JP2018003106A - Steel for machine structural use for cold working and production method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel for machine structural use for cold working capable of exhibiting excellent cold workability, even when treatment time of spheroidizing is largely shortened.SOLUTION: The steel for machine structural use for cold working is provided that contains C:0.3 mass% to 0.6 mass%, Si:0.05 mass% to 0.5 mass%, Mn:0.2 mass% to 1.7 mass%, P:over 0 mass% and 0.03 mass% or less, S:0.001 mass% to 0.05 mass%, Al:0.01 mass% to 0.1 mass%, N:0 mass% to 0.015 mass% and the balance iron with inevitable impurities, and that has metallographic structure having proeutectoid ferrite and pearlite, a total area ratio of the proeutectoid ferrite and the pearlite to all structure being 90% or more, an area ratio of the proeutectoid ferrite to all structure being (1-1.25×[C%])×80% or more and an average block size of the pearlite being 25 μm or less. [C%] represents C content by mass%.SELECTED DRAWING: None

Description

  The present invention relates to a machine structural steel for cold working and a method for producing the same. More specifically, the present invention relates to a machine structural steel having a low deformation resistance after spheroidizing annealing and excellent cold workability, and a method for producing the machine structural steel.

Steel used for machine structural parts such as automobile parts and construction machine parts is desired to have good cold workability, particularly low deformation resistance. If the deformation resistance of steel is low, the processing becomes easy and the life of the cold working mold can be improved.
In the manufacturing process of machine structural parts, hot rolled materials such as carbon steel and alloy steel are spheroidized and annealed. By spheroidizing annealing, cementite contained in pearlite in the steel is spheroidized, and the cold workability of the steel is improved. The spheroidized annealed rolled material is cold worked by cold forging, cold forging, cold rolling, etc., and further formed into a predetermined shape by machining such as cutting, and finally by quenching and tempering treatment After final strength adjustment, a machine structural component is obtained.

In order to improve the cold workability of steel, a steel has been proposed in which cementite in pearlite is easily spheroidized during spheroidizing annealing.
For example, Patent Document 1 discloses a steel wire whose metal structure is composed of a pro-eutectoid ferrite structure, a pearlite structure, and a bainite structure. The volume fraction of the pearlite structure is 1.40 × C (%) × 100% or more, the volume fraction of pro-eutectoid ferrite is (1-1.25 × (C%)) × 50% or less (including 0%), and The volume fraction of the bainite structure is regulated to 20% or less (including 0%). With this steel wire, the annealing temperature of spheroidizing annealing can be lowered. Further, the average block size of the pearlite structure is preferably 20 μm or less, and the processing time for spheroidizing annealing can be shortened.

  In Patent Document 1, as a method for obtaining the above metal structure, winding is performed after hot rolling, and then immersed in a molten salt bath of 400 ° C. or higher and 600 ° C. or lower for 10 seconds or longer, and then 500 ° C. or higher and 600 ° C. or lower. It is disclosed that the molten salt bath is kept at a constant temperature for 20 seconds or more and 150 seconds or less and then cooled, and wire drawing is performed so that the area reduction rate is 25% or more and 50% or less.

  In Patent Document 2, the metal structure has pearlite and pro-eutectoid ferrite, the total area ratio of pearlite and pro-eutectoid ferrite with respect to the entire structure is 90% by area or more, and the area ratio A of pro-eutectoid ferrite is predetermined. The steel for machine structure for cold work larger than the Ae value represented by the relational expression is disclosed. In this machine structural steel, since the area ratio A of pro-eutectoid ferrite is large, it is said that it can be made softer than ordinary steel when spheroidizing annealing is performed at the same annealing temperature and processing time as usual.

  In Patent Document 2, as a method for obtaining the metal structure, after finish rolling at a temperature of 1050 ° C. or more and 1200 ° C. or less, a temperature of 700 ° C. or more and less than 800 ° C. at an average cooling rate of 10 ° C./second or more. And then cooled to a temperature range of 580 to 660 ° C. at an average cooling rate of 10 ° C./second or more, and further cooled to 1 ° C. It is disclosed to cool or hold for at least 20 seconds at an average cooling rate of less than / second.

Japanese Patent No. 5257082 Japanese Patent No. 5357994

  In recent years, from the viewpoint of energy saving, it has been required to shorten the processing time of spheroidizing annealing. If the processing time for spheroidizing annealing can be reduced, a reduction in energy consumption corresponding to that, that is, a reduction in CO2 emissions can be expected. However, if the treatment time for spheroidizing annealing is shortened, cementite is not properly spheroidized and cold workability deteriorates. Therefore, it has not been easy to significantly reduce the processing time for spheroidizing annealing (specifically, 20-30% reduction) while maintaining sufficient cold workability.

  In the steel wire described in Patent Document 1, the processing time for spheroidizing annealing can be shortened by setting the average block size of the pearlite structure to 20 μm or less. However, if the processing time is significantly shortened, spherical cementite does not grow sufficiently, and appropriate cold workability cannot be obtained.

  In the steel for machine structure described in Patent Document 2, when the treatment time is significantly shortened and spheroidizing annealing is performed, pearlite remains without cementite being appropriately spheroidized, or pearlite reprecipitates after spheroidizing annealing. That is, if annealing is performed in a short time, the pearlite is not properly spheroidized and the cold workability of the steel is reduced.

  It is an object of the present invention to provide a machine structural steel for cold working that can exhibit excellent cold workability even if the treatment time for spheroidizing annealing is greatly shortened, and a method for producing the same.

