High-carbon steel sheet with excellent durability and its manufacturing method, industrial or automotive parts

Abstract

According to one aspect of the present invention, it is possible to provide a high carbon steel sheet having excellent durability and an industrial or automotive part manufactured using the same.

Description

[Technical Field]

[0001] The present invention relates to a high-carbon steel sheet, a method for producing the same, and industrial or automotive parts. More specifically, it relates to a high-carbon steel sheet excellent in durability and particularly suitable for use as parts in various industries, a method for producing the same, and industrial or automotive parts produced using the high-carbon steel sheet. [Background Art]

[0002] High-carbon steel sheets are widely used throughout all industrial fields, and are mainly used particularly as raw materials for automotive or industrial parts subjected to repeated stress or deformation. Examples of automotive parts that use high-carbon steel sheets include clutch parts or spring parts for seat belts, and examples of industrial parts that use high-carbon steel sheets include industrial spring parts or tool parts. The reason why high-carbon steel sheets are widely used in various parts is that high-carbon steel sheets can simultaneously ensure durability while supporting strength.

[0003] In order to ensure the desired physical properties during the production of high-carbon steel, it is common to perform heat treatment after hot rolling or cold rolling. Patent Documents 1 to 3 are patent documents relating to high-carbon steel sheets excellent in durability. Patent Document 1 discloses a method for producing spring steel through a dipping step and a heat treatment step, Patent Document 2 discloses a method for securing a martensite structure through high-temperature heat treatment after cold rolling, and Patent Document 3 discloses a method for increasing the strength of spring steel through an induction heat treatment step.

[0004] However, recently, in connection with rapidly changing climate change, carbon neutrality has emerged as an important issue across all industries, and the reduction of carbon emissions is required without exception in the steel industry as well. Therefore, there is an urgent need for research on solutions that can ensure the desired physical properties while omitting the heat treatment step in the production of high-carbon steel sheets. [Prior Art Documents] [Patent Documents]

[0005] [Patent Document 1] International Publication Number WO2011-115255A1 (Published September 22, 2011) [Patent Document 2] European Patent No. EP3814536A1 (Published May 5, 2021) [Patent Document 3] European Patent No. EP2192201A1 (Published June 2, 2010) [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] One aspect of the present invention is to provide a highly durable high-carbon steel sheet and industrial or automotive parts manufactured using the same.

[0007] Another aspect of the present invention is to provide a method for manufacturing high-carbon steel sheets that can shorten or eliminate the heat treatment process by appropriately utilizing hot rolling and cold rolling processes, thereby reducing manufacturing costs and carbon emissions.

[0008] The problems that the present invention addresses are not limited to those described above. A person of ordinary skill should have no difficulty understanding further problems that the present invention addresses from the general contents of this specification. [Means for solving the problem]

[0009] A high-carbon steel sheet according to one aspect of the present invention contains, by weight %, C: 0.11-0.30%, Mn: 0.1-3.0%, Si: 0.5% or less (excluding 0%), Al: 0.1% or less (excluding 0%), P: 0.05% or less (including 0%), S: 0.03% or less (including 0%), N: 0.03% or less (including 0%), and the remaining Fe and unavoidable impurities, 90 area% The above martensite is included as a microstructure, and the residual stress of the martensite in the rolling direction can be 70 MPa or more.

[0010] The above high-carbon steel sheet may further contain Ti: 0.005~0.1% by weight.

[0011] The above high-carbon steel sheet may further contain one or more of the following in weight percent: Nb: 0.05% or less, V: 0.05% or less, Cr: 1.0% or less, Mo: 1.0% or less, and B: 0.005% or less.

[0012] The residual stress in the martensite at a 45° angle to the rolling direction can be 30 MPa or more.

[0013] Of the martensite mentioned above, the proportion of martensite stretched along the rolling direction may be 50% or more.

[0014] Of all the packets in the above-mentioned martensite, the proportion of packets with a length-to-short axis ratio of 2:1 or greater may be 50% or more.

[0015] The above high-carbon steel sheet contains one or more types selected from ferrite and retained austenite. area% The following (including 0%) are included in the total fraction, and one or more selected from perlite and bainite are included in 5 area% The following can be included as a total fraction (including 0%).

[0016] The yield strength of the above steel plate can be 1300 MPa or higher, and the tensile strength can be 1500 MPa or higher.

