Claims:1. A process for making a Cold Rolled Dual Phase (CRDP) 780 steel, the process comprising:
casting a steel of thickness >200-250 mm with following composition:
Carbon (C) = 0.06-0.12, Manganese (Mn) = 1.2-2.4, Sulphur (S) = 0.003-0.012, Phosphorus (P) = 0.007-0.012, Silicon (Si) = 0.02-0.08, Aluminum (Al) = 0.03-0.1, Nitrogen (N) = 0.002-0.008, Chromium (Cr) = 0.3-0.7, rest Iron (Fe) and incidental ingredients (all in wt. %);
soaking the steel at 1200-1220 °C for 1.5-2.5 hours;
hot rolling the steel at Finishing Rolling Temperatures (FRT) ranging from 870-920 °C achieving reduced thickness of 3-6 mm;
coiling the steel at 640-700 °C;
cold rolling the steel with 50-65 % reduction with speed of 80-90 meters per minute (mpm);
continuous annealing the steel at 770 - 850 °C at a heating rate of 2-6 °C;
soaking the steel at the soaking temperature for 40-100 seconds;
cooling either at a cooling rate of 1-5 °C/s to an intermediate temperature (700-800 °C) or at a cooling rate of = 20 °C/s to room temperature; and
if cooled to intermediate temperature, further cooling at a cooling rate of = 20 °C/s to the room temperature.
2. The process as claimed in claim 1, wherein composition of the steel is Carbon (C) = 0.09, Manganese (Mn) = 1.7, Sulphur (S) = 0.006, Phosphorus (P) = 0.010, Silicon (Si) = 0.05, Aluminum (Al) = 0.04, Nitrogen (N) = 0.003, Chromium (Cr) = 0.6, rest Iron (Fe) and incidental ingredients (all in wt. %).

3. The process as claimed in claim 1, wherein the steel is continuously cast after refining in basic oxygen furnace (BOF) converter.

4. The process as claimed in claim 1, comprising pickling of the steel after step of coiling and before cold rolling.

5. The process as claimed in claim 4, wherein the pickling is carried out in concentration of Hydro Chloride (10% by volume).

6. The process as claimed in claim 1, wherein the hot rolling of the steel is carried out by 6-7 stands.

7. The process as claimed in claim 1, wherein the cold rolling of the steel is carried out by 5 stands.

8. The process as claimed in claim 1, wherein the steel has ultimate tensile strength (UTS) > 780 MPa.

9. The process as claimed in claim 1, wherein the steel has yield strength (YS) of 450-550 MPa.

10. The process as claimed in claim 1, wherein the steel has yield ratio (YR) 0.60-0.65.

11. The process as claimed in claim 1, wherein the steel has 30-45% martensite and rest is ferrite (by volume)

12. The process as claimed in claim 1, wherein the % Elongation of the steel is 19-23.

13. A Cold Rolled Dual Phase (CRDP) comprising:
composition of Carbon (C) = 0.06-0.12, Manganese (Mn) = 1.2-2.4, Sulphur (S) = 0.003-0.012, Phosphorus (P) = 0.007-0.012, Silicon (Si) = 0.02-0.08, Aluminum (Al) = 0.03-0.1, Nitrogen (N) = 0.002-0.008, Chromium (Cr) = 0.3-0.7, rest Iron (Fe) and incidental ingredients (all in wt. %); and
minimum ultimate tensile strength (UTS) of 780MPa.
14. The Cold Rolled Dual Phase (CRDP) as claimed in claim 13, wherein the yield strength (YS) is 450-550 MPa.

15. The Cold Rolled Dual Phase (CRDP) as claimed in claim 13, wherein the steel has yield ratio (YR) 0.60-0.65.

16. The Cold Rolled Dual Phase (CRDP) as claimed in claim 13, wherein the steel has 30-45% martensite and rest is ferrite (by volume).

