CN107406949B

CN107406949B - Steel wire for machine structural parts

Info

Publication number: CN107406949B
Application number: CN201680019276.1A
Authority: CN
Inventors: 佐佐木雄基; 高知琢哉; 千叶政道; 坂田昌之
Original assignee: Kobe Steel Ltd
Current assignee: Kobe Steel Ltd
Priority date: 2015-03-31
Filing date: 2016-03-16
Publication date: 2020-07-14
Anticipated expiration: 2036-03-16
Also published as: EP3279355A4; JP2016194100A; US20180105894A1; JP6479538B2; MX2017012063A; CN107406949A; KR20170118902A; TWI586814B; TW201641709A; EP3279355A1; WO2016158428A1

Abstract

Provided is a steel wire for machine structural parts, which has reduced deformation resistance during cold working, is improved in crack resistance, and exhibits excellent cold workability. The steel wire for machine structural parts of the present invention contains, in mass%, C: 0.3-0.6%, Si: 0.05-0.5%, Mn: 0.2-1.7%, P: above 0% and below 0.03%, S: 0.001 to 0.05%, Al: 0.005-0.1% and N: 0 to 0.015% and the balance iron and unavoidable impurities, wherein the steel has a microstructure composed of ferrite and cementite, and the proportion of cementite present in the ferrite grain boundary is 40% or more relative to the total amount of cementite.

Description

Steel wire for machine structural parts

Technical Field

The present invention relates to a steel wire used as a material for machine structural parts. More particularly, the present invention relates to a steel wire for machine structural parts, which has low deformation resistance during cold working, good crack resistance, and excellent cold workability when a wire rod produced by temper rolling is cold worked after spheroidizing annealing. In the present specification, the term "wire rod" refers to a wire-shaped steel material that is used in the meaning of a rolled wire rod and is cooled to room temperature after hot rolling. The "steel wire" refers to a wire-shaped steel material obtained by subjecting a rolled wire rod to a heat treatment such as spheroidizing annealing.

Background

In the production of various parts for machine structures such as automobile parts and parts for construction machines, hot rolled wire rods of carbon steel, alloy steel, and the like are generally subjected to spheroidizing annealing for the purpose of imparting cold workability thereto. Then, the spheroidizing annealed rolled wire rod, i.e., the steel wire is cold-worked, thereafter subjected to machining such as cutting to be formed into a predetermined shape, and subjected to quenching and tempering to finally adjust the strength, thereby being used as a machine structural part.

In cold working, the reduction of the deformation resistance of the steel wire can be expected to improve the die life. In addition, the crack resistance of the steel wire is improved, and the yield of various parts can be expected to be improved.

Various methods have been proposed as techniques for improving the cold workability of steel wires. As such a technique, for example, patent document 1 discloses the following technique: "the metal structure substantially consists of ferrite grains having an average grain diameter of 15 μm or more and spherical carbides having an average grain diameter of 0.8 μm or less and a maximum grain diameter of 4.0 μm or less and per 1mm²Is 0.5 × 10⁶×C％～5.0×10⁶× C%, and the maximum distance between spherical carbides having a particle size of 0.1 μm or more among the spherical carbides is 10 μm or less ".

Patent document 2 discloses the following technique: "a steel wire having a metal structure of steel, wherein the metal structure has cementite and ferrite, the total area ratio of the cementite and ferrite is 95 area% or more, an aspect ratio of 90% or more of the cementite is 3 or less, an average center-of-gravity distance of the cementite is 1.5 μm or more, and an average grain size of the ferrite is 5 to 20 μm" with respect to the entire structure ".

Patent document 2 discloses a method for obtaining the above-mentioned metallic structure, which comprises raising the temperature to a temperature range from a point a1 to a point a1 +50 ℃, maintaining the temperature at a point a1 to a point a1 +50 ℃ for 0 to 1hr, then performing an annealing treatment of cooling the metallic structure at an average cooling rate of 10 to 200 ℃/hr for 2 or more times from the temperature range from the point a1 to a1 +50 ℃ to a point a 1-100 to a 1-30 ℃, then raising the temperature to a temperature range from a point a1 to a point a1 +30 ℃, and maintaining the metallic structure at a point a1 to a point a1 +30 ℃ and then cooling the metallic structure. That is, a method is disclosed in which, when the temperature is raised to a point A1 and then the mixture is cooled after being held in a temperature range from point A1 to point A1 +30 ℃, the residence time is 10 minutes to 2 hours in the temperature range from point A1 to point A1 +30 ℃ to point A1, the mixture is cooled at an average cooling rate of 10 to 100 ℃/hr from the temperature range from point A1 to point A1 +30 ℃ to a cooling temperature range from point A1 to point-100 ℃ to point A1-20 ℃, and then the mixture is held in the cooling temperature range for 10 minutes to 5 hours and then cooled.

On the other hand, patent document 3 discloses the following technique: "A steel wire having a structure in which the standard deviation of the distance between cementites divided by the average value of the distance between cementites is 0.50 or less". In this method, cementite is distributed at substantially uniform intervals, and a large amount of cementite exists even in ferrite grains.

[ Prior art documents ]

[ patent document ]

[ patent document 1 ] International publication No. 2011/108459

[ patent document 2 ] Japanese patent laid-open No. 2012 and 140674

[ patent document 3 ] Japanese patent laid-open No. 2006 and 316291

The techniques proposed so far are useful as techniques for steel wires that improve cold workability such as cold forging, but it is desired to develop techniques for steel wires that further improve cold workability.

Disclosure of Invention

The present invention has been made under such circumstances, and an object thereof is to provide a steel wire for machine structural parts, which has reduced deformation resistance during cold working, has improved crack resistance, and can exhibit excellent cold workability.

