CN117043374A

CN117043374A - Cold-rolled steel sheet and method for producing cold-rolled steel sheet

Info

Publication number: CN117043374A
Application number: CN202280020985.7A
Authority: CN
Inventors: 小岛真由美; 樱井康广; 船川义正; 木户章雅
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2021-03-31
Filing date: 2022-03-29
Publication date: 2023-11-10
Also published as: EP4317484A1; JP7472992B2; WO2022210761A1; TWI792968B; JPWO2022210761A1; TW202239975A

Abstract

The invention provides a cold-rolled steel sheet with excellent punching performance. A cold-rolled steel sheet having a predetermined composition and having the following steel structure: the average grain size of ferrite is 10 [ mu ] m or less, the average grain size of cementite existing in ferrite grain boundaries is 5 [ mu ] m or less, the average grain size of NaCl-type carbide containing at least one of Nb, ti and V existing in ferrite grains is 0.5 [ mu ] m or less, and the average interval of the NaCl-type carbide is 710nm or less.

Description

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

Technical Field

The present invention relates to a cold-rolled steel sheet, and more particularly, to a cold-rolled steel sheet having excellent punching properties. The present invention also relates to a method for producing the cold-rolled steel sheet.

Background

As a method of forming a cold-rolled steel sheet into a member shape, press blanking is widely used. For example, in the production of a textile 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 press blanking, a final textile machine component is produced by heat treatment such as cutting, drawing, grinding, and the like, quenching and tempering.

However, in the press blanking process, burrs are generated on the end surfaces when blanking the material. If burrs are generated, dimensional accuracy is lowered, and the burrs are a cause of failure when the parts are used in textile machines such as braiding machines. Therefore, although the burrs are removed by grinding or polishing after the press blanking process, it is difficult to sufficiently remove the burrs according to the complexity of the size and shape of the member.

Therefore, the cold-rolled steel sheet is required to have excellent punching properties, that is, to have as little burrs as possible in the press punching process.

In order to meet the above requirements, various techniques for improving the punching properties of cold-rolled steel sheets have been proposed.

For example, patent document 1 proposes a medium-high carbon cold-rolled steel sheet in which occurrence of bending and sagging of a punched end surface due to punching is suppressed by controlling a structure.

Patent document 2 proposes a method of producing a high-carbon steel sheet excellent in soft and formability by optimizing the composition and production conditions.

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

Prior art literature

Patent literature

Patent document 1: japanese patent laid-open publication No. 2019-039056

Patent document 2: japanese patent laid-open No. 05-171288

Patent document 3: international publication No. 2019/163828

Disclosure of Invention

According to the technique of patent document 1, the quality of the punched end surface is improved by increasing the pearlite structure ratio of the metal structure and decreasing the spheroidized carbide ratio so that the crack direction is uniform. However, since ferrite in pearlite is coarse and the deformation direction is various, burrs become high depending on the shearing direction. Therefore, the punching property is still insufficient.

In addition, the technique proposed in patent document 2 is to suppress a decrease in workability due to a variation in material characteristics in a coil by reducing the variation, and does not improve the essential blanking property of a steel sheet.

On the other hand, according to the technique proposed in patent document 3, although a certain improvement in the punching properties is seen, further improvement in the punching properties is demanded.

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

The present inventors have studied a method for further improving the punching properties of a cold-rolled steel sheet, and as a result, have found the following findings.

(1) When a blank is punched in punching, voids are generated from ferrite grain boundaries, and a large amount of ferrite grains undergo plastic deformation during the growth and connection of the voids, so that burrs on the punched end face become high.

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

(3) Further, the reduction in the plastic deformation amount of ferrite grains has an effect of reducing the residual stress. That is, when the plastic deformation amount of the ferrite grain is small, both the shape failure due to the burr and the dimensional change due to the residual stress become small, and as a result, the residual stress is reduced.

(4) In order to reduce the plastic deformation amount of ferrite grains, it is necessary to harden the ferrite grains themselves. Hardening of ferrite grains can be achieved by micronizing ferrite grains and dispersing fine carbides within ferrite grains.

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

The present invention has been completed based on the above-described findings, and has the following gist.

