High-carbon hot-rolled steel sheet, high carbon steel product using same and method of manufacturing same

Abstract

The high-carbon steel product according to the present invention can be manufactured by cold working a high-carbon hot-rolled steel sheet comprising, by weight%, C: 0.70–0.90%, Mn: 0.2–0.6%, Si: 0.1–0.5%, Cr: 0.1–0.5%, Al: 0.007–0.015%, Ni: 0.4–2.0%, V: 0.1% or less (including 0%), and the balance of Fe and inevitable impurities; subsequently heating and maintaining same at a temperature of 850–950℃ for 1–90 minutes; cooling same at an average cooling rate of 7℃ / sec or higher; and tempering same.

Description

High-carbon hot-rolled steel sheets, high-carbon steel products using the same, and methods for manufacturing the same

[0001] The present invention mainly relates to a technology for manufacturing high carbon steel products such as cutting tools such as saw blades.

[0002] More specifically, the present invention relates to a high-carbon hot-rolled steel sheet suitable for manufacturing a high-carbon steel product having excellent weldability and impact toughness, and a high-carbon steel product using the same.

[0003] In addition, the present invention relates to a method for manufacturing a high-carbon hot-rolled steel sheet and a method for manufacturing a high-carbon steel product.

[0004] Demand for saw blade materials for cutting wood, stone, metal, and other materials used in civil engineering, construction, and structural applications is increasing. Blades must be able to rotate at high speeds to cut materials, and must withstand axial and radial stresses during the cutting process. Therefore, materials suitable for saw blades require high levels of hardness, rigidity, fatigue strength, and impact toughness.

[0005] Additionally, since the teeth of the saw blade are locally welded with cemented carbide to maximize hardness and wear resistance, weldability is required.

[0006] In the manufacture of high-carbon steel products for cutting, a technique for refining tempered martensite grains to achieve both hardness and impact toughness is known. To achieve this, the austenite grain size (prior austenite grain size) must be refined during quenching heat treatment. Grain refinement is known to improve impact toughness by effectively impeding crack propagation along grain boundaries.

[0007] Methods for refining austenite grain size can be broadly divided into two: utilizing precipitates and lowering the quenching temperature. The method utilizing large amounts of precipitates refines grains by exploiting the phenomenon of precipitate pinning at grain boundaries, while the method of lowering the heat treatment temperature suppresses austenite grain growth.

[0008] However, since the precipitates formed along the grain boundaries have significantly higher hardness than the parent material, cracks caused by property deviations can easily propagate, which may actually reduce the impact toughness. In addition, if the austenite grain size decreases, it is difficult to secure sufficient hardness during the quenching heat treatment process. This is because pearlite transforms at the grain boundaries during the quenching heat treatment process, so not all of the austenite transforms into martensite, and some pearlite is formed.

[0009] The problem to be solved by the present invention is to provide a high-carbon hot-rolled steel sheet and a method for manufacturing the same, which are suitable for manufacturing high-carbon steel products having excellent weldability and impact toughness through control of alloy components and microstructure.

[0010] In addition, the problem to be solved by the present invention is to provide a high-carbon steel product with excellent weldability and impact toughness and a method for manufacturing the same by controlling the heat treatment process along with controlling the alloy composition and microstructure.

[0011] A high-carbon hot-rolled steel sheet according to an embodiment of the present invention for solving the above problem contains, in wt%, C: 0.70-0.90%, Mn: 0.2-0.6%, Si: 0.1-0.5%, Cr: 0.1-0.5%, Al: 0.007-0.015%, Ni: 0.4-2.0%, V: 0.1% or less (including 0%) and the remainder of Fe and unavoidable impurities, and has a microstructure containing pearlite at 90% or more in area%.

[0012] It is preferable that the above high-carbon hot-rolled steel sheet satisfies the following equation 1.

[0013] [Formula 1]

[0014] [C] + [Si] / 14 + [Ni] / 24 + [V] / 10 ≤ 0.94,

[0015] ([ ] is the weight % of the corresponding ingredient)

[0016] In the steel, AlN precipitates are present, and precipitates containing at least one of V, Nb, and Ti may not be present.

[0017] A method for manufacturing a high-carbon hot-rolled steel sheet according to an embodiment of the present invention for solving the above problem comprises the steps of reheating a steel material containing, in wt%, C: 0.70-0.90%, Mn: 0.2-0.6%, Si: 0.1-0.5%, Cr: 0.1-0.5%, Al: 0.007-0.015%, Ni: 0.4-2.0%, V: 0.1% or less (including 0%) and the remainder of Fe and unavoidable impurities at 1050-1300°C; hot-rolling it under a finishing rolling temperature condition of 800-1150°C; and cooling it to 550-750°C and coiling it.

[0018] It is preferable that the above steel material satisfies the following equation 1.

