TWI792968B - Cold-rolled steel sheet and method for manufacturing cold-rolled steel sheet - Google Patents

Abstract

To provide a cold-rolled steel sheet excellent in stampability. This cold-rolled steel sheet has a predetermined composition, and has: the average particle size of ferrite (Ferrite) is 10 μm or less, and the average particle size of cementite (Cementite) existing in the ferrite grain boundary is 5 μm or less A steel structure in which the average particle size of NaCl-type carbides containing at least one of Nb, Ti, and V present in ferrite grains is 0.5 μm or less, and the average interval of the aforementioned NaCl-type carbides is 710 nm or less.

Description

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

The present invention relates to a cold-rolled steel sheet, in particular to a cold-rolled steel sheet excellent in pressability. Moreover, this invention relates to the manufacturing method of the said cold-rolled steel plate.

The method of processing cold-rolled steel sheets into the shape of parts is widely used in stamping. For example, in the manufacture of fiber machinery parts including plain needles used in knitting machines, cold-rolled steel sheets are processed into the shape of parts by stamping, and then processed by cutting, drawing, grinding, or heat treatment such as quenching and tempering. And make the final fiber mechanical parts.

However, in the stamping process, there is a problem that burrs are generated on the end faces when stamping the material. When burrs are generated, not only does the dimensional accuracy decrease, but it also causes loss when parts with burrs are used in textile machines such as knitting machines. Therefore, cutting or grinding is performed after stamping to remove burrs, but it is difficult to sufficiently remove burrs due to the complexity of the size or shape of the parts.

Therefore, cold-rolled steel sheets are required to be excellent in stampability, that is, extremely difficult to generate burrs during press stamping.

In order to meet the above-mentioned demands, various techniques for improving the stampability of cold-rolled steel sheets have been proposed.

For example, Patent Document 1 proposes a medium-high-carbon cold-rolled steel sheet that suppresses bending caused by stamping and sagging of the stamped end surface by controlling the structure.

In addition, Patent Document 2 proposes a method of producing a soft high-carbon steel sheet with excellent formability by optimizing the composition and production conditions.

In Patent Document 3, a high-carbon cold-rolled steel sheet that improves fine blanking workability is proposed by optimizing the particle size of snow-white carbon iron and ferrite. prior art literature patent documents

Patent Document 1: Japanese Patent Laid-Open No. 2019-039056 Patent Document 2: Japanese Patent Application Laid-Open No. 05-171288 Patent Document 3: Japanese International Publication No. 2019/163828

[Problem to be Solved by the Invention]

According to Patent Document 1, by increasing the perlite structure ratio of the metal structure and reducing the spherical carbide ratio, the direction of cracks is aligned to improve the stamping end surface properties. However, the ferrite iron in perlite is thicker and has more deformation directions, so the burrs are increased due to the different shear directions. Therefore, punchability is still insufficient.

In addition, the technique proposed in Patent Document 2 suppresses the decrease in workability caused by the variation by reducing the variation in material properties in the coil, and does not improve the essential punchability of the steel sheet.

On the other hand, according to the technology proposed in Patent Document 3, although a certain degree of improvement in stampability is observed, further improvement in stampability is required.

The present invention was made in view of the above circumstances, and an object of the present invention is to provide a cold-rolled steel sheet excellent in stampability. [Technical means to solve the problem]

The inventors of the present invention studied methods for further improving the stampability of cold-rolled steel sheets, and found the following findings.

(1) When the raw material is stamped by stamping, pores are formed from the grain boundaries of ferrite grains, and during the growth and connection of pores, ferrite grains undergo a large amount of plastic deformation, so the burrs on the stamping end face increase.

(2) Therefore, if the plastic deformation of ferrite grains is suppressed, burrs can be reduced. That is to say, in the case of large plastic deformation of fertilized iron grains, many pores are formed and connected at the grain boundary of fertilized iron grains, resulting in increased burrs, but if the plastic deformation of fertilized iron grains is small, then The rough edges are reduced.

(3) Moreover, the reduction of the plastic deformation of fertilized iron particles also has the effect of reducing the residual stress. That is, when the amount of plastic deformation of ferrite grains is small, both the shape defect due to burrs and the dimensional change due to residual stress are reduced, and as a result, the residual stress is reduced.

(4) When reducing the amount of plastic deformation of the fertilizer iron pellets, it is necessary to harden the fertilizer pellets themselves. The hardening of the ferrite grains can be achieved by making the ferrite grains finer and dispersing fine carbides in the ferrite grains.

(5) When the fertilizer iron particles are refined and fine carbides are dispersed in the fertilizer iron particles, the snowy carbon iron that exists in the fertilizer grain iron grain boundary (hereinafter sometimes referred to as "grain boundary snow bright carbon iron") has to be nuanced. In addition, by suppressing the generation of coarse grain boundary snow-white carbon iron, the generation of coarse pores on the grain boundary can be suppressed, and as a result, the generation of burrs can be reduced.

The present invention has been accomplished based on the above findings, and the gist thereof is as described below.

