JP6479538B2 - Steel wire for machine structural parts - Google Patents

Description

  The present invention relates to a steel wire used as a material for machine structural parts. More specifically, when cold working after spheroidizing annealing on a wire manufactured by temper rolling, deformation resistance during cold working is low, crack resistance is good, and cold workability is excellent. The present invention relates to a steel wire for machine structural parts that exhibits excellent characteristics. In addition, in this specification, a "wire" is used for the meaning of a rolled wire, and points out the linear steel material cooled to room temperature after hot rolling. The “steel wire” refers to a linear steel material obtained by subjecting a rolled wire material to a tempering treatment such as spheroidizing annealing.

  In manufacturing various machine structural parts such as automobile parts and construction machine parts, spheroidizing annealing is usually applied to hot rolled wire rods such as carbon steel and alloy steel for the purpose of imparting cold workability. Is done. Then, the rolled wire rod after the spheroidizing annealing, that is, the steel wire is cold-worked, and then formed into a predetermined shape by machining such as cutting, and the final strength is obtained by quenching and tempering treatment. Adjustments are made into machine structural parts.

  In cold working, it is possible to expect an improvement in mold life by lowering the deformation resistance of the steel wire. Moreover, the yield improvement of various components can be expected by improving the crack resistance of the steel wire.

So far, various methods have been proposed as techniques for improving the cold workability of steel wires. As such a technique, for example, Patent Document 1 states that “the metal structure is substantially composed of ferrite grains and spherical carbides, and the ferrite grains have an average particle diameter of 15 μm or more, and the spherical carbides have an average particle diameter of 0. .8 μm or less, the maximum particle size is 4.0 μm or less, and the number per 1 mm ² is 0.5 × 10 ⁶ × C% to 5.0 × 10 ⁶ × C%, Among them, a technique of “steel wire whose maximum distance between spherical carbides having a particle size of 0.1 μm or more is 10 μm or less” is disclosed.

  Patent Document 2 states that “the steel metal structure has cementite and ferrite, and the total area ratio of cementite and ferrite to the entire structure is 95% by area or more, and the aspect ratio of 90% or more of the cementite. 3 or less, and the average center-of-gravity distance of the cementite is 1.5 μm or more, and the average crystal grain size of the ferrite is 5 to 20 μm.

  In Patent Document 2, as a means for obtaining the metal structure, the temperature is raised to a temperature range of A1 point to A1 point + 50 ° C., and maintained for 0 to 1 hr in the temperature range of A1 point to A1 point + 50 ° C. after the temperature rise. Then, the annealing process is performed twice, in which the temperature range from the A1 point to the A1 point + 50 ° C. to the A1 point−100 ° C. to the A1 point−30 ° C. is cooled at an average cooling rate of 10 to 200 ° C./hr. After performing the above, it is disclosed that the temperature is controlled to the temperature range of A1 point to A1 point + 30 ° C. and the cooling condition is controlled as follows after the temperature is maintained in the temperature range of A1 point to A1 point + 30 ° C. ing. That is, the temperature stays in the temperature range from A1 point to A1 point + 30 ° C until it reaches the A1 point when it is cooled after being held in the temperature range of A1 point to A1 point + 30 ° C after reaching the A1 point when raising the temperature. The time is 10 minutes to 2 hours, and the cooling temperature range from the A1 point to A1 point + 30 ° C temperature range from A1 point to -100 ° C to A1 point to -20 ° C is 10 to 100 ° C / hr average cooling rate. After cooling, a method of further cooling after holding for 10 minutes to 5 hours in the cooling temperature range is disclosed.

  On the other hand, Patent Document 3 discloses a technique of “a steel wire having a structure in which a value obtained by dividing a standard deviation of a distance between cementites by an average value of the distances between cementites is 0.50 or less”. In this method, cementite is distributed at substantially uniform intervals, and a large amount of cementite is also present in the ferrite grains.

International Publication No. 2011/108459 JP 2012-140673 A JP 2006-316291 A

  The technologies proposed so far are useful as steel wire technologies with improved cold workability such as cold forging, but it is hoped that steel wire technologies with further improved cold workability will be developed. It is rare.

  The present invention has been made under such circumstances, and its purpose is to reduce mechanical resistance during cold working, improve crack resistance, and provide mechanical structure parts that can exhibit excellent cold workability. The purpose is to provide steel wire.

The steel wire for machine structural parts of the present invention that has achieved the above-mentioned problems is in mass%,
C: 0.3-0.6%
Si: 0.05 to 0.5%,
Mn: 0.2 to 1.7%,
P: more than 0%, 0.03% or less,
S: 0.001 to 0.05%,
Al: 0.005 to 0.1% and N: 0 to 0.015%, respectively, the balance consisting of iron and inevitable impurities,
The metal structure of the steel is composed of ferrite and cementite, and the number ratio of cementite existing at the ferrite grain boundary is 40% or more with respect to the total cementite number.

  The steel wire for machine structural parts of the present invention, if necessary, in mass%, Cr: more than 0%, 0.5% or less, Cu: more than 0%, 0.25% or less, Ni: more than 0%, It is preferable to contain at least one selected from the group consisting of 0.25% or less, Mo: more than 0%, 0.25% or less, and B: more than 0%, 0.01% or less.

  In the steel wire for machine structural parts of the present invention, it is preferable that an average equivalent circle diameter of bcc (body-centered cubic) -Fe crystal grains in the metal structure is 30 μm or less.