17. The Cold Rolled Dual Phase (CRDP) as claimed in claim 13, wherein the % Elongation of the Cold Rolled Dual Phase (CRDP) is 19-23. , Description:TECHNICAL FIELD
[0001] The present disclosure relates to a cold rolled and continuously annealed (CRCA) dual phase steel. Particularly, the present disclosure relates to a process for manufacturing cold rolled and continuously annealed (CRCA) dual phase steel.
BACKGROUND
[0002] Background description includes information that may be useful in understanding the present subject matter. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed subject matter, or that any publication specifically or implicitly referenced is prior art.
[0003] Dual phase steels are one of the most important varieties of Advanced High Strength Steels (AHSSs) in the automotive market. It is extensively used for reducing the car body weight and thereby improving on the fuel economy, greenhouse gas emission without compromising on the safety of the passengers. Hot rolled dual phase steels are mainly used for automobile structures such as wheels, suspensions, etc. However, cold rolled dual phase steels are mainly suited for structural applications such as A-pillar or B-pillars as well as front and rear bumper reinforcements requiring high tensile strength, high impact energy absorption capacity and good formability.
[0004] Nowadays, dual phase steels are also used as the inner and outer panel of an automobile door due to improved forming techniques. Cold Rolled Dual Phase (CRDP) steel is produced by inter-critical annealing to develop the microstructure having ferrite and austenite phase and it is followed by quenching to transform the austenite into martensite. Therefore, dual phase steels have the microstructure consisting of soft ferrite and hard martensite. This microstructure allows the steel to yield continuously and to have higher tensile strength than HSLA with similar yield strength. Furthermore, good elongation leads to better formability.
[0005] US patent US7879160B2 discloses a cold rolled steel sheet having a dual phase microstructure with the martensite content of less than 35 vol%; a composition containing by percent weight: 0.01?C?0.2; 0.3?Mn?3; 0.05?Si?2; 0.1?Cr?2; 0.01?Al?0.10;0.0005?Ca?0.01, with the balance iron and incidental ingredients. The steel was made by batch annealing method and have tensile strength 400 MPa (min) and n value of at least 0.175. It may be appreciated that in context of weight reduction in automotive components, dual phase steels gain with higher strength level in excess of 780 MPa is most important rather than only 400 MPa.
[0006] US 20120255654 A1 relates to a dual phase steel sheet and a method for manufacturing the same. The steel sheet possesses a chemical composition of C: 0.05˜0.10 % wt %, Si: 0.03˜0.50 wt %, Mn: 1.50˜2.00 wt %, P: greater than 0 wt %˜0.03 wt %, S: greater than 0 wt %˜0.003 wt %, Al: 0.03˜0.50 wt %, Cr:0.1˜0.2 wt %, Mo: 0.1˜0.20 wt %, Nb: 0.02˜0.04 wt %, B: greater than 0 wt %˜0.005 wt %, N: greater than 0 wt %˜0.01 wt %, and the balance of Fe and other unavoidable impurities. The steel was made such that it possessed excellent formability, bake hardenability, dent resistance, high R value and plating characteristics to the steel sheet for exterior and interior panels of automobiles, the steel sheet is processed to have a dual phase structure through cold rolling, annealing, and hot-dip galvanizing. However, the chemistry of the material appears to be very rich containing the expensive miroalloying elements. Further, it contains silicon which is notoriously known to deteriorate the surface finish of the product.
OBJECTS OF THE DISCLOSURE
[0007] Some of the objects of the present disclosure, which at least one embodiment herein satisfy, are listed hereinbelow.
[0008] In view of the foregoing limitations inherent in the above cited prior-arts, it is an object of the present disclosure to develop a new process for manufacturing cold rolled and continuous annealed (CRCA) dual phase steel.
[0009] Another object of the present disclosure is to develop the galvannealed dual phase steel such that the ultimate tensile strength (UTS)> 780MPa.
[0010] Still another object of the present disclosure is to develop the CRCA 780 dual phase steel with a similar composition of DP600, having minimal silicon (Si) content and use comparatively less expensive alloys.