1. A cold-rolled steel plate, which has a content of C: 0.60 to 1.25%, Si: 0.1 to 0.55%, Mn: 0.5 to 2.0%, P: 0.0005 to 0.05%, S: 0.0001 to 0.01%, Al: 0.001 to 0.10%, N: 0.001 to 0.009%, Cr: 0.05 to 0.65%, and At least one selected from the group consisting of Ti: 0.001 to 0.30%, Nb: 0.01 to 0.1%, and V: 0.005 to 0.5%, And the remainder is composed of Fe and unavoidable impurities; And it has: the average particle size of ferrite is less than 10 μm, The average particle size of Xueming carbon iron existing in the ferrite grain boundary is 5 μm or less, The average particle size of the NaCl-type carbides containing at least one of Nb, Ti, and V present in the fertilizer iron particles is 0.5 μm or less, and A steel structure in which the average spacing of the aforementioned NaCl-type carbides is 710 nm or less.

2. The cold-rolled steel sheet as described in the above 1, wherein the aforementioned composition further contains Sb: less than 0.1%, Hf: less than 0.5%, REM: less than 0.1%, Cu: 0.5% or less, Ni: 3.0% or less, Sn: 0.5% or less, Mo: less than 1%, and Zr: At least one type of the group consisting of 0.5% or less.

3. A method for manufacturing cold-rolled steel sheets, which involves heating a steel billet having the composition described in 1 or 2 above, Under the conditions of starting temperature of hot rolling: Ac3 point or higher and processing and rolling exit temperature: 800°C or higher, the heated steel billet is hot-rolled to form a hot-rolled steel plate, The hot-rolled steel sheet is cooled under the conditions of the time from the end of the hot rolling to the start of cooling: 5.0 seconds or less, the average cooling rate: 25°C/s or more, and the cooling stop temperature: 620°C to 740°C, The above-mentioned hot-rolled steel plate after coiling and cooling, The first annealing is performed on the hot-rolled steel sheet after coiling under the conditions of annealing temperature: 730°C or less, annealing time: 5 hours or more, Bending and reverse bending is applied to the hot-rolled steel sheet after the first annealing, The annealing temperature: 600°C or higher is applied to the hot-rolled steel sheet after the above-mentioned bending back, To the hot-rolled steel sheet after the above-mentioned second annealing, the cold rolling of the rolling ratio: 15% or more and the third annealing of the annealing temperature: 600° C. or more are repeated twice or more. [Effect of Invention]

According to the present invention, a cold-rolled steel sheet excellent in drawability can be provided. The cold-rolled steel sheet of the present invention suppresses the generation of burrs during press stamping, and because the residual stress is small, it is very suitable for use as a raw material for press stamping, especially as a raw material for fiber machine parts including plain needles. .

The following describes the present invention in detail. But the present invention is not limited to this implementation type.

[ingredient composition] The cold-rolled steel sheet of the present invention has the above composition. The reason for this limitation is explained below. In the following description, "%" which is a unit of content means "mass %" unless otherwise specified.

C: 0.60 to 1.25% C is an element having an effect of increasing hardness by quenching, and plays an important role in stampability. C and Fe form snowy carbon iron, and as a result, a junction occurs between the formed snowy carbon iron and ferrite. This junction becomes the starting point of pores during punching. In the situation caused by shearing with the pores as the starting point, the plastic deformation of ferrite is suppressed, and the height of the burr is reduced. When the C content is less than 0.60%, the formation of Xueming carbon iron consumes carbon and becomes unable to form carbides in the grains, thus promoting the plastic deformation of ferrite. As a result, burrs are increased, residual stress is increased, and accuracy of shape and size is reduced. Therefore, the C content is set to be 0.60% or more, preferably 0.65% or more, and most 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 easily causes brittle fracture, so cracking occurs on the sheared end surface during stamping. Therefore, the C content is set to be 1.25% or less, preferably 1.20% or less, particularly preferably 1.15% or less.

Si: 0.1 to 0.55% Si is an element that has the effect of increasing the strength of the ferrite structure through solid solution strengthening, and the stampability can be improved by adding Si. In order to obtain the aforementioned effects, the Si content is set to be 0.1% or more, preferably 0.12% or more, and most preferably 0.14% or more. On the other hand, when the Si content is excessive, the formation and grain growth of ferrite will be promoted, and the strength of ferrite will decrease. In addition, due to the promotion of the formation of fat grain iron, the precipitation of coarse snow-white carbon iron to the grain boundary is promoted, and the number of pores is reduced. As a result, the amount of plastic deformation increases and the stampability decreases. Therefore, the Si content is set to be 0.55% or less, preferably 0.52% or less, particularly preferably 0.50% or less.

Mn: 0.5 to 2.0% Mn is an element that is mixed in the snow carbon to suppress the growth of the snow carbon. By miniaturizing the snow-white carbon iron formed at the grain boundary of ferrite, the plastic deformation of ferrite can be suppressed and the punchability can be improved. In order to obtain the aforementioned effects, the Mn content is set to be 0.5% or more, preferably 0.52% or more, and most preferably 0.54% or more. On the other hand, when the Mn content exceeds 2.0%, a banded structure is formed in a wide range in the rolling direction due to the segregation of Mn sulfide, and the formation of the structure becomes abnormal. As a result, the abnormal grain growth of fertilized iron grains is promoted, and the precipitation of Xueming carbon iron becomes inhomogeneous, which reduces the punchability. Therefore, the Mn content is set to be 2.0% or less, preferably 1.95% or less, particularly preferably 1.90% or less, more preferably 1.85% or less.