The steel wire for machine structural parts according to the present invention to achieve the above object is characterized by comprising, in mass%, C: 0.3-0.6%, Si: 0.05-0.5%, Mn: 0.2-1.7%, P: above 0% and below 0.03%, S: 0.001 to 0.05%, Al: 0.005-0.1% and N: 0 to 0.015% and the balance iron and unavoidable impurities, wherein the steel has a microstructure composed of ferrite and cementite, and the proportion of cementite present in the ferrite grain boundary is 40% or more relative to the total amount of cementite.

The steel wire for machine structural parts of the present invention preferably contains, as necessary, a steel wire made from Cr: above 0% and below 0.5%, Cu: above 0% and below 0.25%, Ni: above 0% and below 0.25%, Mo: above 0% and below 0.25% and B: more than 0% and less than 0.01%.

In the steel wire for machine structural parts of the present invention, it is preferable that the 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 microstructure is composed of ferrite and cementite, and the ratio of the number of cementite existing in the ferrite grain boundary to the total number of cementite satisfies a predetermined value, whereby a steel wire with reduced deformation resistance and improved crack resistance can be provided. The steel wire for machine structural parts according to the present invention can suppress wear and breakage of a tool or a jig for plastic working such as a die because of a reduced deformation resistance, and can suppress occurrence of cracks during upsetting because of an improved crack resistance, thereby exhibiting excellent cold workability.

Detailed Description

The present inventors have studied from various viewpoints in order to realize a steel wire having both reduced deformation resistance during cold working and improved crack resistance. As a result, it was found that the deformation resistance was increased by cementite in ferrite grains at the time of cold working, and that voids, which are the cause of cracks, originated from cementite in ferrite grains.

Since cementite existing in ferrite grain boundaries receives a smaller amount of strain during cold working than cementite existing in the grain boundaries, deformation resistance can be reduced and the starting points for forming voids can be suppressed. Namely, the following idea can be obtained: in order to achieve the combination of the reduction in deformation resistance and the improvement in crack resistance, it is important to increase the ratio of the number of cementite existing at the ferrite grain boundary with respect to the total number of cementite, that is, to decrease the ratio of the number of cementite existing within the ferrite grain with respect to the total number of cementite.

In the techniques proposed so far, as a method for improving the deformation resistance and the crack resistance, a method of controlling the ferrite grain size is known, and no attention is paid to cementite aggregated in the grain boundary.

Hereinafter, each requirement defined in the present invention will be described.

The metal structure of the steel wire for machine structural parts (hereinafter simply referred to as "steel wire") of the present invention is a so-called spheroidized structure, and is composed of ferrite and cementite. The spheroidized structure is a metal structure that reduces the deformation resistance of the steel and contributes to the improvement of cold workability. The metal structure of the present invention may partially contain a pearlite structure. In addition, if the adverse effect on cold workability is small, precipitates such as AlN can be allowed to fall below 3% in area percentage.

However, the cold workability cannot be improved only by forming a microstructure composed of ferrite and cementite. Thus, as described in detail below, it is necessary to appropriately control the quantitative ratio of cementite existing at ferrite grain boundaries to the total number of cementite in the metal structure.

In the present specification, the ratio of the number of cementite present at ferrite grain boundaries (grain boundary cementite) to the total number of cementite is referred to as "grain boundary cementite ratio". The ratio of the number of cementite (intragranular cementite) present in ferrite grains to the total number of cementite is referred to as "intragranular cementite ratio". The "grain boundary cementite ratio" and "intragranular cementite ratio" are defined as follows.

In microscopic observation of the metal structure, the number of grain boundary cementite and the number of intragranular cementite were measured by a predetermined method in a predetermined visual field.

When the number of grain boundary cementite is "Na", the number of intragranular cementite is "Nb", and the total number of cementite (the sum of the number of grain boundary cementite and the number of intragranular cementite) is "Na + Nb", the grain boundary cementite ratio and the intragranular cementite ratio can be obtained as follows.

Proportion (%) of Na/(Na + Nb) × 100 of grain boundary cementite

Intragranular cementite ratio (%) - (% Nb/(Na + Nb)) × 100

The number of cementite can be measured in 1 visual field, or in a plurality of visual fields. In the measurement in a plurality of visual fields, the grain boundary cementite ratio and the intragranular cementite ratio are calculated using the total value of the number of grain boundary cementite and the number of intragranular cementite measured in each visual field.

The details of the measurement method will be described later.

If the proportion of grain boundary cementite decreases and the proportion of intragranular cementite increases, dislocations introduced into ferrite grains during cold working are trapped by the intragranular cementite, causing an increase in dislocations, and showing work hardening. As a result, the deformation resistance increases and the cold workability decreases. In addition, cementite within the grain is more likely to accumulate strain around cementite during cold working than grain boundary cementite. As a result, the intragranular cementite is likely to become a starting point of the crack. This shows that it is extremely effective to precipitate cementite at the ferrite grain boundary to improve cold workability.

From this viewpoint, the proportion of the number of cementite existing at ferrite grain boundaries (i.e., the proportion of cementite at grain boundaries) needs to be 40% or more with respect to the total number of cementite. By setting the proportion of grain boundary cementite to 40% or more, the deformation resistance can be reduced, and the occurrence of cracks at the starting points of cementite can be suppressed.

The morphology of cementite to be measured as the number of grain boundary cementite and the number of intragranular cementite is not particularly limited. For example, the shape of the cementite is not limited, and includes, in addition to spherical cementite, rod-like cementite having a large aspect ratio, lamellar cementite forming a pearlite structure, and the like. The size of the cementite to be measured is not limited, but a standard for the size is determined according to the measurement method. In the method of measuring the ratio of grain boundary cementite described later, the size of cementite that can be discriminated by an optical microscope at a magnification of 1000 times is the minimum size. Specifically, cementite having a size of 0.3 μm or more in circle-equivalent diameter is a measurement target.

