JP6089131B2 - High carbon cold rolled steel sheet and method for producing the same - Google Patents

Description

  The present invention relates to a high-carbon cold-rolled steel sheet used as a raw material for various machine parts produced by quenching and tempering, and is particularly hardened by a short-time solution treatment and sufficiently hard after low-temperature tempering (600 to 750 HV) and impact characteristics (toughness), and further 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 that have severe requirements for durability, wear resistance and the like. Here, the short-time solution treatment refers to a treatment in the range of 760 to 820 ° C. for 3 to 15 minutes, and the low temperature tempering treatment refers to a treatment in the temperature range of 200 to 350 ° C.

  In general, carbon steel materials for machine structures (SxxC) and carbon tool steel materials (SK) specified in JIS are used for various types of machine parts. When used as a wrought material, a predetermined hardness and toughness (impact characteristics) are imparted by performing quenching and tempering treatment after forming into a part shape through punching and various plastic processing. Among them, knitted fabric knitting needles knit the knitted fabric by pulling back the yarn while repeating reciprocating motion at a high speed, so that the butt portion of the needle body that comes into contact with the rotary drive part has sufficient strength and wear resistance. In addition, the hook portion that rubs against the yarn is required to have excellent impact characteristics at the tip portion due to reciprocation in addition to sufficient wear resistance.

  The high carbon cold-rolled steel sheet used as the material for knitting needles is used for knitting needles for flat knitting machines when the thickness is 1.0 mm or more, and when the thickness is less than 1.0 mm, a circular knitting machine or Used for knitting needles for warp knitting machines. In the latter needle, since a thin yarn is knitted at high speed, the thickness of the material used is often 0.4 to 0.7 mm. Furthermore, in addition to excellent cold workability (also called secondary workability), the material has sufficient hardness and sufficient toughness at the needle tip when quenched and tempered after secondary processing into a needle shape. Is required.

  In addition, so-called high carbon steel plates such as carbon steel materials for machine structures (SxxC) and carbon tool steel materials (SK) specified in JIS are finely classified according to the amount of C. In the region where the amount of C is less than 0.8 mass%, that is, the steel sheet of hypoeutectoid composition, the ferrite phase has a high fraction and is excellent in cold workability, but it is difficult to obtain sufficient quenching hardness, It is not suitable for knitting needle applications that require wear resistance and durability of the needle body. On the other hand, among the hypereutectoid composition of 0.8 mass% or more, the high carbon steel sheet having a C content of 1.1 mass% or more has excellent hardenability, but cold workability due to a large amount of carbide (cementite). However, it is not suitable for knitting needles where precision and fine processing such as grooving is performed, and is limited to parts that require a simple shape and high hardness, such as blades and cold dies.

  Conventionally, carbon material steel or alloy tool steel of C: 0.8 to 1.1 mass% or a steel composition material added with a third element based on these steel compositions has 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 when actually used 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. A case where the tempering temperature is a low temperature of 200 to 350 ° C. is generally adopted. 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, low temperature tempering treatment in the temperature range of 200 to 350 ° C. However, the tempering of the martensite phase is not sufficiently performed, and the impact characteristics (toughness) are not sufficiently improved or the toughness value may vary.
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 a technique for adding a carbonitride-forming element such as Ti, Nb, or V and refining the metal structure using a fine carbonitride of these 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 Patent Document 1, by adding carbonitride-forming elements such as V, Ti, and Nb to hypoeutectoid steel of C: 0.5 to 0.7 mass%, the prior austenite grains are refined. And a technique for improving the toughness value (impact characteristics) is disclosed.

  In Patent Document 2, C: 0.60 to 1.30 mass% hypoeutectoid steel to hypereutectoid steel with a wide range of carbon content, Ni ≦ 1.8 mass%, Cr as necessary. ≦ 2.0 mass%, V ≦ 0.5 mass%, Mo ≦ 0.5 mass%, Nb ≦ 0.3 mass%, Ti ≦ 0.3 mass%, B ≦ 0.01 mass%, Ca ≦ 0.01 mass% or A technique for improving impact characteristics by adding two or more kinds and controlling the volume fraction (Vf) of undissolved carbide in the range of 8.5 <15.3 × Cmass% −Vf <10.0 is disclosed. ing.