  According to the steel wire for machine structural parts of the present invention, the chemical composition is appropriately adjusted, the steel metal structure is composed of ferrite and cementite, and the number ratio of cementite existing at the ferrite grain boundary is a specified value. By satisfying the requirements, it is possible to provide a steel wire that realizes an improvement in crack resistance as well as a reduction in deformation resistance. Since the steel wire for machine structural parts of the present invention has reduced deformation resistance, it can suppress wear and breakage of plastic working tools such as molds, and has improved crack resistance, so that forging It can also suppress the occurrence of cracking at the time, and exhibits excellent properties in cold workability.

  The present inventors have studied from various angles in order to realize a steel wire that has both deformation resistance reduction during cold working and improved crack resistance. As a result, it has been found that cementite in ferrite grains increases deformation resistance during cold working, and that voids that cause cracks originate from cementite in ferrite grains.

  The cementite present in the ferrite grain boundaries is less strained during cold working than cementite present in the grains, so that the deformation resistance can be reduced and the origin of voids can be suppressed. That is, in order to achieve both reduction in deformation resistance and improvement in crack resistance, it is important to increase the cementite number ratio present in the ferrite grain boundaries, that is, to reduce the cementite number ratio present in the ferrite grains. The idea was obtained.

  In the techniques proposed so far, a method for controlling the ferrite grain size is known as a method for improving the deformation resistance and crack resistance, but none has focused on cementite accumulated at the grain boundaries.

  Hereinafter, each requirement prescribed | regulated by this invention is demonstrated.

  The metal structure of the steel wire for machine structural parts of the present invention (hereinafter sometimes simply referred to as “steel wire”) is a so-called spheroidized structure, and is composed of ferrite and cementite. The spheroidized structure is a metal structure that contributes to improving cold workability by reducing the deformation resistance of steel. A part of the pearlite structure may be included in the metal structure of the present invention. Moreover, if the adverse effect on cold workability is small, precipitates such as AlN can be allowed to be less than 3% in terms of area ratio.

  However, it is impossible to improve cold workability simply by using a metal structure composed of ferrite and cementite. For this reason, it is necessary to appropriately control the grain boundary cementite ratio in this metal structure, as will be described in detail below.

  When the grain boundary cementite ratio is reduced and the cementite ratio in the grains is increased, the dislocations introduced into the ferrite grains during cold working are trapped in the intragranular cementite, causing an increase in dislocations and exhibit work hardening. As a result, deformation resistance increases and cold workability decreases. In addition, the cementite in the grains tends to accumulate strain around the cementite during cold working, compared to the grain boundary cementite. As a result, cementite present in the grains tends to be the starting point of cracking. For this reason as well, it is very effective to precipitate cementite on the grain boundaries for improving the cold workability.

  From such a viewpoint, the ratio of the number of cementites existing in the ferrite grain boundaries needs to be 40% or more with respect to the total number of cementites. By making the number ratio of cementite present in ferrite grain boundaries to the total cementite number (hereinafter sometimes referred to simply as “grain boundary cementite ratio”) 40% or more, deformation resistance is reduced and cracking of the cementite origin Occurrence can be suppressed.

  The form of the whole cementite is not particularly limited, and includes, in addition to spherical cementite, rod-like cementite having a large aspect ratio, layered cementite that forms a pearlite structure, and the like, and the shape of cementite is not limited. In addition, although the reference | standard of the magnitude | size of the cementite used as a measuring object is not limited, the size of the cementite which can be discriminate | determined with the measuring method of the grain-boundary cementite ratio mentioned later becomes the minimum size. Specifically, a size of 0.3 μm or more is a measurement target.

  The minimum with a preferable grain boundary cementite ratio is 45% or more, More preferably, it is 50% or more. The higher the grain boundary cementite ratio, the more effective the reduction of deformation resistance and the suppression of cracking, and the most preferable is 100%. However, as will be described later, an increase in the grain boundary cementite ratio is not easy in terms of production, and the current technology may have disadvantages such as a decrease in hot rolling temperature and a longer spheroidizing annealing time. In the current technology, from the viewpoint of manufacturability, approximately 80% or less is preferable, and more preferably 70% or less.

  In the steel wire of the present invention, the average equivalent circle diameter of the bcc-Fe crystal grains in the metal structure is preferably 30 μm or less. By setting the average equivalent circle diameter of the bcc-Fe crystal grains (hereinafter sometimes simply referred to as “bcc-Fe crystal grain diameter”) to 30 μm or less, the ductility is improved and the occurrence of cracks during cold working is further increased. Can be suppressed. The upper limit with preferable bcc-Fe crystal grain diameter is 25 micrometers or less, More preferably, it is 20 micrometers or less. In addition, although the reference | standard of the magnitude | size of the bcc-Fe crystal grain which is a measuring object is not limited, The size which can be discriminate | determined with the measuring method mentioned later becomes the minimum size similarly to the said cementite. Specifically, a size of 1 μm or more is a measurement target.

  The above-described structure to be controlled for the bcc-Fe crystal grain size is a bcc-Fe crystal grain surrounded by a large-angle grain boundary whose orientation difference is larger than 15 °. This is because the effect on cold workability is small at the small-angle grain boundary where the orientation difference is 15 ° or less. The above-mentioned “crystal orientation difference” is also referred to as “shift angle” or “inclination angle”, and the EBSP method (Electron Backscattering Pattern Method) may be employed to measure the orientation difference. In addition, bcc-Fe surrounded by large-angle grain boundaries for measuring the average particle diameter includes ferrite contained in the pearlite structure in addition to pro-eutectoid ferrite.