A cold-rolled steel sheet having the following composition:

comprises the following components in percentage by mass: 0.6 to 1.25 percent of Si:0.1 to 0.55 percent of Mn:0.5 to 2.0 percent, P:0.0005 to 0.05 percent, S: 0.0001-0.01%, al:0.001 to 0.1 percent, N:0.001 to 0.009%, cr:0.05 to 0.65%, selected from Ti:0.001 to 0.3 percent of Nb:0.01 to 0.1 percent and V:0.005 to 0.5%,

the remainder is composed of Fe and unavoidable impurities;

and, has the following steel structure:

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

the average grain size of cementite present in ferrite grain boundaries is 5 μm or less,

the NaCl-type carbide containing at least one of Nb, ti and V, which is present in ferrite grains, has an average grain size of 0.5 [ mu ] m or less,

the average interval of the NaCl-type carbide is 710nm or less.

2. The cold-rolled steel sheet according to the above 1, wherein the above composition further comprises, in mass%, a composition selected from the group consisting of Sb:0.1% or less, hf: below 0.5%, REM:0.1% or less, cu: less than 0.5%, ni:3.0% or less, sn: less than 0.5%, mo:1% below and Zr: at least one of 0.5% or less.

3. A method for manufacturing a cold-rolled steel sheet,

a billet having the composition of 1 or 2 above is heated,

heating the heated billet at a hot rolling start temperature: ac3 point or higher and finish rolling outlet side temperature: hot-rolled steel sheet is produced by hot-rolling at 800 ℃ or higher,

and (c) the hot rolled steel sheet is cooled from the end of the hot rolling to the start of cooling: average cooling rate of 5.0 seconds or less: 25 ℃/s or more, cooling stop temperature: cooling at 740-620 deg.c,

the cooled hot rolled steel sheet is wound,

annealing temperature is applied to the coiled hot rolled steel sheet: 730 ℃ or lower, annealing time: a first annealing under conditions of 5 hours or more,

bending back (bending and reverse bending) the first annealed hot-rolled steel sheet,

annealing temperature is carried out on the hot rolled steel plate after bending and back: a second anneal above 600 c,

the rolling ratio of the second annealed hot-rolled steel sheet is repeated twice or more: cold rolling and annealing temperatures above 15%: and a third anneal above 600 ℃.

According to the present invention, a cold-rolled steel sheet having excellent punching properties can be provided. The cold-rolled steel sheet of the present invention is excellent in suitability for use as a preform for press blanking, particularly as a preform for a textile machine component represented by a knitting needle, because it suppresses the occurrence of burrs during press blanking and has a small residual stress.

Detailed Description

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

[ composition of ingredients ]

The cold-rolled steel sheet of the present invention has the above-described composition. The reason for this 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 having an effect of improving hardness by quenching, and plays an important role in blanking. C forms cementite with Fe, and as a result, a boundary is generated between the generated cementite and ferrite. This boundary becomes the starting point of the void during punching. When shearing occurs with the void as a starting point, plastic deformation of ferrite is suppressed and the burr height is reduced. When the C content is less than 0.60%, carbon is consumed in cementite formation, carbide is not formed in the grains, and thus plastic deformation of ferrite grains is promoted. As a result, burrs become high, and residual stress becomes large, and accuracy of shape and dimension is lowered. 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, so that cracks occur in the sheared edge during blanking. Therefore, the C content is 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 having an effect of improving strength of a ferrite structure by solid solution strengthening, and can improve punching properties by adding Si. In order to obtain the above effect, the Si content is set to 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, the generation of ferrite and grain growth are promoted, and the ferrite strength is lowered. In addition, by promoting ferrite formation, precipitation of coarse cementite into grain boundaries is promoted, and the frequency of void generation is reduced. As a result, the plastic deformation amount increases, and the punching property decreases. Therefore, the Si content is 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 is mixed into cementite and inhibits cementite growth. By miniaturizing cementite generated in ferrite grain boundaries, plastic deformation of ferrite can be suppressed, and punching performance can be improved. In order to obtain the above effect, the Mn content is set to 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 band-like structure is generated in the rolling direction due to segregation of Mn sulfide, and the structure generation becomes abnormal. As a result, abnormal grain growth of ferrite grains is promoted, cementite precipitation becomes uneven, and the punching property is lowered. Therefore, the Mn content is 2.0% or less, preferably 1.95% or less, more preferably 1.90% or less, and even more preferably 1.85% or less.

