QT heat treated high carbon hot rolled steel sheet, high carbon cold rolled steel sheet, QT heat treated high carbon cold rolled steel sheet and manufacturing method thereof

Abstract

The present invention provides a QT heat-treated high-carbon hot-rolled steel sheet, a high-carbon cold-rolled steel sheet, and a QT heat-treated high-carbon cold-rolled steel sheet, which contain, by weight, 1.0-1.4% C, 0.1-0.4% Si, 0.1-0.8% Mn, 0.3-11% Cr, 0.05-2.5% W, 0.03% or less P, 0.03% or less S, 0.02% or less Al, the balance being Fe and other unavoidable impurities, and have an average carbide size of 0.1-20 μm, as well as methods for producing these.

Description

[Technical Field]

[0001] The present invention relates to a QT heat-treated high-carbon hot-rolled steel sheet, a high-carbon cold-rolled steel sheet, a QT heat-treated high-carbon cold-rolled steel sheet, and methods for producing these. [Background technology]

[0002] High carbon steel refers to steel that contains 0.3% or more carbon, or about 0.15% carbon along with other alloying elements. Generally, as the carbon content increases, the hardness and strength of steel increases, making carbon the most economical and effective element for adjusting the physical properties of steel. JIS standards classify steel types according to their carbon content, and among the steel types currently produced in electric furnaces, SK120 has the highest carbon content, with a carbon content of 1.15-1.25%.

[0003] The SK120 alloy can achieve even higher hardness by transforming its microstructure into martensite through rapid cooling at high temperatures in the austenite single-phase region. However, because martensite is highly brittle, it is reheated in the austenite region and then tempered to maintain toughness. This series of heat treatments is commonly referred to as QT (Quenching-Tempering).

[0004] However, although the SK120 has the advantage of being excellent in hardness and toughness after QT heat treatment due to the C content of 1.15 to 1.25%, it has the disadvantage of being composed of a single tempered martensite phase and therefore having low wear resistance.

[0005] To overcome these drawbacks, a method was developed to allow some cementite to remain during QT heat treatment using spheroidized annealed SK120 by adjusting the reheating temperature and time. However, cementite has a hardness of 1300Hv, which is not significantly different from the tempered martensite base material, making it difficult to expect excellent wear resistance. Another drawback is that cementite dissolves completely in the reheating temperature range during QT heat treatment, requiring advanced heat treatment technology. Summary of the Invention [Problem to be solved by the invention]

[0006] One aspect of the present invention is to provide a QT heat-treated high-carbon hot-rolled steel sheet, a QT heat-treated high-carbon cold-rolled steel sheet, and methods for producing these. [Means for solving the problem]

[0007] One embodiment of the present invention provides a QT heat-treated high carbon hot rolled steel sheet containing, by weight %, C: 1.0 to 1.4%, Si: 0.1 to 0.4%, Mn: 0.1 to 0.8%, Cr: 0.3 to 11%, W: 0.05 to 2.5%, P: 0.03% or less, S: 0.03% or less, Al: 0.02% or less, the balance being Fe and other unavoidable impurities, and the microstructure contains, by area %, 0.1 to 20% carbides, the balance being tempered martensite, and the carbides have an average size of 0.1 to 20 μm.

[0008] Another embodiment of the present invention provides a high carbon cold rolled steel sheet containing, by weight%, C: 1.0 to 1.4%, Si: 0.1 to 0.4%, Mn: 0.1 to 0.8%, Cr: 0.3 to 11%, W: 0.05 to 2.5%, P: 0.03% or less, S: 0.03% or less, Al: 0.02% or less, the balance being Fe and other unavoidable impurities, and having a microstructure containing, by area%, 20 to 99.9% ferrite, 10% or less cementite, 50% or less pearlite, and 0.1 to 20% carbides, wherein the carbides have an average size of 0.1 to 20 μm.

[0009] Yet another embodiment of the present invention provides a QT heat-treated high carbon cold rolled steel sheet containing, by weight%, C: 1.0 to 1.4%, Si: 0.1 to 0.4%, Mn: 0.1 to 0.8%, Cr: 0.3 to 11%, W: 0.05 to 2.5%, P: 0.03% or less, S: 0.03% or less, Al: 0.02% or less, the balance being Fe and other unavoidable impurities, and the microstructure contains, by area%, 0.1 to 20% carbides, the balance being tempered martensite, and the carbides have an average size of 0.1 to 20 μm.

