High-carbon steel plate 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 and a manufacturing method thereof, and an industrial or automotive part, and more specifically to a high carbon steel sheet which has excellent durability and is particularly suitable for use in parts in various industries, a manufacturing method thereof, and an industrial or automotive part manufactured using the high carbon steel sheet. [Background technology]

[0002] High carbon steel sheets are widely used throughout the industrial field, and are primarily used as materials for automobiles or industrial parts that are subject to repeated stress or deformation. Examples of automobile parts for which high carbon steel sheets are used include clutch parts and seat belt spring parts, and examples of industrial parts for which high carbon steel sheets are used include industrial spring parts and tool parts. High carbon steel sheets are widely used for various parts because they can provide strength while ensuring durability at the same time.

[0003] In order to ensure the desired physical properties when producing high carbon steel, it is common to carry out heat treatment after hot rolling or cold rolling. Patent Documents 1 to 3 are patent documents related to high carbon steel sheets with excellent durability, and Patent Document 1 discloses a method for producing spring steel through a dipping process and a heat treatment process, 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 process.

[0004] However, in light of recent rapid climate change, carbon neutrality has emerged as an important issue across all industries, and the steel industry is no exception when it comes to reducing carbon emissions. As a result, research is being conducted urgently into ways to ensure the desired physical properties while eliminating the heat treatment process when manufacturing high carbon steel sheets. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. WO2011-115255A1 (Published on September 22, 2011) [Patent Document 2] European Patent No. EP3814536A1 (Published 2021.05.05) [Patent Document 3] European Patent No. EP2192201A1 (Published 2010.06.02) Summary of the Invention [Problem to be solved by the invention]

[0006] One aspect of the present invention is to provide a high carbon steel sheet having excellent durability and an industrial or automotive part manufactured using the same.

[0007] Another aspect of the present invention is to provide a method for manufacturing a high carbon steel sheet, which can reduce manufacturing costs and carbon emissions by appropriately utilizing hot rolling and cold rolling processes and shortening or omitting a heat treatment process.

[0008] The object of the present invention is not limited to the above-mentioned contents. A person skilled in the art will have no difficulty in understanding further object of the present invention from the entire 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 percent, 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 balance being Fe and unavoidable impurities, and the content of the high carbon steel sheet is 90% or less. area% The above martensite is contained as a microstructure, and the residual stress of the martensite in the rolling direction can be 70 MPa or more.

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

[0011] The high carbon steel plate may further contain, by weight percent, one or more of 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 martensite may have a residual stress in a direction at 45° to the rolling direction of 30 MPa or more.

[0013] In the martensite, the proportion of martensite elongated along the rolling direction may be 50% or more.

[0014] Of all the packets of martensite, the ratio of packets having a long / short axis ratio of 2:1 or more can be 50% or more.

[0015] The high carbon steel plate is made of ferrite and / or retained austenite. area% Contains the total fraction of 0% or less (including 0%) and contains 5% or more of pearlite and bainite. area% It may contain the following total percentages (including 0%):

[0016] The steel plate may have a yield strength of 1300 MPa or more and a tensile strength of 1500 MPa or more.

[0017] The carbon (C) content of the steel plate may exceed 0.20 wt %, and the residual stress of the martensite in the rolling direction may be 190 MPa or more.

[0018] The steel plate may have a yield strength of 1590 MPa or more and a tensile strength of 1640 MPa or more.

[0019] Industrial or automotive parts manufactured using the high carbon steel plate can have a durability test result of 100,000 times or more.

[0020] A method for producing a high carbon steel sheet according to one aspect of the present invention includes the steps of heating a slab containing, 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 inevitable impurities, at a temperature range of 1100°C or more; hot rolling the heated slab at a rolling end temperature of 800 to 950°C to provide a hot rolled steel sheet; quenching the hot rolled steel sheet at a cooling rate of 50 to 1000°C / sec to a cooling end temperature of 350°C or less within 5 seconds after completion of the hot rolling, and coiling the hot rolled steel sheet; and omitting the heat treatment after the coiling, cold rolling the hot rolled steel sheet at a rolling reduction of 20 to 50%.

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

[0022] The slab may further contain, by weight percent, one or more of 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] The carbon (C) content in the slab may be more than 0.20% by weight.

[0024] The above quenched hot-rolled steel sheet is 90 area% The above martensite may be included.

[0025] The method for producing the high carbon steel sheet does not require quenching after the cold rolling.

