Cold-rolled steel sheet and method for manufacturing cold-rolled steel sheet

Abstract

Provided is a cold-rolled steel sheet having excellent blanking properties. A cold-rolled steel sheet having a prescribed composition and having a steel structure in which the average grain size of ferrite is 10 μm or less, the average grain size of cementite present at the grain boundaries of the ferrite is 5 μm or less, the average grain size of NaCl-type carbides containing at least one of Nb, Ti, and V present within the ferrite grains is 0.5 μm or less, and the average spacing of the NaCl-type carbides is 710 nm or less.

Description

TECHNICAL FIELD

[0001] The present application relates to a cold-rolled steel sheet, and particularly to a cold-rolled steel sheet excellent in punching and blanking properties. In addition, the present application relates to a method for manufacturing the above cold-rolled steel sheet. BACKGROUND

[0002] As a method for processing a cold-rolled steel sheet into a component shape, punching and blanking processing is widely used. For example, in the manufacture of a fiber machine component represented by a knitting needle used in a knitting machine, after a cold-rolled steel sheet is processed into a component shape by punching and blanking processing, the final fiber machine component is manufactured by processing such as cutting, wire drawing, and polishing, and heat treatment such as quenching and tempering.

[0003] However, in punching and blanking processing, there is a problem that burrs are generated on the end surface when a material is blanked. If burrs are generated, dimensional accuracy is reduced, and when a component having burrs is used in a fiber machine or the like, it becomes a cause of failure. Therefore, burrs are removed by grinding or polishing after punching and blanking processing, but depending on the size and complexity of the shape of the component, it is difficult to sufficiently remove burrs.

[0004] Therefore, a cold-rolled steel sheet is required to have excellent blanking properties, that is, to generate as few burrs as possible in punching and blanking processing.

[0005] In order to meet the above requirements, various techniques for improving the blanking properties of a cold-rolled steel sheet have been proposed.

[0006] For example, in Patent Literature 1, a medium-high carbon cold-rolled steel sheet that suppresses the generation of bending and sagging of a blanked end surface caused by blanking processing by controlling the structure is proposed.

[0007] In addition, in Patent Literature 2, a method for manufacturing a high-carbon steel sheet that is soft and excellent in formability by optimizing the composition and manufacturing conditions is proposed.

[0008] In Patent Literature 3, a high-carbon cold-rolled steel sheet that improves the precision blanking processability by optimizing the particle diameter of cementite and ferrite and the like is proposed.

[0009] PRIOR ART DOCUMENTS

[0010] PATENT LITERATURE

[0011] Patent Literature 1: Japanese Patent Application Laid-Open No. 2019-039056

[0012] Patent Literature 2: Japanese Patent Application Laid-Open No. H05-171288

[0013] Patent Literature 3: International Publication No. 2019 / 163828 SUMMARY

[0014] According to the technology of Patent Document 1, by increasing the pearlite ratio of the metal structure and reducing the spheroidized carbide ratio, the crack direction is uniform, and thus the punched end surface property is improved. However, since the ferrite in the pearlite is coarse and the deformation direction is various, the burr becomes high depending on the shearing direction. Thus, the punching property is still insufficient.

[0015] In addition, the technology proposed in Patent Document 2 is to suppress the reduction in workability caused by the deviation in the material properties within the coil by reducing the deviation, and is not to improve the essential punching property of the steel sheet.

[0016] On the other hand, according to the technology proposed in Patent Document 3, although a certain improvement in the punching property is seen, further improvement in the punching property is required.

[0017] The present application was completed in view of the above circumstances, and aims to provide a cold-rolled steel sheet with excellent punching property.

[0018] The present inventors and others have studied a method for further improving the punching property of a cold-rolled steel sheet, and as a result, have obtained the following insights.

[0019] (1) When a punched base material is punched in a punching process, voids are generated from the ferrite grain boundaries, and a large amount of plastic deformation of the ferrite grains occurs in the growth and connection of the voids, and thus the burr of the punched end surface becomes high.

[0020] (2) Therefore, if the plastic deformation of the ferrite grains is suppressed, the burr can be reduced. That is, when the ferrite grains undergo a large plastic deformation, a large number of voids are generated from the ferrite grain boundaries and connected, and as a result, the burr becomes high, but if the plastic deformation amount of the ferrite grains is small, the burr becomes small.

[0021] (3) Furthermore, the reduction in the plastic deformation amount of the ferrite grains also has the effect of reducing the residual stress. That is, in the case where the plastic deformation amount of the ferrite grains is small, both the shape defects caused by the burr and the dimensional variation caused by the residual stress are small, and as a result, the residual stress is reduced.

[0022] (4) In order to reduce the plastic deformation amount of the ferrite grains, it is necessary to make the ferrite grains themselves hard. The hardening of the ferrite grains can be achieved by making the ferrite grains fine and dispersing fine carbides in the ferrite grains.

[0023] (5) In order to make the ferrite grains fine and disperse fine carbides in the ferrite grains, cementite present at the ferrite grain boundaries (hereinafter sometimes referred to as "grain boundary cementite") must be fine. In addition, by suppressing the generation of coarse grain boundary cementite, the generation of coarse voids in the grain boundaries can be suppressed, and as a result, the generated burr can be reduced.

[0024] The present invention is based on the above insight, and its gist is as follows.