The machine structural steel for cold working according to the present invention is C: 0.3% by mass to 0.6% by mass, Si: 0.05% by mass to 0.5% by mass, Mn: 0.2% by mass to 1.7% by mass, P: more than 0% by mass, 0.03% by mass or less, S: 0.001% by mass to 0.05% by mass, Al: 0.01% by mass to 0.1% by mass, and N : 0% by mass to 0.015% by mass, the balance being iron and inevitable impurities, the steel microstructure has pro-eutectoid ferrite and pearlite, and the total area ratio of pro-eutectoid ferrite and pearlite with respect to the entire structure Is 90% or more, the area ratio of pro-eutectoid ferrite to the whole structure is (1-1.25 × [C%]) × 80% or more, and the average block size of the pearlite is 25 μm or less. It is steel for machine structure for cold working.
However, [C%] indicates the C content expressed in mass%.

  In the steel for cold working machine structure according to the present invention, the average equivalent circle diameter of the bcc-Fe crystal grains is preferably 15 to 35 μm.

The steel for machine structure for cold working according to the present invention, if necessary, Cr: more than 0 mass%, 0.5 mass% or less, Cu: more than 0 mass%, 0.25 mass% or less, Ni: 0 mass 1% or more selected from the group consisting of more than%, 0.25% by weight or less, Mo: more than 0% by weight, 0.25% by weight or less and B: more than 0% by weight, 0.01% by weight or less In addition, the following formula (1) may be satisfied.
[Cr%] + [Cu%] + [Ni%] + [Mo%] ≦ 0.75 (1)
However, [Cr%], [Cu%], [Ni%], and [Mo%] respectively indicate the contents of Cr, Cu, Ni, and Mo expressed in mass%.

  The machine structural steel for cold working of the present invention may further contain Ti: more than 0% by mass and 0.1% by mass or less.

  The method of manufacturing the above-described cold-working machine structural steel of the present invention includes a step of finish rolling at a finish rolling temperature of 950 ° C. to 1100 ° C., and a temperature of 7 ° C./second or more from the finish rolling temperature to 800 ° C. Cooling at an average cooling rate, cooling from 800 ° C to 710 ° C at an average cooling rate of 1 ° C / second or less, and cooling from 710 ° C to 600 ° C at an average cooling rate of 4 to 20 ° C / second And a step of performing.

  The steel for machine structural use for cold working of the present invention has an appropriate adjustment of chemical composition, total area ratio of pro-eutectoid ferrite and pearlite, total area ratio of pro-eutectoid ferrite, pro-eutectoid ferrite area ratio, and pearlite Each block size is in an appropriate range. Thereby, even if it shortens the processing time of spheroidizing annealing, it becomes a favorable spheroidizing structure and can be sufficiently softened, and as a result, good cold workability is obtained. Moreover, the manufacturing method of this invention can manufacture the steel for cold-work machine structure which has the characteristics as mentioned above.

  The inventors of the present application can obtain a degree of spheroidization equal to or higher than that of conventional spheroidizing annealing (hereinafter, referred to as “short-time spheroidizing annealing”), which is significantly shorter than usual, and sufficiently soft. In order to realize cold-working machine structural steels that can be made into steel, we studied from various angles.

As a result, the total area ratio of pro-eutectoid ferrite and pearlite with respect to the entire structure is as high as 90% or more as a metal structure before spheroidizing annealing (hereinafter referred to as “pre-structure”), and the pro-eutectoid ferrite with respect to the entire structure It was found that when the area ratio is as high as (1-1.25 × [C%]) × 80% or more, the growth of spherical cementite is promoted during spheroidizing annealing. Furthermore, if the area ratio of proeutectoid ferrite in the previous structure is high, the distance between the particles of spherical cementite becomes longer in the metal structure after spheroidizing annealing, and ferrite grains that do not contain spherical cementite in the grains (that is, soft grains) It has been found that the ratio of ferrite grains) increases. In other words, the steel having a pre-structure with a high area ratio of pro-eutectoid ferrite and pearlite and a high pro-eutectoid ferrite area ratio can be sufficiently reduced in hardness (that is, sufficiently softened) by short-time annealing. I found.
Moreover, when the pearlite block of the previous structure was as fine as an average block size of 25 μm or less, it was found that spheroidization of cementite was promoted during spheroidizing annealing, and the degree of spheroidization after short-time annealing was improved. .

  From these findings, in order to obtain a steel that can achieve both softening of the steel and spheroidization of the cementite while shortening the treatment time of spheroidizing annealing, in the metal structure before annealing, the proeutectoid ferrite area ratio is set. The idea that it is important to make the pearlite block fine while increasing the height was obtained, and the present invention was completed.

In the present invention, the “degree of spheroidization” means the spheroidized structure photograph shown in the attached drawing of JIS G3539: 1991. 1-No. It is a numerical value determined based on 4. A photograph of the metal structure of the target steel is shown as a photograph of a spheroidized structure. 1-No. No. 4 is the closest to the spheroidized structure. Was defined as “spheroidization degree”.
Photograph No. of spheroidized structure No. 1 is the best spheroidized structure. No. 4 is not spheroidized and a lot of pearlite is present.
Therefore, the smaller the degree of spheroidization (that is, the closer the degree of spheroidization is to 1), the better the spheroidized structure and the better the cold workability.

Details of each requirement defined by the present invention are shown below.
In addition, in this specification, a "wire" is used for the meaning of a rolled wire, and points out the linear steel material cooled to room temperature after hot rolling. The “steel wire” refers to a linear steel material obtained by subjecting a rolled wire material to a tempering treatment such as spheroidizing annealing.

