KR20210113668A - Machine structural steel for cold working and manufacturing method thereof - Google Patents

Abstract

C: 0.32 to 0.44 mass%, Si: 0.15 to 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 to 1.25 mass%, Mo: 0.15 to 0.35 mass %, Al: 0.01 to 0.1 mass %, balance: iron and unavoidable impurities, the area ratio of proeutectoid ferrite is 30% or more and 70% or less, and the average grain size of ferrite grains is 5 to 15 µm for cold working machine structure it's a river

Description

Machine structural steel for cold working and manufacturing method thereof

The present disclosure relates to a steel for machine structural use for cold working and a method for manufacturing the same.

In manufacturing various parts, such as parts for automobiles and construction machines, spheroidizing annealing is usually performed for the purpose of providing cold workability to hot-rolled materials, such as carbon steel or alloy steel. Then, the rolled material after the spheroidizing annealing is subjected to cold working, and then formed into a predetermined shape by machining such as cutting, followed by quenching and tempering, and final strength adjustment is performed.

In recent years, from a viewpoint of energy saving, the conditions of a spheroidizing annealing are reviewed, and especially shortening of a spheroidizing annealing is calculated|required. If the treatment time of the spheroidizing annealing can be reduced by 20 to 30%, reduction in _{energy consumption and CO 2 emission can be expected accordingly.}

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

For example, in Patent Document 1, the chemical composition is, in terms of 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%, balance: iron and unavoidable impurities. A steel for mechanical structure excellent in cold forgeability after spheroidization annealing is disclosed, wherein the average particle diameter of prior austenite in the metal structure is 100 μm or more, and the ferrite fraction is 20% or less. It is said that cold forgeability of the said steel for machine structure can fully be ensured even by a comparatively short-time spheroidizing annealing.

Japanese Patent No. 3783666

However, although Cr is contained in the steel for mechanical structure described in patent document 1, Mo is not contained as an essential component. Since the strength of steel can be significantly increased by including Cr and Mo together, such an increase in strength cannot be expected in the steel of Patent Document 1. In addition, in steel containing both Cr and Mo, it may be difficult to soften after spheroidizing annealing. not.

The embodiment of the present invention has been made in view of such a situation, and one of its objects is that it contains Cr and Mo, and even when spheroidizing annealing is performed for a relatively short time, the spheroidization degree is excellent, and it can be sufficiently softened. It is to provide a steel for mechanical structure for cold working that can .

Aspect 1 of the present invention is,

C: 0.32 to 0.44 mass %;

Si: 0.15-0.35 mass %;

Mn: 0.55-0.95 mass %;

P: 0.030 mass % or less;

S: 0.030 mass % or less;

Cr: 0.85 to 1.25 mass%;

Mo: 0.15-0.35 mass %;

Al: 0.01 to 0.1 mass%;

balance: consisting of iron and inevitable impurities,

The area ratio of proeutectoid ferrite is 30% or more and 70% or less,

It is a steel for cold working mechanical structure with an average grain size of ferrite grains of 5 to 15 μm.

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

Aspect 3 of this invention is the steel for machine structural use for cold working as described in aspect 1 or 2 whose hardness is HV300 or less.

Aspect 4 of the present invention,

Cu: 0.25 mass % or less (0 mass % is not included), and

Ni: 0.25 mass % or less (0 mass % is not included) It is the steel for machine structure for cold working in any one of aspects 1-3 which further contains 1 or more types selected from the group which consists of Ni: 0.25 mass % or less.

Aspect 5 of the present invention,

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 not included) It is the steel for machine structural use for cold working in any one of aspects 1-4 further containing 1 or more types selected from the group.

Aspect 6 of the present invention is,

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 or less (not including 0% by mass), and

REM: 0.05 mass % or less (0 mass % is not included) It is the steel for machine structural use for cold working in any one of aspects 1-5 which further contains 1 or more types selected from the group.

Aspect 7 of the present invention prepares a steel having the chemical composition according to any one of aspects 1 to 6,

(a) performing pre-processing with a compression ratio of 20% or more and a holding time of 10 seconds or less;

(b) after the step (a), a step of performing finish processing at a compression ratio of 20% or more at a compression ratio of more than 800°C and 1050°C or less;

(c) after the step (b), a step of cooling to 750 ° C. or more and 840 ° C. or less in 10 seconds or less;

(d) after the step (c), cooling to 500° C. or less at an average cooling rate of 0.1° C./sec or more and less than 10° C./sec.