[0017] The carbon (C) content of the above steel sheet exceeds 0.20% by weight, and the residual stress in the rolling direction of the martensite can be 190 MPa or more.

[0018] The yield strength of the above steel plate can be 1590 MPa or higher, and the tensile strength can be 1640 MPa or higher.

[0019] Industrial or automotive parts manufactured using the above-mentioned high-carbon steel sheets can pass durability tests with a result of 100,000 cycles or more.

[0020] A method for producing a high-carbon steel sheet according to one aspect of the present invention comprises the steps of: heating, in a temperature range of 1100°C or higher, a slab comprising, by weight%, C: 0.11 to 0.30%, Mn: 0.1 to 3.0%, Si: 0.5% or less (excluding 0%), Al: 0.1% or less (excluding 0%), P: 0.05% or less (including 0%), S: 0.03% or less (including 0%), N: 0.03% or less (including 0%), with the remainder being Fe and unavoidable impurities; hot-rolling the heated slab at a finishing rolling temperature of 800 to 950°C to obtain a hot-rolled steel sheet; quenching and coiling the hot-rolled steel sheet within 5 seconds after completion of the hot rolling at a cooling rate of 50 to 1000°C / sec to a finishing cooling temperature of 350°C or lower; and omitting heat treatment after the coiling, and cold-rolling the hot-rolled steel sheet at a reduction ratio of 20 to 50%.

[0021] The slab may further comprise Ti: 0.005 to 0.1% by weight.

[0022] The slab may further comprise one or more selected from the group consisting of, by weight%, Nb: 0.05% or less, V: 0.05% or less, Cr: 1.0% or less, Mo: 1.0% or less, and B: 0.005% or less.

[0023] A content of carbon (C) contained in the slab may be more than 0.20% by weight.

[0024] The quenched hot-rolled steel sheet has 90 area% % or more of martensite.

[0025] In the method for producing the high-carbon steel sheet, quenching does not need to be performed after the cold rolling.

[0026] The above solution to the problem does not list all features of the present invention, and various features of the present invention and the accompanying advantages and effects can be understood in more detail with reference to the following specific implementation examples and working examples. Effects of the Invention

[0027] According to one aspect of the present invention, it is possible to manufacture high-carbon steel sheets with excellent durability while shortening or eliminating the heat treatment process. This not only effectively reduces the energy and manufacturing costs required for the production of high-carbon steel sheets, but also ensures environmental friendliness by reducing the amount of carbon emitted in the high-temperature heat treatment process.

[0028] According to one aspect of the present invention, in order to provide a high-carbon steel plate with excellent durability, the lifespan of spring members manufactured using it can be effectively improved.

[0029] The effects of the present invention are not limited to those described above and can be interpreted to include matters that a person of the ordinary skill could reasonably infer from the matters described herein. [Brief explanation of the drawing]

[0030] [Figure 1] This is a photograph of the microstructure of specimen 1 observed using a scanning electron microscope (SEM). [Modes for carrying out the invention]

[0031] This invention relates to a highly durable high-carbon steel sheet, a method for manufacturing the same, and a spring member, and preferred examples of the invention are described below. These examples of the invention can be modified in various forms, and the scope of the invention should not be construed as being limited to the examples described below. These examples are provided to those with ordinary skill in the art to which the invention pertains, in order to further elaborate on the invention.

[0032] The following describes in more detail a high-carbon steel sheet according to one aspect of the present invention.

[0033] A high-carbon steel sheet according to one aspect of the present invention contains, by weight %, C: 0.11-0.30%, Mn: 0.1-3.0%, Si: 0.5% or less (excluding 0%), Al: 0.1% or less (excluding 0%), P: 0.05% or less (including 0%), S: 0.03% or less (including 0%), N: 0.03% or less (including 0%), and the remaining Fe and unavoidable impurities, 90 area% The above martensite is included as a microstructure, and the residual stress of the martensite in the rolling direction can be 70 MPa or more.

[0034] The steel composition contained in the high-carbon steel sheet of the present invention will be described in more detail below. Unless otherwise specified, the percentages representing the content of each element are based on weight.

