Cold-rolled steel sheet, steel components, manufacturing methods of cold-rolled steel sheet and steel components

Abstract

This invention provides a cold-rolled steel sheet with excellent toughness. The cold-rolled steel sheet has a specified composition, wherein the average grain size of carbides containing at least one of Nb, Ti, and V present within the ferrite grains is 0.10 μm or more, and the number density of carbides with a grain size of 0.10 μm or more is 100 particles / mm². 2 above.

Description

Technical Field

[0001] This invention relates to cold-rolled steel sheets, and more particularly to cold-rolled steel sheets capable of manufacturing steel components with excellent toughness. Furthermore, this invention relates to steel components using the aforementioned cold-rolled steel sheets, methods for manufacturing the aforementioned cold-rolled steel sheets, and methods for manufacturing the aforementioned steel components. Background Technology

[0002] Cold-rolled steel sheets are widely used as materials for manufacturing various steel components. Among them, cold-rolled steel sheets made of high-carbon steel are used for applications requiring wear resistance, such as parts for fiber machinery, bearing components, and mechanical and household knives, due to their high hardness.

[0003] On the other hand, steel components such as those used in fiber machinery, bearings, and mechanical / household knives are repeatedly subjected to impacts from reciprocating motion during use. Therefore, steel components are required to have excellent toughness to prevent damage caused by the impacts of reciprocating motion.

[0004] However, because metallic materials become more brittle with increasing hardness, it is difficult to balance hardness and toughness. For example, to improve the toughness of steel parts, quenching and tempering are usually performed. However, quenching and tempering reduce the hardness of the steel. Therefore, in the past, quenching and tempering processes could not achieve a high level of balance between hardness and toughness.

[0005] Therefore, various methods have been proposed to balance hardness and toughness.

[0006] For example, patent documents 1 and 2 disclose a technique for improving the toughness of high-carbon cold-rolled steel sheets by utilizing the grain refinement effect of Nb addition.

[0007] In addition, Patent Document 3 proposes a technique to improve the wear resistance of cold-rolled steel sheets by dispersing coarse Nb-containing carbides at high density in a matrix composed of ferrite phase and to improve toughness by utilizing the grain refinement effect of Nb addition.

[0008] Patent document 4 proposes a technique to improve the wear resistance and toughness of cold-rolled steel sheets by dispersing coarse Nb·Ti-based carbides at high density in the matrix and reducing the number and density of voids.

[0009] Patent document 5 proposes the following technique: by annealing a steel plate containing 0.5 to 0.7% by mass of carbon before final quenching and tempering, the spheroidization rate of carbides such as cementite is increased, resulting in improved toughness.

[0010] Patent document 6 proposes the following technology: by placing the material in a final annealing state before the final quenching and tempering, the density of generated voids in the material is increased, thereby generating a soft high-carbon steel plate with excellent punching properties.

[0011] Patent document 7 proposes the following technology: In high carbon steel plates, the formation of cementite carbides that do not contain niobium, titanium, or vanadium carbides is controlled so that the spheroidization rate and number density of cementite carbides are at the desired values, thereby improving impact toughness and wear resistance.

[0012] Existing technical documents

[0013] Patent documents

[0014] Patent Document 1: Japanese Patent Application Publication No. 05-345952

[0015] Patent Document 2: Japanese Patent Application Publication No. 2017-036492

[0016] Patent Document 3: Japanese Patent Application Publication No. 2015-190036

[0017] Patent Document 4: Japanese Patent Application Publication No. 2017-190494

[0018] Patent Document 5: Japanese Patent Application Publication No. 2009-024233

[0019] Patent Document 6: Japanese Patent Application Publication No. 2011-012316

[0020] Patent Document 7: Japanese Patent No. 6880245 Summary of the Invention

[0021] In the technologies proposed in Patent Documents 1 and 2, the toughness of high-carbon cold-rolled steel sheets is improved by utilizing the grain refinement effect of Nb addition. However, the grain refinement effect of Nb saturates when the Nb content is around 0.1% by mass, therefore the necessary toughness cannot be obtained solely through the grain refinement effect.

[0022] Furthermore, the technology proposed in Patent Document 3 also utilizes the grain refinement effect of Nb addition to improve toughness. However, in Patent Document 3, Nb-containing carbides are used to improve wear resistance, but these Nb-containing carbides become the main cause of reduced toughness. Therefore, the effects of Nb addition and Nb-containing carbides cancel each other out, failing to achieve the necessary toughness.

[0023] Similar to Patent Document 3, the technology proposed in Patent Document 4 also utilizes the effect of improving wear resistance by dispersing hard Nb·Ti-based carbides at a high density. However, when the Nb·Ti-based carbides are dispersed at a high density, voids are generated between the matrix and the carbides during cold rolling, resulting in reduced toughness. Therefore, in Patent Document 4, the generation of voids is suppressed by limiting the rolling rate during cold rolling. However, in this method, since the rolling rate is limited, the thickness and mechanical properties of the cold-rolled steel sheet that can be manufactured are inevitably limited, and it cannot be considered a fundamental solution.

