JP2020125538A - Steel for cold working machine structures, and method for producing same - Google Patents

Abstract

To provide a steel for cold working machine structures that can be softened sufficiently even when the processing time of spheroidizing is shortened, and a method of producing the same.SOLUTION: A steel for cold working machine structures, comprising 0.32 to 0.44 mass% of C, 0.15 to 0.35 mass% of Si, 0.55 to 0.95 mass% of Mn, 0.030 mass% or less of P, 0.030 mass% or less of S, 0.85 to 1.25 mass% of Cr, 0.15 to 0.35 mass% of Mo, 0.01 to 0.1 mass% of Al and the balance Fe with inevitable impurities, containing proeutectoid ferrite at an area ratio of 30% or more and 70% or less, and having an average ferrite crystal grain diameter of 5 to 15 μm.SELECTED DRAWING: None

Description

The present invention relates to a cold work machine structural steel and a method for producing the same.

In the production of various parts such as parts for automobiles and parts for construction machinery, usually, a hot rolled material such as carbon steel or alloy steel is subjected to spheroidizing annealing for the purpose of imparting cold workability. Then, the rolled material after spheroidizing annealing is subjected to cold working, and then subjected to mechanical processing such as cutting to be formed into a predetermined shape, and quenching and tempering processing is performed to finally adjust strength. ..

In recent years, from the viewpoint of energy saving, the conditions for spheroidizing annealing have been reviewed, and in particular spheroidizing annealing has been required to be shortened. If the processing time of the spheroidizing annealing can be reduced by 20 to 30%, the energy consumption and CO ₂ emission can be expected to be reduced accordingly.

However, when the spheroidizing annealing treatment with a shorter treatment time than usual (hereinafter sometimes referred to as “short-time annealing”) is performed, the spheroidizing degree, which is an index of the degree of spheroidizing of cementite, deteriorates, It is known that it is difficult to sufficiently soften the steel and that the cold workability is deteriorated, and it is not easy to shorten the spheroidizing annealing time. Therefore, a technique for sufficiently softening the steel without deteriorating the degree of spheroidization even when annealed for a short time has been studied.

For example, in Patent Document 1, the chemical composition by mass ratio is C: 0.3 to 0.6%, Mn: 0.2 to 1.5%, Si: 0.05 to 2.0%, and Cr: 0. 0.04 to 2.0%, balance: iron and unavoidable impurities , spheroidizing characterized by having an average grain size of prior austenite of 100 μm or more and a ferrite fraction of 20% or less in the metal structure. A mechanical structural steel having excellent cold forgeability after annealing is disclosed. It is said that the steel for machine structural use can sufficiently secure cold forgeability even with a relatively short time spheroidizing annealing.

Patent No. 3782666

However, the steel for machine structure disclosed in Patent Document 1 contains Cr but does not contain Mo as an essential component. Although the strength of the steel can be remarkably increased by including both Cr and Mo, such strength increase cannot be expected in the steel of Patent Document 1. Furthermore, although steel containing both Cr and Mo may be difficult to soften after spheroidizing annealing, in Patent Document 1, steel containing both Cr and Mo is sufficiently softened when annealed for a short time. It does not disclose that.

The present invention has been made in view of such a situation, and one of the objects thereof is to spheroidize even when Cr and Mo are included and spheroidizing annealing is performed for a relatively short time. Another object of the present invention is to provide a machine structural steel for cold working which is excellent in degree and capable of being sufficiently softened. Another object is to contain Cr and Mo and shorten the processing time of spheroidizing annealing. Even if it makes it possible to sufficiently soften, it is to provide a method for producing a steel for machine structural use for cold working.

Aspect 1 of the present invention is
C: 0.32-0.44 mass%,
Si: 0.15-0.35 mass%,
Mn: 0.55 to 0.95 mass%,
P: 0.030 mass% or less,
S: 0.030 mass% or less,
Cr: 0.85-1.25% by mass,
Mo: 0.15-0.35 mass%,
Al: 0.01 to 0.1% by mass,
The balance: consisting of iron and unavoidable impurities,
The area ratio of proeutectoid ferrite is 30% or more and 70% or less,
It is a machine structural steel for cold working in which the average grain size of ferrite crystal grains is 5 to 15 μm.

Aspect 2 of the present invention is the steel for cold working machine structural use according to Aspect 1, wherein the ratio of the area ratio of pearlite to the total area ratio of the structures other than the proeutectoid ferrite is 80% or less.

Aspect 3 of the present invention is the steel for machine working for cold working according to aspect 1 or 2, which has a hardness of HV 300 or less.

Aspect 4 of the present invention is
Aspect 1 further containing one or more selected from the group consisting of Cu: 0.25% by mass or less (not including 0% by mass), and Ni: 0.25% by mass or less (not including 0% by mass). The steel for machine work for cold working according to any one of 3 above.

Aspect 5 of the present invention is
Ti: 0.2% by mass or less (not including 0% by mass),
Aspect 1 further containing one or more selected from the group consisting of Nb: 0.2% by mass or less (not including 0% by mass), and V: 1.5% by mass or less (not including 0% by mass). The steel for machine structural use for cold working according to any one of 4 above.

Aspect 6 of the present invention is
N: 0.01% by mass or less (not including 0% by mass),
Mg: 0.02% by mass or less (not including 0% by mass),
Ca: 0.05% by mass or less (not including 0% by mass),
Aspect 1 further containing one or more selected from the group consisting of Li: 0.02% by mass or less (excluding 0% by mass), and REM: 0.05% by mass or less (excluding 0% by mass). The steel for machine work for cold working according to any one of 5 above.