[0035] Carbon (C): 0.11~0.30%

[0036] Since carbon (C) is an element that effectively contributes to improving the strength of steel, the present invention may include a certain level of carbon (C) or higher in order to ensure the strength of the steel sheet. Furthermore, if the carbon (C) content does not reach a certain level, a large amount of low-temperature structures such as pearlite and bainite may be formed during cooling after hot rolling, and it may not be possible to secure the fine structure targeted by the present invention. Therefore, the present invention can limit the lower limit of the carbon (C) content to 0.11%. The carbon (C) content can be 0.15% or higher, and can be 0.20% or higher. A preferable carbon (C) content can exceed 0.20%. On the other hand, if too much carbon (C) is added, the strength of the steel may improve, but the durability may decrease. Therefore, the present invention can limit the carbon (C) content to 0.3% or less. A preferable upper limit of carbon (C) content can be 0.295%.

[0037] Manganese (Mn): 0.1-3.0%

[0038] Manganese (Mn) is an element that effectively contributes to improving the strength and hardening ability of steel. Furthermore, since manganese (Mn) combines with sulfur (S), which is inevitably incorporated during the steel manufacturing process, to form MnS, it is also an element that can effectively prevent crack formation caused by sulfur (S). To achieve these effects, the present invention may contain 0.1% or more of manganese (Mn). A preferred manganese (Mn) content can be 0.3% or more, and a more preferred manganese (Mn) content can be 0.5% or more. On the other hand, if manganese (Mn) is added in excess, there is a concern that the tensile strength will decrease due to retained austenite, and it is undesirable from the standpoint of durability and economics. Therefore, the present invention can limit the manganese (Mn) content to 3.0% or less. The upper limit of the preferred manganese (Mn) content can be 2.9%, and the upper limit of the more preferred manganese (Mn) content can be 2.8%.

[0039] Silicon (Si): 0.5% or less (excluding 0%)

[0040] Since silicon (Si) is an element with a strong affinity for oxygen, adding large amounts can induce a decrease in surface quality due to surface scaling, which is undesirable from a weldability standpoint. Therefore, the present invention can limit the silicon (Si) content to 0.5%. The preferred upper limit for silicon (Si) content can be 0.45%. On the other hand, since silicon (Si) not only acts as a deoxidizing agent but also contributes to improving the strength of steel, the present invention does not completely exclude the addition of silicon (Si), and the lower limit of its content can exclude 0%.

[0041] Aluminum (Al): 0.1% or less (excluding 0%)

[0042] Aluminum (Al) is an element that combines with oxygen in steel to deoxidize it. This invention allows for the addition of aluminum (Al) for this effect, and the lower limit of its content can exclude 0%. On the other hand, if too much aluminum (Al) is added, it can not only increase inclusions but also reduce the workability of the steel sheet; therefore, this invention limits the aluminum (Al) content to 0.1% or less. A preferred upper limit for aluminum (Al) content can be 0.08%.

[0043] Phosphorus (P): 0.05% or less (including 0%)

[0044] Since phosphorus (P) is the main element that segregates at grain boundaries and induces a decrease in steel toughness, it is preferable to control the phosphorus (P) content to be as low as possible. Therefore, theoretically, it is most advantageous to suppress the phosphorus (P) content to 0%. However, phosphorus (P) is an impurity that is inevitably introduced during the steelmaking process, and controlling its content to 0% may induce an excessive process load. Taking this into consideration, the present invention can limit the upper limit of the phosphorus (P) content to 0.05%. A preferable upper limit for the phosphorus (P) content can be 0.03%.

[0045] Sulfur (S): 0.03% or less (including 0%)

[0046] Sulfur (S) is an element that forms MnS, increasing the amount of precipitates and making steel brittle; therefore, it is preferable to control the sulfur (S) content to be as low as possible. Accordingly, theoretically, limiting the sulfur (S) content to 0% is the most advantageous. However, sulfur (S) is an impurity that is inevitably incorporated during the steelmaking process, and controlling its content to 0% may induce an excessive process load. Taking this into consideration, the present invention can limit the upper limit of the sulfur (S) content to 0.03%. A preferable upper limit for sulfur (S) content can be 0.01%.

[0047] Nitrogen (N): 0.03% or less (including 0%)

[0048] Nitrogen (N) is an element that generates nitrides during continuous casting, causing cracks in the slab; therefore, it is preferable to control its content to be as low as possible. Theoretically, limiting the nitrogen (N) content to 0% is the most advantageous. However, nitrogen (N) is an impurity that is inevitably incorporated during the steelmaking process, and controlling its content to 0% may induce excessive process load. Considering these points, the present invention can limit the upper limit of the nitrogen (N) content to 0.03% or less. A preferable upper limit for nitrogen (N) content can be 0.01%.