[0019] [Formula 1]

[0020] [C] + [Si] / 14 + [Ni] / 24 + [V] / 10 ≤ 0.94, ([ ] is the weight% of the corresponding component)

[0021] During the cooling, precipitates containing at least one of V, Nb and Ti may not be formed.

[0022] A high-carbon steel product according to an embodiment of the present invention for solving the above problem comprises, in wt%, C: 0.70-0.90%, Mn: 0.2-0.6%, Si: 0.1-0.5%, Cr: 0.1-0.5%, Al: 0.007-0.015%, Ni: 0.4-2.0%, V: 0.1% or less (including 0%) and the remainder of Fe and unavoidable impurities, and has a microstructure including tempered martensite, wherein the average grain size of the tempered martensite is 17 ㎛ or more.

[0023] The average grain size of the above tempered martensite may be 30-40㎛.

[0024] It may have a microstructure composed of the above tempered martensite single phase.

[0025] The above high-carbon steel product can satisfy the following equation 1.

[0026] [Formula 1]

[0027] [C] + [Si] / 14 + [Ni] / 24 + [V] / 10 ≤ 0.94, ([ ] is the weight% of the corresponding component)

[0028] In the steel, AlN precipitates are present, and precipitates containing at least one of V, Nb, and Ti may not be present.

[0029] A method for manufacturing a high-carbon steel product according to an embodiment of the present invention for solving the above problem comprises a cold working step of cold working a hot-rolled steel sheet, which comprises, in wt%, C: 0.70-0.90%, Mn: 0.2-0.6%, Si: 0.1-0.5%, Cr: 0.1-0.5%, Al: 0.007-0.015%, Ni: 0.4-2.0%, V: 0.1% or less (including 0%) and the remainder of Fe and unavoidable impurities; a step of heating and maintaining the cold-worked resultant at a temperature of 850-950°C for 1-90 minutes, and then cooling it at an average cooling rate of 7°C / sec or more; and a step of tempering the quenched resultant.

[0030] Prior to the above cold working, a step of spheroidizing the high-carbon hot-rolled steel sheet may be additionally included.

[0031] The above high-carbon hot-rolled steel sheet can satisfy the following equation 1.

[0032] [Formula 1]

[0033] [C] + [Si] / 14 + [Ni] / 24 + [V] / 10 ≤ 0.94, ([ ] is the weight% of the corresponding component)

[0034] The average grain size of the austenite produced by the above heating and maintaining may be 17 µm or more. The average grain size of the austenite produced by the above heating and maintaining may be 30-40 µm.

[0035] According to the present invention, a steel product having excellent weldability and impact toughness can be manufactured by controlling alloy components such as C, Al, and Ni, preventing austenite grain size refinement due to precipitates, and controlling quenching heat treatment temperature.

[0036] Specifically, according to the present invention, a steel product manufactured through the above-described alloy composition, microstructure, and heat treatment control can exhibit excellent impact toughness along with high hardness characteristics of 650 Hv or higher, and is therefore suitable for use as a cutting material such as a saw blade.

[0037] In addition to the effects described above, specific effects of the present invention are described below while explaining specific details for carrying out the invention.

[0038] The advantages and features of the present invention, and the methods for achieving them, will become clearer with reference to the embodiments described in detail below. However, the present invention is not limited to the embodiments disclosed below, but may be implemented in various different forms. These embodiments are provided solely to ensure that the disclosure of the present invention is complete and to fully inform those skilled in the art of the scope of the invention. The present invention is defined solely by the scope of the claims.

[0039] Hereinafter, a high-carbon hot-rolled steel sheet according to a preferred embodiment of the present invention, a high-carbon steel product using the same, and a manufacturing method thereof will be described in detail.

[0040] The present invention relates to a high-carbon hot-rolled steel sheet suitable for manufacturing steel products such as saw blades for cutting wood, cutting stone, and circular saws, a high-carbon steel product having excellent weldability and toughness using the same, and a method for manufacturing the same.

[0041] For example, a saw blade must be able to rotate at high speeds to cut materials and withstand axial and radial stresses during the cutting process. Therefore, materials suitable for saw blades require high levels of hardness, rigidity, fatigue strength, and impact toughness. Furthermore, since the teeth of a saw blade are locally welded with cemented carbide to maximize hardness and wear resistance, weldability is essential. In summary, cutting materials such as saw blades must satisfy high hardness, excellent impact toughness, and weldability.

[0042] Hardness and weldability are significantly influenced by the alloying composition of the material. As the content of alloying elements such as carbon (C), silicon (Si), and nickel (Ni) increases, hardness improves due to the solid solution strengthening effect. Adding vanadium (V) also enhances hardness by forming hard carbides. However, these elements can affect the structure of the heat-affected zone when exposed to high temperatures, promoting property discrepancies between the melt zone and the base metal, thereby impairing weldability.