P: 0.0005 to 0.05% P is an element that has the effect of strengthening fertilizer grain iron. 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 set to 0.0005% or more, preferably 0.0010% or more. On the other hand, when the P content exceeds 0.05%, because P produces grain boundary segregation and inhibits the formation of snowy carbon iron on the grain boundary, the plastic deformation of ferrite increases, resulting in a decrease in punchability. Therefore, the P content is set to 0.05% or less, preferably 0.04% or less.

S: 0.0001 to 0.01% The S system forms sulfides with Mn contained in the steel. When MnS is formed at the ferrite grain boundary, it becomes the starting point of the pores at the junction of ferrite and precipitates similarly to iron carbon, so the punchability is improved. Therefore, the S content is set to 0.0001% or more, preferably 0.0005% or more. On the other hand, when the S content exceeds 0.01%, a large amount of stretched band-shaped MnS is generated to promote abnormal grain growth, thereby causing local deformation and degrading the stampability. Therefore, the S content is set to be 0.01% or less, preferably 0.008% or less.

Al: 0.001 to 0.10% The Al system is dispersed as an oxide in the steel and solid-soluted to strengthen the ferrite, thereby suppressing the plastic deformation of the ferrite and improving the stampability. 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 to increase the amount of plastic deformation, resulting in a decrease in stampability. Therefore, the Al content is set to be 0.10% or less, preferably 0.08% or less, particularly preferably 0.06% or less.

N: 0.001 to 0.009% N is bonded to Al in steel to form AlN. When the N content is less than 0.001%, the ferrite grains are coarsened and the punchability is reduced. Therefore, the N content is made 0.001% or more. On the other hand, when the N content exceeds 0.009%, AlN is precipitated at the ferrite grain boundary of the hot-rolled steel sheet as an intermediate product, and the ferrite grains are expanded and coarsened, so the stampability is reduced. Therefore, the N content is set to be 0.009% or less, preferably 0.006% or less.

Cr: 0.05 to 0.65% Cr is an element that improves the hardenability of steel to increase the strength, and also affects the stampability. When the Cr content is less than 0.05%, the Xueming carbon iron is easy to coarsen, and the pore density decreases to reduce the punchability. Therefore, the Cr content is set to 0.05% or more, preferably 0.08% or more, particularly preferably 0.10% or more, more preferably 0.15% or more. On the other hand, when the Cr content is excessive, coarse Cr carbides or Cr nitrides will be formed, and before the pores formed at the interface between Xueming carbon iron and fertile iron, Cr carbides or Cr nitrides and Cr nitrides will be formed first. Pores are generated on the interface of fertilized iron. In addition, due to the formation of coarse Cr carbides, the formation of carbides in the grains is suppressed, and the strength of ferrite is reduced. This results in localization of deformation and reduction in punchability. Therefore, the Cr content is set to be 0.65% or less, preferably 0.60% or less.

The above component composition contains at least one kind 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%.

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

Nb: 0.01 to 0.1% Nb forms fine NbC in ferrite grains, strengthens ferrite grains and suppresses plastic deformation. Therefore, the stampability can be improved by adding Nb. However, when the Nb content is less than 0.01%, since the precipitation amount of NbC is small, the effect of improving the stampability 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 formed, and pores locally exist around the coarse Nb(CN), which localizes deformation and reduces punchability. Therefore, the Nb content is set to be 0.1% or less, preferably 0.09% or less.

V: 0.005 to 0.5% The V system forms fine VC in the ferrite grains, strengthens the ferrite grains and inhibits plastic deformation. Therefore, by adding V, stampability can be improved. However, when the V content is less than 0.005%, since the precipitated amount of VC is small, the effect of improving the stampability cannot be obtained. Therefore, when V is added, the V content is set to be 0.005% or more, preferably 0.010% or more. On the other hand, if the V content exceeds 0.5%, coarse V(CN) will be formed, and pores will locally exist around the coarse V(CN), and the amount of deformation will be biased, so the punchability will decrease. Therefore, the V content is set to be 0.5% or less, preferably 0.45% or less, particularly preferably 0.40% or less.

A cold-rolled steel sheet according to an embodiment of the present invention has a composition consisting of the above-mentioned components and the remainder of Fe and unavoidable impurities.

In addition, in other implementation forms of the present invention, the above-mentioned component composition may optionally further contain an element selected from the group consisting of Sb: 0.1% or less, Hf: 0.5% or less, REM: 0.1% or less, Cu: 0.5% or less, Ni: 3.0% At least one of the group consisting of Sn: 0.5% or less, Mo: 1% or less, and Zr: 0.5% or less.

Sb: less than 0.1% Sb is an element effective in improving corrosion resistance, but when added excessively, a Sb-rich layer is formed under the scale formed during hot rolling, and peeling (damage) of the surface of the steel sheet occurs after hot rolling. Therefore, the Sb content is made 0.1% or less. On the other hand, the lower limit of the Sb content is not particularly limited, but from the viewpoint of enhancing the addition effect, the Sb content is preferably 0.0003% or more.