[0049] In addition to the alloy components described above, a high-carbon steel sheet according to one aspect of the present invention may further contain titanium (Ti): 0.005 to 0.1%, and one or more of the following: niobium (Nb): 0.05% or less, vanadium (V): 0.05% or less, chromium (Cr): 1.0% or less, molybdenum (Mo): 1.0% or less, and boron (B): 0.005% or less.

[0050] Titanium (Ti): 0.005~0.1%

[0051] Generally, titanium (Ti) is an element known to combine with carbon (C) and nitrogen (N) to form carbides and nitrides. While this invention adds boron (B) to steel to ensure hardening ability, if the nitrogen (N) contained in the steel combines with the boron (B), the intended effect of adding boron (B) cannot be achieved. When titanium (Ti) is added to steel, the nitrogen (N) before combining with boron (B) combines with the titanium (Ti) to form nitrides, thus more effectively improving the effect of adding boron (B). Therefore, this invention allows for the addition of 0.005% or more of titanium (Ti) to achieve this effect. A preferred lower limit for titanium (Ti) content can be 0.010%, and a more preferred lower limit can be 0.015%. On the other hand, if titanium (Ti) is added excessively, the continuous casting properties may decrease during the slab manufacturing stage; therefore, this invention limits the upper limit of the titanium (Ti) content to 0.1%. The upper limit for a preferred titanium (Ti) content can be 0.09%, and the upper limit for a more preferred titanium (Ti) content can be 0.08%.

[0052] Niobium (Nb): 0.05% or less, Vanadium (V): 0.05% or less, Molybdenum (Mo): 1.0% or less

[0053] Generally, niobium (Nb), vanadium (V), and molybdenum (Mo) are known elements that combine with carbon (C) and nitrogen (N) to form carbides and nitrides. Therefore, when niobium (Nb), vanadium (V), and molybdenum (Mo) are added, the strength is increased by the formation of carbides and nitrides. In this invention, one or more of niobium (Nb), vanadium (V), and molybdenum (Mo) can be added to achieve this effect. However, if niobium (Nb), vanadium (V), and molybdenum (Mo) are added in excess, the rolling load may become excessively large, and manufacturing costs may increase excessively. Therefore, in this invention, the upper limits of the niobium (Nb), vanadium (V), and molybdenum (Mo) content can be limited to 0.05%, 0.05%, and 1.0%, respectively.

[0054] Chromium (Cr): 1.0% or less

[0055] Since chromium (Cr) is an element that contributes to improving the hardening ability of steel, the present invention may include chromium (Cr) to achieve this effect. The lower limit of the preferred chromium (Cr) content can be 0.005%. On the other hand, excessive addition of chromium (Cr), which is an expensive element, is undesirable from an economic standpoint, and excessive addition of chromium (Cr) can reduce weldability, so the present invention may limit the upper limit of the chromium (Cr) content to 1.0%. The upper limit of the preferred chromium (Cr) content can be 0.5%.

[0056] Boron (B): 0.005% or less

[0057] Boron (B) is an element that effectively contributes to improving the hardening ability of steel, and even with the addition of a small amount, it can effectively suppress the transformation to low-temperature structures such as ferrite and pearlite during cooling after hot rolling. In this invention, it is possible to add 0.0005% or more of boron (B) to achieve such effects. The lower limit of the preferred boron (B) content can be 0.001%. On the other hand, if boron (B) is added in excess, it may react with iron (Fe) and induce grain boundary brittleness, so in this invention, the upper limit of the boron (B) content can be limited to 0.005%. The upper limit of the preferred boron (B) content can be 0.0045%.

[0058] A high-carbon steel sheet according to one aspect of the present invention may contain the remaining Fe and other unavoidable impurities in addition to the components described above. However, since unintended impurities may inevitably be mixed in from raw materials or the surrounding environment during the normal manufacturing process, it is not possible to completely eliminate them. Such impurities are recognizable to anyone with ordinary skill in the art, and therefore, their full details are not specifically mentioned in this specification. Furthermore, the addition of additional effective components other than those described above is not entirely excluded.

[0059] The microstructure contained in high-carbon steel sheets according to one aspect of the present invention will be described in more detail below.

