TWI586814B - Steel wire for mechanical structure parts - Google Patents

Description

Steel wire for mechanical structural parts

The present invention relates to steel wires used as materials for mechanical structural parts. More specifically, in the case of cold rolling processing after spheroidizing and blunting of a wire manufactured by quenching and temper rolling, the deformation resistance during cold rolling is low, the crack resistance is good, and the wire can be cooled. A steel wire for mechanical structural parts with excellent rolling workability. In the present specification, the term "wire material" is used in the sense of a rolled wire, and refers to a linear steel material which is cooled to room temperature after hot rolling and rolling. The term "steel wire" refers to a linear steel material which is subjected to a tempering treatment such as spheroidizing and blunting on a rolled wire.

When manufacturing various parts for mechanical structures such as parts for automobiles and parts for construction machinery, the hot-rolled rolled wires such as carbon steel and alloy steel are usually spheroidally burnt for the purpose of imparting cold-rolling workability. Then, the steel wire which is a spheroidized burnt wire is subjected to cold rolling, and then subjected to machining by cutting or the like to be formed into a specific shape, quenched and tempered, and finally subjected to strength adjustment to prepare. Parts for mechanical construction.

In cold rolling, by reducing the deformation resistance of the steel wire, Expect an increase in the life of the mold. Moreover, by increasing the crack resistance of the steel wire, it is expected that the yield of various parts will be improved.

Heretofore, various methods have been proposed as techniques for improving the cold rolling workability of steel wires. As such a technique, for example, Patent Document 1 discloses a steel wire in which a metal structure consists essentially of ferrite iron particles and spherical carbides, and the average grain size of the ferrite particles is 15 μm. In the above, the spherical carbide-based average particle diameter is 0.8 μm or less, and the maximum particle diameter 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%. In the spherical carbide, the maximum distance between the spherical carbides having a particle diameter of 0.1 μm or more is 10 μm or less.

Further, Patent Document 2 discloses a technique of "a steel wire in which a metal structure of a steel has cementite and ferrite iron, and a total area ratio of cementite and ferrite iron to all tissues is 95% or more, and The aspect ratio of 90% or more of the cementite is 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 iron is 5 to 20 μm.

In Patent Document 2, as a means for obtaining the above-described metal structure, it is revealed that the temperature is raised in the temperature range from A1 to A1 + 50 ° C, and after the temperature rise, the temperature is maintained at 0 to 1 hr in the temperature range of A1 to A1 + 50 ° C. Then, from the temperature range of A1 point to A1 point +50 °C to the temperature range of A1 point -100 °C~A1 point -30 °C, the blunt cooling treatment is performed at an average cooling rate of 10 to 200 ° C / hr for 2 times or more. Thereafter, the temperature is raised in the temperature range from A1 to A1 + 30 ° C, and the conditions for maintaining the post-cooling in the temperature range of A1 to A1 + 30 ° C are controlled as follows. That is, after the temperature rise reaches A1 point, it is in A1. When the temperature range from point ~A1 +30 °C is maintained and cooled, the retention time in the temperature range from A1 point to A1 point +30 °C before reaching point A1 is set to 10 minutes to 2 hours, from the above point A1~A1 The temperature region from +30 ° C to the cooling temperature region of A1 point -100 ° C ~ A1 point -20 ° C is cooled at an average cooling rate of 10 to 100 ° C / hr, and after maintaining the cooling temperature region for 10 minutes to 5 hours, The method of further cooling.

On the other hand, Patent Document 3 discloses a technique of "a steel wire having a structure in which the standard deviation of the distance between cementites is divided by the average value of the distance between the cementites to be 0.50 or less." In this way, cementite is distributed at substantially uniform intervals, and a majority of cementite is also present in the ferrite particles.

[Previous Technical Literature] [Patent Literature]

[Patent Document 1] International Publication No. 2011/108459

[Patent Document 2] Japanese Patent Laid-Open Publication No. 2012-140674

[Patent Document 3] Japanese Laid-Open Patent Publication No. 2006-316291

The technology proposed so far is useful as a technique for improving the cold-rolling workability of steel wire such as cold-rolling forging, but it is desired to develop a steel wire having improved cold-rolling workability.

The present invention is based on the completion of such a situation, the purpose of which is A steel wire for mechanical structural parts that can reduce the deformation resistance during cold rolling and improve the crack resistance and exhibit excellent cold rolling workability.

The steel wire for the mechanical structural component of the present invention which achieves the above-mentioned problem contains C: 0.3 to 0.6%, Si: 0.05 to 0.5%, Mn: 0.2 to 1.7%, and P: more than 0% and 0.03% or less, respectively, by mass%. , S: 0.001~0.05%, Al: 0.005~0.1% and N: 0~0.015%. The rest is made of iron and inevitable impurities. The metal structure of steel consists of ferrite and cementite. The proportion of cementite in the ferrite grain boundary is 40% or more relative to the total number of cementite.

The steel wire for the mechanical structural component of the present invention is preferably one by mass or more, and further contains one or more selected from the group consisting of: Cr: more than 0% and 0.5% or less, and Cu: more than 0% and 0.25% or less. Ni: more than 0% and 0.25% or less, Mo: more than 0% and 0.25% or less, and B: more than 0% and 0.01% or less.

In the steel wire for a mechanical structural part of the present invention, it is preferred that the bcc (body-centered cubic)-Fe crystal grain in the metal structure has an average equivalent circle diameter of 30 μm or less.