Aspect 7 of the present invention provides a steel having the chemical composition as described in any one of Aspects 1 to 6,
(A) a step of performing pre-processing at a compression rate of 20% or more and a holding time of 10 seconds or less,
(B) after the step (a), a step of performing a finishing process at a temperature higher than 800° C. and lower than or equal to 1050° C. and a compression rate of 20% or higher;
(C) after the step (b), cooling to 750° C. or higher and 840° C. or lower in 10 seconds or less,
(D) After the step (c), a step of cooling to 500° C. or less at an average cooling rate of 0.1° C./second or more and less than 10° C./second is a method for producing a steel for cold working machine structural steel. ..

Aspect 8 of the present invention is a steel wire for cold working mechanical structural steel produced by the method of Aspect 7, which is subjected to one or more steps of annealing, spheroidizing annealing, wire drawing, forging and quenching and tempering. It is a manufacturing method.

In one embodiment of the present invention, a mechanical structure for cold working, which contains Cr and Mo and is excellent in spheroidization degree and can be sufficiently softened even if the spheroidizing annealing time is shortened more than usual. A steel for cold working, which is capable of providing a steel for use, and, in another embodiment, includes Cr and Mo, and is capable of sufficiently softening even if the processing time of spheroidizing annealing is shortened. It is possible to provide a method for manufacturing structural steel.

The present inventors provide a steel for cold working mechanical structure which contains Cr and Mo and which is excellent in spheroidization degree and can be sufficiently softened even if the spheroidizing annealing time is shortened more than usual. We examined it from various angles to realize it.

As a result, the chemical composition including Cr and Mo is appropriately adjusted, and the proeutectoid ferrite is contained, and the area ratio of the proeutectoid ferrite and the average grain size of the ferrite crystal grains are controlled to be predetermined values. As a result, it has been found that even if the processing time of spheroidizing annealing is shortened, it is possible to realize a steel for mechanical working for cold working which is excellent in spheroidizing degree and can be sufficiently softened.

Furthermore, by controlling the area ratio of pro-eutectoid ferrite and the average grain size of ferrite crystal grains, a steel for cold working mechanical structures that can be sufficiently softened even if variations occur in the temperature during spheroidizing annealing. At the same time, I also found out what can be achieved. This is very useful when spheroidizing annealing is processed in a large furnace. That is, in a large furnace, the temperature varies considerably due to the presence of a place where the temperature is lower than the set temperature and the temperature rising rate is delayed, but the cold working machine structural steel according to the embodiment of the present invention is It has been found that even if spheroidizing annealing is performed in such a furnace, it can be sufficiently softened.

The details of each requirement defined by the embodiment of the present invention will be described below.
In the present specification, the term “wire material” is used to mean a rolled wire material, and refers to a linear steel material that has been cooled to room temperature after hot rolling. Further, the "steel wire" refers to a linear steel material in which the rolled wire material is annealed or the like to adjust its characteristics.

<1. Chemical composition>
The steel for machine structural use for cold working according to the embodiment of the present invention has C: 0.32 to 0.44 mass%, Si: 0.15 to 0.35 mass%, Mn: 0.55 to 0.95. % By mass, P: 0.030% by mass or less, S: 0.030% by mass or less, Cr: 0.85 to 1.25% by mass, Mo: 0.15 to 0.35% by mass, Al: 0. 01 to 0.1% by mass, and the balance: iron and inevitable impurities.
Hereinafter, each element will be described in detail.

(C: 0.32-0.44 mass%)
C is a strength imparting element, and if it is less than 0.32% by mass, the required strength of the final product cannot be obtained. On the other hand, if it exceeds 0.44% by mass, the cold workability and toughness of steel deteriorate. Therefore, the content of C is set to 0.32 to 0.44% by mass. Further, when the C content is less than 0.40% by mass, more proeutectoid ferrite can be precipitated, which is preferable.

(Si: 0.15 to 0.35 mass%)
Si is useful as a deoxidizing element and as an improving element to be contained for the purpose of increasing the strength of the final product by solid solution hardening. In order to effectively exhibit such effects, the Si content is set to 0.15 mass% or more. On the other hand, if Si is contained excessively, the hardness is excessively increased and the cold workability of steel deteriorates. Therefore, the Si content is set to 0.35 mass% or less.

(Mn: 0.55 to 0.95 mass%)
Mn is an element effective in increasing the strength of the final product through improving the hardenability. In order to effectively exhibit such effects, the Mn content is set to 0.55 mass% or more. On the other hand, if Mn is excessively contained, the hardness increases and the cold workability of steel deteriorates. Therefore, the Mn content is set to 0.95 mass% or less.

(P: 0.030 mass% or less)
P is an element that is inevitably contained in the steel and causes grain boundary segregation in the steel, which causes deterioration of the ductility of the steel. Therefore, the P content is set to 0.030 mass% or less.

(S: 0.030 mass% or less)
S is an element that is inevitably contained in the steel, exists as MnS in the steel and deteriorates the ductility of the steel, and is a harmful element that deteriorates the cold workability of the steel. Therefore, the S content is 0.030 mass% or less.

(Cr: 0.85 mass% or more and 1.25 mass% or less)
Cr is an element effective in increasing the strength of the final product by improving the hardenability of the steel material. In order to effectively exhibit such effects, the Cr content is 0.85 mass% or more. Such an effect increases as the Cr content increases. However, if the Cr content becomes excessive, the strength becomes too high and the cold workability of the steel deteriorates, so the content is made 1.25 mass% or less.

