High-strength cold-rolled and annealed steel and method for manufacturing the same

Abstract

The present invention relates to high-strength cold-rolled annealed steel and a method (100) for producing high-strength cold-rolled annealed steel. The high-strength cold-rolled annealed steel has the following composition expressed in weight percent: C: 0.25~0.35, Mn: 1.0~2.5, S: <0.006, P: <0.02, Si: 0.02~0.50, Al: 0.02~0.50, Mo: 0.00~0.30, N (ppm) < 80, with the remainder being iron (Fe) and unavoidable impurities. The high-strength cold-rolled annealed steel has a microstructure of 10~30% bainite and 70~90% martensite. The high-strength cold-rolled annealed steel exhibits a tensile strength in the range of 1400~1800 MPa and a minimum elongation of 7%.

Description

Technical Field

[0001] The present invention relates to a high-strength cold-rolled and annealed steel sheet or strip having an excellent combination of tensile strength and ductility at room temperature, and more particularly, to a method for producing a high-strength cold-rolled and annealed steel suitable for manufacturing structural and safety parts including automotive battery casings.

Background Art

[0002] Conventional steels such as IF (interstitial free), BH (bake hardenable), HSLA (high-strength low-alloy) are well documented in the literature and have been analyzed in various aspects such as processing, mechanical behavior, microstructure, and texture (shown in the following image). Their mechanical properties (e.g., strength and ductility) depend on conventional strengthening mechanisms including grain refinement, solid solution strengthening, etc. Advanced high-strength steels (AHSS) are multiphase steels that utilize advanced concepts of deformation of "composite" microstructures together with conventional strengthening mechanisms to provide a better combination of mechanical properties.

[0003]

Chemical Formula

[0004] The AHSS family mainly includes duplex (DP) steel, composite (CP) steel, transformation-induced plasticity (TRIP) steel, martensite (MS or MART) steel, and twinning-induced plasticity (TWIP) steel, which improve the balance between strength and formability by utilizing composite effects, separate from conventional strengthening mechanisms. The microstructure of DP steel consists of a mixture of ferrite and martensite, while composite steel may have a mixture of ferrite, martensite, bainite, and retained austenite. TRIP steel typically has a certain amount of retained austenite with different matrix phases such as ferrite, bainite, and martensite, which transform into martensite under the influence of stress (transformation-induced plasticity (TRIP) effect), improving the balance between strength and ductility. TWIP steel has a fully austenitic microstructure that generates twins when external stress is applied, thereby giving a better combination of strength and ductility.

[0005] Among the various types of steel mentioned above, TRIP steels are perhaps the most sought-after and studied. However, a minimum amount of Si (>1 wt%) must be used to retain a viable amount of retained austenite in the final microstructure and extract the advantages of high strength with a better combination of strength and elongation. Si is well known to cause problems during casting and degrade surface quality. These two factors have limited the applications of such steels. In this study, an alternative method is proposed that primarily utilizes composite effects to achieve a better combination of strength and elongation. This has led to the development of a new type of steel identified as MB steel, where MB means that the microstructure is a mixture of martensite and bainite.

[0006] This disclosure aims to overcome one or more of the limitations described above, or any other limitations related to the prior art. [Overview of the project] [Problems that the invention aims to solve]

[0007] The object of the present invention is to solve the problems of the prior art and to provide a new chemical composition for cold-rolled grade steel to obtain a microstructure mainly consisting of a mixture of martensite (M) phase and bainite (B) phase in the final microstructure. The developed steel is a new type of steel identified as MB steel.

[0008] Another object of the present invention is to develop a high-strength bendable steel grade having a tensile strength exceeding 1400 MPa and a tensile elongation value exceeding 7%, and having a moderate forming amount that can be made into steel strips, steel plates, and blanks.

[0009] Another objective of the present invention is to develop high-strength cold-rolled and annealed steel suitable for the manufacture of automobiles and structural components that require a moderate amount of forming.

[0010] Another object of the present invention is to provide a novel and easier manufacturing method that combines thermomechanical, hot rolling, cold rolling, and continuous annealing processes for proposed chemical compositions under controlled conditions. [Means for solving the problem]

[0011] This summary is provided to introduce the concept of a method for manufacturing high-strength cold-rolled annealed steel and high-strength cold-rolled annealed steel sheets or strips. The concept is further described below in a detailed description. This summary is not intended to identify any important or essential features of the claimed subject matter, nor is it intended to limit the scope of the claimed subject matter.