1. Metal Structure The machine structural steel for cold working of the present invention (hereinafter sometimes simply referred to as “steel”) contains proeutectoid ferrite and pearlite as a metal structure.
The metal structure of the steel of the present invention contains proeutectoid ferrite and pearlite. These structures are metal structures that contribute to the improvement of cold workability by reducing (that is, softening) the deformation resistance of the steel after spheroidizing annealing. However, if the steel simply has a metal structure containing pro-eutectoid ferrite and pearlite, the desired softening cannot be achieved after spheroidizing the steel. In order to achieve the desired softening, it is necessary to appropriately control the total area ratio of pro-eutectoid ferrite and pearlite, the area ratio of pro-eutectoid ferrite, the size of pearlite block, and the like.

Total area ratio of pro-eutectoid ferrite and pearlite: 90% or more When there are many microstructures such as bainite and martensite in the microstructure before steel, even in the metal structure after spheroidizing annealing, the microstructure ( In particular, ferrite) is locally refined. If the strength of the steel increases due to the refinement of the structure, the steel will not be sufficiently softened even if spheroidizing annealing is performed. In order to promote softening by spheroidizing annealing, it is necessary to reduce the amount of fine structure in the previous structure. Specifically, the total area ratio of pro-eutectoid ferrite and pearlite with respect to the entire structure is 90% or more. The total area ratio is preferably 95% or more, more preferably 97% or more, and most preferably 100%.

Examples of metal structures (mainly fine structures) other than proeutectoid ferrite and pearlite contained in steel include martensite, bainite, and retained austenite. As described above, when the content of the microstructure increases, the strength of the steel after spheroidizing annealing increases. Thus, steel does not have to contain any microstructure.
Steel may contain carbides other than cementite, nitrides, oxides, sulfides and the like as other structure factors.

Area ratio of pro-eutectoid ferrite: (1-1.25 × [C%]) × 80% or more If the area ratio of pro-eutectoid ferrite in the previous structure of steel is increased, it will be spheroidized for a short time for the following reasons Sufficient softening can be achieved by annealing.
When the area ratio of proeutectoid ferrite increases, the precipitation site of cementite during spheroidizing annealing is limited. As a result, the number density of cementite decreases, and the growth and coarsening of cementite are promoted. Thereby, in the steel after spheroidizing annealing, the inter-particle distance of spherical cementite is increased, and as a result, a sufficiently softened metal structure (soft structure) is obtained.

  The area ratio of pro-eutectoid ferrite not only changes depending on the heat treatment conditions in the steel manufacturing method, but also changes depending on the carbon content of the steel. Generally, when the amount of carbon increases, the area ratio of pro-eutectoid ferrite decreases. Conversely, when the amount of carbon decreases, the area ratio of pro-eutectoid ferrite increases. Therefore, the maximum value of the area ratio of pro-eutectoid ferrite differs depending on the amount of carbon contained in the steel. In the present invention, the optimum area ratio of pro-eutectoid ferrite can be determined with respect to the maximum value of pro-eutectoid ferrite in the steel. Therefore, the optimum area ratio of pro-eutectoid ferrite may vary depending on the type of steel, particularly the amount of carbon in the steel.

  According to a number of experimental results, the area ratio of pro-eutectoid ferrite with respect to the entire structure in the previous structure satisfies (1-1.25 × [C%]) × 80% or more, so that softening can be achieved. I found it. The technical significance of this equation can be explained as follows.

In hypoeutectoid steel, the weight of each structure in the two-phase region of ferrite and austenite is expressed by the following formula (2).
W = Wα + Wγ (2)
here,
W: Mass of the whole material Wα: Mass of ferrite Wγ: Mass of austenite

Moreover, the following formula | equation (3) is materialized from the relationship between the weight of each structure | tissue, and the carbon content of each structure | tissue.
([C%] / 100) W = ([Cα%] / 100) Wα + ([Cγ%] / 100) Wγ (3)
here,
[C%]: Carbon content of the entire material (mass%)
[Cα%]: Carbon content of ferrite (mass%)
[Cγ%]: Carbon content (% by mass) of austenite

Here, when the volume fraction of pro-eutectoid ferrite is just above the A1 transformation point, it can be considered that [Cα%] = 0.0 mass% and [Cγ%] = 0.8 mass%. From these values and the formulas (2) and (3), the following formula (4) is obtained.
Wγ / W = [C%] / 0.8 (4)

Equation (4) is the mass ratio of austenite when the volume fraction of pro-eutectoid ferrite is maximum. When the temperature becomes lower than the A1 transformation point, austenite transforms into pearlite, and the mass of austenite becomes the mass of pearlite as it is. Therefore, the mass ratio of austenite (Wγ / W) can be regarded as the mass ratio of pearlite. Furthermore, since the specific gravity of ferrite and pearlite is substantially the same, the pearlite mass ratio Wγ / W is equal to the pearlite volume fraction Vp / V at room temperature. That is,
Vp / V = Wγ / W = [C%] / 0.8 (5)
here,
V: Volume of the whole material Vp: Volume of pearlite

And the maximum volume ratio (V (alpha) / V) of pro-eutectoid ferrite can be represented by the following formula | equation (6) by using Formula (5).
Vα / V = (V−Vp) / V = 1− [C%] / 0.8 = 1−1.25 [C%] (6)
here,
Vα: volume of pro-eutectoid ferrite

  The volume ratio of pro-eutectoid ferrite can be regarded substantially as the area ratio of pro-eutectoid ferrite. Therefore, the maximum area ratio of pro-eutectoid ferrite contained in the steel is 1-1.25 [C%].