(Mo: 0.15 mass% or more and 0.35 mass% or less)
Mo is an element effective in increasing the strength of the final product by improving the hardenability of the steel material. In particular, by including Mo in the steel together with Cr, the strength of the final product can be significantly increased. In order to effectively exert such effects, the Mo content is set to 0.15 mass% or more. Such an effect increases as the Mo content increases. However, if the Mo content is excessive, the strength becomes too high and the cold workability of steel deteriorates. In particular, by including Mo in the steel together with Cr, the steel may be hard to be significantly softened after the spheroidizing annealing. Therefore, Mo is 0.35 mass% or less.

(Al: 0.01 mass% or more and 0.1 mass% or less)
Al is an element which is useful as a deoxidizing agent, and at the same time, binds with N to precipitate AlN and prevents the crystal grains from abnormally growing during processing to lower the strength. In order to effectively exhibit such effects, the Al content is 0.01% by mass or more, preferably 0.015% by mass or more, and more preferably 0.020% by mass or more. However, if the Al content becomes excessive, Al ₂ O ₃ is excessively generated and the cold forgeability deteriorates. Therefore, the Al content is 0.1% by mass or less, preferably 0.090% by mass or less, and more preferably 0.080% by mass or less.

The balance is iron and inevitable impurities. As unavoidable impurities, it is permissible to mix in elements (for example, B, As, Sn, Sb, Ca, O, H, etc.) that are brought in depending on the conditions of raw materials, materials, manufacturing equipment, and the like.
Note that, for example, as in P and S, it is usually preferable that the content is small, and thus it is an unavoidable impurity, but there are elements whose composition range is separately specified as described above. Therefore, in the present specification, the term “unavoidable impurities” constituting the balance is a concept excluding elements whose composition range is separately defined.

Further, the cold working machine structural steel according to the embodiment of the present invention may optionally contain the following optional elements, and the properties of the steel are further improved depending on the contained components. It

(One or more selected from the group consisting of Cu: 0.25% by mass or less (not including 0% by mass) and Ni: 0.25% by mass or less (not including 0% by mass))
Cu and Ni are elements that effectively act to improve hardenability and product strength. Such action increases as the contents of these elements increase, but in order to effectively exhibit, Cu and Ni are preferably each 0.05 mass% or more, more preferably 0.08 mass% or more, It is preferably 0.10 mass% or more. However, if it is contained excessively, a supercooled structure is excessively generated, the strength becomes too high, and the cold forgeability deteriorates. Therefore, Cu and Ni are preferably each 0.25 mass% or less. It is more preferably 0.22% by mass or less, and even more preferably 0.20% by mass or less. In addition, Cu and Ni may be contained individually, or may be contained in two or more kinds, and in the case of containing two or more kinds, the content may be any content within the above range. ..

(Ti: 0.2 mass% or less (0 mass% is not included), Nb: 0.2 mass% or less (0 mass% is not included), and V: 1.5 mass% or less (0 mass% is included. One or more selected from the group consisting of
Ti, Nb, and V are elements that combine with N to form a compound (nitride), reduce the amount of solute N in steel, and obtain a deformation resistance reducing effect. In order to exert such an effect, each of Ti, Nb, and V is preferably 0.05 mass% or more, more preferably 0.06 mass% or more, still more preferably 0.08 mass% or more. However, if these elements are excessively contained, the amount of nitride increases, the deformation resistance increases, and the cold forgeability deteriorates. Therefore, Ti and Nb are each preferably 0.2 mass% or less, more preferably 0% by mass or less. 0.1% by mass or less, more preferably 0.15% by mass or less, V is preferably 1.5% by mass or less, more preferably 1.3% by mass or less, still more preferably 1.0% by mass or less. is there. Each of Ti, Nb and V may be contained alone, or may be contained in two or more kinds, and the content in the case of containing two or more kinds is any content within the above range. Good.

(N: 0.01 mass% or less (0 mass% is not included), Mg: 0.02 mass% or less (0 mass% is not included), Ca: 0.05 mass% or less (0 mass% is not included) ), Li: 0.02% by mass (not including 0% by mass), and rare earth element (Rare Earth Metal: REM): 0.05% by mass or less (not including 0% by mass). One or more)
N is an impurity that is unavoidably contained in steel, but when solid solution N is contained in steel, the hardness increases and the ductility decreases due to strain aging, and cold forgeability deteriorates. Therefore, N is preferably 0.01% by mass or less, more preferably 0.009% by mass or less, and further preferably 0.008% by mass or less. Further, Mg, Ca, Li, and REM are effective elements for making the sulfide compound inclusions such as MnS spherical and improving the deformability of the steel. Such an effect increases as the content thereof increases, but in order to effectively exhibit, Mg, Ca, Li and REM are preferably each 0.0001 mass% or more, more preferably 0.0005 mass% or more. is there. However, even if excessively contained, the effect is saturated and the effect commensurate with the content cannot be expected, so that Mg and Li are preferably 0.02% by mass or less, more preferably 0.018% by mass or less, and further preferably 0.015 mass% or less, Ca and REM are each preferably 0.05 mass% or less, more preferably 0.045 mass% or less, still more preferably 0.040 mass% or less. Each of N, Ca, Mg, Li and REM may be contained alone, or may be contained in two or more kinds, and the content in the case of containing two or more kinds is each within the above range. Any content may be used.