[0012] In one embodiment, a high-strength cold-rolled annealed steel is provided. The high-strength cold-rolled annealed steel has the following composition expressed in weight percent (wt%): carbon (C): 0.25-0.35%, manganese (Mn): 1.0-2.0%, silicon (Si): 0.02-0.5%, sulfur (S): <0.006%, phosphorus (P): <0.02%, aluminum (Al): 0.02-0.5%, molybdenum (Mo): <0.3%, nitrogen (N): <80 ppm, and the remainder being substantially iron and incidental impurities. The high-strength cold-rolled annealed steel has a microstructure of 10-30% bainite and 70-90% martensite.

[0013] In one embodiment, the high-strength cold-rolled annealed steel exhibits an ultimate tensile strength (UTS) in the range of 1400 to 1800 MPa, a yield strength (YS) of 1000 to 1300 MPa, a minimum uniform elongation of 4%, and a minimum total elongation of 7%.

[0014] In one embodiment, the carbon content in the high-strength cold-rolled annealed steel is optimally maintained to improve the hardenability and hardness of the martensite.

[0015] In one embodiment, the Mn content in high-strength cold-rolled annealed steel further improves hardenability and delays ferrite transformation.

[0016] In one embodiment, the Mo content improves the hardenability of the steel and also increases the rate of bainite formation during continuous annealing.

[0017] In another aspect of the present invention, a method is provided for producing a high-strength cold-rolled annealed steel sheet or strip having a thickness in the range of 0.5 mm to 2 mm. The method comprises casting molten steel having a composition expressed in weight percent: C: 0.25 to 0.35, Mn: 1.0 to 2.5, S: < 0.006, P: < 0.02, Si: 0.02 to 0.50, Al: 0.02 to 0.50, Mo: 0.00 to 0.30, N (ppm) < 80, with the remainder being iron (Fe) and unavoidable impurities, to obtain a steel slab. The method also comprises reheating the steel slab to a temperature in the range of 1200 to 1280°C. The method further comprises roughing the steel slab in a roughing mill having an exit temperature in the range of 1000 to 1080°C. The method comprises finishing rolling at a temperature (T) that changes in the range of 830°C to 880°C. FRT The method includes producing a steel sheet by hot rolling a roughly rolled steel slab, as performed in ( ). The method also includes obtaining a first intermediate hot-rolled steel by cooling the hot-rolled steel to a first intermediate temperature in the range of 640 to 680°C at a first intermediate cooling rate in the range of 0.5°C / sec to 50°C / sec. The method further includes winding the first intermediate hot-rolled steel at a winding temperature in the range of 640 to 680°C. The method includes pickling the first intermediate hot-rolled steel to remove scale from the surface of the steel strip. The method also includes cold rolling the steel strip for further thinning to obtain cold-rolled steel having a predetermined thickness. The method further includes obtaining heated cold-rolled steel by heating the cold-rolled steel from ambient temperature to a first holding temperature in the range of 800 to 900°C at a heating rate of 20°C / sec or less. This method involves holding heated cold-rolled steel in a furnace at a temperature range of 800 to 900°C for 30 to 300 seconds. The method also involves cooling to room temperature at a cooling rate in the range of 20°C / second to 60°C / second to obtain high-strength cold-rolled annealed steel sheet or high-strength cold-rolled annealed steel strip. The high-strength cold-rolled annealed steel sheet contains a microstructure of 10 to 30% bainite and 70 to 90% martensite, and the high-strength cold-rolled annealed steel exhibits an ultimate tensile strength in the range of 1400 to 1800 MPa.

[0018] In one embodiment, the high-strength cold-rolled annealed steel has a composition expressed in weight percent: C: 0.28~0.32, Mn: 1.4~1.5, Si: 0.0~0.2, Al: 0.3~0.5, Mo: 0.25~0.27, with the remainder being iron (Fe) and unavoidable impurities. The resulting high-strength cold-rolled annealed steel exhibits a yield strength (YS) of 1000~1260 MPa, an ultimate tensile strength (UTS) of 1630~1750 MPa, a minimum uniform elongation of 4~5.2%, a yield ratio (YS / UTS) of 0.72~0.75, and a minimum total elongation of 7~9%.

[0019] In one embodiment, the high-strength cold-rolled annealed steel has a composition expressed in weight percent: C: 0.28, Mn: 1.4, Si: 0.2, Al: 0.3, Mo: 0.27, with the remainder being iron (Fe) and unavoidable impurities. The resulting high-strength cold-rolled annealed steel exhibits a yield strength (YS) of 1188 MPa, an ultimate tensile strength (UTS) of 1640 MPa, a minimum uniform elongation of 5.2%, and a minimum total elongation of 8.6%.