[0011] These and other objects and advantages of the present invention will be apparent to those skilled in the art after a consideration of the following detailed description taken in conjunction with the accompanying drawings in which a preferred form of the present invention is illustrated.
SUMMARY
[0012] This summary is provided to introduce concepts related to a process for manufacturing a cold rolled and continuously annealed (CRCA) dual phase steel. The concepts are further described below in the detailed description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.
[0013] The present disclosure relates to a process for making a CRCA dual phase steel the process comprising steps of casting a steel of thickness >200-250 mm with composition of Carbon (C) = 0.06-0.12, Manganese (Mn) = 1.2-2.4, Sulphur (S) = 0.003-0.012, Phosphorus (P) = 0.007-0.012, Silicon (Si) = 0.02-0.08, Aluminum (Al) = 0.03-0.1, Nitrogen (N) = 0.002-0.008, Chromium (Cr) = 0.3-0.7, rest Iron (Fe) and incidental ingredients (all in wt. %), soaking the steel at 1200-1220°C for 1.5-2.5hrs, hot rolling the steel at FRT 870-920 °C with reduced thickness achieved of 3-6mm with speed of 80-90 meters per minute (mpm), coiling the steel at 640-700 ° C, cold rolling the steel with 50-65% reduction with speed of 80-90 meters per minute (mpm), continuous annealing the steel at 750 - 850 °C followed by continuous cooling at a cooling rate of = 20oC/s to room temperature.
[0014] In an aspect, the composition of the steel is Carbon (C) = 0.09, Manganese (Mn) = 1.7, Sulphur (S) = 0.006, Phosphorus (P) = 0.010, Silicon (Si) = 0.05, Aluminum (Al) = 0.04, Nitrogen (N) = 0.003, Chromium (Cr) = 0.6, rest Iron (Fe) and incidental ingredients (all in wt. %).
[0015] In an aspect, the steel has ultimate tensile strength (UTS) > 780 MPa.
[0016] In an aspect, the steel has yield strength (YS) of 450-550 MPa.
[0017] In an aspect, the steel has a yield ratio (YR) 0.60-0.65.
[0018] In an aspect, the steel has 30-45% martensite and rest is ferrite (by volume).
[0019] In an aspect, the steel is continuously cast after refining in basic oxygen furnace (BOF) converter.
[0020] In an aspect, the method further includes pickling of the steel after the step of coiling and before cold rolling.
[0021] In an aspect, the pickling is carried out in concentration of Hydro Chloride (10% by volume).
[0022] In an aspect, the hot rolling of the steel is carried out by 6-7 stands.
[0023] In an aspect, the cold rolling of the steel is carried out by 5 stands.
[0024] In an aspect, the % Elongation of the steel is 19-23.
[0025] The present disclosure relates to a Cold Rolled Dual Phase (CRDP) comprising a composition of Carbon (C) = 0.06-0.12, Manganese (Mn) = 1.2-2.4, Sulphur (S) = 0.003-0.012, Phosphorus (P) = 0.007-0.012, Silicon (Si) = 0.02-0.08, Aluminum (Al) = 0.03-0.1, Nitrogen (N) = 0.002-0.008, Chromium (Cr) = 0.3-0.7, rest Iron (Fe) and incidental ingredients (all in wt. %); and a minimum ultimate tensile strength (UTS) of 780MPa.
[0026] Various objects, features, aspects, and advantages of the inventive subject matter will become more apparent from the following detailed description of preferred embodiments, along with the accompanying drawing figures in which like numerals represent like components.
[0027] It is to be understood that the aspects and embodiments of the disclosure described above may be used in any combination with each other. Several of the aspects and embodiments may be combined to form a further embodiment of the disclosure.
[0028] The foregoing summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features will become apparent by reference to the drawings and the following detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] The accompanying drawings, which are incorporated in and constitute a part of this disclosure, illustrate exemplary embodiments and, together with the description, serve to explain the disclosed principles. In the figures, the left-most digit(s) of a reference number identifies the figure in which the reference number first appears. The same numbers are used throughout the figures to reference like features and components. Some embodiments of system and/or methods in accordance with embodiments of the present subject matter are now described, by way of example only, and with reference to the accompanying figures, in which:
[0030] FIG. 1 illustrates a flow diagram of a process illustrating various steps for manufacturing a cold rolled and continuously annealed (CRCA) dual phase steel, in accordance with an embodiment of the present disclosure;
