High carbon hot rolled steel sheets, high carbon cold rolled steel sheets, high carbon cold rolled steel sheets subjected to quenching and tempering heat treatment, and methods for manufacturing the same

Abstract

Provided are a high-carbon hot-rolled steel sheet, a high-carbon cold-rolled steel sheet, a high-carbon cold-rolled steel sheet subjected to quenching and tempering heat treatment, and a method for manufacturing the same, the high-carbon hot-rolled steel sheet comprising, in terms of % by mass, C: 1.0-1.4%, Si: 0.1-0.4%, Mn: 0.1-0.8%, Cr: 0.3-11%, W: 0.05-2.5%, P: 0.03% or less, S: 0.03% or less, Al: 0.02% or less, the balance of Fe and other unavoidable impurities, and the average size of carbides being 0.1-20 µm.

Description

TECHNICAL FIELD

[0001] The present application relates to a high carbon hot-rolled steel sheet, a high carbon cold-rolled steel sheet, a high carbon cold-rolled steel sheet subjected to quenching-tempering (QT) heat treatment, and a method of manufacturing the same. BACKGROUND

[0002] High carbon steel refers to steel containing 0.3% or more of carbon or containing about 0.15% of carbon together with other alloying elements. In general, as the carbon content increases, the hardness and strength of the steel increase, and thus carbon is the most economical and effective element for adjusting the physical properties of the steel. In the JIS standard, steel types are classified according to the content of carbon, and among the steel types currently produced in a converter, the steel type having the highest carbon content is SK120, which has a carbon content of 1.15-1.25%.

[0003] The SK120 can obtain higher hardness by phase-transforming a fine structure into martensite through rapid cooling heat treatment at a high temperature in an austenite single phase region. However, martensite is strong in brittleness, and thus is subjected to tempering after reheating in an austenite region to secure toughness. In general, this series of heat treatment processes is called quenching-tempering (QT).

[0004] However, the SK120 has the advantage of excellent hardness and toughness after quenching-tempering heat treatment due to containing 1.15-1.25% of C, but has the disadvantage of low wear resistance due to being composed of a single phase of tempered martensite.

[0005] In order to compensate for this disadvantage, a method of allowing a portion of cementite to remain by adjusting the reheating temperature and time when the SK120 subjected to spheroidizing annealing heat treatment is subjected to quenching-tempering heat treatment has been developed. However, the hardness of the cementite is at a level of 1300 Hv, and there is no large difference in hardness from the tempered martensite as a base material, and thus excellent wear resistance cannot be expected. In addition, the cementite is completely solid-solved in the reheating temperature range during quenching-tempering heat treatment, and thus has the disadvantage of requiring a high level of heat treatment technology. SUMMARY

[0006] Technical Problem to be Solved

[0007] An object of one aspect of the present application is to provide a high carbon hot-rolled steel sheet, a high carbon cold-rolled steel sheet, a high carbon cold-rolled steel sheet subjected to quenching-tempering (QT) heat treatment, and a method of manufacturing the same.

[0008] Technical Solution

[0009] One embodiment of the present invention provides a high-carbon hot-rolled steel sheet subjected to quenching and tempering heat treatment, comprising, in weight %, C: 1.0-1.4 %, Si: 0.1-0.4 %, Mn: 0.1-0.8 %, Cr: 0.3-11 %, W: 0.05-2.5 %, P: 0.03 % or less, S: 0.03 % or less, Al: 0.02 % or less, the balance of Fe and other inevitable impurities, and a microstructure comprising, in area %, 0.1-20 % of carbides and the balance of tempered martensite, and the average size of the carbides being 0.1-20 µm.

[0010] Another embodiment of the present invention provides a high-carbon cold-rolled steel sheet, comprising, in weight %, C: 1.0-1.4 %, Si: 0.1-0.4 %, Mn: 0.1-0.8 %, Cr: 0.3-11 %, W: 0.05-2.5 %, P: 0.03 % or less, S: 0.03 % or less, Al: 0.02 % or less, the balance of Fe and other inevitable impurities, and a microstructure comprising, in area %, ferrite: 20-99.9 %, cementite: 10 % or less, pearlite: 50 % or less, and carbides: 0.1-20 %, and the average size of the carbides being 0.1-20 µm.

[0011] Another embodiment of the present invention provides a high-carbon cold-rolled steel sheet subjected to quenching and tempering heat treatment, comprising, in weight %, C: 1.0-1.4 %, Si: 0.1-0.4 %, Mn: 0.1-0.8 %, Cr: 0.3-11 %, W: 0.05-2.5 %, P: 0.03 % or less, S: 0.03 % or less, Al: 0.02 % or less, the balance of Fe and other inevitable impurities, and a microstructure comprising, in area %, 0.1-20 % of carbides, the balance of tempered martensite, and the average size of the carbides being 0.1-20 µm.

