WO2017029922A1

WO2017029922A1 - High-carbon cold-rolled steel sheet and method for manufacturing same

Info

Publication number: WO2017029922A1
Application number: PCT/JP2016/071133
Authority: WO
Inventors: 栄司土屋; 雄太松村; 佳弘細谷; 友佳宮本; 崇小林; 長滝　康伸; 瀬戸　一洋
Original assignee: 株式会社特殊金属エクセル; Ｊｆｅスチール株式会社
Priority date: 2015-08-14
Filing date: 2016-07-19
Publication date: 2017-02-23
Also published as: KR20170075783A; TW201712130A; KR101953495B1; CN107208224B; EP3216889A1; JP2017036492A; JP6089131B2; EP3216889A4; EP3216889B1; CN107208224A; TWI591187B

Abstract

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

Description

High carbon cold rolled steel sheet and method for producing the same

The present invention relates to a high carbon cold-rolled steel sheet used as a material for various machine parts produced by quenching and tempering. In particular, it has been hardened with a solution treatment for a short time, has both sufficient hardness (600 to 750HV) and excellent impact properties (toughness) after low temperature tempering treatment, and has further demands for durability and wear resistance. The present invention relates to a high-carbon cold-rolled steel sheet with a thickness of less than 1.0 mm that can be applied to severe knitting needles. Here, short-time solution treatment means treatment in a temperature range of 760 to 820 ° C. for 3 to 15 minutes, and low-temperature tempering treatment means treatment in a temperature range of 200 to 350 ° C. .

Generally, carbon steel materials for machine structure (SxxC) and carbon tool steel materials (SK) specified in JIS are used for various types of machine parts. When used as a wrought material, it is formed into a part shape through punching and various plastic processing, and then subjected to quenching and tempering. Thereby, predetermined | prescribed hardness and toughness (impact characteristic) are provided. Among them, for example, knit fabric knitting needles knitting the knit fabric by pulling back the yarn while repeating reciprocating motion at high speed, so that the bat portion of the needle body in contact with the rotational drive unit has sufficient strength and resistance. Abrasion is required, and the hook portion that rubs against the yarn is required to have excellent impact characteristics in addition to sufficient wear resistance.

High carbon cold-rolled steel sheets used as material for knitting needles are used for knitting needles for flat knitting machines when the thickness is 1.0 mm or more, and circular knitting machines and warp knitting machines when the thickness is less than 1.0 mm. Used for knitting needles. In knitting needles for circular knitting machines and warp knitting machines, thin yarns are knitted at high speed, so the thickness of the material used is often 0.4 to 0.7 mm. Furthermore, in addition to having excellent cold workability (hereinafter also referred to as secondary workability), the material for knitted needles is sufficiently processed after being processed into a needle shape (secondary work) and quenched and tempered. It is required to have sufficient hardness and toughness at the needle tip.

In addition, so-called high carbon steel sheets such as carbon steel materials for machine structures (SxxC) and carbon tool steel materials (SK) specified in JIS are classified in detail according to the amount of C. A steel sheet having a C content of less than 0.8 mass%, that is, a hypoeutectoid composition, is excellent in cold workability because of its high ferrite phase fraction, but it is difficult to obtain sufficient quenching hardness. For this reason, a steel sheet having a hypoeutectoid composition is not suitable for knitting needles and the like that require wear resistance of the hook portion and durability of the needle body. On the other hand, in the region of 0.8 mass% or higher, that is, in the hypereutectoid composition steel plate, the high carbon steel plate with a C content greater than 1.1 mass% has excellent hardenability, but it contains a large amount of carbide (cementite). However, the cold workability is extremely inferior, and it is not suitable for knitting needle applications where precise and fine machining such as grooving is performed. High carbon steel sheets with a C content greater than 1.1 mass% are limited to parts applications that require a simple shape and high hardness, such as blades and cold dies.

Conventionally, carbon tool steels and alloy tool steels with C: 0.8 to 1.1 mass% or steel compositions with a third element added based on these steel compositions have been widely used for knitted needles. In the manufacturing process of the knitted needle, the material is subjected to various plastic workings such as punching (shearing), cutting, wire drawing, caulking, bending, and the like. Therefore, this material for manufacturing knitted needles is required for actual use as a needle, in addition to having sufficient workability (secondary workability) during 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 knitted needles, the material is subjected to quenching and tempering in order to ensure a predetermined hardness characteristic. In this tempering treatment, a low temperature tempering treatment in a temperature range of 200 to 350 ° C. is generally employed. However, if the amount of Mn or Cr effective for hardenability is increased with emphasis on hardness characteristics, or if a large amount of other third elements are added, the above-mentioned low-temperature tempering treatment will sufficiently temper the martensite phase. In some cases, the impact characteristics (toughness) are not sufficiently improved or the toughness value varies.
On the other hand, for the purpose of improving the impact characteristics of knitted needles, P and S, which are impurity elements, are reduced in the chemical composition of the material, and the grain boundary segregation of P and the generation of MnS inclusions are minimized. Reducing the adverse effects of this is also an effective measure. However, there is a limit in reducing the P and S to improve the impact characteristics of the knitted needle from the viewpoint of steelmaking technology and cost economy.

Further, it has been conventionally known that the refinement of the metal structure is effective as means for improving the impact characteristics. For example, Patent Documents 1 and 2 disclose techniques for adding a carbonitride-forming element such as Ti, Nb, V, etc., and refining the metal structure using the fine carbonitride of those elements. . However, these elements are generally added as a measure for improving the toughness of steel having a hypoeutectoid composition with C of 0.8 mass% or less.
In particular, the influence (particularly the interaction) of the individual third elements on the impact properties of the martensite phase in the low-temperature tempered state at 200 to 350 ° C. has not been fully elucidated, and the effects of the individual elements are regarded as equivalent. In many cases, the ingredients were designed.

