KR101953495B1

KR101953495B1 - High carbon cold-rolled steel sheet and method of manufacturing the same

Info

Publication number: KR101953495B1
Application number: KR1020177014614A
Authority: KR
Inventors: 에이지 쓰치야; 유타 마쓰무라; 요시히로 호소야; 유카 미야모토; 다카시 고바야시; 야스노부 나가타키; 가즈히로 세토
Original assignee: 가부시키가이샤 도쿠슈 긴조쿠 엑셀; 제이에프이 스틸 가부시키가이샤
Priority date: 2015-08-14
Filing date: 2016-07-19
Publication date: 2019-02-28
Also published as: KR20170075783A; TW201712130A; CN107208224B; EP3216889A1; JP2017036492A; JP6089131B2; EP3216889A4; WO2017029922A1; EP3216889B1; CN107208224A; TWI591187B

Abstract

A high carbon cold rolled steel sheet having a plate thickness of less than 1.0 mm, capable of having good impact properties and hardness characteristics after a short time of solution treatment, subsequent quenching and low temperature tempering treatment.
the steel contains 0.85 to 1.10% of C, 0.50 to 1.0% of Mn, 0.10 to 0.35% of Si, 0.030% or less of P, 0.030% or less of S and 0.35 to 0.45% of Cr, : 0.005 to 0.020 mass% and the balance Fe and inevitable impurities, wherein the average particle diameter (d _av ) of the carbide dispersed in the steel sheet is 0.2 to 0.7 (μm) and the spheroidization rate is 90% Plate thickness with structure: High carbon cold-rolled steel sheet of less than 1.0 mm. As a result, even if the solution treatment is performed for a short time of 3 to 15 minutes, the hardness characteristics such as an impact property of an impact value of 5 J / cm ² or more and a hardness characteristic of 600 to 750 HV are obtained by the subsequent quenching and low temperature tempering treatment Mechanical properties can be expressed.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a high carbon steel sheet,

The present invention relates to a high carbon cold-rolled steel sheet as a material for various mechanical parts manufactured by tempering tempering treatment. Particularly, it is quenched by a short-time solution treatment process, applied with hardness (600 to 750 HV) and excellent impact property (toughness) after low-temperature tempering treatment and further applied to knitting needles with strict demands on durability and abrasion resistance Carbon steel sheet having a thickness of less than 1.0 mm. Here, the short term solution treatment refers to treatment in a temperature range of 760 to 820 占 폚 for 3 to 15 minutes, and the low temperature treatment refers to treatment in a temperature range of 200 占 폚 to 350 占 폚.

Generally, carbon steel steels (S × C) and carbon steel steels (SK) for mechanical structures specified in JIS are used for various mechanical parts, large and small. When it is used as a general material, it is subjected to punching or various kinds of plasticizing to form a part, followed by quenching and tempering. As a result, predetermined hardness and toughness (impact characteristics) are imparted. In the knitting needles for knitting a knit fabric, for example, knit fabrics are squeezed by pulling a yarn while repeating reciprocating motion at a high speed, so that the butt portion of the knitting main body, which is in contact with the rotation driving portion, Abrasion resistance is required, and a hook portion that is in contact with the yarn is required to have a sufficient impact resistance at the tip portion in addition to sufficient abrasion resistance.

The high carbon cold-rolled steel sheets used as knitting needles are used for knitting knitting for horizontal knitting when the thickness is 1.0 mm or more and for circular knitting or knitting if the thickness is less than 1.0 mm. It is used as knitting needles. In the knitting needles for a circular knitting machine and an end piece knitting machine, since a small diameter yarn is squeezed at a high speed, the thickness of the material used is often 0.4 to 0.7 mm. In addition to having excellent cold workability (hereinafter also referred to as secondary workability), the knitting material for knitting has a sufficient hardness and a sufficient toughness at the tip of the needle after quenching and tempering, .

The so-called high-carbon steel sheets such as carbon steel steels (S 占 폚 C) and carbon tool steel steels (SK) specified in JIS are classified finely by C content. If the C content is less than 0.8% by mass, that is, a steel sheet having an acetylacetone composition, the cold workability is excellent because of the high fraction of the ferrite phase, but it is difficult to obtain sufficient quenching hardness. Therefore, the steel sheet having an abacetal composition is not suitable for a knitting needle application in which wear resistance of the hook portion and durability of the needle body are required. On the other hand, a high carbon steel sheet having a C content of more than 1.1% by mass among the steel sheets having a porosity of 0.8% by mass or more, that is, a steel sheet having a high porosity, has excellent quenchability, while the cold workability is extremely inferior due to a large amount of carbides (cementite) It is not suitable for a knitting needle application in which precise and minute processing such as groove cutting processing is performed. The high carbon steel sheet having a C content of 1.1% by mass or more is limited to a component having a simple shape such as a cutter or a cold-formed metal and requiring a high strength.

Conventionally, for the knitting needles, a material of carbon tool steel or alloy tool steel of C: 0.8-1.1 mass% or steel composition based on the steel composition and the third element added is widely used. In the manufacturing process of the knitting needles, the material is provided for various kinds of plastic working such as punching (shearing), cutting, drawing, caulking, and bending. Therefore, the material for knitting needles has satisfactory processability (secondary processability) at the time of material processing in the step of manufacturing needles, and also has hardness characteristics and impact properties (toughness .

In the production of knitting needles, quenching and tempering treatment is applied to the material to ensure predetermined hardness characteristics. In this tempering treatment, a low temperature tempering treatment in a temperature range of 200 to 350 캜 is generally employed. However, when the addition amount of Mn or Cr which is effective for the hardenability and the addition of a large amount of other third elements to the hardenability is emphasized, tempering on the martensite is not sufficiently performed in the above-mentioned low temperature tempering treatment, (Toughness) is insufficient or the toughness value is not uniform.