[0012] Another embodiment of the present invention provides a method of manufacturing a high-carbon hot-rolled steel sheet subjected to quenching and tempering heat treatment, comprising the steps of: preparing a hot-rolled steel sheet comprising, in weight %, C: 1.0-1.4 %, Si: 0.1-0.4 %, Mn: 0.1-0.8 %, Cr: 0.3-11 %, W: 0.05-2.5 %, P: 0.03 % or less, S: 0.03 % or less, Al: 0.02 % or less, the balance of Fe and other inevitable impurities; re-heating the prepared hot-rolled steel sheet at 740-1100 °C; cooling the re-heated hot-rolled steel sheet at a cooling rate of 10 °C / sec or more; and tempering the cooled hot-rolled steel sheet at 150-600 °C.

[0013] Another embodiment of the present application provides a method of manufacturing a high-carbon cold-rolled steel sheet, including the steps of: preparing a hot-rolled steel sheet including, in weight %, C: 1.0-1.4 %, Si: 0.1-0.4 %, Mn: 0.1-0.8 %, Cr: 0.3-11 %, W: 0.05-2.5 %, P: 0.03 % or less, S: 0.03 % or less, Al: 0.02 % or less, a balance of Fe and other inevitable impurities; and cold-rolling the prepared hot-rolled steel sheet to obtain a cold-rolled steel sheet.

[0014] Another embodiment of the present application provides a method of manufacturing a high-carbon cold-rolled steel sheet, including the steps of: preparing a hot-rolled steel sheet including, in weight %, C: 1.0-1.4 %, Si: 0.1-0.4 %, Mn: 0.1-0.8 %, Cr: 0.3-11 %, W: 0.05-2.5 %, P: 0.03 % or less, S: 0.03 % or less, Al: 0.02 % or less, a balance of Fe and other inevitable impurities; and cold-rolling the prepared hot-rolled steel sheet to obtain a cold-rolled steel sheet.

[0015] Advantageous Effects

[0016] According to one aspect of the present application, a high-carbon hot-rolled steel sheet, a high-carbon cold-rolled steel sheet, a high-carbon cold-rolled steel sheet subjected to a quenched and tempered heat treatment, and a method of manufacturing the same can be provided.

[0017] Best Mode for Carrying Out the Invention

[0018] Hereinafter, a high-carbon steel according to the present application will be described. First, an alloy composition of the high-carbon steel according to the present application will be described. Unless otherwise specified, the content of the alloy composition described below indicates weight %.

[0019] C: 1.0-1.4 %

[0020] C is the most influential alloying element for increasing the strength and hardness of steel. C is an element that stably forms austenite, and the atomic size is small, so it has a solid solution strengthening effect when present in a solid solution state. In addition, the solid solubility limit of C in ferrite is low, so it forms precipitates in combination with alloying elements that form carbides, or forms cementite (Fe3C) in combination with Fe, thereby exhibiting a strengthening effect. C has a fast diffusion rate, so it rapidly redistributes even when kept at a high temperature for a short time. Therefore, the C has the greatest influence on increasing the hardness of martensite, while increasing the wear resistance of the steel. When the C is added in an amount less than 1.0%, the above-mentioned effects of increasing the strength and wear resistance are insufficient. On the other hand, when the C exceeds 1.4%, eutectoid cementite is formed in the grain boundaries of austenite, so the toughness can be reduced. Therefore, the content of the C preferably has a range of 1.0 to 1.4%. The lower limit of the content of the C is more preferably 1.05%. The upper limit of the content of the C is more preferably 1.35%, and further preferably 1.3%.

[0021] Si: 0.1 to 0.4%

[0022] Si is an element that stably forms ferrite, and the Si is solid-solved in ferrite to increase the strength. When the Si is less than 0.1%, the solid solution strengthening effect is insufficient, and when the Si exceeds 0.4%, the hot workability and toughness are reduced. Therefore, the content of the Si preferably has a range of 0.1 to 0.4%. The upper limit of the content of the Si is more preferably 0.35%.

[0023] Mn: 0.1 to 0.8%

[0024] Mn has an effect of increasing the cleanliness of steel as a deoxidizing and desulfurizing agent. In addition, Mn is added to ensure hardenability in consideration of the cooling level. When the Mn is less than 0.1%, the effect is insufficient, and when the Mn exceeds 0.8%, a segregation layer is formed in the center portion of the thickness, resulting in reduced workability. Therefore, the content of the Mn preferably has a range of 0.1 to 0.8%. The upper limit of the content of the Mn is more preferably 0.7%, and further preferably 0.6%.

[0025] Cr: 0.3 to 11%

[0026] Cr is a ferrite stabilizing element, and Cr is an element that is solid-solved in the matrix structure to ensure hardenability. In addition, Cr forms hard Cr7C3 carbides in combination with C, so it has an effect of increasing the hardness and wear resistance. When the Cr is less than 0.3%, the effect is insufficient, and when the Cr exceeds 11%, the toughness can be reduced due to excessive hardenability and the formation of coarse Cr7C3 carbides. Therefore, the content of the Cr preferably has a range of 0.3 to 11%. The upper limit of the content of the Cr is more preferably 10.5%.