  In the present invention, it is intended for steel wires used as raw materials for machine structural parts, and may have a normal chemical composition as a steel wire for machine structural parts, but C, Si, Mn, P, About S, Al, and N, it is good to adjust to an appropriate range. From this point of view, the appropriate ranges of these chemical components and the reasons for their limitations are as follows. In the present specification, “%” for the chemical component composition means mass%.

C: 0.3 to 0.6%
C is an element useful for ensuring the strength of the steel, that is, the strength of the final product. In order to exhibit such an effect effectively, the C content needs to be 0.3% or more. The C content is preferably 0.32% or more, and more preferably 0.34% or more. However, if C is excessively contained, the strength is increased and the cold workability is lowered, so that it is necessary to be 0.6% or less. The C content is preferably 0.55% or less, more preferably 0.50% or less.

Si: 0.05-0.5%
Si is contained as a deoxidizing element and for the purpose of increasing the strength of the final product by solid solution hardening. In order to effectively exhibit such an effect, the Si content was set to 0.05% or more. The Si content is preferably 0.07% or more, and more preferably 0.10% or more. On the other hand, when Si is contained excessively, the hardness is excessively increased and the cold workability is deteriorated. Therefore, the Si content is set to 0.5% or less. The Si content is preferably 0.45% or less, more preferably 0.40% or less.

Mn: 0.2 to 1.7%
Mn is an effective element for increasing the strength of the final product through improvement of hardenability. In order to effectively exhibit such an effect, the Mn content is set to 0.2% or more. The Mn content is preferably 0.3% or more, and more preferably 0.4% or more. On the other hand, when Mn is contained excessively, the hardness increases and the cold workability deteriorates. Therefore, the Mn content is set to 1.7% or less. The Mn content is preferably 1.5% or less, and more preferably 1.3% or less.

P: more than 0% and 0.03% or less P is an element inevitably contained in steel, causes segregation of grain boundaries in steel, and causes ductility deterioration. Therefore, the P content is set to 0.03% or less. The P content is preferably 0.02% or less, more preferably 0.017% or less, and particularly preferably 0.01% or less. The smaller the P content, the better. However, there may be a case where approximately 0.001% remains due to restrictions on the manufacturing process.

S: 0.001 to 0.05%
S is an element inevitably contained in the steel and is present as MnS in the steel and deteriorates the ductility. Therefore, S is an element harmful to cold workability. Therefore, the S content is set to 0.05% or less. The S content is preferably 0.04% or less, and more preferably 0.03% or less. However, since S has the effect | action which improves a machinability, it is made to contain 0.001% or more. The S content is preferably 0.002% or more, and more preferably 0.003% or more.

Al: 0.005 to 0.1%
Al is useful as a deoxidizing element and is useful for fixing solute N present in steel as AlN. In order to exhibit such an effect effectively, the Al content is set to 0.005% or more. The Al content is preferably 0.008% or more, and more preferably 0.010% or more. However, when the Al content is excessive, Al ₂ O ₃ is excessively generated and the cold workability is deteriorated. Therefore, the Al content is determined to be 0.1% or less. Al content becomes like this. Preferably it is 0.090% or less, More preferably, it is 0.080% or less.

N: 0 to 0.015%
N is an element inevitably contained in the steel. When solid solution N is contained in the steel, the hardness increases due to strain aging and the ductility decreases, and the cold workability deteriorates. Therefore, the N content is set to 0.015% or less. The N content is preferably 0.013% or less, and more preferably 0.010% or less. The N content is preferably as low as possible, and is most preferably 0%, but it may remain about 0.001% due to restrictions on the manufacturing process.

  The basic components of the steel wire of the present invention are as described above, and the balance is substantially iron. In addition, “substantially iron” can accept trace components (eg, Sb, Zn, etc.) that do not impair the characteristics of the present invention in addition to iron, and inevitable impurities other than P, S, N (eg, O). , H, etc.). Furthermore, in this invention, the following arbitrary elements may be contained as needed, and the characteristic of a steel wire is further improved according to the component to contain.

Cr: more than 0%, 0.5% or less, Cu: more than 0%, 0.25% or less, Ni: more than 0%, 0.25% or less, Mo: more than 0%, 0.25% or less and B: One or more selected from the group consisting of more than 0% and less than 0.01% Cr, Cu, Ni, Mo and B all increase the strength of the final product by improving the hardenability of the steel. It is an effective element and contained alone or in combination of two or more as required. Such an effect increases as the content of these elements increases, and the preferable content for effectively exhibiting the above-described effect is such that the Cr content is 0.015% or more, more preferably 0.020% or more. It is. The preferable contents of Cu, Ni and Mo are all 0.02% or more, more preferably 0.05% or more. The preferable content of B is 0.0003% or more, more preferably 0.0005% or more.

  However, when the contents of Cr, Cu, Ni, Mo and B are excessive, the strength becomes too high and the cold workability is deteriorated. Therefore, the Cr content is preferably 0.5% or less, the Cu, Ni and Mo contents are preferably 0.25% or less, and the B content is preferably 0.01% or less. The more preferable content of these elements is such that the Cr content is 0.45% or less, more preferably 0.40% or less. The upper limit of the amount of Cu, Ni and Mo is more preferably 0.22% or less, and still more preferably 0.20% or less. The upper limit with more preferable B amount is 0.007% or less, More preferably, it is 0.005% or less.