P：0.0005～0.05％

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

S：0.0001～0.01％

S forms sulfides with Mn contained in the steel. When MnS is formed in ferrite grain boundaries, the ferrite becomes the starting point of a void at the boundary between ferrite and precipitates, similarly to cementite, and thus the punching performance is improved. Therefore, the S content is 0.0001% or more, preferably 0.0005% or more. On the other hand, when the S content exceeds 0.01%, elongated band-like MnS is generated in large amounts, and abnormal grain growth is promoted, so that local deformation is caused and punching properties are deteriorated. Therefore, the S content is 0.01% or less, preferably 0.008% or less.

Al：0.001～0.10％

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

N：0.001～0.009％

N combines with Al in steel to form AlN. When the N content is less than 0.001%, ferrite grains coarsen and the punching property is lowered. Therefore, the N content is set to 0.001% or more. On the other hand, when the N content exceeds 0.009%, alN precipitates at the ferrite grain boundaries of the hot-rolled steel sheet as an intermediate product, and ferrite grains are elongated and coarsened, and the punching performance is lowered. Therefore, the N content is 0.009% or less, preferably 0.006% or less.

Cr：0.05～0.65％

Cr is an element that improves the hardenability and strength of steel and affects the blanking property. When the Cr content is less than 0.05%, cementite tends to coarsen, the void density decreases, and the punching properties decrease. Accordingly, the Cr content is 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 carbide and Cr nitride are formed, and voids are generated at the interface between Cr carbide and Cr nitride and ferrite before voids are generated at the interface between cementite and ferrite. In addition, the formation of coarse Cr carbide suppresses carbide formation in the grains, and strength of ferrite is lowered. This locally deforms and reduces the punching performance. Therefore, the Cr content is 0.65% or less, preferably 0.60% or less.

The composition of the components comprises Ti:0.001 to 0.30 percent of Nb:0.01 to 0.1 percent and V:0.005 to 0.5%.

Ti：0.001～0.30％

Ti forms fine TiC in ferrite grains, strengthens ferrite grains, and suppresses plastic deformation. Therefore, the punching property can be improved by adding Ti. However, when the Ti content is less than 0.001%, tiN precipitates earlier than TiC and consumes Ti, so that the effect of improving the punching property cannot be obtained. Therefore, when Ti is added, 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 void formation and growth locally occur around the coarse TiC. As a result, plastic deformation is localized, and the punching property is lowered. Therefore, the Ti content is 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 ferrite grains, and suppresses plastic deformation. Therefore, the punching property can be improved by adding Nb. However, when the Nb content is less than 0.01%, the precipitation amount of NbC is small, and thus the effect of improving the punching property cannot be obtained. Therefore, when Nb is added, the Nb content is set to 0.01% or more, preferably 0.015% or more. On the other hand, when the Nb content exceeds 0.1%, coarse Nb (CN) is generated, voids are locally present around the Coarse Nb (CN), and deformation is localized, so that the punching property is lowered. Therefore, the Nb content is 0.1% or less, preferably 0.09% or less.

V：0.005～0.5％

V forms fine VC in ferrite grains, strengthens ferrite grains, and suppresses plastic deformation. Therefore, the punching property can be improved by adding V. However, when the V content is less than 0.005%, the deposition amount of VC is small, and thus the effect of improving the punching property cannot be obtained. Therefore, when V is added, the V content is set to 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 locally present around the coarse V (CN), and the deformation amount is biased, so that the punching property is lowered. Therefore, the V content is 0.5% or less, preferably 0.45% or less, and more preferably 0.40% or less.

The cold-rolled steel sheet according to one embodiment of the present invention has a composition composed of the above components, and the balance of Fe and unavoidable impurities.

In another embodiment of the present invention, the above-described composition of components may optionally further contain a compound selected from the group consisting of Sb:0.1% or less, hf: below 0.5%, REM:0.1% or less, cu: less than 0.5%, ni:3.0% or less, sn: less than 0.5%, mo:1% below and Zr: at least one of 0.5% or less.

Sb: less than 0.1%

Sb is an element effective for improving corrosion resistance, but when added in excess, sb-rich layers are formed under the scale formed during hot rolling, and surface scars (scratches) of the steel sheet are formed after hot rolling. Therefore, the Sb content is 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 addition effect, the Sb content is preferably 0.0003% or more.

Hf: less than 0.5%

Hf is an element effective for improving corrosion resistance, but when added in excess, it forms a Hf-rich layer under the scale formed during hot rolling, and after hot rolling, scars (scratches) on the surface of the steel sheet occur. Therefore, the Hf content is 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 addition effect, the Hf content is preferably 0.001% or more.