On the other hand, for the purpose of improving the impact characteristics of the knitting needles, the impurity elements P and S in the chemical composition of the material are reduced to minimize the generation of grain boundary segregation of P and MnS inclusions, It is also considered to be an effective countermeasure. However, from the viewpoint of the steelmaking technology and cost-effectiveness, there is a limit in reducing the P or S and improving the impact characteristics of the knitting needles.

Further, it has been known that miniaturization of metal structure is effective as a means for improving impact characteristics. For example, Patent Documents 1 and 2 disclose a technique of adding carbonitride-forming elements such as Ti, Nb, and V, and refining a metal structure using the fine carbonitride of these elements. However, these elements were generally added as a countermeasure for improving the toughness of the steel having an acetylacetone composition with C of 0.8 mass% or less.

Particularly, the effect (in particular interaction) of individual third elements with respect to the impact characteristics on the martensite under a low-temperature tempering condition of 200 to 350 캜 is not sufficiently clarified, and the effects of individual elements are regarded as equivalent There are many cases of component design.

For example, in the technique described in Patent Document 1, a carbonitride-forming element such as V, Ti, or Nb is added to a quartz quartz steel of C: 0.5 to 0.7 mass%, whereby the old austenitic grains are refined, Thereby improving the toughness value (impact property).

In the technique described in Patent Document 2, a steel having a wide carbon content of 0.60 to 1.30 mass% of cobalt quarry from C: 0.60 to 1.30 mass% is used as a target, and if necessary, Ni: 1.8 mass% or less, Cr: 2.0 mass% : Not more than 0.5 mass%, Mo: not more than 0.5 mass%, Nb: not more than 0.3 mass%, Ti: not more than 0.3 mass%, B: not more than 0.01 mass%, and Ca: not more than 0.01 mass% (15.3 x Cmass% -Vf) is controlled to fall within the range of more than 8.5 to less than 10.0, thereby improving the impact characteristics.

Japanese Patent Application Laid-Open No. 2009-24233 Japanese Unexamined Patent Application Publication No. 2006-63384

However, the technique described in Patent Document 1 is limited to porcelain steels and is a technique expected to exert an effect of refining old austenitic grains by the addition of carbonitride-forming elements such as V, Ti, and Nb . Further, the technique described in Patent Document 1 is also a technique for improving the formability of the ferrite parent phase because the carbon level is a sub-vacancy composition. Therefore, it is difficult to apply this technique to machine parts requiring high hardness such as knitting needles.

Further, in the technique described in Patent Document 2, Mo, V, Ti, Nb, B and the like are added to the porcelain quartz having a carbon content of 0.67-0.81 mass%. The addition of Mo, V, Ti, Nb, B and the like is clearly interpreted as an addition intended to improve the properties of the sintered steel. In Patent Document 2, there is no disclosure concerning the action of each of the third elements in the carbon having a carbon content exceeding 0.81 mass% and optimization thereof.

In addition, in the technique described in Patent Document 2, the upper limit value which does not adversely affect the impact value is specified only for the addition amount of the third element, and the lower limit value thereof is not specified. From this, it can be said that Patent Document 2 does not disclose a technique in which the third element is added in an intended range to actively improve the impact characteristics by the action of the additive element.

Patent Document 1 and Patent Document 2 disclose a method for manufacturing a high carbon steel cold-rolled steel sheet having desired impact properties and predetermined hardness by quenching and low-temperature tempering at a temperature of 200 to 350 캜 for a short- There is no disclosure of a technique for improving the steel sheet with a thickness less than 1.0 mm nor a technique for evaluating the impact characteristics.

Therefore, the present invention provides a method of manufacturing a plate having an impact value of 5 J / cm ² or more and a hardness in a range of 600 to 750 HV after quenching and low-temperature tempering treatment after a short- (Hereinafter, simply referred to as " cold rolled steel sheet ") having a thickness of less than 1.0 mm.

In order to solve the above problems, the inventors of the present invention have extensively studied the appropriate range of addition of the chemical components of the high carbon cold-rolled steel sheet and the grain size and existence form of carbides in the steel.

The present invention is limited to a carbon content of not less than 0.85 mass% and not more than 1.10 mass% C suitable for knitting needles in view of workability, hardenability, low temperature tempering, hardness and toughness. However, It is the core of the present technology that it is found that controlling the average grain size and spheroidization degree of carbide by adding Nb to a predetermined range is effective for manifesting desired properties.

In particular, the inventors of the present invention have developed a new test method (new impact test method) for toughness evaluation of a steel sheet having a thickness of less than 1.0 mm, which was difficult to evaluate toughness. A new test method (new impact test method) is shown in Fig. 1 and Fig.

Using this new impact test method, the impact value of the quenched low temperature tempered state was investigated for a high carbon cold rolled steel sheet having a plate thickness of less than 1.0 mm, to which various third elements were added. As a result, it was found that the addition of a predetermined amount of Nb satisfies the above-mentioned desired characteristics uniquely. The present invention is based on this finding.

That is, the present inventors have conducted intensive studies in order to solve the above problems, and have found out that the basic components are composed of 0.85 to 1.10 mass% of C, 0.50 to 1.0 mass% of Mn, 0.10 to 0.35 mass% of Si, : 0.030 mass% or less and Cr: 0.35 to 0.45 mass%, it is essential to add 0.005 to 0.020 mass% of Nb to the high carbon steel, and to control the spheroidization of the carbide and the average grain size to a predetermined range It is possible to obtain a high carbon cold-rolled steel sheet having excellent quenching properties and excellent toughness, and it is also possible to shorten the quenching treatment time and lower the tempering temperature. In addition, by adopting a test method for appropriately evaluating the impact characteristics of the thin plate, it is possible to specify the appropriate chemical composition and the spheroidization ratio and average particle diameter of the carbide.