  The steel wire of the present invention defines the structure form after spheroidizing annealing, and in order to obtain such a structure form, it is preferable to appropriately control the spheroidizing annealing conditions described later. However, in order to ensure the structure form as described above, the condition at the stage of producing the rolled wire is also appropriately controlled, and the grain form cementite is likely to precipitate during the spheroidizing annealing of the structure form in the rolled wire. More preferably.

In the rolled wire rod manufacturing stage, the steel rolling which satisfies the above-mentioned composition can be adjusted to the final rolling temperature when hot-rolling, and the cooling speed and temperature range can be appropriately adjusted by setting the subsequent cooling speed to three stages. preferable. By producing a rolled wire rod under these conditions, the structure before spheroidizing annealing has pearlite and ferrite as the main phase, the bcc-Fe crystal grain size is in a predetermined range, and the pro-eutectoid ferrite crystal grains are equiaxed. And the interval at the narrowest part of the pearlite can be set to a predetermined value or less. By subjecting such a structure to spheroidizing annealing under the conditions described later, it becomes easy to obtain a steel wire in which grain boundary cementite is sufficiently precipitated. Specifically, the rolled wire manufacturing conditions for this are as follows:
Finish rolling at 800 ° C or higher and 1050 ° C or lower,
A first cooling with an average cooling rate of 7 ° C./second or more;
Second cooling with an average cooling rate of 1 ° C./second or more and 5 ° C./second or less;
A third cooling in which the average cooling rate is faster than the second cooling and is 5 ° C./second or more is performed in this order,
The end of the first cooling and the start of the second cooling are performed within a range of 700 to 750 ° C., the end of the second cooling and the start of the third cooling are performed within a range of 600 to 650 ° C., 3 The end of cooling is preferably 400 ° C. or lower. The finish rolling temperature and the first to third cooling will be described in detail.

(A) Finish rolling temperature: 800 ° C. or higher and 1050 ° C. or lower In order to make the bcc-Fe crystal grain size of the structure before spheroidizing annealing small, for example, 15 μm or less, it is preferable to appropriately control the finish rolling temperature. . When the finish rolling temperature exceeds 1050 ° C., it is difficult to reduce the bcc-Fe crystal grain size. However, when the finish rolling temperature is less than 800 ° C., the bcc-Fe crystal grain size becomes too small, for example, less than 5 μm, and softening becomes difficult. The minimum with more preferable finish rolling temperature is 850 degreeC or more, More preferably, it is 900 degreeC or more. The upper limit with more preferable finish rolling temperature is 1000 degrees C or less, More preferably, it is 950 degrees C or less.

(B) 1st cooling In the 1st cooling which starts from 800 degreeC or more and 1050 degrees C or less which are finishing rolling temperature, and complete | finishes in the temperature range of 700-750 degreeC, when a cooling rate becomes slow, bcc of the structure | tissue before spheroidizing annealing There is a possibility that the -Fe crystal grains become coarse and the bcc-Fe crystal grain size becomes large. Therefore, the average cooling rate in the first cooling is preferably 7 ° C./second or more. The average cooling rate of the first cooling is more preferably 10 ° C./second or more, and further preferably 20 ° C./second or more. The upper limit of the average cooling rate of the first cooling is not particularly limited, but is preferably 200 ° C./second or less as a realistic range. In the cooling in the first cooling, the cooling rate may be changed as long as the average cooling rate is 7 ° C./second or more.

(C) Second cooling In order to equiax the pro-eutectoid ferrite crystal grains, that is, to make the average aspect ratio of the pro-eutectoid ferrite crystal grains small, for example, 3.0 or less, start from a temperature range of 700 to 750 ° C. In the second cooling which is finished in the temperature range of 600 to 650 ° C., it is preferable to slowly cool at an average cooling rate of 5 ° C./second or less. A more preferable upper limit of the average cooling rate of the second cooling is 4 ° C./second or less, and further preferably 3.5 ° C./second or less. On the other hand, if the average cooling rate in the second cooling is too slow, the bcc-Fe crystal grains may become coarse and the bcc-Fe crystal grain size may become too large. Therefore, the average cooling rate in the second cooling is preferably 1 ° C./second or more. A more preferable lower limit of the average cooling rate of the second cooling is 2 ° C./second or more, and further preferably 2.5 ° C./second or more. In the cooling in the second cooling, the cooling rate may be changed as long as the average cooling rate is 1 ° C./second or more and 5 ° C./second or less.

(D) Third Cooling In this third cooling, the average lamellar spacing of pearlite is made as narrow as possible to facilitate the dissolution of cementite, so that no spherical cementite nuclei remain in the grains. Thereby, the number ratio of the cementite which exists in a ferrite grain boundary is increased by performing the subsequent appropriate spheroidizing annealing process. In order to make the average lamellar interval of pearlite narrow, for example 0.20 μm or less, in the third cooling that starts from a temperature range of 600 to 650 ° C. and ends at 400 ° C. or less, it is faster than the second cooling and 5 It is preferable to cool at an average cooling rate of at least ° C / second. Cooling slower than 5 ° C./second makes it difficult to narrow the average lamellar spacing of pearlite. The average cooling rate of the third cooling is more preferably 10 ° C./second or more, and further preferably 20 ° C./second or more.

  The upper limit of the average cooling rate of the third cooling is not particularly limited, but is preferably 200 ° C./second or less as a practical range. In the third cooling, the cooling rate may be changed as long as the average cooling rate is 5 ° C./second or more. Although the minimum of the completion | finish temperature of 3rd cooling is not specifically limited, For example, it is preferable that it is 200 degreeC. After the third cooling, normal cooling such as cooling is performed to cool to room temperature.