[0010] Yet another embodiment of the present invention provides a method for manufacturing a QT heat-treated high carbon hot rolled steel sheet, the method including the steps of: preparing a hot rolled steel sheet containing, by weight%, 1.0 to 1.4% C, 0.1 to 0.4% Si, 0.1 to 0.8% Mn, 0.3 to 11% Cr, 0.05 to 2.5% W, 0.03% or less P, 0.03% or less S, 0.02% or less Al, with the balance being Fe and other unavoidable impurities; reheating the prepared hot rolled steel sheet at 740 to 1100°C; cooling the reheated hot rolled steel sheet at a cooling rate of 10°C / s or more; and tempering the cooled hot rolled steel sheet at 150 to 600°C.

[0011] Yet another embodiment of the present invention provides a method for producing a high carbon cold rolled steel sheet, the method including the steps of: preparing a hot rolled steel sheet containing, by weight, C: 1.0 to 1.4%, Si: 0.1 to 0.4%, Mn: 0.1 to 0.8%, Cr: 0.3 to 11%, W: 0.05 to 2.5%, P: 0.03% or less, S: 0.03% or less, Al: 0.02% or less, the balance being Fe and other unavoidable impurities; and cold rolling the prepared hot rolled steel sheet to obtain a cold rolled steel sheet.

[0012] Yet another embodiment of the present invention provides a method for manufacturing a QT heat-treated high carbon cold-rolled steel sheet, the method including the steps of: preparing a hot-rolled steel sheet containing, by weight, 1.0 to 1.4% C, 0.1 to 0.4% Si, 0.1 to 0.8% Mn, 0.3 to 11% Cr, 0.05 to 2.5% W, 0.03% or less P, 0.03% or less S, 0.02% or less Al, and the balance being Fe and other unavoidable impurities; cold-rolling the prepared hot-rolled steel sheet to obtain a cold-rolled steel sheet; reheating the cold-rolled steel sheet at 740 to 1100°C; cooling the reheated cold-rolled steel sheet at a cooling rate of 10°C / s or more; and tempering the cooled cold-rolled steel sheet at 150 to 600°C. [Effects of the Invention]

[0013] According to one aspect of the present invention, there can be provided a QT heat-treated high-carbon hot-rolled steel sheet, a high-carbon cold-rolled steel sheet, a QT heat-treated high-carbon cold-rolled steel sheet, and methods for manufacturing these. DETAILED DESCRIPTION OF THE INVENTION

[0014] The high carbon steel of the present invention will be described below. First, the alloy composition of the high carbon steel of the present invention will be described. The contents of the alloy composition described below are in weight percent unless otherwise specified.

[0015] C: 1.0 to 1.4% Carbon (C) is the alloying element that has the greatest impact on improving the strength and hardness of steel. C is an element that stably forms austenite. Due to its small atomic size, C exerts a solid-solution strengthening effect when present in a solid solution state. Meanwhile, due to its low solubility limit in ferrite, C exerts a strengthening effect by forming precipitates with alloying elements that form carbides or by combining with Fe to form cementite (Fe3C). Due to its fast diffusion rate, C rapidly redistributes even when maintained at high temperatures for a short period of time. Therefore, it exerts the greatest influence on increasing the hardness of martensite and simultaneously improving the wear resistance of steel. If C is added in an amount less than 1.0%, the aforementioned effects of improving strength and wear resistance are insufficient. On the other hand, if C exceeds 1.4%, pro-eutectoid cementite may form at austenite grain boundaries, resulting in a decrease in toughness. Therefore, the C content is preferably in the range of 1.0 to 1.4%. The lower limit of the C content is more preferably 1.05%. The upper limit of the C content is more preferably 1.35%, and even more preferably 1.3%.

[0016] Si: 0.1 to 0.4% Si is an element that stably forms ferrite and dissolves in ferrite to improve strength. If the Si content is less than 0.1%, the solid solution strengthening effect is insufficient, and if it exceeds 0.4%, the hot workability and toughness decrease. Therefore, the Si content is preferably in the range of 0.1 to 0.4%. The upper limit of the Si content is more preferably 0.35%.

[0017] Mn: 0.1 to 0.8% Mn has the effect of improving the cleanliness of steel as a deoxidizing and desulfurizing agent. It is also added to ensure hardenability, taking into account the cooling level. If the Mn content is less than 0.1%, the above effect is insufficient, and if it exceeds 0.8%, a segregation layer is formed in the center of the thickness, reducing workability. Therefore, the Mn content is preferably in the range of 0.1 to 0.8%. The upper limit of the Mn content is more preferably 0.7%, and even more preferably 0.6%.

[0018] Cr: 0.3 to 11% Cr is a ferrite stabilizing element and is an element that dissolves in the matrix structure to ensure hardening ability. Furthermore, Cr combines with C to form hard Cr7C3 carbides, thereby improving hardness and wear resistance. If the Cr content is less than 0.3%, the above effect is insufficient, while if it exceeds 11%, there is a risk of excessive hardening ability and the formation of coarse Cr7C3 carbides, resulting in a decrease in toughness. Therefore, the Cr content is preferably in the range of 0.3 to 11%. It is more preferable that the upper limit of the Cr content is 10.5%.