[0027] W: 0.05-2.5%

[0028] W combines with C to form hard carbides with a wear resistance of 2300-2800 Hv, thereby improving wear resistance. For this effect, it is preferable to add 0.05% or more of W. However, when the W content exceeds 2.5%, there is a risk of brittleness due to excessive hardenability. Therefore, the W content is preferably in the range of 0.05-2.5%. The upper limit of the W content is more preferably 2.45% or less, and even more preferably 2.35% or less.

[0029] P: below 0.03%

[0030] P is an impurity that cannot be filtered out during the steelmaking process; the lower the content, the better the cleanliness and processability. However, in this invention, considering economic factors, the upper limit of P is controlled at 0.03%.

[0031] S: below 0.03%

[0032] S is an impurity that cannot be filtered out during the steelmaking process; the lower the content, the better the cleanliness and processability. However, in this invention, considering economic factors, the upper limit of S is controlled at 0.03%.

[0033] Al: below 0.02%

[0034] Al is an element commonly used as a deoxidizer in steelmaking processes, and its addition ensures cleanliness. However, in this invention, considering both effectiveness and economy, its content is controlled to below 0.02%.

[0035] In addition to the steel composition described above, the remaining components may include Fe and unavoidable impurities. Unavoidable impurities may be unintentionally introduced during conventional steel manufacturing processes, therefore these impurities cannot be completely eliminated, and their meaning is readily understood by those skilled in the art of steel manufacturing. Furthermore, the addition of other components besides the steel composition described above is not entirely excluded in this invention.

[0036] In addition, in this invention, besides the alloy composition described above, it may further contain one or more of V: ​​less than 0.8% (excluding 0%), Mo: less than 2.5% (excluding 0%), and Nb: less than 1.5% (excluding 0%).

[0037] V: Below 0.8% (excluding 0%)

[0038] V combines with C to form hard carbide of about 2300 Hv, thereby improving wear resistance. However, when V exceeds 0.8%, there is a risk of causing a brittleness disadvantage due to the presence of coarse V carbide. Therefore, the content of V is preferably in the range of 0.8% or less. The lower limit of the V content is more preferably 0.01%, and further preferably 0.05%. The upper limit of the V content is more preferably 0.7%.

[0039] Mo: 2.5% or less (except 0%)

[0040] Mo alone or together with V, Nb, etc. combines with C to form hard carbide, thereby improving wear resistance. In addition, it has the same effect as Cr to improve hardenability. However, when the Mo exceeds 2.5%, there is a risk of causing brittleness due to excessive hardenability. Therefore, the content of Mo is preferably 2.5% or less. The lower limit of the Mo content is more preferably 0.1%, and further preferably 0.2%. The upper limit of the Mo content is more preferably 2.4%.

[0041] Nb: 1.5% or less (except 0%)

[0042] Nb combines with C to form hard carbide to improve wear resistance. However, the precipitation temperature of Nb is about 1300°C and is high, so when added in large amounts, coarse carbide is formed, so there is a risk of reducing toughness, so it is preferable to add 1.5% or less of Nb. Therefore, the content of Nb is preferably 1.5% or less. The lower limit of the Nb content is more preferably 0.05%, and further preferably 0.1%. The upper limit of the Nb content is more preferably 1.2%.

[0043] Hereinafter, the quenched and tempered heat-treated high-carbon hot-rolled steel plate of the present application will be described.

[0044] The microstructure of the quenched and tempered high carbon hot-rolled steel sheet of the present application preferably contains 0.1 to 20% of carbides and the balance of tempered martensite in terms of area %. In the present application, by containing the tempered martensite as a matrix structure, excellent wear resistance and impact resistance can be ensured. Further, the present application improves the wear resistance by ensuring an appropriate fraction of carbides. When the fraction of the carbides is less than 0.1%, there is a disadvantage in that it is difficult to expect the wear resistance to be ensured by hard carbides, and when the fraction of the carbides exceeds 20%, there is a disadvantage in that the material is easily damaged due to brittleness. The lower limit of the fraction of the carbides is more preferably 0.2%, and further preferably 0.5%. The upper limit of the fraction of the carbides is more preferably 18%, and further preferably 16%. In addition, in the present application, the type of the carbides is not particularly limited, and for example, can be a single carbide or a complex carbide containing one or more of W, V, Mo, and Nb. In addition, the microstructure of the quenched and tempered high carbon hot-rolled steel sheet of the present application can inevitably contain one or more of ferrite, pearlite, bainite, and residual austenite in a total amount of less than 10% due to the manufacturing process. When the total amount of one or more of the ferrite, the pearlite, the bainite, and the residual austenite is 10% or more, the hardness can be reduced. The total amount of one or more of the ferrite, the pearlite, the bainite, and the residual austenite is more preferably 7% or less, and further preferably 5%.