[0031] FIG. 2 illustrates a microstructure of the CRCA dual-phase steel processed in accordance with an implementation of the present disclosure; and
[0032] FIG. 3 illustrates a strain curve of Cold Rolled Dual Phase (CRDP) 780 grade exhibiting continuous yielding in accordance with an embodiment of the present disclosure.
DETAILED DESCRIPTION
[0033] an example, instance, or illustration." Any embodiment or implementation of the present subject matter described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other embodiments.
[0034] While the disclosure is susceptible to various modifications and alternative forms, specific embodiment thereof has been shown by way of example in the drawings and will be described in detail below. It should be understood, however that it is not intended to limit the disclosure to the forms disclosed, but on the contrary, the disclosure is to cover all modifications, equivalents, and alternatives falling within the scope of the disclosure.
[0035] The terms “comprises”, “comprising”, “includes” or any other variations thereof, are intended to cover a non-exclusive inclusion, such that a setup, device or method that includes a list of components or steps does not include only those components or steps but may include other components or steps not expressly listed or inherent to such setup or device or method. In other words, one or more elements in a system or apparatus proceeded by “comprises… a” does not, without more constraints, preclude the existence of other elements or additional elements in the system or method.
[0036] Described herein a process for making a CRCA dual phase steel, in accordance with an embodiment of the present disclosure. The process comprising steps of casting a steel of thickness >200-250 mm with following composition Carbon (C) = 0.06-0.12, Manganese (Mn) = 1.2-2.4, Sulphur (S) = 0.003-0.012, Phosphorus (P) = 0.007-0.012, Silicon (Si) = 0.02-0.08, Aluminum (Al) = 0.03-0.1, Nitrogen (N) = 0.002-0.008, Chromium (Cr) = 0.3-0.7, rest Iron (Fe) and incidental ingredients (all in wt. %); soaking the steel at 1200-1220 °C for 1.5-2.5hrs; hot rolling the steel at FRT 870-920 °C with thickness achieved of 3-6mm with speed of 80-90 meters per minute (mpm); coiling the steel at 640-700 °C; cold rolling the steel with 50-65% reduction with speed of 80-90 meters per minute (mpm); continuous annealing the steel at 750 - 850 °C at speed of 60-80 mpm; continuous cooling from soaking temperature at a cooling rate of = 20 °C/s.
[0037] FIG. 1 illustrates a flow a diagram of a process 100 illustrating various steps for manufacturing a CRCA dual-phase steel (DP), in accordance with an embodiment of the present disclosure. The process comprises various steps from 104 to 132. The order in which the process 100 is described is not intended to be construed as a limitation, and any number of the described method blocks can be combined in any appropriate order to carry out the process 100 or an alternative method. Additionally, individual blocks may be deleted from the process 100 without departing from the scope of the subject matter described herein.
[0038] At block 102, the process 100 includes casting a steel of thickness >200-250 mm with composition of Carbon (C) = 0.06-0.12, Manganese (Mn) = 1.2-2.4, Sulphur (S) = 0.003-0.012, Phosphorus (P) = 0.007-0.012, Silicon (Si) = 0.02-0.08, Aluminum (Al) = 0.03-0.1, Nitrogen (N) = 0.002-0.008, Chromium (Cr) = 0.3-0.7, rest Iron (Fe) and incidental ingredients (all in wt. %).
[0039] Then, the steel is refined in basic oxygen furnace (BOF) converter and casted in continuous casting strands. In some embodiments, the steel can be manufactured in other furnaces as per requirement.
[0040] At block 104, the process 100 includes soaking the steel at 1200-1220 °C for about 1.5-2.5 hrs.
[0041] At block 106, the process 100 includes hot rolling the steel at Finishing Rolling Temperatures (FRT) ranging from 870-920 °C with reduced thickness achieved as 3-6 mm. The speed is maintained at 80-90 meters per minute (mpm). The hot rolling of the steel is done by 6 stands. The number of the stands can be varied as per the requirement keeping the reduction of steel up to 3-6 mm.
[0042] At block 108, the process 100 includes coiling the steel at 640-700 °C. The coiling of the steel, at block 108, is performed in a real hot strip mill condition.