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 for improving the formability of the ferrite matrix because the carbon level is a hypoeutectoid composition, and is suitable for machine parts that require high hardness such as knitted needles. It is difficult to apply.

Moreover, the technique described in patent document 2 is about the steel grade in which the carbon content which added Mo, V, Ti, Nb, B etc. is 0.67-0.81 mass%, The technique is Obviously, this addition is intended to improve the properties of hypoeutectoid steel. In Patent Document 2, there is no disclosure regarding the action and optimization of individual third elements in steel having a carbon content exceeding 0.81 mass%.
Furthermore, in the technique described in Patent Document 2, only the upper limit value that does not adversely affect the impact value as described in the specification is specified for the addition amount of the third element. Also, since there is no lower limit, there is no disclosure of 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 disclose that high-carbon cold-rolled steel sheets are quenched with a short solution treatment holding time such as 3 to 15 minutes, and desired impact characteristics are obtained by low-temperature tempering at 200 to 350 ° C. In addition, there is no disclosure of a technology that advantageously improves the predetermined hardness, and there is no disclosure of a technology that evaluates impact characteristics of a steel plate having a thickness of less than 1.0 mm.
Accordingly, the present invention is, after a short time of solution treatment, subjected to quenching and low temperature tempering treatment, the impact characteristics are evaluated in thickness near that is actually used, the impact value is 5 J / cm ² or more, and hardness The object 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, which can exhibit mechanical properties having a thickness of 600 to 750 HV. And

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 a carbon amount of 0.85 mass% ≦ C ≦ 1.10 mass% suitable for a knitted needle from the viewpoint of workability, hardenability, hardness and toughness after low temperature tempering, etc. Thus, the core of the present technology is that it has been found that it is effective to add Nb as a third element in a predetermined range and control the average particle size of carbide and the degree of spheroidization.

  In particular, in the present invention, a new test method (FIGS. 1 and 2), which will be described later, has been devised, focusing on the impact value in a low-temperature tempering state of 200 to 350 ° C., and targeting a steel sheet of less than 1.0 mm, which has conventionally been difficult to evaluate toughness. . As a result of investigation using the new test method, it was made based on a novel finding that a predetermined amount of Nb addition satisfies only the required characteristics.

