JPWO2021090472A1 - High carbon cold rolled steel sheet and its manufacturing method and high carbon steel machine parts - Google Patents

Abstract

After quenching (quenching) treatment and low-temperature tempering treatment (quenching and tempering treatment) after a short-time solution heat treatment, it is possible to have good impact characteristics and hardness characteristics, as well as excellent wear resistance, and tempering and tempering. Provided is a high carbon cold-rolled steel sheet having a plate thickness of less than 1.0 mm, which has little deterioration in secondary workability before processing. By mass%, C: 0.85 to 1.10%, Mn: less than 0.60%, Si: 0.10 to 0.35%, P: 0.030% or less, S: 0.030% or less, Cr: less than 0.60%, and Mn + Cr is less than 1.0% Further, a high carbon cold-rolled steel sheet containing Nb: 0.005 to 0.020 mass% and having a steel sheet chemical composition consisting of the balance Fe and unavoidable impurities is obtained. As a result, there is less deterioration in secondary workability before quenching and tempering compared to conventional steel materials. In addition, by forming a steel plate structure with an average particle size of carbides of 0.2 to 0.7 (μm) and a spheroidization rate of 90% or more, the impact value is 9 J / even even by quenching and tempering for a short time of 3 to 15 min. It is possible to make mechanical parts with excellent impact characteristics of cm2 or more, sufficient hardness characteristics in the range of 600 to 750 HV, and excellent wear resistance.

Description

The present invention relates to a high carbon cold rolled steel sheet used as a material for various machine parts manufactured by quenching and tempering, and a method for manufacturing the high carbon cold rolled steel sheet, and particularly relates to a high carbon cold rolled steel sheet having a thickness of less than 1.0 mm, which is applied to knitting needles and the like. It is a thing.

Generally, carbon steel materials for machine structure (SXXC) and carbon tool steel materials (SK) specified in JIS are used for various large and small machine parts. When used as a wrought material, it is given a predetermined hardness and toughness (impact characteristics) by quenching and tempering after forming a part shape through punching and various plastic workings. Among them, the knitted needle that knits the knitted fabric has sufficient strength and abrasion resistance for the butt part of the needle body that comes into contact with the rotary drive part because the knitted fabric is knitted by pulling the yarn while repeating the reciprocating motion at high speed. Further, the hook portion that rubs against the thread is required to have sufficient wear resistance and excellent impact characteristics of the tip portion due to the reciprocating motion.

The high carbon cold-rolled steel sheet used as a material for knitting needles is suitable for knitting needles for flat knitting machines when the plate thickness is 1.0 mm or more, and for circular knitting machines or vertical knitting when the plate thickness is less than 1.0 mm. Used for machine knitting needles. Since the latter needle knits a small diameter thread at high speed, the thickness of the material used is often 0.4 to 0.7 mm. Further, in addition to excellent cold workability (also referred to as secondary workability), the material has sufficient hardness at the needle tip and sufficient toughness when quenching and tempering after secondary processing into a needle shape. Is required.

Further, so-called high carbon steel sheets such as carbon steel materials for machine structure (SXXC) and carbon tool steel materials (SK) specified in JIS are classified in detail according to the amount of C. In a region where the amount of C is less than 0.8% by mass, that is, in a steel sheet having a sub-eutectic composition, the ferrite phase fraction is high, so that it is excellent in cold workability, but it is difficult to obtain sufficient quenching hardness, and it is difficult to obtain sufficient quenching hardness. It is not suitable for knitting needle applications where wear resistance and durability of the needle body are required. On the other hand, among the hypereutectoid compositions of 0.8% by mass or more, high carbon steel sheets having a C content of more than 1.1% by mass have excellent hardenability, but the cold workability is extremely high due to the carbide (cementite) contained in a large amount. It is not suitable for knitting needles that perform precise and fine machining such as grooving, and is limited to parts such as cutting tools and cold dies that require high hardness with a simple shape.

Conventionally, for knitting needles, carbon tool steels and alloy tool steels having a C: 0.8 to 1.1% by mass, or materials having a steel composition based on these steel compositions and to which a third element is added have been widely used. In the manufacturing process of this knitting needle, the material is subjected to a wide variety of plastic working such as punching (shearing), cutting, wire drawing, caulking, and bending. Therefore, this material for manufacturing a knitted needle is required when it is actually used as a needle, in addition to having sufficient workability (secondary workability) at the time of material processing in the needle manufacturing process. It is necessary to have hardness characteristics and impact characteristics (toughness) after quenching and tempering.

In the manufacture of knitting needles, the material is quenched and tempered in order to ensure a predetermined hardness characteristic. The temperature of this tempering process is generally as low as 200 to 350 ° C. However, if the content of Mn and Cr, which is effective for hardenability, is increased with an emphasis on hardness characteristics, or if a large amount of other third element is contained, the above-mentioned temperature range of 200 to 350 ° C. In the low-temperature tempering treatment, the martensite phase was not sufficiently tempered, and the impact characteristics (toughness) were not sufficiently improved, or the toughness value was sometimes varied.

On the other hand, for the purpose of improving the impact characteristics of knitting needles, P and S, which are impurity elements in the chemical composition of the material, are reduced, and grain boundary segregation of P and formation of inclusions (MnS) are suppressed. It is also considered to be an effective measure to reduce the adverse effects of elements. However, from the viewpoint of steelmaking technology and economy, there is a limit to reducing P and S to improve the impact characteristics of knitting needles.

Further, it has been conventionally known that miniaturization of a metal structure is effective as a means for improving impact characteristics.

For example, Patent Document 1 describes "a high carbon steel sheet having excellent hardenability, fatigue characteristics, and toughness, and a method for producing the same". The high carbon steel sheet described in Patent Document 1 has a mass% of C: 0.5 to 0.7%, Si: 0.5% or less, Mn: 1.0 to 2.0%, P: 0.02% or less, S: 0.02% or less, Al: With a component composition containing 0.001 to 0.10%, one or more of V: 0.05 to 0.50%, Ti: 0.02 to 0.20%, Nb: 0.01 to 0.50%, and the balance being Fe and unavoidable impurities. A high carbon steel sheet having a structure in which carbides having a spheroidization rate of 95% or more and a maximum particle size of 2.5 μm or less are dispersed. In the technique described in Patent Document 1, fine carbonitrides are formed by adding V, Ti, and Nb, which are carbonitride forming elements, to subeutectoid steel, and these fine carbonitrides are formed. It is said that the effect of pinning will be used to refine the old austenite grains to improve toughness.