[0060] One aspect of the present invention is a high-carbon steel sheet containing martensite as a matrix structure. The fraction of martensite is: Total area of ​​the cross-section at the point where the plate thickness is 1 / 4 90 area% The above is possible, and the preferred fraction of martensite is 95 area% The above can be achieved. A high-carbon steel sheet according to one aspect of the present invention contains martensite, which is a hard structure, as its base structure, and therefore can simultaneously ensure high strength and yield ratio.

[0061] The martensite contained in the high-carbon steel sheet according to one aspect of the present invention is formed by rapid cooling after hot rolling and then stretched by subsequent cold rolling, so that the proportion of stretched martensite among the total martensite contained in the steel sheet can be 50% or more. Stretched martensite can mean martensite arranged in a direction where the long axis of the packet is within 45° from the rolling direction. Furthermore, in the high-carbon steel sheet according to one aspect of the present invention, the proportion of packets with a long-short axis ratio of 2:1 or more among the total martensite packets can be 50% or more.

[0062] The martensite contained in the high-carbon steel sheet according to one aspect of the present invention is formed by cooling after hot rolling and then stretched by subsequent cold rolling. Therefore, unlike conventionally produced martensite, it can satisfy a residual stress level of 70 MPa or higher in the rolling direction and a residual stress level of 30 MPa or higher in the direction 45° to the rolling direction. Preferably, the residual stress in the rolling direction of the martensite can be 190 MPa or higher. The residual stress of the martensite can be measured by X-ray analysis, and a person with ordinary skills in the art to which the present invention belongs can measure the residual stress of the martensite without any special technical difficulty.

[0063] One aspect of the present invention involves a high-carbon steel sheet that not only contains martensite, a hard structure, as its base structure, but also controls the martensite contained in the steel sheet to be stretched by cold rolling so that it has a residual stress above a certain level or has an elongated form. This makes it possible to more effectively improve the durability of the steel sheet and the parts manufactured using it.

[0064] One aspect of the present invention does not completely exclude the presence of structures other than martensite in high-carbon steel sheets. However, since ferrite, pearlite, bainite, and retained austenite are undesirable for ensuring strength and durability, it is necessary to control their proportions within a certain range. The total proportion of ferrite and / or retained austenite is 10 area% Preferably, the total fraction of perlite and / or bainite is 5 area% The following is preferable: The present invention may include a case where the total fraction of ferrite, retained austenite, pearlite, and bainite is 0%.

[0065] On the other hand, a high-carbon steel sheet according to one aspect of the present invention may further contain cementite and precipitates as the remaining structure, in addition to the microstructure described above.

[0066] According to one aspect of the present invention, the yield strength (YS) of the high-carbon steel sheet can be 1300 MPa or higher, and the tensile strength (TS) can be 1500 MPa or higher. A preferred yield strength (YS) can be 1590 MPa or higher, and a preferred tensile strength (TS) can be 1640 MPa or higher.

[0067] When a spring member in coil form is manufactured using a high-carbon steel plate according to one aspect of the present invention, and a durability test is conducted in which the coiled spring is pulled to a certain length and then re-wound to its original state, the durability test results show that it has excellent durability, exceeding 100,000 cycles.

[0068] The following describes in more detail a method for manufacturing high-carbon steel sheets according to one aspect of the present invention.

[0069] A method for manufacturing a high-carbon steel sheet according to one aspect of the present invention is a step of heating a slab containing, by weight %, C: 0.11~0.30%, Mn: 0.1~3.0%, Si: 0.5% or less (excluding 0%), Al: 0.1% or less (excluding 0%), P: 0.05% or less (including 0%), S: 0.03% or less (including 0%), N: 0.03% or less (including 0%), the remaining Fe and unavoidable impurities, in a temperature range of 1100°C or higher; the above heated The process may include: hot-rolling a slab at a rolling completion temperature of 800-950°C to provide a hot-rolled steel sheet; rapidly cooling the hot-rolled steel sheet at a cooling rate of 50-1000°C / sec within 5 seconds after the completion of the hot-rolling to a cooling completion temperature of 350°C or lower; winding the rapidly cooled hot-rolled steel sheet in a temperature range of 350°C or lower; and omitting the heat treatment after winding and cold-rolling the hot-rolled steel sheet with a reduction ratio of 20-50%.