  After cooling to room temperature, wire drawing may be performed at room temperature as necessary, and the area reduction rate at that time may be, for example, 30% or less. When the wire is drawn, the carbides in the steel are destroyed, and the agglomeration of the carbides can be promoted by the subsequent spheroidizing annealing, which is effective in shortening the soaking time of the spheroidizing annealing. However, if the area reduction ratio of the wire drawing process exceeds 30%, the strength after annealing increases and the cold workability may be deteriorated. Therefore, the area reduction ratio of the wire drawing process is preferably 30% or less. The lower limit of the area reduction rate is not particularly limited, but the effect is preferably obtained by setting it to 2% or more.

  In the rolled wire manufactured under the preferable conditions as described above, the pearlite in the structure is transformed into austenite by the subsequent spheroidizing annealing treatment, and then the original pearlite size is reduced while transforming into ferrite + cementite. That is, by suppressing the grain growth of the metal structure, the intragranular precipitation of cementite is reduced and the grain boundary cementite is likely to precipitate.

  As such spheroidizing annealing conditions, when heating a rolled wire from room temperature to 730 ° C. in an air furnace, for example, SA1 described later, an average heating rate of 50 ° C./hour is at least from 500 ° C. to 730 ° C. Heat up to above, then heat up to 740 ° C. at an average heating rate of 2-5 ° C./hour, hold at 740 ° C. for 1-3 hours, then cool to 720 ° C. at an average cooling rate of 20 ° C./hour or more, average cooling It is preferable to cool to 640 ° C. at a rate of 8 to 12 ° C./hour and then to cool.

  Under the above spheroidizing annealing conditions, when heating from room temperature to 730 ° C., the average heating rate from at least 500 ° C. to 730 ° C. is set to 50 ° C./hour or more to suppress the grain growth of the metal structure. The average heating rate at this time is more preferably 60 ° C./hour or more. However, if the average heating rate is too high, it becomes difficult to follow the temperature of the rolled wire, and therefore it is preferably 200 ° C./hour or less, more preferably 150 ° C./hour or less.

  The average heating rate when heating from room temperature to 500 ° C. is usually 100 ° C./hour or more, but the average heating rate in this temperature range has little influence on the grain growth of the metal structure. Considering productivity, it is preferable that the heating rate at this time is fast, for example, 120 ° C./hour or more, and more preferably 140 ° C./hour or more. The upper limit of the average heating rate at this time is preferably 200 ° C./hour or less, more preferably 150 ° C./hour or less, similarly to the average heating rate from 500 ° C. to 730 ° C. The average heating rate at the time of heating from room temperature to 500 ° C. may be the same as or different from the average heating rate from at least 500 ° C. to 730 ° C. In short, the average heating rate from at least 500 ° C. to 730 ° C. is 50 ° C. in order to reduce the intra-grain precipitation of cementite by reducing the original pearlite size and to easily precipitate grain boundary cementite. As long as it is secured more than / hour.

  In addition, by controlling the average heating rate from 730 ° C to 740 ° C immediately above the A1 point from 2 to 5 ° C / hour, it is possible to sufficiently decompose and dissolve cementite in the pearlite structure while suppressing the grain growth of the metal structure as much as possible. It can be carried out. When the average heating rate is higher than 5 ° C / hour, it is difficult to secure a sufficient time for decomposition and solid solution of cementite in the pearlite structure, and from 730 ° C when the average heating rate is lower than 2 ° C / hour. The heating time up to 740 ° C. becomes longer, and it becomes difficult to suppress the grain growth of the metal structure. The average heating rate at this time is more preferably 3 ° C./hour or more and 4 ° C./hour or less.

  It is preferable to hold at 740 ° C. for 1 to 3 hours. When this holding temperature is shorter than 1 hour, decomposition and solid solution of cementite in the pearlite structure is insufficient, and when it is longer than 3 hours, it becomes difficult to suppress grain growth of the metal structure. The holding time at this time is more preferably 1.5 hours or more and 2.5 hours or less.

  After holding as described above, the grain growth of the metal structure can be suppressed by setting the preferable average cooling rate up to 720 ° C. to 20 ° C./hour or more. The average cooling rate at this time is more preferably 30 ° C./hour or more. However, if the average cooling rate is too high, it becomes difficult to follow the temperature of the rolled wire, and therefore it is preferably 100 ° C./hour or less.

  Thereafter, by controlling the average cooling rate from 720 ° C. to 640 ° C. at 8 to 12 ° C./hour, cementite is preferentially precipitated at the ferrite grain boundaries, thereby suppressing the precipitation of cementite having a large aspect ratio such as a pearlite structure. can do. When the average cooling rate is lower than 8 ° C./hour, it is difficult to suppress the grain growth of the metal structure, and when the average cooling rate is higher than 12 ° C./hour, cementite with a large aspect ratio such as a pearlite structure is formed. Reprecipitates a lot. The average cooling rate at this time is more preferably 9 ° C./hour or more and 11 ° C./hour or less.

  The spheroidizing annealing as described above may be repeated a plurality of times, and by performing such repetition, the individual aspect ratio of cementite is reduced and the grain boundary cementite ratio is increased. For example, test Nos. In Examples to be described later. As shown in 7, 12, 14, 19, and 27, even when steel grades C, E, F, H, and K whose rolling wire manufacturing conditions are not properly controlled are used, a predetermined spheroidization is performed thereafter. By repeating the annealing, the grain boundary cementite ratio is within an appropriate range, and both deformation resistance and crack generation rate can be reduced.