Hf: less than 0.5% Hf is an element effective in improving the corrosion resistance, but when added excessively, a Hf-rich layer will be formed under the scale formed during hot rolling, and the surface of the steel sheet will peel off (damage) after hot rolling. 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 the Hf content is preferably 0.001% or more from the viewpoint of enhancing the addition effect.

REM: less than 0.1% REM (rare earth metal) is an element that increases the strength of steel. However, when added excessively, the refinement of carbides is delayed, and heterogeneous deformation is promoted during cold working, which may cause deterioration of 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 it is preferable to set the REM content to 0.005% or more from the viewpoint of enhancing the effect of addition.

Cu: less than 0.5% Cu is an element effective in improving corrosion resistance, but when added excessively, a Cu-rich layer is formed under the scale generated during hot rolling, and peeling (damage) of the surface of the steel sheet occurs after hot rolling. Therefore, the Cu content is made 0.5% or less. On the other hand, the lower limit of the Cu content is not particularly limited, but the Cu content is preferably 0.01% or more from the viewpoint of enhancing the effect of addition.

Ni: less than 3.0% Ni is an element that increases the strength of steel. However, when added excessively, the refinement of carbides is delayed, and heterogeneous deformation is promoted during cold working, which may cause deterioration of surface properties. Therefore, the Ni content is made 3.0% or less. On the other hand, the lower limit of the Ni content is not particularly limited, but from the viewpoint of enhancing the addition effect, the Ni content is preferably 0.01% or more.

Sn: less than 0.5% Sn is an element effective in improving corrosion resistance, but when added excessively, a Sn-rich layer is formed under the scale formed during hot rolling, and peeling (damage) of the surface of the steel sheet occurs after hot rolling. 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 enhancing the addition effect, the Sn content is preferably 0.0001% or more.

Mo: less than 1% Mo is an element that increases the strength of steel. However, when added excessively, the refinement of carbides is delayed, and heterogeneous deformation is promoted during cold working, which may cause deterioration of surface properties. Therefore, the Mo content is made 1% or less. On the other hand, the lower limit of the Mo content is not particularly limited, but it is preferable to make the Mo content 0.001% or more from the viewpoint of enhancing the addition effect.

Zr: less than 0.5% Zr is an element effective in improving the corrosion resistance, but when added excessively, a Zr-rich layer is formed under the scale formed during hot rolling, and the surface of the steel sheet peels off (damage) after hot rolling. Therefore, the Zr content is made 0.5% or less. On the other hand, the lower limit of the Zr content is not particularly limited, but from the viewpoint of enhancing the addition effect, the Zr content is preferably 0.01% or more.

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

The average particle size of fertilizer grain iron: below 10μm The finer the particle size of ferrite, the more plastic deformation of ferrite is inhibited. In order to obtain excellent punchability, the average particle size of ferrite ferrite is set to be 10 μm or less. On the other hand, since the finer the iron fertilizer, the better, so the lower limit of the average particle size is not limited. However, from the viewpoint of industrial production, the aforementioned average particle diameter may be 0.5 μm or more. The average particle size of ferrite ferrite can be measured by the method described in the examples.

The average particle size of Xueming carbon iron existing in the ferrite grain boundary: 5 μm or less Xueming carbon iron exists in both the ferrite granule and the ferritic grain boundary. Compared with the ferric iron in the ferritic grain, the ferric iron in the ferritic grain boundary is relatively coarser. The inventors of the present invention have found that punchability can be improved by controlling the average grain size of the snow-white carbon iron existing in the ferrite grain boundary.

That is, when the cold-rolled steel sheet is subjected to press working, shearing proceeds due to the generation of voids between the grain boundaries and the snow-white carbon iron. At this time, pores are being formed at the junction formed by coarse snow-white carbon iron, and when local deformation occurs, the height of the burrs will increase. Therefore, in order to improve the punchability, the snow-white carbon iron existing in the ferrite grain boundary must be finer. Therefore, the average particle size of the snow-bright carbon iron existing in the grain boundary of ferritic iron is set to be 5 μm or less. On the other hand, since the aforementioned average particle diameter is preferably as small as possible, the lower limit of the average particle diameter is not particularly limited. However, since the annealing is repeated in the production method described later, snowy carbon iron at grain boundaries tends to grow. Therefore, the above-mentioned average particle diameter is actually 0.5 μm or more. The average grain size of the Xueming carbon iron present in the ferrite grain boundary can be measured by the method described in the examples.

As mentioned above, it is important in the present invention that the grain boundary snow bright iron is fine, and the snow bright iron carbon is spheroidized as a result. The spheroidization rate of the grain-boundary snow bright iron carbon is not particularly limited, but is preferably 2.5 or less. The spheroidization rate of the above-mentioned grain boundary snow-bright carbon iron is defined by the following formula. Spheroidization rate=La/Lb Here, La: the average value of the long diameters of Xueming carbon iron, and Lb: the average value of the short diameters of Xueming carbon iron. For La and Lb, a scanning electron microscope (SEM: Scanning Electron Microscope) was used to observe three fields of view of a cross-section of a cold-rolled steel sheet cut in the thickness direction at a magnification of 1000 times, and to measure the area in the obtained image. Observing the long and short diameters of all grain boundary snow-bright carbon iron, and then calculate the average value. At this time, the above-mentioned major axis and minor axis are the values when Xueming carbon iron is used as an ellipsoid or a sphere.