<2. Metallization>
The steel for cold working mechanical structures according to the embodiment of the present invention contains proeutectoid ferrite. The proeutectoid ferrite contributes to softening the steel after spheroidizing annealing. However, especially when Cr and Mo are contained, it is not possible to realize a steel which is excellent in spheroidization degree and can be sufficiently softened after a short time annealing by only including proeutectoid ferrite.
Therefore, as described in detail below, in the steel for machine work for cold working according to the embodiment of the present invention, the area ratio of proeutectoid ferrite is 30% or more and 70% or less, and the average grain size of ferrite crystal grains is It is controlled to be 5 to 15 μm.

[2-1. Area ratio of proeutectoid ferrite: 30% or more and 70% or less]
The presence of a large amount of proeutectoid ferrite can promote the agglomeration/spheroidization of carbides such as cementite during spheroidization annealing, and as a result, the spheroidization degree of steel can be improved and the hardness of steel can be reduced. From this point of view, the area ratio of proeutectoid ferrite needs to be 30% or more. The area ratio of proeutectoid ferrite is preferably more than 30%, more preferably more than 35%, and further preferably more than 40%. On the other hand, if a large amount of proeutectoid ferrite is made to exist, the manufacturing time increases. Considering a practical manufacturing time, the area ratio of proeutectoid ferrite needs to be suppressed to 70% or less.

[2-2. Average grain size of ferrite crystal grains: 5 to 15 μm]
By refining the average grain size of the ferrite crystal grains, it is possible to promote the agglomeration and spheroidization of carbides such as cementite after spheroidization annealing, and as a result, it is possible to improve the spheroidization degree and reduce the hardness of steel. realizable. From this viewpoint, it is necessary to control the average grain size of ferrite crystal grains to 15 μm or less. It is preferably 13 μm or less. On the other hand, if it is made too fine, the hardness will increase, so it is necessary to control to 5 μm or more. It is preferably at least 7 μm.
The term “ferrite crystal grain” as used herein means a boundary (also called a large-angle grain boundary) having a crystal orientation difference (oblique angle) of more than 15° as a crystal grain boundary as a result of backscattering electron diffraction image (Electron backscattering pattern; , A ferrite region surrounded by the crystal grain boundaries. Further, the average grain size referred to here is an average value of diameters when the area of the region surrounded by the crystal grain boundaries is converted into a circle, that is, an average equivalent circle diameter.
The average grain size of the ferrite crystal grains is measured by using, for example, a field emission high-resolution scanning electron microscope (FE-SEM) and an EBSP analyzer.

Further, the cold working machine structural steel according to the embodiment of the present invention may have any of the following metallographic structures as needed, whereby the properties of the steel after spheroidizing annealing are further improved. ..

[2-3. Ratio of area ratio of pearlite to total area ratio of structures other than proeutectoid ferrite: 80% or less]
From the viewpoint of improving the degree of spheroidization of steel after spheroidizing and annealing, it is effective to reduce the proportion of pearlite in the structure other than proeutectoid ferrite (hereinafter sometimes referred to as "residual structure"). If the proportion of pearlite in the balance structure is too high, rod-shaped carbides are likely to be present even after spheroidizing annealing, and the spheroidization degree of the steel is likely to deteriorate. The ratio of the area ratio of pearlite to the total area ratio of the structures other than the pro-eutectoid ferrite is preferably 80% or less, more preferably 70% or less.
Examples of the structure other than pearlite in the balance structure include bainite, martensite, austenite, etc., but all of bainite is more preferable for improving the spheroidization degree of steel. Specifically, when the area ratio of pearlite in the residual structure is 80% or less, the area ratio of bainite in the residual structure is more preferably 20% or more, and the area of pearlite in the residual structure is more preferably 20% or more. When the ratio of the ratio is 70% or less, the ratio of the area ratio of bainite in the balance structure is more preferably 30% or more.

<3. Hardness>
Further, the cold working machine structural steel according to the embodiment of the present invention may have any of the following hardness as necessary, thereby further improving the properties of the steel after spheroidizing annealing. To be done.

[3. Hardness is 300 HV or less]
In order to soften the steel after the spheroidizing annealing, it is effective to lower the hardness of the steel. Therefore, the hardness of the steel is set to HV350 or less, preferably HV300 or less. More preferably, it is HV290 or less.

<4. Manufacturing method>
In the method for producing a steel for cold working mechanical structure according to the embodiment of the present invention, a steel material satisfying the above-described chemical composition is used, and working and cooling after working are performed. The processing and the cooling after the processing are each performed in two stages.
Specifically, the method for manufacturing a cold working machine structural steel according to an embodiment of the present invention, a steel material having the above chemical composition,
(A) a step of performing pre-processing at a compression rate of 20% or more and a holding time of 10 seconds or less,
(B) after the step (a), a step of performing a finishing process at a temperature higher than 800° C. and lower than or equal to 1050° C. and a compression rate of 20% or higher;
(C) after the step (b), cooling to 750° C. or higher and 840° C. or lower in 10 seconds or less,
(D) After the step (c), cooling to 500° C. or lower at an average cooling rate of 0.1° C./second or more and less than 10° C./second is included.
Hereinafter, each step will be described in detail. The processing here may be any processing as long as the above-mentioned requirements are satisfied, and for example, press processing and rolling processing may be included therein. Further, the steps (c) and (d) may be referred to as first cooling and second cooling, respectively.

