Claims:WE CLAIM:
1. A bake hardenable (BH) steel with high bake hardenability and shelf life, the steel comprises:
Carbon (C) 0.001 to 0.003 wt.%
Silicon (Si) = 0.01 to 0.04 wt.%;
Sulphur (S) = less than 0.1 wt.%;
Phosphorus (P) 0.02 to 0.08 wt.%;
Manganese (Mn) 0.2 to 0.5 wt.%;
Nitrogen (N) less than 0.004 wt.%;
Aluminum (Al) 0.03 to 0.06 wt.%;
Niobium (Nb) = less than 0.01 wt.%;
Titanium (Ti) = less than 0.01 wt.%; and
Boron (B) 0.0002 to 0.002 wt.%.

2. The bake hardenable (BH) steel as claimed in claim 1, wherein the steel has a shelf life greater than seven months.
3. The bake hardenable (BH) steel as claimed in claim 1, wherein the steel has a bake hardenable (BH) value in the range 39-57 MPa.
4. The bake hardenable (BH) steel as claimed in claim 1, wherein the steel having a yield strength (YS) in the range 266-279 MPa.
5. The bake hardenable (BH) steel as claimed in claim 1, wherein the steel having an elongation length (EL) in the range 36-48%.
6. The bake hardenable (BH) steel as claimed in claim 1, wherein the steel having maximum flow stress in the range 329-339 MPa.
7. A method (100) for making a bake hardenable (BH) steel, the method comprising:
casting (101) a steel with a composition
Carbon (C) 0.001 to 0.003 wt.%,
Silicon (Si) = 0.01 to 0.04 wt.%,
Sulphur (S) = less than 0.1 wt.%,
Phosphorus (P) 0.02 to 0.08 wt.%,
Manganese (Mn) 0.2 to 0.5 wt.%,
Nitrogen (N) less than 0.004 wt.%,
Aluminum (Al) 0.03 to 0.06 wt.%,
Niobium (Nb) = less than 0.01 wt.%,
Titanium (Ti) = less than 0.01 wt.%,
Boron (B) 0.0002 to 0.002 wt.%;
hot rolling (102) the steel at a reheat furnace temperature in the range 1200-1250 degree C;
rolling (103) the steel in roughing mill at an exit temperature in the range 1050-1090 degree C;
finish rolling (104) the steel at a temperature in the range 890-930 degree C;
coiling (105) the steel at a temperature in the range 680-720 degree C; and
cold rolling (106) the steel with a reduction in the range 70-90 %.
8. The method (100) as claimed in claim 7, wherein continuous annealing, heating and soaking furnace temperature is in the range 760-800 degree C.
9. The method (100) as claimed in claim 7, wherein slow cooling furnace temperature is in the range 655-695 degree C.
10. The method (100) as claimed in claim 7, wherein fast cooling zone temperature is in the range 380-420 degree C.
11. The method (100) as claimed in claim 7, wherein over aging zone temperature is in the range 330-370 degree C.
12. The method (100) as claimed in claim 7, wherein second cooling zone temperature is in the range 130-170 degree C.
13. The method (100) as claimed in claim 7, wherein a percentage of elongation of bake hardened steel during temper rolling after continuous annealing is 1.3%-1.7%. , Description:TECHNICAL FIELD
[0001] The present disclosure, in general, relates to steel grade having a range of composition for improving a range of properties. In particular, it relates to a bake hardenable steel with a high bake hardenability and a higher shelf life.
