WO2022210761A1

WO2022210761A1 - Cold-rolled steel sheet and cold-rolled steel sheet manufacturing method

Info

Publication number: WO2022210761A1
Application number: PCT/JP2022/015630
Authority: WO
Inventors: 真由美小島; 康広櫻井; 義正船川; 章雅木戸
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2021-03-31
Filing date: 2022-03-29
Publication date: 2022-10-06
Also published as: CN117043374A; JP7472992B2; TWI792968B; JPWO2022210761A1; EP4317484A1; TW202239975A

Abstract

Provided is a cold-rolled steel sheet having excellent punching properties. This cold-rolled steel sheet has a predetermined component composition and a steel structure in which the average grain diameter of ferrite is 10 μm or less, the average grain diameter of cementite existing in a ferrite grain boundary is 5 μm or less, the average grain diameter of NaCl-type carbides including at least one of Nb, Ti, and V and existing in ferrite grains is 0.5 μm or less, and the average interval between the NaCl-type carbides is 710 nm or less.

Description

COLD-ROLLED STEEL AND METHOD FOR MANUFACTURING COLD-ROLLED STEEL

The present invention relates to cold-rolled steel sheets, and more particularly to cold-rolled steel sheets with excellent press punchability. The present invention also relates to a method for manufacturing the cold-rolled steel sheet.

Press punching is widely used as a method for processing cold-rolled steel sheets into component shapes. For example, in the manufacture of textile machine parts such as knitting needles used in knitting machines, cold-rolled steel sheets are processed into parts by press punching, and then processed by cutting, wire drawing, polishing, etc., and heat treatment such as quenching and tempering. through which the final textile machine parts are manufactured.

However, in press punching, there is a problem that burrs occur on the end face when punching the material. The occurrence of burrs reduces dimensional accuracy and causes troubles when parts with burrs are used in textile machines such as knitting machines. Therefore, the burrs are removed by grinding or polishing after the press punching process, but it is difficult to sufficiently remove the burrs depending on the complexity of the dimensions and shapes of the parts.

Therefore, cold-rolled steel sheets are required to have excellent punchability, that is, to minimize the occurrence of burrs during press punching.

In order to meet the above requirements, various techniques have been proposed to improve the punchability of cold-rolled steel sheets.

For example, Patent Document 1 proposes a medium- and high-carbon cold-rolled steel sheet in which bending due to punching and sagging at the punched end face are suppressed by controlling the structure.

In addition, Patent Document 2 proposes a method of manufacturing a high-carbon steel sheet that is soft and has excellent formability by optimizing the chemical composition and manufacturing conditions.

Patent Document 3 proposes a high-carbon cold-rolled steel sheet with improved fine blanking workability by optimizing the grain sizes of cementite and ferrite.

JP 2019-039056 A JP-A-05-171288 WO2019/163828

According to the technique of Patent Document 1, by increasing the ratio of pearlite structure in the metal structure and decreasing the ratio of spheroidized carbide, the crack directions are aligned and the punched end surface properties are said to be improved. However, since ferrite in pearlite is coarse and deforms in various directions, burrs increase depending on the shearing direction. Therefore, the punchability was still insufficient.

In addition, the technique proposed in Patent Document 2 is to suppress the deterioration of workability caused by the variation by reducing the variation in material properties in the coil. does not improve

On the other hand, according to the technique proposed in Patent Document 3, although a certain improvement in punchability can be seen, there is a demand for further improvement in punchability.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a cold-rolled steel sheet with excellent punchability.

As a result of studying methods for further improving the punchability of cold-rolled steel sheets, the inventors of the present invention obtained the following knowledge.

(1) When the material is punched by punching, voids are generated from the ferrite grain boundaries, and as the voids grow and connect, the ferrite grains undergo a large amount of plastic deformation, resulting in high burrs on the punched end face.

(2) Therefore, burrs can be reduced by suppressing plastic deformation of ferrite grains. That is, when the ferrite grains undergo large plastic deformation, a large number of voids are generated and connected at the ferrite grain boundaries, resulting in high burrs.

(3) Furthermore, reduction in the amount of plastic deformation of ferrite grains also has the effect of reducing residual stress. That is, when the amount of plastic deformation of the ferrite grains is small, both the shape defect due to burrs and the dimensional fluctuation due to residual stress are reduced, resulting in a decrease in residual stress.

(4) In order to reduce the amount of plastic deformation of ferrite grains, it is necessary to harden the ferrite grains themselves. Ferrite grains can be hardened by making the ferrite grains finer and by dispersing fine carbides in the ferrite grains.

(5) In order to make the ferrite grains finer and to disperse fine carbides in the ferrite grains, the cementite present at the ferrite grain boundaries (hereinafter sometimes referred to as “grain boundary cementite”) must be fine. In addition, by suppressing the formation of coarse grain boundary cementite, it is possible to suppress the formation of coarse voids at the grain boundaries, and as a result, it is possible to reduce the amount of burrs that are generated.

