, Description:Technical Field
The present invention relates to developing steel for automotive application. More particularly the invention relates to developing steel with low alloys.
Background
High Strength Low Alloy (HSLA) steels are used very often in manufacture of lightweight automobiles for the advantages of improving fuel economy and reducing emissions. These steels are used mainly in the structural parts and must possess good forming properties in addition to the high strength. Forming properties are mainly dependent on the total elongation or the ductility of the material. Additionally, a low Yield Strength to Ultimate Tensile Strength Ratio or Yield Ratio (YR) (YS/UTS) is generally considered to be an advantage from the viewpoint of low forming load requirement that normally reduces the chance of crack formation or generation of other forming defects. This is often found to be a limitation of HSLA steels that generally record a high YR, mostly exceeding 0.85.
By contrast, dual phase (DP) steels show a low yield ratio (YR), typically in the range of 0.6-0.65, which is generally considered to be an advantage for similar application in automotive manufacture.
HSLA steels primarily rely on the fine grain size of ferrite for achieving high strength through Hall-Petch type hardening. The other contribution to the strength of these steels comes from the precipitation hardening that depends on the volume fraction and size of the precipitates that are mostly carbide or carbonitride of titanium, niobium, vanadium or molybdenum. The high strength therefore originates mostly due to the impedance provided to the motion of the dislocations inside the ferrite grains. As a result, both YS and UTS of these steels are found to be relatively high, consequently reducing the YR value. By contrast, DP steels possess a two-phase microstructure where the dislocation motion is very easy in the clean ferrite grains of an optimum size. The onset of yielding is therefore relatively easy whereas the hard phase of martensite raises the ultimate tensile strength. As a result, these steels are found to have a low YR.
Objects:
An object of the invention is to provide a HSLA with low YR.
Another object of the invention is to provide a process for developing HSLA with low YR.
Disclosure of the invention
The present invention provides a method for developing of High Strength Low Alloy Steel comprising:
developing a steel slab/ingot with composition carbon (C) 0.07 to 0.20, manganese (Mn) 1.0-2.0, silicon (Si) 0.05 to 0.30, titanium (Ti) 0.12 to 0.22, Niobium (Nb) 0.12 to 0.17; Vanadium (V) or molybdenum (Mo) 0.3 to 0.4, Nitrogen-150 ppm (max), rest Fe and unavoidable impurities (all in wt.%),
reheating of the steel slab/ingot to 1250-1400 deg. C in a reheating furnace;
rolling the steel slab/ingot with percentage reduction of 55 to 70% with FRT of 950 to 1050 deg. C; and
coiling the steel sheet at 550 to 650 deg. C.
The combination of composition with process steps above mentioned ensures an appropriate microstructure of the HSLA steel that have helped to reduce the YR. A relatively coarser grain size of ferrite (6+0.9 to 10 + 1 micro m) is favourable to increase the mean free path of dislocation motion, eventually reducing the yielding stress.
In another embodiment, the present invention describes a High Strength Low Alloy Steel comprising:
carbon (C) 0.07 to 0.20, manganese (Mn) 1.0-2.0, silicon (Si) 0.05 to 0.30, titanium (Ti) 0.12 to 0.22, Niobium (Nb) 0.12 to 0.17, Vanadium or molybdenum 0.30 to 0.40, Nitrogen-150 ppm (max), rest Fe and unavoidable impurities (all in wt.%) with yield ratio 0.7 to 0.85.
Brief description of Drawings:
The novel features and characteristic of the disclosure are set forth in the appended claims. The disclosure itself, however, as well as a mode of use, further objectives and advantages thereof, will best be understood by reference to the following detailed description of an embodiment when read in conjunction with the accompanying drawings. One or more embodiments are now described, by way of example only, with reference to the accompanying drawings wherein like reference numerals represent like elements and in which:
Fig. 1 shows a process in accordance with an embodiment of the invention.
Fig. 2 shows thermomechanical cycle of the two steels in laboratory scale rolling mill as per the process of Fig. 1.