As a result of examining a large number of experimental results, the inventors have found that when the area ratio of pro-eutectoid ferrite in the steel front structure is 80% or more of the maximum area ratio of pro-eutectoid ferrite in the steel, short-time spheroidizing annealing is performed. I found that steel can be softened sufficiently. That is, in the present invention, the area ratio of pro-eutectoid ferrite in the front structure of the steel is determined to be (1-1.25 [C%]) × 80% or more.
The area ratio of pro-eutectoid ferrite is preferably 85% or more of the maximum area ratio of pro-eutectoid ferrite, and more preferably 90% or more.

  In Patent Document 1, since the area ratio of pro-eutectoid ferrite is low, the growth of spherical cementite hardly occurs during spheroidizing annealing. Therefore, in the steel after spheroidizing annealing, the interparticle distance of spherical cementite becomes short, and as a result, it cannot be sufficiently softened. That is, the steel wire of Patent Document 1 cannot sufficiently reduce the hardness by short-time spheroidizing annealing.

The size of the pearlite block: 25 μm or less The smaller the size of the pearlite block in the previous structure, the more spheroidization is promoted and the reprecipitation of pearlite is suppressed. When the pearlite block is refined, the interface of the pearlite block increases. Since the interface of the pearlite block becomes a cementite precipitation site, the cementite is precipitated at the interface during the spheroidizing annealing. Spheroidizing cementite grows during cooling after spheroidizing annealing. Therefore, cementite is hardly precipitated as pearlite.
When the size of the pearlite block (more precisely, the average block size of the pearlite block) is 25 μm or less, a good spheroidized structure can be obtained in the metal structure after spheroidizing annealing for a short time. When the average block size of the pearlite block exceeds 25 μm, a large amount of pearlite is precipitated during cooling of the spheroidizing annealing, and a good spheroidized structure cannot be obtained. The average block size of the pearlite block is preferably 23 μm or less, and more preferably 20 μm or less.

  In Patent Document 2, heat treatment necessary for reducing the size of the pearlite block is not performed in the steel manufacturing process. For example, in patent document 2, the stop temperature of rapid cooling after finish rolling is 700-800 degreeC, and is gradually cooled after that. When the stop temperature is higher than the Ps point (approximately 710 ° C.), the cooling is gradually performed from the stop temperature to the Ps point, and the size of the pearlite block is increased. Therefore, when the steel of Patent Document 2 is spheroidized and annealed for a short time, the cementite in the pearlite is not appropriately spheroidized, and as a result, the pearlite remains or reprecipitates. Therefore, the steel of Patent Document 2 cannot sufficiently progress spheroidization (that is, the degree of spheroidization cannot be reduced) by short-time spheroidizing annealing.

Average equivalent circle diameter of bcc-Fe crystal grains: 15 to 35 μm
When the bcc-Fe crystal grains are relatively large, strengthening due to crystal grain refinement hardly occurs. Therefore, the strength of the steel before and after spheroidizing annealing can be lowered. Thereby, even if it is short-time spheroidizing annealing, it becomes easy to obtain steel with low strength (that is, soft). In the present specification, “bcc-Fe” includes pro-eutectoid ferrite and ferrite contained in the pearlite structure.

In order to exhibit the effect of reducing the strength of the steel, the average equivalent circle diameter of the bcc-Fe crystal grains (hereinafter referred to as “bcc-Fe average particle diameter”) is preferably 15 to 35 μm. In the present specification, the “equivalent circle diameter of crystal grains” means the diameter of the circle when a circle having the same area as the area of each crystal grain appearing in the cross section of the structure is defined. The “average equivalent circle diameter of bcc-Fe crystal grains” is an average value of equivalent circle diameters of a plurality of bcc-Fe crystal grains.
From the viewpoint of reducing the strength of the steel, the bcc-Fe average particle diameter may exceed 35 μm. However, when the bcc-Fe average particle diameter exceeds 35 μm, it is difficult to reduce the size of the pearlite block. Accordingly, the average particle diameter of bcc-Fe is preferably 35 μm or less. The bcc-Fe average particle diameter is more preferably 17 to 33 μm, and particularly preferably 20 to 30 μm.

The structure to be controlled in the average bcc-Fe grain size is bcc-Fe crystal grains surrounded by a large-angle grain boundary in which the orientation difference between two adjacent crystal grains is larger than 15 °. The pre-structure before spheroidizing annealing also includes small-angle grain boundaries with an orientation difference of 15 ° or less. However, the small-angle grain boundary has little influence on the spheroidized structure after spheroidizing annealing. In order to obtain a desired spheroidized structure after spheroidizing annealing, it is desirable to appropriately control the large-angle grain boundaries contained in the previous structure. By setting the bcc-Fe average particle size surrounded by the large-angle grain boundaries to 15 to 35 μm, the steel can be sufficiently softened even in a short spheroidizing annealing, and the steel can be well spheroidized. Can be an organization.
“Directional difference” is also referred to as “shift angle” or “bevel angle”. The EBSP method (Electron Back Scattering Pattern Method) can be employed for measuring the azimuth difference.

2. Chemical composition The present invention is a steel for machine structure for cold working suitable for cold working. The steel type has a normal chemical composition as a machine structural steel for cold working, and in particular, C, Si, Mn, P, S, Al, and N are adjusted to the following appropriate ranges. . The appropriate ranges of these chemical components and the reasons for their limitations are described below. In the present specification, “%” used to indicate the chemical component composition means mass%.