[0024] Furthermore, the toughness of the technologies proposed in patent documents 5-7 is still insufficient.

[0025] The present invention was made in view of the above circumstances, and its object is to achieve further superior toughness in cold-rolled steel sheets with increased hardness using carbides such as Nb.

[0026] The inventors have studied a method for solving the above-mentioned problems and have obtained the following insights.

[0027] (1) By appropriately controlling the size and density of Nb·Ti·V carbides in cold-rolled steel sheets, the toughness of the cold-rolled steel sheets after quenching and tempering can be effectively improved. As a result, steel components with both high hardness and toughness can be manufactured.

[0028] (2) By appropriately controlling the composition of the steel slab and the manufacturing conditions of the cold-rolled steel plate, the size and density of Nb·Ti·V carbides in the cold-rolled steel plate can be appropriately controlled.

[0029] This invention is based on the above insights, and its main points are as follows.

[0030] 1. A cold-rolled steel sheet comprising the following components: (in mass%):

[0031] C: 0.6–1.25%

[0032] Si: 0.10–0.55%

[0033] Mn: 0.20~2.0%

[0034] P: 0.0005~0.05%

[0035] S: below 0.03

[0036] Al: 0.001~0.1%

[0037] N: 0.001~0.009%

[0038] Cr: 0.1–1.0%, and

[0039] One or more of the following: Ti: 0.01–1.0%, Nb: 0.05–0.5%, and V: 0.01–1.0%.

[0040] The remainder consists of Fe and unavoidable impurities;

[0041] The average grain size of carbides containing at least one of Nb, Ti, and V present within ferrite grains is greater than 0.1 μm.

[0042] Furthermore, the number density of carbides with a particle size of 0.1 μm or larger in the aforementioned carbides is 100 particles / mm. 2 above.

[0043] 2. The cold-rolled steel sheet according to claim 1, wherein the composition further comprises, in weight percent, a subset selected from:

[0044] Sb: below 0.1%

[0045] Hf: below 0.5%

[0046] REM: below 0.1%

[0047] Cu: less than 0.5%

[0048] Ni: below 3.0%

[0049] Sn: less than 0.5%

[0050] Mo: 1% or less,

[0051] Zr: below 0.5%

[0052] B: Below 0.005%, and

[0053] W: One or more of the following: less than 0.01%.

[0054] 3. A steel component, which is made by quenching and tempering the cold-rolled steel sheet described in 1 or 2 above.

[0055] 4. The steel component according to claim 3 above, wherein the steel component is any one of a component for fiber machinery, a bearing component, and a cutting tool.

[0056] 5. A method for manufacturing cold-rolled steel sheet, comprising heating a steel slab having the composition described in 1 or 2 above,

[0057] The heated steel slab is hot-rolled at a temperature above Ac3 on the finishing mill inlet side to produce a hot-rolled steel plate.

[0058] The hot-rolled steel sheet was cooled under the following conditions: a time of less than 2 seconds from the end of hot rolling to the start of cooling; an average cooling rate of more than 25°C / s; and a cooling stop temperature of less than 720°C.

[0059] The cooled hot-rolled steel sheet is then wound up.

[0060] The hot-rolled steel sheet after winding is subjected to a first annealing at an annealing temperature of 650℃~780℃ and an annealing time of more than 3 hours.

[0061] The hot-rolled steel sheet after the first annealing is subjected to cold rolling with a rolling rate of more than 15% and a second annealing at an annealing temperature of 600-800℃, and then further subjected to final cold rolling with a rolling rate of more than 20%.

[0062] 6. The method for manufacturing cold-rolled steel sheet according to 5 above, wherein the heating rate in the second annealing is 50°C / h or higher.

[0063] 7. A method for manufacturing a steel component, wherein a cold-rolled steel sheet manufactured by the manufacturing method described in 5 or 6 above is quenched at a quenching temperature of 700°C to 800°C and a holding time of more than 1 minute and less than 60 minutes, and then tempered at a tempering temperature of 150°C to 300°C and a holding time of 20 minutes to 3 hours.

[0064] According to the present invention, further superior toughness after quenching and tempering can be obtained in cold-rolled steel sheets whose hardness is increased by using carbides such as Nb. Therefore, the cold-rolled steel sheet of the present invention is very suitable for use as a material for various steel parts, such as components for fiber machinery, bearing components, and mechanical and household knives. Furthermore, according to the present invention, steel parts using the aforementioned cold-rolled steel sheet can be provided. Detailed Implementation

[0065] The present invention will now be described in detail. It should be noted that the present invention is not limited to this embodiment. Furthermore, in the present invention, consideration is given to carbides containing at least one of Nb, Ti, and V present within ferrite grains. Therefore, in the following description, "carbides containing at least one of Nb, Ti, and V present within ferrite grains" will sometimes be simply referred to as "carbides".

[0066] [Ingredients]

[0067] The cold-rolled steel sheet of the present invention has the above-described composition. The reasons for this limitation will be explained below. It should be noted that, unless otherwise specified, the "%" in the following description refers to "mass %" as the unit of content.