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

0.01%, 0.020% and 0.055% of Nb in terms of mass%, 1.01% C-0.26% Si-0.73% Mn-0.42% Cr-0.02% Cold rolling (reduction rate: 25 to 65%, final amount: 3 to 50%), softening annealing and spheroidizing annealing (640 to 700 占 폚) were carried out at a temperature of 5 (Less than 1 mm). The obtained cold-rolled steel sheet was subjected to a solution treatment in which the heating temperature was changed to two levels of 780 ° C and 800 ° C and the holding time was changed in the range of 0 to 16 minutes, followed by oil quenching and measurement of Vickers hardness (HV). The obtained results are shown in Fig. 3 (heating temperature: 800 占 폚) and Fig. 4 (heating temperature: 780 占 폚) in relation to the heating holding time (minute) and the hardness of hardening (HV) of the solution treatment.

3 and 4, it can be seen that the quenched hardness of the cold-rolled steel sheet having the Nb content of 0.010% by mass can secure the quenching hardness exceeding 700 HV at the shortest heating holding time. If the Nb content exceeds 0.010 mass%, the hardness increase in the short-time heating and holding becomes slow. From the results of Fig. 4, the heating holding time at which the quenching hardness reaches 700 HV when the heating temperature of the solution treatment is 780 占 폚 is determined, and is shown in Fig. 5 in relation to the Nb content.

In Fig. 5, when the Nb content is 0.020 mass% or more, the heating and holding time of the solution treatment process in which the quenching hardness reaches 700 HV is almost constant. When the Nb content is in the range of 0.005 to 0.015 mass%, the heating and holding time of the solution treatment for ensuring the desired quenching hardness (700 HV) is the shortest, and stable quenchability can be ensured. Moreover, if the Nb content is within this range, the heating and holding time of the solution treatment can be shortened. From this, it has been found that setting the Nb content in the range of 0.005 to 0.015 mass% is effective as a countermeasure for preventing the quenching height variation and the quenching bending which are regarded as problems in the needle processing maker.

Further, a solution treatment was applied to a cold-rolled steel sheet having various Nb contents at a heating temperature of 800 DEG C and a heating and holding time of 10 minutes, followed by oil quenching and further tempering treatment. In the tempering treatment, the tempering temperature was set at various temperatures of 150 ° C, 200 ° C, 250 ° C, 300 ° C and 350 ° C, and the holding time was set to 1 hour. After the tempering treatment, the impact characteristics were investigated. On the other hand, the impact characteristics were measured using the new test method shown in Figs. 1 and 2. The obtained results are shown in Fig. The impact value was the highest when the Nb content was 0.010 mass% when the tempering temperature was 200 ° C or higher.

From FIG. 6, the tempering temperature at which the impact value: 5 J / cm ² is obtained is determined and is shown in FIG. 7 in relation to the Nb content. 7, the tempering temperature at which the impact value: 5 J / cm ² is obtained is the lowest in the case of the steel sheet having the Nb content of 0.010 mass%. If the Nb content is increased beyond the 0.020mass%, the impact value: this is 5J / cm ² is obtained by the tempering temperature is the high temperature side. When the tempering temperature becomes high, the hardness decreases and the durability as a needle decreases. When the Nb content is less than 0.005% by mass, it has been found that it is necessary to set the tempering temperature to a high temperature in order to secure a desired impact value.

In Fig. 5 and Fig. 7, in order to combine high hardness after tempering and excellent impact characteristics, the Nb content is 0.005 mass% and the upper limit is 0.020 mass%. In order to shorten the heating and holding time of the solution treatment, the upper limit of the Nb content is preferably 0.015 mass%.

The present invention is based on this finding and is further complicated by further studies. That is, the gist of the present invention is as follows.

The steel sheet has a chemical composition of 0.85 to 1.10 mass% of C, 0.50 to 1.0 mass% of Mn, 0.10 to 0.35 mass% of Si, 0.030 mass% or less of P, 0.030 mass% or less of S, 0.45mass%, Nb: containing 0.005~0.020mass%, and the balance Fe and inevitable impurities, is made from, and the average grain size of the carbide dispersed in the steel sheet (d _av) and nodularity (N _SC / _TC N) × 100% Satisfy the following formulas (1) and (2), respectively, and the thickness of the steel sheet is less than 1.0 mm.

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0.2? D _av ≤ 0.7 (μm) ... (One)

(N _SC / N _TC ) x 100 > 90% (2)

Here, the average grain diameter (d _av ) of the formula (1) is an average value of the diameters of the respective circles (circle equivalent diameter) when assuming a circle having an area equivalent to that of the individual carbides observed in the cross section of the steel sheet.

N _TC and N _SC in the formula (2) are the number of carbides satisfying the condition of N _TC : total number of carbides per 100 μm ² observation area and N _SC : d _L / d _S of 1.4 or less, a major diameter d of the _L, and the minor axis d to the _S.

[2] The chemical composition according to the above [1], wherein the chemical composition further contains one or two kinds selected from Mo and V, and the content of each of them is 0.001 mass% or more and less than 0.05 mass% Cold rolled steel sheet.

[3] A method for producing a high carbon cold-rolled steel sheet by repeatedly performing cold rolling and spheroidizing annealing on a hot-rolled steel sheet having the chemical composition as described in [1] or [2] _{Characterized in that the} average particle diameter (d _av ) and the spheroidization ratio (N _SC / N _TC ) satisfy the following formulas (1) and (2) and the plate thickness of the high carbon cold- A method for manufacturing a cold rolled steel sheet.

