TW201712130A - High-carbon cold-rolled steel sheet and method for manufacturing same - Google Patents

Abstract

Provided is a high-carbon cold-rolled steel sheet having a thickness of less than 1.0 mm and capable of having good impact and hardness characteristics after a short solution treatment, and thereafter quenching and low-temperature tempering. A high-carbon cold-rolled steel sheet having a steel sheet chemical composition containing, in terms of mass%, C: 0.85-1.10%, Mn: 0.50-1.0%, Si: 0.10-0.35%, P: 0.030% or less, S: 0.030% or less, and Cr: 0.35-0.45%, and furthermore containing Nb: 0.005-0.020 mass% with the remainder being Fe and unavoidable impurities, having a steel sheet structure in which the average particle diameter (dav) of carbide dispersed in the steel sheet is 0.2-0.7 ([mu]m) and the spheroidization ratio is 90% or higher, and having a thickness of less than 1.0 mm. Mechanical characteristics having an excellent impact characteristic in which the impact value is 5 j/cm2 or higher and a sufficient hardness characteristic within the range of 600-750 HV can thereby be manifested by a short solution treatment of 3-15 minutes, and thereafter quenching and low-temperature tempering.

Description

High carbon cold rolled steel sheet and manufacturing method thereof

The present invention relates to a high carbon cold rolled steel sheet which is produced by quench hardening and tempering treatment as a material for various mechanical parts. It is about quenching and hardening by a short-time solution treatment, and after performing low-temperature tempering treatment, it has both sufficient hardness (600 to 750 HV) and excellent impact resistance (toughness). Further, it is also applicable to a high carbon cold-rolled steel sheet having a thickness of less than 1.0 mm for applications such as knitting needles having strict requirements for durability and abrasion resistance. The short-term solution treatment referred to herein means that the treatment is carried out for 3 to 15 minutes in the temperature range of 760 to 820 ° C; the so-called low-temperature tempering treatment means: in the temperature range of 200 to 350 ° C. Implementation of the process.

In general, carbon steel steel materials (S××C) and carbon tool steel materials (SK) for mechanical structures specified in Japanese Industrial Standard JIS are used for various mechanical parts of various sizes. If it is used as a stretched material, it is subjected to quenching hardening and tempering treatment after being subjected to punching and various plastic working to form a part shape. Thereby, the steel can be given a predetermined hardness and toughness (impact resistance). Among them, for example: used for knitting knitting The knitting needle of the cloth is reciprocated at a high speed, and the yarn is pulled together to be knitted into a knitted fabric. Therefore, the needle end portion of the needle body that is in contact with the rotary driving portion is required to have sufficient The strength and wear resistance of the hook portion which is rubbed against the yarn are required to have sufficient wear resistance as well as excellent impact resistance at the front end portion.

The high-carbon cold-rolled steel sheet used as the material for knitting needles is suitable for the production of knitting needles for flat knitting machines if the thickness is 1.0 mm or more. If the thickness is less than 1.0 mm, it is suitable for the production of circular knitting machines and Knitting needles for the knitting machine. The knitting needles for the circular knitting machine and the vertical knitting machine are used for high-speed knitting of the yarn of a small diameter. Therefore, the thickness of the material to be used is usually 0.4 to 0.7 mm. In addition, the material for knitting needles is required to have excellent cold-working properties (hereinafter also referred to as secondary workability), and is also required to be processed into needle-like (secondary processing) and implemented. After quench hardening and tempering, it has sufficient hardness and sufficient toughness at the tip end of the needle.

In addition, the so-called high-carbon steel sheets such as carbon steel materials (S××C) for mechanical structures and carbon steel-steel steels (SK) for mechanical structures specified in Japanese Industrial Standards JIS are classified into uses according to the C content. In the field where the C content is less than 0.8% by mass, that is, the steel sheet of the hypoeutectoid composition, since the percentage of the ferrite grain iron phase is high, although it has excellent cold workability, it is difficult to obtain sufficient quenching. Hardening hardness. Therefore, the steel sheet composed of the hypoeutectoid composition is not suitable for use in a knitting needle which is required to have abrasion resistance of the hook portion and durability of the needle body. On the other hand, if it is in C In the field of 0.8% by mass or more, that is, a high carbon steel sheet having a C content of more than 1.1% by mass in a steel sheet having a hypereutectoid composition, although it has excellent quench hardenability, it contains more carbonization. The material (Xueming Iron) is extremely poor in cold workability, and is not suitable for use in knitting needles which are subjected to precise and fine processing such as groove processing. A high-carbon steel sheet having a C content of more than 1.1% by mass is limited to the use of parts such as tools and cold-working metal molds which are simple in shape and required to have high hardness.

Conventionally, knitting needles are mostly made of C: 0.8 to 1.1% by mass of carbon tool steel and alloy tool steel, or materials which are added to the steel composition of the third element based on these steel compositions. In the manufacturing process of such a knitting needle, the material is subjected to various plastic working such as punching (cutting), cutting, drawing, riveting, flexing, and the like. Therefore, in addition to the sufficient workability (secondary workability) required for the material to be processed in the manufacturing process of the needle, the material for the production of the knitting needle must have: Hardness characteristics and impact resistance (toughness) after quench hardening tempering required for practical use.

At the time of manufacture of the knitting needle, in order to ensure a predetermined hardness characteristic, the material is subjected to quench hardening and tempering treatment. This kind of tempering treatment generally adopts low temperature tempering treatment in a temperature range of 200 to 350 °C. However, in order to increase the amount of Mn and Cr added to the quench hardenability in order to pay attention to the hardness characteristics, or to add a large amount of other third elements, if only the above-mentioned low-temperature tempering treatment is carried out, the iron phase of the granules is returned. The fire is not enough, and the impact resistance (toughness) is not improved enough. There will be cases where the toughness value becomes jagged.

On the other hand, for the purpose of improving the impact resistance of the knitting needle, P and S of the impurity elements in the chemical composition of the material are reduced, so that the grain boundary segregation of P and the formation of MnS inclusions are minimized, thereby reducing these. The method of adverse effects of elements is also an effective countermeasure. However, in terms of technical and cost-effective steelmaking, there is a limit to improving the impact resistance of the knitting needle by reducing P and S.

Further, it has been known in the past that miniaturization of metal structures is an effective means for improving impact resistance. For example, the techniques disclosed in Patent Documents 1 and 2 are techniques in which a carbonitride forming element such as Ti, Nb, or V is added, and a fine carbonitride of these elements is used to make the metal structure fine. However, it is a common practice to add these elements as the toughness of steel for improving the sub-eutectoid composition having a C content of 0.8% by mass or less.