[0020] In one embodiment, the high-strength cold-rolled annealed steel has a composition expressed in weight percent: C: 0.32, Mn: 1.5, Si: 0.2, Al: 0.3, Mo: 0.25, with the remainder being iron (Fe) and unavoidable impurities. The resulting high-strength cold-rolled annealed steel exhibits a yield strength (YS) of 1204 MPa, an ultimate tensile strength (UTS) of 1660 MPa, a minimum uniform elongation of 5.1%, and a minimum total elongation of 8.8%.

[0021] In one embodiment, the high-strength cold-rolled annealed steel has a composition expressed in weight percent: C: 0.28, Mn: 1.5, Si: 0.0, Al: 0.5, Mo: 0.25, with the remainder being iron (Fe) and unavoidable impurities. The resulting high-strength cold-rolled annealed steel exhibits a yield strength (YS) of 1147 MPa, an ultimate tensile strength (UTS) of 1600 MPa, a minimum uniform elongation of 5.1%, and a minimum total elongation of 7.3%.

[0022] In one embodiment, the high-strength cold-rolled annealed steel has a composition represented by weight %: C: 0.32, Mn: 1.5, Si: 0.0, Al: 0.5, Mo: 0.25, and the balance is iron (Fe) and unavoidable impurities. The obtained high-strength cold-rolled annealed steel exhibits a yield strength (YS) of 1210 MPa, an ultimate tensile strength (UTS) of 1660 MPa, a minimum uniform elongation of 5.2%, and a minimum total elongation of 7.7%.

[0023] In one embodiment, parts manufactured from the high-strength cold-rolled annealed steel are used for automotive applications.

[0024] Other features and aspects of the present disclosure will become apparent from the following description and the accompanying drawings.

Brief Description of the Drawings

[0025] [Figure 1] A flowchart of a method for manufacturing a high-strength cold-rolled annealed steel according to an embodiment of the present invention is shown.

[0026] [Figure 2a] A graph showing stress versus elongation obtained during a tensile test of a high-strength cold-rolled annealed steel having the composition of MB-1 according to an embodiment of the present invention is shown.

[0027] [Figure 2b] A graph showing stress versus elongation obtained during a tensile test of a high-strength cold-rolled annealed steel having the composition of MB-2 according to an embodiment of the present invention is shown.

[0028] [Figure 2c] A graph showing stress versus elongation obtained during a tensile test of a high-strength cold-rolled annealed steel having the composition of MB-3 according to an embodiment of the present invention is shown.

[0029] [Figure 2d] A graph showing stress versus elongation obtained during a tensile test of a high-strength cold-rolled annealed steel having the composition of MB-4 according to an embodiment of the present invention is shown.

[0030] [Figure 3a] An optical image of a high-strength cold-rolled annealed steel having the composition MB-2, according to one embodiment of the present invention, is shown.

[0031] [Figure 3b] The image shows a SEM image of a high-strength cold-rolled annealed steel having the composition MB-2 according to one embodiment of the present invention.

[0032] [Figure 3c] The image shows a SEM image of a high-strength cold-rolled annealed steel having the composition MB-4 according to one embodiment of the present invention. [Modes for carrying out the invention]

[0033] Unless otherwise specified, the drawings referenced herein should not be understood to be drawn to scale, and such drawings are essentially illustrative.

[0034] A detailed description of various exemplary embodiments of this disclosure is given herein with reference to the accompanying drawings. It should be noted that the embodiments are described herein in detail to clearly convey the disclosure. However, the amount of detail provided herein is not intended to limit the expected variations of the embodiments; rather, the intention is to cover all modifications, equivalents, and alternatives that fall within the spirit and scope of this disclosure as defined by the accompanying claims.

[0035] It should be understood that various configurations embodying the principles of this disclosure may be devised, even if not expressly described or shown herein. Furthermore, all descriptions herein describing the principles, aspects and embodiments of this disclosure, as well as specific examples, are intended to encompass their equivalents.

[0036] The terms used herein are for the purpose of describing specific embodiments and are not intended to limit the exemplary embodiments. Where used herein, the singular forms "a," "an," and "the" are intended to be inclusive of the plural forms unless the context clearly indicates otherwise. Where used herein, the terms "comprises," "comprising," "includes," and / or "including" identify the presence of the described features, integers, processes, operations, elements, and / or components, but do not exclude the presence or addition of one or more other features, integers, processes, operations, elements, components, and / or groups thereof.