[0026] The above solutions to the problems described above are not intended to be exhaustive, and the various features of the present invention and their associated advantages and benefits can be more fully understood with reference to the following specific implementations and examples. Effect of the Invention

[0027] According to one aspect of the present invention, a high carbon steel sheet having excellent durability can be manufactured while shortening or omitting a heat treatment process, which not only effectively reduces the energy and manufacturing costs required for manufacturing the high carbon steel sheet, but also reduces the amount of carbon emitted in the high temperature heat treatment process, thereby ensuring environmental friendliness.

[0028] According to one aspect of the present invention, a high carbon steel sheet having excellent durability is provided, so that the life of a spring member manufactured using the same can be effectively improved.

[0029] The effects of the present invention are not limited to the above-mentioned matters, but can be interpreted as including matters that a person of ordinary skill in the art can reasonably infer from the matters described in this specification. [Brief description of the drawings]

[0030] [Figure 1] 1 is a photograph of the microstructure of test piece 1 observed using a scanning electron microscope (SEM). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031] The present invention relates to a high carbon steel plate having excellent durability, a manufacturing method thereof, and a spring member, and preferred embodiments of the present invention will be described below. The embodiments of the present invention may be modified in various forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. The embodiments are provided to provide further details of the present invention to those skilled in the art to which the present invention pertains.

[0032] Hereinafter, the high carbon steel sheet according to one aspect of the present invention will be described in more detail.

[0033] A high carbon steel sheet according to one aspect of the present invention contains, by weight percent, 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 balance being Fe and unavoidable impurities, and the content of the high carbon steel sheet is 90% or less. area% The above martensite may be contained as a microstructure, and the martensite may have a residual stress in the rolling direction of 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, % representing the content of each element is based on weight.

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

[0036] Carbon (C) is an element that effectively contributes to improving the strength of steel, so the present invention can contain carbon (C) at a certain level or more to ensure the strength of the steel sheet. In addition, if the carbon content (C) does not reach a certain level, a large amount of low-temperature structures such as pearlite and bainite are formed during cooling after hot rolling, and the microstructure targeted by the present invention may not be secured, so 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 more, and can be 0.20% or more. A preferred carbon (C) content can be more than 0.20%. On the other hand, if carbon (C) is added in excess, the strength of the steel is improved but the durability may be reduced, so the present invention can limit the carbon (C) content to 0.3% or less. A preferred upper limit of the carbon 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 hardenability of steel. In addition, manganese (Mn) is an element that can effectively prevent cracks caused by sulfur (S) from occurring because it combines with sulfur (S) that is inevitably introduced during the steel manufacturing process to form MnS. The present invention can contain 0.1% or more of manganese (Mn) to achieve such effects. 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, when manganese (Mn) is added in excess, not only is there concern about a decrease in tensile strength due to retained austenite, but it is also undesirable from the standpoint of durability and economy. Therefore, the present invention can limit the manganese (Mn) content to 3.0% or less. A preferred upper limit of the manganese (Mn) content can be 2.9%, and a more preferred upper limit of the manganese (Mn) content can be 2.8%.

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

[0040] Silicon (Si) is an element with a strong affinity for oxygen, so if a large amount is added, it may induce a deterioration in surface quality due to surface scale, which is undesirable from the perspective of weldability. Therefore, the present invention can limit the silicon (Si) content to 0.5%. A preferred upper limit for the silicon (Si) content can be 0.45%. On the other hand, silicon (Si) not only acts as a deoxidizer but also contributes to improving the strength of steel, so the present invention does not completely exclude the addition of silicon (Si), and the lower limit for the 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 act as a deoxidizer. In the present invention, aluminum (Al) can be added for this effect, and the lower limit of the content can exclude 0%. On the other hand, if aluminum (Al) is added in excess, not only will the amount of inclusions increase, but the workability of the steel sheet can also decrease, so the present invention can limit the aluminum (Al) content to 0.1% or less. A preferred upper limit of the aluminum (Al) content can be 0.08%.

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

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

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

[0046] Since sulfur (S) is an element that forms MnS, increases the amount of precipitates, and embrittles steel, it is preferable to control the sulfur (S) content as low as possible. Therefore, it is theoretically most advantageous to limit the sulfur (S) content to 0%. However, sulfur (S) is also an impurity that is inevitably introduced during the steelmaking process, and controlling the content to 0% may induce excessive process load. In consideration of this point, the present invention can limit the upper limit of the sulfur (S) content to 0.03%. A preferable upper limit of the 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 and causes cracks in the slab, so it is preferable to control the content as low as possible. Therefore, it is theoretically most advantageous to limit the nitrogen (N) content to 0%. However, nitrogen (N) is also an impurity that is inevitably introduced during the steelmaking process, and controlling the content to 0% may induce excessive process load. In consideration of this point, the present invention can limit the upper limit of the nitrogen (N) content to 0.03% or less. A preferable upper limit of the nitrogen (N) content can be 0.01%.