That is, the present inventors have intensively studied in order to solve the above problems, and the basic components are C: 0.85 to 1.10 mass%, Mn: 0.50 to 1.0 mass%, Si: 0.10. Add 0.005 to 0.020 mass% of Nb to high carbon steel defined in the range of 0.35 mass%, P ≦ 0.030 mass%, S ≦ 0.030 mass%, 0.35 ≦ Cr ≦ 0.45 mass% By controlling the spheroidization of carbide and the average particle size within a predetermined range, it is possible to obtain a high carbon cold-rolled steel sheet that has both hardenability and toughness, and further shortens the quenching time and tempering. It was found that the temperature could be lowered. 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, carbide spheroidization rate, and average particle size.
First, the 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 0%, 0.010%, 0.020 %, 0.055%, and cold rolled (rolling ratio: 25 to 65%, final 3 to 50%) to a hot rolled steel sheet (4 mm thick) having the composition of the balance Fe and inevitable impurities. ) And softening annealing and spheroidizing annealing (640 to 700 ° C.) were each 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, and then it was oil-quenched to obtain 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 the 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 by heating and holding for a short time is slowed down. From the result of FIG. 4, when the heating temperature of the solution treatment is 780 ° C., the relationship between the heating holding time at which the quenching hardness reaches 700 HV and the Nb content is shown in FIG.
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 the shortest, 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 that can prevent the variation in firing elongation and the bending that are problems in the needle processing manufacturer.
Further, the cold-rolled steel sheets having various Nb contents were 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, further tempering was performed. 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. When the tempering temperature was 200 ° C. or higher, the impact value was highest when the Nb content was 0.010 mass%.
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 an impact value of 5 J / cm ² is obtained is the lowest in the case of a steel plate with Nb content: 0.010 mass%. When the Nb content is increased beyond 0.020 mass%, the tempering temperature at which an impact value of 5 J / cm ² is obtained becomes higher. When the tempering temperature becomes high, the hardness decreases, and the durability as a needle decreases. Further, it has been found that when the Nb content is less than 0.005 mass%, it is necessary to increase the tempering temperature in order to secure a desired impact value.
From FIG. 5 and FIG. 7, in order to combine high hardness after tempering and impact characteristics, 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%, S ≦ 0.030 mass%, Cr: 0.35 to 0.45 mass%, Nb: 0.005 to 0.020 mass%, comprising the balance Fe and inevitable impurities, and the average particle size of carbides dispersed in the steel sheet ( d _av ) and the spheroidization ratio (N _SC / N _TC ) × 100% satisfy the following formulas (1) and (2), respectively, and the plate thickness of the steel sheet is less than 1.0 mm. Carbon cold rolled steel sheet.
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.
N _TC and N _{SC in the} formula (2) are respectively N _TC : the total number of carbides per observation area of 100 μm ² , and N _SC : the number of carbides satisfying the condition of d _L / d _S ≦ 1.4, Here, the major axis of the carbide is d _L and the minor axis is d _S.
[2] The chemical component 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 diameter (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 plate thickness of the steel sheet is less than 1.0 mm A method for producing a high carbon cold-rolled steel sheet, comprising:
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.
N _TC and N _{SC in the} formula (2) are respectively N _TC : the total number of carbides per observation area of 100 μm ² , and N _SC : the number of carbides satisfying the condition of d _L / d _S ≦ 1.4, Here, 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. By controlling to a size of 2 to 0.7 μm and controlling the spheroidization rate described later to 90% or more, good impact characteristics (impact) can be obtained by quenching and tempering even in a solution treatment for a short time of 3 to 15 minutes. Value; 5 J / cm ² or more) and hardness properties (600-750 HV).

  Furthermore, the high carbon cold-rolled steel sheet according to the present invention is hardened into a full martensite structure after a short-time solution treatment, and then compared to conventional high-carbon steel under so-called low-temperature tempering conditions of 200 to 350 ° C. Demonstrates a clear advantage in terms of the balance between hardness and impact properties (toughness). That is, it is possible to obtain a machine tool part made of high carbon steel having excellent toughness after quenching and tempering while ensuring excellent hardenability. 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 in 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. It is a graph which shows the relationship between the tempering temperature and Nb content from which an impact value: 5 J / cm < ² > 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. Is. Thereafter, the high-carbon cold-rolled steel sheet is subjected to predetermined secondary processing and solution treatment, subjected to quenching and tempering treatment, and is provided to a member such as a knitted needle.

  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%, The reasons for defining S ≦ 0.030 mass%, Cr: 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 hinder a wide variety of cold working. Were determined. That is, the lower limit value was defined as 0.85 mass% in order to stably secure a hardness of 600 HV by a short-time quenching and tempering treatment. Further, the upper limit value is defined as 1.10 mass% as an upper limit that can withstand various plastic workings such as punchability, swaging properties, bendability, and machinability. When the carbide spheroidization treatment is performed by repeating cold rolling and spheroidizing annealing, the cold workability is improved, but if it exceeds 1.10 mass%, the rolling load in the hot rolling process and the cold rolling process is improved. In addition, problems in the manufacturing process, such as the frequency of cracks at the coil ends becoming extremely high, become obvious. Preferably, it is 0.95-1.05 mass%.

Mn: 0.50 to 1.0 mass%
Mn is an element effective for deoxidation of steel, and can improve the hardenability of steel and stably obtain a predetermined hardness. When the target is a high carbon steel plate applied to a severe use, the effect of the present invention becomes remarkable at 0.50 mass% or more. 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 parts processing. Therefore, the upper limit of Mn is defined as 1.0 mass%. Preferably it is 0.50 to 0.80 mass%.

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

P ≦ 0.030 mass%, S ≦ 0.030 mass%
P and S are unavoidably present in the steel as impurity elements, and both have an adverse effect on impact properties (toughness), so it is preferable to reduce them as much as possible. P contains up to 0.030 mass%, and S contains up to 0.030 mass%. 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 is defined as 0.45 mass%. From the balance between hardness and impact characteristics after quenching and tempering, the lower limit was set to 0.35 mass%.