Further, Patent Document 2 describes "a high carbon steel member having excellent impact characteristics". The high carbon steel member described in Patent Document 2 has a mass% of C: 0.60 to 1.30%, Si: 1.0% or less, Mn: 0.2 to 1.5%, P: 0.02% or less, S: 0.02% or less, Fe. : Substantially has the composition of the balance, and the matrix after quenching and tempering is based on the following formula.
8.5 <15.3 x C% -Vf <10.0
Undissolved carbide remains at a volume ratio of Vf (volume%) that satisfies the above, and undissolved carbide with a particle size of 1.0 μm or more is restricted to 2 or less per ^{100 μm 2 observation area.} It is a steel member. In the high carbon steel member described in Patent Document 2, in addition to the above composition, Ni: 1.8% or less, Cr: 2.0% or less, V: 0.5% or less, Mo: 0.5% or less, Nb in mass%. : 0.3% or less, Ti: 0.3% or less, B: 0.01% or less, Ca: 0.01% or less may be contained in one or more. The technique described in Patent Document 2 targets steels having a wide carbon content from sub-eutectic to hyper-eutectic, but the impact value is 25 J / cm ² or more while maintaining the target hardness: 600 to 900 HV. It is said that a high carbon steel member exhibiting excellent impact characteristics can be obtained.

Further, Patent Document 3 describes "high carbon cold-rolled steel sheet and its manufacturing method". The high carbon cold-rolled steel sheet described in Patent Document 3 has a mass% of C: 0.85 to 1.10%, Mn: 0.50 to 1.0%, Si: 0.10 to 0.35%, P: 0.030% or less, S: 0.030% or less. , Cr: 0.35 to 0.45% Nb: containing 0.005 to 0.020 percent, the balance being Fe and inevitable impurities, an average particle size (d _av) and spheroidization ratio of carbides dispersed in the steel sheet (N _SC / N _TC ) × 100% is the following equation (1), respectively.
0.2 ≤ d _av ≤ 0.7 (μm)… (1)
And the following equation (2)
(N _SC / N _TC ) × 100 ≧ 90%… (2)
It is a cold-rolled steel sheet that meets the above requirements and has a thickness of less than 1.0 mm. In addition, in the technique described in Patent Document 3, in addition to the above-mentioned composition, one or two kinds selected from Mo and V are further contained, and the content of each is 0.001% or more and less than 0.05%. It is said that it is preferable. According to the technique described in Patent Document 3, the content of Nb: 0.005 to 0.020% is effective for improving the hardenability and impact characteristics (toughness) after the short-time solution treatment and the low-temperature tempering treatment.

Further, Patent Document 4 describes "abrasion resistant steel sheet having excellent toughness". The wear-resistant steel plate described in Patent Document 4 has a mass% of C: 0.60 to 1.25%, Si: 0.50% or less, Mn: 0.30 to 1.20%, P: 0.030% or less, S: 0.030% or less, Cr. : 0.30 to 1.50%, Nb: 0.10 to 0.50%, Ti: 0 to 0.50%, Mo: 0 to 0.50%, V: 0 to 0.50%, Ni: 0 to 2.00%, balance Fe and unavoidable impurities It has a metal structure in which cementite particles and carbide particles containing one or more of Nb and Ti are dispersed in a metal substrate of a ferrite phase, and has a cross section parallel to the rolling direction and the plate thickness direction. In the L section), the number density of Nb / Ti carbide particles with a circle equivalent diameter of 0.5 μm or more is 3000 to 9000 particles / mm ² , and the number density of voids with a circle equivalent diameter of 1.0 μm or more is 1250 ^{particles / mm 2} or less. It is a steel plate. The wear-resistant steel sheet described in Patent Document 4 is said to be a steel sheet having both excellent wear resistance and toughness.

Japanese Unexamined Patent Publication No. 2009-24233 Japanese Unexamined Patent Publication No. 2006-63384 Japanese Unexamined Patent Publication No. 2017-36492 Japanese Unexamined Patent Publication No. 2017-190494

The high carbon cold-rolled steel sheet used as a material for knitting needles is required to have sufficient hardness and sufficient impact characteristics (toughness) after quenching and tempering. In recent years, in order to improve productivity, further speeding up of the knitting machine has been required, and as a result, the load applied to the knitting needle has increased, and the knitting needle often breaks or its life is shortened in a shorter time than before. It's a problem. Therefore, there is a demand for knitting needles having improved impact characteristics and wear resistance. It is considered that such a knitted needle can be realized by adding a third element or increasing the amount of alloying elements such as Cr, Mn, and Mo, but the secondary processability in the needle manufacturing process may be hindered. I am concerned. For these reasons, there is a demand for a material for knitted needles that can improve wear resistance and impact characteristics (toughness) after quenching and tempering without lowering the secondary workability.

However, the technique described in Patent Document 1 is difficult to apply to machine parts that require high hardness. The technique described in Patent Document 1 is limited to the composition of sub-eutectic steel, and by adding a carbonitride forming element such as V, Ti, Nb as a third element, fine charcoal thereof is added. It is a technology that is expected to have the effect of improving toughness by refining old austenite grains with nitrides. Further, the technique described in Patent Document 1 is also a technique for improving the moldability of the ferrite matrix because the carbon level is a sub-eutectic composition.

Further, Patent Document 2 shows only an example of steel having a carbon content in the range of 0.67 to 0.81% by mass for the addition of the third elements Mo, V, Ti, Nb and B. It is presumed that in the technique described in Patent Document 2, a third element such as Mo, V, Ti, Nb, and B is added with the intention of improving the characteristics of the subeutectoid steel. Moreover, Patent Document 2 does not describe the action of third elements such as Mo, V, Ti, Nb, and B and their optimization for steels having a carbon content of more than 0.81% by mass.

Further, in Patent Document 1 and Patent Document 2, high carbon cold-rolled steel sheets are subjected to short-time solution heat treatment such as 3 to 15 min, quenching, and low-temperature tempering at 200 to 350 ° C. to obtain desired impact characteristics and predetermined values. There is no description of a technique for advantageously improving hardness.

Further, in the technique described in Patent Document 3, it is effective to contain Nb: 0.005 to 0.020% for quenching after holding the solution for a short time and improving hardenability and impact characteristics (toughness) after low-temperature tempering treatment. However, Patent Document 3 describes the secondary workability of the high carbon cold-rolled steel sheet before quenching (quenching) after holding the solution for a short time and before low-temperature tempering treatment (hereinafter, also referred to as tempering tempering treatment). There is no specific mention. Patent Document 3 describes a high carbon cold-rolled steel sheet that can have both excellent toughness and excellent wear resistance after quenching and tempering, but this high-carbon cold-rolled steel sheet is hardened. There was a problem that the secondary workability before the tempering process was insufficient and it was not possible to meet the recent demand for improvement in productivity.

Further, according to the technique described in Patent Document 4, it is possible to increase both wear resistance and toughness after quenching and tempering in a high carbon cold-rolled steel plate, but secondary processing before quenching and tempering is performed. There is no description about the property, and Patent Document 4 does not mention that the wear resistance and toughness after quenching and tempering can be improved without deteriorating the secondary processability before quenching and tempering.