According to the steel wire for mechanical structural parts of the present invention, the chemical composition is appropriately adjusted, and the metal structure of the steel is composed of ferrite iron and cementite, and the proportion of cementite present in the iron grain boundary of the fat grain relatively When the total number of cementites satisfies a predetermined value, a steel wire which can achieve a reduction in deformation resistance and an improved crack resistance can be provided. Since the steel wire for the mechanical structural part of the present invention is reduced in deformation resistance, it is possible to suppress wear and damage of the plastic working tool such as a mold, and since crack resistance is improved, cracking during press working can be suppressed. When it occurs, it can exhibit the characteristics of excellent cold rolling workability.

The inventors of the present invention reviewed various angles in order to realize a steel wire having reduced deformation resistance and improved crack resistance during cold rolling. As a result, it was found that in the cold rolling process, the cementite in the ferrite grains increases the deformation resistance, and the pores which cause the cracks start from the cementite in the ferrite grains.

The cementite present in the ferrite grain boundary is smaller than the cementite present in the grain, and the amount of deformation during cold rolling is small, so that the deformation resistance can be reduced and the starting point of the hole can be suppressed. That is, it is assumed that in order to achieve both the deformation resistance reduction and the crack resistance improvement, it is important to increase the proportion of the cementite present in the ferrite grain boundary with respect to the total number of cementites, that is, Reduce the amount of cementite present in the ferrite particles relative to the total number of cementites.

Among the techniques proposed so far, a method of controlling the deformation resistance and the crack resistance is known as a method of controlling the particle size of the ferrite, but it is not focused on the cementite accumulated in the grain boundary.

The various requirements specified in the present invention are described below.

Steel wire for mechanical structural parts of the present invention (hereinafter sometimes referred to as abbreviated The metal structure of the "steel wire" is a so-called spheroidized structure composed of ferrite iron and cementite. The spheroidized structure contributes to reducing the deformation resistance of steel and improving the metal structure of cold rolling workability. In the metal structure of the present invention, a part of the metal structure may also contain a perlite structure. Further, if the adverse effect on the cold rolling workability is small, the precipitate such as AlN can be allowed to be less than 3% in terms of the area ratio.

However, it is only a metal structure composed of ferrite iron and cementite, and it is impossible to improve the cold rolling workability. Therefore, as described in detail below, it is necessary to appropriately control the amount ratio of the cementite present in the iron grain boundary of the ferrite grain to the total number of cementite in the metal structure.

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

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

The number of grain boundary cementite is "Na", the number of cementite in the grain is set to "Nb" and the total number of cementite (the total number of cementites in the grain boundary and the number of cementite in the grain) When it is "Na+Nb", the ratio of the grain boundary cementite to the intragranular cementite can be obtained as follows.

Grain boundary cementite ratio (%) = Na / (Na + Nb) × 100

Proportion of intragranular cementite (%) = Nb / (Na + Nb) × 100

The measurement of the number of cementite can be carried out in one field of view or in multiple fields of view. When measuring in the plural field of view, the ratio of the number of grain boundary cementites to the number of intragranular cementites measured by each field of view is used to calculate the ratio of grain boundary cementite to intragranular cementite.

The details of the measurement method will be described later.

When the proportion of cementite in the grain boundary is reduced and the proportion of cementite in the grain increases, the misalignment of the ferrite particles introduced into the cold rolling process shifts to the intragranular cementite, causing an increase in dislocation and showing work hardening. As a result, the deformation resistance increases and the cold rolling workability decreases. Moreover, the intragranular cementite tends to be deposited and deformed around the cementite in the cold rolling process as compared with the grain boundary cementite. As a result, the intragranular cementite tends to be the starting point of the crack. Therefore, precipitation of cementite on the ferrite grain boundary is extremely effective for improving cold rolling workability.

From this point of view, the proportion of cementite present in the ferrite grain boundary (ie, the ratio of grain boundary cementite) must be 40% or more relative to the total number of cementite. By setting the grain boundary cementite ratio to 40% or more, the deformation resistance can be reduced, and cracking of the cementite origin can be suppressed.

The morphology of the cementite to be measured for the number of grain boundary cementite and the number of intragranular cementite is not particularly limited. For example, in addition to the spherical cementite, the cementite including the rod-like cementite having a relatively large length and a large width and the layered cementite forming the perlite structure are not limited to the shape of the cementite. Further, the size of the cementite to be measured is not limited, but the size is determined by the measurement method. The method for measuring the ratio of the grain boundary cementite described later can be set to a minimum size by the size of the cementite which can be discriminated by an optical microscope at a magnification of 1000 times. Specifically, carburizing is performed in a size having a substantially circular diameter of 0.3 μm or more. The body is the object of measurement.

The preferred lower limit of the ratio of grain boundary cementite is 45%, more preferably 50%. The higher the ratio of grain boundary cementite, the lower the deformation resistance and the more effective the crack inhibition, preferably 100%. However, the increase in the proportion of the grain boundary cementite described later is not easy in terms of the production surface, and there is a possibility that the hot rolling calendering temperature is lowered and/or the spheroidizing blunt time is prolonged due to the current state of the art. In the prior art, the ratio of the grain boundary cementite is preferably about 80% or less, more preferably 70% or less, based on the viewpoint of manufacturability.