REM: less than 0.1%

REM (rare earth metal) is an element that improves the strength of steel. However, excessive addition of REM sometimes delays carbide refinement, and promotes uneven deformation during cold working, which deteriorates surface properties. Therefore, 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 addition effect, the REM content is preferably 0.005% or more.

Cu: less than 0.5%

Cu is an element effective for improving corrosion resistance, but when excessively added, a Cu-rich layer is formed under the scale formed during hot rolling, and surface scars (scratches) of the steel sheet are formed 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 addition effect, 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 carbide, and promotes uneven deformation during cold working, which 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 addition effect, the Ni content is preferably 0.01% or more.

Sn: less than 0.5%

Sn is an element effective for improving corrosion resistance, but when added in excess, sn-rich layers are formed under the scale formed during hot rolling, and surface scars (scratches) of the steel sheet are formed 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 additive effect, the Sn content is preferably 0.0001% or more.

Mo: less than 1%

Mo is an element that improves the strength of steel. However, excessive addition delays the refinement of carbide, and promotes uneven deformation during cold working, which 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 addition effect, the Mo content is preferably 0.001% or more.

Zr: less than 0.5%

Zr is an element effective for improving corrosion resistance, but when added in excess, zr-rich layers are formed under the scale formed during hot rolling, and surface scars (scratches) of the steel sheet are formed 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 addition effect, the Zr content is preferably 0.01% or more.

[ tissue ]

Next, the structure of the cold-rolled steel sheet of the invention will be described.

Average particle diameter of ferrite: less than 10 mu m

The finer the grain size of ferrite, the more plastic deformation of ferrite is suppressed. In order to obtain excellent punching properties, the average grain size of ferrite is set to 10 μm or less. On the other hand, since finer ferrite is more preferable, the lower limit of the average grain size is not limited. However, from the viewpoint of industrial production, the average particle diameter 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 existing in ferrite grain boundary: 5 μm or less

Cementite exists in ferrite grains and ferrite grain boundaries, and cementite in ferrite grain boundaries is coarser than cementite in ferrite grains. The inventors of the present invention found that the punching properties can be improved by controlling the average grain size of cementite present in the ferrite grain boundaries.

That is, when the cold-rolled steel sheet is subjected to punching, a gap is formed between the grain boundary and cementite, thereby shearing the cold-rolled steel sheet. At this time, voids are formed at the boundary formed by coarse cementite, and if local deformation occurs, the burr height becomes high. Therefore, cementite present in ferrite grain boundaries must be fine in order to improve the punching properties. Therefore, the average grain size of cementite present in ferrite grain boundaries is set to 5 μm or less. On the other hand, the smaller the average particle diameter is, the better, so the lower limit of the average particle diameter is not particularly limited. However, in the manufacturing method described later, annealing is repeatedly performed, so that cementite at the grain boundary is liable to grow. Therefore, in practice, the average particle diameter is 0.5 μm or more. The average grain size of cementite present in ferrite grain boundaries 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, but the cementite is spheroidized as a result of the fine refinement. The spheroidization ratio of the grain boundary cementite is not particularly limited, but is preferably 2.5 or less. The spheroidization ratio of the above-mentioned grain boundary cementite is defined by the following formula.

Spheroidization ratio=la/Lb

Here, la: average value of long diameter of cementite, lb: average value of cementite short diameter. La and Lb were obtained by taking three field-of-view images of a cross section obtained by cutting a cold-rolled steel sheet in the sheet thickness direction at a magnification of 1000 times by using a Scanning Electron Microscope (SEM), measuring the long diameter and the short diameter of all grain boundary cementite observed in the obtained images, and obtaining respective average values. In this case, the major axis and the minor axis are values obtained when cementite is formed into an ellipsoid or a sphere.

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

Further, 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 within ferrite grains and ferrite grain boundaries. By finely dispersing the NaCl-type carbide in ferrite grains, 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.

In the present invention, therefore, the average particle diameter of the NaCl-type carbide containing at least one of Nb, ti and V, which is present in ferrite grains, is set to 0.5 μm or less. On the other hand, the smaller the average grain size is, the higher the effect of strengthening ferrite is, and therefore the lower limit of the average grain size is not particularly limited. However, in the production method described later, annealing is repeatedly performed, so that precipitates are likely to grow. Therefore, in practice, the average particle diameter is 0.01 μm or more. The average particle diameter can be measured by the method described in examples. In the following description, the NaCl type carbide containing at least one of Nb, ti, and V existing in ferrite grains is sometimes simply referred to as "NaCl type carbide".