[0068] C: 0.6–1.25%

[0069] Carbon (C) is an element required to increase the hardness after quenching and tempering. Furthermore, C is also required for the formation of cementite and carbides with elements such as Nb, Ti, and V. To generate the necessary carbides to achieve the required strength after quenching and tempering, the C content needs to be 0.6% or more. Therefore, a C content of 0.6% or more is preferred, and 0.7% or more is preferable. On the other hand, if the C content exceeds 1.25%, the hardness increases excessively, leading to embrittlement. Additionally, if the C content exceeds 1.25%, the surface oxide scale hardens during heating, resulting in deterioration of surface properties. Therefore, a C content of 1.25% or less is preferred, and 1.20% or less is preferable.

[0070] Si: 0.10–0.55%

[0071] Si is an element that enhances strength through solid solution strengthening. To achieve this effect, the Si content is 0.10% or more, preferably 0.12% or more, and more preferably 0.14% or more. On the other hand, if the Si content is excessive, Si oxide is formed, resulting in reduced toughness. Furthermore, if the Si content is excessive, it promotes ferrite formation and grain growth, promotes carbide precipitation at grain boundaries, and inhibits carbide precipitation into the grains. Additionally, if the Si content is excessive, the surface oxide layer hardens upon heating, resulting in deterioration of surface properties. Therefore, the Si content is 0.55% or less, preferably 0.50% or less, and more preferably 0.45% or less.

[0072] Mn: 0.20~2.0%

[0073] Mn is an element that increases hardness by promoting quenching and suppressing temper softening. To suppress temper softening, it is necessary to inhibit the formation of carbide (C) into cementite or delay dislocation recovery. Mn has both of these effects; by adding Mn, a high-hardness microstructure with high dislocation density can be maintained even after tempering. To achieve these effects, the Mn content is 0.20% or more, preferably 0.25% or more. On the other hand, if the Mn content exceeds 2.0%, banded microstructure is formed due to Mn segregation. Especially in the segregated areas of MnS, abnormal grain growth and microstructure inhomogeneity are easily generated, leading to local precipitation towards ferrite grain boundaries, thus suppressing carbide formation within the grains. Furthermore, this becomes a cause of cracks and poor shape during processing. Therefore, the Mn content is 2.0% or less, preferably 1.95% or less.

[0074] P: 0.0005~0.05%

[0075] By adding trace amounts of phosphorus (P), the strength can be improved through solid solution strengthening. To achieve this effect, the P content is 0.0005% or more, preferably 0.0008% or more. On the other hand, if the P content exceeds 0.05%, the toughness decreases due to grain boundary embrittlement. Therefore, the P content is 0.05% or less, preferably 0.045% or less.

[0076] S: below 0.03%

[0077] Sulfur (S) reduces toughness by forming sulfides with manganese (Mn). Therefore, the S content is 0.03% or less, preferably 0.02% or less. On the other hand, from the viewpoint of improving toughness, the lower the S content, the better; therefore, the lower limit of the S content is not particularly limited and can be 0%. However, since excessive reduction leads to an increase in manufacturing costs, from the viewpoint of industrial production, it is preferable to make the S content 0.0005% or more, more preferably 0.001% or more.

[0078] Al: 0.001–0.1%

[0079] Al is an element required for deoxidation during steelmaking. Therefore, the Al content is 0.001% or more. On the other hand, if Al is excessive, nitrides are formed, promoting the formation of cracks and voids originating from these nitrides, resulting in reduced toughness. Therefore, the Al content is 0.1% or less, preferably 0.08% or less, and more preferably 0.06% or less.

[0080] N: 0.001~0.009%

[0081] Nitrogen is an element that improves toughness by refining particle size through the formation of fine nitrides. Therefore, the N content is 0.001% or more. On the other hand, if N is excessive, it combines with Al to form nitrides, promoting the formation of cracks and voids originating from these nitrides, resulting in reduced toughness. Therefore, the N content is 0.009% or less, preferably 0.008% or less.

[0082] Cr: 0.1–1.0%

[0083] Cr is an element that improves the hardenability and strength of steel. To achieve these effects, the Cr content is 0.1% or more, preferably 0.12% or more. On the other hand, if Cr is excessive, coarse Cr carbides and Cr nitrides are formed, creating voids around these Cr carbides and Cr nitrides, resulting in reduced toughness. Therefore, the Cr content is 1.0% or less, preferably 0.95% or less.

[0084] The above composition contains one or more of the following: Ti: 0.01–1.0%, Nb: 0.05–0.5%, and V: 0.01–1.0%. To obtain the desired carbide number density, at least one of the above-mentioned amounts of Ti, Nb, and V needs to be added.