In the steel wire according to 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 circular diameter of the bcc-Fe crystal grains (hereinafter sometimes abbreviated as "bcc-Fe crystal grain size") to 30 μm or less, ductility can be improved, and cracking during cold rolling can be suppressed. The upper limit of the bcc-Fe crystal grain size is 25 μm, more preferably 20 μm. Moreover, the size of the bcc-Fe crystal grain to be measured is not limited, and the size of the material is determined in accordance with the measurement method as in the case of the cementite. In the measurement method described later, the size which can be determined by the EBPS analysis device and the FE-SEM is set to the minimum size. Specifically, a bcc-Fe crystal grain having a size of a circle having a diameter of 1 μm or more is a measurement object.

The structure to be controlled by the above-mentioned bcc-Fe crystal grain size is a bcc-Fe crystal grain surrounded by a large large-angle grain boundary having a difference in orientation of more than 15°. The reason for this is that when the azimuth difference of 15° or less is small, the influence on the cold rolling workability is small. Further, the above-mentioned "crystal orientation difference" is also referred to as "shift angle" or "tilt angle", and the measurement of the azimuth difference may be performed by the EBSP method (Electron Backscattering Pattern method). And, The bcc-Fe surrounded by the large-angle grain boundary, which measures the average particle diameter, contains the ferrite iron contained in the perlite structure in addition to the initial precipitated iron.

The present invention is directed to a steel wire used in a material of a mechanical structural part, as long as it has a usual chemical composition as a steel wire for a mechanical structural part, but regarding C, Si, Mn, P, S, Al, and N. Should be adjusted to the appropriate range. Based on this point of view, the appropriate range of such chemical components and the reasons for their limitation are as follows. In the present specification, the term "%" as used in the chemical composition means mass%.

C: 0.3~0.6%

The C system is an element that ensures the strength of the steel, that is, the strength of the final product. In order to effectively exert this effect, it is necessary to set the C content to 0.3% or more. The C content is preferably 0.32% or more, more preferably 0.34% or more. However, if C is excessively contained, the strength becomes high and the cold rolling workability is lowered. Therefore, it is necessary to set it to 0.6% or less. The C content is preferably 0.55% or less, more preferably 0.50% or less.

Si: 0.05~0.5%

The Si system is contained as a deoxidizing element and is intended to increase the strength of the final product by solid solution hardening. In order to effectively exert this effect, the Si content is set to be 0.05% or more. The Si content is preferably 0.07% or more, more preferably 0.10% or more. On the other hand, if Si is excessively contained, the hardness is excessively increased to deteriorate the cold rolling workability. Therefore, the Si content is set to be 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 effective element for increasing the strength of the final product by improving the quenching property. In order to effectively exert this effect, the Mn content is set to 0.2% or more. The Mn content is preferably 0.3% or more, more preferably 0.4% or more. On the other hand, when Mn is excessively contained, the hardness increases and the cold rolling workability deteriorates. Therefore, the Mn content is set to 1.7% or less. The Mn content is preferably 1.5% or less, more preferably 1.3% or less.

P: more than 0% and less than 0.03%

An element that is inevitably contained in the P-based steel causes a segregation of the grain boundary and deterioration of ductility in the steel. Therefore, the P content is set to be 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. If the P content is small, the smaller the better, and the remaining amount may be about 0.001% depending on the limitation of the production steps.

S: 0.001~0.05%

An element which is inevitably contained in the S-based steel, which is present as MnS in steel and deteriorates ductility, is an element harmful to cold-rolling workability. Therefore, the S content is set to be 0.05% or less. The S content is preferably 0.04% or less, more preferably 0.03% or less. However, since S has an effect of improving the machinability, it contains 0.001% or more. The S content is preferably 0.002% or more, more preferably 0.003% or more.

Al: 0.005~0.1%

Al is useful as a deoxidizing element and is useful when the solid solution N existing in steel is fixed as AlN. In order to effectively exert this effect, the Al content is set to be 0.005% or more. The Al content is preferably 0.008% or more, more preferably 0.010% or more. However, when the Al content is excessive, Al ₂ O _{3 is} excessively formed to deteriorate the cold rolling workability. Therefore, the Al content is set to be 0.1% or less. The Al content is preferably 0.090% or less, more preferably 0.080% or less.

N: 0~0.015%

An element which is inevitably contained in the N-based steel. When the steel contains a solid-melting N, the hardness is increased by the deformation aging, and the ductility is lowered to deteriorate the cold-rolling workability. Therefore, the N content is set to be 0.015% or less. The N content is preferably 0.013% or less, more preferably 0.010% or less. If the N content is small, the less is preferably as small as possible, and it is preferably 0%, and the residual amount in the manufacturing step may be about 0.001%.

The basic components of the steel wire of the present invention are as described above, and the remainder is substantially iron. In addition, "substantially iron" means that a small amount of components (for example, Sb and Zn) which do not inhibit the characteristics of the present invention, in addition to iron, may be contained, and may also contain inevitable impurities other than P, S and N (for example, O and H, etc.). Further, in the present invention, any of the following elements may be contained as needed, and the characteristics of the steel wire may be further improved depending on the components contained therein.

One or more selected from the group consisting of: Cr: more than 0% and 0.5% or less, Cu: more than 0% and 0.25% or less, Ni: more than 0% and 0.25% or less, Mo: more than 0% and 0.25% Below, and B: Super Over 0% and below 0.01%

Each of Cr, Cu, Ni, Mo, and B is an effective element for increasing the strength of the final product by improving the quenching property of the steel material, and may be contained alone or in combination of two or more kinds as needed. These effects are increased as the content of the elements increases, and the amount of Cr is preferably 0.015% or more, more preferably 0.020% or more, in order to effectively exhibit the above effects. The content of Cu, the amount of Ni, and the amount of Mo are preferably 0.02% or more, 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 content of Cr, Cu, Ni, Mo, and B is excessive, the strength is excessively increased and the cold rolling workability is deteriorated. Therefore, the amount of Cr is preferably 0.5% or less, and the contents of Cu, Ni, and Mo are preferably 0.25% or less, and the content of B is preferably 0.01% or less. A more preferable content of the elements is such that the amount of Cr is 0.45% or less, more preferably 0.40% or less. A more preferable upper limit of the amounts of Cu, Ni and Mo is 0.22%, more preferably 0.20%. The upper limit of the amount of B is preferably 0.007%, more preferably 0.005%.