For example, in the technique described in Patent Document 1, by adding carbonitride-forming elements such as V, Ti, Nb, etc. to hypoeutectoid steel with C: 0.5 to 0.7 mass%, the prior austenite grains are formed. It has been refined to improve the toughness value (impact characteristics).

In the technique described in Patent Document 2, C: 0.60 to 1.30 mass% hypoeutectoid steel to hypereutectoid steel with a wide carbon content, Ni: 1.8 mass% or less as required, Cr: 2.0 mass% or less, V: 0.5 mass% or less, Mo: 0.5 mass% or less, Nb: 0.3 mass% or less, Ti: 0.3 mass% or less, B: 0.01 mass% or less, Ca: 0.01 mass% or less Or, by adding two or more kinds, the impact characteristics are improved by controlling the volume fraction (Vf) of undissolved carbide so that (15.3 × Cmass% −Vf) is in the range of more than 8.5 to less than 10.0. .

JP 2009-24233 A JP 2006-63384 A

However, the technique described in Patent Document 1 is limited to hypoeutectoid steel, and by adding carbonitride-forming elements such as V, Ti, Nb, etc., these fine carbonitrides are used in the past. This technology is expected to have the effect of refining austenite grains. The technique described in Patent Document 1 is also a technique that improves the formability of the ferrite matrix because the carbon level is a hypoeutectoid composition. For this reason, it is difficult to apply this technique to mechanical parts such as knitted needles that require high hardness.

In the technique described in Patent Document 2, Mo, V, Ti, Nb, B, etc. are added to hypoeutectoid steel having a carbon content in the range of 0.67 to 0.81 mass%. This addition of Mo, V, Ti, Nb, B, etc. is clearly understood as an addition intended to improve the properties of hypoeutectoid steel. In Patent Document 2, there is no disclosure regarding the action of each third element in the steel having a carbon amount exceeding 0.81 mass% and its optimization.
Furthermore, in the technique described in Patent Document 2, regarding the addition amount of the third element, only an upper limit value that does not adversely affect the impact value is specified, and the lower limit value is not specified. Therefore, it can be said that Patent Document 2 does not disclose a technique in which the third element is added within the intended range and the impact characteristics are positively improved by the action of the added element.

Further, Patent Document 1 and Patent Document 2 describe desired impact characteristics of high carbon cold rolled steel sheets by quenching with a short solution treatment holding time of 3 to 15 minutes and low temperature tempering at 200 to 350 ° C. Further, there is no disclosure of a technique that advantageously improves the predetermined hardness, and there is no disclosure of a technique that evaluates impact characteristics of a steel sheet having a thickness of less than 1.0 mm.
Therefore, the present invention provides mechanical properties having an impact value of 5 J / cm ² or more and a hardness in the range of 600 to 750 HV after quenching and low temperature tempering treatment after a short time solution treatment. An object of the present invention is to provide a high carbon cold-rolled steel sheet (hereinafter also simply referred to as “cold-rolled steel sheet”) having a thickness of less than 1.0 mm that can be expressed.

In order to solve the above-described problems, the present inventors diligently studied the appropriate addition range of 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 C: 0.85 mass% or more and 1.10 mass% or less of carbon suitable for knitting needles from the viewpoint of workability, hardenability, hardness and toughness after low temperature tempering, etc. In this technology, it was found that adding Nb as a third element within a predetermined range and controlling the average particle size and degree of spheroidization of carbides is effective in developing the desired characteristics. At the heart.

In particular, the present inventors have developed a new test method for evaluating toughness (new impact test method) for steel sheets with a thickness of less than 1.0 mm, which has been difficult to evaluate toughness. A new test method (new impact test method) is shown in FIGS.
Using this new impact test method, the impact value in a quenched and tempered state of a high carbon cold rolled steel sheet with a thickness of less than 1.0 mm to which various third elements were added was investigated. As a result, a new finding was obtained that the addition of a predetermined amount of Nb only satisfies the above-mentioned target characteristics. The present invention has been made based on such knowledge.

