TWI606124B - Steel for mechanical structure for cold room processing and manufacturing method thereof - Google Patents

Description

Steel for mechanical structure for cold room processing and manufacturing method thereof

The present invention relates to a steel for machine structural use for cold working and a method for producing the same. In particular, it is a steel for machine structural use which is excellent in deformation resistance after spheroidizing annealing, and which is excellent in cold workability, and a useful method for producing the steel for machine structural use. The steel for machine structural use for cold working of the present invention is suitably used for parts for automobiles and parts for construction machinery which are manufactured by cold work such as cold forging, cold press, and cold roll forming. Various parts such as. The form of the steel is not particularly limited, and may be, for example, a rolled wire or the like as an object. The various components described above include, in particular, bolts, screws, nuts, sleeves, ball joints, inner tubes, torsion bars, clutch housings, cages, housings, hubs, housings, boxes, and adjustments. Base, tappet, saddle, valve, inner casing, clutch plate, collar, outer ring, sprocket, core, stator, anvil, star wheel, rocker arm, body, flange, drum, joint, Connector, pulley, hardware, yoke, bulb head, valve tappet, spark plug, rack pinion, steering column and common rail mechanical parts and electrical components. In addition, in this specification, the wire material is a linear steel material which is a rolling wire, and is cooled to room temperature after hot rolling. Further, in the present specification, the term "steel wire" refers to a process of applying a wire drawing and/or annealing to the above-mentioned rolled wire rod. A linear steel material after its characteristics.

In the case of manufacturing various parts such as parts for automobiles and construction of parts for machinery, a spheroidizing annealing treatment for the purpose of imparting cold workability is usually performed on hot-rolled wires such as carbon steel and alloy steel. Then, the spheroidized and annealed steel wire is subjected to cold working, and then subjected to machining such as cutting to form a predetermined shape, and then quench hardening and tempering are performed to adjust the final shape. strength.

In recent years, the conditions of spheroidizing annealing have been reconsidered from the viewpoint of energy saving, and in particular, short-time spheroidizing annealing time is required. For example, if the spheroidizing annealing time can be reduced by two to three percent, it is expected to reduce energy consumption and reduce CO ₂ emissions.

However, it is known that when the spheroidizing annealing time is shortened, the index of the spheroidization of the carbide, that is, the degree of spheroidization, becomes large (that is, the spheroidized structure is deteriorated), and the cold workability is deteriorated. Therefore, it is not easy to shorten the spheroidizing annealing time.

Several technical solutions have been proposed in the past to shorten the spheroidizing annealing time. For example, as disclosed in Patent Document 1, by controlling the metal structure before spheroidizing annealing, it is possible to achieve softening of the steel for mechanical construction for cold working, even if only a short time of spheroidizing annealing is performed. And its manufacturing method. Specifically, the steel for mechanical structure for cold working disclosed is a total surface of iron and ferrite iron with respect to the entire structure. The product ratio is controlled to be 95 area% or more, and the area ratio of the ferrite iron is controlled to be a predetermined value or more, and the bcc-Fe crystal grain size is controlled within a suitable range. The method for producing steel for cold working machining disclosed in the present invention is to perform a finishing process at a temperature of 750 to 950 ° C, and then to a temperature range of 600 to 660 ° C at an average cooling rate of 5 ° C /sec or more. Then, the film was cooled at an average cooling rate of 1 ° C /sec or less for 20 seconds or more.

Further, the technical solution disclosed in Patent Document 2 is a steel wire material containing a preliminary precipitated iron structure, a ferrite structure, and a toughened iron structure, and a method for producing the same. Such a steel wire can shorten the softening annealing time, and after the soft annealing, excellent cold forgeability can be achieved. The method for producing a steel wire disclosed therein is first hot-rolled, coiled, and then immersed in a molten salt bath of 500 ° C or more and 600 ° C or less for 10 seconds or more, followed by molten salt of 530 ° C or more and 600 ° C or less. The inside of the tank is cooled after being kept at a constant temperature of 20 seconds or more and 150 seconds or less.

In addition, the technical solution disclosed in Patent Document 3 is that the percentage of the iron structure of the ferrite-grained iron crystal having a grain size number of 9 or more is 30% by area or more, and the rest is made of Bored iron, toughened iron, and granulated iron or The composition of these mixed structures, the percentage of the toughened iron + the granulated iron structure is the hot-rolled wire for cold forging which occupies 50% by area or more of the remaining portion, and a method for producing the same. The method for producing a hot-rolled wire for cold forging disclosed in the method is a temperature range of Ar ₃ to Ar ₃ + 150 ° C, and after refining and rolling, between Ar ₁ and 300 ° C, 5 Cooling is performed at a cooling rate of ~40 ° C / sec.

[Previous Technical Literature] [Patent Document]

[Patent Document 1] Japanese Patent Laid-Open Publication No. 2013-7091

[Patent Document 2] Japanese Patent No. 5195009

[Patent Document 3] Japanese Patent No. 4299744

However, in the method of Patent Document 2 described above, since the area ratio of the initial precipitated iron is low, the hardness of the steel wire after the spheroidizing annealing may become too hard. Further, in the method of Patent Document 3, since the area ratio of the ferrite iron is low and the toughened iron or the granulated iron is contained, it is expected that the hardness of the steel wire after the spheroidizing annealing is sure to become Very hard.

The technique of the prior art proposed is useful for shortening the spheroidizing annealing time. However, the more desirable technology is to develop a ball that is better than the conventional technology. The technique of structuring the structure and softening the steel.

The present invention has been developed under such circumstances, and its object is to provide that the same degree of conventional manufacturing methods can be achieved even if the time for performing spheroidizing annealing is shortened compared with the conventional manufacturing method. Or, it is better to be spheroidized, and it is possible to obtain a steel for cold-working mechanical structure which is softer than the conventional production method, and a method for producing the same.

