
I. Casting Speed 4.5- 5.5 m/min;
II. Slab Thickness 55-65 mm;
III. Slab Cutting Temp 980-1050 0C;
IV. Homogenization Temp (Tunnel Furnace) 1080-1150 0C;
V. Homogenization Time 8-15 min;
VI. Finish Rolling Temp 750-850 0C;
VII. Standwise reduction/time/temp comprising:

Parameters F1 F2 F3 F4 F5 F6
Relative Reduction 40-50 40-50 30-40 20-30 20-30 15-20
Inter stand time 5-15 sec 4-15 sec 3-10 sec 3-10 sec 2-10 sec 2-10 sec
Stand Entry Temp(°C) 1050-1080 1000-1040 970-1000 900-950 870-900 800-850

VIII. Temp before 2nd Ultrafast cooling- 650-700 0C;
IX. Coiling Temp <200 0C;
X. 1st Ultrafast Cooling Rate 60-800C;
XI. 2nd ultrafast cooling rate Cooling rate 150-2000C.

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

BREIF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

Figure 1: is the flow chart showing the details of different steps involved in producing the high strength hot-rolled dual phase steel sheets through thin slab caster route according to the present invention.
Figure 2: is the micrographs of the Microstructure of the steel grade according to the present invention wherein Ferrite ~55% by volume, and Martensite + Bainite ~45% by volume at mid thickness of sheet have been shown.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE ACCOMPANYING DRAWINGS

The present invention is directed to providing high strength hot-rolled dual phase steel sheets through thin slab caster route used for automotive structural and other load bearing application said steel being characterized by good combination of TS = 780 Mpa with hole expansion ratio >70%.

Thus according to the present invention, a cost effective composition of low carbon hot rolled dual phase steel sheets and a method of making such steel sheets is provided. Steel is processed using a thin slab caster and 6 high tandem rolling mill and the cost effective composition of said steel comprises low carbon along with Mn, Cr, Si and Boron as strengthening elements from which sheets are produced following thermo mechanical hot rolling without any further heat treatment but involving two stage ultrafast cooling after finish rolling and prior to coiling. Mechanical properties achieved are tensile strength 780 MPa(minimum) with hole expansion ratio of more than 70%.The microstructure obtained comprises a combination of ferrite (50-70)% and Martensite + Bainite (30-50)%.

In order to produce the low carbon hot rolled dual phase steel grade/sheets with above stated properties, the selective steel composition used for processing through thin slab caster is as follows:
C: 0.04- 0.08wt%
Mn: 1.0-2.0 wt%
Si: 0.2-1.0 wt%
Al < 0.05 wt%
Cr: 0.2-1.0 wt%
B: 20-60 ppm
N: < 100 ppm
Ca: 0-50 ppm
S & P as impurities and balance iron.

The detailed consideration for selecting the above chemical composition are as below:

C: 0.04- 0.08%: Carbon is essential for solute strengthening, propmotes martensite formation, strengthening of martensite phases and formation of carbide with Cr, but upper limit is restricted because of poor effect on weldability and surface quality. Higher ‘C’ at peritectic range increases crack sensitivity, which is more prevalent in thin slab caster due to very high casting speed. Carbon level above 0.08% is particularly prone to surface defect in thin slab caster.

Mn: 1.0-2.0 %: Mn is an important element for solid solution strengthening and also increases hardenability inexpensively. Higher ‘Mn/S’ ratio reduces central segregation and improves castability. Higher ‘Mn/Si’ improves surface finish of HR coil by reducing stickiness of scale. Upper values are restricted because its poor effect on weldability, castability in thin slab caster and load during hot rolling apart from cost implication. Higher amount of Mn is also believed to affect fatigue performance.

Si: 0.2-1.0 %: Silicon acts as a ferrite stabilizer. Minimum amount of Si is required to get desire amount of ferrite in Micro structure. It acts as a de oxidiser and also increases strength by substitution solid solution hardening effect in low carbon steel. Upper value is restricted because it has detrimental effect on surface quality because of stickier scale formation.

Al: < 0.06 %: It is used predominantly as a de oxidizer. But it also restrict the grain growth by pinning grain boundaries with ‘AlN’ precipitation as well as improve formability by fixing free ‘N’. Upper limit is restricted to reduce ‘alumina’ inclusion & improve castability particularly in thin slab caster.

Cr: 0.2-1.0%: In increases hardenability and strength. Upper value is restricted because along with high Si, it has detrimental effect on surface quality.

B: 20-60 ppm: Boron significantly increases hardenability without loss of ductility. Boron is most effective at lower Carbon level. Upper level is restricted due to detrimental effect on surface quality as formation of transverse corner cracking in CSP.
N< 100 ppm: Nitrogen contributes to some extent in tensile strength by formation of nitride and carbonitride precipitates; however upper limit is restricted because of its poor effect on formability of steel and strain aging.

Ca: 0-50 ppm: Steel has to be Calcium treated to improve castability particularly in thin slab casting by converting remaining inclusions in steel at LF to lower melting eutectic Ca-compounds and scavenging upto slag. Remnant Ca-bearing oxides / suphides are also round shaped , which are much less harmful in finished steel.

