JP7247078B2 - Mechanical structural steel for cold working and its manufacturing method - Google Patents

Description

TECHNICAL FIELD The present invention relates to steel for machine structural use for cold working and a method for producing the same.

BACKGROUND ART In manufacturing various parts such as automobile parts and construction machine parts, hot-rolled materials such as carbon steel or alloy steel are usually subjected to spheroidizing annealing for the purpose of imparting cold workability. After the spheroidizing annealing, the rolled material is cold-worked, then machined into a desired shape by machining such as cutting, and then quenched and tempered for final strength adjustment. .

In recent years, the conditions for spheroidizing annealing have been reviewed from the viewpoint of energy saving, and in particular, shortening of the spheroidizing annealing time is required. If the treatment time for spheroidizing annealing can be reduced by 20% to 30%, a corresponding reduction in energy consumption and CO ₂ emissions can be expected.

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

For example, in Patent Document 1, the chemical composition is, by mass ratio, C: 0.3 to 0.6%, Mn: 0.2 to 1.5%, Si: 0.05 to 2.0%, Cr: 0 .04 to 2.0%, the balance being composed of iron and unavoidable impurities, and 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 machine structural steel having excellent cold forgeability after annealing is disclosed. It is said that the steel for machine structural use can sufficiently ensure cold forgeability even with spheroidizing annealing for a relatively short time.

Patent No. 3783666

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

The present invention has been made in view of such circumstances, and one of its objects is to provide a spheroidizing method that contains Cr and Mo and is spheroidized even when 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 has excellent hardness and can be sufficiently softened, and which contains Cr and Mo and has a shortened treatment time for spheroidizing annealing. It is an object of the present invention to provide a method for producing a steel for machine structural use for cold working, which can be sufficiently softened even when the steel is used for cold working.

Aspect 1 of the present invention is
C: 0.32 to 0.44% by mass,
Si: 0.15 to 0.35% by 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,
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.

A second aspect of the present invention is the steel for cold working machine structural use according to the first aspect, wherein the ratio of the area ratio of pearlite to the total area ratio of the structures other than the pro-eutectoid ferrite is 80% or less.

Aspect 3 of the present invention is the steel for cold working machine structural use according to aspect 1 or 2, which has a hardness of HV300 or less.

Aspect 4 of the present invention is
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). 4. Steel for machine structural use for cold working according to any one of 3.

Aspect 5 of the present invention is
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). 5. Steel for machine structural use for cold working according to any one of 4.

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 (excluding 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). 6. The steel for machine structural use for cold working according to any one of 5.

Aspect 7 of the present invention prepares steel having the chemical composition according to any one of aspects 1 to 6,
(a) a step of pre-processing at a compression ratio of 20% or more and a holding time of 10 seconds or less;
(b) After the step (a), a step of finishing at a compression ratio of 20% or more at a compression ratio of 20% or more, and 800° C. or more and 1050° C. or less;
(c) a step of cooling to 750° C. or higher and 840° C. or lower in 10 seconds or less after the step (b);
(d) cooling to 500° C. or lower at an average cooling rate of 0.1° C./sec or more and less than 10° C./sec after the step (c); .

Aspect 8 of the present invention is a steel wire in which one or more steps of annealing, spheroidizing annealing, wire drawing, heading, and quenching and tempering are performed on the steel for cold working machine structural use manufactured by the method of aspect 7. manufacturing method.

In one embodiment of the present invention, a machine structure for cold working that contains Cr and Mo and is excellent in the degree of spheroidization and can be sufficiently softened even if the spheroidization annealing time is shortened than usual. A machine for cold working, which in another embodiment contains Cr and Mo, and which can be sufficiently softened even with a shortened treatment time for spheroidizing annealing. It is possible to provide a method of manufacturing structural steel.

The inventors of the present invention have developed a steel for machine structural use for cold working, which contains Cr and Mo, has an excellent degree of spheroidization, and can be sufficiently softened even when the spheroidization annealing time is shortened. In order to realize this, we examined it from various angles.