Nb: 0.005-0.020 mass%
Nb has been conventionally known to be an element that expands the non-recrystallization temperature range of steel during hot rolling and at the same time precipitates as NbC and contributes to the refinement of austenite grains. It may be added in anticipation of the effect of refining the structure after the hot rolling process. In the present invention, 0.005 to 0.020 mass% of Nb is added mainly for the purpose of toughness recovery by tempering at a low temperature after quenching. If a small amount of Nb is added, NbC that contributes to the 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 a ferrite phase and a martensite phase having a BCC structure is promoted. That is, the diffusion of C dissolved in the ferrite phase into the austenite phase during heating in the quenching process and the diffusion and precipitation of supersaturated solid solution C in the martensite phase during heating in the tempering process are promoted. At the present time, it is thought that improvement in hardenability by time heating and recovery of toughness by low-temperature tempering treatment can be achieved at the same time. 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 recognized. It becomes impossible. For this reason, the upper limit of Nb addition amount was limited to 0.020 mass%. In addition, Preferably it is 0.015 mass% or less. If the Nb addition amount is less than 0.005 mass%, the above-described effects cannot be expected.

  Mo and V may inevitably contain Mo <0.001 mass% and V <0.001 mass%, respectively. 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, if Mo or V is added in a predetermined amount or more, the effect of Nb addition is lost. Therefore, in order to maximize the effect of Nb addition, the contents of Mo and V are limited within the following ranges. It is preferable to do.

0.001 mass% ≦ Mo <0.05 mass%
Mo is an element effective for improving the hardenability of steel, but if the addition amount is large, the low temperature tempering at 200 to 350 ° C. may deteriorate the impact characteristics. Therefore, when added, it is specified to be less than 0.05 mass% within a range that does not inhibit the impact characteristics. Preferably, the addition of Mo is 0.01 to 0.03 mass%.

0.001 mass% ≦ V <0.05 mass%
V is an element effective for improving the impact characteristics by refining the steel structure, but may deteriorate the hardenability. Therefore, when added, it is specified to be less than 0.05 mass% within a range not impairing the hardenability. Preferably, the addition of V is 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.
The average particle size (d _av ) and the spheroidization rate (N _SC / N _TC ) of the carbide dispersed in the high carbon cold-rolled steel sheet of the present invention satisfy the following expressions (1) and (2), respectively. The plate thickness needs to be 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 an 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. is there. 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 secondary processing load on the needle shape increases, and when it exceeds 0.7 μm, a short time solution treatment is achieved. It becomes difficult and 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, N _TC 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, and satisfies the condition carbides number of d L _/ d _S ≦ 1.4. 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, and the satisfying carbide d L _/ d _S ≦ 1.4 defines the 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 that 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. A commercially available image analysis software winroof was used for these measurements and calculations.

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 production conditions. For example, a steel piece (slab) having the above-described chemical composition is heated to 1050 to 1250 ° C. and a finishing temperature of 800 to 950 ° C. It can manufacture by carrying out hot rolling and setting it as a coil by the coiling temperature of 600-750 degreeC. 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.

  By repeating cold rolling (25 to 65%) and spheroidizing annealing (640 to 720 ° C.) a plurality of times, a high carbon cold rolled steel sheet having a thickness of less than 1.0 mm is produced. This cold rolling (25 to 65%) and spheroidizing annealing (640 to 720 ° C.) are preferably repeated 2 to 5 times.