The average particle size of NaCl-type carbides existing in fertilized iron particles: 0.5 μm or less Furthermore, the cold-rolled steel sheet of the present invention contains at least one of Nb, Ti, and V. These elements form NaCl-type carbides and precipitate in the ferrite grains and the ferrite grain boundaries. By finely dispersing the aforementioned NaCl-type carbides in the ferrite grains to harden the ferrite grains, the amount of plastic deformation of the ferrite grains can be reduced. As a result, the height of burrs at the time of press stamping can be reduced.

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

Average interval of NaCl-type carbides: 710nm or less The strengthening of ferrite by the above-mentioned NaCl-type carbides is based on the fact that the finely dispersed NaCl-type carbides function as rearrangement hindrances, and this strengthening is called precipitation strengthening. In precipitation strengthening, the smaller the distance between the precipitates, the greater the strengthening. When the average interval of the aforementioned NaCl-type carbides is greater than 710nm, the reduction in the plastic deformation of ferrite due to precipitation strengthening is insufficient, resulting in reduced stampability. Therefore, in the present invention, the average spacing of the above-mentioned NaCl-type carbides present in the ferrite grains is set to be 710 nm or less, preferably 250 nm or less. On the other hand, the lower limit of the average interval is not particularly limited, but is 30 nm or more in the actual manufacturing range. The average interval of NaCl-type carbides present in ferrite grains can be measured by the method described in the examples.

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

The number density of grain boundary snow-white carbon iron with a particle size of 0.5 μm or more is not particularly limited, but is preferably 5 particles/100 μm ² or more. On the other hand, the upper limit of the number density of the grain boundary snowy carbon iron with a particle size of 0.5 μm or more is not particularly limited, but is preferably 50 pieces/100 μm ² or less.

In the invention of this application, as mentioned 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. The area ratio of ferrite is not particularly limited, and the aforementioned cold-rolled steel sheet preferably has a structure mainly composed of ferrite. The so-called "mainly fertilized iron" here is defined as those whose area ratio is 50% or more. The fertilized iron area ratio is preferably above 68%.

In addition, the aforementioned tissue may contain any tissue other than ferric iron. However, from the viewpoint of reducing coarse iron carbon, it is preferable to set the area ratio of iron carbon to less than 30%.

A cold-rolled steel sheet according to an embodiment of the present invention may have, for example, an area ratio of 68% or more of ferrite, less than 30% of snow-white carbon iron, and the rest of precipitates other than snow-white carbon iron. organization. The aforementioned "precipitates other than iron carbon" include, for example, carbides, nitrides, carbonitrides, sulfides, and carbon sulfides excluding iron carbon (Fe ₃ C). More specific examples include carbides, nitrides, and carbonitrides of at least one of Ti, V, and Nb, Mn-based sulfides, Ti-based composite carbosulfides, and the like.

[board thickness] The thickness of the cold-rolled steel sheet is not particularly limited, and may be any thickness. Considering the use of raw materials for fiber machine parts by press processing, it is preferable to set the plate thickness to 0.1 mm or more and 1.6 mm or less. In particular, considering the raw materials for flat needles, the plate thickness is preferably set to 0.2 mm or more and 0.8 mm or less.

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

The aforementioned cold-rolled steel sheet can be manufactured by sequentially subjecting a billet having the above composition to the following steps. (1) Heating (2) hot rolling (3) cooling (4) coiling (5) 1st annealing (6) Bending back (7) Second annealing (8) cold rolling (9) The third annealing The above steps (8) and (9) are repeated two or more times. Each step will be described in order below.

(1) Heating Firstly, a steel billet having the above composition is heated. The aforementioned steel billet is not particularly limited, and may be manufactured by any method. For example, the composition adjustment of the aforementioned billet can be carried out by the blast furnace converter method or by the electric furnace method. In addition, casting from molten steel to billet can be carried out by continuous casting or block rolling.

The heating temperature of the billet is not particularly limited, and it may be adjusted so that the temperature of the billet becomes in the Austenite region at the beginning of the next hot rolling as described later.

(2) hot rolling Next, the heated billet is hot-rolled to become a hot-rolled steel sheet. In the aforementioned hot rolling, rough rolling and working rolling can be carried out according to common methods.

在此，上述式(1)中的元素符號意指各元素的含量(質量%)，未含有該元素時為零。 Hot-rolling start temperature: above the Ac3 point in the above-mentioned hot-rolling process, when the hot-rolling start temperature is less than Ac3 point, the stretched ferrite will be produced in the hot-rolled steel sheet of the intermediate product and remain in the final product , thus increasing the burrs. Therefore, the hot rolling start temperature is set to Ac3 point or more. The aforementioned Ac3 point (° C.) is obtained by the following formula (1).

Here, the symbol of the element in said formula (1) means content (mass %) of each element, and it is zero when the said element is not contained.