In particular, the influence of each third element on the impact resistance (especially interaction) of the granules of the granules in the low temperature tempering state at 200 to 350 ° C has not been fully understood, and therefore, most of the examples are When designing the component time, the effect of each third element is regarded as an equivalent effect.

For example, in the technique described in Patent Document 1, the sub-eutectoid steel of C: 0.5 to 0.7% by mass is used, and an element is formed by adding a carbonitride such as V, Ti, or Nb to form the old Worthfield. The iron particles are refined to increase the toughness value (impact resistance).

The technique described in the patent document 2 is based on a wide range of carbon content steels from the hypoeutectoid steel of C: 0.60 to 1.30 mass%, and the Ni: 1.8 mass is added as needed. %the following, Cr: 2.0% by mass or less, V: 0.5% by mass or less, Mo: 0.5% by mass or less, Nb: 0.3% by mass or less, Ti: 0.3% by mass or less, B: 0.01% by mass or less, and Ca: 0.01% by mass or less. One or two or more types are used to improve the impact resistance by controlling the volume fraction (Vf) of the undissolved carbide to be (15.3 × C mass% - Vf) falling within a range of more than 8.5 to less than 10.0.

[Previous Technical Literature]

[Patent Literature]

Patent Document 1: Japanese Patent Laid-Open Publication No. 2009-24233

Patent Document 2: Japanese Laid-Open Patent Publication No. 2006-63384

However, the technique described in Patent Document 1 is limited to the technique of hypoeutectoid steel, and it is expected to obtain an element by adding a carbonitride such as V, Ti, or Nb: fine carbon nitrogen using these elements. The technique of miniaturizing the old Worthfield iron particles. Further, in the technique described in Patent Document 1, since the carbon content is a sub-eutectoid composition, it is considered to be a technique for improving the formability of the ferrite-grain metal phase. Therefore, it is difficult to apply this technique to mechanical parts requiring high hardness such as knitting needles.

Moreover, the technique described in Patent Document 2 is directed to: A hypoeutectoid steel having a carbon content falling within the range of 0.67 to 0.81% by mass is added to add Mo, V, Ti, Nb, B, and the like. The addition of Mo, V, Ti, Nb, B, etc. here is considered to be apparently an addition intended to improve the properties of the hypoeutectoid steel. Patent Document 2 does not disclose at all the effect of the respective third elements in the steel having a carbon content of more than 0.81% by mass and the optimum addition amount thereof.

Further, in the technique described in Patent Document 2, the amount of addition of the third element is defined only by the upper limit value which does not adversely affect the impact resistance value, and the lower limit value is not defined. Therefore, for these reasons, it can be said that Patent Document 2 does not disclose a technique of actively adding a third element within an intended range to enhance impact resistance by the action of an additive element.

Further, in Patent Document 1 and Patent Document 2, it is not disclosed that for a high-carbon cold-rolled steel sheet, quenching hardening is performed using only a short time of the solution treatment holding time of 3 to 15 minutes, and 200 to 350 ° C is used. The low-temperature tempering technique can effectively improve the desired impact resistance and the predetermined hardness, and does not reveal a technique for evaluating the impact resistance of a steel sheet having a thickness of less than 1.0 mm. Therefore, it is an object of the present invention to provide an impact value of 5 J/cm ² or more and a hardness of only 5 J/cm ² or more after performing a short-time solution treatment and performing quench hardening and low-temperature tempering treatment. A high-carbon cold-rolled steel sheet having a thickness of less than 1.0 mm in the range of 600 to 750 HV (hereinafter sometimes referred to simply as "cold-rolled steel sheet").

In order to solve the above problems, the inventors of the present invention have made an effort to review the suitable range of addition of the chemical components of the high carbon cold-rolled steel sheet and the particle size and existing form of the carbide in the steel.

The core technology of the present invention obtains a novelty, which is based on the viewpoints of workability, quench hardenability, hardness and toughness after low-temperature tempering, and the like, and the carbon content of the knitting needle is limited to The C content is preferably in the range of 0.85 mass% or more and 1.10 mass% or less, and in the range of the carbon content, Nb is added as the third element in accordance with a predetermined range to control the average particle diameter of the carbide and the spherical shape. The degree of chemical conversion can effectively achieve the desired mechanical properties.

In particular, the inventors of the present invention have developed a new test method (new impact test method) for which a toughness evaluation of a steel sheet having a thickness of less than 1.0 mm has not been easily performed. The new test method (new impact test method) is shown in Fig. 1 and Fig. 2.

With this new impact test method, the impact value of the low-temperature tempering state in the quench hardening state was investigated for the high-carbon cold-rolled steel sheet to which the thickness of each of the third elements was less than 1.0 mm. As a result, a new concept is obtained, that is, only by adding a certain amount of Nb can meet the above-mentioned desired characteristics. The present invention has been developed based on this new concept.

In other words, the inventors of the present invention have been working hard to solve the above problems, and have found that a basic component is set to C: 0.85 to 1.10% by mass and Mn: 0.50 to 1.0% by mass. Si: 0.10 to 0.35 mass%, P: 0.030 mass% or less, S: 0.030 mass% or less, and Cr: 0.35 to 0.45 mass% of high carbon steel. It is necessary to add 0.005 to 0.020 mass% of Nb. By controlling the spheroidization of the carbide and the average particle size within a predetermined range, a high-carbon cold-rolled steel sheet having excellent quench hardenability and excellent toughness can be obtained, and the quench hardening treatment time can be shortened and the time can be reduced. Fire temperature. In addition, by adopting a test method that can properly evaluate the impact resistance of a thin plate, it is possible to formulate a chemical composition of a high carbon cold-rolled steel sheet and a spheroidization rate and an average particle diameter of the carbide.

First, the experimental results performed by the inventors of the present invention will be described.