The reason why cold rolling (25 to 65%) and spheroidizing annealing (640 to 720 ° C.) are repeated a plurality of times is as follows. The average particle size (d _av ) of carbides and the spheroidizing rate (N _SC / N _TC) ) × 100 is controlled so as to satisfy the above-described expressions (1) and (2).
First, cracks are introduced into the carbide by cold rolling, and the carbide started only by spheroidizing annealing is spheroidized, but the spheroidizing rate of the carbide is increased to 90% or more by only one spheroidizing annealing. It is difficult and a 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 increase the spheroidization rate of carbide (N _SC / N _TC ) × 100 to 90% or more, it is optimal to alternately repeat cold rolling and spheroidizing annealing, resulting in fine and spheroidizing in the steel sheet. A high-rate carbide distribution is obtained.
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 are coarsened. On the other hand, if the cold rolling reduction ratio exceeds 65%, the cold rolling operation load may be too large, and therefore the cold rolling reduction ratio is preferably in the range of 25 to 65%.
In addition, about the last cold rolling which does not give spheroidizing annealing after cold rolling, the minimum of a rolling reduction is not specifically 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 coarsen, so the spheroidizing annealing temperature is in the range of 640 to 720 ° C. It is preferable to do. The holding time for the spheroidizing annealing can be appropriately selected at a temperature in 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 with spheroidization of 90% or more 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. Furthermore, it is 250-300 degreeC more preferably. Thereby, various machine parts with hardness 600-750HV can be manufactured.

  If the retention time of the solution treatment is longer than 15 minutes, the carbide is excessively dissolved, and the austenite grains are coarsened, so that the martensite phase after quenching becomes coarse and impact characteristics are deteriorated. Therefore, the upper limit of the solution treatment holding time is preferably 15 minutes. On the other hand, if 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 the range for 5 to 10 minutes.

  When 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 the tempering temperature is preferably 200 to 350 ° C. More preferably, it is 250-300 degreeC. The tempering holding time can be appropriately selected within the range of 30 minutes to 3 hours.

  Steels having various chemical compositions were melted in vacuum and cast into 30 kg steel ingots. This steel ingot was subjected to hot rolling under 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 several production conditions shown in Table 1 to obtain a cold rolled material having a plate thickness of 0.4 mm or more and less than 1.0 mm. The cold-rolled material was inserted into an 800 ° C. furnace for 10 minutes under the conditions shown in Table 2 and subjected to a solution treatment, followed by oil quenching and tempering at 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 according to JIS Z 2244 under a condition of a load of 5 kg (test force: 49.0 N) with a Vickers hardness tester.

  Impact properties 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 targeted by the present invention, a conventional Charpy impact test apparatus for metal materials has a rated capacity of 50 J or more, which is an accurate evaluation. There was a problem that 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 a Charpy impact test method (JIS K 7077) for carbon fiber reinforced plastic. This test apparatus was improved and the distance between the support bases 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 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, the deflection of the plate during the Charpy impact test is minimized by increasing the stress concentration factor. 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 determined that the impact characteristics were excellent when the numerical value of the impact value was 5 J / cm ² or more.

Example 1
Tables 3 and 4 show the test results of the effects of various additive elements on the cross-sectional hardness and impact value after oil quenching and tempering, together with chemical components, after solution treatment. The manufacturing conditions of the cold-rolled steel sheet were both 5A conditions (Table 1). 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 HV satisfies 600 to 750 is evaluated as “、”, and the case where any of the above target values of the impact value and the hardness is not satisfied is evaluated as “X”.

When the amount of C deviated from the lower limit (steel type No. 1), the impact value and the quenching and tempering hardness deviated from the target values. In the case where the amount of C deviated from the upper limit (steel type No. 6), the quenching and tempering hardness exceeded the target value, and the impact value was less than 5 J / cm ² . C: 0.85mass% (steel types No2, Comparative Example), C: 1.10mass% (steel types No4, Comparative Example) Each impact value falls below 5 J / cm ^2, evaluation was ×. On the other hand, the steel plates (steel types Nos. 3, 5, 7, 8, 9, 10) corresponding to the chemical components of the examples of the present invention had quenching and tempering hardness within the target range and excellent impact characteristics.

  In the examples shown in Table 4, all the steels having the chemical components corresponding to the inventive examples had the quenching and tempering hardness satisfying the target values and excellent impact properties (steel types Nos. 15, 16, 17, 19, and 21). V addition amount exceeding 0.05 mass% (steel grade No. 12), Mo addition amount exceeding 0.05 mass% (steel grade No. 13), Nb addition amount less than 0.005 mass% (steel grade No. 14), Nb addition Amount exceeding 0.020 mass% (steel grade No. 18), Nb + Mo composite addition with Mo addition amount greater than 0.05 mass% (steel grade No. 20), Nb + V composite addition with V addition amount greater than 0.05 mass% ( Steel type No. 22) is in the hardness target value, but the impact characteristics are inferior, the target impact value is satisfied, but the hardness is reduced, or both the hardness and impact characteristics are lower than the lower limit of the target value. It was below.