[0045] The average size of the carbides can be 0.1 to 20 μm. When the size of the carbides is less than 0.1 μm, the effect of increasing the hardness is little, and when the size of the carbides exceeds 20 μm, brittleness of the steel material can be induced. The lower limit of the average size of the carbides is more preferably 0.3 μm, and further preferably 0.5 μm. The upper limit of the average size of the carbides is more preferably 17 μm, and further preferably 15 μm.

[0046] The quenched and tempered high carbon hot-rolled steel sheet according to one embodiment of the present application provided as described above has a hardness of 350 Hv or more. Further, when the wear resistance is tested by the ASTM G99 method, in the quenched and tempered high carbon hot-rolled steel sheet, the wear reduction amount can be 35 mg or less when the reheating temperature before quenching and tempering is 800°C, 27 mg or less when the reheating temperature before quenching and tempering is 850°C, and 25 mg or less when the reheating temperature before quenching and tempering is 900°C. Thereby, excellent hardness and wear resistance can be simultaneously ensured.

[0047] Hereinafter, a high carbon cold-rolled steel sheet of the present application will be described.

[0048] The microstructure of the high carbon cold rolled steel sheet of the present application can include, in area %, ferrite: 20 to 99.9%, cementite: 10% or less, pearlite: 50% or less, and carbide: 0.1 to 20%. When the ferrite is less than 20%, low hardness characteristics cannot be ensured, and thus there is a disadvantage in that workability such as cold rolling is poor, and when the ferrite exceeds 99.9%, cementite or hard carbide cannot be ensured, and thus there is a disadvantage in that wear resistance after quenching and tempering heat treatment is reduced. The lower limit of the fraction of the ferrite is more preferably 30%, and further preferably 40%. The upper limit of the fraction of the ferrite is more preferably 99.8%, and further preferably 99.5%. When the cementite exceeds 20%, brittleness of the material is induced, and thus there is a disadvantage in that workability is poor. The lower limit of the fraction of the cementite is more preferably 0.1%, and further preferably 0.3%. The upper limit of the fraction of the cementite is more preferably 8%, and further preferably 7%. When the pearlite exceeds 50%, low hardness characteristics cannot be ensured, and thus there is a disadvantage in that workability such as cold rolling is poor. The lower limit of the fraction of the pearlite is more preferably 1%, and further preferably 5%. The upper limit of the fraction of the pearlite is more preferably 40%, and further preferably 30%. When the fraction of the carbide is less than 0.1%, there is a disadvantage in that wear resistance cannot be ensured due to hard carbide, and when the fraction of the carbide exceeds 20%, there is a disadvantage in that the material is easily damaged due to brittleness. The lower limit of the fraction of the carbide is more preferably 0.2%, and further preferably 0.5%. The upper limit of the fraction of the carbide is more preferably 18%, and further preferably 16%.

[0049] The average size of the carbide can be 0.1 to 20 μm. When the size of the carbide is less than 0.1 μm, the effect of increasing hardness is small, and when the size of the carbide exceeds 20 μm, brittleness of the steel material can be induced. The lower limit of the average size of the carbide is more preferably 0.3 μm, and further preferably 0.5 μm. The upper limit of the average size of the carbide is more preferably 17 μm, and further preferably 15 μm.

[0050] The high carbon cold rolled steel sheet according to one embodiment of the present application provided as described above can have a hardness of 350 Hv or less. By ensuring low hardness as described above, high formability can be ensured, and thus a member as a subsequent process can be smoothly formed.

[0051] Hereinafter, a high carbon cold rolled steel sheet of the present application which is subjected to quenching and tempering heat treatment will be described.

[0052] The microstructure of the quenched and tempered high carbon cold rolled steel sheet according to the present application preferably contains 0.1 to 20% of carbides, the balance of tempered martensite, in terms of area %. The present application can ensure excellent wear resistance and impact resistance by containing tempered martensite as a matrix structure. In addition, the present application improves wear resistance by ensuring an appropriate fraction of carbides. When the fraction of the carbides is less than 0.1%, there is a disadvantage in that it is difficult to expect to ensure wear resistance due to hard carbides, and when the fraction of the carbides exceeds 20%, there is a disadvantage in that the material is easily damaged due to brittleness. The lower limit of the fraction of the carbides is more preferably 0.2%, and further preferably 0.5%. The upper limit of the fraction of the carbides is more preferably 18%, and further preferably 16%. In addition, in the present application, the type of the carbides is not particularly limited, and for example, can be a single carbide or a complex carbide containing one or more of W, V, Mo, and Nb. In addition, the microstructure of the quenched and tempered high carbon hot rolled steel sheet according to the present application can inevitably contain one or more of ferrite, pearlite, bainite, and residual austenite in a total amount of less than 10% due to a manufacturing process. When the total amount of one or more of the ferrite, the pearlite, the bainite, and the residual austenite is 10% or more, the hardness can be reduced. The total amount of one or more of the ferrite, the pearlite, the bainite, and the residual austenite is more preferably 7% or less, and further preferably 5%.