[0085] Ti: 0.01~1.0%

[0086] Ti is an element that has the effect of forming carbides within grains and improving toughness. When adding Ti, to obtain the above-mentioned effect, the Ti content should be 0.01% or more, preferably 0.015% or more. On the other hand, if excessive Ti is added, the austenitizing temperature becomes higher, and therefore ferrite is more likely to form on the surface of the steel sheet due to the lower temperature during hot rolling. The ferrite formed on the surface remains after subsequent cold rolling and annealing, and preferably forms carbides at grain boundaries, thus suppressing the formation of carbides within the grains. Therefore, the Ti content should be 1.0% or less, preferably 0.9% or less.

[0087] Nb: 0.05–0.5%

[0088] Nitrogen (Nb) is an element that forms carbides within grains and improves toughness. Furthermore, Nb also has a significant effect on grain refinement. To achieve these effects, the Nb content is kept at 0.05% or more when adding Nb. On the other hand, if excessive Nb is added, carbides form at grain boundaries, and the density of carbides formed within the grains decreases. Since carbides formed at grain boundaries become the initiation point for voids and cracks, toughness decreases. Therefore, the Nb content is 0.5% or less, preferably 0.45% or less.

[0089] V: 0.01~1.0%

[0090] Vitamin V (V) is an element that helps form carbides within grains and improves toughness. Additionally, V improves hardenability, increasing the strength of the steel. Furthermore, to suppress tempering softening, it is necessary to inhibit carbon formation into cementite or delay dislocation recovery; V has both of these effects. By adding V, the processing structure can be maintained after tempering, thus improving toughness. To achieve the above effects, the V content should be 0.01% or more. On the other hand, if excessive V is added, the carbides formed at grain boundaries become coarse, becoming initiators of voids and cracks, thus reducing toughness. Therefore, the V content is 1.0% or less, preferably 0.95% or less.

[0091] One embodiment of the present invention provides a cold-rolled steel sheet comprising the above-mentioned components plus the remaining portion of Fe and unavoidable impurities.

[0092] In addition, in other embodiments of the present invention, the above-mentioned composition may optionally contain one or more of the following: Sb: less than 0.1%, Hf: less than 0.5%, REM: less than 0.1%, Cu: less than 0.5%, Ni: less than 3.0%, Sn: less than 0.5%, Mo: less than 1%, Zr: less than 0.5%, B: less than 0.005%, and W: less than 0.01%.

[0093] Sb: below 0.1%

[0094] Sb is an effective element for improving corrosion resistance, but if added in excess, a Sb-rich layer will form under the oxide scale generated during hot rolling, resulting in surface peeling (scratching) of the steel sheet after hot rolling. Therefore, the Sb content should be 0.1% or less. On the other hand, there is no particular limit to the lower limit of the Sb content, but from the viewpoint of improving the addition effect, it is preferable to make the Sb content 0.0003% or more.

[0095] Hf: below 0.5%

[0096] Hf is an effective element for improving corrosion resistance, but if added in excess, a Hf-rich layer will form under the oxide scale generated during hot rolling, resulting in surface peeling (scratching) of the steel sheet after hot rolling. Therefore, the Hf content should be 0.5% or less. On the other hand, there is no particular limit to the lower limit of the Hf content, but from the viewpoint of improving the addition effect, it is preferable to make the Hf content 0.001% or more.

[0097] REM: below 0.1%

[0098] Rare earth metals (REMs) are elements that improve the strength of steel. However, excessive addition of REMs can sometimes delay the spheroidization of cementite and promote uneven deformation during cold working, leading to deterioration of surface properties. Therefore, the REM content is kept below 0.1%. On the other hand, there is no particular limitation on the lower limit of REM content, but from the viewpoint of improving the addition effect, it is preferable to have a REM content of 0.005% or more.

[0099] Cu: less than 0.5%

[0100] Cu is an effective element for improving corrosion resistance, but if added in excess, a Cu-rich layer will form under the oxide scale generated during hot rolling, resulting in surface peeling (scratching) of the steel sheet after hot rolling. Therefore, the amount of Cu added should be 0.5% or less. On the other hand, there is no particular limit to the lower limit of Cu content, but from the viewpoint of improving the addition effect, it is preferable to make the Cu content 0.01% or more.

[0101] Ni: below 3.0%

[0102] Ni is an element that increases the strength of steel. However, if added in excess, it can sometimes promote uneven deformation during cold working, leading to a deterioration of surface properties. Therefore, the Ni content is 3.0% or less. On the other hand, there is no particular limit to the lower limit of the Ni content, but from the viewpoint of improving the effect of addition, it is preferable to have a Ni content of 0.01% or more.

[0103] Sn: less than 0.5%

[0104] Sn is an effective element for improving corrosion resistance, but if added in excess, a Sn-rich layer will form under the oxide scale generated during hot rolling, resulting in surface peeling (scratching) of the steel sheet after hot rolling. Therefore, the Sn content should be 0.5% or less. On the other hand, there is no particular limit to the lower limit of the Sn content, but from the viewpoint of improving the addition effect, it is preferable to make the Sn content 0.0001% or more.