[0043] After cooling, pickling of the steel is performed. The pickling is done with Hydro Chloride (HCl) acid to remove scales from the surface. The concentration of the HCl used is 10% by volume.
[0044] At block 110, the process 100 includes cold rolling the steel with 50-65% reduction with speed of 80-90 meters per minute (mpm). The cold rolling of the steel is done by 5 stands. The number of the stands can be varied as per the requirement keeping the reduction upto 50-65%.
[0045] At block 112, the process 100 includes continuously annealing the steel at 750 - 850 °C at a heating rate of 3-10 °C/s.
[0046] At block 114, the process 100 includes continuously cooling the steel at intermediate temperature at a slow cooling rate (~1-5 °C/s), or a fast cooling (cooling rate =20 °C/s) to room temperature (up to 100 °C). Ideally, the final cooling temperature can be anything well below the martensite start temperature of inter-critical annealed austenite composition.
[0047] At block 116, the process 100 includes further cooling of the steel at intermediate temperature at a slower cooling rate (1-5 °C/s), then a final cooling is done at =20 °C/s to room temperature (up to 100 °C).
[0048] As a result of the process 100, the present disclosure provides a steel with preferred composition (all in wt. %) shown following Table 1:
C Mn S P Si Al N Cr Fe + incidental
0.09 1.7 0.006 0.010 0.05 0.04 0.003 0.6 Rest
Table 1: Preferred composition of steel
[0049] The composition of the steel as mentioned in the block 102 of the process 100 provides the extra hardenability requirement in a situation where the rate of cooling from continuous annealing temperature to that of galvanizing, as mentioned in blocks 1122 to 116, is not very fast such as less than 20 °C/s. In accordance with an embodiment of the present disclosure, the composition of the steel is selected in such a way that the required dual phase microstructure containing ferrite and 30-50 vol.% martensite (by vol.) is formed even with slow cooling rate of =20 °C/s.
[0050] The strength of the steel primarily depends on the volume fraction of the hard martensite phase dispersed in the soft and ductile ferrite. In order to ensure that sufficient amount of martensite produced after continuous cooling of the austenite, sufficient quantities of manganese and chromium in the steel. These elements increase the hardenability and avoid the formation of the high temperature phases efficiently by shifting the continuous cooling transformation (CCT) curve to the right.
[0051] However, it is important to note that other elements such as silicon or phosphorous also play a similar role but are restricted in accordance with an embodiment of the present disclosure. This is mainly to avoid brittle fracture and unnecessary scale formation on the surface of the steel.
[0052] Further, concentration of silicon is controlled by maintaining the optimum slag basicity during steel making process, where lime is added to the liquid bath for reacting with silica thereby facilitating extra removal of Silicon. It is done to ascertain the superior product quality such as strength, ductility as well surface appearance after annealing.
[0053] The obtained mechanical properties of the CRDP 780 obtained by means of the process 100 are shown in following Table 2:
Yield Strength YS (MPa) Ultimate Tensile Strength UTS (MPa) %El Yield ratio
450-550 780 – 830 19-23 0.60-0.65
Table 2: Mechanical properties of the CRDP 780
[0054] The CRCA dual phase steel obtained from the process has 30-45% martensite and rest is ferrite (by vol.).
[0055] It is important to note that yield ratio (YR) which dictates the forming characteristics of the material should be around 0.6-0.7 for such steels. Although this steel will have higher amount of martensite than DP 600 grade, the carbon content in the martensite will be less. This will provide better formability for the grade.
EXAMPLES
[0056] The above mentioned process for making CRCA dual phase steel and its properties can be validated by the following examples. The following examples should not be interpreted to limit the scope of present disclosure.
[0057] New steel with the optimum chemical composition mentioned in (Table 1) was produced and processed. The steel was made in BOF converter without any vacuum treatment and cast into slabs of 250 mm thickness. The material was then soaked at 1200 °C for about 2 hours in reheating furnace and hot-rolled into 3.2 mm thick strips. The speed maintained was 84 mpm. The finish rolling temperature was maintained at 900 °C and coiling was done at 660 °C.