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0.2? D _av ≤ 0.7 (μm) ... (One)

(N _SC / N _TC ) x 100 > 90% (2)

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

[5] The method for producing a high-carbon cold-rolled steel sheet according to [3] or [4], wherein the reduction rate of the cold rolling is 25 to 65% and the temperature of the spheroidizing annealing is 640 to 720 ° C.

The high-carbon cold-rolled steel sheet according to the present invention is a thin, high-carbon, cold-rolled steel sheet having a plate thickness of less than 1.0 mm, particularly a plate thickness of 0.4 to 0.7 mm. The average grain size of the carbide is controlled to be 0.2 to 0.7 m, It is a steel plate whose rate is controlled to 90% or more. (Impact value: 5 J / cm ² or more) and hardness characteristics (600 (hardness)) are obtained by quenching and low-temperature tempering heat treatment even with a short time of 3 to 15 minutes of heat treatment treatment, To 750 HV) is obtained.

The high-carbon cold-rolled steel sheet according to the present invention is quenched after a short time of solution treatment to obtain a martensitic phase containing an inevitable residual? Phase, and then subjected to so-called low temperature tempering at 200 to 350 占 폚. Of the high-carbon cold-rolled steel sheet exhibits a clear superiority in terms of balance of hardness and impact properties (toughness). That is, by using the high carbon cold-rolled steel sheet according to the present invention, it is possible to obtain a high-carbon steel-made mechanical tool part having excellent toughness after tempering tempering while ensuring excellent quenchability. In particular, the cold-rolled steel sheet disclosed in the present invention is suitable for applications requiring excellent durability under a severe use environment such as knitting needles, as well as a balance between hardness and toughness as well as wear resistance and endurance.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is an explanatory view showing an example of a test apparatus for an impact test used in the evaluation of the present invention. Fig.
Fig. 2 is an explanatory view showing the shape of a test piece of an impact test used in the evaluation of the present invention. Fig.
3 is a graph showing the relationship between the quenching hardness and the heating holding time of the solution treatment (heating temperature: 800 ° C).
4 is a graph showing the relationship between the quenching hardness and the heating holding time of the solution treatment (heating temperature: 780 DEG C).
5 is a graph showing the relationship between the heating holding time and the Nb content in the solution treatment for obtaining a quenching hardness of 700 HV.
6 is a graph showing the relationship between the impact value and the tempering temperature.
7 is a graph showing the relationship between the tempering temperature at which the impact value: 5 J / cm ² is obtained and the Nb content.

Hereinafter, embodiments of the present invention will be described.

First, the steel sheet according to the present invention is obtained as a high carbon cold rolled steel sheet having a sheet thickness of less than 1.0 mm by alternately repeating the cold rolling and the spheroidizing annealing by performing softening annealing as necessary. Thereafter, the high-carbon and cold-rolled steel sheet is subjected to a predetermined secondary treatment and solution treatment, quenching and tempering treatment to provide it to members (machine parts) such as knitting needles.

First, the chemical composition of the steel sheet of the present invention is adjusted so that the chemical composition of C: 0.85-1.10 mass%, Mn: 0.50-1.0 mass%, Si: 0.10-0.35 mass%, P: 0.030 mass% or less, 0.35 to 0.45 mass% and Nb: 0.005 to 0.020 mass% will be described below.

C: 0.85 to 1.10 mass%

C is an essential element for obtaining sufficient hardness after heat treatment of a high carbon cold-rolled steel sheet. Its lower limit is determined so that a hardness of 600 to 750 HV can be secured in precision parts such as knitting needles and its upper limit value can be controlled by the amount of carbonization at a level that does not inhibit various types of cold working. That is, the lower limit was specified to be 0.85% by mass in order to secure 600HV hardness stably in the quenching tempering treatment for a short time. The upper limit is defined as 1.10% by mass as an upper limit capable of withstanding plasticity over various surfaces including swaging, swaging, bending, and cutting. Cold rolling and spheroidizing annealing are repeated so that the spheroidizing treatment of the carbide improves the cold workability. However, when C is more than 1.10 mass%, the rolling load in the hot rolling step and the cold rolling step becomes high, and the frequency of the crack at the coil end portion becomes remarkably high, . For this reason, C is specified in the range of 0.85 to 1.10 mass%. On the other hand, it is preferably 0.95 to 1.0 mass%.

Mn: 0.50 to 1.0 mass%

Mn is an element effective for deoxidation of steel and is an element capable of stably obtaining a predetermined hardness by improving the hardenability of steel. When the high carbon steel sheet is applied to a severe use, the effect of the present invention becomes remarkable at not less than 0.50 mass%. Therefore, the lower limit was specified as 0.50% by mass. On the other hand, if it exceeds 1.0% by mass, MnS is precipitated and coarsened in the hot rolling, so that cracks and the like occur frequently during part processing. Therefore, the upper limit value is defined as 1.0% by mass. For this reason, Mn is specified in the range of 0.50 to 1.0 mass%. On the other hand, it is preferably 0.50 to 0.80 mass%.

Si: 0.10 to 0.35 mass%

Since Si is a deoxidizing element of steel, it is an effective element in solventing clean steel. Further, Si is an element having tempering softening resistance of martensite. For this reason, the lower limit is specified as 0.10 mass%. Further, when a large amount is added, the tempering of the martensite in the low temperature tempering becomes insufficient and the impact characteristics are deteriorated. Therefore, the upper limit value is specified to be 0.35 mass%. For this reason, Si is specified in the range of 0.10 to 0.35 mass%.