For: % by mass, containing 1.01% of C-0.26% of Si-0.73% of Mn-0.42% of Cr-0.02% of Mo, and the content is 0%, 0.010%, 0.020%, 0.055% Four kinds of hot-rolled steel sheets (thickness: 4 mm) in which the content of Nb is changed and the remainder is Fe and inevitable impurities are respectively carried out five times: cold rolling (rolling reduction ratio: 25 to 65%, final It is 3~50%), softened and spheroidized (640~700 °C), and made into cold-rolled steel sheet (less than 1mm). For the cold-rolled steel sheet to be produced, the heating temperature is adjusted to a temperature of 780 ° C and 800 ° C, and the solution is subjected to a solution treatment in which the holding time is changed in the range of 0 to 16 minutes, and then oil quenching is performed. And the Vickers hardness (HV) was measured. The obtained result is shown in the figure of the relationship between the heating retention time (minute) of the solution treatment and the hardening hardness (HV), and is shown in Fig. 3 (heating temperature: 800 ° C), Fig. 4 (heating temperature: 780 ° C).

As can be seen from Fig. 3 and Fig. 4, the cold-rolled steel sheet having a Nb content of 0.010% by mass can ensure a quench hardening hardness exceeding 700 HV with a minimum heating retention time. Once the Nb content exceeds 0.010% by mass and continues to increase, the increase in hardness which can be achieved by simply heating for a short period of time will tend to passivate. From the results of Fig. 4, the heating retention time required for the quench hardening hardness of 700 °C in the case where the heating temperature of the solution treatment is 780 ° C can be obtained, and the relationship with the Nb content is indicated in the fifth. Figure.

As can be seen from Fig. 5, when the Nb content is 0.020% by mass or more, the heat retention time of the solution hardening treatment in which the quench hardening hardness reaches 700 HV is almost constant. In the range of the Nb content of 0.005 to 0.015 mass%, the heat retention time for ensuring the desired hardening hardness (700 HV) of the hardening treatment is the shortest, and stable quench hardenability can be ensured. Further, as long as it is the Nb content in this range, the heat retention time of the solution treatment can be set to be short. From this result, a kind of originality is obtained, that is, the method of setting the Nb content in the range of 0.005 to 0.015 mass% can be used as: the heating elongation amount which is regarded as a problem by the needle processing manufacturer is uneven and the heating anti-warping Effective countermeasures for deformation.

Further, for the cold-rolled steel sheets having various Nb contents, a solution treatment at a heating temperature of 800 ° C and a heating holding time of 10 minutes was carried out, and after oil quenching and hardening, tempering treatment was further carried out. In the tempering treatment, various temperatures of 150 ° C, 200 ° C, 250 ° C, 300 ° C, and 350 ° C were used as the tempering temperature, and the holding time was set to 1 hour. After tempering, the impact resistance was investigated.

Further, the impact resistance was carried out by the new test method shown in Figs. 1 and 2 . The results obtained are shown in Fig. 6. When the tempering temperature is 200 ° C or more, the impact value is the highest when the Nb content is 0.010% by mass.

From Fig. 6, the tempering temperature at which the impact value was 5 J/cm ^{2 was} obtained, and the relationship between the Nb content and the Nb content was shown in Fig. 7. From Fig. 7, a novelty was obtained in which a tempering temperature of an impact value of 5 J/cm ² was obtained, and a steel sheet having a Nb content of 0.010% by mass was used, and the temperature was the lowest. If the Nb content is further increased in excess of 0.020% by mass, a tempering temperature at which an impact value of 5 J/cm ² can be obtained will be turned to the high temperature side. When the tempering temperature is turned to a high temperature, the hardness is lowered, and the durability as a needle is lowered. Further, if the Nb content is less than 0.005% by mass, in order to secure a desired impact value, it is necessary to set the tempering temperature at a high temperature.

As can be seen from Fig. 5 and Fig. 7, it is understood that the high hardness after tempering and the excellent impact resistance are required, and the lower limit of the Nb content is 0.005 mass%, and the upper limit is 0.020 mass%. Further, in order to shorten the heating retention time of the solution treatment, the upper limit of the Nb content is preferably 0.015 mass%.

The present invention has been developed based on these findings and reviewed. That is, the gist of the present invention is as follows.

[1] A high carbon cold-rolled steel sheet having a chemical composition of C: 0.85 to 1.10% by mass, Mn: 0.50 to 1.0% by mass, Si: 0.10 to 0.35% by mass, and P: 0.030% by mass or less, S. : 0.030 mass% or less, Cr: 0.35 to 0.45 mass%, Nb: 0.005 to 0.020 mass%, the balance being Fe and unavoidable impurities, and the average particle diameter (d _av ) of the carbide dispersed in the steel sheet and the ball The rate of formation (N _SC /N _TC )×100% respectively conforms to the following formula (1) and formula (2), and the thickness of the aforementioned steel sheet is less than 1.0 mm, 0.2 ≦d _av ≦ 0.7 (μm)... Formula (1) (N _SC /N _TC ) × 100 ≧ 90%... Formula (2) Here, the average particle diameter (d _av ) in the formula (1) is observed from the cross section of the steel sheet. The average value of the diameters (circle equivalent diameters) of the respective circles when each carbide is determined to be a circle of the same area; N _TC and N _SC in the formula (2) are respectively N _TC : carbides in an observation area of 100 μm ² The total number; N _SC : the number of carbides satisfying the condition that d _L /d _S is 1.4 or less, where d _L represents the long diameter of the carbide, and d _S represents the short diameter.

[2] The high carbon cold-rolled steel sheet according to the above [1], wherein the chemical composition component further contains one or two selected from the group consisting of Mo and V, each of which is 0.001% by mass or more and not Up to 0.05% by mass.

[3] A method for producing a high-carbon cold-rolled steel sheet, which is obtained by subjecting a hot-rolled steel sheet having the chemical composition described in the above [1] or [2] to cold rolling and spheroidizing annealing A method of carbon cold-rolled steel sheet, characterized in that: the average particle diameter (d _av ) and the spheroidization ratio (N _SC /N _TC ) of the carbide dispersed in the high carbon cold-rolled steel sheet respectively satisfy the following formulas ( 1) and the formula (2), and the thickness of the high carbon cold-rolled steel sheet is less than 1.0 mm, 0.2 ≦d _av ≦ 0.7 (μm)... Formula (1) (N _SC /N _TC ) × 100 ≧ 90% In the formula (1), the average particle diameter (d _av ) in the formula (1) is the diameter of each circle when each of the carbides observed on the cross section of the steel sheet is determined to be a circle of the same area ( The average value of the circle-equivalent diameter); N _TC and N _SC in the formula (2) are N _TC : the total number of carbides in the observation area of 100 μm ² ; N _SC : the d _L /d _S is 1.4 or less. The number of carbides in the condition, where d _L represents the long diameter of the carbide, and d _S represents the short diameter.