Average spacing of NaCl carbides: 710nm or less

The reinforcement of ferrite by the NaCl carbide is called precipitation hardening because the finely dispersed NaCl carbide functions as a barrier to dislocation. In precipitation strengthening, the smaller the distance between precipitates, the larger the strengthening. When the average interval of the NaCl-type carbide is more than 710nm, the decrease in the plastic deformation amount of ferrite grains due to precipitation strengthening becomes insufficient, and as a result, the press-punching property is lowered. Therefore, in the present invention, the average interval of the NaCl-type carbide existing in ferrite grains is made 710nm or less, preferably 250nm or less. On the other hand, the lower limit of the average interval is not particularly limited, but is 30nm or more in the practical production range. The average interval of NaCl-type carbide present in ferrite grains can be measured by the method described in the examples.

The number density of NaCl-type carbide containing at least one of Nb, ti and V in ferrite grains is not particularly limited, but is preferably less than 100/μm ² 。

The number density of the grain boundary cementite having a grain size of 0.5 μm or more is not particularly limited, but is preferably 5/100. Mu.m ² The above. On the other hand, the upper limit of the number density of the grain boundary cementite having a grain size of 0.5 μm or more is not particularly limited, but is preferably 50/100. Mu.m ² The following is given.

In the present invention, as described above, the punching performance is improved by reducing the plastic deformation amount of ferrite. Accordingly, the cold-rolled steel sheet of the present invention has a structure including ferrite. The area ratio of ferrite is not particularly limited, but the cold-rolled steel sheet preferably has a structure mainly composed of ferrite. Here, "ferrite is defined as a main body" and the area ratio of ferrite is 50% or more. The ferrite area ratio is more preferably 68% or more.

The above structure may include any structure other than ferrite. However, from the viewpoint of reducing coarse cementite, it is preferable to reduce the area ratio of cementite to less than 30%.

The cold-rolled steel sheet according to one embodiment of the present invention may have a structure composed of 68% or more ferrite, less than 30% cementite, and the remainder of precipitates other than cementite, for example, in terms of area ratio. Examples of the "precipitate other than cementite" include precipitate other than cementite (Fe ₃ C) Other carbides, nitrides, carbonitrides, sulfides, carbosulfides, and the like. More specific examples include at least one carbide, nitride, and carbonitride of Ti, V, and Nb, mn-based sulfide, ti-based composite carbosulfide, and the like.

[ plate thickness ]

The thickness of the cold-rolled steel sheet is not particularly limited, and may be any thickness. In view of the blank material which is punched and cut and used as the textile machine component, the plate thickness is preferably 0.1mm to 1.6mm. In particular, in view of the blank used as a knitting needle, the plate thickness is preferably 0.2mm to 0.8mm.

[ method of production ]

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

The cold-rolled steel sheet can be produced by sequentially subjecting a steel billet having the above-described composition to the following steps.

(1) Heating

(2) Hot rolling

(3) Cooling

(4) Winding up

(5) First annealing

(6) Bending back

(7) Second annealing

(8) Cold rolling

(9) Third annealing

Then, the steps (8) and (9) are repeated two or more times. The steps will be described in order below.

(1) Heating

First, a billet having the above composition is heated. The billet may be manufactured by any method without particular limitation. For example, the composition of the billet may be adjusted by a blast furnace converter method or an electric furnace method. The casting of the ingot from the molten steel may be performed by continuous casting or billet rolling.

The heating temperature of the slab is not particularly limited, but as described later, the temperature of the slab may be adjusted to be in the austenite region at the stage of starting the subsequent hot rolling.

(2) Hot rolling

Subsequently, the heated slab is hot-rolled to produce a hot-rolled steel sheet. In the above hot rolling, rough rolling and finish rolling may be performed in accordance with a conventional method.

Hot rolling start temperature: ac3 point or higher

In the above-mentioned 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, and thus the burr height becomes high. Therefore, the hot rolling start temperature is set to Ac3 point or higher. The Ac3 point (. Degree. C.) is determined by the following formula (1).

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

Here, the element symbol in the above formula (1) refers to the content (mass%) of each element, and zero is set when the element is not included.

Finish rolling outlet side temperature: 800 ℃ above

Also, when the finish rolling outlet side temperature is less than 800 ℃, elongated ferrite is generated in the hot rolled steel sheet of the intermediate product and remains in the final product, and thus the burr height becomes high. Therefore, the finish rolling outlet side temperature is set to 800 ℃ or higher.