BACKGROUND
[0002] Background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
[0003] Bake Hardenable (BH) steel has always been a preferred choice over Interstitial Free (IF) steel in the automobile industry especially with respect to automotive body panels, where, dent resistance property is highly desirable along with the ease of forming ability. The Bake Hardening (BH) effect is an outcome of the reaction between dislocation and a small number of interstitial carbon atoms in the steel (ferrite phase) at the paint baking temperature of a press-formed panel. This work hardening process is supposed to be very slow at room temperature during its transportation and storage to obtain sufficient shelf life between the stage of temper rolling and press forming during the casting and rolling of the steel. The typical shelf life of BH steel at room temperature (25?C) is around three months, which is sometimes inadequate for certain automakers depending upon their specific requirements. Such a problem faced by these automakers using this steel is further aggravated in Indian conditions due to the predominant summer season and therefore, Bake Hardenable (BH) steel panel after forming is susceptible to the stretcher stain marks within the guarantee period. Therefore, there is a need for enhancing the shelf life without compromising bake hardening (BH) value which is a great challenge to the steelmakers. The strict control of chemical compositions and process parameters to keep a certain amount of solute carbon in the final Bake Hardenable (BH) steel is difficult to achieve.
[0004] In the view of the above-cited problems, there is a need to develop such a steel grade having a high bake hardenable value and an improved shelf life over and above the existing state of the art.
[0005] Accordingly, there is a need for a steel grade that can overcome one or more limitations stated above or any other limitation associated with the conventional prior arts.
OBJECTS OF THE DISCLOSURE
[0006] Some of the objects of the present disclosure, which at least one embodiment herein satisfy, are listed hereinbelow.
[0007] It is a general object of the present disclosure to provide a bake hardenable steel having a high bake hardenable value and an improved shelf life.
[0008] It is another object of the present disclosure to provide a method of producing and processing a bake hardenable steel having a high bake hardenable value and an improved shelf life.
[0009] It is another object of the present disclosure to provide a bake hardenable steel having a high bake hardenable value and an improved shelf life without the addition of any micro-alloying elements.
[0010] It is another object of the present disclosure to provide a dynamic design of process parameters for bake hardenable (BH) steel to optimize the content of solute Carbon (C), responsible for imparting the desired Bake Hardened value (BH) and achieving a high shelf life.
[0011] It is another object of the present disclosure to provide for a methodology by utilizing the change in process parameters, so as to manipulate the grain size of the steel which can accommodate more or less amount of solute carbon near the grain boundary in the steel in order to achieve the desired Bake Hardenable (BH) value and high shelf life when the accurate chemistry of the heat is not in the desired range.
[0012] 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
[0013] This summary is provided to introduce concepts related to a bake hardenable steel having a high bake hardenable value and an improved shelf life.
[0014] 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.
[0015] In an embodiment, the present disclosure relates to a bake hardenable (BH) steel with high bake hardenability and shelf life, the steel having a composition of carbon (C) 0.001 to 0.003 wt.%, Silicon (Si) 0.01 to 0.04 wt.%, Sulphur (S) less than 0.1 wt.%, Phosphorus (P) 0.02 to 0.08 wt.%, Manganese (Mn) 0.2 to 0.5 wt.%, Nitrogen (N) less than 0.004 wt.%, Aluminum (Al) 0.03 to 0.06 wt.%, Niobium (Nb) less than 0.01 wt.%, Titanium (Ti) less than 0.01 wt.% and Boron (B) 0.0002 to 0.002 wt.%.
[0016] In an aspect, the bake hardenable (BH) steel has a shelf life greater than seven months.
[0017] In an aspect, the bake hardenable (BH) steel has a bake hardenable (BH) value in the range 39-57 MPa.
[0018] In an aspect, the BH steel has a yield strength (YS) in the range 266-279 MPa.
[0019] In an aspect, the steel has an elongation length (EL) in the range 36-48%.
[0020] In yet another aspect, the BF steel has maximum flow stress in the range 329-339 MPa.
