Low specific gravity composite steel sheet having excellent formability and fatigue characteristics and method for manufacturing the same

Abstract

One aspect of the present invention relates to a low-specification composite steel sheet with excellent formability and fatigue properties, characterized in that the composite steel sheet comprises a base material and a cladding material disposed on two sides of the base material. The base material, by weight%, is a lightweight steel sheet comprising C: 0.3-1.0%, Mn: 4.0-16.0%, Al: 4.5-9.0%, with the balance being Fe and unavoidable impurities. The cladding material, by weight%, is a martensitic carbon steel comprising C: 0.1-0.45%, Mn: 1.0-3.0%, with the balance being Fe and unavoidable impurities.

Description

Cross-reference to related applications

[0001] This application is a divisional application of Chinese Patent Application No. 201980045390.5, filed on July 8, 2019, entitled "Low-Specific-Compound Composite Steel Plate with Excellent Formability and Fatigue Properties". This application claims priority to KR10-2018-0087721 (July 27, 2018). Technical Field

[0002] This invention relates to a low-specific-weight composite steel sheet with excellent formability and fatigue properties, which can be used in chassis structural components of automobiles, and a method for manufacturing the same. Background Technology

[0003] In recent years, due to carbon dioxide regulations aimed at reducing global warming, there has been a strong demand for lightweight automobiles. At the same time, in order to improve passenger safety, there is an ongoing trend towards ultra-high strength steel sheets for automobiles.

[0004] In automobiles, chassis components such as lower control arms and subframes are located at the lower end of the vehicle's center of gravity, making them particularly effective for reducing fuel costs through weight reduction. To maximize this weight reduction, lightweight materials like aluminum can be considered. However, aluminum and similar materials have poor formability and weldability, requiring casting for manufacturing. This results in high manufacturing costs and low strength, potentially compromising passenger safety.

[0005] Furthermore, chassis components require excellent fatigue properties to prevent fatigue-induced damage during driving. The fatigue properties of steel typically increase proportionally with its yield strength; therefore, high yield strength combined with excellent formability is required for cold pressing. Typically, low-temperature phase transformation structures are used to produce steel sheets for automotive chassis components; however, this presents a challenge in simultaneously ensuring high levels of strength and formability suitable for cold pressing.

[0006] In the case of structural components used in machinery, the following method is usually used: after forming, high-frequency quenching or surface flame treatment is performed to form a local martensitic layer with high yield strength only on the surface, thereby significantly improving fatigue performance. However, automotive parts are characterized by thin thickness and complex shape, so there is a problem that it is practically difficult to apply the above surface hardening treatment to automotive structural components.

[0007] Furthermore, Korean Patent Publication No. 2012-0065464 discloses a lightweight steel that, by adding large amounts of manganese and aluminum to carbon steel, achieves an austenite content of over 90% in its microstructure, resulting in excellent strength and formability while maintaining a low specific gravity, thus exhibiting superior lightweighting effects. However, existing lightweight steels, including the steel described in Korean Patent Publication No. 2012-0065464, only consider tensile strength, elongation, and low specific gravity, without addressing the improvement of fatigue characteristics for automotive components subjected to long-term stress concentration, thereby ensuring vehicle stability.

[0008] Fatigue failure of chassis components has the disadvantage of being difficult to confirm during use, and when it occurs during driving, it has a very serious impact on passenger safety. Therefore, a higher safety factor and conservative application are necessary, ideally designed to be below the fatigue limit in the high-cycle fatigue mode of automotive structural components. Therefore, if chassis components are lightweighted using materials with high fatigue limits and low specific gravity, a very significant reduction in fuel costs can be expected.

[0009] Therefore, in order to maximize the lightweighting of chassis components, it is necessary to develop an automotive steel with excellent formability and fatigue properties and low specific gravity. Summary of the Invention

[0010] (a) Technical problems to be solved One aspect of the present invention is to provide a composite steel plate with excellent formability, low specific gravity and excellent fatigue properties, and a method for manufacturing the same.

[0011] Furthermore, the technical problem of the present invention is not limited to the above description. The technical problem of the present invention can be understood from the entire contents of this specification, and those skilled in the art will have no difficulty in understanding the additional technical problems of the present invention.

[0012] (II) Technical Solution One aspect of the present invention relates to a low-specification composite steel sheet with excellent formability and fatigue properties, characterized in that the composite steel sheet comprises a base material and a cladding material disposed on two sides of the base material. The base material, by weight%, is a lightweight steel sheet comprising C: 0.3-1.0%, Mn: 4.0-16.0%, Al: 4.5-9.0%, with the balance being Fe and unavoidable impurities. The cladding material, by weight%, is a martensitic carbon steel comprising C: 0.1-0.45%, Mn: 1.0-3.0%, with the balance being Fe and unavoidable impurities.

[0013] Another aspect of the invention relates to a method for manufacturing a low-specification composite steel sheet with excellent formability and fatigue properties, comprising the steps of: preparing a base material, by weight%, said base material being a lightweight steel sheet comprising C: 0.3-1.0%, Mn: 4.0-16.0%, Al: 4.5-9.0%, with the balance being Fe and unavoidable impurities; preparing a cladding material, by weight%, said cladding material being a martensitic carbon steel comprising C: 0.1-0.45%, Mn: 1.0-3.0%, with the balance being Fe and unavoidable impurities; and arranging said base material between two said cladding materials to obtain The laminate is prepared by welding the edges of the laminate and then heating it to a temperature range of 1050-1350°C; the reduction rate of the first pass is set to 30% or more, and the heated laminate is finished rolled at a temperature range of 750-1050°C to obtain a hot-rolled steel sheet; the hot-rolled steel sheet is coiled at 400-700°C; the coiled hot-rolled steel sheet is pickled and then cold-rolled at a cold rolling reduction rate of 35-90% to obtain a cold-rolled steel sheet; and the cold-rolled steel sheet is annealed at a temperature range of 550°C or higher and below A3+200°C of the cladding material.