The steel for machine structural structure for cold working according to the present invention which is capable of solving the above-mentioned technical problems is characterized by containing C: 0.07% or more and less than 0.3%, Si: 0.05 to 0.5%, and Mn: 0.2, respectively, by mass%. ~1.7%, P: above 0% and below 0.03%, S: 0.001~0.05%, Al: 0.01~0.1%, and N: 0~0.015%, the rest is iron and inevitable impurities, while steel The metal structure is composed of the initial precipitated ferrite iron and the Borne iron, and the total area ratio of the precipitated ferrite iron and the ferrite is 90% or more with respect to the whole structure, and the area ratio Af of the preliminary precipitated ferrite iron is borrowed and borrowed. The relationship between the A values represented by the following formula (1) is in accordance with the relationship of Af≧A, and the average equivalent circle diameter of the bcc-Fe crystal grains is 15 to 30 μm, and the interval between the ferrite sheets is on average. 0.20 μm or less, A = (103 - 128 × [C (%))) × 0.80 (%) ‧ ‧ ‧ (1)

In the above formula (1), [C(%)] represents the C content in mass%.

In a preferred embodiment of the present invention, the steel for mechanical construction for cold working has a mass percentage of 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 one selected from the group of 0% and 0.01% or less, and conforming to the following formula ( Relationship of X): [Cr%]+[Cu%]+[Ni%]+[Mo%]≦0.75‧‧‧数(X)

[Cr%], [Cu%], [Ni%], and [Mo%] represent the contents of Cr, Cu, Ni, and Mo in mass%, respectively.

In a preferred embodiment of the present invention, the steel for mechanical construction for cold working has a Ti content of more than 0% and 0.1% or less in mass%.

In the method for producing steel for cold working machining according to the present invention, the refining rolling is performed at a temperature of 950° C. or higher and 1150° C. or lower, and then the first cooling step and the second cooling step are sequentially performed. The first cooling step is performed at an average cooling rate of 3 ° C /sec or less until the first cooling end temperature of 700 to 750 ° C. The second cooling step is an average cooling of 5 to 30 ° C / sec. At the speed, cooling is performed from a temperature range of at least 600 ° C from the first cooling end temperature.

In the steel for machine structural use for cold working of the present invention, the chemical composition is appropriately adjusted, and the total area ratio of the precipitated iron and the ferrite to the whole structure and the initial precipitation of the ferrite The area ratio is controlled to be a predetermined value or more, and the average equivalent circle diameter of bcc (body-centered cubic)-Fe crystal grains (hereinafter, sometimes referred to as "bcc-Fe average particle diameter"), and wave The spacing of the iron sheets is controlled to be within the appropriate range. Thereby, even if the spheroidizing annealing time is shorter than the usual manufacturing method, the spheroidized structure of the same degree or better as the conventional manufacturing method can be obtained, and can be manufactured by a conventional method. Steel is softer. Therefore, the steel for mechanical construction for cold working of the present invention is subjected to spheroidizing annealing at room temperature or in the field of processing heat generation. In the case of the various parts described above, the deformation resistance of the steel is low, and cracking of the mold for processing and steel (material) can be suppressed, and excellent cold workability can be exhibited.

1‧‧‧Bollite sheet organization

2‧‧‧Sheet Snow Ming

3‧‧‧Sheet ferrite

4‧‧‧ line segment (orthogonal to the layered structure and the starting and ending ends are thin The center of the thickness of the piece of snow

Fig. 1 is an explanatory view showing a method of measuring the interval of the ferrite sheets; Fig. 1(a) is a schematic view showing the structure 1 of the ferrite sheets; and Fig. 1(b) is an enlarged view of the structure 1 of the ferrite sheets. Figure.

The inventors of the present invention have been able to obtain a conventional manufacturing method even when the spheroidizing annealing time is shorter than the usual manufacturing method (hereinafter referred to as "short-time spheroidizing annealing"). The spheroidized structure of the same degree or better, and the steel for cold-working mechanical construction which is softer than the steel produced by the conventional method, is tried to be examined from various angles. As a result, a finding was found that, in the metal structure (spheroidized structure) of the steel wire after the spheroidizing annealing, the grain size of the ferrite iron crystal is coarsened, and the average particle size of the carbide is enlarged. The distance can be softened by steel. Further, it has been found that it is important to control the metal structure before spheroidizing annealing (hereinafter referred to as "pre-tissue") in order to obtain such a spheroidized structure. And found a kind of originality, that is, in order to obtain the metal structure (spheroidized structure) of the steel wire mentioned above, in addition to controlling the front structure to a structure mainly composed of fermented iron and wave iron, And try to improve the initial precipitation The area ratio of iron, the bcc-Fe crystal grain is controlled to be coarser than the bcc-Fe crystal grain in the steel of the conventional manufacturing method, and the interval of the ferrite sheet is controlled to be less than a predetermined value, and the The steel of the pre-structured steel can obtain the spheroidized structure of the same degree or better in the spheroidized structure in the spheroidized structure after a short spheroidizing annealing, and can be made more than the steel produced by the conventional method. The present invention has been completed in terms of softening.

The various requirements specified by the present invention will be described below.

The metal structure of the steel of the present invention contains the primary precipitated iron and the ferrite. These structures can reduce the deformation resistance of the steel after spheroidizing annealing, and are metal structures that contribute to the improvement of cold workability. However, if it is simply a metal structure containing precipitated ferrite and Borne iron, it is not possible to achieve the desired softening. Therefore, as described below, factors such as the area ratio of these tissues and the average particle diameter of the bcc-Fe crystal grains must also be appropriately controlled.