[0037] In some alternative embodiments, the described functions / operations may be performed in a different order than that shown in the figures. For example, two figures shown consecutively may actually be performed simultaneously, or sometimes in reverse order, depending on the related functions / operations.

[0038] Unless otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as commonly understood by those skilled in the art in the field to which the exemplary embodiments belong. For example, terms defined in commonly used dictionaries should be interpreted as having the same meaning as their meanings in the context of the relevant art, and it will be further understood that they should not be interpreted in an idealized or overly formal sense unless expressly defined herein.

[0039] This disclosure provides high-strength cold-rolled annealed steel suitable for the manufacture of structural and safety components, including battery casings for automobiles. The high-strength cold-rolled annealed steel has the following composition, expressed in weight percent: carbon (C): 0.25-0.35%, manganese (Mn): 1.0-2.0%, silicon (Si): 0.02-0.5%, sulfur (S): <0.006%, phosphorus (P): <0.02%, aluminum (Al): 0.02-0.5%, molybdenum (Mo): <0.3%, nitrogen (N): <80 ppm, with the remainder being substantially iron and incidental impurities.

[0040] High-strength cold-rolled annealed steel has a structure that includes bainite and martensite phases. In one embodiment, the high-strength cold-rolled annealed steel has a microstructure containing 10-30% bainite and 70-90% martensite. The high-strength cold-rolled annealed steel exhibits an ultimate tensile strength (UTS) in the range of 1400-1800 MPa, a yield strength (YS) of 1000-1300 MPa, a minimum uniform elongation of 4%, and a minimum total elongation of 7%.

[0041] In one embodiment, the high-strength cold-rolled annealed steel contains C: 0.28-0.32, Mn: 1.4-1.5, Si: 0.0-0.2, Al: 0.3-0.5, Mo: 0.25-0.27, with the remainder being iron (Fe) and unavoidable impurities.

[0042] High-strength cold-rolled annealed steel exhibits a yield strength (YS) in the range of 1000-1260 MPa, an ultimate tensile strength (UTS) in the range of 1430-1750 MPa, an elongation in the range of 7-9%, a uniform elongation in the range of 4-5%, and a yield ratio (YS / UTS) in the range of 0.7-0.75.

[0043] Referring to Figure 1, an exemplary thermomechanical method (100) for producing high-strength cold-rolled annealed steel strip, high-strength cold-rolled annealed steel sheet, or blank is shown. Each step shown in Figure 1 represents one or more processes, methods, or subroutine steps performed in the method. Furthermore, the order of the blocks is illustrative only, and the blocks can be modified in accordance with the disclosure. Additional blocks can be added or fewer blocks can be used without departing from the disclosure. The method (100) for producing high-strength cold-rolled annealed steel strip, high-strength cold-rolled annealed steel sheet, or blank begins with step (102). In process (102), molten steel having a composition expressed in weight percent as follows: C: 0.25~0.35, Mn: 1.0~2.5, S: <0.006, P: <0.02, Si: 0.02~0.50, Al: 0.02~0.50, Mo: 0.00~0.30, N (ppm) < 80, with the remainder being iron (Fe) and unavoidable impurities, is cast in a casting apparatus to obtain a steel slab.

[0044] In step (104), the steel slab (cast ingot) is reheated in a furnace maintained at a temperature in the range of 1200 to 1280°C, more preferably in the range of 1200 to 1250°C. In a preferred embodiment, the steel slab is reheated to a temperature of 1200°C.

[0045] In step (106), the heated steel slab obtained in step (104) is rolled / deformed in a roughing mill having an outlet temperature in the range of 1000 to 1080°C. In a preferred embodiment, the outlet temperature is 1070°C.

[0046] In process (108), the roughly rolled steel slab obtained in process (106) is subjected to finish rolling at a temperature (T FRT The steel is subjected to a hot working process such as a hot rolling process, as performed in the ) process, to obtain a hot-rolled steel sheet or hot-rolled steel strip. FRTThe temperature varies in the range of 830°C to 880°C, which is higher than the Ar3 temperature of the steel. The hot rolling process may be carried out by passing the steel through a pair of rolls, and the rolling may be carried out at least five times to reduce the thickness of the steel to the required size in the range of 2 to 6 mm. In one embodiment, the hot rolling process is carried out by passing the steel through a pair of rolls, and the rolling is carried out at least five times.