[4] The method for producing a high carbon cold-rolled steel sheet according to the above [3], wherein the number of times of cold rolling and spheroidizing annealing repeatedly performed on the hot-rolled steel sheet is set to two to five times.

[5] The method for producing a high carbon cold-rolled steel sheet according to the above [3], wherein the cold rolling has a rolling reduction ratio of 25 to 65%, and the spheroidizing annealing temperature is 640 to 720. °C.

The high carbon cold-rolled steel sheet of the present invention is a very thin high-carbon cold-rolled steel sheet having a thickness of less than 1.0 mm, especially a thickness of 0.4 to 0.7 mm, and the average particle size of the carbide is controlled to be 0.2 to 0.7 μm. The size of the steel plate with a spheroidization rate of 90% or more. When the heat treatment by quench hardening or tempering is carried out on such a steel sheet, even if it is a soaking treatment for 3 to 15 minutes, it is possible to obtain good resistance by heat treatment by quench hardening or low temperature tempering. Impact (impact value: 5 J/cm ² or more) and hardness characteristics (600 to 750 HV).

Further, in the high-carbon cold-rolled steel sheet according to the present invention, after a short-time solution treatment, quenching and hardening is performed to form a granulated iron phase containing an unavoidable residual γ phase, and a so-called low temperature of 200 to 350 ° C is performed. Under tempering conditions, compared with the conventional high-carbon cold-rolled steel sheet, it is clearly superior to the conventional high-carbon cold-rolled steel sheet at the balance between hardness and impact resistance (toughness). In other words, as long as the high carbon cold-rolled steel sheet of the present invention is used, it is possible to obtain a high carbon steel machine tool part which can ensure excellent quench hardenability and excellent toughness after quench hardening and tempering. In particular, the cold-rolled steel sheet disclosed in the present invention can be applied not only to attain a balance point between hardness and toughness, but also to be used in a severe use such as a knitting needle having wear resistance and fatigue resistance. The use of excellent durability in the environment (that is, the use for knitting needles).

Fig. 1 is an explanatory view showing an example of a test apparatus used in the impact test of the present invention.

Fig. 2 is an explanatory view showing the shape of a test piece used in the impact test of the present invention.

Figure 3 shows the quench hardening hardness and the heat retention of the solution treatment. A graph of the relationship between time (heating temperature is 800 ° C).

Fig. 4 is a graph showing the relationship between the hardening hardness and the heat retention time of the solution treatment (heating temperature: 780 ° C).

Fig. 5 is a graph showing the relationship between the heat retention time and the Nb content of the solution treatment in which the quench hardening hardness is 700 HV.

Figure 6 is a graph showing the relationship between the impact value and the tempering temperature.

Fig. 7 is a graph showing the relationship between the tempering temperature and the Nb content at which an impact value of 5 J/cm ² can be obtained.

The following describes the embodiments of the present invention.

First, in the steel sheet according to the present invention, the hot-rolled steel sheet is first subjected to softening annealing as necessary, and then alternately and alternately subjected to cold rolling and spheroidizing annealing to form a high-carbon cold-rolled steel sheet having a thickness of less than 1.0 mm. Then, the high-carbon cold-rolled steel sheet is subjected to predetermined secondary processing and solution treatment, quench-hardening, and tempering treatment to provide a member (mechanical component) such as a knitting needle.

Hereinafter, the chemical composition of the steel sheet of the present invention will be described as C: 0.85 to 1.10% by mass, Mn: 0.50 to 1.0% by mass, Si: 0.10 to 0.35% by mass, P: 0.030% by mass or less, and S: 0.030% by mass or less. Cr: 0.35 to 0.45 mass%, and Nb: 0.005 to 0.020 mass%.

C: 0.85~1.10% by mass

C is an essential element for obtaining a sufficient hardness after heat treatment of a high carbon cold rolled steel sheet. The lower limit is controlled to ensure the hardness of 600 to 750 HV required for precision parts such as knitting needles, and the upper limit is controlled: for all kinds of cold processing The amount of carbide that causes obstruction. That is, the lower limit value is specified to be 0.85 mass%, so that the hardness of 600 HV can be stably ensured as long as the quench hardening tempering treatment is performed for a short time. In addition, the upper limit is defined as 1.10% by mass, and this content can withstand the upper limit of various plastic working such as punching property, compressive diameter workability, flexibility, and machinability. By performing cold rolling and spheroidizing annealing in reverse, if the spheroidization treatment of the carbide is performed, the cold workability can be improved. However, if the C content exceeds 1.10% by mass, the rolling load in the hot rolling step and the cold rolling step will become high, and the frequency of occurrence of cracks in the end portion of the steel coil will be significantly increased, and problems in the manufacturing process will occur. It will also become more prominent. Therefore, the C content is specified in the range of 0.85 to 1.10% by mass. Further, it is preferably 0.95 to 1.05 mass%.

Mn: 0.50 to 1.0% by mass

Mn is an element effective for deoxidation of steel, and is also an element which can improve the quench hardenability of steel and stably obtain a predetermined hardness. In the case where the Mn content is 0.50% by mass or more, when the Mn content is 0.50% by mass or more, the effect of the present invention is remarkable. Therefore, the lower limit is made 0.50 mass%. On the other hand, when the Mn content exceeds 1.0% by mass, a large amount of MnS is precipitated and coarsened during hot rolling, so that cracks are likely to occur during the processing of parts. The situation. Therefore, the upper limit is made 1.0% by mass. For this reason, the Mn content is specified in the range of 0.50 to 1.0% by mass. Further, it is preferably 0.50 to 0.80% by mass.

Si: 0.10 to 0.35 mass%

Si is a deoxidizing element of steel and, therefore, is an effective element for melting clean steel. Further, Si is an element having a temper softening resistance of the granulated iron. For this reason, the lower limit is made 0.10% by mass. In addition, if the amount of addition is too large, the tempering of the granulated iron in the tempering at the time of low-temperature tempering is insufficient, and the impact resistance is deteriorated. Therefore, the upper limit is made 0.35 mass%. Therefore, the Si content is specified in the range of 0.10 to 0.35 mass%.

P: 0.030% by mass or less, and S: 0.030% by mass or less

P and S are inevitably present in steel as an impurity element, and both of them have an adverse effect on impact resistance (toughness), so it is preferable to reduce it as much as possible. When the P content is 0.030% by mass or less and the S content is 0.030% by mass, there is no problem in practical use. For this reason, the P content is made 0.030% by mass or less, and the S content is made 0.030% by mass or less. Further, in order to maintain more excellent impact resistance, the P content is set to 0.020% by mass or less, and the S content is preferably set to 0.010% by mass or less.