The preferable lower limit of the proportion of the grain boundary cementite is 45%, and more preferably 50%. The higher the proportion of grain boundary cementite is, the lower the deformation resistance is, and the more effective the suppression of cracks is, and the most preferable is 100%. However, as described later, the increase in the proportion of grain boundary cementite is not easy in terms of production, and conventional techniques have disadvantages such as a decrease in hot rolling temperature and a long spheroidizing annealing time. In the conventional technique, the proportion of the grain boundary cementite is preferably about 80% or less, more preferably 70% or less, from the viewpoint of manufacturability.

In the steel wire of the present invention, the mean equivalent circle diameter of 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, simply referred to as "bcc-Fe crystal grain diameter") to 30 μm or less, the ductility can be improved, and the occurrence of cracks during cold working can be further suppressed. A preferred upper limit of the bcc-Fe grain diameter is 25 μm, more preferably 20 μm. The size of the bcc-Fe crystal grains to be measured is not limited, but the standard of the size is determined by the measurement method as in the case of the cementite. In the measurement method described later, the size that can be discriminated by the EBPS analyzer and the FE-SEM is the minimum size. Specifically, bcc-Fe crystal grains having a circle equivalent diameter of 1 μm or more are measured.

The structure to be controlled for the bcc-Fe grain size is a bcc-Fe grain surrounded by a high angle grain boundary having a misorientation larger than 15 °. This is because the small-angle grain boundary having the misorientation of 15 ° or less has little influence on cold workability. The "crystal orientation difference" is also referred to as "off-angle" or "tilt angle", and the EBSP method (Electron Back Shift positioning Pattern method) may be used for the measurement of the orientation difference. In addition, bcc-Fe surrounded by high angle grain boundaries for measuring the average grain size includes ferrite included in the pearlite structure in addition to proeutectoid ferrite.

In the present invention, the steel wire used as a material for the mechanical component is preferably a steel wire having a general chemical composition, but C, Si, Mn, P, S, Al, and N are preferably adjusted to an appropriate range. From this viewpoint, the appropriate ranges of these chemical components and the reasons for their limitations are as follows. In the present specification, the term "%" as to the chemical composition means% by mass.

C：0.3～0.6％

C is an element useful in ensuring the strength of steel, i.e., the strength of the final product. In order to effectively exhibit such an effect, the C content needs to be 0.3% or more. The C content is preferably 0.32% or more, more preferably 0.34% or more. However, if C is contained excessively, the strength is high and the cold workability is reduced, 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 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 is 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, if Si is excessively contained, the hardness excessively increases, and cold workability deteriorates. Therefore, the Si content is 0.5% or less. The Si content is preferably 0.45% or less, more preferably 0.40% or less.

Mn：0.2～1.7％

Mn is an element effective for increasing the strength of the final product by improving the 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, if Mn is excessively contained, hardness increases and cold workability deteriorates. Therefore, the Mn content is 1.7% or less. The Mn content is preferably 1.5% or less, more preferably 1.3% or less.

P: higher than 0% and less than 0.03%

P is an element inevitably contained in steel, and grain boundary segregation occurs in steel, which causes ductility deterioration. Therefore, the P content is 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 lower the P content, the more preferable, but about 0.001% may remain due to the restrictions on the production process.

S：0.001～0.05％

S is an element inevitably contained in steel, and is an element harmful to cold workability because it is present as MnS in steel and deteriorates ductility. Therefore, the S content is 0.05% or less. The S content is preferably 0.04% or less, more preferably 0.03% or less. However, S is contained in an amount of 0.001% or more because it has an effect of improving machinability. The S content is preferably 0.002% or more, and more preferably 0.003% or more.

Al：0.005～0.1％

Al is useful as a deoxidizing element, and is useful for fixing solid-solution N present in steel as AlN. In order to effectively exhibit such an effect, 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, if the Al content is excessive, Al will be present₂O₃Excessively generated, and the cold workability is deteriorated. Therefore, the Al content is 0.1% or less. The Al content is preferably 0.090% or less, more preferably 0.080% or less.

N：0～0.015％

N is an element inevitably contained in steel, and when solid solution N is contained in steel, hardness increases due to strain aging, ductility decreases, and cold workability deteriorates. Therefore, the N content is 0.015% or less. The N content is preferably 0.013% or less, more preferably 0.010% or less. The smaller the N content, the more preferable the N content is, and the most preferable is 0%, but about 0.001% may remain due to the restrictions on the production process.

The steel wire of the present invention has the basic composition as described above, and the balance is substantially iron. The term "substantially iron" means that a trace amount of components (e.g., Sb, Zn, etc.) can be allowed to the extent that the characteristics of the present invention are not impaired, in addition to iron, and unavoidable impurities (e.g., O, H, etc.) other than P, S and N can be contained. In the present invention, the following optional elements may be contained as necessary, and the properties of the steel wire are further improved depending on the components contained.

From Cr: above 0% and below 0.5%, Cu: above 0% and below 0.25%, Ni: above 0% and below 0.25%, Mo: above 0% and below 0.25% and B: more than 0% and less than 0.01%

Cr, Cu, Ni, Mo, and B are elements that improve the hardenability of the steel material and are effective for increasing the strength of the final product, and are contained singly or in two or more kinds as necessary. Such effects become larger as the content of these elements increases, and a preferable content for effectively exerting the effects is a Cr content of 0.015% or more, more preferably 0.020% or more. The preferable content of each of the Cu amount, the Ni amount, and the Mo amount is 0.02% or more, and more preferably 0.05% or more. The content of B is preferably 0.0003% or more, more preferably 0.0005% or more.