[0049] In addition to the alloy components described above, the high carbon steel sheet according to one aspect of the present invention may further contain titanium (Ti): 0.005 to 0.1%, and may further contain one or more of 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] In general, titanium (Ti) is an element known to combine with carbon (C) and nitrogen (N) to form carbides and nitrides. In the present invention, boron (B) is added to steel to ensure hardening ability, but if nitrogen (N) contained in steel combines with boron (B), the effect of adding boron (B) targeted by the present invention cannot be achieved. When titanium (Ti) is added to steel, nitrogen (N) before combining with boron (B) combines with titanium (Ti) to form nitrides, so that the effect of adding boron (B) can be more effectively improved. Therefore, in the present invention, 0.005% or more of titanium (Ti) can be added to achieve such an effect. The preferred lower limit of the titanium (Ti) content may be 0.010%, and the more preferred lower limit of the titanium (Ti) content may be 0.015%. Meanwhile, if titanium (Ti) is added excessively, continuous castability may be reduced in the slab manufacturing stage, so the present invention may limit the upper limit of the titanium (Ti) content to 0.1%. A preferred upper limit of the titanium (Ti) content may be 0.09%, and a more preferred upper limit of the titanium (Ti) content may be 0.08%.

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

[0053] In general, niobium (Nb), vanadium (V) and molybdenum (Mo) are elements known to combine with carbon (C) and nitrogen (N) to form carbides and nitrides. Therefore, when niobium (Nb), vanadium (V) and molybdenum (Mo) are added, there is an effect of increasing strength due to carbides and nitrides. In the present invention, one or more of niobium (Nb), vanadium (V) and molybdenum (Mo) can be added to achieve such an effect. However, when niobium (Nb), vanadium (V) and molybdenum (Mo) are added excessively, the rolling load becomes excessively large, and the manufacturing cost may increase excessively. Therefore, in the present invention, the upper limits of the niobium (Nb), vanadium (V) and molybdenum (Mo) contents can be limited to 0.05%, 0.05% and 1.0%, respectively.

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

[0055] Chromium (Cr) is an element that contributes to improving the hardenability of steel, so the present invention may contain chromium (Cr) to achieve such an effect. A preferred lower limit of the chromium (Cr) content may be 0.005%. On the other hand, excessive addition of chromium (Cr), which is an expensive element, is not preferred from an economical perspective, and if chromium (Cr) is added in excess, it may reduce weldability, so the present invention may limit the upper limit of the chromium (Cr) content to 1.0%. A preferred upper limit of the chromium (Cr) content may be 0.5%.

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

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

[0058] The high carbon steel sheet according to one aspect of the present invention may contain the remaining Fe and other inevitable impurities in addition to the above-mentioned components. However, since unintended impurities may be inevitably mixed in from raw materials or the surrounding environment during normal manufacturing processes, it is not possible to completely exclude them. Since such impurities are known to anyone having ordinary skill in the art, the entire contents of the impurities will not be specifically mentioned in this specification. In addition, the addition of further effective components in addition to the above-mentioned components is not completely excluded.

[0059] Hereinafter, the microstructure contained in the high carbon steel sheet according to one aspect of the present invention will be described in more detail.

[0060] The high carbon steel plate according to one aspect of the present invention contains martensite as a matrix structure. The fraction of martensite is Total area of ​​cut surface at 1 / 4 of plate thickness 90 against area% The preferred martensite fraction is 95% or more. area% The high carbon steel sheet according to one aspect of the present invention includes martensite, which is a hard structure, as a matrix 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 quenching after hot rolling and elongated by subsequent cold rolling, so that the proportion of elongated martensite among all martensite contained in the steel sheet may be 50% or more. Elongated martensite may mean martensite in which the major axis direction of packets is aligned in a direction within 45° from the rolling direction. In addition, in the high carbon steel sheet according to one aspect of the present invention, the proportion of packets having a major axis ratio of 2:1 or more among all martensite packets may 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 elongated by subsequent cold rolling, and therefore, unlike conventional martensite, the residual stress in the rolling direction can satisfy a level of 70 MPa or more, and the residual stress in a 45° direction to the rolling direction can satisfy a level of 30 MPa or more. The residual stress in the rolling direction of the preferred martensite can be 190 MPa or more. The residual stress of martensite can be measured by X-ray analysis, and a person skilled in the art to which the present invention pertains can measure the residual stress of martensite without any special technical difficulties.

