TECHNICAL FIELD
[0001] The present disclosure, in general, relates to the field of metal processing 5 and, more particularly, to a method for improving the edge formability of cold rolled dual phase steel.
BACKGROUND
[0002] Background description includes information that may be useful in 10 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] From the modern-day point of view of stringent environmental norms on decreasing car body weight, the importance of high strength steels, especially of dual 15 phase (DP) grade with ferrite-martensite combination has reached its peak. The star attraction of DP steel is its low yield strength to tensile strength ratio, good elongation properties, high strain hardening rate, good crashworthiness, etc. But its low edge formability restricts its use in many components, especially the ones requiring flanging around holes, etc. It is normally seen that in steel having more than one phase, the edge 20 formability reduces to some extent. Among such steels, bainitic-ferritic steels show best hole expansion properties whereas ferritic-martensitic are comparatively much inferior. The low hole expansion ratio is attributed to the hardness difference between hard phase martensite and soft phase ferrite.
[0004] Currently, to increase the edge formability in the DP steels, very fine ferrite 25 grain is produced by thermo-mechanical reduction. However, in this approach, the martensite island size is decreased along with the ferrite grain size (as compared to
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plant-produced samples), through simple thermal cycling as observed the effects on the edge formability of the same grade.
[0005] Accordingly, there is a need in the art to provide a modified strategy for improving the edge formability of the DP steel.
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OBJECTS OF THE DISCLOSURE
[0006] In view of the foregoing limitations inherent in the state of the art, some of the objects of the present disclosure, which at least one embodiment herein satisfy, are listed herein below.
[0007] It is an object of the present disclosure to propose a modified method for 10 improving the edge formability of the DP steel.
[0008] It is another object of the present disclosure to propose a novel DP steel with improved Hole Expansion Ratio (HER).
[0009] 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 15 description taken in conjunction with the accompanying drawings in which a preferred form of the present invention is illustrated.
SUMMARY 20
[0010] This summary is provided to introduce concepts related to a method for improving the edge formability of cold rolled 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. 25
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[0011] The present disclosure relates to a method for improving the edge formability of cold rolled DP steel. The method includes two-step heat treatment process having a first heat treatment step and a second heat treatment step. The first heat treatment step includes quenching the DP steel from 875-930 °C to room temperature, and the second heat treatment step includes inter-critical annealing of the 5 quenched DP steel at a temperature ranging from 725 to 790 °C for a predefined time period.
[0012] In an aspect, the predefined time period ranges from 1 to 5 minutes.
[0013] In an aspect, the DP steel is a DP 590 steel.
[0014] In an aspect, the first heat treatment step is performed at 900 °C for 2 10 minutes and the second heat treatment step is performed at 700 °C for 2 minutes.
[0015] In an aspect, after the second heat treatment step, tempering of the annealed steel is performed at 275 °C for 2 minutes.
[0016] In an aspect, the second heat treatment being followed by the first heat treatment. 15
[0017] The present disclosure further relates to a DP 590 steel. The DP 590 steel includes tensile ranging from 590-700 MPa; yield ranging from 305-470 MPa; elongation ranging from 16 to 27 percentage; and hole Expansion Ratio ranging from 55-71 percentage.
[0018] Various objects, features, aspects, and advantages of the inventive subject 20 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.
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BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
[0019] The illustrated embodiments of the subject matter will be best understood by reference to the drawings, wherein like parts are designated by like numerals throughout. The following description is intended only by way of example, and simply illustrates certain selected embodiments of devices, systems, and methods that are 5 consistent with the subject matter as claimed herein, wherein:
[0020] FIG. 1 illustrates a method for improving the edge formability of DP steel, in accordance with an embodiment of the present disclosure;
[0021] FIG. 2 illustrates a graph representing holding time and holding temperature at different steps of the method for improving the edge formability of cold 10 rolled DP steel, in accordance with an embodiment of the present disclosure;
[0022] FIG. 3 illustrates a graphical representation of the tensile stress-strain curve of DP 590 annealed steel collected from the steel plant, in accordance with exemplary implementations of the present disclosure;
[0023] FIG. 4 illustrates a graphical representation of the tensile stress-strain curve 15 of ‘900-2 min - 770-2 min - tempering’ cycle, in accordance with exemplary implementations of the present disclosure;
[0024] FIG. 5 illustrates scanning electron microscopy (SEM) image at 1000x of DP 590 annealed steel collected from the steel plant, in accordance with exemplary implementations of the present disclosure; and 20
[0025] FIG. 6 illustrates a SEM image at 1000x of ‘900- 2 min- 770- 2min- tempering’ cycle, in accordance with exemplary implementations of the present disclosure.