[0053] The average size of the carbides can be 0.1 to 20 μm. When the size of the carbides is less than 0.1 μm, the effect of increasing the hardness is insignificant, and when the size of the carbides exceeds 20 μm, brittleness of the steel material can be induced. The lower limit of the average size of the carbides is more preferably 0.3 μm, and further preferably 0.5 μm. The upper limit of the average size of the carbides is more preferably 17 μm, and further preferably 15 μm.

[0054] The quenched and tempered high carbon hot rolled steel sheet according to one embodiment of the present application provided as described above has a hardness of 350 Hv or more. In addition, when wear resistance is tested by the ASTM G99 method, in the quenched and tempered high carbon hot rolled steel sheet, when the reheating temperature before quenching and tempering is 900°C, the wear reduction amount can be 25 mg or less. Thereby, excellent hardness and wear resistance can be simultaneously ensured.

[0055] Hereinafter, a method of manufacturing a quenched and tempered high carbon hot rolled steel sheet according to one embodiment of the present application will be described.

[0056] First, a hot-rolled steel plate having the above alloy composition is prepared. The step of preparing the hot-rolled steel plate can include the steps of heating a slab at 1100-1300°C, and hot-rolling the heated slab at 700-1100°C. When the heating temperature of the slab is less than 1100°C, the degree of after-heating is low, and thus it can be difficult to roll, and when the heating temperature of the slab exceeds 1300°C, high-temperature oxidation occurs, or depending on whether an in-furnace temperature deviation is generated, the slab can have a disadvantage in that it can be locally melted. When the hot-rolling temperature is less than 700°C, since the strength of the material is high, there is a disadvantage in that the hot-rolling load can increase, and when the hot-rolling temperature exceeds 1100°C, since high-temperature oxidation occurs, the surface quality can be deteriorated.

[0057] The hot-rolled steel plate prepared as described above can have a microstructure of one or more of pearlite, pearlite in which a portion of cementite is precipitated on a grain boundary, bainite, and martensite. In addition, the prepared hot-rolled steel plate can have a hardness of 200 Hv or more.

[0058] Thereafter, the hot-rolled steel plate is reheated at 740-1100°C. When the reheating temperature of the hot-rolled steel plate is less than 740°C, austenite cannot be ensured, and thus there is a disadvantage in that a martensite phase change does not occur after rapid cooling, and when the reheating temperature of the hot-rolled steel plate exceeds 1100°C, since the grains are excessively grown, it can be impossible to ensure desired physical properties. The lower limit of the reheating temperature of the hot-rolled steel plate is more preferably 800°C. The upper limit of the reheating temperature of the hot-rolled steel plate is more preferably 1050°C.

[0059] Thereafter, the reheated hot-rolled steel plate is cooled at a cooling rate of 10°C / sec or more. When the cooling rate is less than 10°C, there is a disadvantage in that low-hardness microstructures such as ferrite and pearlite can be generated during the cooling process after reheating. The cooling rate is more preferably 40°C or more, more preferably 90°C / sec or more, and most preferably 100°C / sec or more. In addition, in the present application, the faster the cooling rate, the more preferable, and thus the upper limit of the cooling rate is not particularly limited. However, due to design limitations, it can be difficult to exceed 200°C / sec.

[0060] Thereafter, the cooled hot-rolled steel plate is tempered at 150-600°C. When the tempering temperature is less than 150°C, the recovery of dislocations is insufficient, and thus there is a disadvantage in that the tempering effect is not obtained, and when the tempering temperature exceeds 600°C, there is a disadvantage in that a phase change can not occur. The lower limit of the tempering temperature is more preferably 170°C, and further preferably 190°C. The upper limit of the tempering temperature is more preferably 500°C, further preferably 450°C, and most preferably 380°C.

[0061] Hereinafter, a method of manufacturing a high-carbon cold-rolled steel sheet according to the present application will be described.

[0062] First, a hot-rolled steel sheet having the above-described alloy composition is prepared. The step of preparing the hot-rolled steel sheet can include the steps of heating a slab at 1100-1300°C, and hot-rolling the heated slab at 700-1100°C. When the heating temperature of the slab is less than 1100°C, the degree of residual heat is low, and thus it can be difficult to roll. When the heating temperature of the slab exceeds 1300°C, high-temperature oxidation occurs, or depending on whether a furnace temperature deviation is generated, there is a disadvantage in that the slab can be locally melted. When the hot-rolling temperature is less than 700°C, the strength of the material is high, and there is a disadvantage in that the hot-rolling load can increase. When the hot-rolling temperature exceeds 1100°C, the surface quality can be deteriorated due to high-temperature oxidation.

[0063] The hot-rolled steel sheet prepared as described above can have a microstructure of one or more of pearlite, pearlite in which a portion of cementite is precipitated on a grain boundary, bainite, and martensite. In addition, the prepared hot-rolled steel sheet can have a hardness of 200 Hv or more.