In the steel wire system of the present invention, it is preferable to appropriately control the spheroidizing burn-off condition to be described later in order to obtain the shape of the structure after the spheroidizing burnt. However, in order to secure the above-described texture, it is better to control the conditions at the stage of producing the rolled wire, and the state of the microstructure in the rolled wire becomes a state in which grain boundary cementite is easily precipitated when spheroidized and burnt. .

In the production stage of the rolled wire, the steel satisfying the above composition is preferably adjusted for the finishing rolling temperature of the hot rolling pressing delay, and the subsequent cooling rate is appropriately adjusted as the three stages to adjust the cooling rate and the temperature range. By By manufacturing the calendered wire under such conditions, the spheroidized blunt tissue can be made of perlite and ferrite iron as the main phase and the bcc-Fe crystal grain size can be set to a specific range, and the initial precipitated iron crystal grain can be obtained. It is equiaxed, and the interval between the narrowest portions of the perlite is set to be specific or less. By spheroidizing and burning the conditions described later for this type of structure, it is easy to obtain a steel wire which is analyzed by the grain boundary cementite. Therefore, after the calendering wire production conditions are specifically 800° C. or more and 1050° C. or less, the following cooling is preferably performed in sequence: the first cooling rate and the average cooling rate are 7° C./sec or more. The second cooling of 1 ° C / sec or more and 5 ° C / sec or less and the third cooling of the average cooling rate faster than the second cooling and 5 ° C / sec or more. The end temperature of the first cooling and the initial temperature of the second cooling are preferably in the range of 700 to 750 °C. The temperature at which the second cooling is completed and the initial temperature of the third cooling are preferably in the range of 600 to 650 °C. The temperature at which the third cooling is completed is preferably 400 ° C or lower. The finishing rolling temperature and the first to third cooling will be described separately.

(a) Finishing rolling temperature: 800 ° C or more, 1050 ° C or less

In order to reduce the particle diameter of the bcc-Fe crystal of the structure before the spheroidizing blunt, for example, 15 μm or less, it is preferred to appropriately control the finishing calendering temperature. When the finishing rolling temperature exceeds 1050 ° C, it is difficult to reduce the particle diameter of the bcc-Fe crystal. However, when the finishing rolling temperature is less than 800 ° C, the bcc-Fe crystal grain size is too small, for example, it is less than 5 μm and it is difficult to soften, so it is preferably 800 ° C or more. A lower limit of the finishing calendering temperature is 850 ° C, more preferably 900 ° C or more. The upper limit of the finishing calendering temperature is 1000 ° C, more preferably 950 °C.

(b) first cooling

The first cooling system is started at a temperature ranging from 700 to 750 ° C from 800 ° C to 1050 ° C from the finishing rolling temperature. In the first cooling, when the cooling rate is slow, the bcc-Fe crystal grains of the structure before the spheroidal blunt are coarsened, and the bcc-Fe crystal grain size becomes large. Therefore, the average cooling rate of the first cooling is preferably set to 7 ° C / sec or more. The average cooling rate of the first cooling is more preferably 10 ° C / sec or more, and still more preferably 20 ° C / sec or more. The upper limit of the average cooling rate of the first cooling is not particularly limited, but the range of reality is preferably 200 ° C / sec or less. Further, in the cooling of the first cooling, if the average cooling rate is 7 ° C /sec or more, the cooling rate may be changed and cooled.

(c) second cooling

The second cooling system starts at a temperature range of 700 to 750 ° C and ends at a temperature range of 600 to 650 ° C. In order to make the initial precipitated iron crystal grains equiaxed, the average aspect ratio of the primary precipitated iron crystal grains is reduced to, for example, 3.0 or less, and in the second cooling, the average is preferably 5 ° C/second or less. The cooling rate is slowly cooled. A more preferable upper limit of the average cooling rate of the second cooling is 4 ° C / sec, more preferably 3.5 ° C / sec or less. On the other hand, if the average cooling rate of the second cooling is too slow, the bcc-Fe crystal grains may be coarsened, and the bcc-Fe crystal grain size may become excessively large. Therefore, the average cooling rate of the second cooling is preferably 1 ° C /sec or more. Better average cooling rate for the second cooling The lower limit is 2 ° C / sec, more preferably 2.5 ° C / sec. Further, in the cooling of the second cooling, if the average cooling rate is 1 ° C / sec or more and 5 ° C / sec or less, the cooling rate may be changed and cooled.

(d) third cooling

The third cooling system starts from a temperature range of 600 to 650 ° C and ends below 400 ° C. The third cooling system is such that the average interlayer spacing of the perlite is only narrow, so that the cementite is easily dissolved, and the core of the spherical cementite remains in the particles. Thereby, the grain boundary cementite ratio is increased by performing the subsequent appropriate spheroidization blanching treatment. In order to make the average interlayer interval of the perlite narrow, for example, 0.20 μm or less, in the third cooling, it is preferably faster than the second cooling and cooled at an average cooling rate of 5 ° C /sec or more. If the cooling is slower than 5 ° C / sec, it is not easy to make the average interlayer spacing of the perlite narrow. The average cooling rate of the third cooling is more preferably 10 ° C / sec or more, and still more preferably 20 ° C / sec or more.