[0070] Slab heating and hot rolling

[0071] Since the steel composition of the slab of the present invention corresponds to the steel composition of the steel plate described above, the description of the steel composition of the slab of the present invention is replaced by the description of the steel composition of the steel plate described above. The manufacturing conditions of the slab are not particularly limited, and the manufacturing conditions of slabs used for the manufacture of ordinary high-carbon steel plates can be applied.

[0072] The prepared slab is heated to a certain temperature range. For sufficient homogenization, the slab can be heated to a temperature range of 1100°C or higher. However, if the heating temperature of the slab is excessively high, it is not only undesirable from an economic standpoint but may also adversely affect the surface quality of the final product. Therefore, the upper limit of the heating temperature of the slab can be limited to 1350°C.

[0073] The heated slab can be hot-rolled under normal hot-rolling conditions, but the finish rolling temperature can be limited to a range of 800-950°C to control the rolling load and reduce surface scale.

[0074] Cooling and winding

[0075] Hot-rolled steel sheets can be subjected to rapid cooling conditions immediately after hot rolling.

[0076] Since this invention aims to strictly control the microstructure of steel sheets, it is preferable that the cooling process described herein be started within 5 seconds immediately after the completion of hot rolling. If the time between the end of hot rolling and the start of cooling exceeds 5 seconds, ferrite, pearlite, and bainite unintended by this invention may be formed by air cooling in the atmosphere. A preferred time between the end of hot rolling and the start of cooling can be 3 seconds or less.

[0077] Hot-rolled steel sheets immediately after hot rolling can be cooled to a cooling completion temperature of 350°C or less at a cooling rate of 50 to 1000°C / s. If the cooling completion temperature exceeds a certain range, transformation to ferrite, pearlite, and bainite is unavoidable; therefore, in order to secure the microstructure targeted by the present invention, the upper limit of the cooling completion temperature can be limited to 350°C. On the other hand, there is no particular lower limit for the cooling completion temperature, but a preferred lower limit for the cooling completion temperature can be 150°C. If the cooling rate is below a certain level, transformation to ferrite, pearlite, and bainite occurs during cooling; therefore, in order to secure the microstructure targeted by the present invention, the lower limit of the cooling rate can be limited to 50°C / s. On the other hand, there is no particular upper limit for the cooling rate; however, considering equipment limitations and economics, the upper limit of the cooling rate can be limited to 1000°C / s.

[0078] This invention provides a method for rapidly cooling a hot-rolled steel sheet immediately after hot rolling, using the hot-rolled steel sheet in its state before cold rolling, at 90 area% The above-mentioned martensite can be secured. In the conventional method of manufacturing high-carbon steel sheets, heat treatment is applied immediately after hot rolling, the heat-treated hot-rolled steel sheet is cold-rolled, and then quenching heat treatment is applied to form a martensite structure. However, the present invention not only allows for the omission of heat treatment immediately after hot rolling by strictly controlling the steel composition system, but also allows for the omission of quenching after cold rolling, thus effectively reducing carbon emissions.

[0079] After cooling is complete, the cold-rolled steel sheet can be wound into a hot-rolled coil.

[0080] cold rolling

[0081] After uncoiling the hot-rolled coil, cold rolling can be performed with a reduction of 20-50%. If the reduction is too low, the martensite will not be sufficiently stretched, and the desired high-strength properties and durability cannot be ensured. Therefore, the present invention can limit the cold rolling reduction to a level of 20% or more. A preferred lower limit for the cold rolling reduction can be 25%. On the other hand, if the cold rolling reduction is excessive, there is a concern that the equipment may be damaged due to the rolling load, and the strength may increase too much, potentially reducing durability. Therefore, the present invention can limit the upper limit for the cold rolling reduction to 50%.

[0082] The high-carbon steel sheet produced by the aforementioned manufacturing method is 90 area% The material contains the above-mentioned martensite as a microstructure, and the residual stress in the rolling direction of the martensite is 70 MPa or higher, the yield strength (YS) is 1300 MPa or higher, and the tensile strength (TS) is 1500 MPa or higher.

[0083] Furthermore, when a spring component is manufactured using high-carbon steel plates produced by the aforementioned manufacturing method, the durability test results for the spring component can exceed 100,000 cycles.