[0014] Furthermore, the above technical solutions do not list all the features of the present invention. A more detailed understanding of the various features, advantages, and effects of the present invention can be obtained by referring to the following specific embodiments.

[0015] (III) Beneficial Effects According to the present invention, a composite steel plate and a method for manufacturing the same are provided, wherein the composite steel plate has a content of 7.4 g / cm³. 3 With its specific gravity and fatigue strength of over 500 MPa, the composite steel sheet is preferably used for automotive chassis components, and its elongation of over 25% provides excellent elongation, making it suitable for cold pressing. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the composite steel plate of the present invention, which uses lightweight steel plate as base material (B) and martensitic carbon steel as cladding material (A and C).

[0017] Figure 2 This is an optical microscope photograph showing the fine structure of the boundary between the base material and the coating material in Example 1 of the invention.

[0018] Figure 3 The image shown is a scanning electron microscope photograph of Example 1 of the invention, which shows the elemental distribution at the boundary between the base material and the coating material.

[0019] Figure 4The image shows a scanning electron microscope photograph of the total thickness of Example 1 of the invention and a graph showing the distribution of microhardness measured at each thickness location.

[0020] Figure 5 These are photographs of the appearance of Comparative Example 5 after hot rolling.

[0021] Figure 6 The graph shows the fatigue test results of Comparative Example 1, Comparative Example 2 and Invention Example 1. Best practice

[0022] The preferred embodiments of the present invention will now be described. However, the embodiments of the present invention can be modified into various other embodiments, and the scope of the present invention is not limited to the embodiments described below. Furthermore, the embodiments of the present invention are provided to provide a more complete explanation of the present invention to those skilled in the art.

[0023] The inventors recognized that while existing lightweight steel plates can produce steel with high tensile strength and low specific gravity, they have low yield strength and therefore poor fatigue performance. In order to solve this problem, the inventors conducted in-depth research.

[0024] Fatigue failure leads to final fracture through the initiation and propagation of fatigue cracks. It is known that the fatigue crack initiation stage accounts for approximately 70% of the total fatigue life. Therefore, to improve fatigue performance, it is crucial to effectively prevent fatigue crack initiation. While fatigue cracks are known to occur on surfaces, within inclusions, and at grain boundaries, in practical components, fatigue cracks primarily occur on the surface in most cases. This is because the stress applied in the bending fatigue mode used in actual operating environments is highest at the surface. Therefore, this invention confirms that by manufacturing a composite steel sheet with an external arrangement of martensitic steel (which has high resistance to fatigue crack initiation due to its high yield strength) and a lightweight steel sheet with excellent formability and low specific gravity as the base material, it is possible to achieve excellent formability and low specific gravity, and to manufacture automotive steel sheets with excellent fatigue properties.

[0025] The following is a detailed description of a low-repetition-weight composite steel sheet with excellent formability and fatigue properties, representing one aspect of the present invention.

[0026] A feature of the low-component composite steel sheet with excellent formability and fatigue properties according to one aspect of the present invention is that the composite steel sheet comprises a base material and a cladding material disposed on two sides of the base material. The base material is a lightweight steel sheet containing C: 0.3-1.0%, Mn: 4.0-16.0%, Al: 4.5-9.0%, with the balance being Fe and unavoidable impurities, by weight%. The cladding material is a martensitic carbon steel containing C: 0.1-0.45%, Mn: 1.0-3.0%, with the balance being Fe and unavoidable impurities, by weight.

[0027] Hereinafter, the base material and the cladding material of the present invention will be described separately, and then the composite steel plate including the cladding material arranged on both sides of the base material will be described.

[0028] Base material (lightweight steel plate) The alloy composition of the lightweight steel sheet, the base material of the composite steel sheet constituting one aspect of the present invention, will be described in detail below. Unless otherwise specified, the unit of each element content is by weight%.

[0029] Carbon (C): 0.3-1.0% Carbon is an element that helps stabilize the austenitic phase; the higher the carbon content, the better the austenitic phase is preserved. Austenite distributed in the fine microstructure of lightweight steel sheets plays a role in simultaneously increasing strength and elongation. When the carbon content is less than 0.3%, it becomes difficult to ensure tensile strength and elongation. Furthermore, when the carbon content exceeds 1.0%, cementite and carbamate carbides form in the steel, thereby increasing strength but significantly reducing the steel's ductility. In particular, in steels with added aluminum, carbamate carbides precipitate at grain boundaries, causing brittleness; therefore, the upper limit of the carbon content is preferably set at 1.0%. Therefore, in this invention, it is preferable to limit the carbon content to 0.3-1.0%.

[0030] Manganese (Mn): 4.0-16.0% Manganese, along with carbon, is an element that stabilizes the austenite phase and inhibits carbide formation by increasing the solid solubility of carbon in the austenite phase. Furthermore, manganese reduces the density of steel by increasing its lattice constant, thus reducing the specific gravity of the steel. When the manganese content is less than 4.0%, it is difficult to expect an effect in inhibiting carbide formation. On the other hand, when the manganese content exceeds 16.0%, banded structures are formed due to center segregation, resulting in reduced ductility. Therefore, in this invention, it is preferable to limit the manganese content to 4.0-16.0%.