[0047] In step (110), the hot-rolled steel sheet or hot-rolled steel strip obtained in step (108) is cooled to a first intermediate temperature in the range of 640 to 680°C at a first intermediate cooling rate in the range of 0.5°C / sec to 50°C / sec to obtain a first intermediate hot-rolled steel sheet or a first intermediate hot-rolled steel strip.

[0048] In step (112), the first intermediate hot-rolled steel sheet or strip is then wound up at 640-680°C. In step (114), the first intermediate hot-rolled steel is pickled to remove scale from the surface of the sheet or strip.

[0049] In step (116), the pickled steel sheet or strip is cold-rolled to further reduce its thickness. The pickled hot-rolled strip is fed into an industrial cold-rolling mill, which may be either a multi-stand tandem rolling mill or a single-stand / two-stand reverse rolling mill. A state-of-the-art pickling facility connected to a multi-stand tandem cold-rolling mill, along with its usual accessories such as an inlet weld section and looper sections at different locations (inlet, after pickling, etc.), is the most desirable option. In a preferred embodiment, the cold-rolling reduction is in the range of 50-60%.

[0050] In step (118), the cold-rolled steel sheet or cold-rolled strip is heated at a heating rate of 20°C / second or less to a first holding temperature in the range of 800 to 900°C from the ambient temperature to obtain a heated cold-rolled steel sheet or cold-rolled strip.

[0051] In step (120), the heated cold-rolled steel is held in the furnace for 30 to 300 seconds at a first holding temperature in the range of 800 to 900°C to obtain immersed cold-rolled steel.

[0052] In step (122), the immersed cold-rolled steel sheet or strip is cooled at a cooling rate in the range of 20°C / sec to 60°C / sec to obtain a high-strength cold-rolled annealed steel sheet or strip having a thickness in the range of 0.5 mm to 2 mm. The obtained high-strength cold-rolled annealed steel sheet or strip contains a microstructure of 10-30% bainite and 70-90% martensite. The high-strength cold-rolled annealed steel exhibits an ultimate tensile strength in the range of 1400-1800 MPa.

[0053] The desired grain size of the hot zone is achieved by the pancake formation of austenite during hot rolling, which is due to substantial deformation of austenite below the Tnr temperature. The hot deformation schedule is T nr A considerable amount of deformation (approximately 50%) should be ensured below. Immediately after finish rolling, the temperature should drop rapidly to limit grain growth. Finally, the hot-rolled strip can be wound at a moderate winding temperature without any special treatment.

[0054] Cold rolling deformation should be within the range of 50-60%, depending on the final thickness requirements. Cold-rolled steel is preferably subjected to annealing using continuous annealing equipment because the formation of the transformation phase requires a faster cooling rate, which is difficult to achieve with batch annealing equipment using dilute chemical compositions.

[0055] The annealing process should primarily include the following steps:

[0056] After holding at an intermediate annealing temperature (first holding temperature), the steel should be rapidly cooled to transform the newly formed austenite into a hard phase such as bainite or martensite. The steel is first rapidly cooled to a temperature higher than the martensitic transformation temperature but lower than the bainite transformation temperature. Holding at this temperature for a certain period facilitates the transformation of austenite into the bainite phase. This may be a partial transformation, and subsequent rapid cooling to room temperature ensures the transformation of retained austenite into martensite.

[0057] The method (100) of the present disclosure includes a simple melting, casting, heat treatment, thermomechanical hot rolling route, cold rolling, and annealing. The slab dropout temperature, finish rolling temperature, coiling temperature, annealing temperature, holding temperature, and holding time all have a significant effect on the final mechanical properties.

[0058] The following portions of this disclosure provide details regarding the proportion of each element in the composition of high-strength cold-rolled annealed steel and their roles in improving its properties.

[0059] Carbon (C) can be used in amounts ranging from approximately 0.25 to 0.35 wt% (weight percent). Carbon is the cheapest and most effective element for improving the hardenability and hardness of martensite.

[0060] Manganese (Mn): 1.0~2.0 wt%: Manganese was selected to further improve hardenability and delay ferrite transformation. As a result, the bainite and martensite regions are exposed in the continuous cooling diagram.

[0061] Si + Al: 0~1 wt%: Silicon and aluminum are effective elements that delay carbide precipitation. This ensures that a higher level of carbon remains in the martensite solid solution, resulting in higher strength.

[0062] Molybdenum (Mo): 0.1~0.30 wt%: Molybdenum was selected to allow bainite to form on a larger processing window during cooling.