The total area ratio of the initial analysis of ferrite and ferrite: more than 90%

In the case where there are many fine structures such as toughened iron and granulated iron in the anterior structure of the steel, even if a general spheroidizing annealing is performed, the spheroidal annealing is subjected to the toughening iron and / or the impact of the granulated iron, resulting in local tissue fine, steel softening will not be sufficient. From this point of view, in order to sufficiently soften the steel, it is necessary to control the total area ratio of the precipitated iron and the ferrite to the total structure to 90% or more. The total area ratio of the initial analysis of ferrite and ferrite is better than 95%, more preferably 97% or more, and the optimum is 100%. In addition, In addition to the metal structure other than the initial analysis of ferrite iron and Borne iron, there are mentioned: Ma Tian loose iron, toughened iron and Worth iron. As described above, if the area ratio of these tissues such as the granulated iron is high, the strength of the steel becomes high, so that it is not necessary to contain these tissues at all. Steel may also contain carbides, nitrides, oxides, and/or sulfides other than ferritic iron as other tissue factors.

Average equivalent circle diameter of bcc-Fe crystal grains: 15~30μm

When the average equivalent circle diameter of the bcc-Fe crystal grains in the anterior structure of the steel, that is, the average particle diameter of the bcc-Fe is controlled to 30 μm or less in advance, even after only a short spheroidal annealing is performed, it is obtained. Good spheroidized tissue (ie, spheroidized tissue with a small degree of spheroidization). When the average particle diameter of bcc-Fe is more than 30 μm, and only a short-time spheroidizing annealing is performed, the spheroidized structure is deteriorated (that is, the degree of spheroidization becomes large), and the desired spheroidized structure cannot be obtained. The average particle diameter of bcc-Fe is preferably 29 μm or less, more preferably 28 μm or less. However, if the average particle diameter of the bcc-Fe in the front structure is too small, the grain size of the ferrite iron crystal after the spheroidizing annealing is refined, and the steel is softened, which makes it difficult to soften the steel. Therefore, the average particle diameter of bcc-Fe is set to 15 μm or more. The average particle diameter of bcc-Fe is more preferably 16 μm or more, and more preferably 17 μm or more. Further, the equivalent circle diameter of the crystal grains referred to means the diameter of a circle having the same area as each crystal grain.

The structure to be controlled by the average particle diameter of bcc-Fe is a large-angle grain boundary in which the orientation difference of two adjacent crystal grains is greater than 15°. The surrounding bcc-Fe crystal grains. Small angular boundaries of 15° or less in the azimuth are also included in the tissue. However, these small angular boundaries have little effect on the spheroidized structure obtained after spheroidizing annealing. If a desired spheroidized structure is to be obtained after spheroidizing annealing, it is necessary to control the large-angle boundary of the spheroidizing annealing structure. By controlling the average particle diameter of bcc-Fe surrounded by the large-angle boundary to a predetermined range, a good spheroidized structure can be achieved even if only a short-time spheroidizing annealing is performed (that is, the degree of spheroidization is small). Spheroidized organization). Further, the aforementioned "azimuth difference" is also referred to as "dislocation angle" or "bevel angle", and when measuring the azimuth difference, an electron backscatter pattern method (EBSP method; Electron Back Scattering Pattern method) can be used. Further, bcc-Fe means: in addition to the initial precipitation of ferrite, it also includes the ferrite iron contained in the Borne iron structure.

Spanier sheet spacing: 0.20μm or less

The metal structure of the steel of the present invention, as described above, has a preliminary precipitated iron and a ferritic iron. If the interval between the ferrite sheets is narrowed (that is, the ferrite sheets are fined), even if only a short-time spheroidizing annealing is performed, the carbides (mainly ferritic iron in the Borne iron) can be promoted. The spheroidization results in a good spheroidized structure. From this point of view, it is necessary to control the interval of the Bronze sheets in the front structure to be equal to or less than (hereinafter, simply referred to as "average sheet interval") of 0.20 μm or less. The average sheet interval is more preferably 0.18 μm or less, more preferably 0.16 μm or less. Although the lower limit of the average sheet interval is not particularly limited, it is usually about 0.05 μm.

In addition, in this specification, "Interval" means the distance between adjacent thin layers of stellite. More precisely, it means the shortest distance from the center position of the thickness of the thin layer of the stellite iron to the center position of the thickness of the adjacent stellite layer.

Initial analysis of the area ratio of ferrite iron Af≧A

Further, in the pre-structure, when the area ratio of the precipitated ferrite is increased, the carbide precipitation point in the spheroidizing annealing is reduced, and thus the number density of carbides is reduced, so that the carbide coarsening is promoted. As a result, the distance between the particles of the carbide becomes large, and the metal structure can be made softer. On the other hand, the area ratio of the initial precipitated ferrite is changed by the influence of the carbon content. If the carbon content is increased, the area ratio of the initial precipitation iron will decrease. Similarly, the optimum initial area of the precipitated ferrite used to obtain a good spheroidal material will also vary with the carbon content. The more carbon content, the more the area ratio of the appropriate initial precipitation iron is reduced. Based on this point of view, after analyzing many experimental results, a finding is found: in the pre-tissue, the area ratio Af of the precipitated iron relative to the whole tissue is compared with the following formula (1) When the relationship of the indicated A value is controlled to conform to the relationship of Af≧A, it is possible to further soften.

A=(103-128×[C(%)))×0.80 (%)‧‧‧数(1)

In the above formula (1), [C(%)] represents the C content in mass%.

Af is preferably (103-128 × [C (%))) × 0.85 or more, more preferably (103 - 128 × [C (%))) × 0.90 or more. Further, based on the above viewpoint, the upper limit of Af is not particularly limited. However, if you improve Af If it is to increase the manufacturing cost, it is preferable to control Af to be (103-128 × [C(%))) × 0.97 or less in consideration of productivity.