[0063] The high carbon steel sheet according to one aspect of the present invention not only includes martensite, which is a hard structure, as a matrix structure, but also controls the martensite included in the steel sheet to have a residual stress of a certain level or more by elongating it through cold rolling, or to have an elongated form, thereby more effectively improving the durability of the steel sheet and parts manufactured using the same.

[0064] The high carbon steel sheet according to one aspect of the present invention does not completely exclude the inclusion of structures other than martensite. However, since ferrite, pearlite, bainite, and retained austenite are not preferable for ensuring strength and durability, it is necessary to control the fractions of these structures within a certain range. The total fraction of ferrite and / or retained austenite is 10 area% Preferably, the total fraction of pearlite and / or bainite is 5 area% It is preferable that the total fraction of ferrite, retained austenite, pearlite and bainite is 0%.

[0065] Meanwhile, the high carbon steel sheet according to one aspect of the present invention may further include cementite, precipitates, etc. as the remaining structure in addition to the above-mentioned microstructure.

[0066] The high carbon steel plate according to one aspect of the present invention may have a yield strength (YS) of 1300 MPa or more and a tensile strength (TS) of 1500 MPa or more. The preferred yield strength (YS) may be 1590 MPa or more and the preferred tensile strength (TS) may be 1640 MPa or more.

[0067] A coil-shaped spring member is manufactured using the high carbon steel sheet according to one aspect of the present invention, and a durability test is conducted in which the coiled spring is stretched to a certain length and then re-wound to its original state. As a result of the durability test, the spring can be stretched more than 100,000 times, demonstrating excellent durability.

[0068] Hereinafter, a method for producing a high carbon steel sheet according to one aspect of the present invention will be described in more detail.

[0069] A method for producing a high carbon steel sheet according to one aspect of the present invention includes the steps of heating a slab containing, by weight, C: 0.11-0.30%, Mn: 0.1-3.0%, Si: 0.5% or less (except 0%), Al: 0.1% or less (except 0%), P: 0.05% or less (including 0%), S: 0.03% or less (including 0%), N: 0.03% or less (including 0%), the balance being Fe and unavoidable impurities, at a temperature range of 1100° C. or more; The method may include the steps of: hot rolling a slab at a rolling end temperature of 800 to 950°C to provide a hot-rolled steel sheet; quenching the hot-rolled steel sheet at a cooling rate of 50 to 1000°C / sec to a cooling end temperature of 350°C or less within 5 seconds after completion of the hot rolling; coiling the quenched hot-rolled steel sheet at a temperature range of 350°C or less; and cold rolling the hot-rolled steel sheet at a rolling reduction of 20 to 50% while omitting the heat treatment after the coiling.

[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 substituted for 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 the slab used in the manufacture of a normal high carbon steel plate may 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 slab is heated to an excessively high temperature, it is not only unfavorable from an economical point of view, but may also have a negative effect on the surface quality of the final product, so the upper limit of the slab heating temperature 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 to 950° C. in order to control the rolling load and reduce surface scale.

[0074] Cooling and winding

[0075] The hot-rolled steel sheet immediately after hot rolling can be subjected to cooling under rapid cooling conditions.

[0076] Since the present invention aims to strictly control the microstructure of the steel sheet, it is preferable that the cooling of the present invention is started within 5 seconds immediately after the end of hot rolling. If the time from the end of hot rolling to the start of cooling exceeds 5 seconds, ferrite, pearlite, and bainite, which are not intended by the present invention, may be formed due to air cooling in the atmosphere. The preferred time from the end of hot rolling to the start of cooling can be within 3 seconds.

[0077] The hot-rolled steel sheet immediately after hot rolling may be cooled to a cooling end temperature of 350°C or less at a cooling rate of 50 to 1000°C / s. If the cooling end temperature exceeds a certain range, transformation into ferrite, pearlite, and bainite is inevitable, so the upper limit of the cooling end temperature may be limited to 350°C in order to ensure the microstructure targeted by the present invention. Meanwhile, the lower limit of the cooling end temperature is not particularly specified, but the preferred lower limit of the cooling end temperature may be 150°C. If the cooling rate is below a certain level, transformation into ferrite, pearlite, and bainite occurs during cooling, so the lower limit of the cooling rate may be limited to 50°C / s in order to ensure the microstructure targeted by the present invention. Meanwhile, the upper limit of the cooling rate is not particularly limited, but the upper limit of the cooling rate may be limited to 1000°C / s in consideration of equipment limitations and economic efficiency.