As a result, the chemical component composition including Cr and Mo is appropriately adjusted, the pro-eutectoid ferrite is included, and the area ratio of the pro-eutectoid ferrite and the average grain size of the ferrite crystal grains are controlled to be predetermined values. As a result, the inventors have found that it is possible to realize a steel for machine structural use for cold working that is excellent in the degree of spheroidization and can be sufficiently softened even if the treatment time for spheroidizing annealing is shortened.

Furthermore, by controlling the area ratio of pro-eutectoid ferrite and the average grain size of ferrite crystal grains, steel for machine structural use for cold working can be sufficiently softened even if the temperature during spheroidizing annealing varies. At the same time, I found that it is possible. This is very beneficial when the spheroidizing anneal is processed in a large furnace. That is, in a large furnace, the temperature varies considerably due to the presence of locations where the temperature is lower than the set temperature and locations where the temperature rise rate is delayed. It has been found that even if spheroidizing annealing is performed in such a furnace, the material can be sufficiently softened.

Details of each requirement defined by the embodiments of the present invention are shown below.
In this specification, the term “wire rod” is used to mean a rolled wire rod, and refers to a linear steel material that has been cooled to room temperature after hot rolling. The term "steel wire" refers to a linear steel material obtained by subjecting the rolled wire material to annealing or the like to adjust its properties.

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

(C: 0.32 to 0.44% by 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 the steel deteriorate. Therefore, the content of C is set to 0.32 to 0.44% by mass. In addition, it is preferable to set the C content to less than 0.40% by mass because a larger amount of pro-eutectoid ferrite can be precipitated.

(Si: 0.15 to 0.35% by mass)
Si is useful as a deoxidizing element and as an enhancing element that is included 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% by mass or more. On the other hand, if Si is contained excessively, the hardness of the steel excessively increases and the cold workability of the steel deteriorates. Therefore, the Si content is set to 0.35% by mass or less.

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

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

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

(Cr: 0.85% by mass or more and 1.25% by mass or less)
Cr is an effective element for 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 set to 0.85% by mass or more. Such an effect increases as the Cr content increases. However, if the Cr content is excessive, the strength becomes too high and the cold workability of the steel deteriorates, so the Cr content is made 1.25% by mass or less.

(Mo: 0.15% by mass or more and 0.35% by 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 steel together with Cr, the strength of the final product can be significantly increased. In order to effectively exhibit these effects, the Mo content is set to 0.15% by 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 the steel deteriorates. In particular, by including Mo in the steel together with Cr, the steel can be significantly less likely to be softened after spheroidizing annealing. Therefore, Mo should be 0.35% by mass or less.

(Al: 0.01% by mass or more and 0.1% by mass or less)
Al is an element that is useful as a deoxidizing agent and combines with N to precipitate AlN, thereby preventing abnormal growth of crystal grains during working and a decrease in strength. In order to effectively exhibit these effects, the Al content is set to 0.01% by mass or more, preferably 0.015% by mass or more, and more preferably 0.020% by mass or more. However, when the Al content becomes excessive, Al ₂ O ₃ is excessively generated and deteriorates the cold forgeability. Therefore, the Al content is set to 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 unavoidable impurities. As unavoidable impurities, contamination of elements (for example, B, As, Sn, Sb, Ca, O, H, etc.) brought in depending on the situation of raw materials, materials, manufacturing equipment, etc. is allowed.
For example, there are elements, such as P and S, whose content is generally preferably as low as possible and thus are unavoidable impurities, but whose composition range is separately defined as described above. For this reason, in this specification, the term "inevitable impurities" constituting the balance is a concept excluding elements whose composition range is separately defined.

Furthermore, the steel for cold working machine structural use according to the embodiment of the present invention may selectively contain the following optional elements as necessary, and the properties of the steel are further improved according to the contained elements. be.