The present invention is a steel for a mechanical structure for cold working, and the steel type thereof may be a steel having a chemical composition of a steel for mechanical construction for cold working, and is for C, Si, Mn, P, S, Al, and N, is adjusted to the following appropriate range. In addition, in this specification, "%" with respect to a chemical component means the mass %.

C: 0.07% or more and less than 0.3%

C is an element that is useful for ensuring the strength of the steel, that is, the strength of the final product. In order to effectively exert this effect, the C content must be 0.07% or more. The C content is preferably at least 0.09%, more preferably at least 0.11%. However, if the C content is too large, the strength becomes too high and the cold workability is deteriorated, and it must be controlled to less than 0.3%. The C content is preferably 0.28% or less, more preferably 0.26% or less.

Si: 0.05~0.5%

Si acts as a deoxidizing element and is also a useful element for enhancing the strength of the final product by solid solution hardening. In order to effectively exert this effect, the Si content must be 0.05% or more. The Si content is preferably at least 0.07%, more preferably at least 0.10%. On the other hand, if the Si content is too large, the hardness will excessively increase and the cold workability will be deteriorated. Therefore, the Si content is set to 0.5% or less. The Si content is preferably at most 0.45%, more preferably at most 0.40%.

Mn: 0.2~1.7%

Mn is an effective element that can increase the strength of the final product by improving quench hardenability. In order to effectively exert this effect, the Mn content must be 0.2% or more. The Mn content is more preferably 0.3% or more, and more preferably 0.4% or more. On the other hand, if the Mn content is too large, the hardness will increase and the cold workability will be deteriorated. Therefore, the Mn content is set to 1.7% or less. The Mn content is more preferably 1.5% or less, more preferably 1.3% or less.

P: higher than 0% and less than 0.03%

P is an element that is inevitably contained in steel, and causes segregation at the grain boundary in steel, which causes deterioration in ductility. Therefore, the P content is set to be 0.03% or less. The P content is preferably 0.02% or less, preferably 0.017% or less, more preferably 0.01% or less. The smaller the P content, the better, so 0% is the best, but it is limited by factors such as constraints in the manufacturing process, and sometimes it remains 0.001%.

S: 0.001~0.05%

S is an element which is inevitably contained in steel, and is present in the form of MnS in steel to deteriorate ductility, and is an element which hinders cold 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 the S system has an effect of improving the machinability, it is useful that the S content is 0.001% or more. The S content is preferably at least 0.002%, more preferably at least 0.003%.

Al: 0.01~0.1%

Al is a useful element as a deoxidizing element, and can fix a form in which solid solution N existing in steel becomes AlN. In order to effectively exert this effect, the Al content must be 0.01% or more. The Al content is preferably 0.013% or more, more preferably 0.015% or more. However, if the Al content is too large, too much Al ₂ O ₃ will be generated, and the cold workability will be deteriorated. Therefore, the Al content is set to be 0.1% or less. The Al content is more preferably 0.090% or less, more preferably 0.080% or less.

N: 0~0.015%

N is an element which is inevitably contained in steel. If the steel contains solid solution N, the hardness is increased due to deformation aging, and the ductility is deteriorated, and the cold workability is also deteriorated. Therefore, the N content is set to 0.015% or less. The N content is preferably 0.013% or less, more preferably 0.010% or less. The smaller the N content, the better, so 0% is the best, but it is limited by factors such as constraints in the manufacturing process, and sometimes it remains 0.001%.

The basic composition of the steel for machine structural use of the present invention is as described above, and as one of the embodiments, the remainder is substantially iron. In addition, "substantially iron" means that an element other than iron may be allowed to have a trace amount of, for example, Sb or Zn, and may also contain P, S, etc., to the extent that the characteristics of the present invention are not inhibited. Other than N, for example, inevitable impurities such as O and H. Furthermore, the invention can also be selected as needed Selectively contains the following elements. The properties of the steel can be further improved in response to the type of element (selected component) that is selectively added.

Further, as described above, P, S, and N are elements (unavoidable impurities) that are inevitably contained, but the range of the component is defined in the above manner. Therefore, in the present specification, the "inevitable impurity" contained as the remaining portion means an element which is inevitably contained other than the elements (P, S, and N) whose composition range is separately defined.

From Cr: above 0% and below 0.5%, Cu: above 0% and below 0.25%, Ni: above 0% and below 0.25%, Mo: above 0% and below 0.25%, and B: above More than one selected from 0% and less than 0.01%

Cr, Cu, Ni, Mo, and B are all effective elements for increasing the strength of the final product by improving the quench hardenability of the steel. These elements may be added individually or in combination of two or more as necessary. This effect becomes larger as the content of these elements increases. The content of Cr is preferably 0.015% or more, more preferably 0.020% or more, in order to effectively exert the above-mentioned effects. The suitable content of the Cu content, the Ni content, and the Mo content is 0.02% or more, more preferably 0.05% or more. The suitable content of the B content is 0.0003% or more, more preferably 0.0005% or more.

However, when the content of Cr, Cu, Ni, and Mo is too large, the strength becomes too high and there is a concern that 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. The preferred content of Cr is below 0.45%. More preferably, it is below 0.40%. The content of Cu, Ni and Mo is preferably 0.22% or less, more preferably 0.20% or less.

Moreover, if the B content is too large, there is a concern that the toughness is deteriorated. Therefore, the B content is preferably 0.01% or less. A preferred content of the B content is 0.007% or less, more preferably 0.005% or less.

[Cr%]+[Cu%]+[Ni%]+[Mo%]≦0.75

When the steel wire according to the embodiment of the present invention contains at least one of Cr, Cu, Ni, and Mo within the above range, it is preferable to satisfy the relationship of the following formula (X).