[0031] Aluminum (Al): 4.5-9.0% In this invention, aluminum is the most important element in reducing the specific gravity of steel. Therefore, it is preferable to add 4.5% or more of aluminum. While adding a large amount of aluminum is preferred to reduce specific gravity, adding a large amount of aluminum increases carbamate or intermetallic compounds such as FeAl and Fe3Al, leading to a decrease in the ductility of the steel. Therefore, it is preferable to limit the upper limit of the aluminum content to 9.0%. Thus, in this invention, it is preferable to limit the aluminum content to 4.5-9.0%.

[0032] The remaining component of the base material is iron (Fe). However, undesirable impurities may inevitably be introduced from the raw materials or the surrounding environment during normal manufacturing processes, and therefore these impurities cannot be excluded. These impurities are well known to those skilled in the art in the ordinary course of manufacturing, and therefore their contents are not specifically mentioned in this specification.

[0033] In addition to the above-mentioned components, the lightweight steel sheet constituting the base material may further contain, by weight %, Si: 0.03-2.0%, Ni: 0.1-4.0%, N: less than 0.04% (excluding 0%), P: less than 0.03% and S: less than 0.03%.

[0034] Silicon (Si): 0.03-2.0% Silicon is a component that can be added to improve the yield strength and tensile strength of steel through solid solution strengthening. Silicon is used as a deoxidizer, and therefore steel typically contains more than 0.03% silicon. When the silicon content exceeds 2.0%, a large amount of silicon oxide forms on the surface during hot rolling, thereby reducing pickling properties and increasing resistivity, thus leading to poor weldability. Therefore, in this invention, the silicon content is preferably limited to 0.03-2.0%.

[0035] Nickel (Ni): 0.1-4.0% Like manganese, nickel increases strength and ductility by improving the stability of austenite. Therefore, it can be added together with manganese to increase the strength and ductility of steel. However, adding large amounts of nickel increases the manufacturing cost of the steel; therefore, the nickel content is preferably 4.0% or less. Furthermore, when less than 0.1% of nickel is added, the effect on increasing strength and ductility is not significant; therefore, in this invention, the nickel content is preferably limited to 0.1-4.0%.

[0036] Nitrogen (N): less than 0.04% (except 0%) Nitrogen is an unavoidable impurity, and it reacts with aluminum to precipitate fine nitrides, thus reducing the workability of steel. Therefore, it is preferable to control the nitrogen content as low as possible. Theoretically, it is preferable to control the nitrogen content as low as possible, but nitrogen will inevitably be present in the manufacturing process. Therefore, it is important to control the upper limit of the nitrogen content; in this invention, the nitrogen content is controlled to be below 0.04%.

[0037] Phosphorus (P): less than 0.03% Phosphorus is an unavoidable impurity, and it is a major cause of reduced workability in steel due to segregation. Therefore, it is preferable to control the phosphorus content as low as possible. Theoretically, limiting the phosphorus content to 0% is advantageous, but phosphorus is inevitably present in the manufacturing process. Therefore, it is important to control the upper limit of the phosphorus content, which is controlled at 0.03% in this invention.

[0038] Sulfur (S): less than 0.03% Sulfur is an unavoidable impurity, and it forms coarse manganese sulfide (MnS), causing defects such as flange cracks and significantly reducing the porosity of the steel sheet. Therefore, it is preferable to control the sulfur content as low as possible. Theoretically, limiting the sulfur content to 0% is advantageous, but sulfur is inevitably present in the manufacturing process. Therefore, it is important to control the upper limit of the sulfur content; in this invention, the upper limit of the sulfur content is controlled at 0.03%.

[0039] Furthermore, the lightweight steel sheet constituting the base material in this invention not only satisfies the above-mentioned composition system, but also preferably contains more than 10% retained austenite in its microstructure by area fraction. Retained austenite induces plastic phase transformation or twinning during deformation, thereby giving the steel excellent formability. Therefore, when the content of retained austenite is less than 10% by area fraction, an elongation of more than 25% cannot be guaranteed. Furthermore, the characteristic of the austenite fraction is that formability becomes excellent with increasing austenite content; therefore, there is no upper limit to the austenite fraction. In this invention, by ensuring the microstructure described above, strength, low specific gravity, and elongation can be simultaneously ensured.

[0040] Coating material (martensitic carbon steel) The following is a detailed description of the alloy composition of the martensitic carbon steel used as the cladding material in the composite steel plate constituting one aspect of the present invention. Unless otherwise specified, the content of each element is expressed in weight%.

[0041] Carbon (C): 0.1-0.45% Carbon is an element that increases the hardenability of steel and facilitates the formation of martensitic structures. Furthermore, carbon resides in interstitial positions within martensite and enhances the strength of steel through solid solution strengthening. When the carbon content is less than 0.1%, martensitic transformation begins at high temperatures, and carbon diffuses into dislocations during cooling, thus preventing the expectation of increased steel strength through solid solution strengthening. On the other hand, when the carbon content exceeds 0.45%, the weldability of the steel sheet may decrease. Therefore, the carbon content described in this invention is preferably limited to 0.1-0.45%.

[0042] Manganese (Mn): 1.0-3.0% Manganese is an element that increases the strength of steel sheets by improving hardenability. To achieve this effect, the manganese content is preferably 1.0% or more. On the other hand, when the manganese content exceeds 3.0%, the formability of the steel sheet may be reduced due to the segregated layer structure. Therefore, the manganese content in this invention is preferably limited to 1.0-3.0%.