The present invention solves the above-mentioned problems of the prior art, suppresses deterioration of secondary workability before quenching (quenching) and low-temperature tempering treatment (quenching tempering treatment) after a short time solution treatment, and for a short time. After quenching (quenching) and low-temperature tempering (quenching and tempering) after the solution treatment, the impact value is 9 J / cm ² or more and the hardness is evaluated by an impact test near the plate thickness actually used. It is an object of the present invention to provide a high carbon hardened steel sheet having a thickness of less than 1.0 mm, which satisfies the range of 600 to 750 HV and has excellent wear resistance.

In order to achieve the above object, the present inventors have a relationship between the composition of the high carbon cold-rolled steel sheet, the secondary workability before the quenching and tempering treatment, the hardness after the quenching and tempering treatment, the impact characteristics, and the abrasion resistance. Diligently examined. As a result, from the viewpoint of hardenability, hardness after quenching and low-temperature tempering, impact characteristics, etc., C is limited to the range of 0.85 to 1.10% by mass, which is suitable for knitting needles, and Nb is 0.005 to 0.020% by mass. It was found that it is effective to adjust the average particle size and the degree of spheroidization of carbides and to secure the desired characteristics after quenching and tempering treatment by specifying the above range and implementing a predetermined manufacturing method. .. Further, by adjusting Mn to less than 0.60% by mass and (Mn + Cr) to less than 1.0%, deterioration of secondary workability before quenching and tempering treatment is suppressed, and hardness after quenching and tempering treatment is suppressed. It was found that the impact characteristics (toughness) and abrasion resistance satisfy the desired characteristics.

The present invention has been completed with further studies based on the above findings. That is, the gist of the present invention is as follows.
(1) By mass%, C: 0.85% or more and 1.10% or less, Mn: less than 0.60%, Si: 0.10% or more and 0.35% or less, P: 0.030% or less, S: 0.030% or less, Cr: less than 0.60%, Nb : It contains 0.005% or more and 0.020% or less, satisfies the total of Mn content and Cr content (Mn + Cr) of less than 1.0%, has a steel sheet composition consisting of the balance Fe and unavoidable impurities, and has a steel sheet thickness of 1.0 mm. High carbon cold rolled steel sheet characterized by being less than.
_{(2) In (1), the average particle size (dav} ) and spheroidization rate ( _NSC / _NTC ) × 100% of the carbides having the steel sheet composition and further dispersed in the steel sheet are as follows (1). )formula
0.2 ≤ d _av ≤ 0.7 (μm)… (1)
And the following equation (2)
(N _SC / N _TC ) × 100 ≧ 90%… (2)
(Here, _dav : average value of the equivalent circle diameter of carbides (average particle size μm), _NTC : total number of carbides per 100 μm ² _{observation area, N SC} : per 100 μm ² observation area, (major axis d _L) ) / (Number of carbides satisfying the condition that (minor diameter d _{S) is 1.4 or less)}
A high carbon cold-rolled steel sheet characterized by having a steel sheet structure satisfying the above conditions.
(3) In (1) or (2), instead of the steel sheet composition, C: 0.85% or more and 1.10% or less, Mn: less than 0.60%, Si: 0.10% or more and 0.35% or less, P: 0.030 in mass%. % Or less, S: 0.030% or less, Cr: less than 0.50%, Nb: 0.005% or more and 0.020% or less, and the total Mn content and Cr content (Mn + Cr) is less than 0.90%, and the balance Fe and A high carbon cold-rolled steel sheet characterized by having a steel sheet composition composed of unavoidable impurities.
(4) In any of (1) to (3), the steel sheet composition is further selected from Mo: 0.001% or more and less than 0.05%, V: 0.001% or more and less than 0.05% by mass, or one of them. A high carbon cold-rolled steel sheet characterized by having a steel sheet composition containing two types.
(5) Manufacture of a high carbon cold rolled steel sheet by repeatedly cold rolling and spheroidizing annealing on a hot rolled steel sheet having the steel sheet composition according to any one of (1) to (4) to manufacture a high carbon cold rolled steel sheet. in the method, the high average particle size of the carbide dispersed in the carbon cold-rolled steel sheet and (d _av), spheroidization ratio (N _SC / N _TC) are respectively the following equations (1) and the following equation (2)
0.2 ≤ d _av ≤ 0.7 (μm)… (1)
(N _SC / N _TC ) × 100 ≧ 90%… (2)
(Here, _dav : average value of the equivalent circle diameter of carbides (average particle size μm), _NTC : total number of carbides per 100 μm ² _{observation area, N SC} : per 100 μm ² observation area, (major axis d _L) ) / (Short diameter d _S ) is the number of carbides satisfying the condition of 1.4 or less.)
A method for manufacturing a high carbon cold-rolled steel sheet, which comprises a high-carbon cold-rolled steel sheet having a thickness of less than 1.0 mm.
(6) A method for producing a high carbon cold-rolled steel sheet, wherein the cold rolling and spheroidizing annealing are repeated 2 to 5 times in (5).
(7) In (5) or (6), the high carbon cold-rolled steel sheet is characterized in that the reduction rate of the cold rolling is 25 to 65% and the temperature of the spheroidizing annealing is 640 to 720 ° C. Production method.
(8) Using the high carbon cold-rolled steel sheet according to any one of (1) to (4) as a material, the material is subjected to secondary processing to obtain a machine part having a predetermined shape, and then the machine part is subjected to a short time. A method for manufacturing machine parts that undergoes rapid cooling after solution treatment and tempering, and the process of quenching after short-time solution treatment is performed at a temperature in the range of 760 to 820 ° C. for a time in the range of 3 to 15 min. It is characterized in that it is a mechanical part having excellent wear resistance and excellent toughness as a process of holding and then quenching, and the tempering process of tempering at a temperature in the range of 200 to 350 ° C. How to manufacture high carbon steel machine parts.
(9) A high carbon steel machine part manufactured by the method for manufacturing a high carbon steel machine part according to (8).

The high carbon cold rolled steel sheet of the present invention suppresses deterioration of secondary workability such as machinability, and the life of tools used for punching, swaging, bending, secondary machining, etc. is longer than that of the conventional high carbon cold rolled steel sheet. It has the same level of hardness and impact characteristics as compared to conventional high carbon steel sheets after being subjected to a short-time solution heat treatment and then quenching treatment and low temperature tempering treatment (quenching tempering treatment). In addition, it has a remarkable industrial effect of being able to manufacture mechanical parts that have excellent wear resistance in a well-balanced manner. Further, the high carbon cold-rolled steel sheet of the present invention is excellent in impact characteristics (toughness), wear resistance, and fatigue resistance after quenching and tempering, and in particular, excellent durability in a harsh usage environment such as a knitted needle. It also has the effect of being suitable as a material for machine parts that require toughness.

It is explanatory drawing which shows the outline of the end mill processing test (secondary processability evaluation test) schematically. It is explanatory drawing which shows the outline of the wear test apparatus schematically. It is explanatory drawing which shows the outline of (a) the shape of the wear test piece, (b) the wear state of the wear test piece schematically. It is explanatory drawing which shows the outline of the outline of the Charpy impact test piece shape used in this invention. It is explanatory drawing which shows typically the installation state of the test piece in the Charpy impact tester used in this invention.