P: not more than 0.030 mass%, S: not more than 0.030 mass%

P and S are inevitably present in the steel as impurity elements and all have an adverse influence on the impact properties (toughness), so that it is preferable to reduce them as much as possible. The content of P up to 0.030 mass% and the content of S up to 0.030 mass% do not pose any practical problems. For this reason, P is defined as 0.030 mass% or less, and S is defined as 0.030 mass% or less. On the other hand, in order to maintain a better impact characteristic, it is preferable that P is contained up to 0.020 mass% and S is up to 0.010 mass%.

Cr: 0.35 to 0.45 mass%

Cr is an element which improves the hardenability of steel, but it is dissolved in carbide (cementite) to retard redissolving of the carbide in the heating step. Therefore, the upper limit of Cr was specified to be 0.45 mass%. From the balance between hardness and impact properties after tempering tempering, the lower limit of Cr was specified to be 0.35% by mass. For this reason, Cr is specified in the range of 0.35 to 0.45 mass%.

Nb: 0.005 to 0.020 mass%

It is known that Nb is an element which has conventionally been expanded in the temperature recrystallization temperature region of steel during hot rolling and at the same time precipitated as NbC to contribute to miniaturization of austenite lips. For this reason, even high carbon steels may be added in anticipation of the effect of making the structure finer after the cold rolling step. In the present invention, 0.005 to 0.020 mass% of Nb is added for the main purpose of toughness recovery by tempering at low temperature after quenching. If a trace amount of Nb is added, NbC is not formed to such an extent that it contributes to the miniaturization of the structure, and Nb is in a lean state. It is considered that the diffusion of C in the ferrite phase and the martensitic phase, which are the BCC structures, is promoted by the fact that Nb is in the lean state. That is, during the heating in the quenching treatment, the diffusion of C from the spherical carbide into the ferrite phase into the austenite phase and the diffusion and precipitation of the supersaturated solution C in the martensite phase during heating in the tempering process are promoted. As a result, it is considered at the present time that improvement in the hardenability of the short-time heating furnace and recovery of the toughness by the low-temperature tempering treatment can be achieved at the same time. If Nb is added in an amount exceeding 0.020 mass%, the precipitation of NbC becomes remarkable, the lean solid state of Nb can not be secured, and the promoting effect of diffusion of C due to the lean solid state of Nb can not be confirmed. Therefore, the upper limit of the amount of Nb added is specified to 0.020 mass%. On the other hand, it is preferably 0.015 mass% or less. On the other hand, if the addition amount of Nb is less than 0.005% by mass, the above effects can not be expected. For this reason, the lower limit of the amount of Nb added is specified to be 0.005 mass%. For this reason, Nb is specified in the range of 0.005 to 0.020 mass%.

Although the above-mentioned components are basic components, in the present invention, as optional elements, if necessary, they may further contain one or two selected from Mo and V.

Mo and V are inevitably contained in an amount of less than 0.001% by mass of Mo and less than 0.001% by mass of V, respectively. In addition, in the present invention, as an optional element, it is possible to add Mo and V inevitably more than the level that contains Mo and V in order to improve the hardenability and the impact property after tempering. However, when Mo or V is added in a predetermined amount or more, the addition effect of Nb is lost. Therefore, in order to maximize the effect of addition of Nb, it is preferable to limit the content of Mo and V to the following range.

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

Mo is an element effective for improving the hardenability of steel, but when the amount is large, the impact characteristics may deteriorate at low temperature tempering at 200 to 350 캜. Therefore, when it is added, it is specified that it is 0.001 mass% or more higher than the level inevitably contained and less than 0.05 mass%, which is a range not hindering impact characteristics. On the other hand, the addition of Mo is preferably 0.01 to 0.03 mass%.

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

V is an element effective for improving the impact characteristics by making the steel structure finer, but it is an element which may deteriorate the hardenability. Therefore, when it is added, it is specified that it is more than 0.001 mass% higher than the level inevitably contained and less than 0.05 mass% in the range not hindering the hardenability. On the other hand, the addition of V is preferably 0.01 to 0.03 mass%. The remainder other than the above-mentioned components are Fe and inevitable impurities.

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

In the high carbon cold-rolled steel sheet of the present invention, it is necessary that the average grain size (d _av ) and the spheroidization ratio (N _SC / N _TC ) of the carbide dispersed in the steel sheet satisfy the following formulas (1) and (2), respectively.

0.2? D _av ≤ 0.7 (μm) ... (One)

(N _SC / N _TC ) x 100 > 90% (2)

Here, the average particle diameter (d _av ) (μm) of the formula (1) is an average value of the diameters of the respective circles (circle equivalent diameter) when a circle having an area equivalent to that of individual carbides observed on the cross section of the steel sheet is assumed. When the average particle diameter (d _av ) is within this range, the impact characteristics are excellent, and the desired quenching hardness can be easily achieved even with a short-time solution treatment. If the average particle diameter (d _av ) is less than 0.2 탆, the load during the secondary working, which is machining to the needle shape, increases. On the other hand, when the average particle diameter exceeds 0.7 탆, the desired hardening improvement It is not preferable because it becomes difficult.

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

Since the carbide is not necessarily formed into a complete sphere and is often observed as an ellipse by the observation plane, the degree of spheroidization is defined by the ratio of the major axis to the minor axis (d _L / d _S ). In view of such circumstances, in the present invention, the carbide satisfying the condition of d _L / d _S : 1.4 or less is regarded as spheroidized and N _SC , the number of carbides, is defined. Further, the reason why the spheroidization ratio (N _SC / N _TC ) x 100 is 90% or more is because the inventors have found that the secondary working property of the steel sheet becomes better in this range.