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DETAILED DESCRIPTION
[0026] The detailed description of various exemplary embodiments of the disclosure is described herein with reference to the accompanying drawings. It should be noted that the embodiments are described herein in such details as to clearly communicate the disclosure. However, the amount of details provided herein is not 5 intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure as defined by the appended claims.
[0027] It is also to be understood that various arrangements may be devised that, although not explicitly described or shown herein, embody the principles of the present 10 disclosure. Moreover, all statements herein reciting principles, aspects, and embodiments of the present disclosure, as well as specific examples, are intended to encompass equivalents thereof.
[0028] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments. As used 15 herein, the singular forms “a”, “an”, and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises”, “comprising”, “includes”, “consisting” and/or “including” when used herein, specify the presence of stated features, integers, steps, operations, elements and/or components, but do not preclude the presence or addition of one or 20 more other features, integers, steps, operations, elements, components and/or groups thereof.
[0029] It should also be noted that in some alternative implementations, the functions/acts noted may occur out of the order noted in the figures. For example, two figures shown in succession may, in fact, be executed concurrently or may sometimes 25 be executed in the reverse order, depending upon the functionality/acts involved.
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[0030] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which example embodiments belong. It will be further understood that terms, e.g., those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art 5 and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
[0031] FIG. 1 illustrates a method 100 for improving the edge formability of cold rolled dual phase (DP) steel, in accordance with an embodiment of the present disclosure. The method 100 is suitable for DP 590 steel. The order in which the method 10 100 is described is not intended to be construed as a limitation, and any number of the described method blocks may be combined in any order to implement the method 100, or an alternative method.
[0032] The method includes two-step heat treatment process having a first heat treatment step and a second heat treatment step. 15
[0033] At block 102, the first heat treatment step includes quenching the DP steel from 875-930 °C to room temperature.
[0034] At block 104, the second heat treatment step includes inter-critical annealing at a temperature ranging from 725 to 790 °C for a predefined time period. In an aspect, the predefined time period ranges from 1 to 5 minutes. 20
[0035] In an aspect, the second heat treatment being followed by the first heat treatment.
[0036] The aim of the present disclosure is to refine the microstructure of DP 590 steel, not only the ferrite grains (which is possible by thermomechanical reduction) but also the martensite islands, so that the strain accumulation at the interface can be 25 distributed to more locations/sites thus reducing the maximum strain partitioning reached at any one interface (ferrite/martensite interface).
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[0037] Accordingly, in the present disclosure, a different technique has been proposed to achieve refinement of the microstructure. In accordance with an implementation of the present disclosure, cold rolled full hard DP steel sample having a chemical composition as in Table 1 is selected.
Table 1: Chemical composition of DP 590 steel material 5
Carbon (C)
Manganese (Mn)
Silicon (Si)
Chromium (Cr)
Nickel (Ni)
0.07
1.83
0.385
0.028
0.028
[0038] Then, the DP steel sample having the chemical composition as in Table 1 has been heated up to a level where it reaches the fully austenite phase stage and is held there for a few minutes to achieve carbon dissolution and homogenization. The DP steel sample is quenched very fast so as to obtain martensite phase in the DP steel 10 sample (referred to as the first heat treatment step).
[0039] Martensite, formed in the first heat treatment step, being a strained phase introduces a huge amount of defects in the microstructure which in a consecutive step helps in achieving refinement. The next second heat treatment step is to heat this DP steel sample to an inter-critical annealing temperature and give it an isothermal holding 15 so that approximately 20% of martensite can form at the end of second heat treatment step (typical to achieve a 590MPa UTS target), after which it is quenched.