[Cr%]+[Cu%]+[Ni%]+[Mo%]≦0.75‧‧‧数(X)

[Cr%], [Cu%], [Ni%], and [Mo%] are the contents of Cr, Cu, Ni, and Mo in terms of % by mass, respectively.

By making the contents of Cr, Cu, Ni, and Mo conform to the above formula (X), the strength of the steel can be suppressed from being too high, and the cold workability can be improved.

Ti: above 0% and below 0.1%

The Ti system forms a compound with N to reduce the solid solution N, so that the effect of softening can be exhibited. Therefore, Ti can be contained as necessary. A suitable Ti content which is intended to effectively exert such an effect is 0.01 or more, more preferably 0.02 or more. However, if the Ti content is too large, the formed compound will cause an increase in hardness. Therefore, a suitable Ti content is 0.08% or less, more preferably 0.05 or less.

In order to produce the steel for machine structural use for cold working according to the present invention, the steel which conforms to the above composition is adjusted for the refining rolling temperature during hot rolling, and the subsequent cooling rate is divided into two stages. It is advisable to adjust the cooling rate and temperature range. Specifically, the refining rolling is performed at a temperature of 950° C. or higher and 1150° C. or lower, and then sequentially performed at an average cooling rate of 3° C./sec or less: from 950° C. to 1150° C. The first cooling step of the cooling operation up to the first cooling end temperature of 700 to 750 ° C; the average cooling rate of 5 to 30 ° C / sec: at least 600 ° C from the first cooling end temperature The second cooling step of the cooling operation up to the temperature range.

The refining rolling temperature, the first cooling step, and the second cooling step will be described in detail below.

(a) Refined rolling temperature: 950 ° C or more and 1150 ° C or less

In order to control the average particle diameter of bcc-Fe to 15 to 30 μm, it is necessary to appropriately control the refining rolling temperature. When the refining rolling temperature is higher than 1150 ° C, it is difficult to control the bcc-Fe average particle diameter to 30 μm or less. Therefore, it is preferable to set the refining rolling temperature to 1150 ° C or lower. However, if the refining rolling temperature is less than 950 ° C, it is difficult to control the bcc-Fe average particle diameter to 15 μm or more. Therefore, it is preferable to set the refining rolling temperature to 950 ° C or higher. The refining rolling temperature is more preferably 970 ° C or higher, more preferably 990 ° C or higher. The refining rolling temperature is better below 1130 ° C, more preferably in Below 1110 °C.

(b) The first cooling process

Average cooling rate in the first cooling step: 3 ° C / sec or less

The first cooling step is started from a temperature at which the finishing rolling temperature is 950 ° C or higher and 1150 ° C or lower, and ends at the first cooling end temperature of 700 to 750 ° C. In the first cooling step, if the cooling rate is too fast, the initial precipitation iron area ratio Af will become small, and the relationship of Af≧A may not be satisfied. Therefore, the average cooling rate in the first cooling step is set to 3 ° C /sec or less. The average cooling rate in the first cooling step is preferably 2.5 ° C / sec or less, more preferably 2 ° C / sec or less. The lower limit of the average cooling rate in the first cooling step is not particularly limited. However, as an achievable range, it is preferable to set it as 0.01 ° C / sec or more. Further, in the first cooling step, if the average cooling rate is 3 ° C / sec or less, the cooling rate may be changed.

(c) second cooling process

Average cooling rate in the second cooling step: 5 to 30 ° C / sec

The second cooling step is started from a temperature range of 700 to 750 ° C and ends at at least 600 ° C. In the second cooling step, if the average cooling rate is slower than 5 ° C / sec, it is difficult to control the average sheet interval of the pulverized iron to 0.20 μm or less. The average cooling rate in the second cooling step is preferably 7 ° C / sec or more, more preferably 10 ° C / sec or more. On the other hand, if it is faster than 30 ° C / sec, it will produce toughened iron and / or 麻田散铁 It is difficult to control the total area ratio of the initial precipitated ferrite and the Borne iron to more than 90%. The average cooling rate in the second cooling step is preferably 28 ° C / sec or less, more preferably 25 ° C / sec or less. Further, in the second cooling step, if the average cooling rate is 5 to 30 ° C / sec, the cooling rate may be changed.

The term "at least 600 ° C" as used herein means that the end temperature of the second cooling step at the time of cooling at the above average cooling rate is at most 600 ° C. The reason for setting "600 ° C" is because the average sheet interval of the ferrite specified in the present invention, and the form of the metal structure such as the total area ratio of the initial precipitated iron and the ferrite, are substantially It is determined by the cooling process before cooling to 600 ° C, and after less than 600 ° C, it is almost no longer affected by the cooling rate. Therefore, the end temperature of the second cooling step is not limited to 600 ° C, and room temperature such as the examples described later may be sufficient. Alternatively, for example, the end temperature of the second cooling step may be set to 400 ° C, and then a general cooling treatment such as cooling may be performed to cool to room temperature. In general, the average cooling rate at the time of cooling is often slower than the average cooling rate in the second cooling step described above.

When the steel for machine structural construction for cold working of the present invention is used, it is required to carry out spheroidal annealing for a short period of time, for example, in a temperature range of about Ac ₁ to Ac ₁ + 30 ° C, for a spherical shape of about 1 to 3 hours. By the annealing, the degree of spheroidization can be controlled to be equal to or lower than the target spheroidization degree to be described later, and the hardness can be controlled to be equal to or lower than the target hardness to be described later. Further, Ac ₁ is a numerical value calculated from the following formula. In the following formula, (% element name) means: the content of each element in mass%.

Ac ₁ (°C)=723-10.7 (%Mn)-16.9 (%Ni)+29.1 (%Si)+16.9 (%Cr)

[Examples]

The invention will be more specifically described below by way of examples. The present invention is not limited by the following embodiments, and may be modified and implemented within the scope of the gist of the present invention and the scope of the present invention, which are also included in the technical scope of the present invention.