[0021] In an embodiment, a method for making a bake hardenable (BH) steel, the method comprising steps including casting steel into a composition carbon (C) 0.001 to 0.003 wt.%, Silicon (Si) 0.01 to 0.04 wt.%, Sulphur (S) less than 0.1 wt.%, Phosphorus (P) 0.02 to 0.08 wt.%, Manganese (Mn) 0.2 to 0.5 wt.%, Nitrogen (N) less than 0.004 wt.%, Aluminum (Al) 0.03 to 0.06 wt.%, Niobium (Nb) less than 0.01 wt. %, Titanium (Ti) less than 0.01 wt. % and Boron (B) 0.0002 to 0.002 wt.%, the method further comprising hot rolling of the steel at reheat furnace temperature in the range 1200-1250 degree C, rolling of the steel in roughing mill at an exit temperature in the range 1050-1090 degree C, finish rolling the steel at a temperature in the range 890-930 degree C, coiling the steel at a temperature in the range 680-720 degree C and cold rolling the steel with a reduction in the range 70-90 %.
[0022] In an aspect, continuous annealing heating and soaking furnace temperature is in the range 760-800 degree C, slow cooling furnace temperature is in the range 655-695 degree C, fast cooling zone temperature is in the range 380-420 degree C, over aging zone temp is in the range 330-370 degree C, second cooling zone temperature is in the range 130-170 degree C.
[0023] In an aspect, the percentage of elongation of bake hardened steel during temper rolling after continuous annealing is in the range of 1.3-1.7%.
[0024] 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.
[0025] 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.
[0026] 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
[0027] 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:
[0028] FIG.1 illustrates a method for making a bake hardenable steel having an improved bake hardenability and a higher shelf-life, in accordance with an embodiment of the present disclosure; and
[0029] FIG. 2 illustrates the microstructure of the Continuous Annealing and Processing Line (CAPL) trial coils at different magnification, in accordance with an embodiment of the present disclosure.
DETAILED DESCRIPTION
[0030] 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 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.
[0031] 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 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.
[0032] 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 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” 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 more other features, integers, steps, operations, elements, components and/or groups thereof.
[0033] 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 be executed in the reverse order, depending upon the functionality/acts involved.
[0034] 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 and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
[0035] Embodiments explained herein pertain to a bake hardenable (BH) steel with high bake hardenability and shelf life, the steel comprises Carbon (C) 0.001 to 0.003 wt.%, Silicon (Si) = 0.01 to 0.04 wt.%, Sulphur (S) = less than 0.1 wt. %, Phosphorus (P) 0.02 to 0.08 wt.%, Manganese (Mn) 0.2 to 0.5 wt.%, Nitrogen (N) less than 0.004 wt.%, Aluminum (Al) 0.03 to 0.06 wt.%, Niobium (Nb) = less than 0.01 wt. %, Titanium (Ti) = less than 0.01 wt. % and Boron (B) 0.0002 to 0.002 wt.%.
[0036] The present invention is also directed to provide a method (100) of making bake hardenable (BH) steel as illustrated in FIG.1. The order in which the method (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 methods or an alternative method.
[0037] The method (100) comprises a series of steps during the processing of the steel where at step 101, casting of the steel is done wherein steel is cast in the composition Carbon (C) 0.001 to 0.003 wt.%, Silicon (Si) = 0.01 to 0.04 wt.%, Sulphur (S) = less than 0.1 wt. %, Phosphorus (P) 0.02 to 0.08 wt.%, Manganese (Mn) 0.2 to 0.5 wt.%, Nitrogen (N) less than 0.004 wt.%, Aluminum (Al) 0.03 to 0.06 wt.%, Niobium (Nb) = less than 0.01 wt. %, Titanium (Ti) = less than 0.01 wt. % and Boron (B) 0.0002 to 0.002 wt.%.
[0038] At step 102, the method (100) comprises hot rolling of the steel at a reheat furnace temperature in the range 1200-1250 degree C.
[0039] At step 103, the method (100) comprises processing the hot rolled steel for rolling in a roughing mill at an exit temperature in the range 1050-1090 degree C.
[0040] At step 104, the rolled steel is then sent for finish rolling to be carried out at a temperature in the range 890-930 degree C.