[(A) Step of performing pre-processing at a compression rate of 20% or more and a holding time of 10 seconds or less]
In order to increase the proportion of proeutectoid ferrite and refine the ferrite crystal grains, pre-processing with a compressibility of 20% or more is performed. The compression rate is preferably 30% or more. The compression rate is calculated as follows.
<Compression rate when press working (in this case, compression rate is also referred to as reduction rate)>
Compression rate (%)=(h1−h2)/h1×100
h1: height of steel before working, h2: height of steel after working <compressibility when a wire rod is obtained by rolling (in this case, compressibility is also referred to as surface reduction ratio)>
Compression rate (%)=(S1−S2)/S1×100
S1: cross-sectional area of steel before working, h2: cross-sectional area of steel after working, the temperature during pre-working may be relatively low due to an increase in the proportion of pro-eutectoid ferrite and refinement of ferrite crystal grains. preferable.
Further, the holding time from the pre-processing to the finishing processing needs to be relatively short in order to suppress the growth of ferrite crystal grains. Therefore, the holding time is 10 seconds or less, preferably 5 seconds or less.

[(B) Step (a) is followed by finishing at a temperature higher than 800°C and lower than or equal to 1050°C and a compression rate of 20% or higher]
In order to increase the proportion of proeutectoid ferrite and refine the ferrite crystal grains, finish processing is performed at a compression rate of 20% or more. The compression rate is preferably 50% or more. Further, the processing temperature is set to more than 800° C. and 1050° C. or less in order to set the average grain size of ferrite crystal grains to 5 to 15 μm. For refining the ferrite crystal grains, the temperature is preferably 1000° C. or lower, more preferably 950° C. or lower. On the other hand, in order to prevent the ferrite crystal grains from being excessively refined, the temperature is preferably 825°C or higher, and more preferably 850°C or higher.

[First cooling: (c) After step (b), a step of cooling from 750°C to 840°C in 10 seconds or less]
In order to increase the proportion of pro-eutectoid ferrite and to make the ferrite crystal grains finer, it is promptly cooled to a predetermined temperature (hereinafter sometimes referred to as a first cooling stop temperature) after finishing. The time for cooling from the finishing temperature to the first cooling stop temperature is 10 seconds or less. It is preferably 5 seconds or less, more preferably 3 seconds or less.
In order to increase the proportion of proeutectoid ferrite and set the average grain size of ferrite crystal grains to 5 to 15 μm, the first cooling stop temperature is set to 750° C. or higher and 840° C. or lower. In order to increase the proportion of proeutectoid ferrite, 775°C or higher is preferable. On the other hand, if the temperature is too high, the average grain size of ferrite crystal grains tends to increase, so 820° C. or less is preferable.

[Second cooling: (d) after step (c), cooling to 500° C. or lower at an average cooling rate of 0.1° C./sec or more and less than 10° C./sec]
In order to increase the proportion of proeutectoid ferrite, refine ferrite grains, reduce the proportion of pearlite in the remaining structure, and reduce hardness, the first cooling is performed at an average cooling rate of 0.1°C/sec or more and less than 10°C/sec. Cool from stop temperature to below 500°C. A preferable average cooling rate is 1 to 3° C./second.

After the step (d), the cooling method in the temperature range of 500° C. or lower is not particularly limited, and may be, for example, standing cooling, or the average cooling rate of the second cooling is relatively slow, for example, less than 1° C./second. In this case, gas quenching or the like may be used to shorten the time.

As described above, the cold working mechanical structure steel according to the embodiment of the present invention can be obtained. The steel for cold working mechanical structures according to the embodiment of the present invention is supposed to be subjected to spheroidizing annealing after that, but in some cases, other processing before or after spheroidizing annealing ( Wire drawing etc.) may be performed.
The steel for mechanical working for cold working according to the embodiment of the present invention is then subjected to spheroidizing annealing for a relatively short time (for example, conventional: spheroidizing annealing for about 15 hours is shortened to about 11 hours). Even in the case of being treated, the degree of spheroidization is excellent and the material can be sufficiently softened. Further, in the present invention, a steel wire can be manufactured by performing one or more steps of annealing, spheroidizing annealing, wire drawing, forging and quenching and tempering on the steel material obtained under the above manufacturing conditions. .. The steel wire here refers to a steel material obtained under the above manufacturing conditions, which is a linear steel material whose properties are adjusted by annealing, spheroidizing annealing, wire drawing, forging, quenching and tempering, etc. In addition to these processes, it also includes linear steel materials that have undergone the processes generally performed by secondary processing manufacturers.

Although the method for manufacturing the cold working machine structural steel according to the embodiment of the present invention has been described above, it is understood that the desired characteristics of the cold working mechanical structural steel according to the embodiment of the present invention are understood. There is a possibility that a method for manufacturing a mechanical working steel for cold working having desired properties according to an embodiment of the present invention by a trader through trial and error, and finding a method other than the above manufacturing method.

Hereinafter, the present invention will be described more specifically with reference to examples. The present invention is not limited to the following examples, and may be implemented with appropriate modifications within a range that can conform to the gist described above and below, and both of them are technical scopes of the present invention. Included in.

Using the steels having the chemical composition indicated by the steel types A and D in Table 1, test pieces for φ10 mm×15 mm for forming master were prepared. Using the produced test piece for processing master, press processing and cooling were performed by the processing master tester under the conditions shown in Table 2. Although not shown in Table 2, when cooling in the temperature range of 500° C. or lower, when the average cooling rate during the second cooling is 1° C./sec or more, the average cooling rate during the second cooling is around room temperature (25 C. to 40.degree. C.), and when the average cooling rate during the second cooling was less than 1.degree. C./sec, gas quenching was performed.