(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 improve hardenability and effectively act to increase product strength. Such action increases as the content of these elements increases. Preferably, it is 0.10% by mass or more. However, if it is contained excessively, a supercooled structure is excessively formed, the strength becomes too high, and the cold forgeability deteriorates. Therefore, it is preferable that each of Cu and Ni is 0.25% by mass or less. It is more preferably 0.22% by mass or less, still more preferably 0.20% by mass or less. In addition, Cu and Ni may be contained alone, or may be contained in two or more kinds, and when two or more kinds are contained, the content may be any content within the above range. .

(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) not) one or more selected from the group consisting of)
Ti, Nb, and V are elements that combine with N to form compounds (nitrides), reduce the amount of dissolved N in the steel, and obtain the effect of reducing deformation resistance. In order to exhibit such effects, each of Ti, Nb and V is preferably 0.05% by mass or more, more preferably 0.06% by mass or more, and still more preferably 0.08% by mass or more. However, if these elements are contained excessively, the amount of nitride increases, the deformation resistance increases, and the cold forgeability deteriorates. .18% by mass or less, more preferably 0.15% by mass or less, and V is preferably 1.5% by mass or less, more preferably 1.3% by mass or less, and still more preferably 1.0% by mass or less. be. Incidentally, Ti, Nb and V may be contained alone, or may be contained in combination of two or more. OK.

(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 mass% (not including 0 mass%), and rare earth element (Rare Earth Metal: REM): selected from the group consisting of 0.05 mass% or less (not including 0 mass%) one or more)
N is an impurity that is inevitably contained in steel, but when solid solution N is contained in steel, it causes an increase in hardness and a decrease in ductility due to strain aging, and deteriorates cold forgeability. Therefore, N is preferably 0.01% by mass or less, more preferably 0.009% by mass or less, and even more preferably 0.008% by mass or less. Moreover, Mg, Ca, Li, and REM are elements effective in making sulfide-based inclusions such as MnS spheroidized and improving the deformability of steel. These actions increase as the content increases, but in order to exhibit them effectively, each of Mg, Ca, Li and REM is preferably 0.0001% by mass or more, more preferably 0.0005% by mass or more. be. However, even if it is contained excessively, the effect is saturated, and the effect corresponding to the content cannot be expected. 0.015% by mass or less, Ca and REM are each preferably 0.05% by mass or less, more preferably 0.045% by mass or less, still more preferably 0.040% by mass or less. Incidentally, N, Ca, Mg, Li and REM may be contained singly or in combination of two or more. Any content is acceptable.

<2. Metal structure>
A cold working machine structural steel according to an embodiment of the present invention includes proeutectoid ferrite. Proeutectoid ferrite contributes to softening of steel after spheroidizing annealing. However, when Cr and Mo are contained in particular, the mere inclusion of proeutectoid ferrite cannot achieve a steel that is excellent in the degree of spheroidization and can be sufficiently softened after short-time annealing.
Therefore, as described in detail below, the steel for machine structural use for cold working according to the embodiment of the present invention has an area ratio of proeutectoid ferrite of 30% or more and 70% or less, and an average grain size of ferrite crystal grains of 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 promotes aggregation and spheroidization of carbides such as cementite during spheroidizing annealing, and as a result, the degree of spheroidization of steel can be improved and the hardness of steel can be reduced. From this point of view, the area ratio of pro-eutectoid ferrite should be 30% or more. The area fraction of proeutectoid ferrite is preferably above 30%, more preferably above 35%, and even more preferably above 40%. On the other hand, if a large amount of pro-eutectoid ferrite is to be present, the manufacturing time increases. Considering a realistic production time, the area ratio of pro-eutectoid ferrite must be suppressed to 70% or less.