The high carbon cold-rolled steel sheet of the present invention has C: 0.85% or more and 1.10% or less, Mn: less than 0.60%, Si: 0.10% or more and 0.35% or less, P: 0.030% or less, S: 0.030% or less, Cr. : Less than 0.60%, Nb: 0.005% or more and 0.020% or less, and the total of Mn content and Cr content (Mn + Cr) is less than 1.0%, and has a steel sheet composition consisting of the balance Fe and unavoidable impurities. It is a high carbon cold-rolled steel sheet with a plate thickness of less than 1.0 mm. First, the reason for limiting the composition of the steel sheet will be described. In the following, the mass% related to the composition is simply expressed as%.

C: 0.85% or more and 1.10% or less
C is an essential element for obtaining sufficient hardness (600 to 750 HV) with precision parts such as knitting needles after heat treatment (quenching and tempering). In order to stably secure a hardness of 600 HV or more after heat treatment (quenching and tempering), C must be contained at 0.85% or more. On the other hand, when the amount of C increases, the amount of carbide increases, the cold workability deteriorates, and it becomes impossible to withstand a wide range of plastic working (cold working) such as punching, swaging, bending, and secondary working. Cold workability is improved by repeating cold rolling and spheroidizing annealing to spheroidize carbides, but if C is contained in excess of 1.10%, it is used in the hot rolling process and cold rolling process. Problems in the manufacturing process become apparent, such as a high rolling load and a significantly high frequency of cracking at the end of the coil. For this reason, C was limited to 0.85% or more and 1.10% or less. It is preferably 0.95 to 1.05%.

Mn: less than 0.60%
Mn is an element that effectively acts on the deoxidation of steel, and can improve the hardenability of steel to stably secure a predetermined hardness. However, a content of 0.60% or more increases MnS inclusions and adversely affects the secondary processability before quenching and tempering. When the cleanliness, especially dA, is 0.10% or more, the probability that inclusions hit the cutting blade increases, the cutting resistance increases, and the deterioration of secondary workability becomes remarkable. Therefore, in the present invention, Mn is limited to less than 0.60% as the range in which dA is less than 0.10%. It should be noted that it is preferably 0.50% or less. Cleanliness shall be measured in accordance with JIS G 0555. Here, we focused on dA, especially for A-type inclusions.

Si: 0.10% or more and 0.35% or less
Si acts as a deoxidizing agent for molten steel and is an effective element for melting clean steel. Si is an element that contributes to the temper softening resistance of martensite. In order to obtain such an effect, the content of 0.10% or more is required. On the other hand, if a large amount of Si exceeding 0.35% is contained, the tempering of martensite becomes insufficient at the time of low-temperature tempering treatment, and the impact characteristics are deteriorated. For these reasons, Si was limited to the range of 0.10% or more and 0.35% or less.

P: 0.030% or less, S: 0.030% or less
Both P and S are inevitably present in steel and have an adverse effect on impact characteristics, and it is desirable to reduce them as much as possible. However, it is practical to contain P up to 0.030% and S up to 0.030%. There is no problem. For this reason, P was limited to 0.030% or less, and S was limited to 0.030% or less. From the viewpoint of maintaining excellent impact characteristics, it is preferable to adjust P to 0.020% or less and S to 0.020% or less.

Cr: less than 0.60%
Cr is an element that contributes to the improvement of wear resistance by improving the hardenability of steel and by dissolving it in carbide (cementite) to harden the carbide. In order to obtain such an effect, it is desirable to contain 0.10% or more. Cr dissolves in carbide (cementite) and delays the redissolution of carbide in the heating stage. Therefore, as the amount of Cr increases, the residual carbide after quenching and tempering increases. Here, the residual carbide refers to the carbide remaining in the substrate after quenching for martensitic transformation of the carbide that was not completely melted in the substrate during heating and holding during the quenching treatment. Abrasion resistance improves as residual carbide increases. However, when Cr is contained in a large amount of 0.60% or more, in addition to the increase in residual carbides, the effect of delaying the dissolution of the carbides during quenching and heating retention becomes large, which inhibits the hardenability and lowers the toughness. For this reason, Cr was limited to less than 0.60%. It should be noted that it is preferably 0.10% or more and less than 0.50%.

Nb: 0.005% or more and 0.020% or less
It has been conventionally known that Nb is an element that expands the unrecrystallized temperature range of steel during hot rolling, and at the same time precipitates as NbC, which contributes to the miniaturization of austenite grains, mainly in low carbon steel. .. Even in high carbon steel, it may be added in anticipation of the effect of microstructuring after the cold rolling process. In the present invention, Nb is contained in an amount of 0.005% or more and 0.020% or less for the main purpose of recovering toughness by low-temperature tempering after quenching. If the amount of Nb is small, NbC is not formed to the extent that it contributes to the miniaturization of the structure, and Nb is in a dilute solid solution state. It is considered that the diffusion of C in the ferrite phase and the martensite phase, which are BCC structures, is promoted because Nb is in a dilute solid solution state. That is, the diffusion of C dissolved in the ferrite phase from the carbide to the austenite phase during heating in the quenching treatment and the diffusion and precipitation of the supersaturated solid-dissolved C in the martensite phase during heating in the tempering treatment are promoted, resulting in a short time. We believe that it is possible to achieve both improvement of quenchability by time heating and restoration of toughness by low-temperature tempering treatment. Such an effect becomes remarkable when Nb is contained in an amount of 0.005% or more, but when Nb is contained in an amount of more than 0.020%, the precipitation of NbC becomes remarkable, the diluted solid solution state of Nb cannot be secured, and the Nb is diluted. The effect of promoting C diffusion due to the solid solution state is not recognized. Therefore, Nb was limited to 0.005% or more and 0.020% or less. It is preferably 0.015% or less.

(Mn + Cr): Less than 1.0% In the present invention, in order to improve the toughness and abrasion resistance after the quenching and tempering treatment while suppressing the deterioration of the secondary workability before the quenching and tempering treatment, the Mn content and the Cr content. Adjust the total (Mn + Cr) to less than 1.0%. According to the study by the present inventors, since both Mn and Cr are easily dissolved in carbides, as the total of Mn content and Cr content (Mn + Cr) increases, the heating stage during quenching and heating increases. The effect of delaying the redissolution of carbides is greater than that of Mn alone and Cr alone, residual carbides increase, and wear resistance also increases. However, when (Mn + Cr) increases to 1.0% or more, the residual carbide becomes 6% or more in area ratio, the influence of the decrease in hardenability becomes large, and the impact value (toughness) after quenching and tempering also decreases. If (Mn + Cr) is less than 1%, the residual carbide is less than 6% in area ratio, and excellent wear resistance and toughness can be combined. On the other hand, if (Mn + Cr) is too small, the amount of residual carbide will be small, and the desired wear resistance cannot be ensured. Therefore, it is preferable that the residual carbide has an area ratio of 3% or more. The (Mn + Cr) for achieving a residual carbide amount of 3% or more in terms of area ratio is preferably 0.15% or more. On the other hand, in the secondary workability before quenching and tempering, the increase in (Mn + Cr), especially the increase in Mn, increases the amount of MnS inclusions that adversely affect the secondary workability, thus suppressing the deterioration of the secondary workability. However, in order to improve both wear resistance and toughness, (Mn + Cr) was limited to less than 1.0% in the present invention. It is preferably less than 0.90%.