Cr: 0.35 to 0.45 mass%

Cr is an element that enhances the quench hardenability of steel, but it will solidify In the carbide (snow iron), the remelting of the carbide in the heating stage is delayed, so that if it is added too much, the quench hardenability is hindered. Therefore, the upper limit of the Cr content is set to 0.45 mass%. The lower limit of the Cr content is set to 0.35 mass% based on the consideration of the balance point between the hardness and the impact resistance after quench hardening and tempering. For this reason, the Cr content is specified in the range of 0.35 to 0.45 mass%.

Nb: 0.005~0.020% by mass

In the past, Nb has been considered to be an element that can expand the range of the non-recrystallization temperature of steel during hot rolling and at the same time can be used as NbC to precipitate and contribute to the fineness of Worthite iron particles. Therefore, even in high carbon steel, it is sometimes added based on the effect of miniaturization of the structure which Nb is expected to bring after the cold rolling process. The main purpose of adding Nb in the present invention is to restore the toughness by low-temperature tempering treatment after quench hardening, so that the Nb content is added in an amount of 0.005 to 0.020% by mass. If only a small amount of Nb is added, NbC which is advantageous for the miniaturization of the structure cannot be formed, and Nb is in a thin solid solution state. It is considered that Nb is in a thin solid solution state, which promotes the diffusion of the iron phase of the BCC structure and the C in the granulated iron phase. That is, it is possible to promote the diffusion from the spheroidal carbide to the iron phase in the ferrite phase to the Vostian iron phase during heating in the quench hardening treatment; and, during the heating in the tempering treatment , the diffusion and precipitation of supersaturated solid solution C in the iron phase of Ma Tian. As a result, it is possible to achieve both effects of improving quench hardenability by using only a short time of heating and recovering toughness by low-temperature tempering treatment (currently it is considered of). When the amount of Nb added exceeds 0.020% by mass, the precipitation of NbC tends to be remarkable, and the thin solid solution state of Nb cannot be confirmed, and the effect of promoting C diffusion due to the thin solid solution state of Nb cannot be confirmed. Therefore, the upper limit of the amount of addition of Nb is specified to be 0.020% by mass. Further, it is preferably 0.015 mass% or less. On the other hand, when the amount of Nb added is less than 0.005% by mass, the above effects cannot be expected. Therefore, the lower limit of the amount of addition of Nb is specified to be 0.005% by mass. For this reason, the Nb content is specified in the range of 0.005 to 0.020% by mass.

The above-mentioned components are basic components, but in the present invention, an element which can be selectively added as necessary may further contain one or two selected from Mo and V.

Sometimes Mo and V inevitably contain content of Mo: less than 0.01 mass; V: less than 0.01% by mass. Further, in the present invention, as an element which can be selectively added, Mo and V may be further added for the reason that the quench hardenability and the impact resistance after tempering are increased so that the content thereof is higher than the unavoidable content. level. However, if the amount of addition of Mo or V is equal to or greater than the amount, the effect due to the addition of Nb will disappear. Therefore, in order to maximize the effect of adding Nb, the contents of Mo and V are limited to the ranges described below. It is better.

Mo: 0.001% by mass or more and less than 0.05% by mass

Mo is an effective element for improving the quench hardenability of steel. However, if the amount of addition is too large, the impact resistance is deteriorated after low-temperature tempering at 200 to 350 °C. Therefore, if you want to add Mo, The content is 0.001% by mass or more more than the unavoidable level, and is less than 0.05% by mass in the range which does not impede the impact resistance. Further, the amount of Mo added is preferably 0.01 to 0.03 mass%.

V: 0.001% by mass or more and less than 0.05% by mass

Since the V system can make the steel structure fine, it is an effective element for improving the impact resistance, but it is also an element which can deteriorate the quench hardenability. Therefore, when V is added, the content thereof is set to be 0.001% by mass or more more than the unavoidable level, and is less than 0.05% by mass in a range that does not impede the quench hardenability. Further, the amount of addition of V is preferably 0.01 to 0.03 mass%. The rest of the above components are Fe and unavoidable impurities.

Next, the carbide in the steel sheet of the present invention will be described.

In the high carbon cold-rolled steel sheet of the present invention, the average particle diameter (d _av ) and the spheroidization ratio (N _SC /N _TC ) of the carbide dispersed in the steel sheet must satisfy the following formula (1) and the formula ( 2) The relationship.

0.2≦d _av ≦0.7(μm)...the formula (1)

(N _SC /N _TC )×100≧90%...Number (2)

Here, the average particle diameter (d _av ) (μm) in the formula (1) is the diameter of each circle when the respective carbides observed on the cross section of the steel sheet are desirably defined as circles of the same area (circle equivalent diameter) )average value. When the average particle diameter (d _av ) falls within this range, it is excellent in impact resistance, and the effect of desired quench hardening hardness can be easily achieved even if only a short-time solution treatment is performed. When the average particle diameter (d _av ) is less than 0.2 μm, the load during the secondary processing for processing into a needle shape is empirically increased, and if it exceeds 0.7 μm, only a short time of solution treatment is performed. It is difficult to achieve the desired improvement in quench hardenability, and therefore it is not suitable.

Further, in the present invention, the ratio of the spheroidization of the carbide, that is, the spheroidization ratio, is defined by N _TC and N _SC of the formula (2). Here, N _TC is the total number of carbides in each observation area of 100 μm ² . Further, N _SC is regarded as the number of spheroidized carbides in the same observation field, that is, the number of carbides satisfying the condition of d _L /d _S : 1.4 or less. Here, d _L represents the long diameter of the carbide, and d _S represents the short diameter of the carbide.

Carbide cannot be said to form a completely spherical shape, and depending on the observation surface, there are many cases in which it is observed as an ellipse. Therefore, the ratio of the long diameter to the short diameter (d _L /d _S ) is used to define the ball. Degree of morphing. Based on this, in the present invention, carbides satisfying the condition of d _L /d _S : 1.4 or less are regarded as spheroidized, and the number thereof is defined as N _SC . Further, the reason why the spheroidization rate (N _SC /N _TC ) × 100 is set to 90% or more is because the inventors have found a proof based on the empirical value that if the spheroidization rate falls within this range The secondary workability of the steel sheet will tend to be good.