(Example 2)
Using a hot-rolled steel sheet having the chemical composition of steel type No. 3 (Table 3), the spheroidizing ratio, carbide average particle diameter, and solution treatment when changing the production conditions of cold rolling and spheroidizing treatment described in Table 1 Table 5 shows the hardness and impact value after oil quenching and tempering after the treatment.

  When the number of spheroidizing annealing was one (manufacturing condition No. 1), the spheroidizing rate was insufficient and the impact characteristics were inferior. When the number of times of spheroidizing annealing was 2, when the spheroidizing annealing temperature was 600 to 635 ° C. and the rolling reduction was 10 to 20% and each was performed twice, spheroidization was insufficient and the impact characteristics were inferior. (Manufacturing condition No2A). If the spheroidizing annealing temperature is 600 to 635 ° C. and the rolling reduction is 70 to 85% and each is repeated twice, the impact characteristics are sufficient, but the average particle size of the carbide is outside the lower limit, and the hardness after quenching and tempering treatment is sufficient. Exceeded the target value (manufacturing condition No. 2C).

  If the spheroidizing annealing temperature is 640 to 720 ° C. and the rolling reduction 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 are 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, when the spheroidizing annealing temperature is 640 to 720 ° C. and the rolling reduction is 25 to 65% and repeated twice each, the spheroidizing ratio, carbide particle size, and hardness after quenching and tempering are within the range of the target set values, respectively. The impact characteristics were excellent (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 are within the target values, and the impact characteristics are excellent. (Manufacturing condition No5A). When the manufacturing conditions No. 5A and the annealing temperature were made the same, and the first to fourth cold rolling reductions were all made 10 to 20%, the carbide particle size exceeded the target value and the impact characteristics were inferior. (Manufacturing condition No5B).

(Example 3)
Using a hot-rolled steel sheet having the chemical composition of steel type No. 16 (Table 4), the spheroidization ratio, carbide average particle diameter when changing the production conditions described in Table 1, and further, oil quenching and tempering after solution treatment. The later hardness and impact values are shown in Table 6. The steel sheets that were subjected to cold rolling and spheroidizing annealing using the manufacturing conditions No2B and No5A corresponding to the manufacturing method of the present invention satisfied the target spheroidization rate and the target impact value.

Steel sheets having 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 hyper-eutectoid 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 a severe use environment such as a knitted needle.

Claims (5)

The chemical composition of the steel sheet was 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%, 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 of carbides dispersed in the steel sheet (d _av ) And spheroidization ratio (N _SC / N _TC ) × 100% satisfy the following formulas (1) and (2), respectively, and the thickness of the steel sheet is less than 1.0 mm, Rolled steel sheet.
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.
N _TC and N _{SC in the} formula (2) are respectively N _TC : the total number of carbides per observation area of 100 μm ² , and N _SC : the number of carbides satisfying the condition of d _L / d _S ≦ 1.4, Here, 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%. Item 5. The high carbon cold rolled steel sheet according to Item 1. The average of the carbide | carbonized_material disperse | distributed in the said high carbon cold-rolled steel sheet in the method of manufacturing a high-carbon cold-rolled steel sheet by repeatedly performing cold rolling and spheroidizing annealing to the hot-rolled steel sheet which consists of the chemical composition of Claim 1 or 2. The particle size (d _av ) and the spheroidization rate (N _SC / N _TC ) satisfy the following formulas (1) and (2), respectively, and the plate thickness of the steel sheet is less than 1.0 mm: Manufacturing method of high carbon cold-rolled steel sheet.
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.
N _TC and N _{SC in the} formula (2) are respectively N _TC : the total number of carbides per observation area of 100 μm ² , and N _SC : the number of carbides satisfying the condition of d _L / d _S ≦ 1.4, Here, 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.

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