[0025] A cold-rolled steel sheet having a composition consisting of, in mass%:

[0026] C: 0.6 to 1.25%, Si: 0.1 to 0.55%, Mn: 0.5 to 2.0%, P: 0.0005 to 0.05%, S: 0.0001 to 0.01%, Al: 0.001 to 0.1%, N: 0.001 to 0.009%, Cr: 0.05 to 0.65%, and at least one selected from the group consisting of Ti: 0.001 to 0.3%, Nb: 0.01 to 0.1%, and V: 0.005 to 0.5%,

[0027] the remainder consisting of Fe and inevitable impurities;

[0028] and having a steel structure in which:

[0029] the average grain size of ferrite is 10 μm or less,

[0030] the average grain size of cementite present at the grain boundaries of ferrite is 5 μm or less,

[0031] the average grain size of NaCl-type carbide containing at least one of Nb, Ti, and V present within the ferrite grains is 0.5 μm or less,

[0032] and the average interval of the above NaCl-type carbide is 710 nm or less.

[0033] 2. The cold-rolled steel sheet according to the above 1, wherein the composition further comprises, in mass%, at least one selected from the group consisting 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, and Zr: 0.5% or less.

[0034] 3. A method of manufacturing a cold-rolled steel sheet,

[0035] heating a steel billet having the composition according to the above 1 or 2,

[0036] hot-rolling the heated steel billet to produce a hot-rolled steel sheet under conditions of a hot-rolling start temperature: Ac3 point or more and a finish rolling exit temperature: 800°C or more,

[0037] cooling the hot-rolled steel sheet under conditions of a time from the end of the hot-rolling to the start of cooling: 5.0 seconds or less, an average cooling rate: 25°C / s or more, and a cooling stop temperature: 740°C to 620°C,

[0038] The cooled hot-rolled steel sheet is wound,

[0039] The hot-rolled steel sheet after the winding is subjected to first annealing under conditions of an annealing temperature of 730°C or lower and an annealing time of 5 hours or longer,

[0040] The hot-rolled steel sheet after the first annealing is subjected to bending and reverse bending,

[0041] The hot-rolled steel sheet after the bending and reverse bending is subjected to second annealing at an annealing temperature of 600°C or higher,

[0042] The hot-rolled steel sheet after the second annealing is repeatedly subjected to cold rolling at a rolling rate of 15% or higher and third annealing at an annealing temperature of 600°C or higher, twice or more.

[0043] According to the present application, a cold-rolled steel sheet having excellent blanking properties can be provided. The cold-rolled steel sheet of the present application suppresses the generation of burrs when subjected to press blanking processing, and has small residual stress, and thus can be very suitably used as a material for press blanking processing, particularly a material for fiber machine parts typified by knitting needles. DETAILED DESCRIPTION

[0044] Hereinafter, the present application will be described in detail. It should be noted that the present application is not limited to this embodiment.

[0045] [Component Composition]

[0046] The cold-rolled steel sheet of the present application has the above component composition. Hereinafter, the reasons for the limitations will be described. It should be noted that in the following description, " % " as a unit of content means " mass % " unless otherwise specified.

[0047] C: 0.60 to 1.25 %

[0048] C is an element having an effect of increasing hardness by quenching, and plays an important role in blanking. C forms cementite with Fe, as a result, a boundary is generated between the generated cementite and ferrite. Moreover, the boundary becomes a starting point of voids at the time of blanking. When shearing occurs with the voids as a starting point, plastic deformation of ferrite is suppressed, and burr height is reduced. When the C content is less than 0.60%, carbon is consumed in cementite generation, and no carbide is generated in the grain, thus plastic deformation of ferrite grains is promoted. As a result, burr becomes high, and residual stress becomes large, and accuracy of shape and size is reduced. Therefore, the C content is 0.60% or more, preferably 0.65% or more, and more preferably 0.70% or more. On the other hand, when the C content exceeds 1.25%, the cold-rolled steel sheet becomes too hard, and brittle fracture easily occurs, thus a crack is generated at the sheared end face at the time of blanking. Therefore, the C content is 1.25% or less, preferably 1.20% or less, and more preferably 1.15% or less.

[0049] Si: 0.1 to 0.55%

[0050] Si is an element having an effect of increasing strength of ferrite structure by solid solution strengthening, and blanking can be improved by adding Si. In order to obtain the above effect, the Si content is 0.1% or more, preferably 0.12% or more, and more preferably 0.14% or more. On the other hand, when the Si content is excessive, generation and grain growth of ferrite are promoted, and ferrite strength is reduced. In addition, by promoting generation of ferrite, precipitation of coarse cementite to the grain boundary is promoted, and void generation frequency is reduced. As a result, the amount of plastic deformation increases, and blanking is reduced. Therefore, the Si content is 0.55% or less, preferably 0.52% or less, and more preferably 0.50% or less.

[0051] Mn: 0.5 to 2.0%

[0052] Mn is an element that is mixed in cementite and suppresses growth of cementite. By refining cementite generated at the ferrite grain boundary, plastic deformation of ferrite can be suppressed, and blanking can be improved. In order to obtain the above effect, the Mn content is 0.5% or more, preferably 0.52% or more, and more preferably 0.54% or more. On the other hand, when the Mn content exceeds 2.0%, a wide range of banded structure is generated in the rolling direction due to segregation of Mn sulfide, and structure generation becomes abnormal. As a result, abnormal grain growth of ferrite grains is promoted, and cementite precipitation becomes non-uniform, and blanking is reduced. Therefore, the Mn content is 2.0% or less, preferably 1.95% or less, more preferably 1.90% or less, and further preferably 1.85% or less.