[0064] In addition, a process of subjecting the prepared hot-rolled steel sheet to spheroidizing annealing heat treatment at 630-850°C can be further included. The purpose of the spheroidizing annealing heat treatment is to inhibit high strength of the hot-rolled steel sheet from causing a cold-rolling process to be unable to be performed or a defect to be generated on equipment. That is, the purpose of the spheroidizing annealing heat treatment is to reduce the strength by spheroidization of cementite having particularly high strength to smoothly perform a cold-rolling process. When the spheroidizing annealing heat treatment temperature is less than 630°C, the time required for spheroidization is excessively long, and there can be a disadvantage in that the economy is reduced. When the spheroidizing annealing heat treatment temperature exceeds 800°C, pearlite is generated during the heat treatment, and thus the effect of reducing the strength or hardness can be insignificant. The lower limit of the spheroidizing annealing heat treatment temperature is more preferably 650°C, and is further preferably 670°C. The upper limit of the spheroidizing annealing heat treatment temperature is more preferably 830°C, and is further preferably 810°C.

[0065] Subsequently, the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet. The cold-rolling process can be performed by a method generally performed in the technical field. Thus, in the present application, the cold-rolling process is not particularly limited as long as a cold-rolled steel sheet having a desired thickness can be obtained.

[0066] In addition, the method of manufacturing a high-carbon cold-rolled steel sheet can include performing the above-described spheroidizing annealing heat treatment and cold-rolling process one or more times.

[0067] Hereinafter, a method of manufacturing a high-carbon cold-rolled steel sheet subjected to quenching and tempering heat treatment according to one embodiment of the present application will be described.

[0068] First, a hot-rolled steel sheet having the above alloy composition is prepared. The step of preparing the hot-rolled steel sheet can include the steps of heating a slab at 1100-1300 °C, and hot-rolling the heated slab at 700-1100 °C. When the heating temperature of the slab is less than 1100 °C, the degree of after-heating is low, and thus it can be difficult to roll, and when the heating temperature of the slab exceeds 1300 °C, high-temperature oxidation occurs, or depending on whether an in-furnace temperature deviation occurs, there is a disadvantage in that the slab can be locally melted. When the hot-rolling temperature is less than 700 °C, there is a disadvantage in that the hot-rolling load can increase due to the high strength of the material, and when the hot-rolling temperature exceeds 1100 °C, the surface quality can be deteriorated due to high-temperature oxidation.

[0069] The hot-rolled steel sheet prepared as described above can have a microstructure of one or more of pearlite, pearlite in which a portion of cementite is precipitated on a grain boundary, bainite, and martensite. In addition, the prepared hot-rolled steel sheet can have a hardness of 200 Hv or more.

[0070] In addition, a process of subjecting the prepared hot-rolled steel sheet to spheroidizing annealing heat treatment at 630-850 °C can be further included. The purpose of the spheroidizing annealing heat treatment is to inhibit high strength of the hot-rolled steel sheet from causing a cold-rolling process to be unable to be performed or a defect to occur on equipment. That is, the purpose of the spheroidizing annealing heat treatment is to reduce the strength by spheroidization of cementite having particularly high strength to smoothly perform a cold-rolling process. When the spheroidizing annealing heat treatment temperature is less than 630 °C, the time required for spheroidization is excessively long, and there can be a disadvantage in that the economy is reduced, and when the spheroidizing annealing heat treatment temperature exceeds 800 °C, pearlite is generated during the heat treatment, and thus the effect of reducing the strength or hardness can be insignificant. The lower limit of the spheroidizing annealing heat treatment temperature is more preferably 650 °C, and further preferably 670 °C. The upper limit of the spheroidizing annealing heat treatment temperature is more preferably 830 °C, and further preferably 810 °C.

[0071] Subsequently, the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet. The cold-rolling process can be performed by a method generally performed in the technical field. Thus, in the present invention, the cold-rolling process is not particularly limited as long as a cold-rolled steel sheet having a desired thickness can be obtained.

[0072] Thereafter, the cold-rolled steel sheet is reheated at 740 to 1100°C. When the reheating temperature of the cold-rolled steel sheet is less than 740°C, austenite cannot be ensured, thus having a disadvantage that martensite transformation does not occur after rapid cooling, and when the reheating temperature of the cold-rolled steel sheet exceeds 1100°C, desired physical properties can not be ensured due to excessive grain growth. 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.

[0073] Thereafter, the reheated cold-rolled steel sheet is cooled at a cooling rate of 10°C / sec or more. When the cooling rate is less than 10°C / sec, there is a disadvantage that low-hardness fine microstructure such as ferrite and pearlite can be generated during cooling after reheating. The cooling rate is more preferably 40°C or more, more preferably 90°C / sec or more, and most preferably 100°C / sec or more. In addition, in the present application, the faster the cooling rate is, the more preferable it is, and thus the upper limit of the cooling rate is not particularly limited. However, it can be difficult to exceed 200°C / sec due to design limitations.