However, if the contents of Cr, Cu, Ni, Mo, and B are excessive, the strength becomes too high, and the cold workability deteriorates. Therefore, the Cr content is preferably 0.5% or less, the Cu, Ni, and Mo contents are each preferably 0.25% or less, and the B content is preferably 0.01% or less. The content of these elements is more preferably 0.45% or less, and still more preferably 0.40% or less of the Cr content. The upper limits of the amounts of Cu, Ni, and Mo are preferably 0.22%, and more preferably 0.20%. The upper limit of the amount of B is more preferably 0.007%, and still more preferably 0.005%.

The steel wire of the present invention defines the structure form after spheroidizing annealing, and in order to achieve such a structure form, spheroidizing annealing conditions described below are preferably appropriately controlled. However, in order to ensure the above-described structure form, it is more preferable to appropriately control the conditions in the rolled wire rod production stage so that the structure form of the rolled wire rod is in a state in which grain boundary cementite is likely to precipitate during spheroidizing annealing.

In the rolled wire rod production stage, it is preferable to adjust the cooling rate and the temperature range appropriately so that the steel satisfying the above composition is adjusted to the finish rolling temperature at the time of hot rolling and the cooling rate thereafter is adjusted to 3 stages. By producing the rolled wire rod under such conditions, the microstructure before spheroidizing annealing can be made to have pearlite and ferrite as main phases, the bcc-Fe crystal grain diameter can be made to be in a predetermined range, the pro-eutectoid ferrite crystal grains can be equiaxed, and the interval of the narrowest portion of pearlite can be made to be equal to or smaller than a predetermined value. By subjecting such a structure to spheroidizing annealing under the conditions described later, a steel wire in which grain boundary cementite is sufficiently precipitated can be easily obtained. For this reason, the rolled wire rod manufacturing conditions are preferably, specifically, the following cooling is performed in order: first cooling at an average cooling rate of 7 ℃/sec or more after finish rolling at 800 ℃ or more and 1050 ℃ or less; a second cooling at an average cooling rate of 1 ℃/sec or more and 5 ℃/sec or less; the average cooling rate is higher than the second cooling rate and is a third cooling rate of 5 ℃/sec or more. The finishing temperature of the first cooling and the starting temperature of the second cooling are preferably in the range of 700 to 750 ℃. The finishing temperature of the second cooling and the starting temperature of the third cooling are preferably in the range of 600 to 650 ℃. The finish temperature of the third cooling is preferably 400 ℃ or lower. The finish rolling temperature and the first to third cooling are described in detail, respectively.

(a) The finishing temperature is as follows: 800 ℃ or higher and 1050 ℃ or lower

In order to reduce the bcc-Fe grain size of the structure before spheroidizing annealing to, for example, 15 μm or less, it is preferable to appropriately control the finish rolling temperature. If the finish rolling temperature is higher than 1050 ℃, it is difficult to reduce the bcc-Fe grain size. However, when the finish rolling temperature is less than 800 ℃, the bcc-Fe crystal grain size becomes too small, for example, less than 5 μm, and softening becomes difficult, and therefore 800 ℃ or higher is preferable. The lower limit of the finish rolling temperature is more preferably 850 ℃ and still more preferably 900 ℃ or higher. The upper limit of the finish rolling temperature is more preferably 1000 ℃ and still more preferably 950 ℃.

(b) First cooling

The first cooling is started from 800 ℃ to 1050 ℃ which is the finish rolling temperature, and is ended at a temperature range of 700 to 750 ℃. In this first cooling, if the cooling rate is slow, the bcc-Fe crystal grains of the structure before spheroidizing annealing become coarse, and the bcc-Fe crystal grain diameter may become large. Therefore, the average cooling rate of the first cooling is preferably 7 ℃/sec or more. The average cooling rate of the first cooling is more preferably 10 ℃/sec or more, and still more preferably 20 ℃/sec or more. The upper limit of the average cooling rate of the first cooling is not particularly limited, but is preferably 200 ℃/sec or less as a practical range. In the first cooling, the cooling may be performed by changing the cooling rate as long as the average cooling rate is 7 ℃/sec or more.

(c) Second cooling

The second cooling is started from the temperature range of 700-750 ℃ and ended from the temperature range of 600-650 ℃. In order to equiaxe the proeutectoid ferrite grains, that is, in order to reduce the average aspect ratio of the proeutectoid ferrite grains, for example, 3.0 or less, it is preferable to perform slow cooling at an average cooling rate of 5 ℃/sec or less in the second cooling. A more preferable upper limit of the average cooling rate of the second cooling is 4 ℃/sec, and still more preferably 3.5 ℃/sec or less. On the other hand, if the average cooling rate of the second cooling is too low, the bcc-Fe crystal grains become coarse, and the bcc-Fe crystal grain size may become too large. Therefore, the average cooling rate of the second cooling is preferably 1 ℃/sec or more. A more preferable lower limit of the average cooling rate of the second cooling is 2 ℃/sec, and still more preferably 2.5 ℃/sec. In the second cooling, the cooling may be performed by changing the cooling rate as long as the average cooling rate is not less than 1 ℃/sec and not more than 5 ℃/sec.

(d) Third cooling

The third cooling is started from the temperature range of 600-650 ℃ and ended below 400 ℃. In the third cooling, the average lamellar spacing of pearlite is reduced as much as possible so that cementite is easily dissolved and nuclei of spherical cementite do not remain in the crystal. Thus, by performing the subsequent spheroidizing annealing treatment appropriately, the proportion of the grain boundary cementite is increased. In order to reduce the average lamellar spacing of pearlite to, for example, 0.20 μm or less, it is preferable that the third cooling is performed at an average cooling rate of 5 ℃/sec or more faster than the second cooling. If the cooling is slower than 5 ℃/sec, it becomes difficult to reduce the average lamellar spacing of pearlite. The average cooling rate of the third cooling is more preferably 10 ℃/sec or more, and still more preferably 20 ℃/sec or more.