The present invention has been completed based on the above findings, and the gist thereof is as follows.

in % by mass,
C: 0.6-1.25%,
Si: 0.1 to 0.55%,
Mn: 0.5-2.0%,
P: 0.0005 to 0.05%,
S: 0.0001 to 0.01%,
Al: 0.001 to 0.1%,
N: 0.001 to 0.009%,
Selected from the group consisting of Cr: 0.05 to 0.65%, and Ti: 0.001 to 0.3%, Nb: 0.01 to 0.1%, and V: 0.005 to 0.5% including at least one that is
Having a component composition in which the balance is Fe and unavoidable impurities,
The average grain size of ferrite is 10 μm or less,
The cementite present at the ferrite grain boundary has an average grain size of 5 μm or less,
It has a steel structure in which the average grain size of NaCl-type carbides containing at least one of Nb, Ti, and V present in ferrite grains is 0.5 μm or less, and the average spacing of the NaCl-type carbides is 710 nm or less. , cold-rolled steel.

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

3. Heating a steel slab having the composition according to 1 or 2 above,
The heated steel slab is hot rolled into a hot rolled steel sheet under the conditions of a hot rolling start temperature of Ac 3 points or more and a finish rolling delivery side temperature of 800° C. or more,
The hot-rolled steel sheet is cooled under the conditions of time from the end of hot rolling to the start of cooling: 5.0 seconds or less, average cooling rate: 25 ° C./s or more, cooling stop temperature: 740 ° C. to 620 ° C.,
Winding the cooled hot-rolled steel sheet,
The coiled hot-rolled steel sheet is subjected to a first annealing under conditions of an annealing temperature of 730° C. or less and an annealing time of 5 hours or more,
Bending and reverse bending are applied to the hot-rolled steel sheet after the first annealing,
The hot-rolled steel sheet after bending and unbending is subjected to a second annealing at an annealing temperature of 600 ° C. or higher,
Cold-rolled steel sheet obtained by repeatedly subjecting the hot-rolled steel sheet after the second annealing to cold rolling at a rolling reduction of 15% or more and third annealing at an annealing temperature of 600 ° C. or more twice or more. Production method.

According to the present invention, it is possible to provide a cold-rolled steel sheet with excellent punchability. The cold-rolled steel sheet of the present invention suppresses the generation of burrs when press punching is performed, and has a small residual stress. It can be very suitably used as a material for

The present invention will be described in detail below. In addition, the present invention is not limited to this embodiment.

[Component composition]
The cold-rolled steel sheet of the present invention has the chemical composition described above. The reason for the limitation will be described below. In the following description, "%" as a unit of content means "% by mass" unless otherwise specified.

C: 0.60-1.25%
C is an element that has the effect of improving hardness by quenching, and plays an important role in punchability. C forms cementite with Fe, resulting in a boundary between the formed cementite and ferrite. This boundary becomes the starting point of voids during punching. When shearing occurs starting from voids, plastic deformation of ferrite is suppressed and the burr height is reduced. When the C content is less than 0.60%, carbon is consumed to form cementite, and carbides do not form inside the grains, thereby promoting plastic deformation of the ferrite grains. As a result, the burr height increases, the residual stress increases, and the accuracy of the shape and dimensions deteriorates. Therefore, the C content should be 0.60% or more, preferably 0.65% or more, and more preferably 0.70% or more. On the other hand, if the C content exceeds 1.25%, the cold-rolled steel sheet becomes too hard and brittle fracture is likely to occur, so that cracks occur at the sheared edge during punching. Therefore, the C content should be 1.25% or less, preferably 1.20% or less, and more preferably 1.15% or less.

Si: 0.1-0.55%
Si is an element that has the effect of increasing the strength of the ferrite structure through solid-solution strengthening, and the addition of Si can improve punchability. In order to obtain the above effect, the Si content should be 0.1% or more, preferably 0.12% or more, more preferably 0.14% or more. On the other hand, when the Si content is excessive, ferrite formation and grain growth are promoted, and ferrite strength is lowered. In addition, the promotion of ferrite formation promotes the precipitation of coarse cementite at grain boundaries, thereby reducing the void generation frequency. As a result, the amount of plastic deformation increases and the punchability decreases. Therefore, the Si content should be 0.55% or less, preferably 0.52% or less, and more preferably 0.50% or less.

Mn: 0.5-2.0%
Mn is an element that mixes into cementite and suppresses the growth of cementite. By refining cementite generated at ferrite grain boundaries, plastic deformation of ferrite can be suppressed and punchability can be improved. In order to obtain the above effects, the Mn content should be 0.5% or more, preferably 0.52% or more, and more preferably 0.54% or more. On the other hand, if the Mn content exceeds 2.0%, segregation of Mn sulfide causes a wide band-like structure in the rolling direction, resulting in abnormal structure formation. As a result, abnormal grain growth of ferrite grains is promoted, and cementite precipitation becomes non-uniform, resulting in a decrease in punchability. Therefore, the Mn content is 2.0% or less, preferably 1.95% or less, more preferably 1.90% or less, still more preferably 1.85% or less.