[0043] The remaining component of the coating material is iron (Fe). However, undesirable impurities may inevitably be introduced from raw materials or the surrounding environment during normal manufacturing processes, and therefore these impurities cannot be excluded. These impurities are well known to those skilled in the art in the ordinary course of manufacturing, and therefore their contents are not specifically mentioned in this specification.

[0044] In addition to the above components, the martensitic carbon steel constituting the cladding material may further include, by weight%, the following: Si: 0.03-2.0%, Al: 0.02-0.3%, N: less than 0.04% (excluding 0%), B: 0.0005-0.005%, P: less than 0.03%, and S: less than 0.03%.

[0045] Silicon (Si): 0.03-2.0% Silicon, dissolved in steel, enhances the steel's strength. Silicon is an element that exists as an impurity in molten steel. Controlling the silicon content to less than 0.03% results in excessively high costs. When the silicon content exceeds 2.0%, surface oxides form during annealing, thus deteriorating the surface quality of the steel plate. Therefore, the preferred silicon content is 0.03-2.0%.

[0046] Aluminum (Al): 0.02-0.3% Aluminum is typically added for deoxidation, but controlling its content to less than 0.02% results in excessively high costs. Furthermore, aluminum raises the martensitic transformation initiation temperature, thus reducing the hardenability of the steel. Additionally, when the aluminum content exceeds 0.3%, surface oxides form during annealing, deteriorating the surface quality of the steel sheet. Therefore, the preferred aluminum content is 0.02-0.3%.

[0047] Nitrogen (N): less than 0.04% (except 0%) Nitrogen is an unavoidable element, and the aluminum nitride (AlN) formed by the reaction of nitrogen with residual aluminum in steel may cause surface cracks during continuous casting. Therefore, it is preferable to control the nitrogen content as low as possible, but nitrogen is inevitably present in the manufacturing process. It is important to control the upper limit of the nitrogen content; in this invention, the upper limit of the nitrogen content is controlled at 0.04%.

[0048] Boron (B): 0.0005-0.005% Boron is an element that segregates at austenite grain boundaries, reducing grain boundary energy, and also improves the hardenability of steel. Therefore, it is preferable to include 0.0005% or more boron; however, when the boron content exceeds 0.005%, oxides form on the surface, thus deteriorating the surface quality of the steel sheet. Therefore, the boron content is preferably 0.0005-0.005%.

[0049] Phosphorus (P): less than 0.03% Phosphorus is an unavoidable impurity, and it is a major cause of reduced workability in steel due to segregation. Therefore, it is preferable to control the phosphorus content as low as possible. Theoretically, limiting the phosphorus content to 0% is advantageous, but phosphorus is inevitably present in the manufacturing process. Therefore, it is important to control the upper limit of the phosphorus content, which is controlled at 0.03% in this invention.

[0050] Sulfur (S): less than 0.03% Sulfur is an unavoidable impurity, and it forms coarse manganese sulfide (MnS), causing defects such as flange cracks and significantly reducing the porosity of the steel sheet. Therefore, it is preferable to control the sulfur content as low as possible. Theoretically, limiting the sulfur content to 0% is advantageous, but sulfur is inevitably present in the manufacturing process. Therefore, it is important to control the upper limit of the sulfur content; in this invention, the upper limit of the sulfur content is controlled at 0.03%.

[0051] In addition to the above-mentioned components, the martensitic carbon steel constituting the coating material may further include, by weight percent, one or more of the following: Cr: 0.1-1.0%, Ni: 0.1-1.0%, Mo: 0.05-1.0%, Ti: 0.005-0.05%, and Nb: 0.005-0.05%.

[0052] Chromium (Cr): 0.1-1.0% Chromium is an element that improves the hardenability of steel and enhances its strength by promoting the formation of low-temperature phase transformations. To achieve this effect, the chromium content is preferably 0.1% or more. When the chromium content exceeds 1.0%, it may lead to an excessive increase in manufacturing costs compared to the desired strength enhancement effect. Therefore, the chromium content is preferably 0.1-1.0%.

[0053] Nickel (Ni): 0.1-1.0% Nickel is an element that improves the hardenability of steel and also increases its strength. To achieve this effect, the nickel content is preferably 0.1% or more. When the nickel content exceeds 1.0%, it may cause an excessive increase in manufacturing costs compared to the desired strength improvement effect. Therefore, the nickel content is preferably 0.1-1.0%.

[0054] Molybdenum (Mo): 0.05-1.0% Molybdenum is an element that improves the hardenability of steel, enhances its strength by promoting the formation of low-temperature phase transformations, and increases steel strength by forming carbides within it. To achieve this effect, the molybdenum content is preferably 0.05% or more. When the molybdenum content exceeds 1.0%, it may cause an excessive increase in manufacturing costs compared to the desired strength improvement. Therefore, the molybdenum content is preferably 0.01-1.0%.

[0055] Titanium (Ti): 0.005-0.05% Titanium reacts with nitrogen and carbon within the steel to form carbonitrides, thus increasing strength. Therefore, a titanium content of 0.005% or more is preferred; however, if the titanium content exceeds 0.05%, excessive precipitates form, thus worsening castability. Therefore, the titanium content is preferably 0.005-0.05%.

[0056] Niobium (Nb): 0.005-0.05% Like titanium, niobium is a carbonitride-forming element. Niobium reacts with nitrogen and carbon within the steel, thereby increasing its strength. Therefore, it is preferable to include 0.005% or more niobium. However, when the niobium content exceeds 0.05%, excessive precipitates are formed, thus worsening the castability. Therefore, the niobium content is preferably 0.005-0.05%.