[0041] At step 105, the method (100) comprises coiling of the rolled steel after completion of finish rolling is carried out at a temperature in the range 680-720 degree C.
[0042] At step 106, the method (100) comprises cold rolling of the steel wherein the obtained steel has a reduction in the range 70-90 %.
[0043] In FIG. 2, 201, 202, 203 and 204 illustrate the microstructure of the Continuous Annealing and Processing Line (CAPL) trial coils at different magnifications. During the processing of the steel through a Continuous annealing and processing line (CAPL), a coil is processed, wherein the carbon wt.% is monitored and recorded at different stages at different temperatures, wherein the continuous annealing heating and soaking furnace temperature is in the range 760-800 degree C, slow cooling furnace temperature is in the range 655-695 degree C, fast cooling zone temperature is in the range 380-420 degree C, over aging zone temp is in the range 330-370 degree C, second cooling zone temperature is in the range 130-170 C and percentage of elongation of bake hardened steel during temper rolling after continuous annealing is 1.3-1.7 % elongation.
[0044] The main factors having a pronounced effect on Bake hardenable (BH) value are the amount of solute carbon present and the density of dislocations which are affected before the bake hardening process and the effect has been described further in the description.
[0045] On one hand, increasing the annealing temperature reduces the corresponding strength increment after the bake hardening of the steel. On the other hand, an increase in the annealing temperature effectively increases the bake hardening (BH) value. An increase in Bake hardening (BH) value by 20 MPa with an increase of temperature from 75 degree C to 85 degree C is seen for interstitial free (IF) steel. With an increase in annealing temperature the grain size also increases and the ratio of grain boundaries area to the total area decreases, which affects the bake hardening (BH) value.
[0046] In addition to the effect of annealing temperature on the grain size, the chemical composition of steel also affects the value of annealing temperature for the achievement of a required bake hardening value (BH) value.
[0047] In under stabilized steel, there is no effect on bake hardening (BH) value with an increase of soaking temperature beyond 900 degree C. However, in Titanium (Ti) stabilized steel without any excess of Titanium (Ti), the bake hardening (BH) value increases from BH 10 MPa to 40 MPa as a result of an increase in annealing temperature from 800 degree C to 900 degree C.
[0048] To evaluate the effect of grain size on bake hardening (BH) value, two main characteristics are important to be considered i.e. the distribution of carbon atoms and dislocations in grain interior in the vicinity of or at grain boundaries. On one hand, it is seen that an increase in grain size is accompanied by the increase in carbon content intragranular resulting in cementite formation during cooling and diminished bake hardening (BH) value. Similarly, for a given carbon content the bake hardening (BH) value increases with a decrease in grain size and the effect becomes more pronounced with an increase in carbon content. This behavior is attributed to the role of grain boundaries as sinks for carbon atoms, which results in the reduction of free carbon available intra-granularly for locking of dislocations by the release of carbon atoms from the boundaries and their contribution to the bake hardening (BH).
[0049] However, it is not clear what is the driving force behind the release of carbon, unless it is strong carbide-forming elements with high affinity to carbon. The time required to saturate grain boundaries with carbon is much longer than that for the formation of Cottrell atmospheres in the grain interior. This is due to the planar geometry of the boundaries. In addition, carbon is required to diffuse from larger distances to achieve the saturation level at grain boundaries comparable to that at statistically stored dislocations within the grain interior.
[0050] With grain size reduction two opposite factors operate, the area fraction of grain boundaries increases and thus requires more carbon for its saturation, on the other hand, the diffusion distances from the center to the boundaries are reduced. In case of sufficiently high Bake hardening temperature and time, when there is enough carbon available for grain boundaries saturation, Bake hardening (BH) value should increase with the decrease in grain size. The reason for this is the inhibited generation of dislocations from grain boundaries due to pinning by carbon segregation, and the increased stress necessary to eject dislocations from the boundary.