The measurement of the average particle size and the spheroidization ratio of the carbide described above was carried out by observing the secondary electron image at a magnification of 2,000 times using a scanning electron microscope.

The carbide was cut from the plate test specimens in a direction perpendicular to the rolling direction of the sample before the heat treatment using the cold-rolled steel sheet, and subjected to a process such as resin filling. In the range of the observation area 100 μm ² near the plate thickness center, d _L / d _S ratio, N _TC and N _SC were measured, and the average value of the five visual fields was calculated. These measurements and calculations were made using commercially available image analysis software "winroof" (trade name).

Next, a method of manufacturing a steel sheet according to the present invention will be described.

The hot-rolled steel sheet used in the present invention may be any one obtained under ordinary production conditions. For example, the steel slab having the chemical composition described above can be produced by heating at 1050 to 1250 占 폚, hot rolling at a finishing temperature of 800 to 950 占 폚, and coiling at a coiling temperature of 600 to 750 占 폚. On the other hand, the thickness of the hot-rolled steel sheet may be appropriately set so that the cold-rolled steel sheet has a proper cold-rolled reduction ratio from the desired thickness of the cold-rolled steel sheet.

Cold rolled (25 to 65%) and spheroidizing annealing (640 to 720 ° C) are repeated a plurality of times to produce a high carbon cold rolled steel sheet having a thickness of less than 1.0 mm. The cold rolling (25 to 65%) and the spheroidizing annealing (640 to 720 ° C) are preferably repeated two to five times, respectively.

In the present invention, cold rolling (25 to 65%) and spheroidizing annealing (640 to 720 캜) are repeated a plurality of times. The reason for this is to control the average particle diameter (d _av ) of the carbide and the spheroidizing rate (N _SC / N _TC ) × 100 to satisfy the above-mentioned formulas (1) and (2) .

First, the carbide is cracked by cold rolling, and the carbide starts to be broken by spheroidization annealing. However, it is difficult to increase the spheroidization ratio of carbide to 90% or more only by the number of times of performing the spheroidizing annealing once, so that the carbide in the form of a bar or plate remains. In such a case, the hardenability is adversely affected and the cold workability of the precision parts is deteriorated. Therefore, it is optimal to alternately perform cold rolling and spheroidizing annealing in order to make the spheroidization ratio (N _SC / N _TC ) of carbide 90% or more. As a result, a distribution of carbide having a high degree of spheroidization is obtained in the steel sheet.

Particularly preferred are cold rolling 2 to 5 times and spheroidizing annealing 2 to 5 times.

When the steel sheet (cold-rolled steel sheet) having a cold rolling reduction ratio of less than 25% is subjected to spheroidizing annealing, the carbide is coarsened. On the other hand, if the cold rolling reduction ratio exceeds 65%, the load of the cold rolling work may be excessively large. For this reason, the cold rolling reduction ratio is preferably in the range of 25 to 65%.

On the other hand, in the final cold rolling, since the spheroidizing annealing is not performed after the cold rolling, the lower limit of the reduction rate is not particularly limited.

If the spheroidizing annealing temperature is lower than 640 캜, the spheroidization tends to become insufficient, and if the spheroidizing annealing is repeated at a temperature higher than 720 캜, the carbide tends to be coarsened. Therefore, the spheroidizing annealing temperature is preferably in the range of 640 to 720 캜. The retention time of the spheroidizing annealing can be appropriately selected within the range of 9 to 30 hours at this temperature range.

On the other hand, the same temperature range is preferable for the softening annealing for softening the hot-rolled steel sheet before cold rolling.

As described above, the method for manufacturing a high-carbon cold-rolled steel sheet according to the present invention is preferably performed after the steel sheet is processed into a predetermined shape and then subjected to the following heat treatment, in order to machine parts such as knitting needles.

The high carbon cold-rolled steel sheet having 90% or more spheroidized carbides distributed therein is processed into various machine parts (press processing, phrase processing, swaging processing, etc.), solution treatment, quenching (quenching) and then tempering treatment . In the solution treatment, the heating temperature is 760 to 820 DEG C and the holding time is 3 to 15 minutes which is a short time. It is preferable to use oil for quenching (quenching). In the tempering treatment, the tempering temperature is preferably 200 to 350 캜. More preferably, it is 250 to 300 캜. As a result, various mechanical parts having a hardness of 600 to 750 HV can be manufactured.

If the holding time of the solution treatment is longer than 15 minutes, the carbide is excessively penetrated and the austenite grains are coarsened, so that the martensitic phase after quenching becomes rough and the impact characteristics deteriorate. Therefore, the upper limit of the holding time of the solution treatment is preferably 15 minutes. On the other hand, if it is shorter than 3 minutes, the penetration of the carbide becomes insufficient and it becomes difficult to quench. Therefore, the lower limit of the holding time of the solution treatment is preferably 3 minutes. More preferably 5 to 10 minutes.

If the tempering temperature is lower than 200 DEG C, the toughness recovery on the martensite is insufficient. On the other hand, when the tempering temperature exceeds 350 DEG C, the impact value is recovered, but the hardness is lower than 600 HV, so that durability and abrasion resistance become a problem. Therefore, an appropriate range of the tempering temperature is preferably 200 to 350 캜. On the other hand, it is more preferably 250 to 300 캜. The holding time of the tempering can be appropriately selected in the range of 30 minutes to 3 hours.