That is, the present inventors have intensively studied to solve the above-mentioned problems, and the basic components are C: 0.85 to 1.10 mass%, Mn: 0.50 to 1.0 mass%, Si: 0.10 to 0.35 mass%, P: 0.030 mass. %, S: 0.030 mass% or less, Cr: Addition of 0.005 to 0.020 mass% Nb to the high-carbon steel specified in the range of 0.35 to 0.45 mass%, It has been found that by controlling to a predetermined range, it is possible to obtain a high-carbon cold-rolled steel sheet that has both excellent hardenability and excellent toughness, and further it is possible to shorten the quenching time and lower the tempering temperature. It was. In addition, by adopting a test method for appropriately evaluating the impact characteristics of a thin plate, it has become possible to define appropriate chemical components, spheroidization rate of carbide, and average particle size.
First, experimental results conducted by the present inventors will be described.
mass%, 1.01% C-0.26% Si-0.73% Mn-0.42% Cr-0.02% Mo, and Nb was added at 0%, 0.010%, 0.020% and 0.055%, and the balance Fe and Cold rolled (rolling ratio: 25 to 65%, final 3 to 50%), soft annealing and spheroidizing annealing (640 to 700 ° C) on hot rolled steel sheet (4mm thickness) composed of inevitable impurities Was repeated 5 times to obtain a cold-rolled steel sheet (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 Vickers hardness. (HV) was measured. The obtained results are shown in FIG. 3 (heating temperature: 800 ° C.) and FIG. 4 (heating temperature: 780 ° C.) in relation to the heat retention time (minutes) of the solution treatment and the quenching hardness (HV).
3 and 4, it can be seen that a cold rolled steel sheet having an Nb content of 0.010 mass% can secure a quenching hardness exceeding 700 HV in the shortest heat holding time. When the Nb content increases beyond 0.010 mass%, the increase in hardness due to short-time heating is slowed down. From the result of FIG. 4, when the heating temperature of the solution treatment is 780 ° C., the heating and holding time at which the quenching hardness reaches 700 HV is obtained and shown in FIG. 5 in relation to the Nb content.
From FIG. 5, when the Nb content is 0.020 mass% or more, the heating and holding time of the solution treatment in which the quenching hardness reaches 700 HV is substantially 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 securing the desired quenching hardness (700 HV) is minimized, and stable hardenability can be secured. Furthermore, if it is Nb content of this range, the heat retention time of solution treatment can be made into a short time. 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 measure for preventing the variation in firing elongation and bending, which are problems in needle processing manufacturers.
Further, a cold-rolled steel sheet having various Nb contents was subjected to a solution treatment with a heating temperature of 800 ° C. and a heating and holding time of 10 minutes, and after oil quenching, the steel was further tempered. In the tempering treatment, the tempering temperature was 150 ° C, 200 ° C, 250 ° C, 300 ° C, 350 ° C, and the holding time was 1 hour. After tempering, the impact properties were investigated. The impact characteristics were measured using the new test method shown in FIGS. The obtained result is shown in FIG. The impact value was 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 an impact value of 5 J / cm ² is obtained is obtained and shown in FIG. 7 in relation to the Nb content. From FIG. 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 with Nb content: 0.010 mass%. When the Nb content increases beyond 0.020 mass%, the tempering temperature at which an impact value of 5 J / cm ² is obtained is on the high temperature side. When the tempering temperature becomes high, the hardness decreases, and the durability as a needle decreases. Moreover, when Nb content was less than 0.005 mass%, in order to ensure a desired impact value, it discovered that it was necessary to make tempering temperature high.
From FIG. 5 and FIG. 7, in order to combine high hardness after tempering and excellent impact properties, the Nb content is 0.005 mass% as the lower limit and 0.020 mass% as the upper limit. Furthermore, in order to shorten the heating and holding time of the solution treatment, it is preferable to set the upper limit of the Nb content to 0.015 mass%.

The present invention has been completed by further studies based on this finding. That is, the gist of the present invention is as follows.
[1] The chemical composition of the steel sheet is C: 0.85 to 1.10 mass%, Mn: 0.50 to 1.0 mass%, Si: 0.10 to 0.35 mass%, P: 0.030 mass% or less, S: 0.030 mass% or less, Cr: 0.35 to 0.45 mass%, Nb: 0.005 to 0.020 mass%, consisting of the balance Fe and inevitable impurities, and the average particle size (d _av ) and spheroidization rate (N _SC / N _TC ) of the carbides dispersed in the steel plate × A high carbon cold-rolled steel sheet characterized in that 100% satisfies the following formulas (1) and (2), respectively, and the thickness of the steel sheet is less than 1.0 mm.
0.2 ≦ d _av ≦ 0.7 (μm) (1)
(N _SC / N _TC ) × 100 ≧ 90% (2)
Here, the average particle diameter (d _av ) in the formula (1) is the average value of the diameters of the individual circles (equivalent circle diameters) when assuming a circle having the same area as each carbide observed in the cross section of the steel sheet. It is.
Further, N _TC and N _{SC in the} formula (2) are N _TC : the total number of carbides per observation area 100 μm ² , N _SC : the number of carbides satisfying the condition that d _L / d _S is 1.4 or less, The major axis of the carbide is d _L , and the minor axis is d _S.
[2] The chemical composition further contains one or two selected from Mo and V, and each content is 0.001 mass% or more and less than 0.05 mass%, The high carbon cold-rolled steel sheet according to [1].
[3] In the method for producing a high carbon cold rolled steel sheet by repeatedly performing cold rolling and spheroidizing annealing on the hot rolled steel sheet having the chemical composition described in [1] or [2], The average particle size (d _av ) and the spheroidization ratio (N _SC / N _TC ) of the carbide dispersed in the above satisfy the following formulas (1) and (2), respectively, and the thickness of the high carbon cold-rolled steel sheet is 1.0 The manufacturing method of the high carbon cold-rolled steel plate characterized by being less than mm.
0.2 ≦ d _av ≦ 0.7 (μm) (1)
(N _SC / N _TC ) × 100 ≧ 90% (2)
Here, the average particle diameter (d _av ) in the formula (1) is the average value of the diameters of the individual circles (equivalent circle diameters) when assuming a circle having the same area as each carbide observed in the cross section of the steel sheet. It is.
Further, N _TC and N _{SC in the} formula (2) are N _TC : the total number of carbides per observation area 100 μm ² , N _SC : the number of carbides satisfying the condition that d _L / d _S is 1.4 or less, The major axis of the carbide is d _L , and the minor axis is d _S.
[4] The method for producing a high carbon cold-rolled steel sheet according to the above [3], wherein the hot-rolled steel sheet is repeatedly subjected to cold rolling and spheroidizing annealing 2 to 5 times.
[5] The high carbon cooling according to [3] or [4], wherein the rolling reduction of the cold rolling is 25 to 65%, and the temperature of the spheroidizing annealing is 640 to 720 ° C. A method for producing rolled steel sheets.