[0019] W: 0.05 to 2.5% W improves wear resistance by bonding with C to form hard carbides with a hardness of 2300 to 2800 Hv. To achieve this effect, it is preferable that 0.05% or more of W be added. However, if the W content exceeds 2.5%, there is a risk of inducing brittleness due to excessive hardening ability. Therefore, the W content is preferably in the range of 0.05 to 2.5%. The upper limit of the W content is more preferably 2.45% or less, and even more preferably 2.35% or less.

[0020] P:0.03% or less P is an impurity that cannot be filtered out during the steelmaking process, and the lower its content, the better the cleanliness and workability. However, in the present invention, taking economical reasons into consideration, the upper limit is controlled at 0.03%.

[0021] S: 0.03% or less S is an impurity that cannot be filtered out during the steelmaking process, and the lower its content, the better the cleanliness and workability. However, in the present invention, taking economical reasons into consideration, the upper limit is controlled at 0.03%.

[0022] Al: 0.02% or less Al is an element that is generally used as a deoxidizer in the steelmaking process and is added to ensure cleanliness, but in the present invention, taking into account its effectiveness and economic efficiency, its content is controlled to 0.02% or less.

[0023] In addition to the above-mentioned steel composition, the remainder may include Fe and inevitable impurities. Unavoidable impurities are those that may be unintentionally mixed in during a typical steel manufacturing process, and cannot be completely eliminated. Those skilled in the art of typical steel manufacturing can easily understand the meaning of these impurities. However, the present invention does not completely exclude the addition of other components than the above-mentioned steel composition.

[0024] Meanwhile, in addition to the above-mentioned alloy composition, the present invention may further include one or more selected from the group consisting of V: ​​0.8% or less (excluding 0%), Mo: 2.5% or less (excluding 0%), and Nb: 1.5% or less (excluding 0%).

[0025] V: 0.8% or less (excluding 0%) V improves wear resistance by combining with C to form hard carbides of approximately 2300 Hv. However, if V exceeds 0.8%, the coarse V-containing carbides may cause brittleness. Therefore, the V content is preferably 0.8% or less. The lower limit of the V content is more preferably 0.01%, and even more preferably 0.05%. The upper limit of the V content is more preferably 0.7%.

[0026] Mo: 2.5% or less (excluding 0%) Mo, alone or in combination with elements such as V and Nb, forms hard carbides with C, improving wear resistance. It also has the effect of improving hardenability, similar to Cr. However, if the Mo content exceeds 2.5%, the excessive hardenability may induce embrittlement. Therefore, the Mo content is preferably 2.5% or less. The lower limit of the Mo content is more preferably 0.1%, and even more preferably 0.2%. The upper limit of the Mo content is more preferably 2.4%.

[0027] Nb: 1.5% or less (excluding 0%) Nb combines with C to form hard carbides, improving wear resistance. However, since the precipitation temperature of Nb is high at approximately 1300°C, adding a large amount of Nb may result in the formation of coarse carbides, which may reduce toughness, so it is preferable to add 1.5% or less. Therefore, the Nb content is preferably 1.5% or less. The lower limit of the Nb content is more preferably 0.05%, and even more preferably 0.1%. The upper limit of the Nb content is more preferably 1.2%.

[0028] The QT heat-treated high-carbon hot-rolled steel sheet of the present invention will be described below.

[0029] The microstructure of the QT heat-treated high-carbon hot-rolled steel sheet of the present invention preferably contains, in area percent, 0.1 to 20% carbides and the remainder tempered martensite. By including tempered martensite as the matrix structure, the present invention can ensure not only excellent wear resistance but also impact resistance. Furthermore, the present invention enhances wear resistance by ensuring an appropriate carbide fraction. If the carbide fraction is less than 0.1%, it is difficult to expect wear resistance due to the hard carbides. If it exceeds 20%, it is difficult to expect the material to break easily due to brittleness. The lower limit of the carbide fraction is more preferably 0.2%, and even more preferably 0.5%. The upper limit of the carbide fraction is more preferably 18%, and even more preferably 16%. Meanwhile, the present invention is not particularly limited by the type of carbide, and may be, for example, a single or composite carbide containing one or more of W, V, Mo, and Nb. Meanwhile, the microstructure of the QT heat-treated high-carbon hot-rolled steel sheet of the present invention may unavoidably contain one or more of ferrite, pearlite, bainite, and retained austenite in a total amount of less than 10% due to the manufacturing process. If the total amount of one or more of ferrite, pearlite, bainite, and retained austenite is 10% or more, the hardness may decrease. The total amount of one or more of ferrite, pearlite, bainite, and retained austenite is more preferably 7% or less, and even more preferably 5%.