The upper limit of the average cooling rate of the third cooling is not particularly limited, but is preferably 200 ℃/sec or less as a practical range. In the third cooling, the cooling may be performed by changing the cooling rate as long as the average cooling rate is 5 ℃/sec or more. The lower limit of the temperature at which the third cooling is completed is not particularly limited, but is preferably 200 ℃. After the third cooling, the resultant may be cooled to room temperature by ordinary cooling such as cooling.

After cooling to room temperature, drawing may be carried out at room temperature if necessary, and the reduction of area at this time may be, for example, 30% or less. When wire drawing is performed, carbides in the steel are broken, and aggregation of carbides can be promoted by subsequent spheroidizing annealing, and therefore, it is effective to shorten the soaking time in the spheroidizing annealing. However, if the reduction of area in wire drawing is higher than 30%, the strength after annealing becomes high, and there is a possibility that cold workability deteriorates, and therefore the reduction of area in wire drawing is preferably 30% or less. The lower limit of the reduction of area is not particularly limited, but is preferably 2% or more, which can provide an effect.

In the rolled wire rod produced under the above-described preferred conditions, the pearlite phase in the structure is transformed into austenite by the subsequent spheroidizing annealing treatment, and the pearlite phase in the structure is transformed into ferrite + cementite, and the size of the original pearlite phase is reduced, that is, the grain growth of the metal structure is suppressed, whereby the intragranular precipitation of cementite is reduced, and the intergranular cementite is easily precipitated.

As such spheroidizing annealing conditions, it is preferable that the rolled wire rod is heated from room temperature to 730 ℃ in an air furnace at least at 500 ℃ to 730 ℃ at an average heating rate of 50 ℃/hr or more, then heated to 740 ℃ at an average heating rate of 2 to 5 ℃/hr, held at 740 ℃ for 1 to 3 hours, then cooled to 720 ℃ at an average cooling rate of 20 ℃/hr or more, cooled to 640 ℃ at an average cooling rate of 8 to 12 ℃/hr, and then cooled.

In the spheroidizing annealing conditions, the grain growth of the metal structure can be suppressed by setting the average heating rate of at least 500 ℃ to 730 ℃ to 50 ℃/hr or more when heating from room temperature to 730 ℃. The average heating rate at this time is more preferably 60 ℃/hr or more. However, if the average heating rate is too high, the temperature of the rolled wire rod becomes difficult to follow, and therefore, it is preferably 200 ℃/hr or less, and more preferably 150 ℃/hr or less.

The average heating rate when the alloy is heated from room temperature to 500 ℃ is usually 100 ℃/hr or more, but the average heating rate in this temperature range has little influence on the grain growth of the metal structure. In consideration of productivity, the heating rate is preferably high, for example, 120 ℃/hr or more, and more preferably 140 ℃/hr or more. The upper limit of the average heating rate in this case is preferably 200 ℃/hr, more preferably 150 ℃/hr, as in the case of the average heating rate of 500 ℃ to 730 ℃. The average heating rate from room temperature to 500 ℃ may be the same as or different from the average heating rate of at least 500 ℃ to 730 ℃. In short, in order to reduce the original pearlite size, reduce the intragranular precipitation of cementite, and make grain boundary cementite easily precipitate, the average heating rate of at least 500 ℃ to 730 ℃ should be secured to 50 ℃/hr or more.

Further, by controlling the average heating rate from 730 ℃ to 740 ℃ above the point a1 to 2 to 5 ℃/hr, decomposition and solid solution of cementite in the pearlite structure can be sufficiently performed while suppressing the grain growth of the metal structure as much as possible. When the average heating rate is higher than 5 ℃/hr, it is difficult to secure a sufficient time for decomposition and solid solution of cementite in the pearlite structure, and when the average heating rate is lower than 2 ℃/hr, the heating time from 730 ℃ to 740 ℃ becomes long, and it is difficult to suppress grain growth of the metal structure. The average heating rate in this case is more preferably 3 ℃/hr or more and 4 ℃/hr or less.

Preferably, the temperature is maintained at 740 ℃ for 1 to 3 hours. If the temperature is kept shorter than 1 hour, decomposition and solid solution of cementite in the pearlite structure are insufficient, and if it is longer than 3 hours, it is difficult to suppress grain growth of the metal structure. The holding time in this case is more preferably 1.5 hours or more and 2.5 hours or less.

After the above-described holding, the average cooling rate to 720 ℃ is preferably set to 20 ℃/hr or more, whereby the grain growth of the metal structure can be suppressed. The average cooling rate at this time is more preferably 30 ℃/hr or more, but if the average cooling rate is too high, the temperature of the rolled wire rod becomes difficult to follow, and therefore, it is preferably 100 ℃/hr or less.

Thereafter, by controlling the average cooling rate from 720 ℃ to 640 ℃ to 8 to 12 ℃/hr, cementite can be preferentially precipitated in the ferrite grain boundary, and precipitation of cementite having a large aspect ratio such as a pearlite structure can be suppressed. When the average cooling rate is slower than 8 ℃/hr, it is difficult to suppress the grain growth of the metal structure, and when the average cooling rate is faster than 12 ℃/hr, a large amount of cementite having a large aspect ratio such as a pearlite structure is re-precipitated. The average cooling rate at this time is more preferably 9 ℃/hr or more and 11 ℃/hr or less.

Such spheroidizing annealing may be repeated a plurality of times, and the aspect ratio of each cementite is decreased by such repetition, and the proportion of grain boundary cementite is increased. For example, as shown in test nos. 7, 12, 14, 19 and 27 of examples described later, when steel grades C, E, F, H and K whose rolled wire rod production conditions are not properly controlled are used, the ratio of grain boundary cementite can be brought within an appropriate range by repeating predetermined spheroidizing annealing thereafter, and both the deformation resistance and the crack occurrence rate can be reduced.