An exeperimental analysis:
[0051] In a series of experiments, during the processing of the steel through a Continuous annealing and processing line (CAPL), a coil is processed, wherein the carbon wt.% is monitored and recorded at different stages at different temperatures.
[0052] The below table 1 illustrates the chemical composition of the steel coil during its processing on the Continuous annealing and processing line (CAPL) at different stages:
Table 1: Chemistry of the steel coils through CAPL
Carbon (C) Silicon (Si) (wt.%) Sulphur (S) (wt.%) Phosphorus (P) (wt.%) Manganese (Mn) (wt.%) Nitrogen (N) (wt.%) Aluminum (Al) (wt.%) Niobium (Nb) (wt.%) Titanium (Ti) Boron (B) (wt.%)
RH- 0.0012
LSA- 0.0017
TPA-0.0014- 0.0021
CAPL- 0.0018 0.03 0.03 0.055 0.35 0.0018 0.047 Less than 0.01 Less than 0.01 0.0002

[0053] where RH- Sample was taken after RH de-gaser, LSA- Ladle Sample Analysis, TPA- Test Piece of samples taken from different coils of the same heat
[0054] Table 2 illustrates the process parameters of the processing of the steel coil on the Continuous annealing and processing line (CAPL) at different zones of CAPL.
Table 2: Details of trial coils at CAPL and their process parameters
Parameters HF SF HCF 1C OA 2C SPM E1
Coil (without Ti) Normal Heating rate at speed @130 m/min 780 degree C 675 degree C 400 degree C 345 degree C 140 degree C 1.5 %

[0055] where HF- Heating Furnace zone of CAPL, SF- Soaking Furnace zone of CAPL, SCF- Slow Cooling Furnace zone of CAPL, 1C- Fast Cooling zone of CAPL, OA- Over Aging zone of CAPL, 2C- 2nd Cooling zone of CAPL, SPM E1- Skin Pass Mill Elongation.
[0056] A steel composition and process parameters are achieved through an experimental analysis wherein, the steel having a composition Carbon (C) 0.0018 (wt.%), Silicon (Si) 0.03 (wt.%), Phosphorus (P) 0.055 (wt.%), Manganese (Mn) 0.35 (wt.%), Nitrogen (N) 0.0018 (wt.%), Aluminum (Al) 0.047 (wt.%), Boron (B) 0.0002 (wt.%) and other elements such as Niobium (Nb), Sulphur (S) and Titanium (Ti) found in traces. Additionally, the method having process parameters comprising steps casting a steel followed by hot rolling of the steel at a reheat furnace temperature of 1240 degree C. The steel being further rolled in a roughing mill at an exit temperature of 1071 degree C with finish rolling at a temperature of 920 degree C. Subsequently, coiling of the rolled steel carried out at a temperature of 700 degree C and cold rolling of the rolled steel with a reduction of 78%. The heating and soaking annealing furnace temperature was kept at 780 degree C, slow cooling furnace temperature at 675 degree C, fast cooling zone temperature at 400 degree C, over aging zone temperature at 345 degree C, second cooling zone temperature at 140 degree C and percentage of elongation of the steel during temper rolling after continuous annealing is 1.5%.
[0057] The properties achieved in the bake hardened steel with such process temperatures and composition mentioned above are as follows:
- Shelf life is greater than seven months.
- Bake hardenable (BH) value 45 MPa (average of six samples measured at different locations of the processed coil).
- Yield strength (YS) 272 MPa (average after BH).
- Total Elongation length (EL) 43% (average).
- Maximum flow stress 334 (average) MPa.
[0058] 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 claims.
Referral numerals
Reference Numeral Description
201 Microstructure of the steel at 500µm
202 Microstructure of the steel at 100µm
203 Microstructure of the steel at 50µm
204 Microstructure of the steel at 20µm