[0063] Sulfur (S) may be used in a range of less than approximately 0.005 wt%, and phosphorus (P) may be used in a range of less than approximately <0.03 wt%: the amounts of sulfur and phosphorus should be kept as low as possible. Sulfur content must be limited, otherwise it will result in very high inclusion levels and reduced formability. Since higher phosphorus levels can lead to reduced toughness and weldability due to segregation of P at grain boundaries, phosphorus content should be limited to a maximum of 0.03%.

[0064] Nitrogen (N) can be used in concentrations of less than 0.008%. Too high a N content will make the steel brittle.

[0065] <Fine structure>

[0066] The phases present in the final microstructure and their attributes are crucial factors in determining the final mechanical properties of steel. The target microstructure after continuous annealing was a mixture of martensite and bainite. As bainite forms, some carbon is released from the bainite into the remaining austenite. The carbon-rich remaining austenite then transforms into martensite upon further cooling to room temperature. With martensite being the hard phase, stress is placed on the bainite, which interacts with external stresses (e.g., during uniaxial tensile testing), effectively giving an excellent combination of strength and ductility. [Examples]

[0067] <Example>

[0068] Next, further embodiments of the present disclosure will be described using examples of specific steel compositions. Experiments were conducted using the methods of the present disclosure on steels with compositional values ​​different from those described above. By comparing the results in various aspects, improvements in steel strength were demonstrated.

[0069] The chemical composition of the proposed alloy is shown in Table 1. [Table 1]

[0070] To investigate the properties of the steel, experiments were conducted on specific compositions reported in Table 1. Tensile test specimens were prepared according to ASTM E8 specifications applicable to specimens with a thickness of 1–1.5 mm and gauge lengths of 25 mm (thermal 1 and thermal 2) and 50 mm (thermal 3 and thermal 4). SEM samples were prepared according to standard methods.

[0071] During method (100), different samples of high-strength cold-rolled annealed steel were prepared using the compositions listed in Table 1. Other parameters were kept constant (reheating 1200°C, finish rolling 850°C, first intermediate temperature 640°C, coiling temperature 640°C, and cold rolling reduction ratio 50-60%).

[0072] Samples were prepared for different mechanical property measurements according to standard techniques. The measured properties of the steel samples are shown in Table 2. [Table 2]

[0073] Referring here to Figures 2a to 2d, which are exemplary embodiments of this disclosure, graphs are shown using stress-to-elongation plots obtained during tensile testing of steel specimens according to Table 1 above. Tensile testing can be performed using standard tensile test specimens. For example, the test specimen may have a gauge length of 25 mm and an E8 / E8M-09 configuration according to ASTM standards. As is clear from the graph shown in Figure 2a, the high-strength cold-rolled annealed steel having composition MB-1 exhibits a yield strength (YS) of 1188 MPa, an ultimate tensile strength (UTS) of 1640 MPa, a minimum uniform elongation of 5.2%, and a minimum total elongation of 8.6%. Referring here to Figure 2b, the steel having composition MB-2 exhibits a yield strength (YS) of 1204 MPa, an ultimate tensile strength (UTS) of 1660 MPa, a minimum uniform elongation of 5.1%, and a minimum total elongation of 8.8%. Referring to Figure 2c, the steel with composition MB-3, which does not contain Si, exhibits a yield strength (YS) of 1147 MPa, an ultimate tensile strength (UTS) of 1600 MPa, a minimum uniform elongation of 5.1%, and a minimum total elongation of 7.3%. Referring to Figure 2d, the steel with composition MB-4, which does not contain Si, exhibits a yield strength (YS) of 1210 MPa, an ultimate tensile strength (UTS) of 1660 MPa, a minimum uniform elongation of 5.2%, and a minimum total elongation of 7.7%.

[0074] From Table 2 and Figures 2a-2d, it is important to note that strengths exceeding 1500 MPa are achieved in combination with a total elongation exceeding 7%. More importantly, parts made from the developed steel exhibit a remarkable uniform elongation, which provides better safety.

[0075] Figure 3a shows an optical image of the high-strength cold-rolled annealed steel having the MB-2 composition, and Figure 3b shows an SEM image of the obtained high-strength cold-rolled annealed steel having the MB-2 composition reported in Table 1. Figure 3c shows an SEM image of the obtained high-strength cold-rolled annealed steel having the MB-4 composition reported in Table 1. From Figures 3b and 3c, it can be observed that the developed steel exhibits a microstructure containing bainite and martensite. The main strength and ductility of the steel originate from the phase mixture of bainite and martensite.