[2-2. Average grain size of ferrite crystal grains: 5 to 15 μm]
Refining the average grain size of ferrite crystal grains can promote the aggregation and spheroidization of cementite and other carbides after spheroidizing annealing, resulting in improved spheroidization and reduced hardness of steel. realizable. From this point of view, 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, excessive miniaturization causes an increase in hardness, so it is necessary to control the grain size to 5 μm or more. It is preferably 7 μm or more.
The ferrite crystal grains referred to here are defined as grain boundaries where the crystal orientation difference (oblique angle) exceeds 15° (also referred to as a large angle grain boundary) as a result of electron backscattering pattern (EBSP) analysis. , refers to the ferrite region surrounded by the grain boundaries. The term "average grain size" as used herein refers to the average value of diameters when the area of a region surrounded by grain boundaries is converted into a circle, that is, the average equivalent circle diameter.
The average grain size of ferrite crystal grains is measured using, for example, a field emission scanning electron microscope (FE-SEM) and an EBSP analyzer.

Furthermore, the steel for cold working machine structural use according to the embodiment of the present invention may have any of the following metal structures as necessary, which further improves the properties of the steel after spheroidizing annealing .

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

<3. Hardness>
Furthermore, the cold working machine structural steel according to the embodiments of the present invention may have any of the following hardnesses as required, which further improves the properties of the steel after spheroidizing annealing: be done.

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

<4. Manufacturing method>
In the method of manufacturing a steel for machine structural use for cold working according to an embodiment of the present invention, a steel material satisfying the chemical composition described above is used, and working and cooling after working are performed. Processing and cooling after processing are each performed in two stages.
Specifically, in the method of manufacturing a steel for machine structural use for cold working according to an embodiment of the present invention, a steel material having the above-described chemical composition is
(a) a step of pre-processing at a compression ratio of 20% or more and a holding time of 10 seconds or less;
(b) After the step (a), a step of finishing at a compression ratio of 20% or more at a compression ratio of 20% or more, and 800° C. or more and 1050° C. or less;
(c) a step of cooling to 750° C. or higher and 840° C. or lower in 10 seconds or less after the step (b);
(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.
Each step will be described in detail below. The processing referred to here may be any processing as long as it satisfies the above requirements, and may include, for example, press processing and rolling processing. Steps (c) and (d) may also be referred to as first cooling and second cooling, respectively.

[(a) Step of performing pre-processing with a compression rate of 20% or more and a holding time of 10 seconds or less]
In order to increase the proportion of pro-eutectoid ferrite and refine the ferrite crystal grains, pre-processing is performed with a compressibility of 20% or more. Preferably, the compressibility is 30% or more. Note that 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 ratio (%) = (h1-h2)/h1 x 100
h1: height of steel before working, h2: height of steel after working
Compression rate (%) = (S1-S2)/S1 x 100
S1: cross-sectional area of steel before working, h2: cross-sectional area of steel after working The temperature during pre-working is relatively low due to an increase in the proportion of pro-eutectoid ferrite and refinement of ferrite grains. preferable.
In addition, the holding time from pre-machining to finish-machining must be relatively short in order to suppress the growth of ferrite crystal grains. Therefore, the holding time should be 10 seconds or less, preferably 5 seconds or less.

[(b) After the step (a), the step of finishing at a compression rate of 20% or more at a compression rate of 20% or more]
In order to increase the proportion of pro-eutectoid ferrite and refine the ferrite crystal grains, finish machining is performed at a compression ratio of 20% or more. Preferably, the compressibility is 50% or more. In addition, the processing temperature should be more than 800° C. and not more than 1050° C. in order to make the average grain size of ferrite crystal grains 5 to 15 μm. For refining 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 excessive refinement of ferrite crystal grains, the temperature is preferably 825° C. or higher, more preferably 850° C. or higher.

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

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

After the step (d), the cooling method in the temperature range of 500 ° C. or less is not particularly limited. In some cases, rapid gas cooling or the like may be used to shorten the time.