  The number of repetitions of the spheroidizing annealing is preferably at least 3 times or more. However, even if it is excessively repeated, the grain boundary cementite ratio does not change so much, and is preferably 10 times or less. When the spheroidizing annealing is repeated a plurality of times, it may be repeated under the same conditions or different conditions within the range of the above preferable conditions.

  Hereinafter, the present invention will be described more specifically with reference to examples. The present invention is not limited by the following examples, and can of course be implemented with appropriate modifications within a range that can be adapted to the above-described gist. Included in the range.

  Using steel having the chemical composition shown in Table 1 below, rolling was performed under the various manufacturing conditions shown in Table 2 below to produce a wire having a diameter of 17.0 mm. In Table 2, cooling 1, cooling 2 and cooling 3 correspond to the first cooling, the second cooling and the third cooling recommended in the present invention. Steel type B is a comparative example in which the chemical component composition deviates from the specified value.

  Steel types C, E, F, H, K, O, P, and Q are examples in which a rolled wire was not manufactured under appropriate manufacturing conditions in the present invention. Among these, steel types C, E, F, and K have higher finish rolling temperatures. Steel type H is an example of producing a rolled wire rod by cooling while maintaining the cooling rate in the cooling 3 corresponding to the third cooling, that is, the cooling rate in the second cooling.

  In the steel type O, after performing the second cooling to 550 ° C., a holding step of holding at 580 ° C. for 120 seconds was performed, and the mixture was allowed to cool to room temperature, and a wire drawing step with a surface reduction rate of 40% was performed. In steel type P, cooling was performed at a monotonous cooling rate of only cooling 1. For steel type Q, after performing cooling 1, a holding step of holding at 550 ° C. for 60 seconds was performed, and the mixture was allowed to cool to room temperature, and rough drawing with a surface reduction rate of 15% was performed.

  Next, in each of the rolled wire rods excluding steel types O, P, and Q, (a) when heating from room temperature to 730 ° C in an atmospheric furnace, the average heating rate from room temperature to 500 ° C is 110 ° C / hour. , Heating from 500 ° C. to 730 ° C. at an average heating rate of 80 ° C./hour, then heating to 740 ° C. at an average heating temperature of 3 ° C./hour, holding at 740 ° C. for 3 hours, and then at an average cooling rate of 30 ° C./hour Cooling to 720 ° C., cooling to 640 ° C. at an average cooling rate of 10 ° C./hour, and then allowing to cool (this annealing condition is hereinafter abbreviated as “SA1”), (b) SA1 is repeated five times. Spheroidizing annealing (this annealing condition is abbreviated as “SA2” hereinafter), (c) When heating from room temperature to 730 ° C., from 500 ° C. to 730 ° C. at an average heating rate of 110 ° C./hour from room temperature to 500 ° C. Up to an average heating rate of 80 ° C / And then heated to 740 ° C. at an average heating rate of 3 ° C./hour, held at 740 ° C. for 3 hours, cooled to 640 ° C. at an average cooling rate of 30 ° C./hour, and then allowed to cool (this spheroidizing annealing) Any of the annealing conditions was abbreviated as “SA3” below. The annealing conditions SA1 and SA2 are preferable annealing conditions in the present invention, and the annealing condition SA3 is an example in which the average cooling rate from 720 ° C. to 640 ° C. is not properly controlled.

  For steel type O, in an atmospheric furnace, (d) after heating from room temperature to 680 ° C. at an average heating rate of 80 ° C./hour and holding at 680 ° C. for 5 hours, then at an average cooling rate of 10 ° C./hour. Spheroidizing annealing (this annealing condition is hereinafter abbreviated as “SA4”) which is cooled to 640 ° C. and then allowed to cool, (e) Heated from room temperature to 700 ° C. at an average heating rate of 80 ° C./hour, at 700 ° C. After holding for 5 hours, one of spheroidizing annealing (this annealing condition is abbreviated as “SA5” hereinafter) is performed by cooling to 640 ° C. at an average cooling rate of 10 ° C./hour and then allowing to cool. The annealing conditions SA4 and SA5 are examples that deviate from the preferable annealing conditions in the present invention.

  For steel type P, in an atmospheric furnace, (f) a step of heating from room temperature to 740 ° C. at an average heating rate of 80 ° C./hour, and immediately cooling to 660 ° C. at an average cooling rate of 80 ° C./hour. Is repeated three times (however, the second and subsequent times are heated from 660 ° C.), then heated from 660 ° C. to 740 ° C. at an average heating rate of 80 ° C./hour, held at 740 ° C. for 30 minutes, and then the average cooling rate Cooling to 660 ° C. at 80 ° C./hour, holding at 660 ° C. for 1 hour, and then allowing to cool down, spheroidizing annealing (this annealing condition is hereinafter abbreviated as “SA6”), (g) average heating rate of 80 ° C. / Heat from room temperature to 740 ° C., hold at 740 ° C. for 10 minutes, and then repeat the process of cooling to 660 ° C. at an average cooling rate of 80 ° C./hour three times (however, from the second time onwards, heat from 660 ° C. ), And then an average heating rate of 80 ° C. / After heating from 660 ° C. to 740 ° C., holding at 740 ° C. for 30 minutes, cooling to 660 ° C. at an average cooling rate of 80 ° C./hour, holding at 660 ° C. for 1 hour, and then allowing to cool down (this is annealed) Any of annealing conditions was abbreviated as “SA7” below. The annealing conditions SA6 and SA7 are examples that deviate from the preferable annealing conditions in the present invention.