[0078] In the present invention, the hot-rolled steel sheet is cooled under quenching conditions immediately after hot rolling, so that the hot-rolled steel sheet is cooled to 90°C before cold rolling. area% In the conventional manufacturing method of high carbon steel sheet, a heat treatment is performed immediately after hot rolling, and the heat-treated hot-rolled steel sheet is cold-rolled and then quenched to form a martensite structure, whereas in the present invention, the steel composition is strictly controlled so that not only the heat treatment immediately after hot rolling but also the quenching after cold rolling can be omitted, thereby effectively reducing carbon emissions.

[0079] After cooling, 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 to 50%. If the reduction is small, the martensite is not sufficiently elongated, and the desired high strength properties and durability cannot be secured. Therefore, the present invention can limit the reduction of the cold rolling to a level of 20% or more. A preferable lower limit of the cold rolling reduction can be 25%. On the other hand, if the reduction of the cold rolling is excessive, not only is there concern about damage to equipment due to the rolling load, but there is also a possibility that the strength increases too much and durability decreases, so the present invention can limit the upper limit of the cold rolling reduction to 50%.

[0082] The high carbon steel plate manufactured by the above-mentioned manufacturing method has a strength of 90 area% The above martensite is contained as a microstructure, and the martensite may have a residual stress in the rolling direction of 70 MPa or more, a yield strength (YS) of 1300 MPa or more, and a tensile strength (TS) of 1500 MPa or more.

[0083] Furthermore, when a spring member is manufactured using the high carbon steel plate manufactured by the above-mentioned manufacturing method, the durability test result of the spring member can satisfy 100,000 times or more.

[0084] Hereinafter, the high carbon steel sheet and its manufacturing method and parts of the present invention will be described in more detail through specific examples. It should be noted that the following examples are for understanding the present invention and are not intended to specify 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 having the composition shown in Table 1 below, steel plate test pieces were manufactured using the process conditions shown in Table 2 below. Each slab was manufactured by a conventional manufacturing method and was heated and homogenized in the temperature range of 1050 to 1350°C.

[0086] [Table 1]

[0087] [Table 2]

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

[0089] The proportion of elongated martensite was determined by the area of ​​martensite packets whose major axis was aligned within 45° of the rolling direction relative to the total area of ​​martensite in a scanning electron microscope (SEM) image. The long / short axis ratio of martensite packets was also determined by the area of ​​martensite packets whose major / short axis ratio was 2:1 or greater relative to the total area of ​​martensite packets in a scanning electron microscope (SEM) image.

[0090] In Table 3, M means martensite, F means ferrite, R-γ means retained austenite, P means pearlite, and B means 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 conducting a tensile test according to the JIS standard, and residual stress was measured on the surface using an XSTRESS 3000 model device manufactured by Stresstech Group using X-ray diffraction method under the measurement parameters of Exp.Time: 40s, Radiation: CrKa, Detector distance: 50s. In addition, coil-shaped spring members were made using each test specimen, and durability was evaluated through a repeated test in which the wound spring was stretched to a length of 1.5m and then wound back to its original state, and the results are shown in Table 4.

[0093] [Table 4]

[0094] As shown in Tables 1 to 4, Test Specimens 1 to 12, which satisfy the alloy composition and manufacturing conditions of the present invention, have a martensite fraction of 90% or less. area% It can be confirmed that the ratio of martensite stretched in the rolling direction is 50% or more, and the ratio of martensite packets with a long / short axis ratio of 2:1 or more is 50% or more. It can also be confirmed that test pieces 1 to 12 have a residual stress in the rolling direction of 70 MPa or more, a residual stress in a 45° direction to the rolling direction of 30 MPa or more, a tensile strength of 1500 MPa or more, a yield strength of 1300 MPa or more, and a durability test result of 100,000 times or more.

[0095] In contrast, it can be confirmed that test pieces 13 to 20, which do not satisfy any one or more of the alloy composition and manufacturing conditions of the present invention, do not satisfy any one or more of the martensite fraction, the proportion of martensite elongated in the rolling direction, and the proportion of martensite packets having an axis-length ratio of 2:1 or more, which are limited by the present invention.