[0058] The hot-rolled material was subsequently pickled in HCL solution 10% by vol. to remove the surface scale. Further, the strips were cold-rolled into thin sheets of thickness 1.2 mm. The cold rolled coils were then annealed in Gleeble 3800 thermo-mechanical simulator for continuous annealing. Special strip annealing mobile conversion unit was used to simulate the continuous cooling to produce the dual phase steel. Details of the processing conditions are: Cold rolled samples were cut into rectangular sample size of 127X254 mm size. Four thermocouples were welded in different position to check the uniformity of temperatures. The steel was then heated at 1-5 °C/s, soaked at 770-850 °C for 40-100s, followed by either intermediate cooling (1-5 °C/s) at a temperature (750-800 °C) and final cooling at = 20 °C/s to room temperature.
[0059] Samples were then tested for mechanical properties and assessed (as per ASTM E8) for microstructural details.
[0060] FIG. 2 shows the microstructure of the steel processed in the Gleeble 3800 strip annealing simulator.
[0061] The following mechanical properties have been obtained for samples following above mentioned process as shown in Table 3:
Sample YS UTS %El YR
Specimen 1 474 806 20 0.59
Specimen 2 476 827 21 0.58
Specimen 3 485 828 19 0.59
Specimen 4 480 795 22 0.60
Specimen 5 525 819 20 0.64
Specimen 6 446 793 23 0.56
Specimen 7 465 810 21 0.57
Table 3: Mechanical properties obtained for samples
TECHNICAL ADVANTAGES
[0062] With the present disclosure, the UTS of the galvannealed dual phase steel is > 780 MPa using low Silicon content. Further, the alloys used are comparatively less expensive in light of prior art.
[0063] It is important to note that same composition of DP 600 has been used to produce through CRDP 780. This is the great achievement and cost of the product will be much lower than the commercially available. Moreover, the carbon content of this material is also low as compared to other available material. This will therefore be better steel from weldability point of view.
[0064] The CRCA dual phase steel developed with the implementation of the present disclosure can be used in manufacture of critical parts of automobiles such as the A-pillar, B-pillar and other structural parts.
[0065] Furthermore, with the present disclosure, high tensile strength of the material would allow usage of thinner gage outer panels and help reduce the weight of the car body.
[0066] Yet further, low yield ratio of the material would ensure critical formability of the parts under high speed forming operations.
Equivalents:
[0067] Yet further, the present disclosure specification has described a process for making a Cold Rolled Dual Phase (CRDP) 780 steel. The illustrated steps are set out to explain the exemplary embodiments shown, and it should be anticipated that on-going technological development will change the manner in which particular functions are performed. These examples are presented herein for purposes of illustration, and not limitation. Further, the boundaries of the functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternative boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed. Alternatives (including equivalents, extensions, variations, deviations, etc., of those described herein) will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein. Such alternatives fall within the scope and spirit of the disclosed embodiments. Also, the words "comprising," "having," "containing," and "including," and other similar forms are intended to be equivalent in meaning and be open-ended in that an item or items following any one of these words is not meant to be an exhaustive listing of such item or items, or meant to be limited to only the listed item or items. It must also be noted that as used herein and in the appended claims, the singular forms “a,” “an,” and “the” include plural references unless the context clearly dictates otherwise.
[0068] Finally, the language used in the specification has been principally selected for readability and instructional purposes, and it may not have been selected to delineate or circumscribe the inventive subject matter. It is therefore intended that the scope of the invention be limited not by this detailed description, but rather by any claims that issue on an application based here on. Accordingly, the embodiments of the present invention are intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the following claims.