(3) Cooling

Time until cooling starts: 5.0 seconds or less

Next, the hot rolled steel sheet is cooled. At this time, if a long period of time passes from the end of hot rolling to the start of cooling, carbides containing at least one of Ti, nb, and V precipitate at austenite grain boundaries, and elongated grains are generated in the final product, as a result, blanking work is reduced. Therefore, the time from the end of the hot rolling to the start of cooling (hereinafter, sometimes simply referred to as "time to start cooling") is set to 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 time until the start of cooling is not particularly limited, but is preferably 0.2 seconds or more, more preferably 0.5 seconds or more, from the viewpoint of being suitable for general production facilities.

Average cooling rate: 25 ℃/s or more

When the average cooling rate during the cooling is less than 25 ℃/s, elongated crystal grains are generated in the cold-rolled steel sheet as a final product, and as a result, the blanking property is lowered. Therefore, the average cooling rate is set to 25 ℃ per second or higher. On the other hand, the upper limit of the average cooling rate is not particularly limited, but is preferably 80 ℃/s or less, more preferably 60 ℃/s or less, and still more preferably 50 ℃/s or less, from the viewpoint of being suitable for general production facilities.

Cooling stop temperature: 620-740 DEG C

When the cooling is stopped at a temperature higher than 740 ℃, carbide precipitates at austenite grain boundaries, and elongated grains are generated in the final product, and the punching properties are reduced. Therefore, the cooling stop temperature was 740 ℃ or lower. On the other hand, when the cooling is stopped at a temperature lower than 620 ℃, ferrite is precipitated and pearlite is biased. This bias causes uneven cementite dispersion in the final product. Therefore, the cooling stop temperature is 620 ℃ or higher, preferably 630 ℃ or higher.

(4) Winding up

After the cooling is stopped, the cooled hot-rolled steel sheet is wound into a coil shape. In this case, the winding temperature is not particularly limited, but is preferably 600 to 730 ℃.

It is preferable that the hot-rolled steel sheet is pickled after the winding and before the subsequent first annealing.

(5) First annealing

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

Annealing temperature: 730 ℃ below

When the annealing temperature in the first annealing is higher than 730 ℃, a part of the ferrite grains undergo transformation preferentially, and therefore the ferrite grains locally coarsen, and as a result, the plastic deformation amount increases. In addition, in a locally coarse structure, the processing becomes uneven, and the component shape accuracy also deteriorates. Therefore, the annealing temperature is 730 ℃ or lower. On the other hand, the lower limit of the annealing temperature is not particularly limited, but from the viewpoint of redissolving cementite in pearlite and promoting decomposition of cementite, the annealing temperature is preferably 450 ℃ or higher, more preferably 500 ℃ or higher, and still more preferably 520 ℃ or higher.

Annealing time: for more than 5 hours

In addition, if the annealing time in the above-mentioned first annealing is less than 5 hours, decomposition of cementite does not proceed. If cementite decomposition does not proceed, plate-like cementite remains, and subsequent cold rolling or the like becomes uneven, resulting in deterioration of the shape accuracy of the member. Therefore, the annealing time is 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 start of cementite decomposition, the annealing temperature is preferably 50 hours or less, more preferably 40 hours or less, from the viewpoint of production efficiency.

It is preferable that the hot-rolled steel sheet is further subjected to acid washing after the first annealing and before the subsequent bending.

(6) Bending back

Then, the hot-rolled steel sheet after the first annealing is subjected to bending. This bending back is extremely important in order to make the structure of the finally obtained cold-rolled steel sheet a desired structure. That is, the cementite is decomposed by the first annealing and then bent back to impart a working strain, thereby introducing strain energy. Then, by performing a second annealing described later, the cementite is promoted to be finer. When bending is not performed, coarse cementite is localized, and the plastic deformation amount is locally increased, so that the punching performance is reduced.

The introduction of the processing strain by bending back may be performed by any method without particular limitation. For example, bending may be performed using a leveler, a skin pass mill, a cutter for cutting a steel sheet, or the like used for shape correction, or bending may be performed when unwinding and rewinding a coil from the coil.