P: 0.0005 to 0.05%
P is an element that has the effect of strengthening ferrite. Therefore, by adding a small amount of P, the plastic deformation of ferrite can be suppressed and the punchability can be improved. Therefore, the P content is made 0.0005% or more, preferably 0.0010% or more. On the other hand, when the P content exceeds 0.05%, the grain boundary segregation of P suppresses the formation of cementite at the grain boundaries, and as a result, the amount of plastic deformation of ferrite increases, resulting in a decrease in punchability. Therefore, the P content should be 0.05% or less, preferably 0.04% or less.

S: 0.0001 to 0.01%
S forms sulfides with Mn contained in steel. When MnS is generated at the ferrite grain boundary, like cementite, it becomes a starting point of voids at the boundary between ferrite and precipitates, thereby improving the punchability. Therefore, the S content should be 0.0001% or more, preferably 0.0005% or more. On the other hand, if the S content exceeds 0.01%, a large amount of stretched band-like MnS is generated, which promotes abnormal grain growth, causing local deformation and deteriorating punchability. Therefore, the S content should be 0.01% or less, preferably 0.008% or less.

Al: 0.001-0.10%
Al disperses in steel as an oxide and solid-solves to strengthen ferrite, thereby suppressing plastic deformation of ferrite and improving punchability. Therefore, the Al content is made 0.001% or more, preferably 0.002% or more. On the other hand, if the Al content exceeds 0.10%, the growth of ferrite grains is accelerated and the amount of plastic deformation increases, resulting in lower punchability. Therefore, the Al content should be 0.10% or less, preferably 0.08% or less, and more preferably 0.06% or less.

N: 0.001 to 0.009%
N combines with Al in steel to form AlN. If the N content is less than 0.001%, the ferrite crystal grains become coarse and the punchability deteriorates. Therefore, the N content is made 0.001% or more. On the other hand, if the N content exceeds 0.009%, AlN precipitates at the ferrite grain boundaries of the hot-rolled steel sheet, which is an intermediate product, and the ferrite grains are extended and coarsened, resulting in a decrease in punchability. Therefore, the N content should be 0.009% or less, preferably 0.006% or less.

Cr: 0.05-0.65%
Cr is an element that enhances the hardenability of steel and improves strength, and also affects punchability. If the Cr content is less than 0.05%, the cementite tends to coarsen, the void density decreases, and the punchability deteriorates. Therefore, the Cr content should be 0.05% or more, preferably 0.08% or more, more preferably 0.10% or more, and still 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 prior to the voids generated at the interface between cementite and ferrite, Voids are generated. In addition, the formation of coarse Cr carbides suppresses the formation of carbides in grains, and the strength of ferrite is lowered. This localizes the deformation and reduces the punchability. Therefore, the Cr content should be 0.65% or less, preferably 0.60% or less.

The above component 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% do.

Ti: 0.001-0.30%
Ti forms fine TiC in ferrite grains, strengthens the ferrite grains, and suppresses the amount of plastic deformation. Therefore, the punchability can be improved by adding Ti. However, if the Ti content is less than 0.001%, Ti is consumed by precipitation of TiN prior to TiC, so the effect of improving the punchability cannot be obtained. Therefore, when Ti is added, the Ti content should be 0.001% or more, preferably 0.005% or more. On the other hand, when the Ti content exceeds 0.30%, coarse TiC is formed, and void formation and growth occur locally around the coarse TiC. As a result, the plastic deformation is localized and the punchability is lowered. Therefore, the Ti content should be 0.30% or less, preferably 0.28% or less, and more preferably 0.26% or less.

Nb: 0.01-0.1%
Nb forms fine NbC in ferrite grains, strengthens the ferrite grains, and suppresses plastic deformation. Therefore, the punchability can be improved by adding Nb. However, if the Nb content is less than 0.01%, the amount of NbC precipitated is so small that the effect of improving punchability cannot be obtained. Therefore, when Nb is added, the Nb content should be 0.01% or more, preferably 0.015% or more. On the other hand, when the Nb content exceeds 0.1%, coarse Nb(CN) is formed, and voids are localized around the coarse Nb(CN), resulting in localized deformation and reduced punchability. Therefore, the Nb content should be 0.1% or less, preferably 0.09% or less.

V: 0.005-0.5%
V forms fine VCs in ferrite grains, strengthens the ferrite grains, and suppresses plastic deformation. Therefore, the punchability can be improved by adding V. However, if the V content is less than 0.005%, the amount of VC precipitated is so small that the effect of improving punchability cannot be obtained. Therefore, when V is added, the V content should be 0.005% or more, preferably 0.010% or more. On the other hand, when the V content exceeds 0.5%, coarse V(CN) is formed, voids are localized around the coarse V(CN), and the amount of deformation is biased, resulting in a decrease in punchability. . Therefore, the V content should be 0.5% or less, preferably 0.45% or less, and more preferably 0.40% or less.

A cold-rolled steel sheet in an embodiment of the present invention has a chemical composition consisting of the above components and the balance of Fe and unavoidable impurities.