  For steel grade Q, (h) spheroidizing annealing in which the furnace is heated from room temperature to 720 ° C. at an average heating rate of 150 ° C./hour, held at 720 ° C. for 1 hour, and then allowed to cool (this annealing condition) (Hereinafter abbreviated as “SA8”), (i) spheroidizing annealing (this annealing condition is heated from room temperature to 730 ° C. at an average heating rate of 150 ° C./hour, kept at 730 ° C. for 1 hour, and then allowed to cool) Hereinafter, abbreviated as “SA9”). The annealing conditions SA8 and SA9 are examples that deviate from the preferable annealing conditions in the present invention.

  Regarding the steel wire after the above spheroidizing annealing, (1) bcc-Fe crystal grain size of metal structure, (2) grain boundary cementite ratio, (3) deformation resistance during cold working, (4) cold The crack generation rate at the time of hot working was measured by the following method.

  In the measurement of the ferrite grain size and grain boundary cementite ratio of the steel wire after spheroidizing annealing, the resin was embedded so that the cross section could be observed, and the cut surface was mirror-polished with emery paper and diamond buff. The position of D / 4 was measured with respect to the radius D of the steel wire.

(1) Measurement of bcc-Fe crystal grain size The bcc-Fe crystal grain size was measured using an EBSP analyzer and an FE-SEM (Field-Emission Scanning Electron Microscope, field emission scanning electron microscope). As an analysis tool, OIM software of TSL Solutions Inc. was used. Boundary where crystal orientation difference (also referred to as “bevel”) exceeds 15 °, that is, “crystal grain” is defined with a large-angle grain boundary as the grain boundary, and the area of the bcc-Fe crystal grain is converted to a circle The average value of the diameters, that is, the average equivalent circle diameter was calculated. At this time, the measurement area was 200 μm × 400 μm, the measurement step was 1.0 μm, and measurement points having a confidence index (Confidence Index) of 0.1 or less indicating the reliability of the measurement direction were deleted from the analysis target.

(2) Measurement of grain boundary cementite ratio In the measurement of grain boundary cementite ratio, ferrite grain boundaries and cementite were made to appear by picral etching for 5 minutes or more, and the structure was observed with an optical microscope. I shot the field of view. On these photographs, 10 horizontal lines with equal intervals are drawn, the number of grain boundary cementites and the number of intragranular cementites existing on the lines are measured, and the total number of grain boundary cementites in three fields is divided by the total number of cementite. The grain boundary cementite ratio was calculated. The minimum equivalent circle diameter of cementite to be measured was 0.3 μm. Here, the particles having an aspect ratio of 3.0 or less in contact with the ferrite grain boundaries were defined as grain boundary cementite. Therefore, even if it is in contact with the ferrite grain boundary, the cementite particles having an aspect ratio exceeding 3.0 were regarded as intragranular cementite.

(3) Measurement of deformation resistance A sample for cold forging test of φ10.0 mm × 15.0 mm was prepared from a steel wire, and processed at a strain rate of 5 / sec to 10 / sec at room temperature using a forging press. A cold forging test at a rate of 60% was performed five times. The deformation resistance was measured by measuring the deformation resistance at 40% processing five times from the data of the processing rate-deformation resistance obtained from the cold forging test at the 60% processing rate, and obtaining the average value of the five times. In addition, the acceptance criteria of the deformation resistance in the steel types A to E and P having a C content in the range of less than 0.3 to 0.4% are 650 MPa or less, and the C content is 0.4 to 0.5. In the steel grades F to J, O, and Q within the range of less than%, the acceptance criteria for deformation resistance is 680 MPa or less, and the C content is within the range of 0.5 to 0.6% in the steel types K to N. The acceptance criteria for deformation resistance is 730 MPa or less.

(4) Measurement of crack occurrence rate A sample for a cold forging test of φ10.0 mm × 15.0 mm was prepared from a steel wire, using a forging press at room temperature at a strain rate of 5 / sec to 10 / sec. Cold forging tests with a processing rate of 60% were performed five times. The crack occurrence rate was measured after the cold forging test at a 60% processing rate, by observing the surface with a stereomicroscope five times, and measuring the presence or absence of surface cracks at a magnification of 20 times. The average was obtained by dividing "by 5". The acceptance criteria for the crack occurrence rate in all steel types is 20% or less.

  These results are shown in Table 3 below together with the spheroidizing annealing conditions. In the column of comprehensive evaluation in Table 3, an example in which both the reduction in deformation resistance and the improvement in crack resistance are good is indicated as “OK”, and at least either the reduction in deformation resistance or the improvement in crack resistance is displayed. An example in which the color has deteriorated is indicated as “NG”.

  From the results in Table 3, it can be considered as follows. Test No. 1, 2, 7-9, 12, 14-16, 19-21, 23, 24, 27-29, 31, 32, 34, 35 are examples that satisfy all of the requirements defined in the present invention. It can be seen that both the reduction of deformation resistance and the improvement of crack resistance are achieved.

  Of these, test no. Nos. 7, 12, 14, 19, and 27 are examples using steel types C, E, F, H, or K that are not manufactured under the preferable rolling wire condition. However, the grain boundary cementite is caused by subsequent annealing of SA2. It is sufficiently deposited and both the deformation resistance and the crack generation rate have passed the acceptance criteria. Among these, test No. In No. 12, although the bcc-Fe crystal grain size, which is a preferable requirement, is slightly larger, both the deformation resistance and the crack generation rate have passed the acceptance criteria.