The above-mentioned components are the basic components, but in addition to the basic components, Mo: 0.001% or more and less than 0.05%, V: 0.001% or more and less than 0.05% are selected as one or 2 elements. Can contain seeds.

Mo: 0.001% or more and less than 0.05%, V: 0.001% or more and less than 0.05%, one or two selected
Both Mo and V are elements that contribute to the improvement of hardenability of steel and the improvement of impact characteristics (toughness) after quenching and tempering, and one or two types are inevitably selected as necessary. It can be contained in excess of the contained level (0.001%).

Mo is an element that is effective in improving the hardenability of steel, but if the content is as high as 0.05% or more, the effect of delaying the penetration of carbides will increase, and the hardenability will decrease, resulting in sufficient hardness. In addition to being unable to do so, the effect of Nb is lost and the impact characteristics after low temperature tempering are reduced. Therefore, when it is contained, it is preferable to limit Mo to 0.001% or more and less than 0.05%, which is inevitably higher than the level contained. More preferably, it is 0.01% or more and 0.03% or less.

V is an element that contributes to the improvement of impact characteristics by refining the steel structure, but if it is contained in a large amount of 0.05% or more, the effect of delaying the dissolution of carbides becomes greater, and the hardenability is rather improved. In addition to the decrease and insufficient hardness, the effect of Nb is lost and the impact characteristics after low temperature tempering are deteriorated. Therefore, when it is contained, it is preferable to limit V to 0.001% or more and less than 0.05%, which is unavoidably higher than the content level. More preferably, it is 0.01% or more and 0.03% or less.

The rest other than the above components consist of Fe and unavoidable impurities.
The high carbon cold-rolled steel sheet of the present invention has the above-mentioned composition and has the following formula (1).
0.2 ≤ d _av ≤ 0.7 (μm)… (1)
The average particle size ( _dav ) (μm) that satisfies the condition and the following equation (2) ( _NSC / _NTC ) × 100 ≧ 90%… (2)
It has a structure in which carbides having a spheroidization rate (N _SC / NT _{C) satisfying the above conditions are dispersed.}

Here, the average particle size ( _dav ) in Eq. (1) is the average value of the diameters (equivalent to circles) of individual circles assuming a circle with the same area as the individual carbides observed in the cross section of the steel sheet. be. When the average particle size ( _dav ) of the dispersed carbide is within the range satisfying the equation (1), the impact characteristics are excellent, and even in the short-time solution heat treatment and then quenching (quenching) treatment, the desired quenching hardness is obtained. Has the effect of being easily secured. When the average particle size ( _dav ) of the dispersed carbide is less than 0.2 μm, the carbide becomes finer and the number of dispersed carbides increases, so that the load of secondary processing on the needle shape increases. Further, if the average particle size ( _dav ) exceeds 0.7 μm, it becomes difficult to secure the desired quenching hardness in the short-time solution heat treatment and then the rapid cooling treatment.

Further, in the present invention, the spheroidization rate is defined by _(NSC / _{NTC) in Eq. (2).} Here, N _TC is the total number of carbides per observation area 100 [mu] m ^2, N _SC is the number of carbide which can be regarded to be spheroidized in the same observation field, the condition of d _L / d _S ≦ 1.4 The number of carbides to be satisfied was used. Here, the major axis of the carbide is d _L and the minor axis is d _S.

Carbides are not completely formed in a spherical shape, and are often observed as an ellipse depending on the observation surface. Therefore, the carbides are spheroidized by the ratio of the major axis to the minor axis (d _L / d _S ). The degree of is specified. In the present _{invention, (d L / d S)} : 1.4 The following conditions are satisfied carbide as carbide is spheroidized (spheroidizing carbides), and the number and N _{SC. In addition, from empirical knowledge, it is necessary that the spheroidization rate (NSC} / _NTC ) x 100 is 90% or more in order to maintain good secondary workability of the steel sheet.

The above-mentioned average particle size and spheroidization rate of the charcoal were calculated by observing a secondary electron image (magnification: 2000 times) using a scanning electron microscope and performing image analysis.

Carbide observation test pieces are collected from the cold-rolled steel plate (central part of the plate thickness), embedded in resin, polished, etched with a corrosive liquid, and the carbides are observed using a scanning electron microscope. The equivalent circle diameter, major axis d _L / minor axis d _S ratio, N _TC , and N _SC of carbides were measured within an observation area of 100 μm ^{2 near the center.} Such measurements were carried out in 5 fields of view, and the average value of each was calculated. Commercially available image analysis software winroof was used for these measurements and calculations.

The high carbon cold-rolled steel sheet of the present invention has the above-mentioned steel plate composition and structure, and while maintaining secondary workability such as machinability, the life of tools used for punching, swaging, bending, secondary processing, etc. is long. Compared to the conventional high carbon steel sheet, it is about the same as the conventional high carbon cold-rolled steel sheet, and after being subjected to a short-time solution heat treatment and then rapid cooling and low temperature tempering treatment (quenching tempering treatment). It is a high carbon cold-rolled steel sheet that can manufacture mechanical parts that have high hardness characteristics, excellent impact characteristics, and excellent wear resistance in a well-balanced manner.

As shown in FIG. 1, "excellent in secondary workability" here means that when a cutting (end milling) test is performed and the force applied to the tool (end mill) is less than 40 N (tool rotation speed). Is low speed (1300 rpm)) or less than 35 N (when the tool rotation speed is high speed (2300 rpm)).

In the present invention, focusing on general end mill processing, as shown in FIG. 1, cutting processing (end mill processing) is performed on a steel plate (work material) using an end mill, and cutting attached to a tool at that time. After measuring the X-direction component force, Y-direction component force, and Z-direction component force as the cutting resistance force applied to the tool (end mill: φ6 mm diameter) with a power meter (not shown), the resultant force is calculated and secondary. It was used as an evaluation index for workability. The conditions for the end mill machining test are: cutting speed: 25 m / min (low speed), 45 m / min (high speed), feed amount per blade: 0.016 mm / touth, depth of cut: 0.2 mm, tool protrusion length: 25 mm. , Cutting distance: 30 mm, no cutting fluid was used.

By adopting such an end mill processing test, it is possible to evaluate the secondary workability in a state closer to the actual usage environment. If the cutting resistance applied to the tool is less than 40 N (or less than 35 N), it means that it has excellent secondary workability equal to or higher than the secondary workability of the conventional high carbon cold-rolled steel sheet.

Further, the term "excellent wear resistance" as used herein means a case where a wear test is carried out using the wear test apparatus shown in FIG. 2 and the obtained wear depth is less than 485 μm.