[0057] Furthermore, the martensitic carbon steel constituting the cladding material in this invention not only satisfies the above-mentioned composition system, but also has a fine microstructure consisting of martensite, with the remainder composed of one or more of retained austenite, ferrite, bainite, and carbides. More preferably, the area fraction of the martensite can be 65% or more. By ensuring the fine microstructure described above, excellent tensile strength and yield strength can be ensured.

[0058] Furthermore, tempering treatment can make the matrix structure of the fine microstructure tempered martensite, and the balance can consist of one or more of retained austenite, ferrite, bainite, and carbides. More preferably, the area fraction of the tempered martensite can be 65% or more. This is to eliminate the residual stress formed inside the steel due to the martensitic phase transformation caused by tempering treatment, thereby improving the toughness of the steel.

[0059] Composite steel plate One aspect of the present invention is a composite steel plate comprising the aforementioned base material and the aforementioned cladding material disposed on two sides of the base material. Figure 1 The diagram shows a composite steel sheet of the present invention, which uses a lightweight steel sheet as the base material (B) and martensitic carbon steel as the cladding material (A and C).

[0060] Composite steel sheets are defined as laminated composite materials that are integrated by bonding the surfaces of two or more metallic materials using metallurgical methods. Typically, composite steel sheets use precious metals such as nickel (Ni) or copper (Cu) as cladding materials and are used for special purposes, such as in extremely corrosive environments.

[0061] The base material for the internal steel of this invention is a lightweight steel sheet with low specific gravity and excellent formability, achieved by adding large amounts of manganese and aluminum. Although the lightweight steel sheet has excellent tensile strength, its yield strength is low, making it unsuitable for use as chassis components requiring fatigue resistance.

[0062] The cladding material used as the outer steel in this invention is martensitic carbon steel, which possesses both excellent yield strength and tensile strength. However, martensitic carbon steel has low elongation, making it difficult to ensure the formability required for cold pressing.

[0063] The low elongation of martensitic steel is due to the phenomenon that deformation is concentrated in a local area during molding, resulting in low uniform elongation. However, the inventors have discovered that when a lightweight steel plate containing a large amount of austenitic phase is arranged inside, the deformation of the martensitic steel is prevented from being concentrated in a local area, thereby improving formability.

[0064] Therefore, by using the aforementioned lightweight steel plate containing austenite as the base material and including the aforementioned martensitic carbon steel on both sides of the base material, the present invention overcomes the respective disadvantages and can obtain the effect of having both excellent formability and fatigue properties as well as low specific gravity.

[0065] At this point, the single-sided thickness of the cladding material can be 20 μm or more, and the sum of the thicknesses on both sides can be less than 30% of the total thickness of the composite steel plate. The single-sided thickness of the cladding material refers to the thickness of a single piece of cladding material disposed on one side of the base material, and the combined thickness of the cladding material refers to the sum of the thicknesses of two pieces of cladding material disposed on both sides of the base material. Furthermore, the total thickness of the composite steel plate refers to the thickness of the base material and the combined thicknesses of the cladding material, including the thickness of the coating when a plating layer is formed on one side of the cladding material. During the hot rolling and annealing processes in the manufacturing process, a gradient of elemental concentrations arises between the base material and the cladding material due to compositional differences, and element diffusion occurs at the interface. At this time, aluminum moves from the base material with a high content to the cladding material, thus forming localized regions with high aluminum content in the cladding material. Aluminum is an element that increases the martensitic transformation initiation temperature and reduces the hardenability of steel; therefore, when aluminum diffuses into the interior of the cladding material, it may reduce the strength of the cladding material. In this invention, considering the diffusion distance of aluminum, it was confirmed that a coating material thickness of 20 μm or more does not affect the martensitic phase transformation.

[0066] Furthermore, when the coating material is too thick, the desired 7.4 g / cm³ cannot be guaranteed according to this invention. 3 The following specific gravities. When the specific gravities are 7.4 g / cm³. 3 Under the following conditions, a weight reduction of more than 5% can be achieved compared to ordinary steel. This varies depending on the aluminum content in the base material, but when the thickness of the cladding material is less than 30% of the thickness of the composite steel plate, the specific gravity of the composite steel plate can be ensured to be 7.4 g / cm³. 3 the following.

[0067] Furthermore, the base material and the cladding material of the composite steel sheet of the present invention are preferably directly bonded together. The base material of the present invention contains a large amount of aluminum, which readily forms an oxide film upon contact with oxygen in the air. When an aluminum oxide film or the like is sandwiched between the base material and the cladding material, the bonding between the base material and the cladding material becomes unstable, and thus a stable multilayer structure cannot be formed. Therefore, in order to manufacture a composite steel sheet with a stable structure, it is necessary to suppress the formation of the oxide film as described above or remove the oxide film as much as possible.

[0068] Therefore, in this invention, during the manufacturing of the laminate, oxygen intrusion from the outside is blocked by welding the edges, thereby suppressing the formation of an oxide film as much as possible. Furthermore, simultaneously, by setting the reduction rate of the first rolling pass to 30% or more, the oxide film formed due to trace amounts of oxygen is crushed, allowing the base material and the cladding material to be directly bonded together. In the composite steel sheet of this invention manufactured by the manufacturing method described above, no oxides such as aluminum oxide are observed at the interface between the base material and the cladding material, and a bonded joint where the base material and the cladding material are directly bonded is observed. Figure 2 A scanning electron microscope image of Example 1 of the invention, which satisfies the conditions of the present invention, is shown. Figure 3 The elemental distribution of C, O, Al, Si, Mn, and CP in the boundary between the base material and the cladding material is shown. According to... Figure 3 The elemental distribution readily confirms that no specific oxide-containing structure was observed at the interface between the base material and the cladding material.