The high carbon cold-rolled steel sheet according to the present invention is a thin high-carbon cold-rolled steel sheet having a thickness of less than 1.0 mm, particularly 0.4 to 0.7 mm, and the average particle size of carbide is set to 0.2 to 0.7 μm. This steel plate is controlled and the spheroidization rate is controlled to 90% or more. This steel sheet, hardening and heat treatment of tempering, quenching in solution treatment period as short as 3 to 15 minutes, good impact properties by heat treatment of low temperature tempering (impact value: 5 J / cm ² or higher) and hardness properties (600-750 HV) is obtained.

Further, the high carbon cold-rolled steel sheet according to the present invention is a condition in which a so-called low temperature tempering at 200 to 350 ° C. is performed after a short solution treatment, quenching to a martensite phase containing an inevitable residual γ phase. Thus, it has a clear advantage over the balance between hardness and impact properties (toughness) over conventional high carbon cold rolled steel sheets. That is, if the high carbon cold-rolled steel sheet according to the present invention is used, a machine tool part made of high carbon steel having excellent toughness after quenching and tempering while ensuring excellent hardenability can be obtained. In particular, the cold-rolled steel sheet disclosed in the present invention requires not only a balance between hardness and toughness but also wear resistance and fatigue resistance, and applications that require excellent durability under harsh usage environments such as knitted needles. It is suitable for.

It is explanatory drawing which shows the example of the test apparatus of the impact test used for evaluation of this invention. It is explanatory drawing which shows the shape of the test piece of the impact test used for evaluation of this invention. It is a graph which shows the relationship between hardening hardness and the heat retention time of solution treatment (heating temperature: 800 degreeC). It is a graph which shows the relationship between hardening hardness and the heat retention time of solution treatment (heating temperature: 780 degreeC). It is a graph which shows the relationship between the heating holding time and Nb content of the solution treatment from which quenching hardness 700HV is obtained. It is a graph which shows the relationship between an impact value and tempering temperature. Impact value: 5 J / cm ² is a graph showing the relationship between the tempering temperature and the Nb content is obtained.

Embodiments of the present invention will be described below.
First, a steel sheet according to the present invention is obtained as a high-carbon cold-rolled steel sheet having a thickness of less than 1.0 mm by subjecting a hot-rolled steel sheet to soft annealing as needed, and repeatedly repeating cold rolling and spheroidizing annealing. It is. Thereafter, the high-carbon cold-rolled steel sheet is subjected to predetermined secondary processing and solution treatment, and is subjected to quenching and tempering treatments and used for members (mechanical parts) such as knitted needles.

First, the chemical composition of the steel sheet of the present invention is as follows: C: 0.85 to 1.10 mass%, Mn: 0.50 to 1.0 mass%, Si: 0.10 to 0.35 mass%, P: 0.030 mass% or less, S: 0.030 mass% or less, Cr: The reasons for specifying 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 the high carbon cold rolled steel sheet. The lower limit value can ensure a hardness of 600 to 750 HV with precision parts such as knitted needles, and the upper limit value can be controlled to a level of carbide that does not impede various types of cold work. Were determined. That is, the lower limit value was defined as 0.85 mass% in order to stably secure a hardness of 600 HV in a short time quenching and tempering treatment. Moreover, the upper limit was defined as 1.10 mass% as the upper limit that can withstand a wide variety of plastic processing such as punchability, swaging, bendability, and machinability. When the spheroidizing treatment of carbide is performed by repeating cold rolling and spheroidizing annealing, the cold workability is improved. However, when C exceeds 1.10 mass%, the rolling load in the hot rolling process and the cold rolling process becomes high, and the problems in the manufacturing process, such as the frequency of cracks at the coil end becoming remarkably high, become obvious. To do. For this reason, C is specified in the range of 0.85 to 1.10 mass%. Preferably, it is 0.95 to 1.05 mass%.

Mn: 0.50-1.0mass%
Mn is an element effective for deoxidation of steel, and is an element that can improve the hardenability of steel and stably obtain a predetermined hardness. When the target is a high carbon steel sheet applied to a severe use, the effect of the present invention becomes remarkable at 0.50 mass% or more. Therefore, the lower limit is defined as 0.50 mass%. On the other hand, if it exceeds 1.0 mass%, a large amount of MnS precipitates and becomes coarse during hot rolling, so that cracks and the like frequently occur during part processing. Therefore, the upper limit is defined as 1.0 mass%. For these reasons, Mn was specified in the range of 0.50 to 1.0 mass%. Preferably, it is 0.50 to 0.80 mass%.

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

P: 0.030 mass% or less, S: 0.030 mass% or less P and S are unavoidably present in the steel as impurity elements, and since both adversely affect impact properties (toughness), it is preferable to reduce as much as possible. P contains up to 0.030 mass% and S contains up to 0.030 mass%. For this reason, P is specified to be 0.030 mass% or less, and S is specified to be 0.030 mass% or less. In order to maintain more excellent impact characteristics, it is preferable to contain P up to 0.020 mass% and S up to 0.010 mass%.

Cr: 0.35-0.45 mass%
Although Cr is an element that improves the hardenability of steel, it dissolves in the carbide (cementite) and delays the remelting of the carbide in the heating stage. Therefore, if added in a large amount, the hardenability is adversely affected. Therefore, the upper limit value of Cr is regulated to 0.45 mass%. From the balance between hardness and impact characteristics after quenching and tempering, the lower limit value of Cr was defined as 0.35 mass%. For these reasons, Cr is specified in the range of 0.35 to 0.45 mass%.