C: 0.3 to 0.6%
C is an element useful for ensuring the strength of the steel, that is, the strength of the final product. In order to exhibit such an effect effectively, the C content needs to be 0.3% or more. The C content is preferably 0.32% or more, and more preferably 0.35% or more. However, if C is contained excessively, the strength becomes too high and the cold workability deteriorates, so it is necessary to make it 0.6% or less. The C content is preferably 0.55% or less, more preferably 0.50% or less.

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

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

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

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

Al: 0.01 to 0.1%
Al is useful as a deoxidizing element and is useful for fixing solute N present in steel as AlN. In order to exhibit such an effect effectively, the Al content is determined to be 0.01% or more. The Al content is preferably 0.013% or more, and more preferably 0.015% or more. However, when the Al content is excessive, Al2O3 is excessively generated and the cold workability is deteriorated. Therefore, the Al content is determined to be 0.1% or less. Al content becomes like this. Preferably it is 0.090% or less, More preferably, it is 0.080% or less.

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

The basic components of the steel of the present invention are as described above, and in one embodiment, the balance is substantially iron. In addition, “substantially iron” means that a trace component (for example, Sb, Zn, etc.) that does not inhibit the characteristics of the present invention can be allowed in addition to iron, and unavoidable impurities other than P, S, and N ( For example, O, H, etc.). In this invention, you may contain the following arbitrary elements as needed. Depending on the optional components contained, the properties of the steel can be further improved.
As described above, P, S, and N are elements inevitably contained (unavoidable impurities), but their composition ranges are separately defined as described above. For this reason, in this specification, “inevitable impurities” included as the balance mean “elements inevitably included” excluding elements whose composition range is separately defined.

  The steel of the present invention may selectively contain other elements in addition to the above-described chemical composition (C, Si, Mn, P, S, Al and N), iron and inevitable impurities. For example, as exemplified below, Cr, Cu, Ni, Mo, B, Ti, and the like can be appropriately included.

Cr: more than 0%, 0.5% or less, Cu: more than 0%, 0.25% or less, Ni: more than 0%, 0.25% or less, Mo: more than 0%, 0.25% or less and B: One or more selected from the group consisting of more than 0% and less than 0.01% Cr, Cu, Ni, Mo and B all increase the strength of the final product by improving the hardenability of the steel. It is an effective element. You may contain 1 type, or 2 or more types selected from Cr, Cu, Ni, Mo, and B as needed. The effect of improving hardenability increases as the content of these elements increases. A preferable content for effectively exhibiting this effect is such that the Cr content is 0.015% or more, more preferably 0.020% or more. The preferable contents of Cu, Ni and Mo are all 0.02% or more, more preferably 0.05% or more. The preferable content of B is 0.0003% or more, more preferably 0.0005% or more.

  However, when the contents of Cr, Cu, Ni and Mo are excessive, the strength becomes too high and the cold workability is deteriorated. Therefore, the Cr content is preferably 0.5% or less, and the Cu, Ni and Mo contents are preferably 0.25% or less. A more preferable content of Cr is 0.45% or less, and further preferably 0.40% or less. The more preferable contents of Cu, Ni and Mo are all 0.22% or less, more preferably 0.20% or less.

  Moreover, when content of B becomes excessive, there exists a possibility of degrading toughness. Therefore, the B content is preferably 0.01% or less. The more preferable content of B is 0.007% or less, and further preferably 0.005% or less.

In summary, when adding any one or more of Cr, Cu, Ni, Mo and B, the content of each element can be defined as follows.
Cr: more than 0%, 0.5% or less, preferably 0.015-0.45%, more preferably: 0.020-0.40%
Cu: more than 0%, 0.25% or less, preferably 0.02 to 0.22%, more preferably 0.05 to 0.20%
Ni: more than 0%, 0.25% or less, preferably 0.02 to 0.22%, more preferably 0.05 to 0.20%
Mo: more than 0%, 0.25% or less, preferably 0.02 to 0.22%, more preferably 0.05 to 0.20%
B: more than 0%, 0.01% or less, preferably 0.0003 to 0.007%, more preferably 0.0005 to 0.005%

Moreover, it is preferable that content of Cr, Cu, Ni, and Mo satisfy | fills following formula (1), and more suitable intensity | strength can be obtained.
[Cr%] + [Cu%] + [Ni%] + [Mo%] ≦ 0.75 (1)
However, [Cr%], [Cu%], [Ni%], and [Mo%] respectively indicate the contents of Cr, Cu, Ni, and Mo expressed in mass%.
Note that, as described above, Cr, Cu, Ni, and Mo are elements that can be selectively added, and among these elements, the content of the element that is not added in the formula (1) is zero.

Ti: more than 0% and 0.1% or less Ti forms a compound with N and reduces the solid solution N, thereby exhibiting the softening effect. You may contain Ti as needed. In order to effectively exhibit this effect, the Ti content is preferably 0.01% or more, and more preferably 0.02% or more. However, when the Ti content is excessive, the formed compound causes an increase in hardness. Therefore, the Ti content is determined to be 0.1% or less. The Ti content is preferably 0.09% or less, and more preferably 0.08%.

3. Manufacturing Method In order to manufacture the steel for machine structural use for cold working of the present invention, the finish rolling temperature at the time of hot rolling the steel satisfying the above component composition is adjusted, and the subsequent cooling is performed in three stages. Separately, it is preferable to appropriately adjust each cooling rate.
Specifically, after (a) finish rolling at 950 ° C. to 1100 ° C., (b) first cooling for cooling the temperature range from the finish rolling temperature to 800 ° C. at an average cooling rate of 7 ° C./second or more, ( c) second cooling in which the temperature range from 800 ° C. to 710 ° C. is cooled at an average cooling rate of 1 ° C./second or less; It is preferable to perform the 3rd cooling which cools with a cooling rate in this order.
The finish rolling temperature and the first to third cooling will be described in detail below. The “temperature” defined in this specification is the temperature of the material.