Example

The steel having various chemical compositions was vacuum-melted and cast into a 30 kg steel ingot. This steel ingot was rolled and then hot rolled under the conditions of a heating temperature of 1150 占 폚 and a finishing temperature of 870 占 폚 to obtain hot-rolled steel sheets of 4 mm and 2 mm. Thereafter, cold rolling and spheroidizing annealing were conducted under the manufacturing conditions shown in Table 1 to obtain a cold-rolled steel sheet having a thickness of 0.4 mm or more and less than 1.0 mm. Subsequently, this cold-rolled steel sheet was subjected to solution treatment (charging in a furnace at 800 DEG C for 10 minutes) under the conditions shown in Table 2, followed by oil quenching and tempering (tempering temperature: 250 DEG C).

A predetermined test piece was taken from the tempered steel sheet and subjected to an impact test and a hardness measurement test. The hardness was measured in accordance with JIS Z 2244 under the condition of a load of 5 kg (test force: 49.0 N) with a Vickers hardness tester.

The impact characteristics were evaluated by the Charpy impact test. The impact test specimen was a U-notch test piece having a notch width of 0.2 mm (notch depth: 2.5 mm, notched radius: 0.1 mm). Fig. 1 shows a state in which a test piece is installed in a test apparatus, and Fig. 2 shows a shape of the test piece. Such a test piece and a test method are adopted for the following reasons.

In the Charpy impact test apparatus for metal materials conventionally used for a steel sheet having a plate thickness of less than 1.0 mm, which is the object of the present invention, the problem that the rated capacity of the test apparatus is excessively over 50 J there was. As the impact test apparatus whose rated capacity of the test apparatus is smaller than 50J, an impact test apparatus of 1J (manufactured by Toyo Seiki Seisaku-Sho, Ltd., type DG-GB) was used. This test apparatus is a Charpy impact tester based on the Charpy impact test method of carbon fiber-reinforced plastic (JIS K 7077). This test apparatus was modified so that the distance between supports was set to 60 mm to 40 mm. In this test apparatus, the distance between the supports is set to 60 mm to 40 mm in order to set the conditions close to the JIS standard (JIS Z 2242), which is the Charpy impact test method of the metal material.

As shown in Fig. 2, the test piece was a test piece having a notch depth of 2.5 mm, a notch radius of 0.1 mm (notch width of 0.2 mm), and a U-notch formed by electrical discharge machining. The reason why the notch radius is reduced is that when the plate is thinner than 1.0 mm in the Charpy impact test, the plate warping becomes a problem. Therefore, by increasing the stress concentration factor, the warpage of the plate at the Charpy impact test is minimized, To get. By adopting the test method and test piece shape, it has been confirmed that the impact characteristics in a state close to the actual use environment can be obtained. In the present invention, when the value of the impact value is 5 J / cm ² or more, it is determined that the impact property is excellent.

(Example 1)

After the solution treatment, the effect of various additive elements on oil hardening and tempering and on the section hardness and impact value was confirmed. The test results are shown in Tables 3 and 4 together with the chemical components. The conditions for producing cold-rolled steel sheets were 5A (Table 1). The reduction ratio was controlled to the range shown in Table 1. [

The section hardness was measured at the center of the plate thickness by burying a test piece cut in the direction perpendicular to the rolling direction into the resin and polishing the cross section. Impact values were measured using impact test specimens taken in the rolling direction. The obtained results (hardness, impact value) are shown in Tables 3 and 4.

A " when the impact value was greater than 5 J / cm < ² > and the hardness was between 600 and 750 HV was rated as "

In the example shown in Table 3, the impact value and the quenching tempering hardness were out of the target value in the case where the C amount was out of the lower limit value (steel type No. 1). The hardened tempering hardness exceeding the target value of 600 to 750 HV and the impact value below the target value of 5 J / cm ² were those in which the C content exceeded the upper limit value (grade 6). Containing no Nb is, it is 0.85mass% amount of C (steel grade No. 2, comparative example), it is 1.10mass% amount of C (steel grade No. 4, comparative example) value 5J / cm ² both the target impact value , And the evaluation was x. On the other hand, the steel plates (grade Nos. 3, 5, 7, 8, 9 and 10) corresponding to the chemical composition of the inventive example were excellent in quenching tempering hardness within the target range and impact characteristics.

In the example shown in Table 4, the steel plates (steel types Nos. 15, 16, 17, 19, and 21) of the chemical composition corresponding to the inventive example all satisfied the quenching tempering hardness of the target value of 600 to 750 HV, . Those having no addition of Nb (grade No. 11), those having an addition amount of V exceeding 0.05 mass% (grade number 12) without addition of Nb, those having an addition amount of Mo exceeding 0.05 mass% (Steel grade No. 13), the addition of Nb + Mo composite, the addition amount of Nb being less than 0.005% by mass (grade number 14), the addition of Nb + Mo composite and the addition amount of Nb exceeding 0.020% ), A combination of Nb + Mo combined with Mo added in an amount of more than 0.05% by mass (Steel No. 20), a combination of Nb + Mo + V and a V content of more than 0.05% by mass (Steel No. 22) If the tempering hardness satisfies the target value of 600 to 750 HV but the impact characteristic is inferior or the impact characteristic satisfies the target value of 5 J / cm ² , the quenching tempering hardness is lowered, or the quenching tempering hardness and impact property together with the target value Of the total.

(Example 2)

Grade Nr. The cold-rolled steel sheet having the chemical composition shown in Table 3 was used to change the production conditions of the cold rolling and the spheroidizing treatment shown in Table 1 to obtain a cold-rolled steel sheet having a thickness shown in Table 5. Table 5 shows the spheroidization ratio and average carbide grain size of the obtained cold-rolled steel sheet. The obtained cold-rolled steel sheet was subjected to solution quenching treatment and oil quenching and low temperature tempering under the conditions shown in Table 2 in the same manner as in Example 1. [ The hardness and impact value of the obtained cold-rolled steel sheet after quenching and tempering after solution treatment were measured in the same manner as in Example 1, and are shown in Table 5.