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

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

The inventors of the present application studied from various angles in order to realize a steel for machine structure for cold working that contains Cr and Mo, and which is excellent in the degree of spheroidization even if the spheroidization annealing time is shortened than usual and can be sufficiently softened.

As a result, by appropriately adjusting the chemical composition containing Cr and Mo, and controlling so that the area ratio of proeutectoid ferrite and the average grain size of ferrite grains containing pro-eutectoid ferrite to a predetermined value, Even if it shortened processing time, it was excellent in a spheroidization degree, and discovered that the steel for machine structural use for cold working which can fully soften is realizable.

Furthermore, it was also discovered that, by controlling the area ratio of proeutectoid ferrite and the average grain size of ferrite grains, it was possible to realize a steel for cold working that can be sufficiently softened even if there is a temperature variation during spheroidizing annealing. This becomes very advantageous when spheroidizing annealing is processed in a large furnace. That is, in a large furnace, the temperature is considerably uneven due to the presence of a place having a temperature lower than the set temperature or a place having a slow temperature increase rate. Even if spheroidizing annealing was performed, it discovered that it could soften enough.

Below, the detail of each requirement which embodiment of this invention prescribes|regulates is shown.

In addition, in this specification, a "wire" is used in the meaning of a rolled wire, and refers to the linear steel material cooled to room temperature after hot rolling. In addition, a "steel wire" refers to the linear steel material which gave annealing etc. to the said rolled wire material, and adjusted the characteristic.

<1. Chemical Composition>

The steel for machine structural use for cold working according to an embodiment of the present invention, C: 0.32 to 0.44 mass%, Si: 0.15 to 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 to 1.25 mass%, Mo: 0.15 to 0.35 mass%, Al: 0.01 to 0.1 mass%, and the remainder: iron and unavoidable impurities.

Hereinafter, each element is described in detail.

(C: 0.32 to 0.44 mass %)

C is a strength imparting element, and if it is less than 0.32 mass %, the required strength of the final product cannot be obtained. On the other hand, when it exceeds 0.44 mass %, the cold workability and toughness of steel will fall. Therefore, the content of C is set to 0.32 to 0.44 mass%. Moreover, since more proeutectoid ferrite can be precipitated by making content of C less than 0.40 mass %, it is preferable.

(Si: 0.15-0.35 mass %)

Si is useful as a deoxidizing element and as an improving element to be contained for the purpose of increasing the strength of a final product by solid solution hardening. In order to exhibit such an effect effectively, Si content shall be 0.15 mass % or more. On the other hand, when Si is contained excessively, hardness will rise excessively and cold workability of steel will deteriorate. Therefore, Si content shall be 0.35 mass % or less.

(Mn: 0.55-0.95 mass %)

Mn is an effective element for increasing the strength of the final product through improvement of hardenability. In order to exhibit such an effect effectively, Mn content shall be 0.55 mass % or more. On the other hand, when Mn is contained excessively, hardness will rise and the cold workability of steel will deteriorate. Therefore, Mn content shall be 0.95 mass % or less.

(P: 0.030 mass% or less)

P is an element unavoidably contained in steel, causes grain boundary segregation in steel, and causes deterioration of ductility of steel. Therefore, P content shall be 0.030 mass % or less.

(S: 0.030 mass % or less)

S is an element unavoidably contained in steel, exists as MnS in steel and deteriorates ductility of steel, and is therefore a harmful element that deteriorates cold workability of steel. Therefore, S content shall be 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. In order to exhibit such an effect effectively, Cr content shall be 0.85 mass % or more. Such an effect increases as the Cr content increases. However, when the Cr content becomes excessive, the strength becomes too high to deteriorate the cold workability of the steel, so it is set to 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. In particular, by incorporating Mo into the steel together with Cr, the strength of the final product can be significantly increased. In order to exhibit such an effect effectively, Mo content shall be 0.15 mass % or more. Such an effect becomes large as Mo content increases. However, when Mo content becomes excessive, intensity|strength will become high too much and cold workability of steel will deteriorate. In particular, by containing Mo together with Cr in the steel, it may become difficult for the steel to be remarkably softened after spheroidizing annealing. Therefore, Mo shall be 0.35 mass % or less.

(Al: 0.01% by mass or more and 0.1% by mass or less)

Al is useful as a deoxidizer, and is an element that binds with N to precipitate AlN, and prevents the crystal grains from abnormally growing during processing and lowering the strength. In order to exhibit such an effect effectively, Al content is made into 0.01 mass % or more, Preferably it is 0.015 mass % or more, More preferably, it is 0.020 mass % or more. However, if the Al content is excessive, Al ₂ O ₃ is generated excessively deteriorates cold monotonicity. Therefore, Al content shall be 0.1 % mass or less, Preferably it is 0.090 mass % or less, More preferably, it is 0.080 mass % or less.