[0084] The high-carbon steel sheet, its manufacturing method, and parts of the present invention will be described in more detail below through specific examples. It should be noted that the following examples are for understanding the present invention and not for defining the scope of the present invention. The scope of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred therefrom. [Examples]

[0085] After manufacturing slabs with the compositions shown in Table 1 below, test specimens of steel plates were produced by applying the process conditions shown in Table 2 below. Each slab was manufactured using a conventional manufacturing method and homogenized by heating in a temperature range of 1050 to 1350°C.

[0086] [Table 1]

[0087] [Table 2]

[0088] Subsequently, the microstructure of each specimen was measured, and the results are shown in Table 3. After cutting each specimen in a direction parallel to the rolling direction, a specimen for microstructure observation was taken from the cut surface at 1 / 4 of the plate thickness. The collected samples were polished and etched with Nital solution, and then the microstructure of each specimen was observed using an optical microscope and a scanning electron microscope (SEM). The fraction of microstructure was determined by image analysis.

[0089] The proportion of stretched martensite was measured from the area of ​​martensite where the long axis of the packets was aligned within 45° of the rolling direction relative to the total area of ​​martensite, as seen in scanning electron microscope (SEM) images. The ratio of the long and short axes of martensite packets was also measured from the area of ​​packets where the ratio of long-axis to short-axis packets relative to the total area of ​​martensite packets was 2:1 or greater, as seen in scanning electron microscope (SEM) images.

[0090] In Table 3, M represents martensite, F represents ferrite, R-γ represents retained austenite, P represents pearlite, and B represents bainite.

[0091] [Table 3]

[0092] The mechanical properties of each test specimen were measured, and the results are shown in Table 4. Tensile strength and yield strength were evaluated by performing tensile tests according to JIS standards. Residual stress was measured using an XSTRESS 3000 model manufactured by Stresstech Group, which employs X-ray diffraction, with the following measurement parameters: Exp.Time: 40s, Radiation: CrKa, Detector distance: 50s. In addition, coiled spring members were fabricated using each test specimen, and their durability was evaluated through repeated tests in which the coiled spring was stretched to a length of 1.5m and then re-coiled to its original state. The results are also shown in Table 4.

[0093] [Table 4]

[0094] As shown in Tables 1 to 4, test specimens 1 to 12 that satisfy all of the alloy composition and manufacturing conditions of the present invention have a martensite fraction of 90% area% The above confirms that the proportion of martensite stretched in the rolling direction is 50% or more, and that the proportion of martensite packets with a long-short axis ratio of 2:1 or more is 50% or more. Furthermore, it can be confirmed that test specimens 1 to 12 have a residual stress of 70 MPa or more in the rolling direction, a residual stress of 30 MPa or more in the 45° direction relative to the rolling direction, a tensile strength of 1500 MPa or more, a yield strength of 1300 MPa or more, and meet the results of a durability test of 100,000 cycles or more.

[0095] In contrast, test pieces 13 to 20 that do not satisfy one or more of the alloy composition and manufacturing conditions of the present invention can be confirmed to not satisfy one or more of the following limitations of the present invention: the fraction of martensite, the proportion of martensite stretched in the rolling direction, and the proportion of martensite packets with a long-short axis ratio of 2:1 or more.

[0096] Test specimen 13 is a specimen in which cooling has begun 5 seconds after the end of rolling, and it can be confirmed that the ferrite fraction is high and the desired strength and durability cannot be ensured.

[0097] Test specimen 14 represents a case where the rolling completion temperature is low, and test specimen 16 represents a case where the cooling rate is slow. In these test specimens, the fractions of pearlite and bainite are high, and it can be confirmed that the martensite fraction targeted by the present invention cannot be secured, and therefore the target strength and durability cannot be secured.

[0098] Test specimen 15 can confirm that when the cooling completion temperature and winding temperature are high, the bainite fraction is high and the desired durability cannot be ensured.

[0099] In the case of test specimen 17, when the cold reduction ratio is low, the proportion of martensite packets with a long-short axis ratio of 2:1 or higher is low, and the residual stress of the martensite is low, which confirms that the desired durability cannot be ensured.

[0100] Test specimen 18 has a low carbon (C) content, and test specimen 19 has a low titanium (Ti) and boron (B) content. In these cases, the fraction of martensite is significantly low, and it can be confirmed that the desired level of strength and durability cannot be secured.

[0101] Test specimen 20, when it has a high manganese (Mn) content, shows that the transformation to martensite is not sufficient and a large amount of retained austenite is formed. This confirms that while the tensile strength and yield strength are excellent, the durability is poor.