[0074] Thereafter, the cooled cold-rolled steel sheet is tempered at 150 to 600°C. When the tempering temperature is less than 150°C, recovery of dislocations is not sufficient, thus having a disadvantage that tempering effect is not obtained, and when the tempering temperature exceeds 600°C, there is a disadvantage that phase transformation can not occur. The lower limit of the tempering temperature is more preferably 170°C, and further preferably 190°C. The upper limit of the tempering temperature is more preferably 500°C, further preferably 450°C, and most preferably 380°C. DETAILED DESCRIPTION

[0075] Hereinafter, the present application will be described in more detail by examples. However, the following examples are only for a more detailed description of the present application, and do not limit the scope of the present application.

[0076] (Example 1)

[0077] A slab having the alloy composition of Table 1 below was heated at 1200°C, and then hot-rolled at 900°C to obtain a hot-rolled steel sheet, the hardness of which was measured and is shown together in Table 1 below. The hot-rolled steel sheet obtained as described above was reheated at 800°C, 850°C, and 900°C, respectively, and then cooled at a cooling rate of 80°C / sec, and tempered at 200°C to manufacture a quenched and tempered hot-rolled steel sheet.

[0078] After measuring the fine microstructure, hardness, and wear resistance of the quenched and tempered hot-rolled steel sheet manufactured as described above, the results thereof are recorded in Table 2 below.

[0079] The fraction of the microstructure was calculated using Thermo Calc software based on thermodynamic properties.

[0080] The size of the carbide was observed by using FE-SEM scanning electron microscope. Specifically, after polishing the test piece to #400 to #2000 using sandpaper, final polishing was performed with 1 μm diamond abrasive, and after processing in 2% nital etchant, observation was performed using an image analysis program.

[0081] The hardness was measured by using a Vickers hardness tester. At this time, the test was repeated 5 times with a measurement load of 10 kg to calculate the average value.

[0082] The wear resistance evaluation was performed by a ball-on-disk test according to the ASTM G99 method. At this time, a test piece processed in a disk shape with a diameter of 31 mm and a thickness of 5 mm and a SiC ball with a diameter of 12.7 mm were rubbed at a force of 50 N and a speed of 1000 rpm for 3600 seconds at room temperature and tested. The wear resistance was expressed as the value obtained by subtracting the weight after wear from the weight before wear of the test piece, that is, as the wear reduction amount, and the smaller the wear reduction amount, the more excellent the wear resistance.

[0083] [Table 1]

[0084]

[0085]

[0086] [Table 2]

[0087]

[0088]

[0089] As can be seen from Table 1 and Table 2, in the case of Inventive Steel 1 to Inventive Steel 15 satisfying the conditions proposed in the present application, since the microstructure and the size of the carbide desired to be obtained in the present application are secured, excellent hardness and wear resistance are obtained.

[0090] On the other hand, in the case of the existing steel or Comparative Steel 1 to Comparative Steel 4 not satisfying the W content condition proposed in the present application, the size of the carbide desired to be obtained in the present application cannot be secured, and thus the hardness and wear resistance are at a low level.

[0091] (Example 2)

[0092] A slab having the alloy composition of Table 1 described in Example 1 was heated at 1200°C, and then hot-rolled at 900°C to obtain a hot-rolled steel sheet, and the hot-rolled steel sheet was subjected to spheroidizing annealing heat treatment at 770°C and then cold-rolled to produce a cold-rolled steel sheet. Further, the cold-rolled steel sheet was reheated at 900°C, and cooled at a cooling rate of 40°C / sec, and then tempered at 210°C to produce a cold-rolled steel sheet subjected to quenching and tempering heat treatment.

[0093] After measuring the microstructure and hardness of the cold-rolled steel sheet produced as described above, the results thereof are shown in Table 3 below. Further, after measuring the microstructure, hardness and wear resistance of the cold-rolled steel sheet subjected to quenching and tempering heat treatment produced as described above, the results thereof are described in Table 4 below.

[0094] The microstructure, hardness and wear resistance were measured by using the same method as in Example 1.

[0095] [Table 3]

[0096]

[0097]

[0098] [Table 4]

[0099]

[0100] From the Tables 3 and 4, it can be known that in the case of Inventive Steel 1 to Inventive Steel 15 satisfying the conditions proposed in the present application, since the microstructure and the size of carbide desired to be obtained in the present application are ensured, excellent hardness and wear resistance can be obtained.

[0101] On the other hand, in the case of the conventional steel or Comparative Steel 1 to Comparative Steel 4 not satisfying the W content condition proposed in the present application, it can be known that the size of carbide desired to be obtained in the present application cannot be ensured, and thus the hardness and wear resistance are at a low level.