Further, the upper limit of the average cooling rate of the third cooling is not particularly limited, but is preferably 200 ° C / sec or less in the actual range. Further, in the third cooling, if the average cooling rate is 5 ° C /sec or more, the cooling rate may be changed and cooled. The lower limit of the end temperature of the third cooling is not particularly limited, but is preferably, for example, 200 °C. After the third cooling, it is sufficient to cool to room temperature by cooling normally or the like.

After cooling to room temperature, the wire drawing process may be further performed at room temperature as needed, and the reduction ratio may be, for example, 30% or less. When the wire is stretched, since the carbide in the steel is destroyed, the subsequent spheroidization can promote the agglomeration of the carbide, so that the uniform heat treatment of the spheroidized burnt blunt can be effectively shortened. between. However, if the reduction ratio of the wire drawing process exceeds 30%, the strength after cold burning becomes high and the cold rolling workability deteriorates. Therefore, the reduction ratio of the wire drawing process is preferably 30% or less. Further, the lower limit of the reduction ratio is not particularly limited, and it is preferred to obtain an effect by setting it to 2% or more.

The calendered wire produced by the above-mentioned preferred conditions is subjected to subsequent spheroidizing blunt treatment to change the morphology of the perlite in the structure to austenite, followed by the ferrite iron + cementite In the morphological change, the original perlite size is reduced, that is, the grain growth of the metal structure is suppressed, and the precipitation of the cementite in the grain is reduced, and the state of the grain boundary cementite is easily precipitated.

As such a spheroidizing and blunt condition, it is preferred that the rolled wire is heated at a temperature of from room temperature to 730 ° C in an atmospheric furnace, for example, as described later, at an average heating rate of 50 ° C from at least 500 ° C to 730 ° C. Heating for more than one hour, then heating to 740 ° C at an average heating rate of 2 to 5 ° C / hour, holding at 740 ° C for 1-3 hours, cooling to an average cooling rate of 20 ° C / hour or more to 720 ° C, with an average cooling rate of 8 ~ Cool to 640 ° C at 12 ° C / hour, then let cool.

In the spheroidizing blunt condition described above, when heating from room temperature to 730 ° C, the average heating rate from at least 500 ° C to 730 ° C is 50 ° C / hour or more, thereby suppressing grain growth of the metal structure. The average heating rate at this time is more preferably 60 ° C / hour or more. However, when the average heating rate is too fast, the temperature of the rolled wire becomes difficult to follow, and therefore it is preferably 200 ° C / hour or less, more preferably 150 ° C / hour or less.

Moreover, the average heating rate when heated from room temperature to 500 ° C Usually 100 ° C / hour or more, the average heating rate in this temperature range has little effect on the grain growth of the metal structure. In consideration of productivity, the heating rate at this time is preferably faster, for example, 120 ° C / hour or more, more preferably 140 ° C / hour or more. The upper limit of the average heating rate at this time is preferably 200 ° C / hour, more preferably 150 ° C / hour, as the average heating rate from 500 ° C to 730 ° C. The average heating rate when heated from room temperature to 500 ° C may be at least the same as the average heating rate from 500 ° C to 730 ° C. In short, by reducing the size of the original perlite, the precipitation of cementite in the grain can be reduced, and the state of the grain boundary cementite is easily precipitated, so that the average heating rate of at least from 500 ° C to 730 ° C is guaranteed to be 50. °C / hour or more can be.

And by controlling the average heating rate from 730 ° C to 740 ° C directly above the A1 point to 2 to 5 ° C / hr, it is possible to suppress the growth of the metal structure as much as possible while sufficiently performing the cementite in the perlite structure. Decomposition and solid solution. When the average heating rate is faster than 5 ° C / hour, it is difficult to ensure sufficient time for the decomposition and solid solution of cementite in the perlite structure, and the average heating rate is slower than 2 ° C / hour, and the heating time is from 730 ° C to 740 ° C. It becomes longer and becomes difficult to control the growth of the grain 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 preferably maintained at 740 ° C for 1 to 3 hours. When the holding temperature is shorter than 1 hour, decomposition and solid solution of cementite in the perlite 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 at this time is preferably 1.5 hours or more and 2.5 hours or less.

After maintaining as above, by comparing to 720 ° C The optimum average cooling rate is set to 20 ° C / hour or more, and the growth of the metal structure can be suppressed. The average cooling rate at this time is more preferably 30 ° C /hr or more. However, when the average cooling rate is too fast, the temperature of the rolled wire is difficult to follow, and therefore it is preferably 100 ° C /hr or less.

Subsequently, by controlling the average cooling rate from 720 ° C to 640 ° C to 8 to 12 ° C / hour, cementite can be preferentially precipitated in the ferrite grain boundary, and the aspect ratio as large as the perlite structure can be suppressed. The cementite precipitates. When the average cooling rate is slower than 8 ° C / hour, the growth of the grain growth of the metal structure is difficult, and when the average cooling rate is faster than 12 ° C / hour, a large amount of cementite having a large aspect ratio like a perlite structure is reprecipitated. The average cooling rate at this time is more preferably 9 ° C / hour or more and 11 ° C / hour or less.