In Tables 1 and 2, and Tables 3 to 5 described later, the underlined numerical values indicate that the numerical values are out of the range of the embodiment of the present invention. In addition, the value calculated by the following formula (1) is described in the column of carbon equivalent of Table 1.

Carbon equivalent (Ceq)=[C]+[Si]/24+[Mn]/6+[Ni]/40+[Cr]/5+[Mo]/4+[V]/14 (1)

Here, [C], [Si], [Mn], [Ni], [Cr], [Mo], and [V] are C, Si, Mn, Ni, Cr, and Mo shown in mass %, respectively. And the contents of V are shown.

The test piece subjected to the thermo-mechanical treatment test was cut along the central axis and divided into four equal parts to obtain four samples including a longitudinal section. One of them was a sample that was not subjected to spheroidizing annealing (hereinafter sometimes referred to as a sample before spheroidizing annealing), and the other was a sample that was subjected to spheroidizing annealing (hereinafter referred to as a sample after spheroidizing annealing). Sometimes). The spheroidizing annealing was performed by placing each test piece in a vacuum sealed tube.
The spheroidizing annealing was performed under the following two conditions (SA1 and SA2).
SA1: After soaking at 760°C for 5 hours, cooled to 685°C at an average cooling rate of 13°C/hour, and then allowed to cool. SA2: After soaking at 2750°C for 2 hours, 660 at an average cooling rate of 13°C/hour. After cooling to SA, the spheroidizing annealing time in the prior art was reduced to about 11 hours, whereas the spheroidizing annealing time in the conventional technique was reduced to about 11 hours. The spheroidizing annealing time referred to here was the time obtained by adding the soaking and holding time and the cooling time until cooling. Further, the condition of SA2 is set at a low temperature in consideration of the delay of temperature follow-up compared with SA1.

For the sample before spheroidizing annealing, resin was embedded so that the longitudinal section could be observed, and (1) area ratio of proeutectoid ferrite, (2) average grain size of ferrite crystal grains, (3) total of structures other than proeutectoid ferrite The ratio of the area ratio of pearlite to the area ratio and (4) hardness before spheroidizing annealing were measured.
Also, for the sample after spheroidizing annealing, resin filling was performed in the same manner as described above so that the longitudinal section could be observed, and (5) hardness after spheroidizing annealing and (6) degree of spheroidizing were measured.
In each of the measurements (1) to (6), the diameter of the test piece was D, and the position of D/4 was measured from the side surface of the test piece toward the central axis.

(1) Measurement of area ratio of pro-eutectoid ferrite The structure of the longitudinal section of the sample before spheroidizing annealing was exposed by nital etching, and the D/4 position was magnified 400 times with an optical microscope (viewing area: width 220 μm× Photographs were taken at a height of 165 μm) and a magnification of 1000 (visual field area: horizontal 88 μm×length 66 μm). In the obtained photograph, 15 vertical lines at regular intervals and 10 horizontal lines at regular intervals were drawn in a grid pattern, and the number of pro-eutectoid ferrite present on 150 intersections was measured, and the number was 150. The value divided by was taken as the area ratio (%) of proeutectoid ferrite.
At this time, a sample having an average grain size of ferrite crystal grains to be described later of 10 μm or more was measured using a photograph of 400 times magnification, and a sample of less than 5 μm was measured using a photograph of 1000 times magnification, 5 μm or more and 10 μm or more. For samples of less than 100, a photograph of 400 times or 1000 times magnification was appropriately selected and measured.

(2) Measurement of average grain size of ferrite crystal grains The average grain size of ferrite crystal grains was measured using an FE-SEM and an EBSP analyzer.
A backscattered electron diffraction image was obtained by FE-SEM at the D/4 position of the longitudinal section of the sample before spheroidization annealing. In the obtained image, the EBSP analyzer is used to define the “grains” with the boundaries where the crystal orientation difference (oblique angle) exceeds 15°, that is, the large-angle grain boundaries as the grain boundaries, and The average particle size was determined. At that time, the measurement area was 200 μm×200 μm, and the measurement step was performed at 0.4 μm intervals, and the 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.

(3) Measurement of ratio of area ratio of pearlite to total area ratio of structures other than proeutectoid ferrite A vertical cross section of a sample before spheroidizing annealing was made to reveal a structure by nital etching, and a D/4 position was observed with an optical microscope. A photograph was taken at a magnification of 400 times (visual field area: 220 μm horizontal×165 μm vertical) and 1000 times (visual field area: 88 μm horizontal×66 μm vertical). With respect to the obtained photograph, 15 vertical lines at regular intervals and 10 horizontal lines at regular intervals were drawn in a grid pattern, and the score A of proeutectoid ferrite present at 150 intersections was measured. Next, the point B of pearlite existing on 150 intersections is measured, and the value obtained by dividing the point B by the point (150-A) is the area ratio of pearlite with respect to the total area ratio of the structures other than proeutectoid ferrite. (%).
At this time, a sample having an average grain size of ferrite crystal grains to be described later of 10 μm or more was measured using a photograph of 400 times magnification, and a sample of less than 5 μm was measured using a photograph of 1000 times magnification, 5 μm or more and 10 μm or more. For samples of less than 100, a photograph of 400 times or 1000 times magnification was appropriately selected and measured.

(4) Measurement of hardness before spheroidizing annealing A vertical section of the sample before spheroidizing annealing was measured at a D/4 position with a load of 1 kgf at 3 to 5 points using a Vickers hardness meter, and the average value (HV ) Was asked.

(5) Measurement of hardness after spheroidizing annealing A vertical section of the sample after spheroidizing annealing was measured at a D/4 position with a load of 1 kgf at 3 to 5 points using a Vickers hardness meter, and the average value (HV ) Was asked.