(A) Finishing rolling temperature: 950 ° C to 1100 ° C
By controlling the finish rolling temperature to 950 ° C. to 1100 ° C., the size of the pearlite block can be controlled to 25 μm or less. When the finish rolling temperature exceeds 1100 ° C., it becomes difficult to make the size of the pearlite block 25 μm or less. On the other hand, when the finish rolling temperature is less than 950 ° C., the bcc-Fe average particle size becomes finer and the strength of the steel increases.
The upper limit of the finish rolling temperature is preferably 1180 ° C, more preferably 1150 ° C. The lower limit of the finish rolling temperature is preferably 970 ° C, more preferably 1000 ° C.

(B) 1st cooling In 1st cooling, it cools to 800 degreeC from 950-1200 degreeC which is finish rolling temperature. If the cooling rate in the first cooling is slow, the size of the pearlite block may exceed 25 μm. Therefore, it is preferable that the average cooling rate of the first cooling is 7 ° C./second or more. The average cooling rate of the first cooling is more preferably 10 ° C./second or more, and further preferably 15 ° C./second or more. Although the upper limit of the average cooling rate of 1st cooling is not specifically limited, It is 200 degrees C / sec or less as a realistic range. In the first cooling, the average cooling rate may be 7 ° C./second or more, and the cooling rate may be changed during the first cooling. Such a cooling rate of the first cooling can be achieved by appropriately cooling the rolled material on the conveyor.

(C) Second cooling In the second cooling, cooling from 800 ° C to 710 ° C is performed. When the cooling rate in the second cooling is fast, the pro-eutectoid ferrite area ratio may be less than (1-1.25 × [C%]) × 80%. Therefore, it is preferable to set the average cooling rate of the second cooling to 1 ° C./second or less. The average cooling rate of the second cooling is more preferably 0.9 ° C./second or less, and further preferably 0.8 ° C./second or less. The lower limit of the average cooling rate of the second cooling is not particularly limited, but is 0.01 ° C./second or more as a realistic range. In the second cooling, the average cooling rate may be 1 ° C./second or less, and the cooling rate may be changed during the second cooling. Such a cooling rate of the second cooling can be achieved by installing a cover on the conveyor for suppressing heat radiation from the rolled material.

(D) Third cooling In the third cooling, cooling from 710 ° C to 600 ° C is performed. If the cooling rate in the third cooling is slow, the size of the pearlite block may exceed 25 μm. Therefore, it is preferable that the average cooling rate of the third cooling is 4 ° C./second or more. The average cooling rate of the third cooling is more preferably 6 ° C./second or more, and further preferably 8 ° C./second or more. If the cooling rate in the third cooling is too fast, a supercooled tissue may be generated. Therefore, it is preferable that the average cooling rate of the third cooling is 20 ° C./second or less. The average cooling rate of the third cooling is more preferably 18 ° C./second or less, and further preferably 16 ° C./second or less. In the third cooling, the average cooling rate may be 4 to 20 ° C./second, and the cooling rate may be changed during the third cooling.

  The cooling after the third cooling, that is, the cooling from 600 ° C. to room temperature does not need to be controlled cooling, and may be, for example, standing to cool. Usually, the average cooling rate by cooling is slower than the average cooling rate of the third cooling.

The steel for machine structural use for cold working according to the present invention can be formed into a good spheroidizing material by simply performing a short spheroidizing annealing, for example, a short time soaking for about 1 to 3 hours in a temperature range of about Ac1 to Ac1 + 30 ° C. A structure (a structure having a spheroidization degree equal to or less than the target spheroidization degree) can be obtained, and can be sufficiently softened (to make the hardness less than the target hardness).
Ac1 is a value calculated from the following equation (7). In formula (7), [% element name] means the content of each element in mass%.
Ac1 (° C.) = 723-10.7 [% Mn] −16.9 [% Ni] +29.1 [% Si] +16.9 [% Cr] (7)

  Hereinafter, the present invention will be described more specifically with reference to examples. The present invention is not limited by the following examples, and can be implemented with appropriate modifications within the scope that can meet the above-mentioned and later-described gist, and they are all within the technical scope of the present invention. Is included.

  Using a steel having the chemical composition shown in Table 1, a wire rod of φ16.0 mm was created by rolling, and a cylindrical (φ8.0 mm × 12.0 mm) processed formaster specimen was created by machining. . Steel type P is a comparative example in which the chemical composition is outside the scope of the present invention. In Table 1, numerical values marked with an asterisk (*) indicate that they are outside the scope of the present invention. In the steel type P, the amount of Cr exceeds 0.5% by mass, which is outside the scope of the present invention. Further, in the steel types A to O, [Cr%] + [Cu%] + [Ni%] + [Mo%] is 0.75 mass% or less, which satisfies the above-described formula (1). In the steel type P, [Cr%] + [Cu%] + [Ni%] + [Mo%] exceeds 0.75 mass% and does not satisfy the formula (1).

  Using the obtained specimen for machining for master, a machining heat treatment test was carried out with a machining for master testing machine under the conditions shown in Table 2. The processing conditions simulate the rolling conditions in the actual machine. “Processing temperature” in Table 2 corresponds to the finish rolling temperature. “First cooling” in Table 2 is cooling from the processing temperature to 800 ° C. “Second cooling” in Table 2 is cooling from 800 ° C. to 710 ° C. “Third cooling” in Table 2 is cooling from 710 ° C. to 600 ° C. The cooling after the third cooling was allowed to cool.