  Here, in test No. 1 in which both annealing conditions of SA1 and SA2 were performed. 1, 2 (steel type A), test no. 6, 7 (steel grade C), test no. 8, 9 (steel type D), test no. 11, 12 (steel type E), test no. 13, 14 (steel type F), test no. 15, 16 (steel grade G), test no. 18, 19 (steel type H), test no. 20, 21 (steel type I), test no. 23, 24 (steel type J), test no. 26, 27 (steel type K), test no. 28, 29 (steel type L), test no. 31, 32 (steel grade M), test no. When paying attention to 34 and 35 (steel type N), it can be seen that both deformation resistance and crack generation rate are further reduced when SA2 is annealed five times in comparison with SA1.

  In contrast, test no. 3 to 6, 10, 11, 13, 17, 18, 22, 25, 26, 30, 33, 36 to 42 are comparative examples lacking any of the requirements defined in the present invention, and deformation resistance and cracking are generated. It can be seen that either or both of the rates do not meet the acceptance criteria.

  That is, test no. 3, 10, 17, 22, 25, 30, 33, and 36 are examples in which spheroidizing annealing was performed with SA3 whose conditions were not appropriate, and the grain boundary cementite ratio was insufficient, and either deformation resistance or crack generation rate was observed. , Or both do not meet the acceptance criteria.

  Test No. Nos. 4 and 5 are examples using the steel type B having an excessive Mn content, and the deformation resistance during cold working remains high.

  Test No. 6, 11, 13, 18 and 26 are examples using steel types C, E, F, H or steel type K that are not manufactured under the preferable conditions at the time of rolling wire manufacturing, and depending on the subsequent spheroidizing annealing of SA1 Grain boundary cementite does not precipitate, and neither deformation resistance nor crack occurrence rate has reached the acceptance standard. However, when SA2 spheroidizing annealing is repeated for these steel types, SA1 is repeated five times thereafter, grain boundary cementite is appropriately precipitated, and both the deformation resistance and the crack generation rate have passed the acceptance criteria. (Test Nos. 7, 12, 14, 19, 27).

  Test No. Nos. 37 and 38 are examples in which spheroidizing annealing was performed with SA4 and SA5 where the conditions were not appropriate using steel type O that was not manufactured under the preferable conditions at the time of manufacturing the rolled wire rod, and fine cementite was uniformly dispersed, The boundary cementite ratio is small, the deformation resistance remains high, and the crack generation rate exceeds the acceptance standard.

  Test No. Nos. 39 and 40 are examples in which spheroidizing annealing was performed with SA6 and SA7 in which the conditions were not appropriate using steel type P that was not manufactured under the preferable conditions at the time of rolling wire manufacturing, and during spheroidizing annealing in ferrite grains The spheroidized cementite is dispersed with the divided layered cementite as the core, the grain boundary cementite ratio is small, the deformation resistance remains high, and the crack generation rate exceeds the acceptance standard.

  Test No. 41 and 42 are examples in which spheroidizing annealing was performed with SA8 and SA9 where the conditions were not appropriate using steel type Q that was not manufactured under the preferable conditions at the time of rolling wire production, and a large amount of layered cementite that was divided during rolling was generated. The grain boundary cementite ratio after spheroidizing annealing is small, the deformation resistance remains high, and the crack generation rate exceeds the acceptance standard.

  The steel wire for machine structural parts of the present invention is suitable for materials of various machine structural parts such as automobile parts and construction machine parts manufactured by cold working such as cold forging, cold forging, and cold rolling. Used for. Specifically, these mechanical structural parts include bolts, screws, nuts, sockets, ball joints, inner tubes, torsion bars, clutch cases, cages, housings, hubs, covers, cases, washers, tappets, saddles, bulgs, Inner case, clutch, sleeve, outer race, sprocket, core, stator, anvil, spider, rocker arm, body, flange, drum, fitting, connector, pulley, metal fitting, yoke, base, valve lifter, spark plug, pinion gear, steering Examples include mechanical parts such as shafts and common rails, and electrical parts. The steel wire of the present invention is industrially useful as a steel wire for a high-strength mechanical structural component that is suitably used as a material for the above-mentioned mechanical structural component, and deformation resistance at room temperature when manufacturing the various mechanical structural components described above. , And excellent cold workability can be exhibited by suppressing cracking of the material.

Claims (2)

% By mass
C: 0.3-0.6%
Si: 0.05 to 0.5%,
Mn: 0.2 to 1.7%,
P: more than 0%, 0.03% or less,
S: 0.001 to 0.05%,
Al: 0.005 to 0.1% and N: 0 to 0.015%, respectively, the balance consisting of iron and inevitable impurities,
Steel metal structure, is composed of ferrite and cementite, the number ratio of cementite present in the ferrite grain boundary state, and are 58% or more relative to the total cementite number,
A steel wire for machine structural parts, wherein an average equivalent circle diameter of bcc-Fe crystal grains in the metal structure is 20 µm or less . Furthermore, in mass%,
Cr: more than 0%, 0.5% or less,
Cu: more than 0%, 0.25% or less,
Ni: more than 0%, 0.25% or less,
The steel wire for machine structural parts according to claim 1, comprising at least one selected from the group consisting of Mo: more than 0% and not more than 0.25% and B: more than 0% and not more than 0.01%.