Processing and rolling outlet temperature: above 800°C Similarly, when the temperature on the exit side of processing and rolling does not reach 800°C, the stretched ferrite will be produced in the hot-rolled steel plate of the intermediate product and remain in the final product, thus increasing the burrs. Therefore, the temperature at the exit side of working and rolling is set at 800° C. or higher.

(3) cooling Time until cooldown starts: 5.0 seconds or less The aforementioned hot-rolled steel sheet is then cooled. At this time, when a long period of time elapses from the end of hot rolling to the beginning of cooling, carbides containing at least one of Ti, Nb, and V will precipitate at the grain boundary of Worth field, and ductile grains will be produced in the final product, resulting in Press workability 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 of cooling") is set to be 5.0 seconds or less, preferably 4.5 seconds or less, particularly preferably 4.0 seconds or less. On the other hand, the lower limit of the time until cooling starts is not particularly limited, but it is preferably 0.2 seconds or more, particularly preferably 0.5 seconds or more, from the viewpoint of suitability for general production equipment.

Average cooling rate: above 25°C/s In addition, when the average cooling rate during the cooling is less than 25° C./s, elongated grains are generated in the cold-rolled steel sheet as a final product, resulting in a decrease in stampability. Therefore, the average cooling rate is set to 25° C./s or more. On the other hand, the upper limit of the aforementioned average cooling rate is not particularly limited, but from the viewpoint of being suitable for general production equipment, it is preferably set at 80° C./s or less, especially preferably 60° C./s or less, and more preferably 50° C./s or less. °C/s or less.

Cooling stop temperature: 620°C to 740°C When the above-mentioned cooling is stopped at a temperature higher than 740°C, carbides will be precipitated at the grain boundaries of the Wostian iron grains, and ductile grains will be produced in the final product, which will reduce the punchability. Therefore, the cooling stop temperature is set to be 740° C. or lower. On the other hand, when the cooling is stopped at a temperature lower than 620°C, ferrite is precipitated and perlite is biased. The existence of this bias will lead to non-uniform dispersion of Xueming carbon iron in the final product. Therefore, the cooling stop temperature is set to be 620°C or higher, preferably 630°C or higher.

(4) coiling After the cooling is stopped, the cooled hot-rolled steel sheet is coiled. The coiling temperature at this time is not particularly limited, but is preferably set at 600°C to 730°C.

After the aforementioned coiling, it is also preferable to pickle the hot-rolled steel sheet before the subsequent first annealing.

(5) 1st annealing The hot-rolled steel sheet after coiling has a perlite structure. Therefore, by applying the first annealing to the above-mentioned hot-rolled steel sheet after coiling, the snow-white carbon iron contained in the perlite is decomposed. By pre-decomposing Xueming carbon iron, Xueming carbon iron becomes finer in the second annealing or cold rolling. As a result, the ferrite grains are made finer and plastic deformation of the ferrite grains can be suppressed.

Annealing temperature: below 730°C When the annealing temperature in the above-mentioned first annealing is higher than 730° C., since a part is preferentially transformed, ferrite is partially coarsened, and as a result, the amount of plastic deformation increases. In addition, in the partially coarsened structure, the processing becomes inhomogeneous, and the precision of the part shape also deteriorates. Therefore, the aforementioned annealing temperature is set to be 730° C. or lower. On the other hand, the lower limit of the above-mentioned annealing temperature is not particularly limited, and from the viewpoint of re-dissolving the snow-white carbon iron in the perlite to promote the decomposition of the snow-white carbon iron, the annealing temperature is preferably set at 450°C or higher. , preferably above 500°C, more preferably above 520°C.

Annealing time: more than 5 hours In addition, when the annealing time in the above-mentioned first annealing is less than 5 hours, the decomposition of snow bright carbon iron does not proceed. When the decomposition of Xueming carbon iron is not carried out, the plate-shaped Xueming carbon iron will remain, and the subsequent processing by cold rolling and the like will become inhomogeneous, resulting in poor shape accuracy of parts. Therefore, the aforementioned annealing time is set to 5 hours or more. On the other hand, the upper limit of the aforementioned annealing time is not particularly limited. However, the microstructure change is saturated after the decomposition of Xueming carbon iron starts, so from the viewpoint of production efficiency, the above-mentioned annealing time is preferably set to 50 hours or less, more preferably 40 hours or less.

After the aforementioned first annealing, it is also preferable to pickle the hot-rolled steel sheet before the subsequent bending back.

(6) Bending back Next, bending is performed on the hot-rolled steel sheet after the first annealing. This bending back is extremely important in order to have a desired structure of the finally obtained cold-rolled steel sheet. That is, after the first annealing is performed to decompose the snow-white carbon iron, it is bent back to perform processing strain, thereby introducing strain energy. Then, by performing the second annealing described later, the miniaturization of snow-white carbon iron is promoted. When bending back is not performed, coarsened snow-white carbon iron is locally present, and the amount of plastic deformation is locally increased, so the stampability is lowered.

Introduction of processing strain by bending back is not particularly limited, and may be performed by any method. For example, a leveler or a leveler rolling machine used in shape correction, a strip cutter for cutting steel plates, etc. can be used to apply bending and bending, and it can also be uncoiled from a coil or recoiled into a coil Bending back when applied.