[0053] P: 0.0005 to 0.05%

[0054] P is an element having the effect of strengthening ferrite. Therefore, by adding a small amount of P, plastic deformation of ferrite can be suppressed, and the blanking property can be improved. Therefore, the content of P is made to be 0.0005% or more, and preferably 0.0010% or more. On the other hand, when the content of P exceeds 0.05%, the formation of cementite at the grain boundaries is suppressed by the grain boundary segregation of P, the amount of plastic deformation of ferrite increases, and as a result, the blanking property decreases. Therefore, the content of P is made to be 0.05% or less, and preferably 0.04% or less.

[0055] S: 0.0001 to 0.01%

[0056] S forms a sulfide with Mn contained in the steel. When MnS is generated at the ferrite grain boundaries, it becomes a starting point of voids at the boundaries between ferrite and precipitates, like cementite, and thus the blanking property is improved. Therefore, the content of S is made to be 0.0001% or more, and preferably 0.0005% or more. On the other hand, when the content of S exceeds 0.01%, elongated band-shaped MnS is generated in large amounts, abnormal grain growth is promoted, and thus local deformation occurs, and the blanking property deteriorates. Therefore, the content of S is made to be 0.01% or less, and preferably 0.008% or less.

[0057] Al: 0.001 to 0.10%

[0058] Al is dispersed in the steel in the form of an oxide, and is solid-solved to strengthen ferrite, thereby suppressing plastic deformation of ferrite and improving the blanking property. Therefore, the content of Al is made to be 0.001% or more, and preferably 0.002% or more. On the other hand, when the content of Al exceeds 0.10%, the growth of ferrite grains is promoted, the amount of plastic deformation increases, and as a result, the blanking property decreases. Therefore, the content of Al is made to be 0.10% or less, and preferably 0.08% or less, and more preferably 0.06% or less.

[0059] N: 0.001 to 0.009%

[0060] N combines with Al to form AlN in the steel. When the content of N is less than 0.001%, the ferrite grains become coarse, and the blanking property decreases. Therefore, the content of N is made to be 0.001% or more. On the other hand, when the content of N exceeds 0.009%, AlN is precipitated at the ferrite grain boundaries of the hot-rolled steel sheet as an intermediate product, the ferrite grains become elongated and coarse, and the blanking property decreases. Therefore, the content of N is made to be 0.009% or less, and preferably 0.006% or less.

[0061] Cr: 0.05 to 0.65%

[0062] Cr is an element that improves the hardenability and strength of steel, but also affects blanking properties. When the Cr content is less than 0.05%, cementite tends to coarsen, void density decreases, and blanking properties decrease. Therefore, the Cr content is 0.05% or more, preferably 0.08% or more, more preferably 0.10% or more, and even more preferably 0.15% or more. On the other hand, when the Cr content is excessive, coarse Cr carbides and Cr nitrides are formed, and voids are generated at the interface between Cr carbides, Cr nitrides, and ferrite before the voids are generated at the interface between cementite and ferrite. In addition, the formation of coarse Cr carbides inhibits the formation of carbides within the grains, reducing the strength of ferrite. As a result, deformation is localized and blanking properties decrease. Therefore, the Cr content is 0.65% or less, preferably 0.60% or less.

[0063] The above-mentioned composition contains at least one selected from the group consisting of Ti: 0.001 to 0.30%, Nb: 0.01 to 0.1%, and V: 0.005 to 0.5%.

[0064] Ti: 0.001~0.30%

[0065] Ti forms fine TiC within ferrite grains, strengthening the ferrite grains and suppressing the amount of plastic deformation. Therefore, adding Ti can improve punchability. However, when the Ti content is less than 0.001%, TiN precipitates before TiC, consuming the Ti, and thus the punchability improvement effect cannot be achieved. Therefore, when adding Ti, the Ti content is set to 0.001% or more, preferably 0.005% or more. On the other hand, when the Ti content exceeds 0.30%, coarse TiC is generated, and voids are locally formed and grow around the coarse TiC. As a result, plastic deformation is localized, and punchability is reduced. Therefore, the Ti content is set to 0.30% or less, preferably 0.28% or less, and more preferably 0.26% or less.

[0066] Nb: 0.01~0.1%

[0067] Nb forms fine NbC within ferrite grains, strengthening them and suppressing plastic deformation. Therefore, adding Nb can improve blanking properties. However, when the Nb content is less than 0.01%, the amount of NbC precipitated is small, and the effect of improving blanking properties is not achieved. Therefore, when adding Nb, the Nb content is set to 0.01% or higher, preferably 0.015% or higher. On the other hand, when the Nb content exceeds 0.1%, coarse Nb(CN) is formed, and localized voids form around the coarse Nb(CN), localizing deformation and reducing blanking properties. Therefore, the Nb content is set to 0.1% or lower, preferably 0.09% or lower.

[0068] V: 0.005~0.5%

[0069] V forms fine Vc in ferrite grains, strengthens the ferrite grains, and suppresses plastic deformation. Therefore, the blanking property can be improved by adding V. However, when the V content is less than 0.005%, the amount of Vc precipitated is small, and thus the effect of improving the blanking property cannot be obtained. Therefore, when V is added, the V content is made to be 0.005% or more, and preferably 0.010% or more. On the other hand, when the V content exceeds 0.5%, coarse V(CN) is generated, and voids are locally present around the coarse V(CN), and thus the deformation amount is biased, and thus the blanking property is reduced. Therefore, the V content is made to be 0.5% or less, and preferably 0.45% or less, and more preferably 0.40% or less.

[0070] The cold-rolled steel sheet of one embodiment of the present application has a composition consisting of the above components and the balance of Fe and unavoidable impurities.

[0071] In another embodiment of the present application, the above composition can further contain at least one selected from 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, and Zr: 0.5% or less, as appropriate.