As described above, the spheroidizing blunt can be performed plural times, and by repeating this, the aspect ratio of the cementite can be reduced, and the ratio of the grain boundary cementite can be increased. For example, as shown in Test Nos. 7, 12, 14, 19, and 27 of the examples described later, even if the steel grades C, E, F, H, and K which are not properly controlled for the production conditions of the rolled wire are used, they are subsequently repeated. When the specific spheroidization is performed, the ratio of the grain boundary cementite can be made to an appropriate range, and both the deformation resistance and the crack occurrence rate can be reduced.

The number of repetitions of the spheroidized burnt blunt is preferably at least three or more, but since the ratio of the excessively manipulated grain boundary cementite does not change much, it is preferably 10 or less. Further, when the spheroidal passivation is repeated a plurality of times, it may be repeated under the same conditions within the above-described preferable conditions, or may be repeated under different conditions.

[Examples]

Hereinafter, the present invention will be more specifically described by way of examples. The present invention is not limited to the following examples, and it is to be understood that the invention can be carried out with appropriate modifications within the scope of the present invention.

The steel having the chemical composition shown in Table 1 below was rolled and produced under various production conditions shown in Table 2 below. 17.0mm wire. In Table 2, cooling 1, cooling 2, and cooling 3 correspond to the first cooling, the second cooling, and the third cooling recommended by the present invention. A comparative example in which the chemical composition of the steel B system deviates from the predetermined value.

The steel grades C, E, F, H, K, O, P and Q are not examples of producing rolled wires under the appropriate manufacturing conditions of the present invention. The finishing calendering temperatures of the steel grades C, E, F and K become high. Further, the steel type H is cooled under the condition that the cooling rate of the cooling 3 corresponding to the third cooling is slow, that is, the state in which the cooling rate of the second cooling is maintained, and the rolled wire is produced.

The steel type O was subjected to a second cooling to 550 ° C, then heated to 580 ° C, and maintained at 580 ° C for 120 seconds, and allowed to cool to room temperature to carry out a wire drawing processing step of 40% reduction. The steel grade P is cooled only by the monotonic cooling rate of cooling 1. After the steel type Q was cooled 1, the holding step was carried out at 550 ° C for 60 seconds, and the mixture was allowed to cool to room temperature to carry out a thick drawing line having a reduction ratio of 15%.

Next, for each of the rolled wires other than the steel types O, P, and Q, the following furnace is used: (a) heating from room temperature to 730 ° C, from room temperature to 500 ° C at an average heating rate of 110 °C / hour, from 500 ° C to 730 ° C is heated at an average heating rate of 80 ° C / hour, then heated to 740 ° C at an average heating temperature of 3 ° C / hour, held at 740 ° C for 3 hours, with an average cooling rate of 30 ° C / After cooling to 720 ° C in an hour, it was cooled to 640 ° C at an average cooling rate of 10 ° C / hour, and then spheroidized and burnt by cooling (the blunt condition is hereinafter referred to as "SA1"), (b) SA1 repeated 5 times ( The blunt condition is hereinafter referred to as "SA2") and (c) heating from room temperature to 730 ° C, from room temperature to 500 ° C at an average heating rate of 110 ° C / hour, from 500 ° C to 730 ° C for average heating The temperature was heated at 80 ° C / hour, then heated to 740 ° C at an average heating temperature 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 cooled and spheroidized. Blunt (this blunt condition is hereinafter referred to as "SA3"). The above-mentioned blunt conditions SA1 and SA2 are preferred blunt conditions of the present invention, and the above-mentioned blunt condition SA3 is an example in which the average cooling rate from 720 ° C to 640 ° C is not properly controlled.

Further, for the steel type O, in the atmospheric furnace, one of the following is performed: (d) heating from room temperature to 680 ° C at an average heating rate of 80 ° C / hour, and maintaining at 680 ° C for 5 hours, at an average cooling rate of 10 ° C /hour cooling to 640 ° C, followed by cooling spheroidal burning blunt (the blunt condition is hereinafter referred to as "SA4") and (e) heating at room temperature to 700 ° C at an average heating rate of 80 ° C / hour, at 700 ° C After 5 hours of maintenance, it was cooled to 640 ° C at an average cooling rate of 10 ° C / hour, and then spheroidized and blunt-cooled (the blunt condition was Hereinafter referred to as "SA5"). The above-mentioned blunt conditions SA4 and SA5 are examples which deviate from the preferred blunt conditions of the present invention.

And for the steel species P, in an atmospheric furnace, any one of the following: (f) heating from room temperature to 740 ° C at an average heating rate of 80 ° C / hour, and then immediately cooling to 660 ° C at an average cooling rate of 80 ° C / hour, These steps were repeated 3 times (but heated from 660 ° C after the second time), then heated from 660 ° C to 740 ° C at an average heating rate of 80 ° C / hour, and maintained at 740 ° C for 30 minutes, at an average cooling rate of 80 ° C. /hour cooling to 660 ° C, maintaining at 660 ° C for 1 hour, followed by cooling spheroidal burning blunt (the blunt condition is hereinafter referred to as "SA6") and (g) heating at room temperature at an average heating rate of 80 ° C / hour After maintaining at 740 ° C for 10 minutes at 740 ° C, it was cooled to 660 ° C at an average cooling rate of 80 ° C / hour. This step was repeated 3 times (but after 260 ° C after the second time), followed by an average heating rate of 80 ° C. /hour heated from 660 ° C to 740 ° C, after maintaining at 740 ° C for 30 minutes, cooled to 660 ° C at an average cooling rate of 80 ° C / hour, held at 660 ° C for 1 hour, and then chilled spheroidized burnt (the burn) The blunt condition is hereinafter referred to as "SA7"). The above-mentioned blunt conditions SA6 and SA7 are examples which deviate from the preferred blunt conditions of the present invention.