Using a steel having the chemical composition shown in Table 1 below, rolling was carried out to obtain a wire having a diameter of 17.0 mm, and a machining deformation test point of 8.0 mm in diameter × 12.0 mm in length was obtained by mechanical processing. Test piece used. The test piece for the test was measured using the obtained processing abnormal point, and the processing heat treatment test was carried out by the processing abnormal point measuring machine according to the conditions described in Table 2. The processing conditions described in Table 2 simulate the actual rolling conditions. Further, the processing temperatures in Table 2 correspond to the refining rolling temperature.

The test piece after the processing heat treatment test was carried out in accordance with the conditions described in Table 2, and the structure was evaluated according to the methods described in the following (1) to (3). Further, for the test piece which was subjected to spheroidizing annealing after the heat treatment test, the spheroidization degree and the hardness were measured in accordance with the methods described in the following (4) and (5). In each of the measurements, the test piece after the heat treatment or the spheroidizing annealing was cut from a plane passing through the central axis of the test piece and parallel to the central axis (axial center section). In the following description, the test piece after cutting may be referred to as a "longitudinal section sample". In order to observe the axial center profile of the test piece, the longitudinal section sample was buried in the resin. For processing heat After the treatment and the spheroidizing annealing test piece, when the diameter of the cylindrical test piece is regarded as D, it is a position D/4 toward the center from the side of the test piece ("D/4 position" ") The measurement was carried out.

(1) Determination of tissue area ratio

A longitudinally sectioned sample after specular polishing was applied to the axial center profile, and the metal structure was formed by etching with a nitric acid etchant. Then, for the tissue at the D/4 position, images of five observation fields were taken with an optical microscope at a magnification of 400 times for a field of 220 μm × 165 μm. Then, for the photographs taken, 10 vertical lines and 10 horizontal lines are equally spaced to form a lattice shape, and then the points of the initial precipitated iron and the Boron iron existing at 100 intersections are measured. number. The area ratio (%) of each tissue in each observation field was determined, and the average value of the five observation fields was calculated.

(2) Determination of the average particle size of bcc-Fe

For the measurement of the average particle diameter of bcc-Fe, an electron backscatter pattern (EBSP) analyzer and an electric field emission type scanning electron microscope (FE-SEM) were used. The boundary of the crystal orientation difference (oblique angle) exceeding 15°, that is, the large-angle grain boundary is defined as a crystal grain boundary to define "crystal grains", and the average particle diameter of bcc-Fe is determined. At this time, the measurement range was 200 μm × 400 μm, and the measurement procedure was performed at intervals of 1.0 μm. The measurement point for indicating the reliability of the measurement orientation with a Confidence Index of 0.1 or less is deleted from the analysis target. In addition, there is a precipitated hemp in the metal structure. The sample of the scattered iron structure was not measured because it could not obtain a suitable average particle size of bcc-Fe.

(3) Determination of the spacing of the ferrite sheets

Fig. 1(a) is a schematic view showing a structure 1 of a ferrite sheet; and Fig. 1(b) is an enlarged view of a structure 1 of a ferrite sheet. The structure 1 of the ferrite sheet is a layered (thin layered) structure in which the sheet ferrite iron 3 and the thin snow iron 2 are arranged as shown in Fig. 1(b). The sheet spacing specified in the present specification means the interval of the sheet of stellite 2.

A longitudinally sectioned sample after specular polishing was applied to the axial center profile, and the metal structure was formed by etching with a bitter acid etchant. Then, the structure at the D/4 position was observed using an electric field emission type scanning electron microscope (FE-SEM) at a magnification of 3000 times, for a field of 42 μm × 28 μm, or at a magnification of 5000 times, for a 25 μm × 17 μm. In the field, we made a total of five observations of photography. At this time, at least one wave of iron must be contained in each observation field. Among the observation fields of the photographed photograph, the most fine sheet interval (that is, the narrowest sheet interval) was selected as the measurement target. For the ferrite of the measurement object, a line segment 4 is drawn which is orthogonal to the layered structure (i.e., orthogonal to the direction in which the layers extend) and the starting end and the end point are located at the center of the thickness of the thin strip of fermented iron. Further, the length L of the line segment 4 and the number n of sheets of the stellite iron 2 in the line segment 4 (in other words, the number n of the sheet of the stellite 2 which intersects the line segment 4) are measured. In addition, the number n of the thin snowy irons includes the starting end and the ending end of the line segment 4. The thin piece of snow-light iron. Using the equation (2), the sheet interval λ is calculated from the length L and the number of sheets n. The sheet interval λ in each observation field was obtained, and the average value of the five observation fields was calculated. Further, in the first diagram (b), an example in which n=5 is shown is not limited thereto. In the present description, when the sheet interval λ is calculated, the line segment 4 is drawn such that the number n of the sheets of the ferritic iron 2 intersecting the line segment 4 is at least five or more.

λ=L/(n-1)‧‧‧数(2)

Further, in the metal structure, the sample in which the granulated iron structure was precipitated and the total area ratio of the precipitated ferrite iron and the ferritic iron was less than 90%, the measurement was not performed because the sheet interval was difficult to calculate.

(4) Determination of spheroidization degree after spheroidizing annealing

For the longitudinal section sample of the test piece after the spheroidizing annealing, the axial center section was mirror-polished, and then the metal structure was formed by etching with a bitter acid etchant. Five fields of view were observed for the tissue at the D/4 position using an optical microscope at a magnification of 400 times. The spheroidization degree of each observation field is compared with the drawings of Japanese Industrial Standard JIS G3539:1991, and the number of stages of No.1 to No.4 is compared, and the average value of five observation fields is calculated. . Further, the smaller the degree of spheroidization, means that it has a good spheroidized structure.