The wear test device 10 shown in FIG. 2 includes a thread unwinding means 11 for unwinding a thread, a tension adjusting means 12 for applying a desired tension to the unwound thread 2, and holes 1a to 1d for passing the tension-applied thread. It is a device having a wear test piece 1 and a thread winding means 13 for winding a thread, and can reproduce the wear of a knitted needle due to knitting yarn in a situation close to that of an actual machine. The wear test device 10 has a structure in which the tension becomes zero when the thread breaks, and the device automatically stops at that point.

The wear test piece 1 to be used is a wear test piece having the shape shown in FIG. 3A, and the thread 2 continuously unwound from the bobbin (thread unwinding means) 11 has an appropriate tension by the tension adjusting means 12. Then, the thread is wound by the thread winding means 13 while passing through, for example, the hole 1a formed in the wear test piece 1 and contacting the hole 1a to wear the hole 1a. Holes were formed at four locations (1a to 1d) with one test piece. The conditions for the wear test are knitted yarn made of polyester full dull (standard 110T48), yarn feeding speed: 160 m / s, tension: 10 ± 2 N / cm, and the length of the yarn is 100,000 in one hole. It was carried out until m was extended, and the wear depth in the hole was measured. Such a wear test is carried out in each of the four holes 1a to 1d formed in one wear test piece, the wear depth of each hole is measured, and the average value thereof is used as the wear depth of the wear test piece. (Average).

As a result of the wear test under the above conditions, if the wear depth is less than 485 μm, it means that the wear resistance is equal to or higher than that of the conventional high carbon cold-rolled steel sheet. By adopting such a wear test, it is possible to evaluate the wear resistance in a state close to the wear caused by the thread of the knitted needle hook portion. By evaluating the wear resistance of the Norias needle hook portion in a state close to the wear caused by the thread, it was found that the presence of residual carbide greatly affects the wear resistance. The wear resistance is proportional to the area ratio of the residual carbide, and if the area ratio of the residual carbide is less than 3%, the desired wear resistance cannot be ensured. The area ratio of residual carbide is preferably 3% or more.

The term "excellent impact characteristics" as used herein refers to the impact test piece shown in FIG. 4 (U notch test piece having a notch width of 0.2 mm (notch depth 2.5 mm, notch radius 0.1 mm)) and JIS K 7077. Impact when tested at room temperature with a Charpy impact tester (Toyo Seiki Seisakusho Co., Ltd., model DG-GB) with a rated capacity of 1J based on the above, with the distance between the supports as 40 mm, as shown in FIG. The case where the value is 9 J / cm ² or more is referred to.

By using such a Charpy impact tester, even if a test piece with a plate thickness of less than 1.0 mm is used, the test can be performed under conditions close to JIS Z 2242, which is a Charpy impact test method for metal materials. By using a flexible impact test piece, the stress concentration coefficient becomes high, the deflection during the impact test is minimized, and a stable impact value can be obtained. By adopting such an impact test method and an impact test piece, it is possible to evaluate the impact characteristics in a state close to the actual usage environment. The impact value tends to be higher when the amount of residual carbide is small, but when the amount of residual carbide exceeds 6% in terms of area ratio, the impact value drops significantly, so the desired impact value is secured. Therefore, the present inventors have found that the residual carbide is less than 6% in area ratio.

In this way, by introducing a new wear test method for evaluating wear resistance and an end mill processing test method for evaluating secondary workability, appropriate chemistry is based on evaluation in an environment close to the actual machine. It has become possible to specify the range of ingredients.

Next, a method for manufacturing the high carbon cold-rolled steel sheet of the present invention will be described.
The high carbon cold-rolled steel sheet of the present invention is produced by repeatedly softening and annealing a hot-rolled steel sheet, cold rolling and spheroidizing annealing, if necessary.

The hot-rolled steel sheet used in the present invention may be obtained under normal manufacturing conditions. For example, a steel piece (slab) having the above composition is heated to 1050 to 1250 ° C. and at a finishing temperature of 800 to 950 ° C. It can be manufactured by hot rolling and forming a coil at a winding temperature of 600 to 750 ° C. The thickness of the hot-rolled steel sheet may be appropriately set so as to have a suitable cold reduction ratio from the desired thickness of the cold-rolled steel sheet.

Cold-rolled and spheroidized annealing are repeatedly applied to the hot-rolled steel sheet to obtain a high-carbon cold-rolled steel sheet with a thickness of less than 1.0 mm. Cold rolling and spheroidizing annealing are preferably repeated 2 to 5 times each.

The rolling reduction in cold rolling is preferably in the range of 25 to 65%. When spheroidizing annealing is applied to a steel sheet (cold-rolled steel sheet) having a reduction ratio of cold rolling of less than 25%, carbides become coarse. On the other hand, if the rolling reduction of cold rolling exceeds 65%, the load of cold rolling operation may be too large. Therefore, the rolling reduction rate for cold rolling is limited to the range of 25 to 65%. The lower limit of the rolling reduction is not particularly limited for the final cold rolling that is not subjected to spheroidizing annealing after cold rolling.

Further, the spheroidizing annealing is preferably performed at a temperature in the range of 640 to 720 ° C. When the spheroidizing annealing temperature is less than 640 ° C, spheroidization tends to be insufficient, while when the temperature is higher than 720 ° C, carbides tend to coarsen. Therefore, spheroidizing annealing was performed at a temperature in the range of 640 to 720 ° C. The holding time for spheroidizing annealing is preferably selected in the range of 9 to 30 hr.

The reason why the repeated cold rolling (25-65%) and spheroidizing annealing (six hundred and forty to seven hundred and twenty ° C.) a plurality of times, the average particle size of the carbide (d _av), spheroidization ratio (N _SC / N _TC) × This is because 100 is controlled so as to satisfy the above-mentioned equations (1) and (2), respectively.

First, cracks are introduced into the carbide by cold rolling, and the carbide that has started to be spheroidized by spheroidizing annealing is spheroidized. It is difficult and rod-shaped or plate-shaped carbide remains. In such a case, the hardenability is also adversely affected, and the cold workability for precision parts is deteriorated. Therefore, in order to increase the spheroidization rate (N _SC / _NTC ) x 100 of carbides to 90% or more, it is optimal to repeat cold rolling and spheroidizing annealing alternately, resulting in fine and spheroidizing in the steel sheet. A high rate of carbide distribution is obtained. Particularly preferred are 2-5 cold rollings and 2-5 spheroidizing annealings. The same temperature range is preferable for softening and annealing for the purpose of softening hot-rolled steel sheets before cold rolling.

The above is the method for manufacturing a high carbon cold-rolled steel sheet of the present invention. In order to make this steel sheet into a mechanical part such as a knitted needle, which is the final purpose, after processing it into a predetermined shape, the following heat treatment is performed. It is preferable to do it.