As described above, the steel for machine structural use for cold working according to the embodiment of the present invention can be obtained. It is assumed that the cold working machine structural steel according to the embodiment of the present invention is then subjected to spheroidizing annealing, but in some cases other processing ( wire drawing, etc.) may be applied.
The cold work machine structural steel according to embodiments of the present invention is then subjected to a relatively shortened spheroidizing anneal (e.g. conventional: about 11 hours of spheroidizing compared to about 15 hours). Even in the case of being spheroidized, it can be sufficiently softened. In the present invention, a steel wire can be produced by subjecting the steel material obtained under the above production conditions to one or more steps of annealing, spheroidizing annealing, wire drawing, heading, and quenching and tempering. . The steel wire here refers to a wire-shaped steel material obtained by subjecting the steel material obtained under the above manufacturing conditions to annealing, spheroidizing annealing, wire drawing, heading, quenching and tempering, etc. to adjust the properties. In addition to the above processes, it also includes linear steel materials that have undergone processes generally performed by secondary processing manufacturers.

As described above, the method of manufacturing the cold-work machine structural steel according to the embodiment of the present invention has been described. Through trial and error, a trader may find a method other than the method of manufacture described above to produce a cold work machine structural steel having the desired properties according to embodiments of the present invention.

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. The present invention is not limited by the following examples, and can be implemented with appropriate modifications within the scope that can match the spirit described above and below. subsumed in

Using the steels having the chemical compositions shown in Steel Types A and D in Table 1, φ10 mm × 15 mm test pieces for working formers were produced. Press working and cooling were performed by a working former tester under the conditions shown in Table 2 using the prepared test piece for working former. Although not listed in Table 2, cooling in the temperature range of 500 ° C. or less is performed when the average cooling rate during the second cooling is 1 ° C./sec or more, and the average cooling rate during the second cooling is around room temperature (25 to 40° C.), and when the average cooling rate during the second cooling was less than 1° C./sec, gas quenching was adopted.

In Tables 1 and 2, and Tables 3-5 below, underlined numbers indicate values outside the scope of embodiments of the present invention. In addition, in the column of carbon equivalent in Table 1, the value calculated by the following formula (1) is described.

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 respectively C, Si, Mn, Ni, Cr, Mo and V content.

The test piece subjected to the thermomechanical test was cut along the central axis and divided into four equal parts to obtain four samples including longitudinal sections. One of them is a sample not subjected to spheroidizing annealing (hereinafter sometimes referred to as a sample before spheroidizing annealing), and the other is a sample subjected to spheroidizing annealing (hereinafter referred to as a sample after spheroidizing annealing). (sometimes). The spheroidizing annealing was carried out by placing each test piece in a vacuum sealed tube.
Spheroidizing annealing was performed under the following two conditions (SA1 and SA2).
SA1: After soaking at 760°C for 5 hours, cool to 685°C at an average cooling rate of 13°C/hour, then allow to cool SA2: After soaking at 750°C for 2 hours, 660 at an average cooling rate of 13°C/hour ℃ and then allowed to cool SA1 was set to a condition in which the spheroidizing annealing time was shortened from about 15 hours in the conventional technology to about 11 hours. The spheroidizing annealing time referred to here was the sum of the soaking holding time and the cooling time until cooling. Also, SA2 was set to be performed at a low temperature assuming a delay in temperature follow-up compared to SA1.

The sample before spheroidizing annealing was embedded in resin so that the longitudinal section could be observed, and (1) the area ratio of pro-eutectoid ferrite, (2) the average grain size of ferrite crystal grains, and (3) the total of structures other than pro-eutectoid ferrite. The ratio of the area ratio of pearlite to the area ratio and (4) the hardness before spheroidizing annealing were measured.
In addition, the samples after spheroidizing annealing were also embedded in resin in the same manner as described above so that the vertical cross section could be observed, and (5) the hardness after spheroidizing annealing and (6) the degree of spheroidization were measured.
For any of the measurements (1) to (6), the diameter of the test piece is D, and the position of D/4 from the side of the test piece toward the central axis was measured.