Claims

1. A high-carbon hot-rolled steel sheet subjected to quenching and tempering heat treatment, comprising, in mass%, C: 1.0-1.4%, Si: 0.1-0.4%, Mn: 0.1-0.8%, Cr: 0.3-11%, W: 0.05-2.5%, P: 0.03% or less, S: 0.03% or less, Al: 0.02% or less, the balance of Fe and other inevitable impurities, a microstructure comprising, in area%, 0.1-20% of carbides and the balance of tempered martensite, the average size of the carbides being 0.1-20 μm, the wear reduction amount being 35 mg or less when the reheating temperature before quenching and tempering is 800°C, 27 mg or less when the reheating temperature before quenching and tempering is 850°C, and 25 mg or less when the reheating temperature before quenching and tempering is 900°C. The hot-rolled steel sheet further comprises one or more selected from the group consisting of V: 0.8% or less except 0%, Mo: 2.5% or less except 0%, and Nb: 1.5% or less except 0%. The hot-rolled steel sheet has a hardness of 350 Hv or more.

4. A high-carbon cold-rolled steel sheet subjected to quenching and tempering heat treatment, comprising, in mass%, C: 1.0-1.4%, Si: 0.1-0.4%, Mn: 0.1-0.8%, Cr: 0.3-11%, W: 0.05-2.5%, P: 0.03% or less, S: 0.03% or less, Al: 0.02% or less, the balance of Fe and other inevitable impurities, a microstructure comprising, in area%, 0.1-20% of carbides, the balance of tempered martensite, the average size of the carbides being 0.1-20 μm, the wear reduction amount being 25 mg or less when the reheating temperature before quenching and tempering is 900°C.

2. The quenched and tempered heat treated high carbon hot rolled steel sheet according to claim 1, wherein, The cold-rolled steel sheet further comprises one or more selected from the group consisting of V: 0.8% or less except 0%, Mo: 2.5% or less except 0%, and Nb: 1.5% or less except 0%.

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

7. A method of manufacturing a high-carbon hot-rolled steel sheet subjected to quenching and tempering heat treatment, comprising the steps of: preparing a hot-rolled steel sheet comprising, in mass%, C: 1.0-1.4%, Si: 0.1-0.4%, Mn: 0.1-0.8%, Cr: 0.3-11%, W: 0.05-2.5%, P: 0.03% or less, S: 0.03% or less, Al: 0.02% or less, the balance of Fe and other inevitable impurities; reheating the prepared hot-rolled steel sheet at 740-1100°C; cooling the reheated hot-rolled steel sheet at a cooling rate of 10°C / sec or more; and tempering the cooled hot-rolled steel sheet at 150-600°C, wherein the prepared hot-rolled steel sheet has a microstructure of one or more of pearlite, pearlite with a part of cementite precipitated on grain boundaries, bainite, and martensite. The hot-rolled steel sheet further comprises one or more selected from the group consisting of V: 0.8% or less except 0%, Mo: 2.5% or less except 0%, and Nb: 1.5% or less except 0%. ​ ​ 5. The high carbon cold rolled steel sheet quenched and tempered heat treated according to claim 4 wherein, ​ 6. The high carbon cold rolled steel sheet quenched and tempered heat treated of claim 4 wherein, ​ ​ ​ ​ ​ ​ ​ ​ 8. The method of manufacturing a quenched and tempered hot rolled high carbon steel plate according to claim 7, wherein, The step of preparing the hot-rolled steel sheet includes the steps of: heating a slab at 1100-1300°C; and hot-rolling the heated slab at 700-1100°C.

9. The method of manufacturing a quenched and tempered hot rolled high carbon steel plate according to claim 7, wherein, The prepared hot-rolled steel sheet has a hardness of 200 Hv or more.

10. A method of manufacturing a high-carbon cold-rolled steel sheet subjected to a quenched and tempered heat treatment, comprising the steps of: preparing a hot-rolled steel sheet containing, in terms of weight %, C: 1.0-1.4%, Si: 0.1-0.4%, Mn: 0.1-0.8%, Cr: 0.3-11%, W: 0.05-2.5%, P: 0.03% or less, S: 0.03% or less, Al: 0.02% or less, the balance of Fe and other inevitable impurities; cold-rolling the prepared hot-rolled steel sheet to obtain a cold-rolled steel sheet; reheating the cold-rolled steel sheet at 740-1100°C; cooling the reheated cold-rolled steel sheet at a cooling rate of 10°C / sec or more; and tempering the cooled cold-rolled steel sheet at 150-600°C, wherein the prepared hot-rolled steel sheet has a microstructure of one or more of pearlite, pearlite with a part of cementite precipitated on grain boundaries, bainite, and martensite.

11. A method of manufacturing a high carbon cold rolled steel sheet quenched and tempered heat treated according to claim 10, wherein, The step of preparing the hot-rolled steel sheet includes the steps of: heating a slab at 1100-1300°C; and hot-rolling the heated slab at 700-1100°C.

12. The method of manufacturing a high carbon cold rolled steel sheet quenched and tempered heat treated according to claim 10, wherein, The prepared hot-rolled steel sheet has a hardness of 200 Hv or more.

13. The method of manufacturing a high carbon cold rolled steel sheet quenched and tempered heat treated of claim 10 wherein, The method further includes a step of subjecting the hot-rolled steel sheet to a spheroidizing annealing heat treatment at 630-850°C before the cold-rolling.

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