In another embodiment of the present invention, the above component composition is optionally Sb: 0.1% or less, Hf: 0.5% or less, REM: 0.1% or less, Cu: 0.5% At least one selected from the group consisting of Ni: 3.0% or less, Sn: 0.5% or less, Mo: 1% or less, and Zr: 0.5% or less can be further included.

Sb: 0.1% or less Sb is an element effective in improving corrosion resistance. scratches). Therefore, the Sb content is set to 0.1% or less. On the other hand, although the lower limit of the Sb content is not particularly limited, the Sb content is preferably 0.0003% or more from the viewpoint of enhancing the effect of addition.

Hf: 0.5% or less Hf is an element that is effective in improving corrosion resistance. scratches). Therefore, the Hf content is set to 0.5% or less. On the other hand, the lower limit of the Hf content is not particularly limited, but from the viewpoint of increasing the effect of addition, the Hf content is preferably 0.001% or more.

REM: 0.1% or less REM (rare earth metal) is an element that improves the strength of steel. However, excessive addition of REM delays the refinement of carbides and promotes uneven deformation during cold working, which may deteriorate the surface properties. Therefore, the REM content is set to 0.1% or less. On the other hand, the lower limit of the REM content is not particularly limited, but from the viewpoint of enhancing the effect of addition, the REM content is preferably 0.005% or more.

Cu: 0.5% or less Cu is an element effective in improving corrosion resistance. scratches). Therefore, the Cu content is set to 0.5% or less. On the other hand, the lower limit of the Cu content is not particularly limited, but from the viewpoint of enhancing the effect of addition, the Cu content is preferably 0.01% or more.

Ni: 3.0% or less Ni is an element that improves the strength of steel. However, excessive addition delays the refinement of carbides, promotes non-homogeneous deformation during cold working, and sometimes deteriorates the surface properties. Therefore, the Ni content is set to 3.0% or less. On the other hand, the lower limit of the Ni content is not particularly limited, but from the viewpoint of increasing the effect of addition, the Ni content is preferably 0.01% or more.

Sn: 0.5% or less Sn is an element that is effective in improving corrosion resistance. scratches). Therefore, the Sn content is set to 0.5% or less. On the other hand, the lower limit of the Sn content is not particularly limited, but from the viewpoint of increasing the effect of addition, the Sn content is preferably 0.0001% or more.

Mo: 1% or less Mo is an element that improves the strength of steel. However, excessive addition delays the refinement of carbides, promotes non-homogeneous deformation during cold working, and sometimes deteriorates the surface properties. Therefore, the Mo content is set to 1% or less. On the other hand, the lower limit of the Mo content is not particularly limited, but from the viewpoint of increasing the effect of addition, the Mo content is preferably 0.001% or more.

Zr: 0.5% or less Zr is an element that is effective in improving corrosion resistance. scratches). Therefore, the Zr content should be 0.5% or less. On the other hand, the lower limit of the Zr content is not particularly limited, but from the viewpoint of increasing the effect of addition, the Zr content is preferably 0.01% or more.

[Organization]
Next, the structure of the cold-rolled steel sheet of the present invention will be explained.

Average grain size of ferrite: 10 µm or less Plastic deformation of ferrite is suppressed as the grain size of ferrite becomes finer. In order to obtain excellent punchability, the average grain size of ferrite is set to 10 μm or less. On the other hand, since the finer the ferrite, the better, the lower limit of the average particle diameter is not limited. However, from the viewpoint of industrial production, the average particle size may be 0.5 μm or more. The average grain size of ferrite can be measured by the method described in Examples.

Average grain size of cementite present at ferrite grain boundaries: 5 μm or less Cementite exists both inside ferrite grains and at ferrite grain boundaries, and cementite at ferrite grain boundaries is relatively coarser than cementite inside ferrite grains. is. The present inventors have found that the punchability can be improved by controlling the average grain size of cementite present in the ferrite grain boundaries.

That is, when punching cold-rolled steel sheets, voids are generated between grain boundaries and cementite, and shear progresses. At that time, void formation progresses at the boundaries formed by coarse cementite, and if local deformation occurs, the burr height increases. Therefore, in order to improve the punchability, the cementite present at the ferrite grain boundaries must be fine. Therefore, the average grain size of cementite present at ferrite grain boundaries is set to 5 μm or less. On the other hand, since the smaller the average particle size, the better, the lower limit of the average particle size is not particularly limited. However, in the manufacturing method described later, since repeated annealing is performed, cementite at grain boundaries tends to grow. Therefore, in reality, the average particle diameter is 0.5 μm or more. The average grain size of cementite existing at the ferrite grain boundary can be measured by the method described in Examples.

As described above, in the present invention, it is important that the grain boundary cementite is fine, and as a result, the cementite becomes spheroidized as a result of the fineness. The spheroidization rate of the grain boundary cementite is not particularly limited, but is preferably 2.5 or less. The spheroidization rate of the grain boundary cementite is defined by the following formula.
Spheroidization rate = La/Lb
Here, La is the average value of the major diameters of cementite, and Lb is the average value of the minor diameters of cementite. La and Lb are obtained by photographing a cross section of the cold-rolled steel sheet in the thickness direction using a scanning electron microscope (SEM) at a magnification of 1000 times for 3 fields of view, and all grain boundary cementite observed in the obtained images. Measure the major axis and the minor axis, and obtain the average value of each. At that time, the major axis and the minor axis are values when the cementite is an ellipsoid or a sphere.