A high-carbon cold-rolled steel sheet in which carbides spheroidized by 90% or more are distributed is processed into various machine parts, and then subjected to a solution heat treatment and then a quenching (quenching) treatment, and then a tempering treatment. In the solution treatment, the heating temperature is 760 to 820 ° C and the holding time is 3 to 15 min for a short time. It is preferable to use oil for quenching (quenching). In the tempering treatment, the tempering temperature is preferably a low temperature of 200 to 350 ° C. The temperature is more preferably 250 to 300 ° C. This makes it possible to make various mechanical parts with a hardness of 600 to 750 HV.

If the retention time of the solution treatment is longer than 15 min, carbides will dissolve too much and the austenite grains will be coarsened, resulting in coarsening of the martensite phase after quenching and deterioration of impact characteristics. On the other hand, if the holding time is shorter than 3 min, the carbides are not sufficiently dissolved, and it becomes difficult to obtain the desired high hardness after quenching. Therefore, the retention time of the solution treatment is preferably 3 min or more and 15 min or less. More preferably, it is 5 to 10 min.

Further, if the tempering temperature is less than 200 ° C., the toughness recovery of the martensite phase becomes insufficient. On the other hand, when the tempering temperature exceeds 350 ° C., the hardness falls below 600 HV and the impact value increases, but the durability and wear resistance decrease, which is a problem. Therefore, the tempering temperature is preferably a temperature in the range of 200 to 350 ° C. The temperature is more preferably 250 to 300 ° C. It is preferable that the holding time of the tempering treatment is appropriately selected in the range of 30 min to 3 hr.

Hereinafter, the present invention will be further described based on examples.

The molten steel having the chemical components shown in Table 1 was melted in a vacuum melting furnace and then cast into a mold to obtain a small ingot (50 kgf). These small ingots were ingot-rolled into steel pieces and then hot-rolled under the conditions of heating temperature: 1150 ° C and rolling finish temperature: 870 ° C to obtain hot-rolled steel sheets (plate thickness: 4 mm). Then, the obtained hot-rolled steel sheet was cold-rolled and spheroidized and annealed under the conditions shown in Table 2 to obtain a cold-rolled steel sheet having a plate thickness of 0.4 mm or more and less than 1.0 mm.

First, a test piece for structure observation was collected from the obtained cold-rolled steel plate, embedded in a resin, polished and corroded, and the structure was observed with a secondary electron image (magnification: 2000 times) of a scanning electron microscope. The images were taken and image analysis was performed _{to calculate the average particle size (dav} ) and spheroidization rate ( _NSC / _NTC ) of the charcoal. ^{Within the range of the observed area of 100 μm 2} near the center of the plate thickness, the circle-equivalent diameter of each carbide and the major axis d _L / minor axis d _S ratio of each carbide are obtained, and the total number of carbides per 100 μm ² _{observation area N TC} , d _L / d _S: 1.4 was measured the total number N _SC of the following conditions are met carbide. Such measurements were carried out in 5 fields of view, and their average values were calculated respectively. Commercially available image analysis software winroof was used for these measurements and calculations. In addition, the cleanliness dA of the test piece for tissue observation was measured for A-based inclusions in accordance with JIS G 0555. The measurement field of view was set to 60 fields of view.

Further, a test piece was collected from the obtained cold-rolled steel plate, and a machinability test (end milling test) was carried out under the conditions shown in Table 3 as shown in FIG. 1, and the tool (end mill: 6 mm diameter) was used. After measuring the forces in the X, Y, and Z directions, the resultant force was calculated and used as the cutting resistance force. There are two types of tool rotation speeds: low speed (1300 rpm) and high speed (2300 rpm).

Then, the obtained cold-rolled steel sheet is charged into a heating furnace under the conditions shown in Table 4, subjected to a short-time solution treatment, then rapidly cooled (oil-quenched), and further subjected to a low-temperature tempering treatment. Heat treatment was performed. Specimens were collected from the heat-treated steel sheet and subjected to residual carbide investigation, hardness test, impact test, and wear test. The test method was as follows.
(1) Investigation of residual carbides Test pieces for structure observation are collected from the heat-treated steel plate, embedded in resin, polished and corroded, and the structure is observed with a secondary electron image (magnification: 2000 times) of a scanning electron microscope. Then, an image was taken, and the area ratio (%) of the residual carbide was calculated for the residual carbide having a size equivalent to a circle of 0.1 μm or more by image analysis. The measured area was 100 μm ² .
(2) Hardness test A hardness test piece was cut out from the heat-treated steel sheet in a direction perpendicular to the rolling direction, embedded in a resin, the cross section was polished, and the hardness was measured at the center of the plate thickness. The hardness was measured at 5 points each using a Vickers hardness tester (test force: 49.0N) in accordance with JIS Z 2244, and the average value was taken as the hardness of the steel sheet.
(3) Impact test An impact test piece (U notch test piece with a notch width of 0.2 mm (notch depth 2.5 mm, notch radius 0.1 mm)) shown in FIG. 4 is placed so as to be parallel to the rolling direction from the heat-treated steel plate. A Charpy impact tester with a rated capacity of 1J (model DG-GB manufactured by Toyo Seiki Seisakusho Co., Ltd.) based on JIS K 7077, with a distance between supports of 40 mm and room temperature as shown in Fig. 5. The Charpy impact test was carried out in 1 and the impact value (J) was determined. The number of test pieces was five, and the average of the obtained impact values was taken as the impact value of the steel sheet.
(4) Wear test A wear test piece having the shape shown in FIG. 3 was collected from the heat-treated steel sheet, and a wear test was carried out using the wear test apparatus shown in FIG. The conditions for the wear test were polyester full dull knitting yarn (standard 110T48), yarn feeding speed: 160 m / s, and tension: 10 ± 2 N / cm. After running the thread 100,000 m in one hole, the test device is stopped, and the wear depth formed in the hole (here, 1a) of the wear test piece 1 is measured with an optical microscope as shown in FIG. 3 (b). bottom. Such a wear test was carried out in each hole (1a to 1d), the wear depth of each hole (4 points) was measured, and the average value thereof was obtained and used as the wear depth of the wear test piece.

The results obtained are shown in Table 5.

In each of the examples of the present invention, the force (cutting resistance) applied to the tool is less than 40 N in low-speed machining and less than 35 N in high-speed machining, and the secondary workability is the same as that of the conventional high-carbon cold-rolled steel sheet. It has high hardness characteristics that satisfy the hardness range of 600 to 750 HV after short-time solution treatment, quenching (oil quenching) treatment and low-temperature tempering treatment, and has an impact value of 9 J / cm ^2. Satisfied with the above, it is a high carbon cold-rolled steel plate with excellent impact characteristics and a wear depth of less than 485 μm, which is excellent in wear resistance, and was evaluated as “◎”. On the other hand, in a comparative example outside the scope of the present invention, the force (cutting resistance) applied to the tool is 40 N or more in low-speed machining and 35 N or more in high-speed machining, resulting in inferior secondary workability or quenching (oil) after short-time solution treatment. After heat treatment, which is subjected to quenching) treatment and further low-temperature tempering treatment, the ^{impact characteristics are deteriorated when the impact value is less than 9 J / cm 2} , or the wear resistance is deteriorated when the wear depth is 485 μm or more. It is a high carbon cold-rolled steel sheet evaluated as "x".