[0043] When manufacturing materials with superior hardness and impact toughness, not only alloy composition but also microstructure significantly influences these properties. Reducing the austenite grain size (prior austenite grain size) and increasing the grain boundary area effectively impedes crack propagation, leading to improved impact toughness.

[0044] The most common method is to utilize large amounts of precipitates. However, this method can actually reduce impact toughness. Precipitates formed along grain boundaries are typically significantly harder than the parent material, making cracks caused by property deviations very susceptible to propagation, potentially reducing impact toughness.

[0045] In addition, when the austenite grain size becomes fine, it is difficult to secure sufficient hardness during the quenching heat treatment process because pearlite is transformed at the grain boundaries during the quenching heat treatment process, so not all of the austenite is transformed into martensite, but some pearlite is formed. During the cooling process, the grain boundaries that can form pearlite rather than martensite increase, so the hardenability decreases, and the critical cooling rate for securing martensite by cooling in the temperature range of the austenite single phase during the quenching heat treatment process increases, resulting in poor heat treatment properties.

[0046] Therefore, securing high hardness requires a high alloying element content, while considering weldability, a low alloying element content is required. While fine austenite grain size is essential for improving impact toughness, excessive use of precipitates to refine grain size can actually reduce impact toughness. Furthermore, small grain sizes may prevent uniform martensite formation during the quenching heat treatment process.

[0047] The present invention provides a high-carbon hot-rolled steel sheet that simultaneously considers alloy elements and microstructure to secure weldability and impact toughness, a high-carbon steel product using the same, and a method for manufacturing the same.

[0048] A high-carbon hot-rolled steel sheet according to an embodiment of the present invention contains, in wt%, carbon (C): 0.70-0.90%, manganese (Mn): 0.2-0.6%, silicon (Si): 0.1-0.5%, chromium (Cr): 0.1-0.5%, aluminum (Al): 0.007-0.015%, nickel (Ni): 0.4-2.0%, vanadium (V): 0.1% or less (including 0%), and the remainder of iron and unavoidable impurities.

[0049] Hereinafter, the role and content of each component included in the high-carbon hot-rolled steel sheet according to the present invention will be described.

[0050] Carbon (C): 0.7-0.9 wt%

[0051] Carbon is an element that stably forms austenite and is the alloying element most influential in enhancing the strength and hardness of steel. It also enhances hardenability and is the most important element in determining the hardness of martensite after quenching.

[0052] As the amount of carbon added increases, brittleness increases and toughness decreases, so it is desirable to adjust the carbon content according to the intended use and required level.

[0053] From the perspective of manufacturing high-hardness steel products such as cutting materials such as saw blades, if the carbon content is less than 0.7 wt%, it is difficult to form hard carbides other than the martensite matrix after quenching heat treatment or quenching and tempering heat treatment, and there is a concern that wear resistance may decrease.

[0054] When the carbon content exceeds 0.9 wt%, a large amount of carbides are precipitated at the grain boundaries, resulting in a rapid decrease in toughness, and it may be difficult to secure the required level of impact toughness even after heat treatment.

[0055] From this point of view, the appropriate carbon content is 0.7-0.9 wt%, more preferably 0.75-0.85 wt%.

[0056] Manganese (Mn): 0.2-0.6 wt%

[0057] Manganese combines with the sulfur impurity to improve the purity of steel. Manganese also slightly increases strength and hardness through solid solution strengthening. It also enhances the steel's hardenability, helping to form martensite even at slow cooling rates. To achieve these benefits, manganese must be added at a level of at least 0.2 wt%. However, if the manganese content exceeds 0.6 wt%, there is a risk of segregation layers forming, which can reduce the steel's workability.

[0058] From this point of view, the appropriate manganese content is 0.2-0.6 wt%, more preferably 0.3-0.5 wt%.

[0059] Silicon (Si): 0.1-0.5 wt%

[0060] Silicon has a high affinity for oxygen and is utilized for deoxidation. It is an element that stably forms ferrite and, when dissolved in ferrite, can enhance its strength. For this effect, silicon must be present in amounts of at least 0.1 wt%. However, excessive silicon additions exceeding 0.5 wt% can reduce hot workability and toughness, and can also impair scale exfoliation at high temperatures, degrading the surface quality of hot-rolled steel.

[0061] From this point of view, the appropriate silicon content is 0.1-0.5 wt%, more preferably 0.2-0.4 wt%.

[0062] Chromium (Cr): 0.1-0.5 wt%

[0063] Chromium is a ferrite-stabilizing element that enhances hardenability. For this purpose, chromium must be added in amounts of at least 0.1 wt%.

[0064] Conversely, if the chromium content is excessive, exceeding 0.5 wt%, chromium oxide may be formed during the welding process, which may lead to concerns about weld embrittlement.