[0069] Furthermore, preferably, the Vickers hardness measured on the surface of the martensitic carbon steel constituting the cladding material is 400 Hv or higher, and the elongation of the composite steel sheet is 25% or higher. By ensuring the surface hardness and elongation as described above, it can be preferably applied to automotive chassis components, etc.

[0070] Furthermore, the composite steel plate may further include a coating formed on the cladding material, the coating being formed by a hot-dip galvanizing method, and the coating being selected from Zn-based, Zn-Fe-based, Zn-Al-based, Zn-Mg-based, Zn-Mg-Al-based, Zn-Ni-based, Al-Si-based, and Al-Si-Mg-based systems.

[0071] The following describes in detail another aspect of the present invention a method for manufacturing a low-repetition-weight composite steel sheet with excellent formability and fatigue properties.

[0072] Another aspect of the present invention describes a method for manufacturing a low-ratio composite steel sheet with excellent formability and fatigue properties, comprising the following steps: preparing a base material, said base material being a lightweight steel sheet satisfying the above-described alloy composition; preparing a cladding material, said cladding material being a martensitic carbon steel satisfying the above-described alloy composition; arranging said base material between two said cladding materials to obtain a laminate; welding the edges of said laminate and then heating it to a temperature range of 1050-1350°C; setting the reduction rate of the first pass to 30% or more, and finishing the heated laminate at a temperature range of 750-1050°C to obtain a hot-rolled steel sheet; coiling said hot-rolled steel sheet at 400-700°C; pickling said hot-rolled steel sheet and then cold-rolling it with a cold-rolling reduction rate of 35-90% to obtain a cold-rolled steel sheet; and annealing said cold-rolled steel sheet at a temperature range of 550°C or higher and below A3+200°C of said cladding material.

[0073] Prepare a base material and a cladding material that meet the above alloy composition, and then arrange the base material between the two cladding materials to obtain a laminate. The surfaces of the base material and the cladding material can be washed before lamination.

[0074] Regarding the methods for manufacturing the base material and the cladding material, conventional manufacturing processes can be applied, and therefore no particular limitation is made. However, as a preferred example, the base material can be manufactured by casting molten steel produced in an electric furnace or blast furnace, and the cladding material can be manufactured by refining and casting molten steel produced in a blast furnace to control the content of unavoidable impurities.

[0075] The edges of the laminate are welded, and then heated to a temperature range of 1050-1350°C. Welding the edges of the laminate prevents oxygen from penetrating between the base material and the cladding material, thus effectively preventing the formation of oxides at the interface during heating.

[0076] When the heating temperature is below 1050°C, it is difficult to ensure the finishing rolling temperature during hot rolling, and the rolling load increases due to the lower temperature, making it difficult to roll to the specified thickness. On the other hand, when the heating temperature exceeds 1350°C, the grain size increases and surface oxidation occurs, tending to reduce strength or deteriorate the surface, which is therefore undesirable. Furthermore, a liquid film forms on the columnar grain boundaries of the continuously cast slab, which may cause cracks during subsequent hot rolling. Therefore, the heating temperature is preferably 1050-1350°C.

[0077] The reduction rate of the first pass is set to 30% or more, and the heated laminate is precision rolled in a temperature range of 750-1050°C to obtain a hot-rolled steel sheet.

[0078] A crucial aspect of this invention is setting the reduction rate of the first pass in the hot rolling process to 30% or more. Even when oxygen intrusion from the outside during the heating process is blocked by welding the edges during laminate manufacturing, the large amount of aluminum contained in the lightweight steel sheet constituting the base material can still form an oxide film due to the trace amounts of oxygen present between the laminates. When this interfacial oxide is trapped between the base material and the cladding material, the bonding strength between the base material and the cladding material weakens, potentially leading to sheet separation.

[0079] More specifically, the inventors discovered that when the reduction rate of the first rolling pass is 30% or more, the oxides are crushed during rolling. Between the crushed aluminum oxides, the unoxidized base material and cladding material bond together at high temperature to form a solid-state bond, thereby obtaining a stable multilayer structure. Furthermore, even with an increase in rolling weight, the area of ​​the interfacial oxides does not increase significantly, but the area of ​​the solid-state bond increases proportionally with the rolling weight. Therefore, it is crucial to maintain the bond in the first rolling pass; if the bond is maintained in the first pass, subsequent rolling can proceed smoothly.

[0080] On the other hand, when the reduction rate of the first pass is less than 30%, insufficient area of ​​the solid-phase joint cannot ensure adequate bond strength, leading to plate separation. Furthermore, continuous oxidation occurs due to oxygen intruding between the separated plates, preventing the formation of a stable multilayer structure. Therefore, controlling the reduction rate of the first pass to 30% or higher is more important than controlling the total reduction rate of hot rolling.

[0081] Furthermore, when the finishing rolling temperature is below 750°C, the rolling load increases, thus causing the mill to overload. On the other hand, when the finishing rolling temperature exceeds 1050°C, surface oxidation may occur during rolling. Therefore, the finishing rolling temperature is preferably 750-1050°C.

[0082] The hot-rolled steel sheet is coiled at 400-700°C. When the coiling temperature is below 400°C, a low-temperature phase transformation occurs during cooling, leading to an excessive increase in the strength of the hot-rolled steel sheet, which may cause an increase in rolling load during cold rolling. On the other hand, when the coiling temperature exceeds 700°C, a thick oxide film forms on the surface of the hot-rolled steel sheet, making it difficult to control the oxide layer during pickling. Therefore, the coiling temperature is preferably limited to 400-700°C.