From the viewpoint of increasing the amount of strain introduced, it is preferable to apply bending back using a small-diameter roll. Specifically, it is preferable to use a roll with a diameter of 1100 mm or less, and it is particularly preferable to use a roll with a diameter of 800 mm or less. By applying bending back using a roll with a diameter of 1100mm or less, sufficient strain necessary to promote the miniaturization of the annealed snowbright carbon iron can be introduced. 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 by cutting or cutting strips in advance, which increases the number of processes. In addition, if the diameter of the roll is too small, meandering or cracking of the sheet will be promoted. Therefore, the diameter of the roll is preferably at least 300 mm, particularly preferably at least 450 mm. The foregoing roll may be a tension roll. In the case of using tension rollers, strain can be introduced by passing the sheet between the tension rollers.

(7) Second annealing The second annealing is applied to the hot-rolled steel sheet after the above-mentioned bending back. As mentioned above, the second annealing is performed after bending back to impart processing strain, thereby promoting the miniaturization of snow-white carbon iron.

Annealing temperature: above 600°C When the annealing temperature in the aforementioned second annealing is lower than 600°C, the refinement of snowflake carbon cannot proceed, and the formation of NaCl-type carbides containing at least one of Nb, Ti, and V is suppressed. When the formation of the aforementioned NaCl-type carbides is suppressed, since the plastic deformation of ferrite cannot be suppressed, the burrs increase. Therefore, the annealing temperature in the aforementioned second annealing is set to 600° C. or higher. On the other hand, the upper limit of the above-mentioned annealing temperature is not particularly limited. If it is too high, the structure will be coarsened and the burrs will increase instead. Therefore, the above-mentioned annealing temperature is preferably set at 790°C or lower, especially preferably set at 770°C or lower.

(8) cold rolling (9) The third annealing The cold rolling and third annealing are repeated two or more times to the hot-rolled steel sheet after the second annealing. The thickness of the final cold-rolled steel sheet is adjusted by the aforementioned cold rolling. In addition, by performing the third annealing after the cold rolling, the strain generated in the aforementioned cold rolling is removed. By performing the above-mentioned cold rolling and third annealing twice or more, the uniformity of the structure is improved, and the ferrite is strengthened due to the refinement of the ferrite structure, and as a result, the stampability can be improved. In order to obtain the aforementioned effects, the reduction ratio in the cold rolling is set to 15% or more, and the annealing temperature in the third annealing is set to 600° C. or more. On the other hand, the upper limit of the above-mentioned rolling ratio is not particularly limited, and if the rolling ratio is too high, the structure will be locally coarsened, and burrs will increase instead. Therefore, the above-mentioned rolling ratio is preferably set to 52% or less, more preferably 50% or less. In addition, the upper limit of the annealing temperature in the above-mentioned third annealing is not particularly limited, and if the annealing temperature is too high, the structure will be coarsened, and the burrs will increase instead. Therefore, the aforementioned annealing temperature is preferably set at 750° C. or lower, particularly preferably set at 720° C. or lower.

After repeating the above-mentioned cold rolling and third annealing two or more times, final cold rolling may be further performed. When the final cold rolling is performed, the rolling ratio in the final cold rolling is not particularly limited, but is preferably 20% or more. The upper limit of the rolling ratio in the aforementioned final cold rolling is also not particularly limited, but is preferably 50% or less.

By satisfying the above conditions, a cold-rolled steel sheet with good formability can be produced. In addition, any surface treatment may be further performed on the finally obtained cold-rolled steel sheet. Example

Hereinafter, in order to confirm the effects of the present invention, cold-rolled steel sheets were produced by the following procedures, and the stampability of the obtained cold-rolled steel sheets was evaluated.

First, steel billets were formed by melting steel having the composition shown in Table 1 in a converter by continuous casting. Then, heating, hot rolling, cooling, coiling, pickling, first annealing, pickling, bending back, second annealing, cold rolling, and third annealing are applied to the aforementioned steel billets in order to form the final sheet. Thickness: about 0.4mm cold-rolled steel plate. Each process was implemented under the conditions shown in Tables 2 and 3, and cold rolling and third annealing were repeated for the number of times shown in Tables 2 and 3. In addition, the said bending back was implemented using the tension roller of the diameter shown in Table 2, 3 at the time of winding up of a coil. In addition, for comparison, bending back was not performed in some examples (comparative example No. 16).

(organize) Then, the structure of the obtained cold-rolled steel sheet was evaluated by the following steps.

The average particle size of fertilizer iron First, test pieces for structure observation were collected from the obtained cold-rolled steel sheets. After grinding the section in the rolling direction (L section) of the test piece for structure observation, the polished surface was etched using 3 vol % nitric acid solution to expose the structure. Next, using a SEM (scanning electron microscope), the surface of the test piece for tissue observation was photographed at a magnification of 3000 times to obtain a tissue image. According to JIS G0551:2020, the ferrite particle size is measured by the cutting method from the obtained tissue image. Calculate the average value of the ferrite particle diameters measured in five fields of view and set it as the average particle diameter.