The number of repetitions of spheroidizing annealing is preferably at least 3 or more, but the number of repetitions is preferably 10 or less because the proportion of grain boundary cementite does not change even when the number of repetitions is too large. When the spheroidizing annealing is repeated a plurality of times, the spheroidizing annealing may be repeated under the same conditions or may be repeated under different conditions within the range of the above-described preferable conditions.

[ examples ] A method for producing a compound

The present invention will be described in more detail with reference to examples. The present invention is not limited to the following examples, and can be modified and implemented as appropriate within the scope that can meet the purpose described above and below, and all of them are included in the technical scope of the present invention.

Steels having chemical compositions shown in Table 1 below were rolled under various production conditions shown in Table 2 below to prepare steels

The wire rod of (1). In table 2, cooling 1, cooling 2, and cooling 3 correspond to the first cooling, second cooling, and third cooling recommended in the present invention. Steel type B is a comparative example in which the chemical composition deviates from the specified values.

Steel grades C, E, F, H, K, O, P and Q are examples in which rolled wire rods were not produced under the appropriate production conditions of the present invention. Wherein the finishing temperature of steel grades C, E, F and K is high. Further, steel type H is an example of producing a rolled wire rod under the condition that the cooling rate of cooling 3 corresponding to the third cooling is slow, that is, an example of producing a rolled wire rod by cooling while maintaining the cooling rate of the second cooling.

In steel type O, the steel is subjected to a second cooling to 550 ℃, then heated to 580 ℃, and then held at 580 ℃ for 120 seconds, and then cooled to room temperature, and then subjected to a drawing process with a reduction in area of 40%. In steel type P, cooling was performed at a monotonous cooling rate of cooling 1. After cooling 1 in steel type Q, a holding step of holding at 550 ℃ for 60 seconds was performed, and the steel type Q was allowed to cool to room temperature to perform rough drawing with a reduction of area of 15%. [ TABLE 1 ]

[ TABLE 2 ]

Next, for each of the rolled wire rods from which the steel grades O, P and Q were removed, any of the following spheroidizing anneals was performed in an atmospheric furnace: (a) heating from room temperature to 730 ℃, heating from room temperature to 500 ℃ at an average heating rate of 110 ℃/h, heating from 500 ℃ to 730 ℃ at an average heating rate of 80 ℃/h, thereafter heating to 740 ℃ at an average heating temperature of 3 ℃/h, holding at 740 ℃ for 3 hours, cooling to 720 ℃ at an average cooling rate of 30 ℃/h, cooling to 640 ℃ at an average cooling rate of 10 ℃/h, and thereafter releasing the cold spheroidizing annealing (this annealing condition is hereinafter referred to as "SA 1"); (b) SA1 was subjected to 5 repetitions of spheroidizing annealing (this annealing condition is hereinafter abbreviated as "SA 2") and (c) heating from room temperature to 730 ℃, heating from room temperature to 500 ℃ at an average heating rate of 110 ℃/hr, heating from 500 ℃ to 730 ℃ at an average heating rate of 80 ℃/hr, thereafter heating at an average heating rate of 3 ℃/hr to 740 ℃, holding at 740 ℃ for 3 hours, then cooling at an average cooling rate of 30 ℃/hr to 640 ℃, and thereafter, spheroidizing annealing by cooling down (this annealing condition is hereinafter abbreviated as "SA 3"). 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 ℃ to 640 ℃ is not properly controlled.

Further, regarding steel grade O, any one of the following spheroidizing anneals is performed in an atmospheric furnace: (d) heating from room temperature to 680 ℃ at an average heating rate of 80 ℃/hr, holding at 680 ℃ for 5 hours, cooling at an average cooling rate of 10 ℃/hr to 640 ℃, and thereafter performing a free-cooling spheroidizing annealing (hereinafter, this annealing condition will be referred to as "SA 4"), and (e) heating from room temperature to 700 ℃ at an average heating rate of 80 ℃/hr, holding at 700 ℃ for 5 hours, cooling at an average cooling rate of 10 ℃/hr to 640 ℃, and thereafter performing a free-cooling spheroidizing annealing (hereinafter, this annealing condition will be referred to as "SA 5"). Annealing conditions SA4 and SA5 are examples of annealing conditions deviating from the preferred ones in the present invention.

In addition, for the steel grade P, any one of the following spheroidizing annealing is carried out by using an atmospheric furnace: (f) heating from room temperature to 740 ℃ at an average heating rate of 80 ℃/hr, immediately thereafter cooling to 660 ℃ at an average cooling rate of 80 ℃/hr, repeating this procedure 3 times (however, heating from 660 ℃ C. after the second time), thereafter heating from 660 ℃ to 740 ℃ at an average heating rate of 80 ℃/hr, after holding at 740 ℃ for 30 minutes, cooling to 660 ℃ at an average cooling rate of 80 ℃/hr, holding at 660 ℃ for 1 hour, thereafter performing a spheroidizing annealing of cooling (hereinafter this annealing condition is abbreviated as "SA 6") and (g) heating from room temperature to 740 ℃ at an average heating rate of 80 ℃/hr, after holding at 740 ℃ for 10 minutes, cooling to 660 ℃ at an average cooling rate of 80 ℃/hr, repeating this procedure 3 times (however, heating from 660 ℃ C. after the second time), thereafter heating from 660 ℃ to 740 ℃ at an average heating rate of 80 ℃/hr, after 30 minutes at 740 ℃, the steel sheet was cooled at an average cooling rate of 80 ℃/hr to 660 ℃ and held at 660 ℃ for 1 hour, and thereafter, the spheroidizing annealing was performed by cooling (this annealing condition is hereinafter abbreviated as "SA 7"). Annealing conditions SA6 and SA7 are examples of annealing conditions deviating from the preferred ones in the present invention.