Nb: 0.005-0.020 mass%
Conventionally, Nb is an element that expands the non-recrystallization temperature range of steel during hot rolling and simultaneously precipitates as NbC to contribute to the refinement of austenite grains. For this reason, high carbon steel may be added in anticipation of the effect of refining the structure after the cold rolling step. In the present invention, Nb is added in an amount of 0.005 to 0.020 mass% mainly for the purpose of restoring toughness by tempering at a low temperature after quenching. If a small amount of Nb is added, NbC that contributes to refinement of the structure is not formed, and Nb is in a dilute solid solution state. When Nb is in a dilute solid solution state, it is considered that diffusion of C in the ferrite phase and martensite phase having a BCC structure is promoted. That is, the diffusion of C dissolved in the ferrite phase from the spherical carbide into the austenite phase during heating in the quenching treatment and the diffusion and precipitation of supersaturated solid solution C in the martensite phase during heating in the tempering treatment are promoted. As a result, at the present time, it is considered that both the improvement of hardenability by short-time heating and the recovery of toughness by low-temperature tempering treatment can be achieved. When Nb is added in excess of 0.020 mass%, the precipitation of NbC becomes remarkable, the Nb dilute solid solution state cannot be secured, and the effect of promoting the diffusion of C due to the Nb dilute solid solution state is observed. Disappear. For this reason, the upper limit of Nb addition amount was prescribed | regulated to 0.020 mass%. In addition, Preferably it is 0.015 mass% or less. On the other hand, when the Nb addition amount is less than 0.005 mass%, the above-described effects cannot be expected. For this reason, the minimum of Nb addition amount was prescribed | regulated to 0.005 mass%. For these reasons, Nb was specified in the range of 0.005 to 0.020 mass%.

Although the above-mentioned components are basic components, in the present invention, one or two selected from Mo and V can be further contained as an optional selection element as required.
Mo and V may inevitably be contained in each of Mo: less than 0.001 mass% and V: less than 0.001 mass%. Furthermore, in this invention, in order to improve hardenability and the impact characteristic after tempering as arbitrary selection elements, it can add more than the level which inevitably contains Mo and V. However, the addition effect of Mo or V exceeds a certain amount, the effect of Nb addition is lost. In order to maximize the effect of Nb addition, the content of Mo and V is limited within the following range. It is preferable to do.

Mo: 0.001 mass% or more and less than 0.05 mass% Mo is an element effective in improving the hardenability of steel. However, if the addition amount is large, low temperature tempering at 200 to 350 ° C may deteriorate the impact characteristics. Therefore, when added, it is defined as 0.001 mass% or more, which is higher than the level of unavoidably contained, and less than 0.05 mass%, which is a range that does not inhibit the impact characteristics. 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 that is effective in improving impact characteristics by refining the steel structure, but it may deteriorate the hardenability. Therefore, when added, it is defined as 0.001 mass% or more higher than the level of unavoidably contained and less than 0.05 mass% within a range that does not impair hardenability. The addition of V is preferably 0.01 to 0.03 mass%. The balance other than the above components is Fe and inevitable impurities.

Next, the carbide of the steel plate according to the present invention will be described.
In the high carbon cold-rolled steel sheet of the present invention, the average particle diameter (d _av ) and spheroidization ratio (N _SC / N _TC ) of the carbide dispersed in the steel sheet satisfy the following expressions (1) and (2), respectively. It is necessary.
0.2 ≦ d _av ≦ 0.7 (μm) (1)
(N _SC / N _TC ) × 100 ≧ 90% (2)
Here, the average particle diameter (d _av ) (μm) of the formula (1) is the diameter of each circle (equivalent circle diameter) when assuming a circle having the same area as each carbide observed in the cross section of the steel sheet. Average value. When the average particle diameter (d _av ) is in this range, the impact properties are excellent, and the desired quenching hardness can be easily achieved even with a solution treatment for a short time. When the average particle size (d _av ) is less than 0.2 μm, experience shows that the load during secondary processing, which is processing into a needle shape, increases. The desired hardenability improvement is difficult to achieve, which is not preferable.

In the present invention, carbides defined in N _TC and N _SC of the spheroidization ratio is a ratio which is spheroidized (2). Here, _NTC is the total number of carbides per 100 μm ² observation area. Further, N _SC is the number of carbide which can be regarded to be spheroidized in the same observation field, _d L _/ d _S: set to 1.4 following conditions are satisfied carbides number. Here, the major axis of the carbide was d _L and the minor axis was d _S.

The carbide is not necessarily formed into a perfect sphere, and is often observed as an ellipse depending on the observation surface. Therefore, the ratio of the major axis to the minor axis (d _L / d _S ) determines the spheroidization. The degree was specified. Under such circumstances, in the present invention, _d L _/ d _S: 1.4 satisfying the following conditions carbides were defined N _SC is its number is regarded as being spheroidized. Further, the reason why the spheroidization ratio (N _SC / N _TC ) × 100 is 90% or more is because empirical knowledge has been found that the secondary workability of the steel sheet is good within this range. is there.

As described above, the measurement of the average particle size and the spheroidization ratio of the carbide described above was performed by observing the secondary electron image at a magnification of 2000 times using a scanning electron microscope.
Carbide cuts a plate-shaped test piece in a direction perpendicular to the rolling direction of the sample before heat treatment using a steel sheet after cold rolling, and performs processing such as resin embedding, and an observation area of 100 μm ² near the center of the plate thickness. The equivalent circle diameter, d _L / d _S ratio, N _TC , and N _SC were measured in the range, and the average value for five fields of view was calculated. For these measurements and calculations, commercially available image analysis software “winroof” (trade name) was used.