[0072] Sb: 0.1% or less

[0073] Sb is an element effective for improving corrosion resistance, but when added in excess, an Sb-rich layer is generated under the scale generated at the time of hot rolling, and surface scarring (scratches) of the steel sheet occur after hot rolling. Therefore, the Sb content is made to be 0.1% or less. On the other hand, the lower limit of the Sb content is not particularly limited, but from the viewpoint of improving the effect of addition, the Sb content is preferably made to be 0.0003% or more.

[0074] Hf: 0.5% or less

[0075] Hf is an element effective for improving corrosion resistance, but when added in excess, an Hf-rich layer is generated under the scale generated at the time of hot rolling, and surface scarring (scratches) of the steel sheet occur after hot rolling. Therefore, the Hf content is made to be 0.5% or less. On the other hand, the lower limit of the Hf content is not particularly limited, but from the viewpoint of improving the effect of addition, the Hf content is preferably made to be 0.001% or more.

[0076] REM: 0.1% or less

[0077] REM (rare earth metal) is an element for improving the strength of steel. However, excessive addition of REM sometimes delays the refinement of carbides, promotes uneven deformation at the time of cold working, and deteriorates the surface properties. Therefore, the REM content is 0.1% or less. On the other hand, the lower limit of the REM content is not particularly limited, but from the viewpoint of improving the effect of addition, it is preferable to make the REM content 0.005% or more.

[0078] Cu: 0.5% or less

[0079] Cu is an element effective for improving corrosion resistance, but when excessively added, a Cu-rich layer is formed under the oxide skin generated at the time of hot rolling, and surface scarring (scratches) of the steel sheet occur after hot rolling. Therefore, the Cu content is 0.5% or less. On the other hand, the lower limit of the Cu content is not particularly limited, but from the viewpoint of improving the effect of addition, it is preferable to make the Cu content 0.01% or more.

[0080] Ni: 3.0% or less

[0081] Ni is an element for improving the strength of steel. However, excessive addition sometimes delays the refinement of carbides, promotes uneven deformation at the time of cold working, and deteriorates the surface properties. Therefore, the Ni content is 3.0% or less. On the other hand, the lower limit of the Ni content is not particularly limited, but from the viewpoint of improving the effect of addition, it is preferable to make the Ni content 0.01% or more.

[0082] Sn: 0.5% or less

[0083] Sn is an element effective for improving corrosion resistance, but when excessively added, a Sn-rich layer is formed under the oxide skin generated at the time of hot rolling, and surface scarring (scratches) of the steel sheet occur after hot rolling. Therefore, the Sn content is 0.5% or less. On the other hand, the lower limit of the Sn content is not particularly limited, but from the viewpoint of improving the effect of addition, it is preferable to make the Sn content 0.0001% or more.

[0084] Mo: 1% or less

[0085] Mo is an element for improving the strength of steel. However, excessive addition sometimes delays the refinement of carbides, promotes uneven deformation at the time of cold working, and deteriorates the surface properties. Therefore, the Mo content is 1% or less. On the other hand, the lower limit of the Mo content is not particularly limited, but from the viewpoint of improving the effect of addition, it is preferable to make the Mo content 0.001% or more.

[0086] Zr: 0.5% or less

[0087] Zr is an element effective for improving corrosion resistance, but when added in excess, a Zr-rich layer is formed under the scale formed during hot rolling, resulting in scarring (scratches) on the surface of the steel sheet after hot rolling. Therefore, the Zr content is 0.5% or less. On the other hand, the lower limit of the Zr content is not particularly limited, but from the viewpoint of improving the effect of addition, the Zr content is preferably 0.01% or more.

[0088] [Organization]

[0089] Next, the organization of the cold-rolled steel sheet of the present application is described.

[0090] Average grain size of ferrite: 10 μm or less

[0091] The finer the grain size of ferrite, the more the plastic deformation of ferrite is suppressed. In order to obtain excellent blanking property, the average grain size of ferrite is made 10 μm or less. On the other hand, the finer the ferrite, the more preferable it is, and therefore the lower limit of the above average grain size is not limited. However, from the viewpoint of industrial production, the above average grain size can be 0.5 μm or more. It should be noted that the average grain size of ferrite can be measured by the method described in the examples.

[0092] Average grain size of cementite present at ferrite grain boundaries: 5 μm or less

[0093] Cementite is present in ferrite grains and at ferrite grain boundaries, and the cementite at the ferrite grain boundaries is coarser than that in the ferrite grains. The present inventors have found that by controlling the average grain size of the cementite present at the ferrite grain boundaries, the blanking property can be improved.

[0094] That is, when a cold-rolled steel sheet is subjected to blanking processing, shearing is performed by creating voids between grain boundaries and cementite. At this time, void formation is performed at the boundaries formed by coarse cementite, and if local deformation occurs, the burr height becomes high. Therefore, in order to improve the blanking property, the cementite present at the ferrite grain boundaries must be fine. Therefore, the average grain size of the cementite present at the ferrite grain boundaries is made 5 μm or less. On the other hand, since the smaller the above average grain size, the better, the lower limit of the average grain size is not particularly limited. However, in the manufacturing method described later, since annealing is repeatedly performed, the cementite at the grain boundaries tends to grow. Therefore, in practice, the above average grain size is 0.5 μm or more. It should be noted that the average grain size of the cementite present at the ferrite grain boundaries can be measured by the method described in the examples.

[0095] As described above, in the present application, it is important that the grain boundary cementite be fine, but as a result of the fining, the cementite is spheroidized. The spheroidization rate of the grain boundary cementite is not particularly limited, but is preferably 2.5 or less. The spheroidization rate of the above grain boundary cementite is defined by the following formula.