[0105] Mo: 1% or less

[0106] Mo is an element that increases the strength of steel. However, if added in excess, it can sometimes delay the spheroidization of cementite and promote uneven deformation during cold working, thus deteriorating the surface properties. Therefore, the Mo content is 1% or less. On the other hand, there is no particular limit to the lower limit of the Mo content, but from the viewpoint of improving the effect of addition, it is preferable to make the Mo content 0.001% or more.

[0107] Zr: below 0.5%

[0108] Zr is an effective element for improving corrosion resistance, but if added in excess, a Zn-rich layer will form under the oxide scale generated during hot rolling, resulting in surface peeling (scratching) of the steel plate after hot rolling. Therefore, the Zr content should be 0.5% or less. On the other hand, there is no particular limit to the lower limit of the Zr content, but from the viewpoint of improving the addition effect, it is preferable to make the Zr content 0.01% or more.

[0109] B: Below 0.005%

[0110] Boron (B) is an element that improves hardenability and can be added arbitrarily. However, if the B content exceeds 0.005%, surface cracks are prone to occur during quenching. Therefore, the B content is 0.005% or less. On the other hand, there is no particular lower limit to the B content, but from the viewpoint of improving the addition effect, it is preferable to make the B content 0.0001% or more.

[0111] W: below 0.01%

[0112] W is an element that improves hardenability and can be added arbitrarily. However, if the W content exceeds 0.01%, surface cracks are prone to occur during quenching. Therefore, the W content is 0.01% or less. On the other hand, there is no particular limit to the lower limit of the W content, but from the viewpoint of improving the addition effect, it is preferable to make the W content 0.001% or more when adding W.

[0113] [carbide]

[0114] Next, the carbides contained in the cold-rolled steel sheet of the present invention will be described.

[0115] Average particle size: ≥0.10μm

[0116] Number density: 100 / mm 2 above

[0117] In the pre-cold rolling stage of the pre-processing stage of component manufacturing, a microstructure containing Nb, Ti, and V-based carbides within the grains is pre-formed. Then, after the processing microstructure generated during cold rolling, some fine Nb, Ti, and V-based carbides precipitate again at the subgrain boundaries during subsequent quenching and tempering. This microstructure increases resistance to strain introduced by repeated deformation, resulting in improved toughness of the final product. To achieve this effect, the average grain size of carbides containing at least one of Nb, Ti, and V within the ferrite grains needs to be 0.10 μm or more. For the same reason, the number density of carbides with a grain size of 0.10 μm or more needs to be 100 particles / mm. 2 above.

[0118] If the average particle size of the carbides is less than 0.10 μm, the amount of fine Nb, Ti, and V carbides precipitated after quenching and tempering will be insufficient, and a high toughness improvement effect cannot be achieved. Furthermore, if the carbide number density is less than 100 particles / mm², the yield will be insufficient. 2 Similar to the case of average particle size, the amount of fine Nb, Ti, and V carbides precipitated after quenching and tempering is insufficient and cannot achieve a high toughness improvement effect.

[0119] [Plate thickness]

[0120] The thickness of the aforementioned cold-rolled steel sheet is not particularly limited and can be any thickness, but is preferably 0.1 mm or more, more preferably 0.2 mm or more. Furthermore, there is no particular limitation on the upper limit of the sheet thickness, but it is preferably 2.5 mm or less, more preferably 1.6 mm or less, and even more preferably 0.8 mm or less. With a sheet thickness of 0.2 mm to 0.8 mm, it is particularly suitable for use as a material for fiber machinery parts such as knitting needles.

[0121] [Manufacturing method of cold-rolled steel sheet]

[0122] Next, a method for manufacturing a cold-rolled steel sheet according to one embodiment of the present invention will be described.

[0123] The above-mentioned cold-rolled steel sheet can be manufactured by sequentially performing the following processes on a steel slab with the above-mentioned composition.

[0124] (1) Heating

[0125] (2) Hot rolling

[0126] (3) Cooling

[0127] (4) Winding

[0128] (5) First annealing

[0129] (6) Cold rolling

[0130] (7) Second annealing

[0131] (8) Final cold rolling

[0132] Furthermore, the steps (6) and (7) above are repeated more than twice. The following is a description of each step.

[0133] (1) Heating

[0134] First, the steel slab with the above-mentioned composition is heated. The steel slab can be manufactured by any method without particular limitation. For example, the composition of the steel slab can be adjusted by a blast furnace converter or by an electric furnace. Furthermore, the slab can be cast from molten steel using continuous casting or by rolling the steel slab.

[0135] The heating described above can be carried out by any method, but it is preferred to use a heating furnace.

[0136] When using a heating furnace for the above heating, there is no particular limitation on the furnace temperature, but from the viewpoint of homogenizing the steel composition and dissolving segregation and undissolved carbides in the steel slab, it is preferable to be 1100°C or higher.

[0137] There is no particular limitation on the holding time during the above heating, but from the viewpoint of fully dissolving the undissolved carbides, it is preferable to hold for more than 1 hour.

[0138] (2) Hot rolling

[0139] Next, the heated steel slab is hot-rolled to produce a hot-rolled steel sheet. In this hot rolling process, roughing and finishing can be performed using conventional methods.