[0076] The present invention provides high-strength cold-rolled annealed steel and a method (100) for producing even higher-strength cold-rolled annealed steel by adding the minimum necessary alloying elements. High-strength cold-rolled annealed steel is a martensite and bainite steel that makes a significant contribution to cost-effective, futuristic, and strategic lightweight applications of steel with a greater safety factor. High-strength cold-rolled annealed steel with high strength and excellent tensile ductility at room temperature is particularly required for automotive and structural applications. The thermomechanical / hot rolling process is very simple and does not require significant energy consumption. Therefore, the method (100) of this disclosure supports a steel manufacturing process that reduces energy consumption and is therefore cost-effective. Furthermore, the method (100) provides a new steel developed by a more lean chemistry to achieve the required properties by utilizing existing hot-rolling equipment in an integrated steel plant to obtain the desired microstructure.

[0077] Please understand that experiments were conducted on specific compositions of high-strength cold-rolled annealed steel reported in Table 1, and the results obtained are reported in Table 2. However, this composition can similarly be extended to other compositions of low-carbon cold-rolled composite phase steel strips, and should not be construed as a limitation to this disclosure.

[0078] Furthermore, the terms used herein are for illustrative purposes only and are not intended to limit the present disclosure. It will be understood that some of the features and other functions disclosed above, or their substitutes, may be combined with other systems or applications. Those skilled in the art may make various currently unforeseen or unexpected substitutions, modifications, variations, or improvements without departing from the scope of this disclosure as encompassed in the following claims.

[0079] The claims, as initially presented and as they may be modified, encompass variations, substitutions, modifications, improvements, equivalents, and substantial equivalents of the embodiments and teachings disclosed herein, including those not currently anticipated or recognized, which may arise, for example, from the applicant / patentee.

[0080] Although various embodiments of the present invention have been described above, other further embodiments of the present invention can be devised without departing from the basic scope of the present invention. The scope of the present invention is determined by the following claims. The present invention is not limited to the embodiments, versions, or examples described herein, which are included to enable those skilled in the art to construct and use the present invention in combination with information and knowledge available to those skilled in the art.

Claims

1. A high-strength cold-rolled annealed steel having the following composition expressed in weight percent: Carbon (C): 0.25-0.35%, Manganese (Mn): 1.0% to 2.0% Silicon (Si): 0.02% to 0.5% Sulfur (S): <0.006% Phosphorus (P): <0.02% Aluminum (Al): 0.02-0.5%, Molybdenum (Mo): <0.3% High-strength cold-rolled and annealed steel containing nitrogen (N) < 80 ppm, with the remainder being substantially iron and associated impurities, and containing a microstructure of 10-30% bainite and 70-90% martensite.

2. The high-strength cold-rolled annealed steel according to claim 1, exhibiting an ultimate tensile strength (UTS) in the range of 1400 to 1800 MPa, a yield strength (YS) of 1000 to 1300 MPa, a minimum uniform elongation of 4%, and a minimum total elongation of 7%.

3. The high-strength cold-rolled annealed steel according to claim 1, wherein the carbon content in the high-strength cold-rolled annealed steel is optimally maintained to improve the hardenability and hardness of martensite.

4. The high-strength cold-rolled annealed steel according to claim 1, wherein the Mn content in the high-strength cold-rolled annealed steel further improves hardenability and delays ferrite transformation.

5. The high-strength cold-rolled annealed steel according to claim 1, wherein the Mo content improves the hardenability of the steel and increases the rate of bainite formation during continuous annealing.

6. A method (100) for producing a high-strength cold-rolled annealed steel sheet or high-strength cold-rolled annealed steel strip having a thickness in the range of 0.5 mm to 2 mm, A process of casting molten steel to obtain a steel slab, having a composition expressed in weight percent: C: 0.25-0.35, Mn: 1.0-2.5, S: <0.006, P: <0.02, Si: 0.02-0.50, Al: 0.02-0.50, Mo: 0.00-0.30, N (ppm) < 80, with the remainder being iron (Fe) and unavoidable impurities; The process involves reheating the steel slab to a temperature in the range of 1200 to 1280°C; A process of roughly processing a steel slab in a rough rolling mill having an outlet temperature in the range of 1000 to 1080°C; A roughly rolled steel slab is hot-rolled, and the finish rolling is performed at a temperature range of 830°C to 880°C (T ＦＲＴ The process of manufacturing steel plates as is done in the following way; A step of obtaining first intermediate hot-rolled steel by cooling hot-rolled steel to a first intermediate temperature in the range of 640 to 680°C at a first intermediate cooling rate in the range of 0.5°C / sec to 50°C / sec; The first step involves winding the intermediate hot-rolled steel at a winding temperature in the range of 640 to 680°C; The first step involves pickling the intermediate hot-rolled steel to remove scale from the surface of the steel strip; A process of further thinning a steel strip by cold rolling it to obtain cold-rolled steel having a predetermined thickness; A step of obtaining heated cold-rolled steel by heating cold-rolled steel from ambient temperature to a first holding temperature in the range of 800 to 900°C at a heating rate of 20°C / second or less; A process of holding heated cold-rolled steel in a furnace at a temperature range of 800 to 900°C for 30 to 300 seconds; A method (100) comprising the steps of: cooling to room temperature at a cooling rate in the range of 20°C / sec to 60°C / sec to obtain a high-strength cold-rolled annealed steel sheet or high-strength cold-rolled annealed steel strip, wherein the high-strength cold-rolled annealed steel sheet contains a microstructure of 10-30% bainite and 70-90% martensite, and the high-strength cold-rolled annealed steel exhibits an ultimate tensile strength in the range of 1400-1800 MPa.