From the viewpoint of increasing the amount of strain introduced, it is preferable to use a small diameter roller to perform bending. Specifically, a roller having a diameter of 1100mm or less is preferably used, and a roller having a diameter of 800mm or less is more preferably used. By bending back using a roller having a diameter of 1100mm or less, it is possible to introduce a considerable deformation required for promoting miniaturization of cementite after annealing. However, if the diameter of the roll is too small, the rolling load is limited, and therefore, it is necessary to reduce the size of the plate by shearing or cutting in advance, and man-hours increase. If the diameter of the roller is too small, meandering of the plate and generation of cracks are promoted. Therefore, the diameter of the roller is preferably 300mm or more, more preferably 450mm or more. The roll may be a tension roll. When tension rolls are used, strain is introduced by passing a plate between the tension rolls.

(7) Second annealing

And (3) performing a second annealing on the hot rolled steel sheet after the bending and the back bending. As described above, the second annealing is performed after the bending back is performed to impart the working strain, thereby promoting the miniaturization of cementite.

Annealing temperature: 600 ℃ above

When the annealing temperature in the second annealing is less than 600 ℃, the cementite is not refined, and the formation of NaCl-type carbide containing at least one of Nb, ti, and V is suppressed. If the formation of the NaCl-type carbide is suppressed, plastic deformation of ferrite grains cannot be suppressed, and thus burrs become high. Therefore, the annealing temperature in the second annealing is set to 600 ℃ 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 coarsens, but instead burrs become high, so the annealing temperature is preferably 790 ℃ or less, more preferably 770 ℃ or less.

(8) Cold rolling

(9) Third annealing

And repeating the cold rolling and the third annealing of the hot-rolled steel sheet after the second annealing twice or more. The thickness of the final cold-rolled steel sheet is adjusted by the cold rolling. Further, by performing the third annealing after the cold rolling, the strain generated in the cold rolling is removed. By performing the cold rolling and the third annealing twice or more, uniformity of the structure is improved, and ferrite is strengthened by miniaturization of the ferrite structure, and as a result, punching property is improved. In order to obtain the above-described effect, the rolling reduction in the cold rolling is 15% or more, and the annealing temperature in the third annealing is 600 ℃. On the other hand, the upper limit of the rolling reduction is not particularly limited, but when the rolling reduction is too high, the structure becomes locally coarse, and the burrs become high. Therefore, the rolling reduction is preferably 52% or less, more preferably 50% or less. The upper limit of the annealing temperature in the third annealing is not particularly limited, but when the annealing temperature is too high, the structure coarsens, and the burr becomes high. Therefore, the annealing temperature is preferably 750 ℃ or lower, more preferably 720 ℃ or lower.

The above cold rolling and the third annealing may be repeated two or more times, and then the final cold rolling may be further 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. The upper limit of the reduction ratio in the final cold rolling is not particularly limited, but is preferably 50% or less.

By satisfying the above conditions, a cold-rolled steel sheet having excellent punching properties can be produced. The finally obtained cold-rolled steel sheet may be further subjected to any surface treatment.

Examples

In order to confirm the effects of the present invention, a cold-rolled steel sheet was produced in the following steps, and the obtained cold-rolled steel sheet was evaluated for punching properties.

First, steel having the composition shown in table 1 was melted in a converter, and a billet was produced by a continuous casting method. Subsequently, the billet is sequentially subjected to heating, hot rolling, cooling, coiling, pickling, first annealing, pickling, bending, second annealing, cold rolling, and third annealing to obtain a final plate thickness: about 0.4 mm. Each step was carried out under the conditions shown in tables 2 and 3, and the cold rolling and the third annealing were repeated the times shown in tables 2 and 3. The above-described bending was performed by using tension rolls having diameters shown in tables 2 and 3 when the coil was wound off. For comparison, in some examples, bending was not performed (comparative example No. 16).

(organization)

Next, the structure of the obtained cold-rolled steel sheet was evaluated according to the following procedure.

Average grain size of ferrite

First, a test piece for tissue observation was collected from the obtained cold-rolled steel sheet. After polishing the rolling direction cross section (L section) of the test piece for observing the structure, the polished surface was etched with a 3vol% nitrate alcohol etching solution, whereby the structure was developed. Next, the surface of the test piece for tissue observation was photographed at a magnification of 3000 times using SEM (scanning electron microscope), and a tissue image was obtained. According to JIS G0551:2020, ferrite grain size was measured by cutting from the obtained tissue image. The average value of ferrite grain sizes measured in five fields of view was calculated as the average grain size.