Specifically, in the comparative example (steel plate No. 1) in which the amount of C is low outside the range of the present invention, the cutting resistance is low, the secondary workability is excellent, and the impact value is 9 J / cm ² or more, which is excellent in impact characteristics. However, the amount of residual carbide was small, and the wear depth was 485 μm or more, resulting in reduced wear resistance. Further, in the comparative example (steel No. 12) in which the amount of C is high outside the range of the present invention, there are many residual carbides and the wear depth is less than 485 μm, which is excellent in wear resistance, but the impact value is less than ^{9 J / cm 2.} And the impact characteristics are reduced. Further, (Mn + Cr) exceeds 1.0%, the cleanliness is poor, the force applied to the tool (cutting resistance) is high, and the secondary workability is lowered. Further, in all the comparative examples (steel plates No. 9, No. 10, and No. 11) in which (Mn + Cr) is 1.0% or more, which greatly exceeds the range of the present invention, the residual carbide is large and the wear depth is less than 485 μm. It has excellent wear resistance, but its impact characteristics are deteriorated ^{when the impact value is less than 9 J / cm 2.} Furthermore, the cleanliness is poor, the force applied to the tool (cutting resistance) is high, and the secondary workability is reduced. Further, the comparative example (steel plate No. 13) in which the amount of V is highly outside the range of the present invention and the comparative example (steel plate No. 14) in which the amount of Mo is highly outside the range of the present invention have a large amount of residual carbide and have abrasion resistance. However, the toughness is reduced. In addition, both the comparative example (steel plate No. 3 and No. 15) in which the amount of Nb is low outside the range of the present invention and the comparative example (steel plate No. 16) in which the amount of Nb is high outside the range of the present invention are impact values. However, the impact characteristics are reduced to less than 9 J / cm ^2. The example of the present invention (steel plate No. 19) having a low (Mn + Cr) of 0.14% shows a tendency for the wear resistance to be slightly lowered, and the example of the present invention (steel plate No. 20) having a high (Mn + Cr) of 0.90% shows. , The secondary workability tends to decrease to some extent. Further, in the comparative example (steel plate No. 21) in which (Mn + Cr) exceeds 1.0% and the comparative example (steel plate No. 22) in which Cr is highly outside the scope of the present invention, the residual carbide exceeds 6% in area ratio and the toughness resistance. It has excellent wear resistance, but its impact toughness is reduced ^{when the impact value is less than 9 J / cm 2.}

1 Abrasion test piece 1a, 1b, 1c, 1d Hole 2 Thread 10 Abrasion test device 11 Thread unwinding means (bobbin)
12 Tension adjusting means 13 Thread winding means

Claims (9)

By mass%,
C: 0.85% or more and 1.10% or less, Mn: less than 0.60%,
Si: 0.10% or more and 0.35% or less, P: 0.030% or less,
S: 0.030% or less, Cr: less than 0.60%,
Nb: Contains 0.005% or more and 0.020% or less, satisfies the total Mn content and Cr content (Mn + Cr) of less than 1.0%, has a steel sheet composition consisting of the balance Fe and unavoidable impurities, and has a steel sheet thickness of 1.0. High carbon cold rolled steel sheet characterized by being less than mm. The following equations (1) and (2) are used for _{the average particle size (dav} ) and spheroidization rate ( _NSC / _NTC ) x 100% of the carbides having the steel sheet composition and dispersed in the steel sheet, respectively. The high carbon cold-rolled steel sheet according to claim 1, wherein the steel sheet structure is satisfied.
Record
0.2 ≤ d _av ≤ 0.7 (μm)… (1)
(N _SC / N _TC ) × 100 ≧ 90%… (2)
Here, _dav : the average value of the equivalent circle diameter of the carbide (average particle size μm),
_NTC : Total number of carbides per 100 μm ^{2 observation area,}
N _SC : The number of carbides satisfying the condition of (major diameter d _L ) / (minor diameter d _S ) of 1.4 or less per 100 μm ^{2 observation area.} Instead of the steel sheet composition, by mass%,
C: 0.85% or more and 1.10% or less, Mn: less than 0.60%,
Si: 0.10% or more and 0.35% or less, P: 0.030% or less,
S: 0.030% or less, Cr: less than 0.50%,
Nb: A claim comprising a steel sheet composition containing 0.005% or more and 0.020% or less, satisfying a total of Mn content and Cr content (Mn + Cr) of less than 0.90%, and having a balance Fe and unavoidable impurities. The high carbon cold-rolled steel sheet according to 1 or 2. The steel sheet composition is further characterized by a steel sheet composition containing one or two selected from Mo: 0.001% or more and less than 0.05% and V: 0.001% or more and less than 0.05% in mass%. The high carbon cold-rolled steel sheet according to any one of claims 1 to 3. In the method for manufacturing a high carbon cold rolled steel sheet, the hot rolled steel sheet having the steel sheet composition according to any one of claims 1 to 4 is repeatedly cold rolled and spheroidized and annealed to manufacture a high carbon cold rolled steel sheet. the average particle size of the carbide dispersed in the carbon cold-rolled steel sheet and (d _av), spheroidization ratio (N _SC / N _TC) satisfies the respective following equations (1) and equation (2), wherein the high carbon cold A method for manufacturing a high-carbon cold-rolled steel sheet, which comprises a high-carbon cold-rolled steel sheet having a thickness of less than 1.0 mm.
Record
0.2 ≤ d _av ≤ 0.7 (μm)… (1)
(N _SC / N _TC ) × 100 ≧ 90%… (2)
Here, _dav : the average value of the equivalent circle diameter of the carbide (average particle size μm),
_NTC : Total number of carbides per 100 μm ^{2 observation area,}
N _SC : The number of carbides satisfying the condition of (major diameter d _L ) / (minor diameter d _S ) of 1.4 or less per 100 μm ^{2 observation area.} The method for producing a high carbon cold-rolled steel sheet according to claim 5, wherein the number of times of repeated cold rolling and spheroidizing annealing is 2 to 5 times. The method for producing a high carbon cold-rolled steel sheet according to claim 5 or 6, 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 any one of claims 1 to 4 is used as a material, and the material is subjected to secondary processing to obtain a machine part having a predetermined shape, and then the machine part is subjected to a short-time solution treatment and then rapidly cooled. It is a manufacturing method of machine parts that are processed and tempered.
The process of quenching after the short-time solution treatment is performed at a temperature in the range of 760 to 820 ° C. for a time in the range of 3 to 15 min, and then the process of quenching is performed, and the tempering process is performed in the range of 200 to 350 ° C. A method for manufacturing a machine part made of high carbon steel, which comprises making a machine part having excellent wear resistance and excellent toughness as a process of tempering at a temperature. A high carbon steel machine part manufactured by the method for manufacturing a high carbon steel machine part according to claim 8.

Images