[0040] The martensite, in the first heat treatment step, transforms to ferrite as the heat treatment process begins the second heat treatment step. New ferrite grains and martensite islands formed on the prior formed martensite, after second heat treatment 20 step, is expected to be finer than ordinarily achieved by single step inter-critical annealing route followed in a steel plant.
[0041] The second heat treatment step is followed by simulation of a tempering step (tempering at 275 °C for 2 minutes) which is also standard industrial practice. Accordingly, in a preferred implementation of the method 100, the first heat treatment 25
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step is performed at 900 °C for 2 minutes, the second heat treatment step is performed at 700 °C for 2 minutes, and the tempering step is performed at 275 °C for 2 minutes, as shown in FIG. 2.
[0042] The implementation or experiment of the present disclosure is performed in an annealing simulator having a heating chamber, intermediate chamber and cooling 5 chamber suitable to simulate different thermal cycles pertaining to industrial continuous annealing line.
[0043] In accordance with the present disclosure, both thermal cycles, namely the first and second heat treatment steps, require isothermal holding at a specific temperature for a specific time. Holding temperature in the first heat treatment step is 10 to be kept just above A3 and not too high, so as to prevent grain coarsening of the austenite. Holding temperature in the second heat treatment step is to be determined based on the amount of martensite required in a final microstructure so as to guarantee the minimum target ultimate tensile strength (UTS), i.e., 590 Megapascal (MPa). Similarly, the holding time in both the first and second heat treatment steps is to 15 adjusted so that it is high enough for homogenization of Carbon, yet low enough to prevent redundant grain coarsening. The challenge in this work is to achieve a balance in terms of the thermal cycle.
[0044] To achieve this, a series of trial runs are conducted on the cold-rolled full hard DP 590 grade steel collected from a steel plant. Table 2 lists down the description 20 of all the trial thermal cycles (where the first heat treatment step is followed by the second heat treatment step) that has been carried out.
Table 2: Description of a thermal cycle of all trials
Trial No.
Step 1
Step 2
Temperature (oC)
Time (minutes)
Temperature (oC)
Time (minutes)
1
900
5
790
5
2
925
5
750
5
10
3
850
5
790
5
4
900
5
725
5
5
900
2
725
5
6
900
2
790
2
7
900
3
770
2
8
900
3
790
1.5
9
900
1
790
1.5
10
900
2
750
1.5
11
900
2
770
2
12
875
2
725
1
13
875
2
790
1
14
900
2
750
1
[0045] Each of these heat treatment cycles is administered initially on the sub-size tensile specimen, followed by tensile testing and microstructural examination. In the tensile stress-strain curve, the post-uniform elongation is a parameter known to have a very good correlation with edge formability (measured by Hole Expansion Ratio or 5 HER). Therefore, from the trials, few cycles are chosen based on the post-uniform elongation in the corresponding tensile specimen. These cycles are simulated again but this time with samples of 100 mm by 100 mm in size. The results are compared with the tensile data and the HER test of cold-rolled, continuous-annealed DP 590 sample collected from the plant. HER tests are done using a conical punch. The process is fine-10 tuned so that the edge formability of the final sample is better than that of the DP 590 collected from the steel plant.