[0140] Finishing mill inlet side temperature: above Ac3 point

[0141] If the finishing mill inlet temperature in the hot rolling process is lower than point Ac3, stretched ferrite is generated in the hot-rolled steel sheet, and this stretched ferrite also remains in the final cold-rolled steel sheet. As a result, the formation of grain boundary carbides is promoted, while the formation of intragranular carbides is inhibited, thus reducing toughness. Therefore, the finishing mill inlet temperature in the hot rolling process is above point Ac3. On the other hand, there is no particular upper limit to the finishing mill inlet temperature, but it is preferably below 1200°C.

[0142] It should be noted that the above Ac3 point (°C) is obtained by the following equation (1).

[0143] Ac3 (°C) = 910 - (203 × C) 1 / 2 )+(44.7×Si)-(30×Mn)-(11×Cr)+(400×Ti)+(460×Al)+(700×P)+(104×V)+38…(1)

[0144] Here, the element symbol in the above formula (1) refers to the content (mass%) of each element, which is zero when the element is not present.

[0145] (3) Cooling

[0146] Time from the end of hot rolling to the start of cooling: less than 2 seconds

[0147] Next, the hot-rolled steel sheet is cooled. If a long time elapses between the end of hot rolling and the start of cooling, coarse ferrite is formed, and carbides containing at least one of Ti, Nb, and V precipitate unevenly at the grain boundaries. This uneven structure will not homogenize during subsequent cold rolling and annealing, hindering carbide formation within the grains. Therefore, the time from the end of hot rolling to the start of cooling should be 2 seconds or less. On the other hand, from the above perspective, the shorter the time from the end of hot rolling to the start of cooling, the better; therefore, there is no particular lower limit. However, from the perspective of industrial production, it can be 0.5 seconds or more, or even 0.8 seconds or more.

[0148] Average cooling rate: 25℃ / s or higher

[0149] If the average cooling rate during the aforementioned cooling process is less than 25°C / s, the ferrite grains coarsen, and the generated carbides become localized. Therefore, during subsequent cold rolling and annealing, carbide formation at grain boundaries becomes concentrated, suppressing the formation of carbides within the grains. Thus, an average cooling rate of 25°C / s or higher is preferred. On the other hand, there is no particular upper limit to the average cooling rate, but if the cooling rate is too high, the winding shape becomes undesirable due to volume expansion caused by phase transformation during subsequent winding. Therefore, from the viewpoint of achieving a good winding shape, an average cooling rate of 160°C / s or less is preferable, and more preferably 150°C / s or less is more suitable.

[0150] Cooling stop temperature: 720℃

[0151] Furthermore, even if the cooling stop temperature is too high, the base metal grains will still coarsen, thus suppressing carbide formation within the grains during repeated cold rolling and annealing. Therefore, the cooling stop temperature is 720°C or lower. On the other hand, there is no particular limitation on the lower limit of the cooling stop temperature, but if the cooling stop temperature is too low, the winding shape will become undesirable due to volume expansion caused by phase transformation during subsequent winding. Therefore, it is preferable to make the cooling stop temperature 620°C or higher, more preferably 640°C or higher.

[0152] (4) Winding

[0153] After the cooling process is stopped, the cooled hot-rolled steel sheet is wound into a coil. The winding temperature is not particularly limited at this time, but is preferably 600–730°C. At this temperature, plate-like cementite precipitates, thereby stabilizing the coil's winding shape.

[0154] (5) First annealing

[0155] Annealing temperature: 650℃~780℃

[0156] Annealing time: 3 hours or more

[0157] The hot-rolled steel sheet after winding is subjected to a first annealing at an annealing temperature of 650°C to 780°C and an annealing time of 3 hours or more. The microstructure of the hot-rolled steel sheet after winding is a pearlite microstructure consisting of plate-like carbides and ferrite. Since the pearlite microstructure is stable, it will not homogenize unless held at a high temperature for a long time. In order to disrupt the pearlite microstructure and generate the desired carbides within the grains during subsequent cold rolling and annealing processes, the annealing temperature needs to be 650°C or higher and the annealing time needs to be 3 hours or higher. On the other hand, if the annealing temperature is higher than 780°C, phase transformation preferentially begins in some areas, resulting in a locally coarse microstructure and an inhomogeneous microstructure, making it difficult to obtain carbides within the grains and thus failing to achieve the desired carbide number density. There is no particular upper limit to the annealing time, but if it is too long, not only will productivity decrease, but the effect will also saturate. Therefore, 20 hours or less is preferred.

[0158] It should be noted that pickling of hot-rolled steel sheets is also preferred before the first annealing.

[0159] (6) Cold rolling

[0160] (7) Second annealing

[0161] After hot rolling, plate-shaped carbides are formed in the steel sheet. Because these plate-shaped carbides are stable, they tend to remain later, ultimately contributing to void formation and cracking, thus reducing toughness. Therefore, to convert the plate-shaped carbides into particle shape, dissolve them again through annealing heating, and precipitate carbides within the grains, the hot-rolled steel sheet after the first annealing is subjected to cold rolling and a second annealing at least twice.