(1) Measurement of area ratio of pro-eutectoid ferrite For the longitudinal section of the sample before spheroidizing annealing, the structure was revealed by nital etching, and the D/4 position was observed with an optical microscope at a magnification of 400 times (field of view: horizontal 220 μm × 165 μm in height) and 1000× (viewing area: 88 μm in width×66 μm in height). On the obtained photograph, 15 equally spaced vertical lines and 10 equally spaced horizontal lines are drawn in a grid pattern, and the number of proeutectoid ferrite present on 150 intersections is measured. The area ratio (%) of the pro-eutectoid ferrite was obtained by dividing the value by .
At this time, samples with an average grain size of ferrite crystal grains of 10 μm or more, which will be described later, are measured using a photograph at a magnification of 400 times, and samples with an average grain size of less than 5 μm are measured using a photograph at a magnification of 1000 times. Samples with less than 100% magnification were measured by appropriately selecting either a 400-fold or 1000-fold photograph.

(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 spheroidizing annealing. In the obtained image, using an EBSP analyzer, a boundary where the crystal orientation difference (oblique angle) exceeds 15°, that is, a large-angle grain boundary is defined as a “crystal grain”, and a “crystal grain” is defined as a crystal grain boundary. Average particle size was determined. At that time, the measurement area was set to 200 μm×200 μm, and the measurement step was performed at intervals of 0.4 μm. Measurement points with a confidence index of 0.1 or less, which indicates the reliability of the measurement direction, were deleted from the analysis target.

(3) Measurement of the ratio of the area ratio of pearlite to the total area ratio of structures other than pro-eutectoid ferrite For the longitudinal section of the sample before spheroidizing annealing, the structure was revealed by nital etching, and the D / 4 position was observed with an optical microscope. Photographs were taken at a magnification of 400 (field of view area: 220 µm horizontal × 165 µm vertical) and 1000 × (field of view area: 88 µm horizontal × 66 µm vertical). On the obtained photograph, 15 equally spaced vertical lines and 10 equally spaced horizontal lines were drawn in a grid pattern, and the number A of proeutectoid ferrite existing on 150 intersections was measured. Next, the score B of pearlite present on 150 intersections is measured, and the value obtained by dividing the score B by the score (150-A) is the area ratio of pearlite with respect to the total area ratio of the structure other than pro-eutectoid ferrite. The ratio (%) of
At this time, samples with an average grain size of ferrite crystal grains of 10 μm or more, which will be described later, are measured using a photograph at a magnification of 400 times, and samples with an average grain size of less than 5 μm are measured using a photograph at a magnification of 1000 times. Samples with less than 100% magnification were measured by appropriately selecting either a 400-fold or 1000-fold photograph.

(4) Measurement of hardness before spheroidizing annealing For the longitudinal section of the sample before spheroidizing annealing, a Vickers hardness tester was used to measure 3 to 5 points with a load of 1 kgf at the D/4 position, and the average value (HV ).

(5) Measurement of hardness after spheroidizing annealing For the longitudinal section of the sample after spheroidizing annealing, using a Vickers hardness tester, 3 to 5 points were measured at the D/4 position with a load of 1 kgf, and the average value (HV ).

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

(Hardness (HV)) < 97.3 x Ceq + 84 (2)

The case where the hardness after SA1 satisfied the above formula (2) was rated as the best (⊚), and the case where the above formula (2) was not satisfied was rated as poor (x).
When the carbon equivalent is 0.70 or more, it is more preferable that the hardness after SA1 is HV 150 or less.

In addition, since SA2 is an annealing condition in which softening is less likely to occur at a lower temperature than in SA1, a standard (loose standard) different from the above formula (2) was set for the hardness after SA2. Specifically, the hardness after SA2 was determined based on whether or not the following formula (3) was satisfied.