[0065] From this point of view, the appropriate chromium content is 0.1-0.5 wt%, more preferably 0.2-0.4 wt%.

[0066] Aluminum (Al): 0.007-0.015 wt%

[0067] Aluminum has a high affinity for oxygen and is used for deoxidation. Furthermore, when precipitate-forming elements such as niobium, titanium, and vanadium are substantially absent, it combines with nitrogen during the hot-rolled steel sheet manufacturing process to form AlN, thereby suppressing grain growth from becoming excessively coarse. To achieve this, aluminum must be added in amounts of at least 0.007 wt%.

[0068] On the other hand, an excessive Al content exceeding 0.015 wt% may increase the content of alumina inclusions, which may result in deterioration of toughness due to undesirable grain refinement.

[0069] From this point of view, the appropriate aluminum content is 0.007-0.015 wt%, more preferably 0.008-0.012 wt%.

[0070] Nickel (Ni): 0.4-2.0 wt%

[0071] Nickel is an element that stably forms austenite. It does not significantly affect the strength and hardness of steel, but because it does not form high-hardness precipitates, adding large amounts does not significantly reduce toughness. In particular, nickel enhances hardenability, enabling the stable formation of martensite even at slow cooling rates after quenching. For this effect, nickel must be added at least 0.4 wt%.

[0072] On the other hand, if nickel is added in large amounts exceeding 2.0 wt%, there is a concern that the residual austenite fraction may increase after heat treatment, thereby reducing hardness.

[0073] From this point of view, the appropriate nickel content is 0.4-2.0 wt%, more preferably 0.8-1.85 wt%.

[0074] Vanadium (V): 0.1 wt% or less (including 0%)

[0075] Vanadium combines with carbon to form hard carbides, thereby improving strength, hardness and wear resistance, and particularly compensates for the decrease in hardness caused by the formation of precipitates during tempering heat treatment.

[0076] However, if vanadium is added in excess, it can significantly promote austenite grain refinement through precipitates, thereby lowering impact toughness.

[0077] From this point of view, it is preferable that vanadium is not added, or if added, its content is 0.1 wt% or less.

[0078] In addition to the composition described above, the steel of the present invention may contain remaining iron (Fe) and unavoidable impurities. Unavoidable impurities may be unintentionally incorporated during the normal manufacturing process, and thus cannot be excluded.

[0079] It is preferable that the high-carbon hot-rolled steel sheet according to the present invention satisfies the following equation 1.

[0080] [Formula 1]

[0081] [C] + [Si] / 14 + [Ni] / 24 + [V] / 10 ≤ 0.94, ([ ] is the weight% of the corresponding component)

[0082] The above formula 1 can be viewed as a type of carbon equivalent (Ceq), and the lower it is, the better the weldability. If the value of the above formula 1 exceeds 0.94, the carbon equivalent may be inhibited due to excessive alloying elements. In other words, if the elements included in the above formula 1 are excessive, when exposed to high temperatures during welding, it affects the structure of the heat-affected zone and promotes a property deviation between the molten zone and the base metal zone, thereby deteriorating the welding characteristics.

[0083] In terms of easily achieving the above formula 1, it is more preferable that the content of C be included at 0.85 wt% or less.

[0084] Meanwhile, the lower limit of the above formula 1 is preferably 0.80 or more in terms of securing hardness, and is more preferably 0.84 or more.

[0085] The high-carbon hot-rolled steel sheet according to the present invention may have a microstructure including pearlite of 90% or more in area% through the hot-rolling process described below in addition to the above alloy components. In addition to pearlite, cementite, austenite, bainite, etc. may be included in an amount of 10% or less. Meanwhile, the high-carbon hot-rolled steel sheet according to the present invention may have AlN precipitates in the steel, and may not have precipitates including at least one of Nb and Ti, which are representative precipitate-forming elements. Furthermore, in some embodiments where V is not included, precipitates including V may not be present.

[0086] A high-carbon hot-rolled steel sheet according to the present invention can be manufactured by a method including the following reheating step, hot rolling step, and coiling step using a steel material having the above alloy component.

[0087] Reheating and hot rolling

[0088] After manufacturing a steel material such as a slab having the aforementioned alloy composition using a furnace or electric furnace, the steel is reheated at a temperature of 1050℃ or higher and 1300℃ or lower for 270 minutes or less. Reheating can promote homogenization of the steel.

[0089] The homogenized steel slab is hot rolled at 800℃ or higher. The finishing rolling temperature is preferably 800-1150℃, preferably 800-1000℃, and more preferably 820-920℃. If the finishing rolling temperature is lower than 800℃, hot rolling may proceed in the austenite and ferrite coexistence region or the austenite and cementite coexistence region, which may cause plate fracture due to deformation imbalance at the phase interface. On the other hand, if the finishing rolling temperature is higher than 1150℃, there is a concern that excessive scale may occur, resulting in poor surface quality.