[0030] The carbides may have an average size of 0.1 to 20 μm. If the size of the carbides is less than 0.1 μm, the effect of improving hardness is minimal, and if it exceeds 20 μm, it may cause brittleness of the steel material. The lower limit of the average size of the carbides is more preferably 0.3 μm, and even more preferably 0.5 μm. The upper limit of the average size of the carbides is more preferably 17 μm, and even more preferably 15 μm.

[0031] The QT heat-treated high-carbon hot-rolled steel sheet according to one embodiment of the present invention may have a hardness of 350 Hv or more. Furthermore, when subjected to an abrasion resistance test according to ASTM G99, the QT heat-treated high-carbon hot-rolled steel sheet may have an abrasion loss of 35 mg or less when the reheating temperature before QT is 800°C, an abrasion loss of 27 mg or less when the reheating temperature before QT is 850°C, and an abrasion loss of 25 mg or less when the reheating temperature before QT is 900°C. This allows for excellent hardness and abrasion resistance to be achieved simultaneously.

[0032] Hereinafter, the high carbon cold rolled steel sheet of the present invention will be described.

[0033] The microstructure of the high-carbon cold-rolled steel sheet of the present invention may contain, by area percentage, 20 to 99.9% ferrite, 10% or less cementite, 50% or less pearlite, and 0.1 to 20% carbide. If the ferrite content is less than 20%, low hardness cannot be ensured, resulting in poor workability in cold rolling and other processes. If the ferrite content exceeds 99.9%, cementite or hard carbides cannot be ensured, resulting in poor wear resistance after QT heat treatment. The lower limit of the ferrite content is more preferably 30%, and even more preferably 40%. The upper limit of the ferrite content is more preferably 99.8%, and even more preferably 99.5%. If the cementite content exceeds 20%, the material becomes brittle, making processing difficult. The lower limit of the cementite content is more preferably 0.1%, and even more preferably 0.3%. The upper limit of the cementite content is more preferably 8%, and even more preferably 7%. If the pearlite content exceeds 50%, low hardness cannot be ensured, resulting in poor workability in cold rolling and other processes. The lower limit of the pearlite content is more preferably 1%, and even more preferably 5%. The upper limit of the pearlite content is more preferably 40%, and even more preferably 30%. If the carbide content is less than 0.1%, it is difficult to expect wear resistance from the hard carbides, and if it exceeds 20%, it is difficult to expect the material to break easily due to brittleness. The lower limit of the carbide content is more preferably 0.2%, and even more preferably 0.5%. The upper limit of the carbide content is more preferably 18%, and even more preferably 16%.

[0034] The carbides may have an average size of 0.1 to 20 μm. If the size of the carbides is less than 0.1 μm, the effect of improving hardness is minimal, and if it exceeds 20 μm, it may induce brittleness of the steel material. The lower limit of the average size of the carbides is more preferably 0.3 μm, and even more preferably 0.5 μm. The upper limit of the average size of the carbides is more preferably 17 μm, and even more preferably 15 μm.

[0035] The high carbon cold rolled steel sheet according to an embodiment of the present invention may have a hardness of 350 Hv or less. By ensuring such low hardness, high formability can be ensured, which can facilitate part forming in the subsequent process.

[0036] The QT heat-treated high-carbon cold-rolled steel sheet of the present invention will be described below.

[0037] The microstructure of the QT heat-treated high-carbon cold-rolled steel sheet of the present invention preferably contains, in area percent, 0.1 to 20% carbides and the remainder tempered martensite. By including tempered martensite as the matrix structure, the present invention can ensure not only excellent wear resistance but also impact resistance. Furthermore, the present invention enhances wear resistance by ensuring an appropriate carbide fraction. If the carbide fraction is less than 0.1%, it is difficult to expect wear resistance due to the hard carbides. If it exceeds 20%, it is difficult to expect the material to break easily due to brittleness. The lower limit of the carbide fraction is more preferably 0.2%, and even more preferably 0.5%. The upper limit of the carbide fraction is more preferably 18%, and even more preferably 16%. Meanwhile, the present invention is not particularly limited by the type of carbide, and may be, for example, a single or composite carbide containing one or more of W, V, Mo, and Nb. Meanwhile, the microstructure of the QT heat-treated high-carbon hot-rolled steel sheet of the present invention may unavoidably contain one or more of ferrite, pearlite, bainite, and retained austenite in a total amount of less than 10% due to the manufacturing process. If the total amount of one or more of ferrite, pearlite, bainite, and retained austenite is 10% or more, the hardness may decrease. The total amount of one or more of ferrite, pearlite, bainite, and retained austenite is more preferably 7% or less, and even more preferably 5%.