The average particle size and number density of grain-boundary snow-bright carbon iron First, test pieces for structure observation were collected from the obtained cold-rolled steel sheets. After polishing the section in the rolling direction (L section) of the test piece for structure observation, the polished surface was etched using a 3 vol % nitric acid solution to expose the structure. Next, the surface of the test piece for tissue observation was photographed at a magnification of 3000 times using a SEM to obtain a tissue image. From the obtained microstructure image, the particle size was measured only for grain boundary snow bright carbon iron by the cutting method. The average particle diameter of the grain boundary snow bright carbon iron measured in the 3 fields of view was calculated and set as the average particle diameter of the grain boundary snow bright carbon iron. In addition, from the above-mentioned structure image, the number density of grain boundary snow-clear carbon iron with a grain size of 0.5 μm or more was obtained.

Average particle size of NaCl-type carbides The average particle size of NaCl-type carbides containing at least one of Nb, Ti, and V existing in ferrite grains is determined by the following steps. Using a transmission electron microscope (TEM: Transmitting Electron Microscope), the surface of the test piece was photographed at a magnification of 80,000 times to obtain tissue images of five fields of view. By using image processing using circle approximation, the particle diameters of NaCl-type carbides containing at least one of Nb, Ti, and V existing in ferrite grains in the tissue image obtained above are obtained, and calculated its average. Whether the carbide contains at least one of Nb, Ti, and V is identified by TEM-EPMA.

Average spacing of NaCl-type carbides The average interval of NaCl-type carbides containing at least one of Nb, Ti, and V present in ferrite grains is calculated by measuring the interval of all NaCl-type carbides that can be confirmed in a field of view of 80,000 times Determined relative to the average of 5 fields of view.

The measurement results are shown in Tables 4 and 5. The so-called 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.

(stamping) Next, in order to evaluate the punchability of the obtained cold-rolled steel sheets, a punch punch test was performed under the following conditions, and the height of burrs was measured.

First, test pieces having a width of 20 mm, a length of 150 mm, and a thickness of 0.4 mm were collected from each cold-rolled steel sheet. Next, punch the aforementioned test piece using a ϕ10 SKD or superhard punch. The clearance in the aforementioned punching was set to 100 μm. In addition, the said punching was performed 10 times with respect to one test piece. At this time, the distance from the end of the test piece to the punched hole was set to 5 mm or more at the time of the first punching. In addition, at the time of punching after the second time, the interval between adjacent punched holes is set to 5 mm or more.

Then, the height of the burrs generated in the peripheral direction was observed with a microscope, and the heights of the burrs at 5 places were equally measured in the circumferential direction with respect to one hole, and the average value of the burrs at the 5 places was calculated. Then, the same measurement was carried out on 10 holes, and the average value of the burr heights calculated for each hole was used as the burr height.

Claims (3)

A cold-rolled steel plate, which contains C: 0.60 to 1.25%, Si: 0.1 to 0.55%, Mn: 0.5 to 2.0%, P: 0.0005 to 0.05%, S: 0.0001 to 0.01%, Al: 0.001 to 0.10%, N: 0.001 to 0.009%, Cr: 0.05 to 0.65%, and At least one selected from the group consisting of Ti: 0.001 to 0.30%, Nb: 0.01 to 0.1%, and V: 0.005 to 0.5%, And the remainder is composed of Fe and unavoidable impurities; And it has: the average particle size of Ferrite is 10 μm or less, The average particle size of Cementite existing in the ferrite grain boundary is 5 μm or less. The average particle size of the NaCl-type carbides containing at least one of Nb, Ti, and V present in the fertilizer iron particles is 0.5 μm or less, and A steel structure in which the average spacing of the aforementioned NaCl-type carbides is 710 nm or less. The cold-rolled steel sheet as described in Claim 1, wherein the aforementioned composition further contains Sb: less than 0.1%, Hf: less than 0.5%, REM: less than 0.1%, Cu: 0.5% or less, Ni: 3.0% or less, Sn: 0.5% or less, Mo: less than 1%, and Zr: At least one type of the group consisting of 0.5% or less. A method for manufacturing cold-rolled steel sheets, comprising heating a steel billet having the composition described in claim 1 or 2, Under the condition that the starting temperature of hot rolling: Ac3 point or higher and the processing and rolling outlet temperature: 800°C or higher, the heated steel billet is hot-rolled to form a hot-rolled steel plate, The hot-rolled steel sheet is cooled under the conditions of the time from the end of the hot rolling to the start of cooling: 5.0 seconds or less, the average cooling rate: 25°C/s or more, and the cooling stop temperature: 620°C to 740°C, The above-mentioned hot-rolled steel plate after coiling and cooling, The first annealing is performed on the hot-rolled steel sheet after coiling under the conditions of annealing temperature: 730°C or less, annealing time: 5 hours or more, Bending back is applied to the hot-rolled steel sheet after the first annealing, The annealing temperature: 600°C or higher is applied to the hot-rolled steel sheet after the above-mentioned bending back, To the hot-rolled steel sheet after the above-mentioned second annealing, the cold rolling of the rolling ratio: 15% or more and the third annealing of the annealing temperature: 600° C. or more are repeated twice or more.