Next, the manufacturing method of the steel plate which concerns on this invention is demonstrated.
The hot-rolled steel sheet used in the present invention may be obtained under normal manufacturing conditions. For example, a steel slab having the above-mentioned chemical composition (slab) is heated to 1050 to 1250 ° C, hot-rolled at a finishing temperature of 800 to 950 ° C, and coiled at a winding temperature of 600 to 750 ° C. it can. In addition, what is necessary is just to set suitably the plate | board thickness of a hot-rolled steel plate so that it may become a suitable cold reduction rate from the plate | board thickness of desired cold-rolled steel plate.

A high-carbon cold-rolled steel sheet with a thickness of less than 1.0 mm is manufactured by repeating cold rolling (25-65%) and spheroidizing annealing (640-720 ° C) multiple times. This cold rolling (25 to 65%) and spheroidizing annealing (640 to 720 ° C.) are preferably repeated 2 to 5 times, respectively.

In the present invention, cold rolling (25 to 65%) and spheroidizing annealing (640 to 720 ° C.) are repeated a plurality of times. The reason for this is that the average particle size (d _av ) and spheroidization rate (N _SC / N _TC ) × 100 of the carbides are controlled so as to satisfy the above-described equations (1) and (2), respectively, as described below. It is to do.
First, cracks are introduced into the carbide by cold rolling, and the carbide that has just started to be spheroidized is spheroidized. However, it is difficult to increase the spheroidization rate of carbide to 90% or more by only one spheroidizing annealing, and rod-like or plate-like carbide remains. In such a case, the hardenability is also adversely affected and the cold workability of precision parts is deteriorated. Therefore, in order to make the spheroidization rate (N _SC / N _TC ) × 100 of carbide 90% or more, it is optimal to alternately repeat cold rolling and spheroidizing annealing. As a result, a fine carbide distribution with a high spheroidization rate is obtained in the steel sheet.
Particularly preferred are 2-5 cold rolling and 2-5 spheroidizing annealing.

When spheroidizing annealing is performed on a steel sheet (cold rolled steel sheet) having a cold rolling reduction of less than 25%, carbides become coarse. On the other hand, if the cold rolling reduction ratio exceeds 65%, the cold rolling operation load may be too large. For this reason, the cold rolling reduction ratio is preferably in the range of 25 to 65%.
In the final cold rolling, since the spheroidizing annealing is not performed after the cold rolling, the lower limit of the rolling reduction is not particularly limited.

When the spheroidizing annealing temperature is lower than 640 ° C., the spheroidizing tends to be insufficient, and when spheroidizing annealing is repeated at a temperature higher than 720 ° C., the carbide tends to be coarsened. Therefore, the spheroidizing annealing temperature is preferably in the range of 640 to 720 ° C. The holding time of the spheroidizing annealing can be appropriately selected at a temperature within this range within a range of 9 to 30 hours.
In addition, the same temperature range is preferable also about the softening annealing aiming at softening of the hot rolled steel sheet before cold rolling.
The above is the method for producing a high carbon cold-rolled steel sheet according to the present invention. In order to make this steel sheet the final object, a mechanical part such as a knitted needle, the following heat treatment is performed after processing into a predetermined shape. It is preferable to carry out.

After processing high carbon cold rolled steel sheets with carbides of 90% or more spheroidized into various machine parts (pressing, grooving, swaging, etc.), solution treatment, rapid cooling (quenching), and tempering Apply processing. In the solution treatment, the heating temperature is 760 to 820 ° C., and the holding time is short 3 to 15 minutes. It is preferable to use oil for quenching (rapid cooling). In the tempering treatment, the tempering temperature is preferably 200 to 350 ° C. Further, it is more preferably 250 to 300 ° C. As a result, various machine parts having a hardness of 600 to 750 HV can be manufactured.

If the retention time of the solution treatment is longer than 15 minutes, the carbide is excessively melted and the austenite grains are coarsened, so that the martensite phase after quenching becomes coarse and the impact characteristics are deteriorated. Therefore, the upper limit of the solution treatment retention time is preferably 15 minutes. On the other hand, when the time is shorter than 3 minutes, the carbide is not sufficiently dissolved and it is difficult to burn, so the lower limit of the solution treatment holding time is preferably 3 minutes. More preferably, it is in the range of 5 to 10 minutes.

If the tempering temperature is less than 200 ° C, the toughness recovery of the martensite phase is insufficient. On the other hand, when the tempering temperature exceeds 350 ° C., the impact value is recovered, but since the hardness is lower than 600 HV, durability and wear resistance become a problem. Therefore, the appropriate range of tempering temperature is preferably 200 to 350 ° C. More preferably, it is 250 to 300 ° C. The tempering holding time can be appropriately selected within the range of 30 minutes to 3 hours.

Steels having various chemical compositions were vacuum melted and cast into 30 kg steel ingots. This steel ingot was subjected to split rolling, and then hot rolled under the conditions of a heating temperature of 1150 ° C and a finishing temperature of 870 ° C to obtain hot rolled steel sheets of 4 mm and 2 mm. Thereafter, cold rolling and spheroidizing annealing were performed under the production 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. Next, the cold-rolled steel sheet was subjected to a solution treatment (charged into an 800 ° C. furnace for 10 minutes) under the conditions shown in Table 2, followed by oil quenching and tempering (tempering temperature: 250 ° C.).
Predetermined specimens were collected 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 with a Vickers hardness tester under a load of 5 kg (test force: 49.0 N).