The measurement of the average particle diameter and the spheroidization ratio of the carbide described above was carried out by observing a secondary electron image at a magnification of two thousand times using a scanning electron microscope.

Using a cold-rolled steel sheet, a plate-shaped test piece is cut out from a direction perpendicular to the rolling direction of the sample sample before the heat treatment, and the test piece is embedded in the resin, and the observation area near the center portion of the plate thickness is observed. In the range of 100 μm ² , the circle equivalent diameter of the carbide, the d _L /d _S ratio, N _TC , and N _SC were measured, and the average value of the five fields of view was calculated. For the measurement and calculation of these numerical values, a commercially available image analysis software "winroof" (trade name) is used.

Next, a method of producing the steel sheet of the present invention will be described.

The hot-rolled steel sheet used in the present invention may be a hot-rolled steel sheet obtained under ordinary production conditions. For example, a steel sheet (steel blank) having the aforementioned chemical composition can be heated to 1050 to 1250 ° C, and then hot rolled at a hot rolling temperature of 800 to 950 ° C to obtain a coiling temperature of 600 to 750 ° C. It is taken up into steel coils to make steel sheets. Further, the thickness of the hot-rolled steel sheet may be appropriately set so as to obtain a desired cold rolling reduction ratio depending on the desired thickness of the cold-rolled steel sheet.

High-carbon cold-rolled steel sheets having a thickness of less than 1.0 mm were produced by cold rolling (25 to 65%) and spheroidizing annealing (640 to 720 ° C) several times in succession. It is advisable to carry out such cold rolling (25~65%) and spheroidizing annealing (640~720 °C) for 2~5 times.

The present invention repeatedly performs cold rolling (25 to 65%) and spheroidizing annealing (640 to 720 ° C) in multiple times. The reason is as follows. In order to control the average particle diameter (d _av ) and the spheroidization ratio (N _SC /N _TC )×100 of the carbide, the above formula (1) and the formula (2) can be respectively satisfied. ).

First, the crack is introduced into the carbide by cold rolling, and the pulverized carbide is spheroidized by spheroidizing annealing. However, it is difficult to increase the spheroidization rate of the carbide to 90% or more by using only one spheroidizing annealing, and carbides in a strip shape or a plate shape may remain. In this case, the quench hardenability is also adversely affected, and the cold workability at the time of processing into a precision part is deteriorated. Therefore, in order to increase the spheroidization ratio (N _SC /N _TC ) × 100 of the carbide to 90% or more, it is preferable to alternately perform cold rolling and spheroidizing annealing alternately. As a result, a distribution of fine carbides having a high spheroidization rate can be obtained in the steel sheet. More preferably, it is subjected to cold rolling 2 to 5 times and spheroidizing annealing 2 to 5 times.

When the spheroidizing annealing is performed on a steel sheet (cold-rolled steel sheet) having a cold rolling reduction ratio of less than 25%, the carbide will be coarsened. On the other hand, if the rolling reduction ratio of cold rolling exceeds 65%, the load at the time of cold rolling will be too large. Therefore, the cold rolling reduction ratio is preferably in the range of 25 to 65%.

Further, at the time of the final cold rolling, since the spheroidizing annealing is not performed after the cold rolling, the lower limit of the rolling reduction ratio is not particularly limited.

When the spheroidizing annealing temperature is lower than 640 ° C, the spheroidization is not sufficiently insufficient, and if the spheroidizing annealing is repeated at a higher temperature than 720 ° C, the carbide tends to be coarsened. Therefore, the spheroidizing annealing temperature is preferably selected in the range of 640 to 720 °C. The holding time of the spheroidizing annealing can be appropriately selected within the range of 9 to 30 hours at this range of temperatures.

Further, the softening annealing which is carried out for the purpose of softening the hot-rolled steel sheet before cold rolling is preferably in the same temperature range.

The above is a description of the method for producing a high carbon cold-rolled steel sheet according to the present invention, and it is intended to produce such a steel sheet as a mechanical zero such as a knitting needle. In the case of processing, it is preferable to carry out the heat treatment described below after processing it into a predetermined shape.

After processing high-carbon cold-rolled steel sheets having 90% or more of spheroidized carbides into various mechanical parts (pressing, groove processing, compression processing, etc.), they are melted and rapidly cooled. (Quenching hardening), and finally tempering treatment. The solution treatment is carried out at a heating temperature of 760 to 820 ° C for a short period of time of 3 to 15 minutes. Quench hardening (rapid cooling) is preferably carried out using oil cooling. The tempering temperature at the time of tempering treatment is preferably set at 200 to 350 °C. Better at 250~300°C. In this way, various mechanical parts with a hardness of 600 to 750 HV can be produced.

If the holding time of the solution treatment exceeds 15 minutes, too much carbide will be dissolved, because the Worthfield iron particles become coarser, and the granulated iron phase of the granules after quenching and hardening becomes thicker and the impact resistance is deteriorated. . Therefore, the upper limit of the holding time of the solution treatment is preferably 15 minutes. On the other hand, if the holding time of the solution treatment is shorter than 3 minutes, the dissolution of the carbide is insufficient and hardening is hard to be quenched. Therefore, it is preferable to set the lower limit of the retention time of the solution treatment to 3 minutes. Better is in the range of 5 to 10 minutes.

If the tempering temperature is less than 200 ° C, the toughness recovery of the Ma Tian iron phase is not sufficient. On the other hand, if the tempering temperature exceeds 350 ° C, although the impact value will recover, the hardness is less than 600 HV, so there is a problem in durability and wear resistance. Therefore, the suitable range of the tempering temperature is preferably set at 200 to 350 °C. Better is 250~300 °C. Tempering retention In the meantime, the system can be appropriately selected within the range of 30 minutes to 3 hours.

[Examples]

Steel having various chemical compositions was vacuum melted to cast a 30 kg steel block. This steel block was subjected to block rolling, and hot rolled at a heating temperature of 1,150 ° C and a refining rolling temperature of 870 ° C to obtain hot-rolled steel sheets having a thickness of 4 mm and 2 mm. Then, cold rolling and spheroidizing annealing were performed under the production conditions shown in Table 1, and a cold rolled steel sheet having a thickness of 0.4 mm or more and less than 1.0 mm was produced. Next, the cold-rolled steel sheet was subjected to a solution treatment (in a furnace at 800 ° C for 10 minutes) under the conditions shown in Table 2, and then subjected to oil quench hardening treatment, followed by tempering ( Tempering temperature: 250 ° C) treatment. A predetermined test piece is taken from the tempered steel sheet for impact test and hardness test. The hardness measurement was carried out under the conditions of a load of 5 kg (test force: 49.0 N) in accordance with the Japanese Industrial Standard JIS Z 2244.