Average grain size and number density of grain boundary cementite

First, a test piece for tissue observation was collected from the obtained cold-rolled steel sheet. After polishing the rolling direction cross section (L section) of the test piece for observing the structure, the polished surface was etched with a 3vol% nitrate alcohol etching solution, whereby the structure was developed. Next, the surface of the test piece for tissue observation was photographed at 3000 times using SEM to obtain a tissue image. From the obtained tissue 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 as the average grain size of the grain boundary cementite. The number density of grain boundary cementite having a grain size of 0.5 mu or more was obtained from the above-mentioned tissue image.

Average particle diameter of NaCl carbide

The average particle diameter of NaCl-type carbide containing at least one of Nb, ti, and V present in ferrite grains was measured as follows. The surface of the test piece was photographed at a magnification of 80000 times using a Transmission Electron Microscope (TEM), and a tissue image of five fields was obtained. By using the image processing of the circle approximation, the respective particle diameters of NaCl carbides containing at least one of Nb, ti, and V existing in the ferrite grains in the above-obtained texture image are obtained, and the average value thereof is calculated. Whether or not the carbide contains at least one of Nb, ti, and V is identified using TEM-EPMA.

Average spacing of NaCl type carbide

The average interval of NaCl carbides containing at least one of Nb, ti, V existing in ferrite grains was determined by measuring the interval of all NaCl carbides identifiable in the 80000-fold field and calculating the average value of five fields.

The measurement results are shown in tables 4 and 5. The NaCl-type carbide in tables 4 and 5 refers to a NaCl-type carbide containing at least one of Nb, ti, and V, which is present in ferrite grains.

(blanking property)

Next, in order to evaluate the punching properties of the obtained cold-rolled steel sheet, a punch punching test was performed under the following conditions to measure the burr height.

First, test pieces having a width of 20mm, a length of 150mm, and a thickness of 0.4mm were collected from each cold-rolled steel sheet. Then, the test piece was punched out using SKD of Φ10 and a superhard punch. The punching gap was 100. Mu.m. Further, the above blanking was performed 10 times on one test piece. In this case, the distance from the end of the test piece to the punched hole during the primary punching was set to 5mm or more. In the second and subsequent times of punching, the interval between adjacent punched holes is set to 5mm or more.

Then, heights of burrs generated in the circumferential direction were observed by a microscope, heights of burrs at 5 positions were measured uniformly in the circumferential direction for one hole, and an average value of the heights of burrs at 5 positions was calculated. Next, the same measurement was performed for 10 wells, and the average value of the burr heights calculated for each well was used as the burr height.

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Claims

1. A cold-rolled steel sheet having the following composition:

comprises the following components in percentage by mass: 0.60 to 1.25 percent of Si:0.1 to 0.55 percent of Mn:0.5 to 2.0 percent, P:0.0005 to 0.05 percent, S: 0.0001-0.01%, al:0.001 to 0.10 percent, N:0.001 to 0.009%, cr:0.05 to 0.65% of a metal selected from Ti:0.001 to 0.30 percent of Nb:0.01 to 0.1 percent and V:0.005 to 0.5%,

the remainder is composed of Fe and unavoidable impurities;

and, has the following steel structure:

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

the average interval of the NaCl-type carbide is 710nm or less.

2. The cold-rolled steel sheet according to claim 1, wherein the component composition further comprises, in mass%, a composition selected from the group consisting of Sb:0.1% or less, hf: below 0.5%, REM:0.1% or less, cu: less than 0.5%, ni:3.0% or less, sn: less than 0.5%, mo:1% below and Zr: at least one of 0.5% or less.

3. A method for manufacturing a cold-rolled steel sheet,

heating a steel slab having the composition of claim 1 or 2,

-bringing the heated steel slab to a hot rolling start temperature: ac3 point or higher and finish rolling outlet side temperature: hot-rolled steel sheet is produced by hot-rolling at 800 ℃ or higher,

and (c) heating the hot rolled steel sheet at a time from the end of the hot rolling to the start of cooling: average cooling rate of 5.0 seconds or less: 25 ℃/s or more, cooling stop temperature: cooling at 620-740 deg.c,

winding the cooled hot rolled steel sheet,

and (3) carrying out annealing temperature on the coiled hot rolled steel plate: 730 ℃ or lower, annealing time: a first annealing under conditions of 5 hours or more,

bending the first annealed hot-rolled steel sheet back,

and (3) carrying out annealing temperature on the hot rolled steel plate after bending and bending: a second anneal above 600 c,

and repeating the rolling ratio of the second annealed hot-rolled steel sheet twice or more: cold rolling and annealing temperatures above 15%: and a third anneal above 600 ℃.