For the steel type Q, in an atmospheric furnace, one of the following is performed: (h) heating from room temperature to 720 ° C at an average heating rate of 150 ° C / hour, holding at 720 ° C for 1 hour, and then cooling the spheroidized blunt (The blunt condition is hereinafter referred to as "SA8") and (i) heating from room temperature to 730 ° C at an average heating rate of 150 ° C / hour, and maintaining at 730 ° C for 1 hour, then cooling the spheroidized blunt (the burning) The blunt condition is hereinafter referred to as "SA9"). The above blunt condition SA8 and SA9 are examples of preferred blunt conditions that deviate from the present invention.

For the steel wire after the spheroidization and blunting, the following methods are used to determine (1) the bcc-Fe crystal grain size of the metal structure, (2) the grain boundary cementite ratio, and (3) the deformation during the cold rolling process. Resistance and (4) Cracking rate during cold rolling.

In addition, in the measurement of the grain size of the ferrite iron and the ratio of the grain boundary cementite in the spheroidized blunt steel wire, the resin is embedded in an observable cross section, and mirror-polished by sanding cloth and diamond polishing. Cut the section. For the radius D of the steel wire, the position from the surface of the steel wire is measured as D/4.

(1) Determination of crystal size of bcc-Fe

The measurement of the bcc-Fe crystal grain size was carried out by using an EBSP analysis apparatus and an FE-SEM (field emission scanning electron microscope, electro-emission type scanning electron microscope). The analysis tool uses OIM software from TSL Solution Co., Ltd. The average of the diameters when the area of the bcc-Fe crystal grain is converted into a circle is calculated by defining the crystal grain boundary as a boundary of the crystal grain boundary with a crystal orientation difference (this is also called "oblique angle") exceeding the boundary of 15. The value is also the diameter of the average equivalent circle. In this case, the measurement area is set to 200 μm×400 μm, the measurement step is set to 1.0 μm, and the measurement point indicating that the reliability index of the measurement orientation is 0.1 or less is deleted from the analysis target.

(2) Determination of the ratio of grain boundary cementite

In the determination of the ratio of grain boundary cementite, picric acid by more than 5 minutes The etch showed a ferrite grain boundary and a cementite, and the tissue was observed by an optical microscope, and three fields of view were taken at a magnification of 1000 times. On these photographs, 10 horizontal lines at equal intervals were drawn, and the number of grain boundary cementite and the number of intragranular cementite present on the line were measured. The ratio of grain boundary cementite in the three fields of view is divided by the total number of cementites present in the same field of view to calculate the ratio of grain boundary cementite. The diameter of the smallest equivalent circle of the cementite measured was set to 0.3 μm. Here, it is defined as grain boundary cementite in contact with the ferrite grain boundary and the aspect ratio of the cementite particles is 3.0 or less. Therefore, even if it is in contact with the ferrite grain boundary, the aspect ratio of the cementite particles exceeding 3.0 is set as the granular cementite.

(3) Determination of deformation resistance

Steel wire production A cold-rolling forging test sample of 10.0 mm × 15.0 mm was subjected to a cold-rolling forging test at a processing rate of 60% at a deformation rate of 5/sec to 10/sec at room temperature using forging pressurization. The measurement of the deformation resistance is the data of the processing rate-deformation resistance obtained by the cold-rolling forging test of 60% of the processing rate, and the deformation resistance at the time of the 40% processing was measured five times, and the average value of the five times was obtained. Further, the qualification standard of the deformation resistance of the steel grades A to E and P in the range of the C content of 0.3 to less than 0.4% is 650 MPa or less. The qualification standard for the deformation resistance of the steel grades F~J, O and Q in the range of 0.4% to less than 0.5% is 680 MPa or less. The qualification standard for the deformation resistance of the steel type K to N in the range of 0.5 to less than 0.6% is 730 MPa or less.

(4) Determination of the incidence of cracks

Steel wire production A cold-rolling forging test sample of 10.0 mm × 15.0 mm was subjected to a cold-rolling forging test at a processing rate of 60% at a deformation rate of 5/sec to 10/sec at room temperature using forging pressurization. The cracking rate was measured after a cold-rolling forging test of 60% processing rate, and each surface observation was performed five times with a solid microscope, and the presence or absence of surface cracks was measured at a magnification of 20 times, and the number of samples having cracks on the surface was measured. Divide by 5 to find the average. The pass rate for cracking of all steel grades is less than 20%.

These results are shown in Table 3 below together with the spheroidized burn-off conditions. In the comprehensive evaluation column of Table 3, the example in which the deformation resistance is reduced and the crack resistance is improved is "OK", and the example in which at least one of the deformation resistance is reduced and the crack resistance is improved is indicated as " NG".

The results from Table 3 can be discussed as follows. Test No. 1, 2, 7~9, 12, 14~16, 19~21, 23, 24, 27~29, 31, 32, 34 And 35 are examples in which all the requirements of the present invention are satisfied, and it is understood that the deformation resistance is reduced and the crack resistance is improved at the same time.

These Test Nos. 7, 12, 14, 19 and 27 use examples of steels C, E, F, H or K which are not manufactured under the conditions of preferred calendering wire conditions, but are subsequently blunt by repeated repetition of SA2. The grain boundary cementite was analyzed and analyzed, and the deformation resistance and crack occurrence rate all reached the qualified standard. Among them, Test No. 12 is a relatively good material, and the bcc-Fe crystal grain size is slightly larger, but the deformation resistance and the crack occurrence rate all meet the qualification criteria.