[0083] The hot-rolled steel sheet is pickled and then cold-rolled with a reduction rate of 35-90% to obtain a cold-rolled steel sheet. When the reduction rate is less than 35%, recrystallization of the base material and cladding material cannot proceed smoothly, resulting in poor processability. On the other hand, when the reduction rate exceeds 90%, the possibility of sheet breakage increases due to the rolling load. Therefore, the preferred reduction rate is 35-90%.

[0084] The cold-rolled steel sheet is annealed within a temperature range of 550°C or higher and below A3+200°C of the cladding material. Many dislocations formed during cold rolling are reduced through static recrystallization during annealing, ensuring the steel's workability. When the annealing temperature is below 550°C, sufficient workability cannot be guaranteed. On the other hand, when annealing is performed at temperatures exceeding A3+200°C of the cladding material, the surface hardness of the cladding material may decrease due to surface decarburization, etc. Therefore, the annealing temperature is preferably within a temperature range of 550°C or higher and below A3+200°C of the cladding material.

[0085] In addition, after the annealing step, a further step may be included to form a coating by hot-dip plating, wherein the coating may be selected from one of Zn-based, Zn-Fe-based, Zn-Al-based, Zn-Mg-based, Zn-Mg-Al-based, Zn-Ni-based, Al-Si-based, and Al-Si-Mg-based systems.

[0086] In addition, after the annealing step, the coating material can be cooled to below the martensitic termination temperature and then heated to below 600°C to temper the matrix structure of the coating material. Detailed Implementation

[0087] The present invention will now be described in more detail through embodiments. However, it should be noted that the following embodiments are merely illustrative of the invention for more detailed explanation and are not intended to limit the scope of the invention. This is because the scope of the invention is determined by the content recorded in the claims and the content reasonably deduced therefrom.

[0088] (Example) Prepare steel ingots containing carbon steel and light steel plates with the composition shown in Table 1 below, and wash the surface of the ingots. Then, arrange the light steel plates between two carbon steel plates to create a three-layer laminate, such that the three-layer laminate has the lamination ratio shown in Table 2 below. Next, perform arc welding along the boundary surfaces of the laminate using welding rods. The laminate with welded boundary surfaces is then reheated in a furnace at 1150°C for 1 hour, and then rolled at a finishing rolling temperature of 900°C to produce a hot-rolled steel sheet. Subsequently, the hot-rolled steel sheet is coiled at 550°C, and then cold-rolled with a 50% cold-rolling reduction after pickling to produce a cold-rolled steel sheet. Finally, annealing is performed at the annealing temperatures shown in Table 2.

[0089] The mechanical properties and coating properties of each manufactured specimen were measured and are shown in Table 3 below. Tensile tests were performed using a universal tensile testing machine, and the yield strength (YS), tensile strength (TS), and total elongation (TEL) were measured. The units for yield strength (YS) and tensile strength (TS) are MPa, and the unit for total elongation (TEL) is %.

[0090] Specific gravity was measured as follows: a steel plate was made into a size of 100×100mm, and its weight was measured at room temperature. Then, it was attached to a 0.05mm diameter iron wire and immersed in a beaker containing room temperature water, and its weight was measured again. The specific gravity of water was set as 1g / cc.

[0091] Surface hardness is measured by pressing the surface with a micro Vickers hardness tester under a load of 300 gf for 10 seconds and then measuring the size of the indentation. To ensure measurement accuracy, the surface is polished to a depth of 10 μm before measurement.

[0092] The fatigue strength of Comparative Example 1, Comparative Example 2, and Invention Example 1 was measured using a bending fatigue testing machine under a stress ratio of -1, with the failure cycle number set to 2,000,000.

[0093] [Table 1] In Table 1, the unit of each element content is by weight.

[0094] [Table 2] [Table 3] As can be confirmed from Tables 1 to 3, in Invention Examples 1 to 7, which satisfy all the components of the present invention, an elongation of more than 25% and a density of 7.4 g / cm³ can be ensured. 3The following specific gravity and surface hardness of 400 Hv or higher. In contrast, Comparative Example 1, a martensitic carbon steel that does not constitute a cladding material, has excellent fatigue characteristics due to its high surface hardness, but cannot guarantee an elongation of 25% or higher.

[0095] Comparative Example 2 is a lightweight steel plate that does not constitute a cladding material. Although it meets the requirements for elongation and specific gravity, it has low surface hardness and therefore poor fatigue characteristics.

[0096] The coating material in Comparative Example 3 has a single-sided thickness of 15 μm, which does not ensure surface hardness.

[0097] The coating material in Comparative Example 4 has a thickness ratio of over 30%, which, while satisfying formability and surface hardness requirements, cannot guarantee 7.4 g / cm³. 3 The following proportions.

[0098] In Comparative Example 5, the reduction rate in the first pass was less than 30%, resulting in plate separation, thus making it impossible to manufacture composite steel.

[0099] Figure 2 This is an optical microscope photograph showing the fine structure of the boundary between the base material and the coating material in Example 1 of the invention. Figure 3 The image shown is a scanning electron microscope photograph of Example 1 of the invention, illustrating the elemental distribution at the boundary between the base material and the cladding material. The lightweight steel sheet serving as the base material is composed of austenitic single-phase material, while the cladding material is composed of martensitic material. No specific oxide-containing microstructure was observed between the base material and the cladding material, which can also be seen from… Figure 3 This was confirmed in the element distribution.