[0038] The average size of the carbides may be 0.1 to 20 μm. If the size of the carbides is less than 0.1 μm, the effect of improving hardness is minimal, and if it exceeds 20 μm, it may induce brittleness of the steel material. The lower limit of the average size of the carbides is more preferably 0.3 μm, and even more preferably 0.5 μm. The upper limit of the average size of the carbides is more preferably 17 μm, and even more preferably 15 μm.

[0039] The QT heat-treated high carbon cold rolled steel sheet according to one embodiment of the present invention may have a hardness of 350 Hv or more. Furthermore, when subjected to an abrasion resistance test according to ASTM G99, the QT heat-treated high carbon cold rolled steel sheet may have an abrasion loss of 25 mg or less when the reheating temperature before QT is 900°C. This allows for excellent hardness and abrasion resistance to be secured simultaneously.

[0040] Hereinafter, a method for manufacturing a QT heat-treated high carbon hot rolled steel sheet according to one embodiment of the present invention will be described.

[0041] First, a hot-rolled steel sheet having the above-mentioned alloy composition is prepared. The step of preparing the hot-rolled steel sheet can include the steps of heating a slab at 1100 to 1300°C and hot-rolling the heated slab at 700 to 1100°C. If the slab is heated to a temperature below 1100°C, the temperature may be low, making rolling difficult. If the temperature exceeds 1300°C, high-temperature oxidation may occur or the slab may melt locally depending on whether or not there is a temperature deviation in the furnace. If the hot-rolling temperature is below 700°C, the material has a high strength, which increases the load during hot rolling. If the temperature exceeds 1100°C, high-temperature oxidation may deteriorate the surface quality.

[0042] The hot-rolled steel sheet thus prepared may have one or more microstructures selected from pearlite, pearlite with some cementite precipitated at grain boundaries, bainite, and martensite, and may have a hardness of 200 Hv or more.

[0043] Thereafter, the hot-rolled steel sheet is reheated at 740 to 1100°C. If the reheating temperature of the hot-rolled steel sheet is less than 740°C, austenite cannot be obtained, resulting in the disadvantage that martensitic transformation does not occur after quenching. If the reheating temperature exceeds 1100°C, crystal grains may grow excessively, making it impossible to obtain the desired physical properties. The lower limit of the reheating temperature of the hot-rolled steel sheet is more preferably 800°C. The upper limit of the reheating temperature of the hot-rolled steel sheet is more preferably 1050°C.

[0044] The reheated hot-rolled steel sheet is then cooled at a cooling rate of 10°C / s or more. If the cooling rate is less than 10°C, there is a drawback in that low-hardness microstructures such as ferrite and pearlite are formed during the cooling process after reheating. The cooling rate is preferably 40°C or more, more preferably 90°C / s or more, and most preferably 100°C / s or more. Meanwhile, in the present invention, the faster the cooling rate, the better, so there is no particular upper limit. However, it may be difficult to exceed 200°C / s due to design limitations.

[0045] The cooled hot-rolled steel sheet is then tempered at 150 to 600°C. If the tempering temperature is less than 150°C, dislocation recovery is insufficient and the tempering effect is ineffective, while if it exceeds 600°C, there is a disadvantage that phase transformation may occur. The lower limit of the tempering temperature is more preferably 170°C, and even more preferably 190°C. The upper limit of the lower limit of the tempering temperature is more preferably 500°C, even more preferably 450°C, and most preferably 380°C.

[0046] The method for producing a high carbon cold rolled steel sheet of the present invention will be described below.

[0047] First, a hot-rolled steel sheet having the above-mentioned alloy composition is prepared. The step of preparing the hot-rolled steel sheet can include heating a slab to 1100-1300°C and hot-rolling the heated slab at 700-1100°C. If the slab is heated to a temperature below 1100°C, the temperature may be low, making rolling difficult. If the temperature exceeds 1300°C, high-temperature oxidation may occur or the slab may melt locally depending on whether or not there is a temperature deviation in the furnace. If the hot-rolling temperature is below 700°C, the material has high strength, resulting in a large hot-rolling load. If the temperature exceeds 1100°C, high-temperature oxidation may cause deterioration of surface quality.

[0048] The hot-rolled steel sheet thus prepared may have one or more microstructures selected from pearlite, pearlite with some cementite precipitated at grain boundaries, bainite, and martensite, and may have a hardness of 200 Hv or more.