Since it is known that the hardness increases as the carbon equivalent of the steel type increases, the criterion for determining the hardness after spheroidizing annealing in this example is set according to the carbon equivalent (Ceq) of the steel type. Specifically, the hardness after SA1 was determined by whether or not the following formula (2) was satisfied.

(Hardness (HV)) <97.3×Ceq+84 (2)

The case where the hardness after SA1 satisfies the above formula (2) is the best (⊚), and the case where the hardness does not satisfy the above formula (2) is the poor (x).
In addition, when carbon equivalent is 0.70 or more, it is more preferable that the hardness after SA1 is HV150 or less.

Further, since SA2 is an annealing condition at a temperature lower than that of SA1 and less likely to be softened, the hardness after SA2 is set to a standard (gentle standard) different from the above formula (2). Specifically, the hardness after SA2 was determined by whether or not the following formula (3) was satisfied.

(Hardness (HV)) <97.3×Ceq+98 (3)

The case where the hardness after SA2 satisfies the above formula (3) is the best (⊚), and the case where the hardness does not satisfy the above formula (3) is the poor (x).
In addition, when the carbon equivalent is 0.70 or more, it is more preferable that the hardness after SA2 is HV165 or less.

(6) Measurement of degree of spheroidization The vertical cross section of the sample after spheroidization was exposed by nital etching to reveal the structure, and a magnification of 400 times was used at a D/4 position using an optical microscope (viewing area: width 220 μm×length). 165 μm). Regarding the observed image, the degree of spheroidization Nos. 1 to 3 was determined according to "degree of spheroidized structure" described in JIS G3509-2. The judgment was the best (⊚) when the degree of spheroidization was 1, the good (∘) when the degree was 2, and the bad (x) when the degree was 3.

Table 3 shows the structure and hardness before the spheroidizing annealing, and the hardness and the spheroidizing degree after the spheroidizing annealing, which were evaluated according to the above procedures (1) to (6). In the comprehensive evaluation after SA1, when the hardness and the degree of spheroidization after SA1 were all ⊚, it was the best (⊚). When ⊚ and ∘ coexisted, it was evaluated as good (∘). When even one x was mixed, it was determined to be defective (x).

In the results of Table 3, all of the remaining structure was bainite except for pearlite.

From the results of Table 3, the following can be considered. Test No. of Table 3 All of 1-1 to 1-4, 1-9 and 1-10 are examples satisfying all the requirements specified in the embodiment of the present invention, and after SA1 in which the spheroidizing annealing time is shortened as compared with the conventional case. In, the hardness and the degree of spheroidization were both good or the best. Test No. Test Nos. 1-1 and 1-2 are test Nos. Unlike 1-3 to 1-4, 1-9 and 1-10, the carbon content is in a preferable range (less than 0.40 mass%), and the average cooling rate during the second cooling is within a preferable range (1 -3°C/sec), and as a result, the preferred requirements (eutectoid ferrite area ratio of more than 40% and pearlite area ratio of the remaining structure of 80% or less) were satisfied, the spheroidization degree after SA1 was the best, and the overall judgment was made. Was the best.
On the other hand, the test No. Nos. 1-5 to 1-8 are examples that did not satisfy the requirements specified in the present invention, and the hardness or spheroidization degree after SA1 was poor.

Test No. Since No. 1-5 had a high finishing temperature of 1200° C., the average grain size of ferrite crystal grains exceeded 15 μm, and the spheroidization degree after SA1 was poor.

Test No. Since No. 1-6 had a low finishing temperature of 800° C., the average grain size of ferrite crystal grains was less than 5 μm, and the hardness after SA1 was poor.

Test No. In No. 1-7, the average cooling rate of the second cooling was as high as 10° C./second, so that the area ratio of proeutectoid ferrite was less than 30%, and the hardness after SA1 was poor.

Test No. In No. 1-8, the finishing temperature was as high as 1200° C., the area ratio of proeutectoid ferrite was less than 30%, the average grain size of ferrite crystal grains was more than 15 μm, and the hardness after SA1 was poor.

In addition, the test No. By satisfying all the requirements defined in the embodiment of the present invention, such as 1-1 to 1-4, 1-9, and 1-10, a low temperature is assumed in consideration of a delay in temperature follow-up as compared with SA1. It was found that even after SA2 in which the spheroidizing annealing was performed, the softening was sufficiently performed.

Using the steel having the chemical composition represented by the steel types B and C in Table 1, rolling and cooling were performed on the rolling line of the actual machine under the conditions of Table 4. In the actual rolling line, a heating furnace, a rough row rolling mill, an intermediate row rolling mill, an intermediate water cooling zone, a block mill rolling machine, a sizing mill rolling machine, a product water cooling zone, a cooling conveyor and a three-dimensional warehouse are connected in this order. The pre-processing was performed by a block mill rolling machine, the finishing processing was performed by a sizing mill rolling machine, and the first cooling and the second cooling were performed by a cooling conveyor. Although not shown in Table 4, the cooling in the temperature range of 500° C. or lower was performed at an average cooling rate during the second cooling up to about 400° C., and then allowed to cool. Samples were cut from the obtained rolled material, one of which was a sample that was not subjected to spheroidizing annealing, and the other was a sample that was subjected to spheroidizing annealing.
The spheroidizing annealing was performed under the following two conditions (SA3 and SA4). SA3 was set to a condition in which the spheroidizing annealing time in the prior art: about 15 hours was shortened to the spheroidizing annealing time: about 9 hours. Further, the condition of SA4 is set at a low temperature in consideration of a delay in temperature follow-up as compared with SA3.
SA3: After soaking and maintaining at 770° C. for 2 hours, it was cooled to 685° C. at an average cooling rate of 13° C./hour and then allowed to cool. SA4: After soaking and holding at 750° C. for 2 hours, an average cooling rate of 13° C./hour was 660. Cool to ℃, then let cool.