[0162] Rolling rate: 15% or more

[0163] If the rolling rate in the above-mentioned cold rolling is less than 15%, the carbides at the grain boundaries become coarser, thus reducing the number density of carbides generated within the grains and decreasing the grain size of the carbides within the grains. Therefore, the rolling rate is made to be 15% or more. On the other hand, there is no particular upper limit to the above-mentioned reduction rate, but it is preferably 70% or less.

[0164] Annealing temperature: 600~800℃

[0165] If the annealing temperature in the second annealing process described above is higher than 800°C, the carbides at the grain boundaries become coarser, thus reducing the number density of carbides generated within the grains and decreasing the grain size of the carbides within the grains. Therefore, the annealing temperature is kept below 800°C. On the other hand, if the annealing temperature is lower than 600°C, the formation of carbides within the grains is suppressed, and the desired grain size cannot be obtained. Therefore, the annealing temperature is kept above 600°C.

[0166] The heating rate in the second annealing process described above is not particularly limited, but if the heating rate is too slow, carbides are easily formed at the ferrite grain boundaries, thus suppressing carbide formation within the grains. Therefore, from the viewpoint of further improving toughness, it is preferable that the heating rate in the second annealing process is 50°C / hr or higher. On the other hand, there is no particular upper limit to the heating rate, but it is preferably 200°C / s or lower.

[0167] The cold rolling and second annealing processes are repeated two or more times. By repeating the cold rolling and annealing processes two or more times, carbide formation can be promoted, ultimately achieving the desired carbide size and number density within the grains. There is no particular upper limit to the number of repetitions, but even if the repetitions exceed 5 times, the effect saturates; therefore, the number of repetitions is preferably 5 times or less.

[0168] (8) Final cold rolling

[0169] Rolling rate of 20% or more

[0170] After repeating cold rolling and second annealing twice or more as described above, a final cold rolling with a rolling ratio of 20% or more is further performed. By performing a final cold rolling with a rolling ratio of 20% or more, carbides of the desired density precipitate within the grains during quenching and tempering, thus improving toughness. A higher rolling ratio in the final cold rolling is better, but if it is 65% or more, the shape of the steel sheet may become unstable. Therefore, the rolling ratio is preferably less than 65%.

[0171] By meeting the above conditions, cold-rolled steel sheets with excellent toughness after quenching and tempering can be manufactured. It should be noted that the final cold-rolled steel sheet can also undergo any further surface treatment.

[0172] [Manufacturing methods for steel components]

[0173] In another embodiment of the present invention, steel components can be manufactured by quenching and tempering the cold-rolled steel sheet manufactured by the above-described manufacturing method. The quenching and tempering conditions are not particularly limited, but to obtain higher toughness, it is preferable to quench at a quenching temperature of 700°C to 900°C and a holding time of 1 minute to less than 60 minutes, followed by tempering at a tempering temperature of 150°C to 400°C and a holding time of 20 minutes to 3 hours. The quenching temperature is more preferably 750°C to 850°C. Furthermore, the tempering temperature is more preferably 200°C to 300°C.

[0174] The cooling process described above is not particularly limited and can be carried out by any method. For example, the cooling can be any of the following: air cooling, water quenching, or oil quenching.

[0175] It should be noted that, prior to the above-mentioned quenching and tempering, the cold-rolled steel sheet can be processed arbitrarily to achieve the desired shape.

[0176] Example

[0177] To confirm the effectiveness of the present invention, cold-rolled steel sheets were manufactured according to the steps described below, and the toughness of the obtained cold-rolled steel sheets after quenching and tempering was evaluated.

[0178] First, steel with the composition shown in Table 1 is melted in a converter and produced into steel slabs by continuous casting. Then, the steel slabs are sequentially subjected to heating, hot rolling, cooling, winding, first annealing, cold rolling, second annealing, and final cold rolling to produce a cold-rolled steel sheet with a final thickness of approximately 0.4 mm. Each process is carried out under the conditions shown in Tables 2 and 3, with the cold rolling and second annealing processes repeated the times shown in Tables 2 and 3.

[0179] (Methods for determining carbides)

[0180] Microstructure observation test pieces were collected from the obtained cold-rolled steel sheets. The rolling direction section (L-section) of the test pieces was ground, and then etched with a 1–3 vol% nitric acid-ethanol etching solution to reveal the microstructure. Next, the surface of the test pieces was photographed using a SEM (Scanning Electron Microscope) at 3000x magnification to obtain microstructure images. Based on the obtained microstructure images, the grain size of Nb, Ti, and V-based carbides formed within the grains was determined by the cut-off method. The carbides within the measured field of view were counted, and the number density was calculated. The average value of the three fields of view was calculated as the grain size and number density. The measurement results are shown in Tables 4 and 5. The identification of Nb, Ti, and V-based carbides was performed using SEM-EDS (Energy Dispersive X-ray Spectroscopy) analysis. Elemental mapping was performed on the field of view to separate cementite and other carbides. The other carbides were then classified as Nb, Ti, and V-based carbides.