Fig. 3 shows microstructures of two steels as per the thermomechanical cycle of Fig. 2.
Fig. 4 shows images of microstructures of two steels as per the thermomechanical cycle of Fig. 2.
Fig. 5 shows Engineering stress-strain curve as per the thermomechanical cycle of Fig. 2.
Fig. 6 shows comparison of YR values of steels per the thermomechanical cycle of Fig. 2 with the conventional HSLA steels.
The figures depict embodiments of the disclosure for purposes of illustration only. One skilled in the art will readily recognize from the following description that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the disclosure described herein.

For a better understanding of the invention and to show how the same may be performed, a preferred embodiment thereof will now be described, by way of non-limiting example only, with reference to accompanying drawings.
Description of Preferred Embodiments
The foregoing has broadly outlined the features and technical advantages of the present disclosure in order that the detailed description of the disclosure that follows may be better understood. Additional features and advantages of the disclosure will be described hereinafter which form the subject of the description of the disclosure. It should also be realized by those skilled in the art that such equivalent methods do not depart from the scope of the disclosure. The novel features which are believed to be characteristic of the disclosure, as to method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present disclosure.

In the present document, the word "exemplary" is used herein to mean "serving as 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.

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 particular forms disclosed, but on the contrary, the disclosure is to cover all modifications, equivalents, and alternative falling within the spirit and the scope of the disclosure.
The terms “comprises”, “comprising”, or any other variations thereof, are intended to cover a non-exclusive inclusion, such that a method that comprises a list of acts does not include only those acts but may include other acts not expressly listed or inherent to such method. In other words, one or more acts in a method proceeded by “comprises… a” does not, without more constraints, preclude the existence of other acts or additional acts in the method.For the above-mentioned object, an appropriate microstructural engineering of HSLA steels may help reduce the YR. A relatively coarser grain size of ferrite can be favourable to increase the mean free path of dislocation motion, eventually reducing the yielding stress. By contrast, the presence of hard particles of the carbide precipitates can be available to raise the ultimate tensile strength of the material, similar as DP steels.
Therefore, in accordance with an embodiment of the invention a process (100) is shown in FIG. 1 with various steps for the development of High Strength Low Alloy Steel (HSLA). The process comprises
developing a steel slab/ingot with composition carbon (C) 0.07 to 0.20, manganese (Mn) 1.0-2.0, silicon (Si) 0.05 to 0.30, titanium (Ti) 0.12 to 0.22, Niobium (Nb) 0.12 to 0.17, Vanadium or molybdenum 0.3 to 0.4, Nitrogen-150 ppm (max), rest Fe and unavoidable impurities (all in wt.%) as per step 104.
The functionalities of the elements are mentioned below
Carbon: Adequate amount of carbon is necessary to ensure that the desired strength levels are reached. For ensuring both strength and ductility to be maximized, carbon content is kept preferably at below 0.2 wt %. Also at this range of Carbon, the weldability of the steel is good.
Manganese: It is necessary to control the austenite to ferrite phase transformation rate. The amount of Mn needs to be 1.0 to 2%, preferably at 1.5 wt%. An excess beyond 2.0% however gives rise to adverse effects such as a centreline segregation and hence Mn is preferably controlled to 1.5 wt. %.
Silicon: Silicon is a ferrite stabilizer. It also helps increase the strength of the steel. However, addition of Si leads to surface scale problems during rolling and therefore should be preferably at 0.15 wt. %.
Titanium: Titanium is added to increase the strength of the steel by grain refinement as well as increasing the strength of the steel through precipitation of carbide and carbonitride during hot rolling. Preferably, the titanium is kept at 0.17 wt. % to avoid an increase in cost or extra processing difficulties (e.g. rolling forces).
Niobium: Niobium is added to increase the strength of the steel by grain refinement. It also plays a role in increasing the strength of the steel through precipitation of carbide and carbonitride during hot rolling. Preferably, the niobium is kept at 0.15 wt. % to avoid an increase in cost or extra processing difficulties (e.g. rolling forces).