  The processed formaster test piece after the heat treatment was cut along a plane orthogonal to the central axis and divided into four equal parts. Of the four cut test pieces (cut test pieces), one was a sample for observing the metal structure, and the other was a sample for spheroidizing annealing.

  All steel types A to O were heat-treated under the above-mentioned preferable rolling conditions (Test Nos. 1 to 32). Furthermore, about the steel types E, G, H, and M, the test heat-processed on the conditions remove | deviated from preferable rolling conditions was also done (test No. 33-40).

  Regarding the cut specimen after the thermomechanical test, (1) area ratio of pro-eutectoid ferrite + pearlite, (2) area ratio of pro-eutectoid ferrite, (3) average block size of pearlite block, (4) after spheroidizing annealing The degree of spheroidization and (5) hardness after spheroidizing annealing were measured by the following methods.

  In the measurement of (1) to (5), the cut specimen after the thermomechanical treatment was cut along the central axis (longitudinal section or axial center section) and embedded with resin so that the longitudinal section could be observed. . When the longitudinal section was mirror-polished, it was mirror-polished with emery paper or diamond buff. When the diameter of the cut specimen was D, the position of D / 4 was measured from the side surface of the cut specimen toward the central axis.

(1) Measurement of area ratio of pro-eutectoid ferrite + pearlite The longitudinal section of the cut specimen was mirror-polished, and then the structure was revealed by nital etching. Five fields of 220 μm × 165 μm areas were photographed at a magnification of 400 times with an optical microscope at a position of D / 4 in the longitudinal section. Ten vertical lines at equal intervals and ten horizontal lines at equal intervals were drawn on the photographed photographs so as to form a lattice pattern. As a result, 100 intersections of vertical lines and horizontal lines were formed. Of the 100 intersections, the number of intersections located on the pro-eutectoid ferrite (number of pro-eutectoid ferrite) and the number of intersections located on the pearlite (number of pearlite) were measured. The total area ratio (%) of pro-eutectoid ferrite + pearlite was determined by dividing the total number of pro-eutectoid ferrite and pearlite by the total number of intersections (100). The same operation was performed for each of the five fields of view, and the average value of the total area ratio (%) was obtained.

(2) Measurement of area ratio of pro-eutectoid ferrite The longitudinal section of the cut specimen was mirror-polished, and the structure was revealed by nital etching. Five fields of 220 μm × 165 μm areas were photographed at a magnification of 400 times with an optical microscope at a position of D / 4 in the longitudinal section. Ten vertical lines at equal intervals and ten horizontal lines at equal intervals were drawn on the photographed photographs so as to form a lattice pattern. As a result, 100 intersections of vertical lines and horizontal lines were formed. Of the 100 intersections, the number of intersections located on the pro-eutectoid ferrite (number of pro-eutectoid ferrite) was measured. By dividing by the number of pro-eutectoid ferrite and the total number of intersections (100), the area ratio (%) of pro-eutectoid ferrite was determined. The same operation was performed for each of the five fields of view, and the average value of the area ratio (%) was obtained.

(3) Measurement of average block size of pearlite An EBSP analyzer and an FE-SEM (Field-Emission Scanning Electron Microscope, field emission scanning electron microscope) were used to measure the size of the pearlite block.
The position of D / 4 in the longitudinal section of the cut specimen was measured with an EBSP analyzer. From the EBSP analysis data, the bcc-Fe average grain size was determined by defining “bcc-Fe crystal grains” with the boundary (large angle grain boundary) where the crystal orientation difference (oblique angle) exceeds 15 ° as the grain boundary. At this time, the measurement area was 200 μm × 400 μm, and the measurement step was measured at 1.0 μm intervals. Measurement points having a confidence index (confidence index) of 0.1 or less indicating the reliability of the measurement direction were deleted from the analysis target.

  Thereafter, the longitudinal section of the cut specimen was mirror-polished and the structure was revealed by nital etching. Tissue observation was performed by FE-SEM in the same visual field as that of the EBSP measurement. From the EBSP analysis data, 20 or more crystal grains corresponding to the pearlite block were randomly selected, and the size of each crystal grain was measured. The average value of crystal grain size was defined as the average size of pearlite blocks (average block size).

(4) Measurement of spheroidizing degree after spheroidizing annealing In order to measure the spheroidizing degree, the cut specimens were spheroidized annealing for a short time. In short-time spheroidizing annealing, a cut specimen is vacuum-sealed in a container, held in an atmosphere furnace at 750 ° C. for 2 hours, cooled to 680 ° C. at an average cooling rate of 10 ° C./hour, and then allowed to cool. Was done. The processing time in short-time spheroidizing annealing is about 9 hours, which is about 30% shorter than the processing time for general spheroidizing annealing (about 13 hours).
The later measurement of the degree of spheroidization is performed by exposing the structure of a mirror-polished longitudinal section sample by picral etching, and observing 5 fields of view at 400 times magnification using an optical microscope at the D / 4 position. The degree of spheroidization of the field of view is No. according to the attached drawing of JIS G3539: 1991. 1-No. The average value of 5 visual fields was calculated. The smaller the degree of spheroidization, the better the spheroidized structure.

(5) Measurement of hardness after spheroidizing annealing The hardness after spheroidizing annealing is measured at 5 points at a load of 1 kgf at a D / 4 position using a Vickers hardness tester on a mirror-polished longitudinal section sample. The average value (HV) was determined by measurement.