[0181] (Toughness after quenching and tempering)

[0182] Next, to evaluate the toughness of the obtained cold-rolled steel sheet after quenching and tempering, tests were conducted according to the following steps, and the impact value in the Charpy impact test was measured. First, the obtained cold-rolled steel sheet was quenched and tempered. The quenching was performed by holding the cold-rolled steel sheet in a furnace preheated to 800°C for 10 minutes, followed by oil quenching at 80°C. The tempering was performed by holding the quenched cold-rolled steel sheet in a furnace preheated to 250°C for 1 hour, followed by air cooling.

[0183] Then, a Charpy impact test was performed to determine the impact value. The results are shown in Tables 4 and 5. The Charpy impact test used test pieces with a notch depth of 2.5 mm and a notch radius of 0.1 mm (notch width of 0.2 mm) collected from quenched and tempered cold-rolled steel sheets. The U-shaped notch of the test pieces was formed by electrical discharge machining. In this invention, when the impact value is 8 J / cm... 2 The above results indicate that the toughness after quenching and tempering is excellent.

[0184]

[0185]

[0186]

[0187] [Table 4]

[0188]

[0189] *1 Carbides containing at least one of Nb, Ti, and V that exist within ferrite grains

[0190] *2 Number density of carbides with a particle size of 0.10 μm or larger in the above-mentioned carbides.

[0191] [Table 5]

[0192]

[0193] *1 Carbides containing at least one of Nb, Ti, and V that exist within ferrite grains

[0194] *2 Number density of carbides with a particle size of 0.10 μm or larger in the above-mentioned carbides.

[0195] As shown in Tables 1-5, the cold-rolled steel sheet meeting the conditions of this invention exhibits excellent toughness after quenching and tempering. According to this invention, both the high hardness and excellent toughness inherent in Nb·Ti·V carbides can be achieved. Therefore, by using the cold-rolled steel sheet of this invention, steel components with both high levels of hardness and toughness can be manufactured. Thus, the cold-rolled steel sheet of this invention is very suitable for use as a material for various steel components such as fiber machinery parts, bearing parts, and cutting tools.

Claims

1. A cold-rolled steel sheet having a composition consisting of, in mass %: containing, in mass %: C：0.6～1.25％、 Si: 0.10 to 0.55%, Mn: 0.20 to 2.0%, P：0.0005～0.05％、 S: 0.03% or less, Al：0.001～0.1％、 N：0.001～0.009％、 Cr: 0.1 to 1.0%, and Ti: 0.01 to 1.0%, one or two or more of Nb: 0.05 to 0.5% and V: 0.01 to 1.0%, the remainder consisting of Fe and inevitable impurities; and, an average particle diameter of a carbide containing at least one of Nb, Ti and V present in ferrite grains is 0.10 μm or more, Also, the number density of the carbides having a particle size of 0.10 μm or more is 100 pieces / mm 2 The above.

2. A cold rolled steel sheet according to claim 1 wherein, the composition further containing, in mass %: one or two or more of Sb: 0.1% or less, Hf: 0.5% or less, REM: 0.1% or less, Cu: 0.5% or less, Ni: 3.0% or less, Sn: 0.5% or less, Mo: 1% or less, Zr: 0.5% or less, B: 0.005% or less, and W: 0.01% or less.

3. A steel member obtained by subjecting the cold-rolled steel sheet of claim 1 or 2 to quenching and tempering.

4. The steel part according to claim 3, wherein, The steel member is any one of a fiber machine member, a bearing member and a tool.

5. A method of manufacturing a cold-rolled steel sheet, the method comprising: heating a steel slab having the composition of claim 1 or 2, hot-rolling the heated steel slab at a finish rolling entry side temperature of Ac3 or higher to produce a hot-rolled steel sheet, cooling the hot-rolled steel sheet at a time from the end of the hot-rolling to the start of the cooling of 2 seconds or less, an average cooling rate of 25°C / s or more and a cooling stop temperature of 720°C or lower, coiling the cooled hot-rolled steel sheet, subjecting the coiled hot-rolled steel sheet to first annealing at an annealing temperature of 650°C to 780°C and an annealing time of 3 hours or more, repeating, two or more times, cold-rolling at a rolling rate of 15% or more and second annealing at an annealing temperature of 600 to 800°C to the hot-rolled steel sheet after the first annealing, and further subjecting to final cold-rolling at a rolling rate of 20% or more.

6. The method of manufacturing a cold rolled steel sheet according to claim 5, wherein, The second annealing has a temperature increase rate of 50°C / h or more.

7. A method of manufacturing a steel member, the method comprising: subjecting the cold-rolled steel sheet manufactured by the method of claim 5 or 6 to quenching at a quenching temperature of 700°C to 900°C and a holding time of 1 minute or more and less than 60 minutes, and then subjecting to tempering at a tempering temperature of 150 to 400°C and a holding time of 20 minutes to 3 hours.

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