(Hardness (HV)) < 97.3 x Ceq + 98 (3)

The case where the hardness after SA2 satisfies the above formula (3) was rated as the best (⊚), and the case where the above formula (3) was not satisfied was rated as poor (x).
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 For the longitudinal section of the sample after spheroidization annealing, the structure was revealed by nital etching, and an optical microscope was used at the D/4 position at a magnification of 400 times (field of view: horizontal 220 μm × vertical 165 μm). For the observed images, the degree of spheroidization Nos. 1 to 3 was determined according to the "degree of spheroidized structure" described in JISG3509-2. When the degree of spheroidization was No. 1, it was judged as the best (⊚).

Table 3 shows the structure and hardness before spheroidizing annealing and the hardness and degree of spheroidization after spheroidizing annealing, which were evaluated according to the above procedures (1) to (6). Regarding the overall judgment after SA1, the hardness and the degree of spheroidization after SA1 were judged to be the best (⊚) when they were all ⊙. When both ⊚ and ∘ coexist, it was rated as good (∘). When even one x was mixed, it was determined as defective (x).

In the results shown in Table 3, all of the residual structures other than pearlite were bainite.

From the results of Table 3, it can be considered as follows. Test No. in Table 3. 1-1 to 1-4, 1-9 and 1-10 are all examples that satisfy all the requirements defined in the embodiments of the present invention, and after SA1 in which the spheroidizing annealing time is shorter than before , both the hardness and the degree of spheroidization were good or best. Especially test no. 1-1 and 1-2 are Test Nos. Unlike 1-3 to 1-4, 1-9 and 1-10, the carbon content is in the preferred range (less than 0.40% by mass), and the average cooling rate during the second cooling is in the preferred range (1 ~ 3 ° C./sec), and as a result, the preferable requirements (pro-eutectoid ferrite area ratio of more than 40% and pearlite area ratio of the remaining structure of 80% or less) were satisfied, so the degree of spheroidization after SA1 was the best, and comprehensive judgment was made. was the best in
On the other hand, Test No. in Table 3. 1-5 to 1-8 are examples that do not satisfy the requirements defined in the present invention, and the hardness or degree of spheroidization after SA1 was poor.

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

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

Test no. In No. 1-7, since the average cooling rate of the second cooling was as high as 10° C./sec, the area ratio of pro-eutectoid 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, Test No. in Table 3. By satisfying all the requirements defined in the embodiments of the present invention such as 1-1 to 1-4, 1-9 and 1-10, the low temperature It was found that the steel was sufficiently softened even after SA2 in which spheroidizing annealing was performed at .

Using the steels having the chemical composition shown in steel grades B and C in Table 1, rolling and cooling were performed on an actual rolling line under the conditions shown in Table 4. In the actual rolling line, the heating furnace, roughing rolling mill, intermediate rolling mill, intermediate water cooling zone, block mill rolling mill, sizing mill rolling mill, product water cooling zone, cooling conveyor and multilevel warehouse are connected in this order. Pre-processing was performed by a block mill rolling mill, finishing processing was performed by a sizing mill rolling mill, and first cooling and second cooling were performed by a cooling conveyor. Although not shown in Table 4, cooling in the temperature range of 500°C or less was performed at the average cooling rate during the second cooling until about 400°C, and then allowed to cool. Samples were cut from the obtained rolled material, one of which was a sample not subjected to spheroidizing annealing, and the other was a sample subjected to spheroidizing annealing.
Spheroidizing annealing was performed under the following two conditions (SA3 and SA4). In SA3, the spheroidizing annealing time was shortened from about 15 hours in the prior art to about 9 hours. SA4 was set to be performed at a low temperature assuming a delay in temperature follow-up compared to SA3.
SA3: After soaking at 770°C for 2 hours, cool to 685°C at an average cooling rate of 13°C/hour, then allow to cool SA4: After soaking at 750°C for 2 hours, 660 at an average cooling rate of 13°C/hour Cool to °C and then stand to cool.