Impact resistance was evaluated by Charpy impact test. The impact test piece was a U-shaped groove test piece having a groove width of 0.2 mm (the groove depth was 2.5 mm, and the groove radius was 0.1 mm). Fig. 1 shows a state in which a test piece is set in a test apparatus; and Fig. 2 shows a shape of a test piece. The reasons why such test pieces and test methods are used are as follows.

The steel sheet having a thickness of less than 1.0 mm which is the object of the present invention, if a conventionally used metal material is used for the Charpy impact test apparatus In this case, the rated capacity of the test device is 50 J or more. Since the rated capacity is too large, there is a problem that a correct evaluation cannot be achieved. Therefore, the impact test apparatus with a rated capacity of less than 50 J, that is, an impact test apparatus (type manufactured by Toyo Seiki Seisakusho Co., Ltd.: DG-GB impact test apparatus) having a rated capacity of 1 J is used instead. This test apparatus is based on the Charpy impact tester of the Charpy impact test method (JIS K 7077) of carbon fiber reinforced plastic. Further, the test apparatus was modified to use the distance between the support tables from 60 mm to 40 mm. In this test apparatus, the reason why the distance between the support stages is changed from 60 mm to 40 mm is to bring the test conditions close to: the Charpy impact test method of the metal material, which is the Japanese industrial standard JIS (JIS Z 2242). Provisions.

As shown in Fig. 2, the test piece was formed into a test piece in which a groove depth was 2.5 mm, a groove radius was 0.1 mm (the groove width was 0.2 mm), and a U-shaped groove was formed by electric discharge machining. The reason why the radius of the groove is made smaller is because it is considered that if the sheet is less than 1.0 mm in the Charpy impact test, the deflection of the sheet will cause a problem, and therefore, by increasing the stress concentration factor, The flexure plate produced by the test piece at the Charpy impact test was minimized to obtain a stable impact value. Further, it has been confirmed that by adopting such a test method and the shape of the test piece, it is possible to obtain an impact characteristic in a state close to the actual use environment. In the present invention, when the value of the impact value is 5 J/cm ² or more, it is judged that the impact resistance is excellent.

(Example 1)

After the solution treatment, oil quench hardening was performed, and the influence of various added elements on the cross-sectional hardness and impact value after tempering was confirmed. The test results are shown together with the chemical components in Tables 3 and 4. The conditions for the production of cold-rolled steel sheets were both 5A (Table 1). The rolling reduction ratio was controlled in the range shown in Table 1.

The measurement of the section hardness is a result obtained by burying the test piece cut in the direction perpendicular to the roll in the resin and polishing the cross section, and measuring the center portion of the sheet thickness. The measurement of the impact value was carried out by using an impact test piece taken in a direction parallel to the rolling. The measured results (hardness, impact value) are shown in Tables 3 and 4.

When the impact value is more than 5 J/cm ² and the hardness is 600 to 750 HV, the evaluation is ◎, and if one of the impact value and the hardness fails to meet the above target value, it is evaluated as ×.

In the example shown in Table 3, the steel sheet (steel type No. 1) having a C content lower than the lower limit value, the impact value and the quench hardening tempering hardness were all lower than the target value. For steel sheets with a C content higher than the upper limit (steel type No. 6), the quench hardening tempering hardness is higher than the target value of 600 to 750 HV, but the impact value is lower than the target value of 5 J/cm ² . In a steel sheet not containing Nb, a steel sheet having a C content of 0.85% by mass (steel type No. 2, a comparative example) and a steel sheet having a C content of 1.10% by mass (steel type No. 4, comparative example) have lower impact values than the target. The value of 5J/cm ² was rated as ×. On the other hand, the steel sheets (steel grades No. 3, 5, 7, 8, 9, 10) corresponding to the chemical components of the invention examples have a quench hardening tempering hardness falling within the target range and excellent impact resistance. .

In the example shown in Table 4, the steel sheets (steel grades No. 15, 16, 17, 19, 21) having the chemical composition equivalent to the invention examples are all quenched and hardened, the tempering hardness is 600 to 750 HV, which is in accordance with the target value, and is resistant to impact. Excellent sex. A steel sheet (steel type No. 11) to which no Nb is added, a steel sheet (steel type No. 12) in which N is added in an amount of more than 0.05% by mass, and a steel sheet (steel type No. 13 in which Nb is not added but the amount of Mo added exceeds 0.05% by mass) ), a steel sheet (steel type No. 14) in which Nb+Mo is added and Nb is added in an amount of less than 0.005% by mass, a steel sheet (steel type No. 18) in which Nb+Mo is added in combination, and Nb is added in an amount exceeding 0.020% by mass, and Nb is added in combination. Steel plate (steel type No. 20) with +Mo and Mo added in an amount of more than 0.05% by mass, steel plate (steel type No. 22) with a compounding amount of Nb+Mo+V and a V addition amount of more than 0.05% by mass, although quench hardening and tempering The hardness is in accordance with the target value of 600~750HV, but it is not the impact resistance is not good, that is, the impact resistance is 5J/cm ² according to the target value, but the quench hardening tempering hardness is reduced, or the quench hardening tempering hardness and resistance The impact is below the lower limit of the target value.

(Example 2)

The hot-rolled steel sheet having the chemical composition of the steel type No. 3 (Table 3) was used, and the production conditions of the cold rolling and the spheroidizing treatment described in Table 1 were changed to produce a cold-rolled steel sheet having the thickness shown in Table 5. The spheroidization ratio and the average carbide particle diameter of the obtained cold-rolled steel sheet are shown in Table 5. Further, with respect to the obtained cold-rolled steel sheet, the conditions shown in Table 2 were used in the same manner as in Example 1, and after the solution treatment, oil quench hardening and low-temperature tempering were performed. The cross-sectional hardness and impact value after hardening and tempering of the obtained cold-rolled steel sheet after quenching treatment were measured in the same manner as in Example 1, and are shown in Table 5.