Average grain size of NaCl-type carbide present in ferrite grains: 0.5 µm or less Furthermore, the cold-rolled steel sheet of the present invention 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 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 during press punching can be reduced.

Therefore, in the present invention, the average grain size of NaCl-type carbide containing at least one of Nb, Ti, and V present in ferrite grains is set to 0.5 μm or less. On the other hand, the smaller the average particle size, the higher the effect of strengthening ferrite, so the lower limit of the average particle size is not particularly limited. However, in the manufacturing method described later, repeated annealing is performed, so precipitates tend to grow. Therefore, in reality, the average particle diameter is 0.01 μm or more. The average particle size can be measured by the method described in Examples. In the following description, NaCl-type carbide containing at least one of Nb, Ti, and V present in ferrite grains may be simply referred to as "NaCl-type carbide".

Average spacing of NaCl-type carbides: 710 nm or less The strengthening of ferrite by the NaCl-type carbides is due to the finely dispersed NaCl-type carbides functioning as dislocation obstacles, and such strengthening is called precipitation strengthening. be. In precipitation strengthening, the smaller the distance between precipitates, the greater the strengthening. If the average spacing of the NaCl-type carbides is larger than 710 nm, the amount of plastic deformation of the ferrite grains is not sufficiently reduced due to precipitation strengthening, and as a result, press punchability is deteriorated. Therefore, in the present invention, the average spacing of the NaCl-type carbides present in ferrite grains is set to 710 nm or less, preferably 250 nm or less. On the other hand, although the lower limit of the average interval is not particularly limited, it is 30 nm or more in a realistic manufacturing range. The average spacing of NaCl-type carbides present in ferrite grains can be measured by the method described in the Examples.

Although the number density of NaCl-type carbides containing at least one of Nb, Ti, and V present in the ferrite grains is not particularly limited, it is preferably less than 100/μm ² .

Although the number density of grain boundary cementite having a grain size of 0.5 μm or more is not particularly limited, it is preferably 5 particles/100 μm ² or more. On the other hand, although the upper limit of the number density of grain boundary cementite having a grain size of 0.5 μm or more is not particularly limited, it is preferably 50 particles/100 μm ² or less.

In the present invention, as described above, the punchability is improved by reducing the amount of plastic deformation of ferrite. Therefore, the cold-rolled steel sheet of the present invention has a structure containing ferrite. Although the area ratio of ferrite is not particularly limited, the cold-rolled steel sheet preferably has a structure mainly composed of ferrite. Here, "mainly composed of ferrite" is defined as having an area ratio of ferrite of 50% or more. More preferably, the ferrite area ratio is 68% or more.

Also, the structure can include any structure other than ferrite. However, from the viewpoint of reducing coarse cementite, the area ratio of cementite is preferably less than 30%.

A cold-rolled steel sheet according to an embodiment of the present invention can have, for example, a structure consisting of 68% or more ferrite, less than 30% cementite, and the balance of precipitates other than cementite, in terms of area ratio. Examples of the "precipitates other than cementite" include carbides, nitrides, carbonitrides, sulfides, and carbosulfides other than cementite (Fe ₃ C). More specific examples include carbides, nitrides, and carbonitrides of at least one of Ti, V, and Nb, Mn-based sulfides, and Ti-based composite carbosulfides.

[Thickness]
The plate thickness of the cold-rolled steel plate is not particularly limited, and may be any thickness. Considering that the sheet is punched by press and used as a material for textile machine parts, the plate thickness is preferably 0.1 mm or more and 1.6 mm or less. In particular, considering the use as a material for knitting needles, the plate thickness is preferably 0.2 mm or more and 0.8 mm or less.

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

The cold-rolled steel sheet can be produced by sequentially subjecting a steel slab having the chemical composition described above to the following steps.
(1) Heating (2) Hot rolling (3) Cooling (4) Winding (5) First annealing (6) Bending back (7) Second annealing (8) Cold rolling (9) Third and the above steps (8) and (9) are repeated two or more times. Each step will be described below in sequence.

(1) Heating First, a steel slab having the above chemical composition is heated. The steel slab can be manufactured by any method without particular limitation. For example, the composition adjustment of the steel slab may be performed by a blast furnace converter method or by an electric furnace method. Casting of molten steel into slabs may be performed by continuous casting or by blooming.

The heating temperature of the steel slab is not particularly limited, but as will be described later, it may be adjusted so that the temperature of the steel slab is in the austenitic region when the next hot rolling is started.

(2) Hot rolling Next, the heated steel slab is hot rolled to form a hot rolled steel sheet. In the hot rolling, rough rolling and finish rolling can be carried out according to a conventional method.