[0096] Spheroidization rate = La / Lb

[0097] Here, La: average value of the long diameter of cementite, Lb: average value of the short diameter of cementite. La and Lb are measured by photographing three fields of view of a cross section of a cold-rolled steel sheet cut in the sheet thickness direction using a scanning electron microscope (SEM) at a magnification of 1000 times, measuring the long diameter and the short diameter of all grain boundary cementites observed in the obtained images, and calculating the average values thereof. At this time, the long diameter and the short diameter are values when the cementite is made into an ellipsoid or a sphere.

[0098] Average particle diameter of NaCl-type carbide present in ferrite grains: 0.5 μm or less

[0099] Further, the cold-rolled steel sheet of the present application contains at least one of Ti, Nb, and V. These elements form NaCl-type carbides and precipitate in ferrite grains and at ferrite grain boundaries. By finely dispersing the above NaCl-type carbides in the ferrite grains, the ferrite can be hardened, and the plastic deformation amount of the ferrite grains can be reduced. As a result, the burr height at the time of press blanking can be reduced.

[0100] Therefore, in the present application, the average particle diameter of the NaCl-type carbide containing at least one of Nb, Ti, and V present in the ferrite grains is 0.5 μm or less. On the other hand, the smaller the above average particle diameter, the higher the effect of strengthening the ferrite, and therefore the lower limit of the above average particle diameter is not particularly limited. However, in the manufacturing method described later, since annealing is repeatedly performed, the precipitates tend to grow. Therefore, in practice, the above average particle diameter is 0.01 μm or more. The above average particle diameter can be measured by the method described in the examples. It should be noted that in the following description, the NaCl-type carbide containing at least one of Nb, Ti, and V present in the ferrite grains is sometimes simply referred to as "NaCl-type carbide".

[0101] Average interval of NaCl-type carbides: 710 nm or less

[0102] The above NaCl-type carbide strengthens the ferrite because the finely dispersed NaCl-type carbide functions as an obstacle to dislocations, and such strengthening is called precipitation strengthening. In the precipitation strengthening, the smaller the distance between the precipitates, the greater the strengthening. When the average interval of the above NaCl-type carbide is greater than 710 nm, the decrease in the plastic deformation amount of the ferrite grains due to the precipitation strengthening becomes insufficient, and as a result, the press blanking property decreases. Therefore, in the present application, the average interval of the above NaCl-type carbide present in the ferrite grains is made to be 710 nm or less, and preferably 250 nm or less. On the other hand, the lower limit of the above average interval is not particularly limited, but is 30 nm or more within the practical manufacturing range. Note that the average interval of the NaCl-type carbide present in the ferrite grains can be measured by the method described in the Examples.

[0103] In addition, the number density of the NaCl-type carbide containing at least one of Nb, Ti, and V present in the ferrite grains is not particularly limited, but is preferably less than 100 / μm 2 .

[0104] The number density of the grain boundary cementite having a particle size of 0.5 μ or more is not particularly limited, but is preferably 5 / 100 μm 2 or more. On the other hand, the upper limit of the number density of the grain boundary cementite having a particle size of 0.5 μ or more is also not particularly limited, but is preferably 50 / 100 μm 2 or less.

[0105] In the present application, as described above, the press blanking property is improved by reducing the plastic deformation amount of the ferrite. Therefore, the cold-rolled steel sheet of the present application has a structure containing ferrite. The area ratio of the ferrite is not particularly limited, but it is preferable that the above cold-rolled steel sheet have a structure in which ferrite is the main component. Here, "ferrite is the main component" is defined as the area ratio of the ferrite being 50% or more. The area ratio of the ferrite is more preferably 68% or more.

[0106] In addition, the above structure can contain any structure other than ferrite. However, from the viewpoint of reducing the coarse cementite, it is preferable that the area ratio of the cementite be less than 30%.

[0107] The cold-rolled steel sheet of one embodiment of the present application can have, for example, a structure composed of 68% or more of ferrite, less than 30% of cementite, and the remaining portion of precipitates other than cementite, in terms of area ratio. As the above "precipitates other than cementite", for example, carbides, nitrides, carbonitrides, sulfides, carbon sulfides, and the like other than cementite (Fe3C) can be given. As more specific examples, at least one of carbides, nitrides, and carbonitrides of Ti, V, and Nb, and Mn-based sulfides, Ti-based complex carbon sulfides, and the like can be given.

[0108] [Plate thickness]

[0109] The plate thickness of the cold-rolled steel sheet is not particularly limited and can be any thickness. In consideration of press blanking processing and use as a base material for a fiber machine component, the plate thickness is preferably 0.1 mm to 1.6 mm. In particular, in consideration of use as a base material for a knitting needle, the plate thickness is preferably 0.2 mm to 0.8 mm.

[0110] [Manufacturing method]

[0111] Next, a manufacturing method of the cold-rolled steel sheet according to an embodiment of the present application will be described.

[0112] The cold-rolled steel sheet described above can be manufactured by sequentially performing the following processes on a steel billet having the composition described above.

[0113] (1) Heating

[0114] (2) Hot rolling

[0115] (3) Cooling

[0116] (4) Coiling

[0117] (5) First annealing

[0118] (6) Bending back

[0119] (7) Second annealing

[0120] (8) Cold rolling

[0121] (9) Third annealing

[0122] Then, the processes of (8) and (9) described above are repeated two or more times. Hereinafter, each process will be described sequentially.

[0123] (1) Heating

[0124] First, a steel billet having the composition described above is heated. The steel billet described above can be manufactured by any method and is not particularly limited. For example, adjustment of the composition of the steel billet can be performed by a blast furnace converter method, or can be performed by an electric furnace method. In addition, casting of the molten steel into a base material can be performed by a continuous casting method, or can be performed by steel billet rolling.