The steel sheet having only one round of spheroidizing annealing (manufacturing condition No. 1) has insufficient spheroidization ratio and poor impact resistance. When the number of spheroidizing annealing times is two, a combination of a spheroidizing annealing temperature of 600 to 635 ° C and a cold rolling reduction ratio of 10 to 20% is used, and each of them is repeated twice, and spheroidized. Not enough, and the impact resistance is not good (manufacturing condition No. 2A). When the spheroidizing annealing temperature is 600 to 635 ° C and the cold rolling reduction ratio is 70 to 85%, the impact resistance is sufficient, but the average particle diameter of the carbide is lower than that. The lower limit, the hardness after the quench hardening tempering treatment is higher than the target value (manufacturing condition No. 2C).

In the case where the spheroidizing annealing temperature is 640 to 720 ° C and the cold rolling reduction ratio is 10 to 20%, each of which is repeated twice, although the spheroidization is sufficient, the average particle diameter of the carbide exceeds The upper limit of the target value is not good in impact resistance (manufacturing condition No. 2D). The reason for this is considered to be that if the carbide is too large, the undissolved carbide of the granulated iron substrate becomes large when quenched and hardened, and it is easy to become the undissolved carbide of the starting point at the time of destruction and the granulated iron substrate. The area of the interface is large, so the impact resistance is deteriorated. On the other hand, when the spheroidizing annealing temperature is 640 to 720 ° C and the cold rolling reduction ratio is 25 to 65%, the spheroidization ratio, the carbide particle diameter, and the quenching are respectively performed twice. The hardness after hardening and tempering falls within the range of the target value, and the impact resistance is excellent (manufacturing condition No. 2B).

When the number of spheroidizing annealing times is set to four, if the rolling reduction ratio of the first to fourth cold rollings is set to 25 to 65%, the spheroidization rate and the carbide particle size may fall. Impact resistance within the range of target values Excellent also (manufacturing condition No. 5A). When the spheroidizing annealing temperature is the same as that of the manufacturing condition No. 5A, and the rolling reduction ratio of the first to fourth cold rolling is set to 10 to 20%, the carbide particle diameter will exceed the target value. It is too large and the impact resistance is not good (manufacturing condition No. 5B).

(Example 3)

The hot-rolled steel sheets having the chemical composition of steel type No. 16 (Table 4) were used, and the manufacturing conditions described in Table 1 were changed to obtain cold-rolled steel sheets having the thickness shown in Table 6. The spheroidization ratio and the average carbide particle diameter of the obtained cold-rolled steel sheet are shown in Table 6. Further, with respect to the obtained cold-rolled steel sheet, the conditions shown in Table 2 were used in the same manner as in Example 1, and after the solution treatment, oil quench hardening and low-temperature tempering were performed. The cross-sectional hardness and impact value after hardening and tempering of the obtained cold-rolled steel sheet after quenching treatment were measured in the same manner as in Example 1, and are shown in Table 6.

The steel sheets subjected to cold rolling and spheroidizing annealing using the manufacturing conditions No. 2B and No. 5A corresponding to the production method of the present invention all conform to the target spheroidization ratio and the target impact value.

[Industrial availability]

The steel sheet having the chemical composition within the range specified by the present invention can improve the quench hardenability by adding Nb, and can improve the impact resistance after heat treatment. Therefore, the hypereutectoid steel can be suitably used in a harsh environment. The use of mechanical tool parts.

The over-eutectoid steel sheet having a C content of 0.85 to 1.10% by mass can be suitably used for a balance point of hardness and toughness in a severe use environment such as a knitting needle.

Claims (5)

A high carbon cold-rolled steel sheet having a chemical composition of C: 0.85 to 1.10% by mass, Mn: 0.50 to 1.0% by mass, Si: 0.10 to 0.35% by mass, P: 0.030% by mass or less, and S: 0.030 mass. % or less, Cr: 0.35 to 0.45 mass%, Nb: 0.005 to 0.020 mass%, the balance being Fe and unavoidable impurities, and the average particle diameter (d _av ) and spheroidization ratio of the carbide dispersed in the steel sheet (N _SC /N _TC )×100% respectively conform to the following formula (1) and formula (2), and the thickness of the aforementioned steel sheet is less than 1.0 mm, 0.2 ≦d _av ≦ 0.7 (μm)... (N _SC /N _TC )×100≧90%...Formula (2) Here, the average particle diameter (d _av ) in the formula (1) is the respective carbides observed from the cross section of the steel sheet. The average of the diameters (circle equivalent diameters) of the circles when the circle of the same area is determined; N _TC and N _SC in the formula (2) are N _TC : the total number of carbides in the observation area of 100 μm ² N _SC : The number of carbides satisfying the condition that d _L /d _S is 1.4 or less, where d _L represents the long diameter of the carbide, and d _S represents the short diameter. The high carbon cold-rolled steel sheet according to claim 1, wherein the chemical component further contains one or two selected from the group consisting of Mo and V, each of which is 0.001% by mass or more and less than 0.05% by mass. . A method for producing a high carbon cold-rolled steel sheet, which is a method for producing a high-carbon cold-rolled steel sheet by subjecting a hot-rolled steel sheet having the chemical composition of claim 1 or 2 to cold rolling and spheroidizing annealing It is characterized in that the average particle diameter (d _av ) and the spheroidization ratio (N _SC /N _TC ) of the carbide dispersed in the high carbon cold-rolled steel sheet respectively satisfy the following formula (1) and the formula ( 2), and the thickness of the aforementioned high carbon cold-rolled steel sheet is less than 1.0 mm, 0.2 ≦d _av ≦ 0.7 (μm)... Equation (1) (N _SC /N _TC ) × 100 ≧ 90%... Equation (2) Here, the average particle diameter (d _av ) in the formula (1) is the average of the diameters (circle equivalent diameters) of the respective circles when the respective carbides observed on the cross section of the steel sheet are determined to be circles of the same area. Values; N _TC and N _SC in the formula (2) are N _TC : the total number of carbides in the observation area of 100 μm ² ; N _SC : carbides satisfying the condition that d _L /d _S is 1.4 or less The number d, where d _L represents the long diameter of the carbide, and d _S represents the short diameter. The method for producing a high-carbon cold-rolled steel sheet according to claim 3, wherein the number of times of cold rolling and spheroidizing annealing repeatedly performed on the hot-rolled steel sheet is set to 2 to 5 times. The method for producing a high carbon cold-rolled steel sheet according to claim 3, wherein the cold rolling has a rolling reduction ratio of 25 to 65%, and the spheroidizing annealing temperature is 640 to 720 °C.