Impact characteristics were evaluated by Charpy impact test. The impact test piece was a U-notch test piece (notch depth 2.5 mm, notch radius 0.1 mm) with a notch width of 0.2 mm. FIG. 1 shows a state where the test piece is installed in the test apparatus, and FIG. 2 shows the shape of the test piece. The reason for adopting such a test piece and test method is as follows.

For a steel plate with a thickness of less than 1.0 mm, which is the subject of the present invention, with the Charpy impact test equipment for metal materials that has been used in the past, the rated capacity of the test equipment is too large, at least 50 J, so accurate evaluation is possible. There was a problem that I could not. As an impact test device having a rated capacity of less than 50 J, a 1 J impact test device (model DG-GB, manufactured by Toyo Seiki Seisakusho Co., Ltd.) was used. This test apparatus is a Charpy impact tester based on the Charpy impact test method (JIS K 7077) of carbon fiber reinforced plastic. This test apparatus was improved and the distance between the supports was changed from 60 mm to 40 mm. In this test apparatus, the distance between the support bases was changed from 60 mm to 40 mm in order to make the conditions close to the JIS standard (JIS Z 2242), which is a Charpy impact test method for metal materials.

As shown in FIG. 2, 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 electric discharge machining was used. The reason why the notch radius is reduced is that the deflection of the plate becomes a problem in the case of a thin plate of less than 1.0 mm during the Charpy impact test. Therefore, increasing the stress concentration factor minimizes the deflection of the plate during the Charpy impact test. In order to obtain a stable impact value. By adopting this test method and test piece shape, it has been confirmed that impact characteristics in a state close to the actual use environment can be obtained. In the present invention, it was judged that the impact characteristics were excellent when the impact value was 5 J / cm ² or more.

Example 1
The effects of various additive elements on the cross-sectional hardness and impact value after oil quenching and tempering after solution treatment were confirmed. The test results are shown in Tables 3 and 4 together with chemical components. As the manufacturing conditions for the cold-rolled steel sheet, the condition of 5A (Table 1) was used for both. The rolling reduction was controlled within the range described in Table 1.
The cross section hardness was measured at the center of the plate thickness by embedding a test piece cut in the direction perpendicular to the rolling direction into a resin, polishing the cross section. The impact value was measured using an impact test piece taken in the rolling parallel direction. The obtained results (hardness, impact value) are shown in Tables 3 and 4.
The evaluation when the impact value is larger than 5 J / cm ² and the hardness satisfies 600 to 750 HV is “◎”, and the case where any of the above target values of the impact value and the hardness is not satisfied is evaluated as “X”.

In the example shown in Table 3, the impact value and the quenching and tempering hardness were outside the target values for the case where the C amount was outside the lower limit (steel type No. 1). When the amount of C deviated from the upper limit (steel type No. 6), the quenching and tempering hardness exceeded the target value of 600 to 750 HV, and the impact value was below the target value of 5 J / cm ² . Nb-free materials have a C amount of 0.85 mass% (steel grade No. 2, comparative example) and a C amount of 1.10 mass% (steel grade No. 4, comparative example). The value was below 5 J / cm ² and the evaluation was x. On the other hand, the steel plates corresponding to the chemical components of the inventive examples (steel types No. 3, 5, 7, 8, 9, 10) had quenching and tempering hardness within the target range and excellent impact characteristics.

In the examples shown in Table 4, the steel plates with the chemical composition corresponding to the inventive examples (steel types No. 15, 16, 17, 19, 21) all have a quenching and tempering hardness satisfying the target value of 600 to 750 HV and excellent impact properties. It was. Nb not added (steel grade No. 11), Nb not added and V added amount exceeding 0.05 mass% (steel grade No. 12), Nb not added and Mo added amount exceeding 0.05 mass% ( Steel type No. 13), Nb + Mo composite addition with Nb addition less than 0.005 mass% (steel grade No. 14), Nb + Mo composite addition with Nb addition over 0.020 mass% (steel type No. 18), Nb + Mo composite When the addition of Mo is greater than 0.05 mass% (steel grade No. 20), and when Nb + Mo + V is added and the addition of V is greater than 0.05 mass% (steel grade No. 22), the quenching and tempering hardness is 600 ~ Although it meets 750 HV, or impact properties are inferior, either but impact properties are satisfied the target value 5 J / cm ² are reduced quenching and tempering hardness, quenching and tempering hardness and impact properties are both desired value It was below the lower limit of.

(Example 2)
Using a hot-rolled steel sheet having the chemical composition of steel type No. 3 (Table 3), the production conditions for cold rolling and spheroidizing treatment shown in Table 1 were changed, and cold-rolled steel sheets having the thicknesses shown in Table 5 were used. Got. Table 5 shows the spheroidization ratio and carbide average particle size of the obtained cold-rolled steel sheet. Furthermore, the obtained cold-rolled steel sheet was subjected to oil quenching and low-temperature tempering after solution treatment under the conditions shown in Table 2 as in Example 1. The cross-sectional hardness and impact value of the obtained cold-rolled steel sheet after solution treatment and quenching and tempering were measured in the same manner as in Example 1, and are shown in Table 5.

When the number of spheroidizing annealing was one (manufacturing condition No. 1), the spheroidizing rate was insufficient and the impact characteristics were inferior. If the number of spheroidizing annealing is two times, combining the spheroidizing annealing temperature at 600 to 635 ° C and the cold rolling reduction ratio of 10 to 20% and performing each twice will result in insufficient spheroidization and impact characteristics. It was inferior (manufacturing condition No. 2A). If the spheroidizing annealing temperature is 600 to 635 ° C and the cold rolling reduction ratio is 70 to 85% and repeated twice, the impact characteristics are sufficient, but the average particle size of the carbide falls outside the lower limit, and quenching and tempering treatment. The later hardness exceeded the target value (Manufacturing condition No. 2C).