[0125] The heating temperature of the steel billet is not particularly limited, but as described later, the temperature of the steel billet at the stage of starting the next hot rolling can be adjusted to be in the austenite region.

[0126] (2) Hot rolling

[0127] Next, the steel billet that has been heated is subjected to hot rolling to produce a hot-rolled steel sheet. In the hot rolling described above, rough rolling and finish rolling can be performed in accordance with conventional methods.

[0128] hot rolling start temperature: Ac3 point or more

[0129] In the above hot rolling, when the hot rolling start temperature is less than the Ac3 point, elongated ferrite is generated in the hot rolled steel sheet of the intermediate product and remains in the final product, so the burr height becomes high. Therefore, the hot rolling start temperature is made to be the Ac3 point or more. Note that the above Ac3 point (°C) is found by the following (1).

[0130] Ac3 (°C) = 910 - (203 x C 1 / 2 ) + (44.7 x Si) - (30 x Mn) - (11 x Cr) + (400 x Ti) + (460 x Al) + (700 x P) + (104 x V) + 38... (1)

[0131] Here, the element symbols in the above (1) mean the content (mass %) of each element, and is zero when the element is not contained.

[0132] finishing rolling exit temperature: 800°C or more

[0133] Also, when the finishing rolling exit temperature is less than 800°C, elongated ferrite is generated in the hot rolled steel sheet of the intermediate product and remains in the final product, so the burr height becomes high. Therefore, the finishing rolling exit temperature is made to be 800°C or more.

[0134] (3) Cooling

[0135] time until cooling starts: 5.0 seconds or less

[0136] Next, the above hot rolled steel sheet is cooled. At this time, if a long time elapses from the end of the hot rolling to the start of the cooling, carbides containing at least one of Ti, Nb, and V are precipitated at the austenite grain boundaries, elongated grains are generated in the final product, and as a result, the blanking workability is reduced. Therefore, the time from the end of the above hot rolling to the start of the cooling (hereinafter sometimes simply referred to as "time until cooling starts") is made to be 5.0 seconds or less, preferably 4.5 seconds or less, and more preferably 4.0 seconds or less. On the other hand, the lower limit of the above time until cooling starts is not particularly limited, but from the viewpoint of being suitable for general production equipment, it is preferably 0.2 seconds or more, and more preferably 0.5 seconds or more.

[0137] average cooling speed: 25°C / s or more

[0138] In addition, when the average cooling rate in the above cooling is less than 25°C / s, elongated grains are generated in the cold-rolled steel sheet as a final product, as a result, the blanking property is reduced. Therefore, the average cooling rate is made to be 25°C / s or more. On the other hand, the upper limit of the above average cooling rate is not particularly limited, but from the viewpoint of being suitable for general production equipment, it is preferable to be 80°C / s or less, more preferable to be 60°C / s or less, and further preferable to be 50°C / s or less.

[0139] Cooling stop temperature: 620°C to 740°C

[0140] When the above cooling is stopped at a temperature higher than 740°C, carbides are precipitated at the austenite grain boundaries, elongated grains are generated in the final product, and the blanking property is reduced. Therefore, the cooling stop temperature is 740°C or less. On the other hand, when the above cooling is stopped at a temperature lower than 620°C, ferrite is precipitated, and the pearlite is biased. This biasing leads to uneven dispersion of cementite in the final product. Therefore, the cooling stop temperature is 620°C or more, and preferably 630°C or more.

[0141] (4) Coiling

[0142] After the above cooling is stopped, the cooled hot-rolled steel sheet is coiled into a coil shape. At this time, the coiling temperature is not particularly limited, but it is preferable to be 600 to 730°C.

[0143] It should be noted that, after the above coiling, it is also preferable to perform pickling of the hot-rolled steel sheet before the first annealing that follows.

[0144] (5) First annealing

[0145] The hot-rolled steel sheet after the above coiling has a pearlite structure. Therefore, by performing the first annealing on the hot-rolled steel sheet after the above coiling, the cementite contained in the pearlite is decomposed. By decomposing the cementite, the cementite becomes fine in the second annealing and the cold-rolling that follow. As a result, the ferrite is refined, and the plastic deformation of the ferrite grains can be suppressed.

[0146] Annealing temperature: 730°C or less

[0147] When the annealing temperature in the above first annealing is higher than 730°C, a part preferentially undergoes phase transformation, and thus the ferrite grains are locally coarsened, as a result, the plastic deformation amount increases. In addition, in the locally coarsened structure, the machining becomes uneven, and the part shape accuracy also deteriorates. Therefore, the annealing temperature is 730°C or less. On the other hand, the lower limit of the above annealing temperature is not particularly limited, but from the viewpoint of promoting the decomposition of the cementite by re-solidifying the cementite in the pearlite, it is preferable to make the annealing temperature 450°C or more, more preferable to be 500°C or more, and further preferable to be 520°C or more.

[0148] Annealing time: 5 hours or more

[0149] In addition, if the annealing time in the above first annealing is less than 5 hours, the decomposition of cementite does not proceed. If the decomposition of cementite does not proceed, plate-shaped cementite remains, and the processing of the subsequent cold rolling and the like becomes uneven, and the part shape accuracy deteriorates. Therefore, the above annealing time is 5 hours or more. On the other hand, the upper limit of the above annealing time is not particularly limited. However, since the microstructure changes saturate after the decomposition of cementite begins, from the viewpoint of manufacturing efficiency, it is preferable to make the above annealing time 50 hours or less, and more preferably 40 hours or less.