[0090] Winding

[0091] Coiling at an appropriate temperature range determines the shape of the material. It is preferable to cool hot-rolled steel immediately after hot rolling to a temperature of 550-750°C, more preferably 600-700°C, before coiling. Coiling at a temperature lower than 550°C can lead to the formation of localized low-temperature microstructures such as bainite and martensite in the width or length direction, which can distort the shape of the material, cause deviations in physical properties, or even cause edge cracks. Coiling at a temperature higher than 750°C can lead to excessive scaling, which can degrade surface quality.

[0092] In the present invention, since the alloy composition does not include Nb and Ti, precipitates containing at least one of Nb and Ti are not formed during cooling. Furthermore, if V is not included, precipitates containing V may not be formed either.

[0093] Through the above process, a microstructure containing pearlite of 90% or more in area% can be obtained, and cementite, austenite, bainite, etc. can be included at 10% or less.

[0094] Hereinafter, a high-carbon steel product using a high-carbon hot-rolled steel sheet according to the present invention and a manufacturing method thereof will be described.

[0095] Spherical annealing

[0096] Materials with a microstructure primarily composed of pearlite can be difficult to process due to their high strength and hardness. Spheroidizing annealing softens the material, creating a microstructure composed of ferrite and spherical cementite. This process significantly reduces strength and hardness, making room-temperature processing, such as rolling, forging, pressing, and punching, much easier.

[0097] Spheroidization annealing can be performed, for example, at temperatures between 650°C and 750°C. At temperatures below 650°C, spheroidization becomes difficult, takes a long time, and has a very low spheroidization rate. Temperatures above 750°C are the temperatures at which some austenite phases form stably, so there is a risk of pearlite transformation.

[0098] Depending on the situation, the annealing of the spheroid may be omitted.

[0099] cold working

[0100] Cold working can be performed at room temperature by known methods such as rolling, forging, pressing, and punching to obtain the desired product shape.

[0101]

[0102] Quenching heat treatment and tempering heat treatment

[0103] The method for manufacturing steel products according to the present invention performs quenching and tempering heat treatments to impart hardness and toughness to the steel product. For example, in the case of steel products for cutting purposes, such as blades and saw blades, high hardness and high toughness are required, so quenching and tempering heat treatments are performed to simultaneously satisfy these characteristics.

[0104] In the present invention, the quenching heat treatment is performed by maintaining heating at a temperature of 850°C or higher to secure an austenite microstructure and then cooling at a rate of 7°C / second or higher to transform into a martensite structure.

[0105] Specifically, the heating and holding is performed at a temperature of 850-950°C, preferably 860-920°C, more preferably 870-900°C, for 1-90 minutes, preferably 10-60 minutes, more preferably 20-50 minutes. If the heating and holding is performed at a temperature lower than 850°C or the heating and holding time is less than 1 minute, austenite cannot be sufficiently secured, making it difficult to secure a martensite microstructure after cooling, and also making it difficult to obtain the target coarse austenite grain size. On the other hand, if the heating and holding temperature exceeds 950°C or the heating and holding time exceeds 90 minutes, there is a concern that the average austenite grain size may grow excessively, for example, exceeding 40 μm.

[0106] The average grain size of the austenite produced by the above heating and maintaining process may be 17 μm or greater. This can be seen as the opposite of conventional techniques aimed at making the austenite grains as fine as possible. Furthermore, in some embodiments, vanadium (V) may not be included, in which case the average grain size of the austenite produced by the heating and maintaining process may be 30-40 μm.

[0107] Cooling can be performed by water cooling, oil cooling, etc., and more preferably by water cooling. What is important in the present invention is to secure the highest possible fraction of martensite structure during cooling. Hardenability, which means the ability to secure martensite, is affected by the microstructure and alloy components. In the present invention, cooling is preferably performed at an average cooling rate of 7°C / sec or more. If the average cooling rate is less than 7°C / sec, there is a risk of formation of structures such as bainite and pearlite. When the average cooling rate is 7°C / sec or more, 100% martensite can be secured. Meanwhile, in the present invention, hardenability can be improved without deteriorating toughness by adding 0.1-0.5 wt% of Cr and 0.4-2.0 wt% of Ni, so that complete quenching heat treatment can be performed even under conditions of an average cooling rate of 20°C / sec or less. Therefore, in the quenching heat treatment applied to the method for manufacturing steel products according to the present invention, it is preferable that the average cooling rate be performed at an average cooling rate of 7°C / second or more, more preferably at an average cooling rate of 7-20°C / second, and even more preferably at an average cooling rate of 10-20°C / second.

[0108] Quenching heat treatment can produce a high-hardness martensite structure with a hardness of 650 Hv or higher. However, materials subjected to quenching heat treatment have poor room-temperature impact toughness, less than 2 J. Therefore, sufficient tempering is required to secure impact toughness.