Hot rolling start temperature: Ac 3 point or higher In the hot rolling, if the hot rolling start temperature is less than Ac 3 point, expanded ferrite is generated in the hot-rolled steel sheet of the intermediate product and remains in the final product. Therefore, the burr height becomes high. Therefore, the hot rolling start temperature is set to Ac3 or higher. The Ac3 point (° C.) is obtained by the following formula (1).
^Ac3 (°C) = 910 - (203 x C1/2) + (44.7 x Si) - (30 x Mn) - (11 x Cr) + (400 x Ti) + (460 x Al) + (700 x P ) + (104 × V) + 38 … (1)
Here, the element symbol in the above formula (1) indicates the content (% by mass) of each element, and is zero when the element is not contained.

Finish rolling delivery side temperature: 800 ° C. or higher Similarly, if the finish rolling delivery side temperature is less than 800 ° C., expanded ferrite is generated in the hot rolled steel sheet of the intermediate product and remains in the final product, so burrs Height increases. Therefore, the finish rolling delivery side temperature is set to 800° C. or higher.

(3) Cooling Time to start of cooling: 5.0 seconds or less Next, the hot-rolled steel sheet is cooled. At that time, if a long period of time elapses from the end of hot rolling to the start of cooling, carbides containing at least one of Ti, Nb, and V precipitate at the austenite grain boundaries, and elongated grains occur in the final product. , the stamping process is degraded. Therefore, the time from the end of hot rolling to the start of cooling (hereinafter sometimes simply referred to as "time to start cooling") is 5.0 seconds or less, preferably 4.5 seconds or less, more preferably 4.0 seconds. seconds or less. On the other hand, the lower limit of the time until the start of cooling is not particularly limited, but from the viewpoint of adaptability to general production equipment, it is preferably 0.2 seconds or more, more preferably 0.5 seconds or more. preferable.

Average cooling rate: 25° C./s or more If the average cooling rate in the cooling is less than 25° C./s, expanded grains are generated in the cold-rolled steel sheet, which is the final product, and as a result, the punchability is lowered. . Therefore, the average cooling rate is set to 25° C./s or higher. On the other hand, the upper limit of the average cooling rate is not particularly limited, but from the viewpoint of suitability for general production equipment, it is preferably 80 ° C./s or less, and more preferably 60 ° C./s or less. It is more preferable to set it to 50° C./s or less.

Cooling stop temperature: 620°C to 740°C
If the cooling is stopped at a temperature higher than 740° C., carbides are precipitated at the austenite grain boundaries, and elongated grains are generated in the final product, resulting in poor punchability. Therefore, the cooling stop temperature is set to 740° C. or lower. On the other hand, when the cooling is stopped at a temperature lower than 620° C., ferrite precipitates and pearlite is unevenly distributed. This uneven distribution leads to uneven cementite distribution in the final product. Therefore, the cooling stop temperature should be 620° C. or higher, preferably 630° C. or higher.

(4) Winding After stopping the cooling, the cooled hot-rolled steel sheet is wound into a coil. At that time, the winding temperature is not particularly limited, but it is preferably 600 to 730°C.

After the coiling, it is also preferable to pickle the hot-rolled steel sheet prior to the next first annealing.

(5) First Annealing The hot-rolled steel sheet after being coiled has a pearlite structure. Therefore, the cementite contained in the pearlite is decomposed by subjecting the coiled hot-rolled steel sheet to the first annealing. By decomposing the cementite, the cementite becomes finer in the subsequent second annealing and cold rolling. As a result, the ferrite becomes finer, and plastic deformation of the ferrite grains can be suppressed.

Annealing temperature: 730° C. or less If the annealing temperature in the first annealing is higher than 730° C., the phase transformation progresses preferentially in one part, so the ferrite grains locally coarsen, and as a result, the amount of plastic deformation increases. . In addition, a locally coarse structure results in non-uniform machining and poor part shape accuracy. Therefore, the annealing temperature is set to 730° C. or lower. On the other hand, the lower limit of the annealing temperature is not particularly limited, but the annealing temperature is preferably 450° C. or higher, and 500° C. or higher, from the viewpoint of promoting the decomposition of cementite by redissolving cementite in pearlite. It is more preferable to set the temperature to 520° C. or higher.

Annealing time: 5 hours or more If the annealing time in the first annealing is less than 5 hours, decomposition of cementite does not progress. If the decomposition of cementite does not progress, plate-like cementite will remain, and subsequent processing such as cold rolling will become non-homogeneous, and the shape accuracy of the part will deteriorate. Therefore, the annealing time is set to 5 hours or longer. On the other hand, the upper limit of the annealing time is not particularly limited. However, since the structural change is saturated after the cementite decomposition starts, the annealing temperature is preferably 50 hours or less, more preferably 40 hours or less, from the viewpoint of production efficiency.

After the first annealing, it is also preferable to pickle the hot-rolled steel sheet prior to the next bending and unbending.