[0150] It should be noted that, after the above first annealing, it is also preferable to perform pickling of the hot-rolled steel sheet before the subsequent bending back.

[0151] (6) Bending back

[0152] Next, the hot-rolled steel sheet after the above first annealing is subjected to bending back. In order to make the microstructure of the finally obtained cold-rolled steel sheet a desired microstructure, this bending back is extremely important. That is, by imparting a processing strain by bending back after the decomposition of cementite by the above first annealing, strain energy is introduced. Then, by performing the second annealing described later, the refinement of cementite is promoted. In the case where bending back is not performed, the coarsened cementite localizes, and the plastic deformation amount locally increases, and thus the blanking property deteriorates.

[0153] The introduction of the processing strain by bending back can be performed by any method without particular limitation. For example, bending back can be performed using a straightening machine used in shape correction, a skin pass mill, a cutting machine used for shearing a steel sheet, or the like, or can be performed when a coil is uncoiled and re-coiled.

[0154] From the viewpoint of increasing the amount of strain introduced, it is preferable to perform bending back using a small-diameter roll. Specifically, it is preferable to use a roll having a diameter of 1100 mm or less, and more preferably a roll having a diameter of 800 mm or less. By performing bending back using a roll having a diameter of 1100 mm or less, a considerable deformation required for promoting the refinement of cementite after annealing can be introduced. However, if the diameter of the roll is too small, the rolling load is limited, and thus it is necessary to reduce the size of the sheet in advance by shearing or cutting, and the working hours increase. In addition, if the diameter of the roll is too small, the snake of the sheet and the generation of cracks are promoted. Therefore, the diameter of the roll is preferably 300 mm or more, and more preferably 450 mm or more. The above roll can be a tension roll. When a tension roll is used, strain is introduced by passing a sheet between the tension rolls.

[0155] (7) Second annealing

[0156] The hot-rolled steel sheet after the bending back is subjected to second annealing. As described above, by performing the second annealing after imparting the working strain by the bending back, the refinement of cementite is promoted.

[0157] Annealing temperature: 600°C or higher

[0158] When the annealing temperature in the above second annealing is less than 600°C, the refinement of cementite does not proceed, and the generation of NaCl type carbide containing at least one of Nb, Ti, and V is inhibited. If the generation of the above NaCl type carbide is inhibited, the plastic deformation of ferrite grains cannot be inhibited, and thus the burr becomes high. Therefore, the annealing temperature in the above second annealing is made to be 600°C or higher. On the other hand, the upper limit of the above annealing temperature is not particularly limited, but when it is too high, the structure becomes coarse, and the burr becomes high, and thus the above annealing temperature is preferably 790°C or lower, and more preferably 770°C or lower.

[0159] (8) Cold rolling

[0160] (9) Third annealing

[0161] The hot-rolled steel sheet after the above second annealing is subjected to cold rolling and third annealing twice or more. By the above cold rolling, the sheet thickness of the final cold-rolled steel sheet is adjusted. In addition, by performing the third annealing after the cold rolling, the strain generated in the above cold rolling is removed. By performing the above cold rolling and the third annealing twice or more, the uniformity of the structure is improved, and the ferrite is strengthened by the refinement of the ferrite structure, as a result, the blanking property is improved. In order to obtain the above effects, the reduction in the above cold rolling is made to be 15% or more, and the annealing temperature in the above third annealing is made to be 600°C or higher. On the other hand, the upper limit of the above reduction is not particularly limited, but when the reduction is too high, the structure locally becomes coarse, and the burr becomes high. Therefore, the above reduction is preferably 52% or lower, and more preferably 50% or lower. In addition, the upper limit of the annealing temperature in the above third annealing is not particularly limited, but when the annealing temperature is too high, the structure becomes coarse, and the burr becomes high. Therefore, the above annealing temperature is preferably 750°C or lower, and more preferably 720°C or lower.

[0162] The final cold rolling can be further performed after the above cold rolling and the third annealing are repeated twice or more. When the final cold rolling is performed, the reduction in the final cold rolling is not particularly limited, but is preferably 20% or more. The upper limit of the reduction in the above final cold rolling is also not particularly limited, but is preferably 50% or lower.

[0163] By satisfying the above conditions, a cold-rolled steel sheet having a good blanking property can be manufactured. In addition, the finally obtained cold-rolled steel sheet can be further subjected to an arbitrary surface treatment.

[0164] Example

[0165] Next, in order to confirm the effects of the present application, cold-rolled steel sheets were produced according to the following steps, and the blanking properties of the obtained cold-rolled steel sheets were evaluated.

[0166] First, a steel having the composition shown in Table 1 was melted in a converter, and a billet was produced by a continuous casting method. Next, the above-described billet was subjected to heating, hot rolling, cooling, coiling, pickling, first annealing, pickling, bending back, second annealing, cold rolling, and third annealing in this order, and a cold-rolled steel sheet having a final sheet thickness of about 0.4 mm was produced. Each process was performed under the conditions shown in Tables 2 and 3, and cold rolling and third annealing were repeated the number of times shown in Tables 2 and 3. In addition, the above-described bending back was performed using a tension roller having a diameter shown in Tables 2 and 3 at the time of uncoiling of the coil. Note that, in order to make a comparison, the bending back was not performed in some examples (Comparative Example No. 16).

[0167] (Microstructure)

[0168] Next, the microstructure of the obtained cold-rolled steel sheet was evaluated according to the following steps.