[0049] Meanwhile, the method may further include a step of subjecting the prepared hot-rolled steel sheet to a spheroidizing annealing heat treatment at 630 to 850°C. The spheroidizing annealing heat treatment is intended to prevent the cold rolling process from being impossible or to prevent equipment defects due to the high strength of the hot-rolled steel sheet. That is, the spheroidizing annealing heat treatment reduces the strength by spheroidizing cementite, which has particularly high strength, thereby facilitating the cold rolling process. If the spheroidizing annealing heat treatment temperature is less than 630°C, the spheroidizing time required is excessively long, resulting in reduced economic efficiency. If the spheroidizing annealing heat treatment temperature exceeds 800°C, pearlite may form during the heat treatment process, resulting in little reduction in strength or hardness. The lower limit of the spheroidizing annealing heat treatment temperature is more preferably 650°C, and even more preferably 670°C. The upper limit of the spheroidizing annealing heat treatment temperature is more preferably 830°C, and even more preferably 810°C.

[0050] The hot-rolled steel sheet is then cold-rolled to obtain a cold-rolled steel sheet. The cold-rolling process may be performed by a method commonly used in the art. Therefore, the present invention is not particularly limited to the cold-rolling process as long as it can obtain a cold-rolled steel sheet having a desired thickness.

[0051] Meanwhile, the method for producing the high carbon cold rolled steel sheet may include carrying out the spheroidizing annealing heat treatment and cold rolling step once or twice or more.

[0052] Hereinafter, a method for manufacturing a QT heat-treated high carbon cold rolled steel sheet according to one embodiment of the present invention will be described.

[0053] First, a hot-rolled steel sheet having the above-mentioned alloy composition is prepared. The step of preparing the hot-rolled steel sheet can include the steps of heating a slab to 1100-1300°C and hot-rolling the heated slab at 700-1100°C. If the slab is heated to a temperature below 1100°C, the temperature may be low, making rolling difficult. If the temperature exceeds 1300°C, high-temperature oxidation may occur or the slab may melt locally depending on whether or not there is a temperature deviation in the furnace. If the hot-rolling temperature is below 700°C, the material has high strength, resulting in a large hot-rolling load. If the temperature exceeds 1100°C, high-temperature oxidation may cause deterioration of surface quality.

[0054] The hot-rolled steel sheet thus prepared may have one or more microstructures selected from pearlite, pearlite with some cementite precipitated at grain boundaries, bainite, and martensite, and may have a hardness of 200 Hv or more.

[0055] Meanwhile, the method may further include a step of subjecting the prepared hot-rolled steel sheet to a spheroidizing annealing heat treatment at 630 to 850°C. The spheroidizing annealing heat treatment is intended to prevent the cold rolling process from being impossible or to prevent equipment defects due to the high strength of the hot-rolled steel sheet. That is, the spheroidizing annealing heat treatment is intended to reduce the strength of the high-strength cementite through spheroidizing, thereby facilitating the cold rolling process. If the spheroidizing annealing heat treatment temperature is less than 630°C, the spheroidizing time required is excessively long, resulting in reduced economic efficiency. If the spheroidizing annealing heat treatment temperature exceeds 800°C, pearlite may form during the heat treatment process, resulting in little reduction in strength or hardness. The lower limit of the spheroidizing annealing heat treatment temperature is more preferably 650°C, and even more preferably 670°C. The upper limit of the spheroidizing annealing heat treatment temperature is more preferably 830°C, and even more preferably 810°C.

[0056] The hot-rolled steel sheet is then cold-rolled to obtain a cold-rolled steel sheet. The cold-rolling process may be performed by a method commonly used in the art. Therefore, the present invention is not particularly limited to the cold-rolling process as long as it can obtain a cold-rolled steel sheet having a desired thickness.

[0057] Thereafter, the cold-rolled steel sheet is reheated at 740 to 1100°C. If the reheating temperature of the cold-rolled steel sheet is less than 740°C, austenite cannot be secured and martensitic transformation does not occur after quenching, while if the reheating temperature exceeds 1100°C, crystal grains may grow excessively, making it difficult to secure desired physical properties. The lower limit of the reheating temperature of the cold-rolled steel sheet is more preferably 800°C. The upper limit of the reheating temperature of the cold-rolled steel sheet is more preferably 1050°C.

[0058] The reheated cold-rolled steel sheet is then cooled at a cooling rate of 10°C / s or more. If the cooling rate is less than 10°C, there is a drawback in that low-hardness microstructures such as ferrite and pearlite are formed during the cooling process after reheating. The cooling rate is preferably 40°C or more, more preferably 90°C / s or more, and most preferably 100°C / s or more. Meanwhile, in the present invention, the faster the cooling rate, the better, and there is no particular upper limit. However, it may be difficult to exceed 200°C / s due to design limitations.