(6) Unbending Next, the hot-rolled steel sheet after the first annealing is subjected to unbending. This bending and unbending is extremely important in order to obtain the desired structure of the finally obtained cold-rolled steel sheet. That is, strain energy is introduced by bending back to give working strain by decomposing cementite by the first annealing. Thereafter, a second annealing, which will be described later, is performed to promote the refinement of cementite. When bending and unbending is not performed, coarsened cementite is localized and the amount of plastic deformation locally increases, resulting in a decrease in punchability.

The introduction of processing strain by bending and unbending can be done by any method without any particular limitation. For example, a leveler or skin pass rolling machine used for shape correction, a slitter for shearing the steel plate, etc. may be used to perform bending and unbending. You may apply a return.

From the point of view of increasing the amount of strain introduced, it is preferable to bend and unbend using a small-diameter roll. Specifically, a roll with a diameter of 1100 mm or less is preferably used, and a roll with a diameter of 800 mm or less is more preferably used. By performing bending and bending back using rolls having a diameter of 1100 mm or less, it is possible to introduce the equivalent strain necessary to promote the refinement of the cementite after annealing. However, if the diameter of the roll is too small, the rolling load is limited, so it is necessary to reduce the size of the plate in advance by shearing or slitting, which increases the number of man-hours. On the other hand, if the diameter of the roll is too small, meandering and cracking of the plate are promoted. Therefore, the diameter of the roll is preferably 300 mm or more, more preferably 450 mm or more. The roll may be a bridle roll. If bridle rolls are used, strain is introduced by passing the sheet through the bridle rolls.

(7) Second Annealing The hot-rolled steel sheet after bending and unbending is subjected to second annealing. As described above, by performing the second annealing after applying the work strain by bending back, the refinement of the cementite is promoted.

Annealing temperature: 600° C. or higher When the annealing temperature in the second annealing is lower than 600° C., the cementite does not become finer, and the formation of NaCl-type carbide containing at least one of Nb, Ti, and V is suppressed. be. If the formation of the NaCl-type carbide is suppressed, the plastic deformation of the ferrite grains cannot be suppressed, resulting in high burrs. Therefore, the annealing temperature in the second annealing is set to 600° C. or higher. On the other hand, the upper limit of the annealing temperature is not particularly limited, but if it is too high, the structure becomes coarse and the burr increases, so the annealing temperature is preferably 790°C or less, more preferably 770°C or less. .

(8) Cold rolling (9) Third annealing The hot-rolled steel sheet after the second annealing is repeatedly subjected to cold rolling and third annealing twice or more. The cold rolling adjusts the thickness of the final cold-rolled steel sheet. Further, by performing the third annealing after the cold rolling, the strain caused by the cold rolling is removed. By performing the cold rolling and the third annealing twice or more, the uniformity of the structure is improved, and the ferrite is strengthened by refining the ferrite structure. As a result, the punchability is improved. In order to obtain the above effects, the rolling reduction in the cold rolling is set to 15% or higher, and the annealing temperature in the third annealing is set to 600° C. or higher. On the other hand, the upper limit of the rolling reduction is not particularly limited, but if the rolling reduction is excessively high, the structure locally coarsens and burrs increase. Therefore, the rolling reduction is preferably 52% or less, more preferably 50% or less. Also, the upper limit of the annealing temperature in the third annealing is not particularly limited, but if the annealing temperature is excessively high, the structure becomes coarse and the burr increases. Therefore, the annealing temperature is preferably 750° C. or lower, more preferably 720° C. or lower.

After the cold rolling and the third annealing are repeated twice or more, final cold rolling may be performed. When the final cold rolling is performed, the rolling reduction in the final cold rolling is not particularly limited, but is preferably 20% or more. Although the upper limit of the rolling reduction in the final cold rolling is not particularly limited, it is preferably 50% or less.

By satisfying the above conditions, cold-rolled steel sheets with good punchability can be produced. Further, the finally obtained cold-rolled steel sheet may be subjected to any surface treatment.

In order to confirm the effects of the present invention, cold-rolled steel sheets were produced according to the procedure described below, and the punchability of the obtained cold-rolled steel sheets was evaluated.

First, steel having the chemical composition shown in Table 1 was melted in a converter and made into a steel slab by continuous casting. Next, the steel slab is sequentially subjected to heating, hot rolling, cooling, coiling, pickling, first annealing, pickling, unbending, second annealing, cold rolling, and third annealing. A cold-rolled steel sheet having a final thickness of about 0.4 mm was obtained. Each step 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. Further, the bending and unbending was performed using bridle rolls having diameters shown in Tables 2 and 3 when the coil was unwound. For comparison, some examples were not subjected to bending and unbending (comparative example No. 16).

(organization)
Next, the structures of the obtained cold-rolled steel sheets were evaluated by the following procedures.

Average Grain Size of Ferrite First, a test piece for structural observation was taken from the obtained cold-rolled steel sheet. After polishing the rolling direction cross section (L cross section) of the test piece for structure observation, the structure was exposed by corroding the polished surface with a 3 vol % nital solution. Next, the surface of the test piece for tissue observation was imaged with a SEM (scanning electron microscope) at a magnification of 3000 to obtain a tissue image. According to JIS G0551:2020, the ferrite grain size was measured by a cutting method from the obtained structure image. The average value of the ferrite grain sizes measured in five fields of view was calculated and used as the average grain size.