When the number of times of spheroidizing annealing was one (manufacturing condition No. 1), the spheroidization rate was insufficient and the impact characteristics were inferior. When the number of times of spheroidizing annealing was 2, the spheroidizing annealing temperature was 600 to 635 캜, and the cold rolling reduction rate was 10 to 20%, respectively, resulting in insufficient spheroidization and inferior impact properties (Manufacturing Conditions No. 2A ). When the sintering temperature is 600 to 635 캜 and the cold rolling reduction rate is 70 to 85%, the sintering temperature is 600 to 635 캜 and the steel material is annealed twice. (Manufacturing Conditions No. 2C).

When the sintering temperature was 640 to 720 캜 and the cold rolling reduction rate was 10 to 20%, the sintering was repeated twice. However, the sintering was sufficient, but the average grain size of the carbides exceeded the upper limit of the target value, (Production Conditions No. 2D). This is because when the carbide is too large, the un-dissolved carbide of the martensite is largely shifted at the time of quenching and the area of the interface between the un-dissolved carbide and the martensite base, which is likely to be a starting point at the time of fracture, . On the other hand, when the spheroidizing annealing temperature is 640 to 720 占 폚 and the cold rolling reduction ratio is 25 to 65%, the spheroidizing rate, the carbide particle diameter and the hardness after quenching tempering fall within the target values, respectively, (Production Condition No. 2B).

When the rounding and annealing times were 4, the sphericalization ratio and the carbide particle diameter were within the target value range and the impact characteristics were excellent when the cold rolling reduction rates of the first to fourth rounds were all set to 25 to 65% 5A). Production condition No. 5A and the spheroidizing annealing temperature were the same, and the first to fourth cold rolling reduction ratios were all in the range of 10 to 20%, the carbide particle size exceeded the target value and became excessively large, and the impact characteristics were also inferior. 5B).

(Example 3)

Grade Nr. 16 (Table 4) were used to change the production conditions described in Table 1 to obtain a cold-rolled steel sheet having a thickness shown in Table 6. [ Table 6 shows the spheroidization ratio and average carbide grain size of the obtained cold-rolled steel sheet. The obtained cold-rolled steel sheet was subjected to solution quenching treatment and oil quenching and low temperature tempering under the conditions shown in Table 2 in the same manner as in Example 1. [ The hardness and impact value of the obtained cold-rolled steel sheet after quenching and tempering after solution treatment were measured in the same manner as in Example 1 and are shown in Table 6. [

Production conditions corresponding to the production method of the present invention are as follows. 2B, No. The steel sheet subjected to cold rolling and spheroidizing annealing using 5A satisfied the target spheroidization ratio and the target impact value.

The steel sheet having the chemical composition within the scope of the present invention is improved in hardenability by addition of Nb and improves the impact characteristics after heat treatment, and thus is suitable for use in machine tool parts used in overbased quarry and harsh environments.

C of 0.85 to 1.10 mass% is suitable for applications requiring hardness and toughness balance under a severe use environment such as knitting needles.

Claims

The steel sheet has a chemical composition of 0.85 to 1.10 mass% of C, 0.50 to 1.0 mass% of Mn, 0.10 to 0.35 mass% of Si, 0.030 mass% or less of P, 0.030 mass% or less of S, 0.35 to 0.45 mass% , Nb: 0.005 to 0.020 mass%, the balance being Fe and inevitable impurities,
The average particle size of the carbide dispersed in the steel sheet (d _av) and nodularity (N _SC / _TC N) × 100% and each satisfy the following equation (1) and equation (2), the plate thickness of the steel plate is 1.0mm By weight or less.
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0.2? D _av ≤ 0.7 (μm) ... (One)
(N _SC / N _TC ) x 100 > 90% (2)
Here, the average grain diameter (d _av ) of the formula (1) is an average value of the diameters of the respective circles (circle equivalent diameter) when assuming a circle having an area equivalent to that of the individual carbides observed in the cross section of the steel sheet.
N _TC and N _SC in the formula (2) are the number of carbides satisfying the condition of N _TC : total number of carbides per 100 μm ² observation area and N _SC : d _L / d _S of 1.4 or less, a major diameter d of the _L, and the minor axis d to the _S.

The method according to claim 1,
Wherein the chemical composition further contains one or two kinds selected from Mo and V, and the content of both of them 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 by repeatedly performing cold rolling and spheroidizing annealing on a hot-rolled steel sheet having the chemical composition according to any one of claims 1 to 3,
Wherein 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 satisfy the following formulas (1) and (2) Is less than 1.0 mm. &Lt; RTI ID = 0.0 > 11. < / RTI >
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0.2? D _av ≤ 0.7 (μm) ... (One)
(N _SC / N _TC ) x 100 > 90% (2)
Here, the average grain diameter (d _av ) of the formula (1) is an average value of the diameters of the respective circles (circle equivalent diameter) when assuming a circle having an area equivalent to that of the individual carbides observed in the cross section of the steel sheet.
N _TC and N _SC in the formula (2) are the number of carbides satisfying the condition of N _TC : total number of carbides per 100 μm ² observation area and N _SC : d _L / d _S of 1.4 or less, a major diameter d of the _L, and the minor axis d to the _S.

The method of claim 3,
Wherein the number of times of cold rolling and spheroidizing annealing is repeatedly performed on the hot-rolled steel sheet is 2 to 5 times.

The method of claim 3,
Wherein the reduction ratio of the cold rolling is 25 to 65% and the temperature of the spheroidizing annealing is 640 to 720 캜.

5. The method of claim 4,
Wherein the reduction ratio of the cold rolling is 25 to 65% and the temperature of the spheroidizing annealing is 640 to 720 캜.