Vanadium or Molybdenum: Both these elements influence the recrystallization behaviour of austenite during hot rolling of steel and thereby help refine the grain size of the material that helps increase the strength of the product. Moreover, these elements also have a strong chemical affinity for carbon and / or nitrogen and form carbide, nitride or carbonitride precipitates either singly or in combination with some other elements such as Titanium present in the steel. These fine precipitates help increase the strength of the steel by interaction with mobile dislocations during plastic deformation. However, excessive addition of these elements increases the cost of steel processing and hence it is preferably restricted to less than 0.4 wt. %.
Nitrogen: The N content is restricted upto 0.005 wt. % maximum, otherwise too much AlN and/or TiN precipitates can form which are detrimental to formability.
At step 108 the steel slab/ingot is reheated to 1250-1400 deg. C in a reheating furnace. The reheating furnace can be muffle furnace in an embodiment.
At step 112 the steel slab/ingot is rolled with percentage reduction of 55 to 70% with FRT of 950 to 1050 deg. C.
At step 116 the steel sheet is coiled at 550 to 650 deg. C.
The microstructure of the steel is ferritic 95-98 % by volume.
The Grain size of the steel obtained 6 + 0.9 to 10 + 1 micro m.
Experimental Analysis
Accordingly, experiments were carried out in laboratory to improve the YR characteristic of HSLA steels as per the process (100).
Two experimental heats (X and Y) were made in 2 kg vacuum induction melting and cast as small size ingots. Chemical composition of the steels is presented in Table 1 shown below.
Table 1
Experiment C Mn Si Ti Nb V Mo N
Exp X 0.1 1.74 0.23 0.16 0.15 0.375 --- 0.011
Exp Y 0.1 1.24 0.05 0.21 0.14 --- 0.38 0.010
As seen, one material was based on Ti-Mo addition whereas the other variety was based on Ti-V addition. Both the steels contained some addition of Si as well as Mn for substitutional solute strengthening as well as Nb for supressing excessive grain coarsening due to exposure to a high reheat temperature before hot rolling.
The cast ingots were subjected to hot rolling in a small experimental rolling mill of reversing type after reheating at 1300 oC for about 50 mins. The steel is air cooled for 15 min after reheating and before rolling. The material was given a total deformation of 56% in 4 passes of thickness reduction. The Finish Rolling Temperature (FRT) was maintained at 1000+10 deg. C. The material after hot deformation was hold at a temperature of 610 oC for 30 mins in a muffle furnace for coiling. The thermomechanical cycle followed during the experimental hot rolling is presented in FIG 2.
Results and Discussion
FIG 3a-3b illustrates the microstructure of the two steels (X and Y) obtained after hot rolling and coiling. It is clear from the optical micrographs, that the two steels are predominantly made of single phase comprising ferrite grains. While Steel Y has shown a great etching response to 2-5% natal solution, Steel X has been observed to be a bit resistance to the etchant.
The above difference in the etching response of the two steels were analysed to be primarily due to the difference in the amounts of the carbide or carbonitiride forming alloying elements viz, Ti, Nb, V and Mo. The composition of Steel X is such that almost the whole of C and N were tied up in the form of precipitates whereas Steel Y might have some residual C and N in solution after the precipitation is complete. This made Steel X essentially free of C and N that eventually reduced the grain boundary energy of the material and made etching more difficult compared with Steel Y. Of course, the precipitates that had formed in the two steels would contribute to the strengthening but the actual magnitude would depend on the size and volume fraction. However, both the steels revealed a very coarse grain size of ferrite that is not commonly observed in the HSLA steels.
A detailed image analysis is carried out using the optical microscope to find out the exact grain size distribution of the two steels (X and Y). The results are presented in FIGs 4 (a)–(b) where it is evident that both the steels mostly record a very similar grain size distribution. A smaller final grain size has been assigned to the Steel Y compared with Steel X shown in below Table 2.