As in Example 1, (1) area ratio of pro-eutectoid ferrite, (2) average grain size of ferrite crystal grains, (3) ratio of area ratio of pearlite to total area ratio of structures other than pro-eutectoid ferrite, (4) Hardness before spheroidizing annealing, (5) Hardness after spheroidizing annealing, and (6) Spheroidizing degree were measured and evaluated. As for the determination of hardness after spheroidizing annealing, the hardness after SA3 satisfying the above formula (2) is the best (⊚), and the case not satisfying the above formula (2) is poor (x). ). Further, when the carbon equivalent is 0.70 or more, the hardness after SA3 is more preferably HV150 or less. Regarding the hardness after SA4, the case where the above formula (3) was satisfied was the best (⊚), and the case where the above formula (3) was not satisfied was the poor (x). In addition, when the carbon equivalent is 0.70 or more, the hardness after SA4 is more preferably HV165 or less.
The results are shown in Table 5.

In the results of Table 5, all of the remaining structure was bainite except for pearlite.

From the results of Table 5, the following can be considered. No. of Table 5 No. 2-2 is an example that satisfies all the requirements defined in the embodiment of the present invention, and the hardness and the degree of spheroidization after SA3 were both the best or the best.
On the other hand, No. In No. 2-1, the cooling stop temperature of the first cooling was more than 840°C, the area ratio of proeutectoid ferrite was less than 30%, and the average grain size of ferrite crystal grains was more than 15 µm. The degree of spheroidization was poor.

The steel for machine working for cold working according to the present invention is suitable as a raw material for various parts manufactured by cold working such as cold forging, cold forging or cold rolling. Although the form of steel is not particularly limited, it may be a rolled material such as a wire rod or a steel bar.
The components include, for example, automobile components and construction machinery components, and specifically, bolts, screws, nuts, sockets, ball joints, inner tubes, torsion bars, clutch cases, cages, housings, hubs, Covers, cases, washers, tappets, saddles, balgs, inner cases, clutches, sleeves, outer races, sprockets, stators, anvils, spiders, rocker arms, bodies, flanges, drums, joints, connectors, pulleys, metal fittings, yokes, It includes bases, valve lifters, spark plugs, pinion gears, steering shafts, and common rails. INDUSTRIAL APPLICABILITY The cold working machine structural steel according to the present invention is industrially useful as a machine structural steel suitably used as a raw material for the above-mentioned components, and after spheroidizing annealing, the above-mentioned various components in a room temperature and a process heat generation region. When it is manufactured, it has low deformation resistance and can exhibit excellent cold workability.

Claims (8)

C: 0.32-0.44 mass%,
Si: 0.15-0.35 mass%,
Mn: 0.55 to 0.95 mass%,
P: 0.030 mass% or less,
S: 0.030 mass% or less,
Cr: 0.85-1.25% by mass,
Mo: 0.15-0.35 mass%,
Al: 0.01 to 0.1% by mass,
The balance: consisting of iron and unavoidable impurities,
The area ratio of proeutectoid ferrite is 30% or more and 70% or less,
A steel for machine working for cold working, wherein an average grain size of ferrite crystal grains is 5 to 15 μm. The steel for cold working machine structural use according to claim 1, wherein the ratio of the area ratio of pearlite to the total area ratio of the structures other than the proeutectoid ferrite is 80% or less. The steel for cold working machine structural use according to claim 1 or 2, which has a hardness of HV 300 or less. Cu: 0.25 mass% or less (0 mass% is not included), and Ni: 0.25 mass% or less (0 mass% is not included) 1 or more types further selected from the group which consists. The steel for machine work for cold working according to any one of 1 to 3. Ti: 0.2% by mass or less (not including 0% by mass),
Nb: 0.2% by mass or less (not including 0% by mass), and V: 1.5% by mass or less (not including 0% by mass), further containing one or more selected from the group consisting of: The steel for machine work for cold working according to any one of to 4. N: 0.01% by mass or less (not including 0% by mass),
Mg: 0.02% by mass or less (not including 0% by mass),
Ca: 0.05% by mass or less (not including 0% by mass),
Li: 0.02% by mass or less (not including 0% by mass), and REM: 0.05% by mass or less (not including 0% by mass), at least one selected from the group consisting of 1. The steel for machine structural use for cold working according to any one of 5. Prepare steel of the chemical composition according to any one of claims 1 to 6,
(A) a step of performing pre-processing at a compression rate of 20% or more and a holding time of 10 seconds or less,
(B) after the step (a), a step of performing a finishing process at a temperature higher than 800° C. and lower than or equal to 1050° C. and a compression rate of 20% or higher;
(C) after the step (b), cooling to 750° C. or higher and 840° C. or lower in 10 seconds or less,
(D) After the step (c), a step of cooling to 500° C. or less at an average cooling rate of 0.1° C./second or more and less than 10° C./second, a method for producing a steel for cold working machine structural steel. A method for producing a steel wire, comprising performing at least one of annealing, spheroidizing annealing, wire drawing, forging and quenching and tempering on the steel for cold working machine structural steel produced by the method of claim 7.