[0102] Although the present invention has been described in detail through the examples above, other forms of embodiment are also possible. Therefore, the technical idea and scope of the claims described below are not limited to the examples.

Claims

1. In weight percent, it contains C: 0.11-0.30%, Mn: 0.1-3.0%, Si: greater than 0% and less than or equal to 0.5%, Al: greater than 0% and less than or equal to 0.1%, P: 0.05% or less (including 0%), S: 0.03% or less (including 0%), and N: 0.03% or less (including 0%), with the remainder consisting of Fe and unavoidable impurities. It contains martensite as a microstructure with an area percentage of 90% or more, The residual stress in the rolling direction of the martensite is 70 MPa or more. A high-carbon steel sheet with a yield strength of 1300 MPa or higher and a tensile strength of 1500 MPa or higher.

2. The high-carbon steel sheet according to claim 1, further comprising Ti: 0.005 to 0.1% by weight.

3. The high-carbon steel sheet according to claim 2, further comprising one or more of the following in weight percent: Nb: 0.05% or less, V: 0.05% or less, Cr: 1.0% or less, Mo: 1.0% or less, and B: 0.005% or less.

4. The high-carbon steel sheet according to claim 1, wherein the residual stress of the martensite in the direction 45° with respect to the rolling direction is 30 MPa or more.

5. The high-carbon steel sheet according to claim 1, wherein the proportion of martensite that is stretched along the rolling direction is 50% or more of the martensite.

6. The high-carbon steel sheet according to claim 1, wherein 50% or more of the martensite packets have a length-to-short axis ratio of 2:1 or more.

7. It contains one or more types selected from ferrite and retained austenite in a total fraction of 10 area percent or less (including 0%), The high-carbon steel sheet according to claim 1, comprising one or more selected from perlite and bainite in a total fraction of 5 area percent or less (including 0%).

8. The carbon (C) content of the steel plate is more than 0.20% by weight. The high-carbon steel sheet according to claim 1, wherein the residual stress in the rolling direction of the martensite is 190 MPa or more.

9. The high-carbon steel sheet according to claim 8, wherein the yield strength of the steel sheet is 1590 MPa or more and the tensile strength is 1640 MPa or more.

10. An industrial or automotive spring component in coil form, manufactured using a high-carbon steel plate as described in any one of claims 1 to 9, wherein when a durability test is performed in which the coiled spring is pulled to a length of 1.5 m and then re-coiled to its original state, the result of the durability test is 100,000 cycles or more.

11. A step of heating a slab consisting of, by weight percent, C: 0.11-0.30%, Mn: 0.1-3.0%, Si: greater than 0% and less than or equal to 0.5%, Al: greater than 0% and less than or equal to 0.1%, P: 0.05% or less (including 0%), S: 0.03% or less (including 0%), and N: 0.03% or less (including 0%), with the remaining Fe and unavoidable impurities, in a temperature range of 1100°C or higher; The step of providing a hot-rolled steel sheet by hot-rolling the heated slab at a rolling completion temperature of 800 to 950°C; Within 5 seconds after the completion of the hot rolling, the hot-rolled steel sheet is rapidly cooled and wound up at a cooling rate of 50 to 1000°C / sec to a cooling completion temperature of 350°C or lower; and A method for manufacturing a high-carbon steel sheet according to claim 1, comprising the step of omitting the heat treatment after winding and cold-rolling the hot-rolled steel sheet with a reduction ratio of 20 to 50%.

12. The method for manufacturing a high-carbon steel sheet according to claim 11, wherein the slab further comprises Ti: 0.005 to 0.1% by weight.

13. The method for manufacturing a high-carbon steel sheet according to claim 12, wherein the slab further comprises one or more of the following in weight percent: Nb: 0.05% or less, V: 0.05% or less, Cr: 1.0% or less, Mo: 1.0% or less, and B: 0.005% or less.

14. The method for manufacturing a high-carbon steel sheet according to claim 11, wherein the carbon (C) content in the slab is more than 0.20% by weight.

15. The method for producing a high-carbon steel sheet according to claim 11, wherein the rapidly cooled hot-rolled steel sheet contains 90% or more martensite by area.

16. The method for manufacturing a high-carbon steel sheet according to claim 11, wherein no quenching is performed after cold rolling.

Images