[0059] The cooled cold-rolled steel sheet is then tempered at 150 to 600°C. If the tempering temperature is less than 150°C, dislocation recovery is insufficient and the tempering effect is ineffective, while if it exceeds 600°C, there is a disadvantage that phase transformation may occur. The lower limit of the tempering temperature is more preferably 170°C, and even more preferably 190°C. The upper limit of the lower limit of the tempering temperature is more preferably 500°C, even more preferably 450°C, and most preferably 380°C. [Example]

[0060] The present invention will be described in more detail below with reference to examples. However, the following examples are intended to explain the present invention in more detail and are not intended to limit the scope of the present invention.

[0061] Example 1 Slabs having the alloy compositions shown in Table 1 below were heated at 1200°C and then hot-rolled at 900°C to obtain hot-rolled steel sheets, and the hardness of these hot-rolled steel sheets was measured and shown in Table 1. The hot-rolled steel sheets thus obtained were reheated at 800°C, 850°C, and 900°C, respectively, cooled at a cooling rate of 80°C / s, and tempered at 200°C to produce QT heat-treated hot-rolled steel sheets.

[0062] The microstructure, hardness and wear resistance of the QT heat-treated hot rolled steel sheets manufactured as described above were measured, and the results are shown in Table 2 below.

[0063] The microstructure fraction was calculated with the aid of ThermoCalc software based on thermodynamic properties.

[0064] The size of the carbides was observed using a FE-SEM scanning electron microscope. Specifically, the test specimen was polished using sandpaper from #400 to #2000, then final polished with 1 μm diamond abrasive, treated with 2% nital etchant, and then observed using an image analysis program.

[0065] The hardness was measured using a Vickers hardness tester, where the test was repeated five times with a measurement load of 10 kg, and the average value was calculated.

[0066] A ball-on-disk test was conducted according to ASTM G99 to evaluate abrasion resistance. A test specimen, machined into a disk shape with a diameter of 31 mm and a thickness of 5 mm, was rubbed against a SiC ball with a diameter of 12.7 mm at room temperature for 3,600 seconds at a force of 50 N and a speed of 1,000 rpm. Abrasion resistance was expressed as the weight of the test specimen before abrasion minus the weight after abrasion, i.e., abrasion loss. The smaller the abrasion loss, the better the abrasion resistance.

[0067] [Table 1]

[0068] [Table 2]

[0069] As can be seen from Tables 1 and 2 above, inventive steels 1 to 15 that satisfy the conditions proposed by the present invention, excellent hardness and wear resistance are achieved by ensuring the microstructure and carbide size that the present invention aims to achieve.

[0070] On the other hand, in the case of the conventional steels and comparative steels 1 to 4, which do not satisfy the W content conditions proposed by the present invention, the size of the carbides that the present invention aims to obtain could not be secured, and therefore, it can be seen that the hardness and wear resistance are at low levels.

[0071] Example 2 A slab having the alloy composition shown in Table 1 in Example 1 above was heated to 1200°C and then hot-rolled at 900°C to obtain a hot-rolled steel sheet, which was then subjected to spheroidizing annealing at 770°C and cold-rolled to produce a cold-rolled steel sheet. This cold-rolled steel sheet was also reheated to 900°C, cooled at a cooling rate of 40°C / s, and tempered at 210°C to produce a QT heat-treated cold-rolled steel sheet.

[0072] The microstructure and hardness of the cold-rolled steel sheets prepared as described above were measured, and the results are shown in Table 3 below. In addition, the microstructure, hardness, and wear resistance of the QT heat-treated cold-rolled steel sheets prepared as described above were measured, and the results are shown in Table 4 below.

[0073] The microstructure, hardness and wear resistance were measured using the same methods as in Example 1.

[0074] [Table 3]

[0075] [Table 4]

[0076] As can be seen from Tables 3 and 4 above, inventive steels 1 to 15 that satisfy the conditions proposed by the present invention, excellent hardness and wear resistance are achieved by ensuring the microstructure and carbide size that the present invention aims to achieve.

[0077] On the other hand, in the case of the conventional steels and comparative steels 1 to 4, which do not satisfy the W content conditions proposed by the present invention, the size of the carbides that the present invention aims to obtain could not be secured, and therefore, it can be seen that the hardness and wear resistance are at low levels.

Claims

1. The alloy contains, by weight, C: 1.0 to 1.4%, Si: 0.1 to 0.4%, Mn: 0.1 to 0.8%, Cr: 0.3 to 11%, W: 0.05 to 2.5%, P: 0.03% or less, S: 0.03% or less, Al: 0.02% or less, with the balance being Fe and other unavoidable impurities; The microstructure contains, by area %, 0.1 to 20% carbides and the remainder is tempered martensite, The carbides have an average size of 0.1 to 20 μm.

2. 2. The QT heat-treated high carbon hot-rolled steel sheet according to claim 1, wherein the hot-rolled steel sheet further contains one or more selected from the group consisting of V: ​​0.8% or less (excluding 0%), Mo: 2.5% or less (excluding 0%), and Nb: 1.5% or less (excluding 0%).