[0109] Tempering heat treatment can be performed by maintaining the temperature in the range of 150-600℃, more preferably in the range of 300-500℃ for 1-180 minutes, preferably for 10-90 minutes, more preferably for 20-60 minutes, and then cooling to secure a tempered martensite microstructure. A temperature range below 150℃ has little effect in annihilating dislocations, and a temperature range above 600℃ causes an excessive decrease in hardness, making it difficult to secure physical properties. If the heat treatment time is less than 1 minute, there is not enough time for the material to be heated evenly, and if it exceeds 180 minutes, the effect converges and becomes meaningless.

[0110] Cooling after tempering heat treatment can be performed, for example, by exposure to air at room temperature.

[0111] A high-carbon steel product according to an embodiment of the present invention contains, in wt%, C: 0.70-0.90%, Mn: 0.2-0.6%, Si: 0.1-0.5%, Cr: 0.1-0.5%, Al: 0.007-0.015%, Ni: 0.4-2.0%, V: 0.1% or less (including 0%) and the remainder of Fe and unavoidable impurities, and has a microstructure including tempered martensite through quenching heat treatment and tempering heat treatment. Preferably, the high-carbon steel product according to the present invention has a microstructure composed of a single phase of tempered martensite.

[0112] In the present invention, the average grain size of austenite is 17 ㎛ or more during the quenching heat treatment, which can be directly transferred to the grain size of tempered martensite. Therefore, in the case of the high-carbon steel product according to the present invention, the average grain size of tempered martensite can be 17 ㎛ or more. In some embodiments where V is not included, the average grain size of tempered martensite can be 30-40 ㎛.

[0113] Example

[0114] Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the examples described below are intended only to illustrate and further concretize the present invention and are not intended to limit the scope of the present invention.

[0115] Table 1 shows the chemical composition of the steel specimen prepared for the present invention, and the remainder is Fe and inevitable impurities.

[0116] [Table 1]

[0117]

[0118] Equation 1(A): [C] + [Si] / 14 + [Ni] / 24 + [V] / 10 ≤ 0.94, ([ ] is the weight% of the corresponding component)

[0119] Each of the steel specimens listed in Table 1 was reheated at 1250°C for 2 hours, hot-rolled to a thickness of 6 mm under finishing rolling conditions at 850°C, and then coiled at 620°C to produce a hot-rolled steel sheet.

[0120] The manufactured hot-rolled steel sheet was subjected to quenching heat treatment by maintaining it at 880°C for 30 minutes and then cooling it in water at room temperature. After the quenching heat treatment was completed, tempering heat treatment was performed by maintaining it at 300°C, 400°C, and 500°C for 40 minutes and then cooling it in air at room temperature.

[0121] The fraction of tempered martensite and the average grain size of tempered martensite in the final tempered heat-treated steel specimens were measured through microscopic observation, and the average grain size of austenite was obtained from the observation of tempered martensite.

[0122] [Table 2]

[0123]

[0124] Equation 1: [C] + [Si] / 14 + [Ni] / 24 + [V] / 10 ≤ 0.94, ([ ] is the weight% of the corresponding component)

[0125] Comparative steel 3 is the result of a specimen that has the same steel composition and manufacturing process as Inventive steel 1, but has an Al content of only 0.005 wt% in the steel composition, and Comparative steel 4 is the result of a specimen that has the same steel composition and manufacturing process as Inventive steel 1, but has an excessive Al content of 0.02 wt% in the steel composition.

[0126] Comparative steel 5 is the result when the steel composition and manufacturing process are the same as those of Inventive steel 1, but the quenching heat treatment temperature is 820℃, and Comparative steel 6 is the result when the steel composition and manufacturing process are the same as those of Inventive steel 1, but the quenching heat treatment temperature is 840℃.

[0127] Referring to Table 2, both the invention steel 1-10 and the comparative steel 1-6 showed pearlite of more than 90% in the hot-rolled state. However, in the case of the invention steel 1-10, the average austenite grain size was more than 17㎛ after quenching heat treatment, whereas in the case of the comparative steel 1-2, the average austenite grain size was only about 5-7㎛. In addition, when comparing the invention steel 1-10, the specimens according to the invention steel 1-6, which did not contain V, had an average austenite grain size of 30-40㎛. On the other hand, the comparative steel 3, which had an Al content of only 0.005 wt%, had an average austenite grain size exceeding 50㎛, and the comparative steel 4, which had an excessive Al content of 0.02 wt%, had an average austenite grain size of less than 5㎛.