If the spheroidizing annealing temperature is 640 to 720 ° C and the cold rolling reduction ratio is 10 to 20% and each is repeated twice, the spheroidization is sufficient, but the average particle size of the carbide exceeds the upper limit of the target value, and the impact characteristics Was inferior (production conditions No. 2D). This is because if the carbide is too large, the undissolved carbide in the martensite substrate becomes larger during quenching, and the area of the interface between the undissolved carbide and the martensite substrate that tends to be the starting point for fracture is large, so the impact characteristics are inferior. It seems to have become. On the other hand, if the spheroidizing annealing temperature is 640 to 720 ° C and the cold rolling reduction ratio is 25 to 65% and repeated twice each, the spheroidizing ratio, carbide grain size, and hardness after quenching and tempering are the target values, respectively. It was within the range and had excellent impact characteristics (manufacturing condition No. 2B).

When the number of spheroidizing annealing is set to 4 and the first to fourth cold rolling reduction ratios are all set to 25 to 65%, the spheroidizing ratio and carbide particle size fall within the target values, and the impact characteristics are excellent. (Manufacturing conditions No. 5A). When manufacturing condition No. 5A and spheroidizing annealing temperature are the same, and the first to fourth cold rolling reductions are all set to 10 to 20%, the carbide particle size becomes too large exceeding the target value, and impact characteristics (Production condition No. 5B).

(Example 3)
Using the hot-rolled steel sheet having the chemical composition of steel type No. 16 (Table 4), the production conditions shown in Table 1 were changed to obtain cold-rolled steel sheets having the thicknesses shown in Table 6. Table 6 shows the spheroidization ratio and carbide average particle size of the obtained cold-rolled steel sheet. Furthermore, the obtained cold-rolled steel sheet was subjected to oil quenching and low-temperature tempering after solution treatment under the conditions shown in Table 2 as in Example 1. The cross-sectional hardness and impact value of the obtained cold-rolled steel sheet after solution treatment and quenching and tempering were measured in the same manner as in Example 1 and are shown in Table 6.
The steel sheets that had been cold-rolled and spheroidized using the production conditions No. 2B and No. 5A corresponding to the production method of the present invention satisfied the target spheroidization rate and target impact value.

Steel sheets with chemical components within the scope of the present invention are hardened by Nb addition and impact properties after heat treatment are improved, so they are suitable for machine tool parts used in harsh environments with hypereutectoid steels. Yes.
A hypereutectoid steel sheet having a C of 0.85 to 1.10 mass% is suitable for applications that require a balance between hardness and toughness in harsh usage environments such as knitted needles.

Claims

The chemical composition of the steel sheet is C: 0.85 to 1.10 mass%, Mn: 0.50 to 1.0 mass%, Si: 0.10 to 0.35 mass%, P: 0.030 mass% or less, S: 0.030 mass% or less, Cr: 0.35 to 0.45 mass% , Nb: 0.005 to 0.020 mass%, comprising the balance Fe and inevitable impurities,
The average particle diameter (d av ) and spheroidization rate (N SC / N TC ) × 100% of the carbides dispersed in the steel sheet satisfy the following expressions (1) and (2), respectively, A high-carbon cold-rolled steel sheet characterized by being less than 1.0 mm.
0.2 ≦ d av ≦ 0.7 (μm) (1)
(N SC / N TC ) × 100 ≧ 90% (2)
Here, the average particle diameter (d av ) in the formula (1) is the average value of the diameters of the individual circles (equivalent circle diameters) when assuming a circle having the same area as each carbide observed in the cross section of the steel sheet. It is.
Further, N TC and N SC in the formula (2) are N TC : the total number of carbides per observation area 100 μm 2 , N SC : the number of carbides satisfying the condition that d L / d S is 1.4 or less, The major axis of the carbide is d L , and the minor axis is d S.
The chemical composition further contains one or two selected from Mo and V, and each content is 0.001 mass% or more and less than 0.05 mass%. The high carbon cold-rolled steel sheet according to 1.
In the 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 claim 1 or 2,
The average particle size (d av ) and spheroidization rate (N SC / N TC ) of the carbide dispersed in the high carbon cold-rolled steel sheet satisfy the following formulas (1) and (2), respectively, A method for producing a high carbon cold-rolled steel sheet, characterized in that the thickness of the rolled steel sheet is less than 1.0 mm.
0.2 ≦ d av ≦ 0.7 (μm) (1)
(N SC / N TC ) × 100 ≧ 90% (2)
Here, the average particle diameter (d av ) in the formula (1) is the average value of the diameters of the individual circles (equivalent circle diameters) when assuming a circle having the same area as each carbide observed in the cross section of the steel sheet. It is.
Further, N TC and N SC in the formula (2) are N TC : the total number of carbides per observation area 100 μm 2 , N SC : the number of carbides satisfying the condition that d L / d S is 1.4 or less, The major axis of the carbide is d L , and the minor axis is d S.
The method for producing a high-carbon cold-rolled steel sheet according to claim 3, wherein the hot-rolled steel sheet is repeatedly subjected to cold rolling and spheroidizing annealing 2 to 5 times.
The method for producing a high carbon cold-rolled steel sheet according to claim 3 or 4, wherein the rolling reduction of the cold rolling is 25 to 65%, and the temperature of the spheroidizing annealing is 640 to 720 ° C.