3. The QT heat-treated high carbon hot-rolled steel sheet according to claim 1, wherein the hot-rolled steel sheet has a hardness of 350 Hv or more.

4. The QT heat-treated high carbon hot-rolled steel sheet according to claim 1, wherein the heat-rolled steel sheet has an abrasion loss of 35 mg or less.

5. The alloy contains, by weight, C: 1.0 to 1.4%, Si: 0.1 to 0.4%, Mn: 0.1 to 0.8%, Cr: 0.3 to 11%, W: 0.05 to 2.5%, P: 0.03% or less, S: 0.03% or less, Al: 0.02% or less, with the balance being Fe and other unavoidable impurities; The microstructure contains, by area %, 0.1 to 20% carbides and the remainder is tempered martensite, The carbides have an average size of 0.1 to 20 μm.

6. The QT heat-treated high carbon cold-rolled steel sheet according to claim 5, wherein the cold-rolled steel sheet further contains one or more selected from the group consisting of V: ​​0.8% or less (excluding 0%), Mo: 2.5% or less (excluding 0%), and Nb: 1.5% or less (excluding 0%).

7. The QT heat-treated high carbon cold-rolled steel sheet according to claim 5, wherein the cold-rolled steel sheet has a hardness of 350 Hv or more.

8. The QT heat-treated high carbon cold-rolled steel sheet according to claim 5, wherein the cold-rolled steel sheet has an abrasion loss of 25 mg or less.

9. preparing a hot-rolled steel sheet containing, by weight %, C: 1.0 to 1.4%, Si: 0.1 to 0.4%, Mn: 0.1 to 0.8%, Cr: 0.3 to 11%, W: 0.05 to 2.5%, P: 0.03% or less, S: 0.03% or less, Al: 0.02% or less, with the balance being Fe and other unavoidable impurities; Reheating the prepared hot-rolled steel sheet at 740 to 1100°C; Cooling the reheated hot-rolled steel sheet at a cooling rate of 10°C / s or more; and The method for manufacturing a QT heat-treated high carbon hot rolled steel sheet according to claim 1, further comprising the step of tempering the cooled hot rolled steel sheet at 150 to 600°C.

10. 10. The method for manufacturing a QT heat-treated high carbon hot rolled steel sheet according to claim 9, wherein the step of preparing the hot rolled steel sheet comprises the steps of: heating a slab at 1100 to 1300°C; and hot rolling the heated slab at 700 to 1100°C.

11. 10. The method for manufacturing a QT heat-treated high carbon hot-rolled steel sheet according to claim 9, wherein the prepared hot-rolled steel sheet has one or more microstructures selected from pearlite, pearlite with some cementite precipitated at grain boundaries, bainite, and martensite.

12. The method for manufacturing a QT heat-treated high carbon hot rolled steel sheet according to claim 9, wherein the prepared hot rolled steel sheet has a hardness of 200 Hv or more.

13. preparing a hot-rolled steel sheet containing, by weight %, C: 1.0 to 1.4%, Si: 0.1 to 0.4%, Mn: 0.1 to 0.8%, Cr: 0.3 to 11%, W: 0.05 to 2.5%, P: 0.03% or less, S: 0.03% or less, Al: 0.02% or less, with the balance being Fe and other unavoidable impurities; cold rolling the prepared hot-rolled steel sheet to obtain a cold-rolled steel sheet; Reheating the cold-rolled steel sheet at 740 to 1100°C; Cooling the reheated cold-rolled steel sheet at a cooling rate of 10°C / s or more; and The method for manufacturing a QT heat-treated high carbon cold rolled steel sheet according to claim 5, further comprising the step of tempering the cooled cold rolled steel sheet at 150 to 600°C.

14. 14. The method for manufacturing a QT heat-treated high carbon cold rolled steel sheet according to claim 13, wherein the step of preparing the hot rolled steel sheet comprises: heating a slab at 1100 to 1300°C; and hot rolling the heated slab at 700 to 1100°C.

15. The method for manufacturing a QT heat-treated high carbon cold rolled steel sheet according to claim 13, wherein the prepared hot rolled steel sheet has one or more microstructures selected from pearlite, pearlite with some cementite precipitated at grain boundaries, bainite, and martensite.

16. The method for manufacturing a QT heat-treated high carbon cold rolled steel sheet according to claim 13, wherein the prepared hot rolled steel sheet has a hardness of 200 Hv or more.

17. The method for manufacturing a QT heat-treated high carbon cold rolled steel sheet according to claim 13, further comprising the step of subjecting the hot rolled steel sheet to a spheroidizing annealing heat treatment at 630 to 850°C before the cold rolling.

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