[0128] Furthermore, when the average austenite grain size (B) was divided by the value (A) according to Equation 1, the B / A of the specimens according to the invention steel 1-10 was 18 or more, while the specimens according to the comparative steel 1-2 showed 5-7. In addition, when comparing the invention steel 1-10, the specimens according to the invention steel 1-6, which does not include V, had a B / A exceeding 30. What this B / A means is that the austenite grain size can be coarsened to a target size using an alloying element that does not significantly change the carbon equivalent, such as Al, for example.

[0129] In addition, in the case of the invention steels 1-10 that satisfy the alloy composition and quenching heat treatment temperature suggested in the present invention, the microstructure of the target tempered martensite single phase was exhibited. However, in the case of the comparative steels 5 and 6, which have the same steel composition and manufacturing process as the invention steel 1 but have quenching heat treatment temperatures of 820°C and 840°C, the target tempered martensite single phase could not be secured.

[0130] Table 3 shows the room temperature impact toughness and hardness values ​​of steels subjected to quenching and tempering heat treatments.

[0131] The room temperature impact toughness was tested three times after processing V-notch specimens according to the ISO 148-1 standard, and the impact toughness value was converted to the value when the material thickness was 10 mm and displayed.

[0132] Hardness was measured through a Vickers hardness test with a load of 10 kgf, and was expressed by performing the experiment 10 times.

[0133] [Table 3]

[0134]

[0135] Referring to Table 3, the hardness of the specimens of Invention Steel 1-10 and Comparative Steel 1-4, which underwent quenching heat treatment, was found to be 697 Hv or higher. Subsequently, the hardness was observed to decrease as a result of tempering heat treatment. In particular, as the tempering temperature increased, the hardness gradually decreased.

[0136] Regarding Vickers hardness, both the invention steel 1-11 and the comparative steel 1-2 possess high hardness characteristics of 697 Hv or more after quenching heat treatment. As the tempering temperature increases, dislocations disappear, and the hardness gradually decreases, showing a result that is lowered to 360 Hv after tempering heat treatment at 500℃. The hardness measurement values ​​show similar values ​​regardless of the invention steel 1-11 and the comparative steel 1-4, which can be considered to be because the carbon content, which is the most dominant factor in the hardness of the tempered martensite microstructure, is at the same level.

[0137] The room temperature impact toughness of the quenched heat-treated material was very poor, less than 2J for both the invention steel 1-10 and the comparative steel 1-6. However, when the tempering heat treatment was performed, the impact toughness was improved, and it could be seen that the impact toughness gradually increased as the tempering temperature increased. Specifically, the room temperature impact toughness of the quenched heat-treated material of the invention steel 1-10 was 1.8J or less, and the impact toughness increased as the tempering temperature increased, and it could be seen that it had an impact toughness of 10J or more after tempering at 500℃.

[0138] However, the comparative steel 1-2 exhibited a relatively high impact toughness compared to other steel specimens, with a room temperature impact toughness of 1.9J for the quenched heat-treated material. This is due to the high fraction of retained austenite, and when tempering heat treatment is performed, the effect disappears as the retained austenite is decomposed into tempered martensite, so that after the final tempering heat treatment, it can be seen that it exhibits an impact toughness value that is inferior to that of the inventive steel 1-10.

[0139] Meanwhile, Comparative Steels 1 and 2 contain 0.4 wt% vanadium (V), and Comparative Steel 4 contains 0.02 wt% aluminum. It is generally known that as the austenite grain size decreases, the grain boundaries that can impede crack propagation increase, thereby improving impact toughness. That is, V precipitates and Al precipitates are very effective in reducing the grain size. As can be seen in Table 2, Comparative Steels 1 and 2 with 0.4 wt% V content and Comparative Steel 4 with 0.02 wt% Al content have the smallest austenite grain sizes, between 4 and 6 μm.

[0140] However, in the case of comparative steels 1-2 and 4 with excessive addition of V and Al, the impact toughness is rather inferior to that of inventive steel 1-10. This can be seen as a result of the excessive vanadium precipitates, which facilitate the occurrence of cracks due to the difference in properties between the base material and the precipitates. In particular, the material tempered at 500℃, where vanadium carbide precipitation is easy, showed a significant decrease in impact toughness.

[0141] In the case of Comparative Steel 3, the Al content was only 0.005 wt%, so the average austenite grain size exceeded 50 ㎛, and the austenite grains were too coarse, resulting in poor impact toughness.

[0142] For comparative steels 5 and 6, the quenching heat treatment temperature was relatively low, and in this case, since it was difficult for carbides to be re-dissolved, some cementite remained even after the quenching heat treatment. As a result, for comparative steels 5 and 6, the impact toughness was low due to the residual cementite. In addition, for comparative steels 5 and 6, the carbon that should have been dissolved in martensite to contribute to the improvement of hardness was consumed in forming cementite, so the dissolved carbon concentration in martensite, which is a relatively large matrix structure, was low, resulting in a decrease in the overall hardness.