As in Example 1, (1) the area ratio of pro-eutectoid ferrite, (2) the average grain size of ferrite crystal grains, (3) the ratio of the area ratio of pearlite to the total area ratio of structures other than pro-eutectoid ferrite, (4) Hardness before spheroidizing annealing, (5) hardness after spheroidizing annealing, and (6) degree of spheroidization were measured and evaluated. As for the hardness after spheroidizing annealing, the hardness after SA3 is best when the above formula (2) is satisfied (⊚), and when it does not satisfy the above formula (2), it is bad (× ). Further, when the carbon equivalent is 0.70 or more, it is more preferable that the hardness after SA3 is HV150 or less. Regarding the hardness after SA4, the case where the above formula (3) was satisfied was rated as the best (⊚), and the case where the above formula (3) was not satisfied was rated as poor (x). When the carbon equivalent is 0.70 or more, it is more preferable that the hardness after SA4 is HV165 or less.
Table 5 shows the results.

In the results shown in Table 5, all of the residual structures other than pearlite were bainite.

From the results of Table 5, it can be considered as follows. No. in Table 5. All of 2-2 are examples that satisfy all the requirements defined in the embodiments of the present invention, and the hardness and degree of spheroidization after SA3 were both the best or best.
On the other hand, No. in Table 5. In 2-1, the cooling stop temperature of the first cooling is over 840 ° C., the area ratio of pro-eutectoid ferrite is less than 30%, the average grain size of ferrite crystal grains is over 15 μm, and the hardness after SA3 and The degree of spheroidization was poor.

The steel for cold working machine structural use according to the present invention is suitable as a material for various parts manufactured by cold working such as cold forging, cold heading, or cold rolling. Although the form of the steel is not particularly limited, it can be a rolled material such as a wire rod or a steel bar.
The parts include, for example, parts for automobiles and parts for construction machinery. Specifically, bolts, screws, nuts, sockets, ball joints, inner tubes, torsion bars, clutch cases, cages, housings, hubs, Covers, cases, receiving washers, tappets, saddles, bulks, inner cases, clutches, sleeves, outer races, sprockets, stators, anvils, spiders, rocker arms, bodies, flanges, drums, joints, connectors, pulleys, metal fittings, yokes, Bases, valve lifters, spark plugs, pinion gears, steering shafts and common rails are included. The steel for machine structural use for cold working according to the present invention is industrially useful as a steel for machine structural use that is suitably used as a raw material for the above parts. When it is manufactured in a low deformation resistance, it can exhibit excellent cold workability.

Claims (8)

C: 0.32 to 0.44% by mass,
Si: 0.15 to 0.35% by 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,
balance: consisting of iron and unavoidable impurities,
The area ratio of proeutectoid ferrite is more than 35% and 70% or less,
Machine structural steel for cold working in which the average grain size of ferrite grains is 5 to 15 μm. 2. 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 structures other than proeutectoid ferrite is 80% or less. 3. The steel for cold working machine structural use according to claim 1 or 2, having a hardness of HV300 or less. 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). 4. Steel for machine structural use 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). 5. Steel for machine structural use for cold working according to any one of -4. N: 0.01% by mass or less (not including 0% by mass),
Mg: 0.02% by mass or less (excluding 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). 6. Steel for machine structural use for cold working according to any one of 5. Prepare steel having the chemical composition according to any one of claims 1 to 6,
(a) a step of pre-processing at a compression ratio of 20% or more and a holding time of 10 seconds or less;
(b) After the step (a), a step of finishing at a compression ratio of 20% or more at a compression ratio of 20% or more, and 800° C. or more and 1050° C. or less;
(c) a step of cooling to 750° C. or higher and 840° C. or lower in 10 seconds or less after the step (b);
(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 , according to any one of claims 1 to 6. of steel for cold working machine structural use. A method of manufacturing a steel wire, wherein the steel for machine structural use for cold working manufactured by the method of claim 7 is subjected to one or more steps of annealing, spheroidizing annealing, wire drawing, heading and quenching and tempering.