The process route followed was : Electric Arc Furnace?Ladle Furnace? thin slab Caster? 6 stand hot rolling mill? coiling route. However, any other combination of processes (before caster) which gives steel of same chemistry can also be used.

Accompanying Figure 1 illustrates the flow chart showing the details of different steps involved in producing the high strength hot-rolled dual phase steel sheets through thin slab caster route according to the present invention.

The process used for making the product according to an embodiment of the present invention is described in details with the help of following example :

EXAMPLE :

(i) Hot metal from blast furnace was refined with the help of CONARC ( combined facilities of Electric Arc Furnace & LD converter)and final chemistry adjustments were done in a ladle refining furnace to obtain a selective cost effective composition as given above involving low carbon along with Mn, Cr, Si and Boron as strengthening elements. The composition of different steel samples obtained on experimental heats are given in following table 1: The composition of different steel samples obtained on trials and experimental heats are given in following table 1:

Table 1: Chemistry of samples
Sample ID C % Mn% Si % P % S % Al % B (ppm) N (ppm) Cr% Ca(ppm)
Sample1 0.06 1.744 0.172 0.012 0.004 0.027 33 57 0.225 19
Sample 2 0.055 1.722 0.168 0.011 0.004 0.025 35 60 0.271 21

(ii) Steel was cast using a thin slab caster with slab thickness ranging from 50-65 mm.
(iii) Slab was further homogenized in a Tunnel furnace at temperature >1075°C. Descaler was used after Tunnel Furnace to remove scales.
(iv) Hot rolling with 6 stand reductions was used to reduce the thickness to the required level with finish rolling temperature in the range of 750-850°C. The details stand wise parameters for hot rolling are as following table 2:
Table 2:
Parameters F1 F2 F3 F4 F5 F6
Relative Reduction 40-50 40-50 30-40 20-30 20-30 15-20
Inter stand time 5-15 sec 4-15 sec 3-10 sec 3-10 sec 2-10 sec 2-10 sec
Stand Entry Temp(°C) 1050-1080 1000-1040 970-1000 900-950 870-900 800-850

(v) After hot rolling, a two stage cooling strategy was adopted in the run-out table. Just after the finishing stand an ultrafast cooling was used to cool the strip fast and then allow the strip to air cool. After certain distance a second ultrafast cooling along with some laminar cooling and trim zone cooling was used to bring down the coiling temperature below the desired level of <200 °C. Initial ultra fast cooling ensures no pearlite formation in the final Micro structure. After first ultrafst cooling, air cooling was adopted for ferrite transformation. The time duration of air cooling decides the amount of ferrite form in the final Micro structure. The second ultrafast cooling along with laminar and trim zone ensures transformation of remaining austenite into martensite and or bainite.
(vi) Coil was subsequently slow cooled in coil yard.
(vii) The resulting steel grade is subjected to testing and inspection to ascertain attainment of desired properties.

The various temperatures used in the above process are as follows:
Finishing Temp (FT)- 750-850 °C;
Temp Before 2nd Ultrafast cooling- 650-750 °C;
Coiling Temp (CT)- <200 °C.

The above process was carried out under controlled operating condition wherein the critical process parameters comprised:
I. Casting Speed 4.5- 5.5 m/min;
II. Slab Thickness 55-65 mm;
III. Slab Cutting Temp 980-1050 °C;
IV. Homogenization Temp (Tunnel Furnace) 1080-1150 °C;
V. Homogenization Time 8-15 min;
VI. Finish Rolling Temp 750-850 °C;
VII. Standwise reduction/time/temp as given in table 2 above;
VIII. Temp before 2nd Ultrafast cooling- 650-700 °C;
IX. Coiling Temp <200 °C;
X. 1st Ultrafast Cooling Rate 60-80 °C;
XI. 2nd ultrafast cooling rate Cooling rate 150-200 °C;

Mechanical properties:

The product obtained was free from any surface defects and the mechanical properties observed for different samples are presented in the following table 3:

Table 3: Mechanical properties:
Sample ID WIDTH THK YS UTS YS/UTS EL HRB HER FT Temp Before 2nd ultrafast cooling CT
Sample 1 1030 6.3 650 874 0.74 9 95 150 805.1 670 150
Sample 2 1030 6.3 593 819 0.72 10 95 130 796.1 680 180

Microstructure:

Accompanying Figure 2 shows the micrographs of the Microstructure of the low carbon hot rolled dual phase steel grade according to the present invention wherein Ferrite ~55% by volume, and Martensite + Bainite ~45% by volume at mid thickness of sheet have been shown.

It is thus possible by way of the present invention to provide novel hot rolled dual phase steel sheet products with very high strength and high hole expansion ratio for application in automotive structural components which will improve crash worthiness as well as passenger safety. On the other hand it also reduces the overall weight of the vehicle by using thinner gauge. The steel sheet product according to the present invention can also be used for other application where weight reduction is a required objective, thus favouring wide industrial application.