[0169] Average grain size of ferrite

[0170] First, a test piece for microstructure observation was taken from the obtained cold-rolled steel sheet. The rolling direction cross section (L cross section) of the above-described test piece for microstructure observation was polished, and the polished surface was etched using a 3 vol% nitric acid alcohol etchant, thereby making the microstructure visible. Next, the surface of the above-described test piece for microstructure observation was photographed at a magnification of 3000 times using an SEM (scanning electron microscope), and a microstructure image was obtained. From the obtained microstructure image, the ferrite grain size was measured by the intercept method. The average value of the ferrite grain sizes measured in five fields of view was calculated as the average grain size.

[0171] Average grain size and number density of grain boundary cementite

[0172] First, a test piece for microstructure observation was taken from the obtained cold-rolled steel sheet. The rolling direction cross section (L cross section) of the above-described test piece for microstructure observation was polished, and the polished surface was etched using a 3 vol% nitric acid alcohol etchant, thereby making the microstructure visible. Next, the surface of the above-described test piece for microstructure observation was photographed at a magnification of 3000 times using an SEM, and a microstructure image was obtained. From the obtained microstructure image, the grain size of the grain boundary cementite was measured by the intercept method. The average value of the grain sizes of the grain boundary cementite measured in three fields of view was calculated as the average grain size of the grain boundary cementite. In addition, the number density of the grain boundary cementite having a grain size of 0.5 μm or more was calculated from the above-described microstructure image.

[0173] Average grain size of NaCl-type carbide

[0174] The average particle diameter of the NaCl-type carbide containing at least one of Nb, Ti, and V present in the ferrite grains is measured according to the following procedure. The surface of the test piece is photographed at a magnification of 80,000 times using a transmission electron microscope (TEM) to obtain five field images of the structure. The particle diameters of the NaCl-type carbide containing at least one of Nb, Ti, and V present in the ferrite grains in the above-obtained structure images are calculated by image processing using a circle approximation, and the average value thereof is calculated. Note that whether the carbide contains at least one of Nb, Ti, and V is identified using TEM-EPMA.

[0175] Average interval of NaCl-type carbide

[0176] The average interval of the NaCl-type carbide containing at least one of Nb, Ti, and V present in the ferrite grains is determined by measuring the intervals of all the NaCl-type carbides that can be confirmed in a field of view at a magnification of 80,000 times and calculating the average value of five fields of view.

[0177] The results of the measurement are shown in Tables 4 and 5. Note that the NaCl-type carbide in Tables 4 and 5 refers to the NaCl-type carbide containing at least one of Nb, Ti, and V present in the ferrite grains.

[0178] (Punchability)

[0179] Next, in order to evaluate the punchability of the obtained cold-rolled steel sheet, a punch test was performed under the following conditions, and the burr height was measured.

[0180] First, a test piece having a width of 20 mm, a length of 150 mm, and a thickness of 0.4 mm was taken from each cold-rolled steel sheet. Next, a punch test was performed on the above test piece using an SKD superhard punch of φ 10. The above punch gap was 100 μm. In addition, the above punch was performed 10 times on one test piece. At this time, the distance from the end of the test piece to the punched hole was set to 5 mm or more at the first punch. In addition, the interval of the adjacent punched holes was set to 5 mm or more at the second and subsequent punches.

[0181] Then, the height of the burr generated in the circumferential direction was observed using a microscope, the height of the burr was measured at five points in the circumferential direction for one hole, and the average value of the burr heights at the above five points was calculated. Next, the same measurement was performed on 10 holes, and the average value of the burr heights calculated in each hole was used as the burr height.

[0182]

[0183]

[0184]

[0185]

[0186]

Claims

1. A cold-rolled steel sheet having the following composition: The composition comprises, in mass%, 0.60-1.25% C, 0.1-0.55% Si, 0.5-2.0% Mn, 0.0005-0.05% P, 0.0001-0.01% S, 0.001-0.10% Al, 0.001-0.009% N, 0.05-0.65% Cr, and at least one selected from 0.001-0.30% Ti, 0.01-0.1% Nb, and 0.005-0.5% V. The remainder consists of Fe and unavoidable impurities; And, it has the following steel structure: The average particle size of ferrite is less than 10 μm. The average particle size of cementite present in the ferrite grain boundaries is 5 μm or less. The average particle size of NaCl-type carbides containing at least one of Nb, Ti, and V present in ferrite grains is 0.5 μm or less. Furthermore, the average spacing between the NaCl-type carbides is 710 nm or less.

2. The cold-rolled steel sheet according to claim 1, wherein: The component composition further includes, in mass%, at least one selected from the group consisting 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, and Zr: 0.5% or less.

3. A method for manufacturing a cold-rolled steel sheet, Heating a steel slab having the composition according to claim 1 or 2, The heated steel slab is hot rolled at a hot rolling starting temperature of Ac3 point or higher and a finishing rolling outlet temperature of 800°C or higher to produce a hot rolled steel sheet. The hot-rolled steel sheet is cooled under the following conditions: the time from the end of hot rolling to the start of cooling is 5.0 seconds or less, the average cooling rate is 25°C / s or more, and the cooling stop temperature is 620°C to 740°C. The cooled hot rolled steel sheet is coiled, The coiled hot-rolled steel sheet is subjected to a first annealing under the conditions of an annealing temperature of 730° C. or less and an annealing time of 5 hours or more. The hot-rolled steel sheet after the first annealing is bent back. The hot-rolled steel sheet after bending and rebending is subjected to a second annealing at an annealing temperature of 600° C. or higher. The hot-rolled steel sheet after the second annealing is subjected to the following steps: cold rolling at a rolling reduction of 15% or more, followed by third annealing at an annealing temperature of 600° C. or higher, which are repeated twice or more.

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