Table 2
Grain Size/Steel Steel X Steel Y
Grain Size 10micron + 1 6micron + 0.9
The deviation might have been due to a very minor population of finer grains in FIG 4(b). The relatively coarse grain size of the material must have been due to the high reheating temperature and the limited deformation that has been applied to the material above the no-recrystallization temperature.
The coarse grain size of ferrite is favourable for reducing the yielding stress of the steels whereas the presence of the precipitates leads to a raised ultimate tensile strength, thereby reducing the YR value favourably. The engineering stress strain curve of the two steels X and Y are presented in FIG 5. The YS and UTS values recorded by the two steels. Interestingly, both the steels displayed a continuous mode of yielding and enhanced total elongation values, contrary to the normal observations usually made in the HSLA type steels. Both these features are advantageous in terms of the forming characteristics of the steels. Additionally, the YS/UTS ratio or the YR can also be seen to be largely reduced as compared to the conventional HSLA steels.
As evident from FIG 5, both the steels recorded an appreciably low YR values namely, 0.73 for steel X and 0.82 for steel Y. The reduction in the YR is significantly large in case of steel X than that for steel Y which might have been a direct consequence of the grain size effect on the hindrance to the dislocation motion.
A comparison of the YR values of the two experimental steels with that of other conventional HSLA steels available in literature is provided in FIG 6.
It is evident form the process 100 and experimental analysis that a High Strength Low Alloy Steel is formed with composition carbon (C) 0.07 to 0.20, manganese (Mn) 1.0-2.0, silicon (Si) 0.05 to 0.30, titanium (Ti) 0.12 to 0.22, Niobium (Nb) 0.12 to 0.17, Vanadium or molybdenum 0.30 to 0.40, Nitrogen-150 ppm (max), rest Fe and unavoidable impurities (all in wt.%) with yield ratio 0.7 to 0.85.
The acceptance of yield ratio is in the range 0.65 to 0.85 by the automotive companies.
The YS of the HSLA obtained is 587 to 674 MPa.
The UTS of the HSLA obtained is 804 to 813 MPa.
The elongation of the HSLA obtained is 23-27%.
The microstructure of HSLA obtained is ferritic 95-98 % by volume.
The grain size of the HSLA obtained size 6 + 0.9 to 10 + 1 micro m.
HSLA steels primarily rely on the fine grain size of ferrite for achieving high strength through Hall-Petch type hardening. The other contribution to the strength of these steels comes from the precipitation hardening that depends on the volume fraction and size of the precipitates that are mostly carbide or carbonitride of titanium, niobium, vanadium or molybdenum.
Due to the process (100) including compositions and process steps, the high strength therefore originates mostly due to the impedance provided to the motion of the dislocations inside the ferrite grains. As a result, both YS and UTS of these steels are found to be relatively high, consequently reducing the YR value.
By contrast, DP steels possess a two-phase microstructure where the dislocation motion is very easy in the clean ferrite grains of an optimum size. The onset of yielding is therefore relatively easy whereas the hard phase of martensite raises the ultimate tensile strength. As a result, these steels are found to have a low YR.
An appropriate microstructure of the HSLA steel have helped to reduce the YR. A relatively coarser grain size of ferrite (6+0.9 to 10 + 1 micro m) is favourable to increase the mean free path of dislocation motion, eventually reducing the yielding stress. By contrast, the presence of hard particles of the carbide precipitates raises the ultimate tensile strength of the material, similar as DP steels.
Equivalents:
With respect to the use of substantially any plural and/or singular terms herein, those having skill in the art can translate from the plural to the singular and/or from the singular to the plural as is appropriate to the context and/or application. The various singular/plural permutations may be expressly set forth herein for sake of clarity.