(5) Determination of hardness after spheroidizing annealing

For the longitudinal section sample of the test piece after spheroidizing annealing, the measurement was performed. The hardness of the D/4 position of the longitudinally sectioned sample after mirror polishing was performed on the axial center profile. The hardness was measured by a Vickers hardness tester at a load of 1 kgf. The measurement was performed for five different points located at the D/4 position, and the average value (HV) thereof was determined.

Example

Using the steel types A to U shown in Table 1 above, the processing temperature (corresponding to the refining rolling temperature) and the cooling rate were changed in accordance with the conditions shown in the following Table 2, and the processing abnormal point measurement test was carried out. Thereby, test pieces for processing abnormal points having different anterior tissues were prepared. Further, the steel species O falls outside the scope of the present invention because the Mn content is higher than 1.7%. The steel species P falls outside the scope of the present invention because the Ti content is more than 0.1%. Further, the steel types A to O and Q to U, [Cr%] + [Cu%] + [Ni%] + [Mo%] are 0.75 mass% or less, and satisfy the above formula (X). On the other hand, the steel type P is [Cr%]+[Cu%]+[Ni%]+[Mo%] higher than 0.75 mass%, which fails to satisfy the formula (X).

In the processing conditions of Table 2, in addition to No. 10, 20, 43, and 44, the "first cooling step" starts from the processing temperature, and ends at the temperature range of 700 to 750 ° C at the first cooling end temperature; The "second cooling step" is started at the room temperature from the first cooling end temperature of the "first cooling step". No. 10, 20, and 44 are cooled at a constant average cooling rate from the processing temperature at the start of the first cooling step to the end temperature of the second cooling step. Therefore, the "first cooling step" is not distinguished. And "second cooling process". In addition, No.44 is from 850 The range from °C up to 300 °C was cooled at an average cooling rate of 40.0 ° C / sec and then allowed to cool to room temperature. In addition, No. 43 sets the end temperature of the "first cooling step" to 650 ° C, sets the end temperature of the "second cooling step" to 550 ° C, and then cools it to room temperature.

The test piece for measuring the above-described processing deformation point was divided into quarters by a cross section orthogonal to the central axis. One of them was taken as a sample for tissue investigation, and the other was used as a sample for spheroidizing annealing. The spheroidizing annealing is performed by separately vacuuming the test piece and then performing heat treatment in an atmospheric furnace. Spheroidizing annealing was carried out at 730 ° C for two hours of soaking, and after cooling at an average cooling rate of 30 ° C / hr, to 710 ° C, the average cooling rate was 10 ° C / hr, and cooled to 680 ° C, Then, let it cool.

The structure before the spheroidizing annealing after the evaluation according to the above (1) to (5), the spheroidization degree after the spheroidizing annealing, and the hardness are shown in Table 3. Further, depending on the C content, the degree of spheroidization obtained is also different. Therefore, the target spheroidization degree (described as "target spheroidization degree" in Table 3) is a numerical value obtained by the following formula (3). Further, the hardness obtained is different depending on the contents of C, Si, and Mn. Therefore, the target hardness (described as "target hardness" in Table 3) is the value obtained by the following formula (4).

Target spheroidization = 5 × [C%] + 1.5‧‧‧ (3)

Target hardness = 88.4 × Ceq + 86.0‧‧‧ (4)

Here, Ceq = [C%] + 0.2 × [Si%] + 0.2 × [Mn%]

[C%], [Si%], and [Mn%] represent the contents of C, Si, and Mn in mass%, respectively.

According to the results of Table 3, the following investigation can be made. No. 1~8, 13, 15~17, 19, 21, 23, 24, 26, 27, 29~32, 35~38 and 45~50 in Table 3 are all the requirements stipulated by the present invention. For example, even if only a short-time spheroidizing annealing is performed, the spheroidization degree after the spheroidizing annealing has reached the target, and the target hardness has also been reached.

On the other hand, No. 9~12, 14, 18 in Table 3, 20, 22, 25, 28, 33, 34, 39-44, which is an example of one of the requirements of the requirements specified in the present invention, after spheroidizing annealing, at least between spheroidization and hardness One failed to achieve the goal.

No. 9, although the steel type A of Table 1 which used the composition of this invention was used, the processing temperature corresponding to the refining rolling temperature was too low. Therefore, the average particle diameter of bcc-Fe is too small, and the hardness does not become soft after spheroidizing annealing.

No. 10 is a steel type A using the composition of the composition according to the present invention, but the cooling rate in the second cooling step is too slow. Therefore, the average sheet spacing of the Borne iron becomes too large, and the spheroidization degree after the spheroidizing annealing is not good.

No. 11 is a steel type A using the composition of the composition according to the present invention, but the cooling rate in the first cooling step is too fast. Therefore, the area ratio of the initial precipitated ferrite is too small, and the hardness is not softened after the spheroidizing annealing.

No. 12, although the steel type A of Table 1 which is in accordance with the composition of the present invention was used, the processing temperature was too high. Therefore, the average particle diameter of bcc-Fe becomes too large, and the degree of spheroidization after spheroidizing annealing becomes too large (that is, the spheroidized structure is not good).

No. 14 is a steel type B using the composition of the composition according to the present invention, but the cooling rate in the second cooling step is too slow. Therefore, the average sheet spacing of the Borne iron becomes too large, and the spheroidization degree after the spheroidizing annealing becomes too large (that is, the spheroidized structure is poor).

No. 18, although the composition according to the present invention is used Steel type D of Table 1, but the processing temperature is too high. Therefore, the average particle diameter of bcc-Fe becomes too large, and the degree of spheroidization after spheroidizing annealing becomes too large (that is, the spheroidized structure is not good).