[0046] The modified DP 590 with a thermal cycle of 2 minutes holding at 900 °C at the first heat treatment step and 2 minutes holding at 770 °C at the second heat treatment step is found is give better results consistently when compared to DP 590 15 collected from the steel plant. The comparative results are given below:
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Table 3: HER results of DP 590 annealed steel collected from the steel plant
Initial diameter1 (mm)
Initial diameter2 (mm)
Final diameter1 (mm)
Final diameter2 (mm)
Punch travel on crack(mm)
Load on crack(kN)
Average HER (%)
10.04
10.03
15.59
15.60
13.23
8.47
55.4
10.03
10.04
15.40
15.49
12.94
8.26
53.9
Table 4: HER results of ‘900- 2 min- 770- 2min- tempering’ cycle:
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[0047] Observing the results in Tables 3, 4, and 5, ‘900- 2 min- 770- 2min- tempering’ cycle is found to have increased the Hole Expansion ratio of DP 590 by greater than 20%. Next, it is to be checked whether this cycle is meeting the mechanical properties of DP 590 steel. 10
[0048] Further, FIG. 3 illustrates a graphical representation of tensile stress-strain curve of DP 590 annealed steel collected from the steel plant and FIG. 4 illustrates a graphical representation of tensile stress-strain curve of ‘900- 2 min- 770- 2min- tempering’ cycle, in accordance with exemplary implementations of the present disclosure. It can be observed from the FIGS. 2 and 3 that, with the modified annealing 15 cycle, the total strain has increased from 23.8% to 27% and the post-uniform strain has increased from 6% to 9% approximately. Also, UTS requirement of 590 MPa is clearly met in the steel sample obtained from a modified cycle. The stain hardening rate has marginally decreased in the modified cycle but that does not affect the objective of the present disclosure in any way. 20
[0049] Yet further, FIG. 5 illustrates scanning electron microscopy (SEM) image at 1000x of DP 590 annealed steel collected from the steel plant, and FIG. 6 illustrates SEM image at 1000x of ‘900- 2 min- 770- 2min- tempering’ cycle, in accordance with
Initial diameter1 (mm)
Initial diameter2 (mm)
Final diameter1 (mm)
Final diameter2 (mm)
Punch travel on crack(mm)
Load on crack(kN)
Average HER (%)
10.03
10.04
16.8
16.84
15.69
9.91
67.62
10.00
10.02
16.84
17.03
15.63
9.77
69.18
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exemplary implementations of the present disclosure. The effect of a modified annealing cycle on the DP 590 steel is clear from the micrographs (FIGS. 4 and 5). Not only the ferrite grain size has reduced but martensite islands are extremely fine and distributed. Amount of interior martensite is also high in case of the modified cycle microstructure. 5
[0050] Furthermore, those skilled in the art can appreciate that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present disclosure. It will be appreciated that several of the above-disclosed and other features and functions, or alternatives thereof, may be combined into other systems or applications. Various presently unforeseen or 10 unanticipated alternatives, modifications, variations, or improvements therein may subsequently be made by those skilled in the art without departing from the scope of the present disclosure as encompassed by the following claims.
[0051] The claims, as originally presented and as they may be amended, encompass variations, alternatives, modifications, improvements, equivalents, and substantial 15 equivalents of the embodiments and teachings disclosed herein, including those that are presently unforeseen or unappreciated, and that, for example, may arise from applicants/patentees and others.
[0052] While the foregoing describes various embodiments of the present disclosure, other and further embodiments of the present disclosure may be devised 20 without departing from the basic scope thereof. The scope of the present disclosure is determined by the claims that follow. The present disclosure is not limited to the described embodiments, versions or examples, which are included to enable a person having ordinary skill in the art to make and use the invention when combined with information and knowledge available to the person having ordinary skill in the art.

Claims:I/We claim:
1. A method for improving the edge formability of cold rolled dual phase (DP) steel, the method comprising two-step heat treatment process having a first heat treatment step and a second heat treatment step, the method comprising:
in the first heat treatment step, quenching the DP steel from 875-930 °C 5 to room temperature; and
in the second heat treatment step, inter-critical annealing of the quenched DP steel at a temperature ranging from 725 to 790 °C for a predefined time period.
2. The method as claimed in claim 1, wherein the predefined time period ranges 10 from 1 to 5 minutes.
3. The method as claimed in claim 1, wherein the DP steel is a DP 590 steel.
4. The method as claimed in claim 1, wherein after the second heat treatment step, the method comprising tempering the annealed steel at 275 °C for 2 minutes.
5. The method as claimed in claim 1, wherein the first heat treatment step is 15 performed at 900 °C for 2 minutes and the second heat treatment step is performed at 700 °C for 2 minutes.
6. The method as claimed in claim 1, wherein the second heat treatment being followed by the first heat treatment.
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7. A dual phase (DP) 590 steel produced by the two-step heat treatment process as claimed in claim 1, wherein the DP 590 steel comprising:
Tensile ranging from 590-700 MPa;
Yield ranging from 305-470 MPa;
Elongation ranging from 16 to27 percentage; and 5
Hole Expansion Ratio (HER) ranging from 55-71 percentage. , Description:METHOD FOR IMPROVING THE EDGE FORMABILITY OF COLD ROLLED DUAL PHASE STEEL