[0143] While the embodiments of the present invention have been described above, it is clear that the present invention is not limited to the embodiments disclosed herein, and that various modifications may be made by those skilled in the art within the scope of the technical concept of the present invention. Furthermore, even if the operational effects according to the configuration of the present invention were not explicitly described and explained while describing the embodiments of the present invention, it is natural that the effects predictable by the corresponding configuration should also be acknowledged.

Claims

1. Contains, in weight%, C: 0.70-0.90%, Mn: 0.2-0.6%, Si: 0.1-0.5%, Cr: 0.1-0.5%, Al: 0.007-0.015%, Ni: 0.4-2.0%, V: 0.1% or less (including 0%) and the remainder of Fe and inevitable impurities. A high-carbon hot-rolled steel sheet having a microstructure containing pearlite at an area % of 90% or more.

2. In paragraph 1, The above high-carbon hot-rolled steel plate is prepared according to the following formula 1. [Formula 1] [C] + [Si] / 14 + [Ni] / 24 + [V] / 10 ≤ 0.94, ([ ] is the weight% of the corresponding component) High-carbon hot-rolled steel sheet satisfying .

3. In paragraph 1, A high-carbon hot-rolled steel sheet having AlN precipitates and no precipitates containing at least one of V, Nb and Ti.

4. A step of reheating a steel material containing C: 0.70-0.90%, Mn: 0.2-0.6%, Si: 0.1-0.5%, Cr: 0.1-0.5%, Al: 0.007-0.015%, Ni: 0.4-2.0%, V: 0.1% or less (including 0%) and the remainder of Fe and unavoidable impurities at 1050-1300℃. A step of hot rolling under the finishing rolling temperature condition of 800-1150℃, A method for manufacturing a high-carbon hot-rolled steel sheet, comprising the step of cooling to 550-750℃ and coiling.

5. In paragraph 4, The above steel material is as follows: Equation 1 [Formula 1] [C] + [Si] / 14 + [Ni] / 24 + [V] / 10 ≤ 0.94, ([ ] is the weight% of the corresponding component) A method for manufacturing a high-carbon hot-rolled steel sheet satisfying the following requirements.

6. In paragraph 4, A method for manufacturing a hot-rolled steel sheet, wherein a precipitate containing at least one of V, Nb and Ti is not generated during the cooling. A high-carbon steel product containing, by weight%, C: 0.70-0.90%, Mn: 0.2-0.6%, Si: 0.1-0.5%, Cr: 0.1-0.5%, Al: 0.007-0.015%, Ni: 0.4-2.0%, V: 0.1% or less (including 0%) and the remainder of Fe and unavoidable impurities, and having a microstructure including tempered martensite, wherein the average grain size of the tempered martensite is 17㎛ or more.

8. In paragraph 7, A high-carbon steel product having an average grain size of the above tempered martensite of 30-40㎛.

9. In paragraph 7, A high-carbon steel product having a microstructure composed of the above tempered martensite single phase.

10. In paragraph 7, The above high carbon steel product is manufactured according to the following formula 1 [Formula 1] [C] + [Si] / 14 + [Ni] / 24 + [V] / 10 ≤ 0.94, ([ ] is the weight% of the corresponding component) High carbon steel products that satisfy .

11. In paragraph 8, A high-carbon steel product, wherein AlN precipitates are present in the steel and precipitates containing at least one of V, Nb and Ti are absent.

12. Cold working step of cold working a high-carbon hot-rolled steel sheet containing, by weight%, C: 0.70-0.90%, Mn: 0.2-0.6%, Si: 0.1-0.5%, Cr: 0.1-0.5%, Al: 0.007-0.015%, Ni: 0.4-2.0%, V: 0.1% or less (including 0%) and the remainder Fe and unavoidable impurities; A step of heating and maintaining the cold-worked result at a temperature of 850-950℃ for 1-90 minutes, and then cooling it at an average cooling rate of 7℃ / sec or more; and A method for manufacturing a high-carbon steel product, comprising the step of tempering the quenched resultant.

13. In paragraph 12, A method for manufacturing a high-carbon steel product, further comprising a step of spheroidizing the high-carbon hot-rolled steel sheet prior to the cold working.

14. In paragraph 12, The above high-carbon hot-rolled steel plate is prepared according to the following formula 1. [Formula 1] [C] + [Si] / 14 + [Ni] / 24 + [V] / 10 ≤ 0.94, ([ ] is the weight% of the corresponding component) A method for manufacturing a high-carbon steel product satisfying the requirements of .

15. In paragraph 12, A method for manufacturing a high-carbon steel product, wherein the average grain size of austenite produced by the above heating maintenance is 17 ㎛ or more.

16. In paragraph 15, A method for manufacturing a high-carbon steel product, wherein the average grain size of austenite produced by the above heating maintenance is 30-40㎛.

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