And for the steel grade Q, carrying out spheroidizing annealing by using an atmospheric furnace, wherein the spheroidizing annealing comprises the following steps: (h) heating from room temperature to 720 ℃ at an average heating rate of 150 ℃/hr, holding at 720 ℃ for 1 hour, and then cold spheroidizing annealing (hereinafter, this annealing condition will be abbreviated as "SA 8"), and (i) heating from room temperature to 730 ℃ at an average heating rate of 150 ℃/hr, holding at 730 ℃ for 1 hour, and then cold spheroidizing annealing (hereinafter, this annealing condition will be abbreviated as "SA 9"). Annealing conditions SA8 and SA9 are examples of annealing conditions deviating from the preferred ones in the present invention.

The steel wire after the spheroidizing annealing was subjected to the following methods to measure (1) the bcc-Fe crystal grain size of the metal structure, (2) the proportion of grain boundary cementite, (3) the deformation resistance at cold working, and (4) the crack occurrence rate at cold working.

In addition, when the ferrite grain size and the grain boundary cementite ratio of the spheroidizing annealed steel wire were measured, resin embedding was performed so that the cross section could be observed, and the mirror-polished surface was polished with sandpaper and a diamond soft-ground. For the radius D of the steel wire, the position D/4 from the surface of the steel wire is measured.

(1) Measurement of bcc-Fe grain diameter

In the measurement of the bcc-Fe crystal grain diameter, measurement was performed using an EBSP analyzer and an Fe-SEM (Field emission scanning Electron Microscope). the analysis tool uses OIM software of seiko TS L ソリューションズ, and boundary of crystal orientation difference (also referred to as "off angle") of more than 15 °, that is, high angle grain boundary, was defined as crystal grain boundary, "crystal grain" was calculated, and the average value of the diameter when the area of bcc-Fe crystal grain was converted into a circle, that is, average equivalent circle diameter was calculated, and at this time, the measurement area was 200 μm × 400 μm, the measurement step was 1.0 μm interval, and the measurement point where the Confidence coefficient (Confidence Index) indicating the reliability of the measurement orientation was 0.1 or less was removed from the analysis object.

(2) Measurement of grain boundary cementite proportion

In the measurement of the ratio of grain boundary cementite, ferrite grain boundaries and cementite were visualized by etching with a picaldehyde etching solution for 5 minutes or more, and the structure was observed with an optical microscope, and 3 fields were photographed at a magnification of 1000 times. On these photographs, 10 transverse lines were drawn at equal intervals, and the number of grain boundary cementite and the number of intragranular cementite present on the lines were measured. The number of grain boundary cementite present in 3 fields was divided by the total number of cementite present in the same field, to thereby calculate the grain boundary cementite ratio. The minimum equivalent circle diameter of the cementite measured was 0.3 μm. Here, the grain boundary cementite is defined as a grain boundary cementite in which the grain boundary cementite is in contact with ferrite grain boundaries and the aspect ratio of the cementite particles is 3.0 or less. Therefore, the cementite particles are in contact with ferrite grain boundaries, and have an aspect ratio exceeding 3.0, and are intracrystalline cementite.

(3) Measurement of deformation resistance

Made of steel wire

The cold forging test specimens of (1) were subjected to cold forging tests at a strain rate of 5/sec to 10/sec at room temperature using a forging press at a working ratio of 60% 5 times each. In the measurement of the deformation resistance, the deformation resistance at 40% working was measured 5 times from the data of the working ratio-deformation resistance obtained by the cold forging test at the working ratio of 60%, and the average value of the 5 times was obtained. The acceptable standard for the deformation resistance of steel grades A to E and P having a C content in the range of 0.3 to less than 0.4% is 650MPa or less. The acceptable standard for the deformation resistance of steel grades F to J, O and Q having a C content in the range of 0.4 to less than 0.5% is 680MPa or less. The qualified standard of the deformation resistance of the steel grades K-N with the C content within the range of 0.5-0.6% is below 730 MPa.

(4) Measurement of crack incidence

Made of steel wire

The cold forging test specimens of (1) were subjected to cold forging tests at a strain rate of 5/sec to 10/sec at room temperature using a forging press at a working ratio of 60% 5 times each. In the measurement of the crack occurrence rate, after the cold forging test at a 60% reduction ratio, surface observation was performed 5 times by a solid microscope, the presence or absence of surface cracks was measured at a magnification of 20 times, and the average was determined by dividing "the number of samples having surface cracks" by 5. The pass standard for the crack occurrence rate of all steel grades is 20% or less.

These results are shown in the following table 3 together with the spheroidizing annealing conditions. In the comprehensive evaluation column in table 3, an example in which both the reduction of the deformation resistance and the improvement of the crack resistance are good is represented by "o.k", and an example in which at least one of the reduction of the deformation resistance and the improvement of the crack resistance is deteriorated is represented by "n.g".

[ TABLE 3 ]

The results of table 3 can be considered as follows. Tests Nos. 1, 2, 7 to 9, 12, 14 to 16, 19 to 21, 23, 24, 27 to 29, 31, 32, 34 and 35 are examples satisfying all requirements specified in the present invention, and it is found that both reduction of deformation resistance and improvement of crack resistance are achieved.

Among them, tests nos. 7, 12, 14, 19 and 27 are examples in which steel grades C, E, F, H or K which were not produced under the preferable conditions for rolling wire rods were used, and by annealing with SA2 repeated thereafter, grain boundary cementite was sufficiently precipitated, and both the deformation resistance and the crack occurrence rate reached the acceptable standards. Among these, test No.12, which is a preferable requirement, had a somewhat large bcc-Fe crystal grain diameter, but both the deformation resistance and the crack occurrence rate reached the acceptable standards.