Average Grain Size and Number Density of Grain Boundary Cementite First, a test piece for structure observation was taken from the obtained cold-rolled steel sheet. After polishing the rolling direction cross section (L cross section) of the test piece for structure observation, the structure was exposed by corroding the polished surface with a 3 vol % nital solution. Next, the surface of the test piece for tissue observation was imaged using an SEM at a magnification of 3000 to obtain a tissue image. From the obtained structure image, the grain size was measured by the cutting method only for the grain boundary cementite. The average grain size of the grain boundary cementite measured in the three fields of view was calculated and used as the average grain size of the grain boundary cementite. Also, the number density of grain boundary cementite having a grain size of 0.5 μm or more was obtained from the texture image.

Average Grain Size of NaCl-Type Carbide The average grain size of NaCl-type carbide containing at least one of Nb, Ti, and V present in ferrite grains was measured by the following procedure. The surface of the test piece was imaged using a transmission electron microscope (TEM) at a magnification of 80000 times to obtain tissue images of 5 fields of view. By image processing using circle approximation, individual particle sizes of NaCl-type carbides containing at least one of Nb, Ti, and V present in the ferrite grains in the obtained structure image are obtained, and the average value thereof was calculated. Whether or not the carbide contains at least one of Nb, Ti, and V was identified using TEM-EPMA.

Average spacing of NaCl-type carbides The average spacing of NaCl-type carbides containing at least one of Nb, Ti, and V present in ferrite grains was obtained by measuring the spacing of all NaCl-type carbides that can be confirmed within a field of view of 80,000 times. , was determined by calculating the average value for 5 fields of view.

The measurement results were as shown in Tables 4 and 5. Note that the NaCl-type carbides in Tables 4 and 5 refer to NaCl-type carbides containing at least one of Nb, Ti, and V present in ferrite grains.

(punchability)
Next, in order to evaluate the punchability of the obtained cold-rolled steel sheet, a punch punch test was conducted under the following conditions to measure the burr height.

First, a test piece with 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, using a φ10 SKD or cemented carbide punch, the test pieces were punched out. The clearance in the punching was set to 100 μm. In addition, the punching was performed 10 times for one test piece. At that time, when punching for the first time, the distance from the edge of the test piece to the punched hole was set to 5 mm or more. In the second and subsequent punching operations, the distance between adjacent punched holes was set to 5 mm or more.

After that, the height of the burr generated in the circumferential direction was observed with a microscope, and the height of the burr was measured at 5 locations evenly in the circumferential direction for one hole. An average value was calculated. Next, the same measurement was performed at 10 holes, and the average value of the burr heights calculated for each hole was adopted as the burr height.

Claims

in % by mass,
C: 0.60 to 1.25%,
Si: 0.1 to 0.55%,
Mn: 0.5-2.0%,
P: 0.0005 to 0.05%,
S: 0.0001 to 0.01%,
Al: 0.001 to 0.10%,
N: 0.001 to 0.009%,
Selected from the group consisting of Cr: 0.05 to 0.65%, and Ti: 0.001 to 0.30%, Nb: 0.01 to 0.1%, and V: 0.005 to 0.5% including at least one that is
Having a component composition in which the balance is Fe and unavoidable impurities,
The average grain size of ferrite is 10 μm or less,
The cementite present at the ferrite grain boundary has an average grain size of 5 μm or less,
It has a steel structure in which the average grain size of NaCl-type carbides containing at least one of Nb, Ti, and V present in ferrite grains is 0.5 μm or less, and the average spacing of the NaCl-type carbides is 710 nm or less. , cold-rolled steel.
The component composition, in mass%,
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,
The cold-rolled steel sheet according to claim 1, further comprising at least one selected from the group consisting of Mo: 1% or less and Zr: 0.5% or less.
Heating a steel slab having the composition according to claim 1 or 2,
The heated steel slab is hot rolled into a hot rolled steel sheet under the conditions of a hot rolling start temperature of Ac 3 points or more and a finish rolling delivery side temperature of 800° C. or more,
The hot-rolled steel sheet is cooled under the conditions of time from the end of hot rolling to the start of cooling: 5.0 seconds or less, average cooling rate: 25 ° C./s or more, cooling stop temperature: 620 ° C. to 740 ° C.,
Winding the cooled hot-rolled steel sheet,
The coiled hot-rolled steel sheet is subjected to a first annealing under conditions of an annealing temperature of 730° C. or less and an annealing time of 5 hours or more,
Bending back the hot-rolled steel sheet after the first annealing,
The hot-rolled steel sheet after bending and unbending is subjected to a second annealing at an annealing temperature of 600 ° C. or higher,
Cold-rolled steel sheet obtained by repeatedly subjecting the hot-rolled steel sheet after the second annealing to cold rolling at a rolling reduction of 15% or more and third annealing at an annealing temperature of 600 ° C. or more twice or more. Production method.