Table 3 shows the structure before spheroidizing annealing and the degree of spheroidization and hardness after spheroidizing annealing evaluated in the manner of (1) to (5) above. The target degree of spheroidization was 3.0. Moreover, the hardness requested | required of steel changes with C, Si, and Mn content. Therefore, the target hardness (described as “target hardness” in Table 3) was defined as shown in Formula (8) based on the contents of C, Si, and Mn.
Target hardness = 88.4 × Ceq + 88.0 (8)
However, Ceq = [C%] + 0.2 × [Si%] + 0.2 × [Mn%], and [C%], [Si%], and [Mn%] are C expressed in mass%, respectively. , Si and Mn contents are shown.

From the results in Table 3, it can be considered as follows. No. in Table 3 Nos. 1 to 30 are examples that satisfy all of the requirements defined in the present invention, and after the short-time spheroidizing annealing, the degree of spheroidization and hardness both achieved the target.
On the other hand, no. 31 to 40 are examples that do not satisfy the requirements defined in the present invention, and the degree of spheroidization and / or hardness did not reach the target after short-time spheroidizing annealing.

  No. Since 31 and 32 used steel type P (Table 1) which has a large Cr content and does not satisfy the formula (1), the hardness after spheroidizing annealing was harder than the target hardness.

  No. Since the processing temperature of 33 and 36 was higher than 1100 ° C., the average block size of the pearlite in the previous structure was larger than 25 μm. Therefore, the degree of spheroidization after spheroidizing annealing exceeded 3.0.

  No. In 34 and 39, the cooling rate of the third cooling was slower than 4 ° C./second, so that the average block size of the pearlite in the previous tissue was larger than 25 μm. Therefore, the degree of spheroidization after spheroidizing annealing exceeded 3.0.

  No. In 35, 37 and 40, the cooling rate of the second cooling was faster than 1 ° C./second, so the area ratio of proeutectoid ferrite in the previous structure was lower than the target position. Therefore, the hardness after spheroidizing annealing was harder than the target hardness.

  No. In No. 38, since the cooling rate of the first cooling was slower than 7 ° C./second, the average block size of pearlite in the previous tissue was larger than 25 μm. Therefore, the degree of spheroidization after spheroidizing annealing exceeded 3.0.

The machine structural steel for cold working of the present invention is suitable as a material for various parts produced by cold working such as cold forging, cold forging, or cold rolling. Although the form of steel is not specifically limited, For example, it can be set as rolling materials, such as a wire rod or steel bar.
The parts include, for example, automobile parts such as electrical parts, and construction machine parts such as various machine parts. Specifically, bolts, screws, nuts, sockets, ball joints, inner tubes, torsion bars, etc. , Clutch case, cage, housing, hub, cover, case, washer, tappet, saddle, bulg, inner case, clutch, sleeve, outer race, sprocket, core, stator, anvil, spider, rocker arm, body, flange, Examples include drums, joints, connectors, pulleys, metal fittings, yokes, caps, valve lifters, spark plugs, pinion gears, steering shafts, common rails, and the like. The steel of the present invention is industrially useful as a steel for machine structures that is suitably used as a material for the above parts, has low deformation resistance at room temperature when producing the above various parts, and has excellent cold workability. It can be demonstrated.

Claims (5)

C: 0.3% by mass to 0.6% by mass,
Si: 0.05 mass% to 0.5 mass%,
Mn: 0.2% by mass to 1.7% by mass,
P: more than 0% by mass, 0.03% by mass or less,
S: 0.001 mass% to 0.05 mass%,
Al: 0.01% by mass to 0.1% by mass and N: 0% by mass to 0.015% by mass,
The balance consists of iron and inevitable impurities,
The steel microstructure has proeutectoid ferrite and pearlite,
The total area ratio of pro-eutectoid ferrite and pearlite with respect to the whole structure is 90% or more,
The area ratio of pro-eutectoid ferrite with respect to the entire structure is (1-1.25 × [C%]) × 80% or more,
A machine structural steel for cold working, wherein the average block size of the pearlite is 25 µm or less.
However, [C%] indicates the C content expressed in mass%.   The steel for machine structure for cold working according to claim 1, wherein the average equivalent circle diameter of the bcc-Fe crystal grains is 15 to 35 µm. Cr: more than 0% by mass, 0.5% by mass or less,
Cu: more than 0% by mass, 0.25% by mass or less,
Ni: more than 0% by mass, 0.25% by mass or less,
Mo: more than 0% by mass, 0.25% by mass or less and B: more than 0% by mass, further containing at least one selected from the group consisting of 0.01% by mass and satisfying the following formula (1) The machine structural steel for cold working according to claim 1 or 2.
[Cr%] + [Cu%] + [Ni%] + [Mo%] ≦ 0.75 (1)
However, [Cr%], [Cu%], [Ni%], and [Mo%] respectively indicate the contents of Cr, Cu, Ni, and Mo expressed in mass%.   The steel for machine structure for cold work according to any one of claims 1 to 3, further containing Ti: more than 0% by mass and 0.1% by mass or less. A method for producing cold-working machine structural steel according to any one of claims 1 to 4,
Finishing rolling at a finishing rolling temperature of 950 ° C. to 1100 ° C .;
Cooling from the finish rolling temperature to 800 ° C. at an average cooling rate of 7 ° C./second or more;
Cooling from 800 ° C. to 710 ° C. at an average cooling rate of 1 ° C./second or less,
A method of manufacturing steel for machine structural use for cold working, wherein the step of cooling from 710 ° C to 600 ° C at an average cooling rate of 4 to 20 ° C / second is performed in this order.