In No. 20, the steel type E of Table 1 which is in accordance with the composition of the present invention was used, but the cooling rate in the first cooling step was too fast. Therefore, the area ratio of the initial precipitated ferrite is too small, and the hardness is not softened after the spheroidizing annealing.

No. 22 is a steel type F using the composition of the composition according to the present invention, but the cooling rate in the second cooling step is too fast. Therefore, there is a precipitation of the granulated iron structure in the field, and the total area ratio of the initial granule iron and the ferritic iron and the area ratio of the initial granule iron become too small. As a result, the hardness did not become soft after spheroidizing annealing.

No. 25, although the steel type H of Table 1 which is in accordance with the composition of the present invention was used, the processing temperature was too low. Therefore, the average particle diameter of bcc-Fe becomes too small, and the hardness does not become soft after spheroidizing annealing.

No. 28 is a steel type I using Table 1 which is in accordance with the composition of the present invention, but the cooling rate in the first cooling step is too fast. Therefore, the area ratio of the initial precipitated ferrite is too small, and the hardness is not softened after the spheroidizing annealing.

No. 33 is a steel type L using the composition of the composition according to the present invention, but the cooling rate in the first cooling step is too fast. Therefore, the area ratio of the initial precipitated ferrite is too small, and the hardness is not softened after the spheroidizing annealing.

No. 34, although the composition according to the present invention is used The steel type L in Table 1 is too fast in the second cooling step. Therefore, there is a precipitation of the granulated iron structure, and the total area ratio of the initial granule iron and the ferritic iron and the area of the initial granule iron become too small. As a result, the hardness did not become soft after spheroidizing annealing.

In No. 39 and 40, the steel type O of Table 1 having a large Mn content was used. Therefore, the hardness was not softened after the spheroidizing annealing.

In No. 41 and 42, the steel type P of Table 1 having a large Cr content and not satisfying the relationship of the formula (X) was used, and therefore, the hardness was not softened after the spheroidizing annealing.

No. 43 is a steel type Q using the composition of the composition according to the present invention, but the processing temperature is too low, and the cooling rate in the first cooling step is too fast. Therefore, the average particle diameter of the bcc-Fe becomes too small, and the precipitation of the granulated iron structure is caused to lower the area ratio of the initial precipitated iron. As a result, the hardness did not become soft after spheroidizing annealing.

In No. 44, the steel type R of Table 1 which is in accordance with the composition of the present invention was used, but the processing temperature was too low, the cooling rate in the first cooling step was too fast, and the cooling rate in the second cooling step was too fast. Therefore, the average particle diameter of the bcc-Fe becomes too small, the area ratio of the precipitated ferrite iron is lowered, and the precipitation of the granulated iron structure is reduced, and the total area ratio of the initial precipitated ferrite iron and the ferrite is lowered. As a result, the hardness did not become soft after spheroidizing annealing.

In addition, No. 1 to 8, 13, 15 to 17, 19, 21, 23, 24, 26, 27, 29 to 32, 35 to 38, and 45 to 50 (fully meeting the requirements specified in the present invention), 2 in the cooling process is cooling to room temperature stop. However, the second cooling step may be performed until the temperature is lowered to 600 ° C, and then the cooling may be performed, and it is expected that substantially the same result can be obtained.

1‧‧‧Bollite sheet organization

2‧‧‧Sheet Snow Ming

3‧‧‧Sheet ferrite

4‧‧‧ line segment (orthogonal to layered tissue and starting and ending ends) Located in the center of the thickness of the thin snowy iron)

Claims (4)

A steel for machine structural use for cold-working, characterized in that it contains C: 0.07% or more and less than 0.3%, Si: 0.05 to 0.5%, Mn: 0.2 to 1.7%, and P: high, respectively, by mass%. 0% and 0.03% or less, S: 0.001 to 0.05%, Al: 0.01 to 0.1%, and N: 0 to 0.015%, the rest is iron and inevitable impurities, and the metal structure of steel contains initial precipitation The total area ratio of the granulated iron and the ferritic iron relative to the initial analysis of the granulated iron and the ferritic iron is more than 90%, and the area ratio Af of the granules of the granules is determined by the following formula (1) The relationship of the A value indicated is in accordance with the relationship of Af≧A, and the average equivalent circle diameter of the bcc-Fe crystal grains is 15 to 30 μm, and the interval of the ferrite sheets is 0.20 μm or less on average, and A = (103- 128 × [C (%)]) × 0.80 (%) ‧ ‧ (1) In the above formula (1), [C (%)] represents the C content in mass%. The steel for machine structural use for cold-working according to claim 1, wherein the steel contains, in mass%, more than 0% and 0.5% or less. Cu: higher than 0% and 0.25% or less, Ni: higher than 0% and 0.25% or less, Mo: higher than 0% and 0.25% or less, and B: higher than 0% and 0.01% or less selected in the group More than one, and in accordance with the relationship of the following formula (X), [Cr%] + [Cu%] + [Ni%] + [Mo%] ≦ 0.75 ‧ ‧ formula (X) [Cr%], Cu%], [Ni%], and [Mo%] represent the contents of Cr, Cu, Ni, and Mo in mass%, respectively. The steel for machine structural use for cold working according to claim 1 or 2, further comprising Ti: more than 0% and 0.1% or less by mass%. A method for producing a steel for machine structural use for cold working, which is characterized in that it is 950 ° C or higher and 1150 when the steel for machine structural use for cold working according to any one of claims 1 to 3 is produced. The refining rolling is performed at a temperature of not higher than ° C, and then, the first cooling step and the second cooling step are sequentially performed, and the first cooling step is performed at an average cooling rate of 3 ° C /sec or less until cooling to 700 ° The first cooling end temperature is 750 ° C. The second cooling step is performed at a temperature of at least 600 ° C from the first cooling end temperature at an average cooling rate of 5 to 30 ° C / sec.