Here, focusing on test nos. 1 and 2 (steel type a), test nos. 6 and 7 (steel type C), test nos. 8 and 9 (steel type D), test nos. 11 and 12 (steel type E), test nos. 13 and 14 (steel type F), test nos. 15 and 16 (steel type G), test nos. 18 and 19 (steel type H), test nos. 20 and 21 (steel type I), test nos. 23 and 24 (steel type J), test nos. 26 and 27 (steel type K), test nos. 28 and 29 (steel type L), test nos. 31 and 32 (steel type M), and test nos. 34 and 35 (steel type N) which were subjected to the annealing conditions of SA1 and SA2, it was found that in any case, compared with the test specimen subjected to the annealing of SA1, the resistance and the crack generation rate of both the SA2 which was subjected to the annealing of SA1 repeated 5 times were further reduced.

On the other hand, test nos. 3 to 6, 10, 11, 13, 17, 18, 22, 25, 26, 30, 33 and 36 to 42 are comparative examples lacking any of the requirements specified in the present invention, and it is found that either or both of the deformation resistance and the crack occurrence rate fail to meet the acceptable standards.

That is, in the case of test nos. 3, 10, 17, 22, 25, 30, 33 and 36 in which spheroidizing annealing was performed with SA3 under inappropriate conditions, the proportion of grain boundary cementite was insufficient, and either or both of the deformation resistance and the crack occurrence rate failed to meet the acceptable standards.

Tests 4 and 5 are examples in which steel type B having an excessive Mn content was used, and the deformation resistance during cold working was high.

Tests 6, 11, 13, 18 and 26 were examples using steel grade C, E, F, H or steel grade K which was not produced under the conditions preferred for the production of rolled wire rods, and after the spheroidizing annealing of SA1, no intergranular cementite was precipitated, and both the deformation resistance and the crack occurrence rate were not satisfactory. However, when these steels were thereafter subjected to SA2 spheroidizing annealing by SA1 repeated 5 times, grain boundary cementite was properly precipitated, and both the deformation resistance and the crack occurrence rate reached the acceptable standards (test nos. 7, 12, 14, 19, and 27).

In the test nos. 37 and 38, in which the steel grade O not produced under the conditions preferred in the production of rolled wire rods was used and the spheroidizing annealing was performed under the improper conditions of SA4 or SA5, fine cementite was uniformly dispersed, the proportion of grain boundary cementite was decreased, and the crack occurrence rate exceeded the acceptable level in the state of high deformation resistance.

In the test nos. 39 and 40, in the case of using the steel type P which was not produced under the conditions preferred in the production of rolled wire rods and carrying out the spheroidizing annealing under the unsuitable conditions SA6 or SA7, cementite spheroidized by using the lamellar cementite broken during the spheroidizing annealing as nuclei is dispersed in ferrite grains, the proportion of the grain boundary cementite is reduced, and the crack occurrence rate is out of the acceptable level in the state where the deformation resistance is high.

In the test nos. 41 and 42, in the case of using the steel type Q not manufactured under the conditions preferable in the manufacture of rolled wire rods and performing the spheroidizing annealing under the improper conditions SA8 or SA9, a large amount of lamellar cementite broken during rolling was generated, the proportion of grain boundary cementite after the spheroidizing annealing became small, and the crack occurrence rate exceeded the acceptable level in the state of high deformation resistance.

[ industrial applicability ]

The steel wire for machine structural parts of the present invention is suitable for use as a material for various machine structural parts such as automobile parts and construction machine parts manufactured by cold working such as cold forging, cold heading, and cold rolling. Specifically, the mechanical component includes a mechanical component, an electrical component, and the like, and more specifically, includes a bolt, a screw, a nut, a socket, a ball joint, a bushing, a torsion bar, a clutch case, a cage, a frame, a hub, a cover, an outer housing, a washer, a tappet, a bracket, a valve (バルグ), an inner housing, a clutch, a sleeve, a bearing outer ring, a sprocket, an iron core, a stator, an anvil, a cross shaft, a rocker arm, a body, a flange, a drum, a joint, a connector, a pulley, a mold, a yoke, a cap, a valve lifter, a spark plug, a pinion, a steering shaft, and a common rail. The steel wire of the present invention is industrially useful as a high-strength steel wire for machine structural parts to which the material for the machine structural parts is applied, and can exhibit excellent cold workability by suppressing cracking of the material and reducing deformation resistance at room temperature in the production of the above-described various machine structural parts.

This application is accompanied with the claims of priority based on the application of the japanese patent application on 2015, 3/31 and the application No. 2015-073776. Patent application No. 2015-073776 is incorporated by reference into this specification.

Claims

1. A steel wire for machine structural parts, which suppresses the occurrence of cracks during cold working, comprising, in mass%, a steel wire having a high tensile strength and a high tensile strength

C：0.3～0.6％、

Si：0.05～0.5％、

Mn：0.2～1.7％、

P: more than 0% and less than 0.03%,

S：0.001～0.05％、

Al: 0.005 to 0.1% and

n: 0 to 0.015% and the balance of iron and unavoidable impurities,

the metal structure of the steel is composed of ferrite and cementite, the proportion of the number of cementite existing in the ferrite grain boundary is more than 58% relative to the total number of cementite,

the average equivalent circle diameter of bcc-Fe crystal grains in the metal structure is 13.5 μm or less.

2. The steel wire for machine structural parts according to claim 1, further comprising at least one selected from the group consisting of

Cr: more than 0% and less than 0.5%,

Cu: more than 0% and less than 0.25%,

Ni: more than 0% and less than 0.25%,

Mo: above 0% and below 0.25% and

b: more than 0% and less than 0.01%.