It will be understood by those within the art that, in general, terms used herein, and especially in the appended claims (e.g., bodies of the appended claims) are generally intended as “open” terms (e.g., the term “including” should be interpreted as “including but not limited to,” the term “having” should be interpreted as “having at least,” the term “includes” should be interpreted as “includes but is not limited to,” etc.). It will be further understood by those within the art that if a specific number of an introduced claim recitation is intended, such an intent will be explicitly recited in the claim, and in the absence of such recitation no such intent is present. For example, as an aid to understanding, the following appended claims may contain usage of the introductory phrases “at least one” and “one or more” to introduce claim recitations. However, the use of such phrases should not be construed to imply that the introduction of a claim recitation by the indefinite articles “a” or “an” limits any particular claim containing such introduced claim recitation to inventions containing only one such recitation, even when the same claim includes the introductory phrases “one or more” or “at least one” and indefinite articles such as “a” or “an” (e.g., “a” and/or “an” should typically be interpreted to mean “at least one” or “one or more”); the same holds true for the use of definite articles used to introduce claim recitations. In addition, even if a specific number of an introduced claim recitation is explicitly recited, those skilled in the art will recognize that such recitation should typically be interpreted to mean at least the recited number (e.g., the bare recitation of “two recitations,” without other modifiers, typically means at least two recitations, or two or more recitations). Furthermore, in those instances where a convention analogous to “at least one of A, B, and C, etc.” is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., “a system having at least one of A, B, and C” would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). In those instances, where a convention analogous to “at least one of A, B, or C, etc.” is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., “a system having at least one of A, B, or C” would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). It will be further understood by those within the art that virtually any disjunctive word and/or phrase presenting two or more alternative terms, whether in the description, claims, or drawings, should be understood to contemplate the possibilities of including one of the terms, either of the terms, or both terms. For example, the phrase “A or B” will be understood to include the possibilities of “A” or “B” or “A and B.” While various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims.

Claims:1. A method for developing of High Strength Low Alloy Steel, comprising:
developing a steel slab/ingot with composition carbon (C) 0.07 to 0.20, manganese (Mn) 1.0-2.0, silicon (Si) 0.05 to 0.30, titanium (Ti) 0.12 to 0.22, Niobium (Nb) 0.12 to 0.17; Vanadium (V) or molybdenum (Mo) 0.3 to 0.4, Nitrogen-150 ppm (max), rest Fe and unavoidable impurities (all in wt.%);
reheating of the steel slab/ingot to 1250-1400 deg. C in a reheating furnace;
rolling the steel slab/ingot with percentage reduction of 55 to 70% with FRT of 950 to 1050 deg. C; and
coiling the steel sheet at 550 to 650 deg. C.
2. The method as claimed in claim 1, wherein microstructure of the steel is ferritic 95-98 % by volume.
3. The method as claimed in claim 1, wherein Grain size is 6 + 0.9 to 10 + 1 micro m.
4. The method as claimed in claim 1, wherein the reheating is done for 50mins.
5. The method as claimed in claim 1, wherein the coiling is done for 30mins.
6. The method as claimed in claim 1, wherein the steel is air cooled for 15 min after reheating and before rolling.
7. A High Strength Low Alloy Steel, comprising:
carbon (C) 0.07 to 0.20, manganese (Mn) 1.0-2.0, silicon (Si) 0.05 to 0.30, titanium (Ti) 0.12 to 0.22, Niobium (Nb) 0.12 to 0.17, Vanadium or molybdenum 0.30 to 0.40, Nitrogen-150 ppm (max), rest Fe and unavoidable impurities (all in wt.%)
with yield ratio 0.7 to 0.85.
8. The High Strength Low Alloy Steel as claimed in claim 7, wherein the YS 587 to 674 MPa.
9. The High Strength Low Alloy Steel as claimed in claim 7, wherein the UTS is 804 to 813 MPa
10. The High Strength Low Alloy Steel as claimed in claim 7, wherein the elongation is 23-27%.
11. The High Strength Low Alloy Steel as claimed in claim 7, wherein the microstructure of the steel is ferritic 95-98 % by volume.
12. The High Strength Low Alloy Steel as claimed in claim 7, wherein the grain size is Grain size 6 + 0.9 to 10 + 1 micro m.