Here, attention is paid to tests No. 1 and 2 (steel type A), test Nos. 6 and 7 (steel type C), and test No. 8 and 9 (steel type D) for performing the blunt conditions of SA1 and SA2, Test No. 11 and 12 (steel type E), test No. 13 and 14 (steel type F), test No. 15 and 16 (steel type G), test No. 18 and 19 (steel type H), test No. 20 and 21 (steel type I), test No. 23 and 24 (steel type J), test No. 26 and 27 (steel type K), test No. 28 and 29 (steel type L), test No. 31, 32 (steel type M), and Test No. 34 and 35 (steel type N), it can be seen that in any case, the blunt test of SA2 in which SA1 is repeated five times is performed, and the deformation resistance and the crack occurrence rate can be reduced. By.

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 of the present invention, and it is known that Either or both of the deformation resistance and the crack occurrence rate do not meet the qualification criteria.

That is, the test No. 3, 10, 17, 22, 25, 30, 33, and 36 are not suitable for the case of spherical passivation with SA3, grain boundary seepage The carbon body ratio is insufficient, and either or both of the deformation resistance and the crack occurrence rate fail to meet the qualification criteria.

Test Nos. 4 and 5 are examples in which steel type B having an excessive Mn content is used, and the deformation resistance at the time of cold rolling processing is high.

Test Nos. 6, 11, 13, 18 and 26 are examples of steel grades C, E, F, H or steel K which are not produced by the preferred conditions for the production of rolled wire, by subsequent spheroidization of SA1 No grain boundary cementite was precipitated, and the deformation resistance and crack occurrence rate did not reach the qualified standard. However, in the case of these steel grades, if the spheroidal blunt of SA2 is repeated five times, the grain boundary cementite is appropriately precipitated, and the deformation resistance and the crack occurrence rate are both up to the acceptable standard (Test No. .7, 12, 14, 19 and 27).

Test Nos. 37 and 38 are examples in which the steel species O produced by the preferred conditions in the production of the rolled wire is spheroidized by the unsuitable SA4 or SA5, and the fine cementite is uniformly dispersed, and the grain boundary is used. The proportion of cementite is small, the deformation resistance is high, and the crack occurrence rate exceeds the qualified standard.

Test Nos. 39 and 40 are examples in which the steel grade P produced by the preferred conditions in the production of the rolled wire material is spheroidized and burnt by the unsuitable SA6 or SA7, and dispersed in the ferrite iron particles. The layered cementite which is divided into spheroidized blunt as a nucleus and spheroidized cementite has a small proportion of grain boundary cementite, high deformation resistance, and the occurrence rate of crack exceeds the qualified standard.

Test Nos. 41 and 42 used a steel type Q which was not produced under the preferable conditions in the production of a rolled wire, and spheroidized burnt blunt with an unsuitable SA8 or SA9, and a large amount of layered seepage of a time-delayed fracture was formed. carbon After the spheroidized blunt, the ratio of grain boundary cementite becomes small, the deformation resistance is high, and the crack occurrence rate exceeds the qualified standard.

[Industrial availability]

The steel wire for a machine structural part of the present invention can be preferably used for various mechanical structural parts such as automobile parts and construction machine parts manufactured by cold rolling processing such as cold rolling forging, cold rolling and cold rolling. Material. Specific examples of such mechanical structural components include mechanical parts and electrical parts, and more specifically bolts, screws, nuts, bushings, ball joints, inner tubes, torsion bars, clutch boxes, housings, housings, hubs, and the like. Cover, box body, adjusting washer, tappet, seat plate, barugu, inner box, clutch, sleeve, outer race, chain gear, core, stator, anvil, star wheel, rocker arm, body ), flanges, rollers, joints, connectors, pulleys, metal fittings, yokes, metal covers, valve lifters, spark plugs, differential pinions, steering shafts, common rails, etc. The steel wire of the present invention is industrially useful as a steel wire for high-strength mechanical structural parts which is suitably used as a material of the above-mentioned mechanical structural component, and has low deformation resistance at room temperature when manufacturing various mechanical structural components described above, and Material cracking is suppressed and cold rolling workability can be exhibited.

The present application is based on the Japanese Patent Application No. 2015-073776, filed on March 31, 2015, and the priority is filed. Japanese Patent Application No. 2015-073776 is incorporated herein by reference.

Claims (3)

A steel wire for mechanical structural parts, which contains, by mass%, C: 0.3 to 0.6%, Si: 0.05 to 0.5%, Mn: 0.2 to 1.7%, P: more than 0% and 0.03% or less, and S: 0.001. ~0.05%, Al: 0.005~0.1% and N:0~0.015%, the rest is made of iron and inevitable impurities. The metal structure of steel consists of ferrite and cementite. The amount of cementite present in the iron grain boundary of the fat grain is 40% or more relative to the total number of cementite. The steel wire for the mechanical structural part of the claim 1 is one or more selected from the group consisting of the following: %: Cr: more than 0% and less than 0.5%, and Cu: more than 0% and 0.25%. Hereinafter, Ni: more than 0% and 0.25% or less, Mo: more than 0% and 0.25% or less, and B: more than 0% and 0.01% or less. A steel wire for a mechanical structural part according to claim 1 or 2, wherein an average equivalent circle diameter of the bcc-Fe crystal grains in the metal structure is 30 μm or less.