[0100] Figure 4 The diagram shows scanning electron microscope images of the total thickness measured in Example 1 of the invention and a graph showing the distribution of microhardness measured at each thickness location. The hardness of the cladding material composed of martensite shows a level of 450 Hv, while the hardness of the base material composed of lightweight steel plate shows a level of 260 Hv.

[0101] Figure 5 These are photographs of the appearance of Comparative Example 5 after hot rolling. The sheet separated immediately after one pass, therefore further rolling was not possible.

[0102] Figure 6 This is a graph showing the fatigue test results of Comparative Example 1, Comparative Example 2, and Invention Example 1. Comparative Example 1 and Invention Example 2, which have high surface hardness, show fatigue strengths of over 500 MPa. This is because fatigue cracks propagate from the surface, thus high surface hardness improves durability. In other words, it can be seen that the fatigue life of steel is proportional to the hardness of the surface material.

[0103] The above embodiments have been described with reference to the present invention. However, those skilled in the art will understand that various modifications and changes can be made to the present invention without departing from the spirit and scope of the invention as set forth in the claims.

Claims

1. A low-specification composite steel plate with excellent formability and fatigue properties, characterized in that, Includes a base material and a cladding material disposed on two sides of the base material. By weight percent, the base material is a lightweight steel plate comprising C: 0.3-1.0%, Mn: 4.0-16.0%, Al: 5.9-9.0%, with the balance being Fe and unavoidable impurities. The coating material, by weight percent, is a martensitic carbon steel comprising C: 0.1-0.45%, Mn: 1.0-3.0%, with the balance being Fe and unavoidable impurities. A solid-phase bond is formed at the interface between the base material and the coating material, in which the base material and the coating material are directly bonded together.

2. The low-component composite steel sheet with excellent formability and fatigue properties according to claim 1, characterized in that, The lightweight steel sheet further comprises, by weight percent: Si: 0.03-2.0%, Ni: 0.1-4.0%, N: less than 0.04% and excluding 0%, P: less than 0.03% and S: less than 0.03%.

3. The low-component composite steel sheet with excellent formability and fatigue properties according to claim 1, characterized in that, The martensitic carbon steel further comprises, by weight percent: Si: 0.03-2.0%, Al: 0.02-0.3%, N: less than 0.04% and excluding 0%, B: 0.0005-0.005%, P: less than 0.03% and S: less than 0.03%.

4. The low-component composite steel sheet with excellent formability and fatigue properties according to claim 3, characterized in that, The martensitic carbon steel further comprises, by weight percent, one or more of the following: Cr: 0.1-1.0%, Ni: 0.1-1.0%, Mo: 0.05-1.0%, Ti: 0.005-0.05%, and Nb: 0.005-0.05%.

5. The low-component composite steel sheet with excellent formability and fatigue properties according to claim 1, characterized in that, The thickness of the coating material on one side is more than 20 μm, and the sum of the thicknesses on both sides is less than 30% of the total thickness of the composite steel plate.

6. The low-component composite steel sheet with excellent formability and fatigue properties according to claim 1, characterized in that, The composite steel plate has a fatigue strength of over 500 MPa, an elongation of over 25%, a surface hardness of over 400 Hv, and a specific gravity of 7.4 g / cm³. 3 the following.

7. The low-component composite steel sheet with excellent formability and fatigue properties according to claim 1, characterized in that, The lightweight steel sheet contains more than 10% retained austenite by area fraction.

8. The low-component composite steel sheet with excellent formability and fatigue properties according to claim 1, characterized in that, The microstructure of the martensitic carbon steel is martensite, and the second phase includes one or more of carbides, ferrite, retained austenite, and bainite.

9. The low-component composite steel sheet with excellent formability and fatigue properties according to claim 1, characterized in that, The microstructure of the martensitic carbon steel is tempered martensite, and the second phase includes one or more of carbides, ferrite, retained austenite, and bainite.

10. The low-component composite steel sheet with excellent formability and fatigue properties according to claim 1, characterized in that, The composite steel plate further includes a coating formed on the cladding material.

11. The low-component composite steel sheet with excellent formability and fatigue properties according to claim 10, characterized in that, The coating is selected from one of the following: Zn-based, Zn-Fe-based, Zn-Al-based, Zn-Mg-based, Zn-Mg-Al-based, Zn-Ni-based, Al-Si-based, and Al-Si-Mg-based.

12. A method for manufacturing a low-specification composite steel sheet with excellent formability and fatigue properties, comprising the following steps: Prepare the base material, which, by weight percent, is a lightweight steel plate containing C: 0.3-1.0%, Mn: 4.0-16.0%, Al: 5.9-9.0%, with the balance being Fe and unavoidable impurities; Prepare a coating material, by weight %, the coating material being a martensitic carbon steel containing C: 0.1-0.45%, Mn: 1.0-3.0%, balance Fe and unavoidable impurities; The base material is arranged between the two cladding materials to obtain a laminate; The edges of the laminate are welded, and then heated to a temperature range of 1050-1350°C; The reduction rate of the first pass is set to be more than 30%, and the heated laminate is precision rolled in a temperature range of 750-1050°C to obtain hot-rolled steel sheet. The hot-rolled steel sheet is coiled at 400-700℃; The hot-rolled steel sheet is pickled and then cold-rolled with a cold rolling reduction of 35-90% to obtain a cold-rolled steel sheet. as well as The cold-rolled steel sheet is annealed within a temperature range of above 550°C and below A3+200°C of the cladding material.

13. The method for manufacturing a low-specification composite steel sheet with excellent formability and fatigue properties according to claim 12, characterized in that, Following the annealing step, a tempering process is further included, in which the composite steel plate is cooled to below the martensitic termination temperature and then reheated to a temperature below 600°C.

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