7. A method (100) for producing a high-strength cold-rolled annealed steel sheet or strip having a thickness in the range of 0.5 mm to 2 mm according to claim 6, wherein the high-strength cold-rolled annealed steel has a composition expressed in weight percent: C: 0.28 to 0.32, Mn: 1.4 to 1.5, Si: 0.0 to 0.2, Al: 0.3 to 0.5, Mo: 0.25 to 0.27, with the remainder being iron (Fe) and unavoidable impurities, and the obtained high-strength cold-rolled annealed steel exhibits a yield strength (YS) of 1000 to 1260 MPa, an ultimate tensile strength (UTS) of 1630 to 1750 MPa, a minimum uniform elongation of 4 to 5.2%, a yield ratio (YS / UTS) of 0.72 to 0.75, and a minimum total elongation of 7 to 9%.

8. A method (100) for producing a high-strength cold-rolled annealed steel sheet or strip having a thickness in the range of 0.5 mm to 2 mm according to claim 7, wherein the high-strength cold-rolled annealed steel has a composition expressed in weight percent: C: 0.28, Mn: 1.4, Si: 0.2, Al: 0.3, Mo: 0.27, with the remainder being iron (Fe) and unavoidable impurities, and the resulting high-strength cold-rolled annealed steel exhibits a yield strength (YS) of 1188 MPa, an ultimate tensile strength (UTS) of 1640 MPa, a minimum uniform elongation of 5.2%, and a minimum total elongation of 8.6%.

9. A method (100) for producing a high-strength cold-rolled annealed steel sheet or strip having a thickness in the range of 0.5 mm to 2 mm according to claim 7, wherein the high-strength cold-rolled annealed steel has a composition expressed in weight percent: C: 0.32, Mn: 1.5, Si: 0.2, Al: 0.3, Mo: 0.25, with the remainder being iron (Fe) and unavoidable impurities, and the resulting high-strength cold-rolled annealed steel exhibits a yield strength (YS) of 1204 MPa, an ultimate tensile strength (UTS) of 1660 MPa, a minimum uniform elongation of 5.1%, and a minimum total elongation of 8.8%.

10. A method (100) for producing a high-strength cold-rolled annealed steel sheet or strip having a thickness in the range of 0.5 mm to 2 mm according to claim 7, wherein the high-strength cold-rolled annealed steel has a composition expressed in weight percent: C: 0.28, Mn: 1.5, Si: 0.0, Al: 0.5, Mo: 0.25, with the remainder being iron (Fe) and unavoidable impurities, and the resulting high-strength cold-rolled annealed steel exhibits a yield strength (YS) of 1147 MPa, an ultimate tensile strength (UTS) of 1600 MPa, a minimum uniform elongation of 5.1%, and a minimum total elongation of 7.3%.

11. A method (100) for producing a high-strength cold-rolled annealed steel sheet or strip having a thickness in the range of 0.5 mm to 2 mm according to claim 7, wherein the high-strength cold-rolled annealed steel has a composition expressed in weight percent: C: 0.32, Mn: 1.5, Si: 0.0, Al: 0.5, Mo: 0.25, with the remainder being iron (Fe) and unavoidable impurities, and the resulting high-strength cold-rolled annealed steel exhibits a yield strength (YS) of 1210 MPa, an ultimate tensile strength (UTS) of 1660 MPa, a minimum uniform elongation of 5.2%, and a minimum total elongation of 7.7%.

12. A part manufactured from high-strength cold-rolled annealed steel according to any one of claims 1 to 11, for use in automotive applications.

Images