[0096] Test piece 13 was a test piece in which cooling began 5 seconds after the end of rolling, and it was confirmed that the ferrite fraction was high and the desired strength and durability could not be secured.

[0097] Test piece 14 is a specimen obtained when the rolling end temperature was low, and test piece 16 is a specimen obtained when the cooling rate was slow. These test pieces had high fractions of pearlite and bainite, and it was confirmed that the martensite fraction targeted by the present invention could not be ensured, and the targeted strength and durability could not be ensured.

[0098] It can be confirmed that test piece 15 is a case in which the cooling end temperature and coiling temperature are high, the proportion of bainite is high, and the desired durability cannot be ensured.

[0099] Test piece 17 is a case where the cold rolling reduction rate is low, the proportion of martensite packets with a long-short axis ratio of 2:1 or more is low, and the residual stress of martensite is low, so it can be confirmed that the desired durability cannot be ensured.

[0100] Test piece 18 is a case where the carbon (C) content is low, and test piece 19 is a case where the titanium (Ti) and boron (B) contents are low. It can be confirmed that the martensite fraction is extremely low and the desired levels of strength and durability cannot be ensured.

[0101] Test piece 20 has a high manganese (Mn) content, and it can be confirmed that the transformation to martensite does not occur sufficiently and a large amount of retained austenite is formed, so that while the tensile strength and yield strength are excellent, the durability is poor.

[0102] Although the present invention has been described in detail with reference to the above exemplary embodiments, other embodiments may be possible, and therefore the spirit and scope of the following claims should not be limited to the exemplary embodiments.

Claims

1. In weight percent, 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. Contains 90% by volume or more of martensite as a microstructure, The high carbon steel plate, wherein the martensite has a residual stress in the rolling direction of 70 MPa or more.

2. The high carbon steel plate according to claim 1, further comprising Ti: 0.005 to 0.1 wt%.

3. The high carbon steel plate according to claim 2, further comprising, by weight percent, one or more of 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 plate according to claim 1, wherein the martensite has a residual stress in a direction at 45° to the rolling direction of 30 MPa or more.

5. The high carbon steel sheet according to claim 1, wherein the martensite has a ratio of martensite elongated along the rolling direction of 50% or more.

6. The high carbon steel plate according to claim 1, wherein a ratio of packets having a long / short axis ratio of 2:1 or more to all packets of martensite is 50% or more.

7. Contains one or more selected from ferrite and retained austenite in a total volume fraction of 10% or less (including 0%); The high carbon steel plate according to claim 1, comprising at least one selected from pearlite and bainite in a total volume fraction of 5% or less (including 0%).

8. 2. The high carbon steel plate according to claim 1, wherein the steel plate has a yield strength of 1300 MPa or more and a tensile strength of 1500 MPa or more.

9. The carbon (C) content of the steel plate is more than 0.20 wt.%; The high carbon steel plate according to claim 1, wherein the martensite has a residual stress in the rolling direction of 190 MPa or more.

10. 10. The high carbon steel plate according to claim 9, wherein the steel plate has a yield strength of 1590 MPa or more and a tensile strength of 1640 MPa or more.

11. 11. An industrial or automotive part using the high carbon steel sheet according to claim 1, the industrial or automotive part having a durability test result of 100,000 cycles or more.

12. a step of heating the slab containing, 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%), the remainder being Fe and unavoidable impurities, in a temperature range of 1100°C or more; hot rolling the heated slab at a rolling finish temperature of 800 to 950°C to obtain a hot rolled steel sheet; Within 5 seconds after the completion of the hot rolling, the hot-rolled steel sheet is quenched to a cooling end temperature of 350° C. or less at a cooling rate of 50 to 1000° C. / sec and coiled; and omitting the heat treatment after coiling and cold rolling the hot-rolled steel sheet at a rolling reduction of 20 to 50%.

13. The method for producing a high carbon steel plate according to claim 12, wherein the slab further contains Ti: 0.005 to 0.1 wt%.

14. The method for producing a high carbon steel plate according to claim 13, wherein the slab further contains, by weight percent, one or more of 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.

15. The method for producing a high carbon steel plate according to claim 12, wherein the carbon (C) content of the slab is more than 0.20 wt%.

16. The method for producing a high carbon steel sheet according to claim 12, wherein the quenched hot-rolled steel sheet contains 90 volume % or more of martensite.

17. The method for producing a high carbon steel sheet according to claim 12, wherein quenching is not performed after the cold rolling.

Images