The remainder is iron and unavoidable impurities. As an unavoidable impurity, incorporation of elements (for example, B, As, Sn, Sb, Ca, O, H, etc.) brought in is allowed depending on the conditions of raw materials, materials, manufacturing facilities, and the like.

On the other hand, for example, there are elements such as P and S, which are usually preferable as their content is small, and therefore are unavoidable impurities, but whose composition range is separately prescribed as described above. For this reason, in this specification, in the case of "unavoidable impurity" constituting the remainder, it is a concept excluding the element whose composition range is separately prescribed.

Furthermore, the steel for machine structural use for cold working which concerns on embodiment of this invention may selectively contain the following arbitrary elements as needed, and the characteristic of steel improves further according to the component contained.

(Cu: 0.25% by mass or less (0 mass% is not included), and Ni: 0.25% by mass or less (at least one selected from the group consisting of 0% by mass)

Cu and Ni are elements which act effectively in improving hardenability and raising product strength. Although this effect increases as the content of these elements increases, in order to effectively exhibit Cu and Ni, each of Cu and Ni is preferably 0.05 mass% or more, more preferably 0.08 mass% or more, still more preferably 0.10 mass% or more. More than that. However, when it contains excessively, a supercooled structure will generate|occur|produce excessively, intensity|strength will become high too much, and cold forgeability will fall. Therefore, it is preferable that Cu and Ni shall each be 0.25 mass % or less. More preferably, it is 0.22 mass % or less, More preferably, it is 0.20 mass % or less. In addition, Cu and Ni may be contained individually, respectively, may be made to contain 2 or more types, and content in the case of containing 2 or more types should just be content arbitrary in the said range, respectively, respectively.

(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 not included) At least one 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 solid solution N in steel, and obtain the effect of reducing deformation resistance. In order to exhibit such an effect, Ti, Nb, and V are each, Preferably it is 0.05 mass % or more, More preferably, it is 0.06 mass % or more, More preferably, it is 0.08 mass % or more. However, when these elements are contained excessively, the amount of nitride increases, deformation resistance rises, and cold forgeability deteriorates. Therefore, Ti and Nb are each preferably 0.2% by mass or less, more preferably 0.18% by mass or less, More preferably, it is 0.15 mass % or less, V becomes like this. Preferably it is 1.5 mass % or less, More preferably, it is 1.3 mass % or less, More preferably, it is 1.0 mass % or less. In addition, Ti, Nb, and V may be contained individually, respectively, may be made to contain 2 or more types, and content in the case of containing 2 or more types should just be content arbitrary in the said range, respectively, respectively.

(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 mass % (not including 0 mass %), and Rare Earth Metal (REM): 0.05 mass % or less (at least one selected from the group consisting of 0 mass %)

N is an impurity that is unavoidably contained in steel, but when solid solution N is contained in steel, hardness increases due to strain aging and ductility decrease, and cold forgeability deteriorates. Therefore, 0.01 mass % or less of N is preferable, More preferably, it is 0.009 mass % or less, More preferably, it is 0.008 mass % or less. In addition, Mg, Ca, Li, and REM are elements effective for spheroidizing sulfide compound-type inclusions, such as MnS, and improving the deformability of steel. Although this effect increases as the content increases, in order to effectively exhibit Mg, Ca, Li and REM, each of Mg, Ca, Li and REM is preferably 0.0001 mass% or more, more preferably 0.0005 mass% or more. However, even if it contains excessively, the effect is saturated, and since the effect suitable for content cannot be expected, Mg and Li are each preferably 0.02 mass% or less, more preferably 0.018 mass% or less, still more preferably 0.015 mass% or less. , Ca and REM are each preferably 0.05 mass% or less, more preferably 0.045 mass% or less, and still more preferably 0.040 mass% or less. In addition, N, Ca, Mg, Li, and REM may be contained individually, respectively, may contain 2 or more types, and in the case of containing 2 or more types, each content may be arbitrary content in the said range, respectively.

<2. metal structure>

The steel for machine structural use for cold working according to an embodiment of the present invention contains proeutectoid ferrite. Proeutectoid ferrite contributes to softening of steel after spheroidizing annealing. However, especially when Cr and Mo are contained, steel which is excellent in the degree of spheroidization and can be sufficiently softened cannot be realized simply by including proeutectoid ferrite after short-time annealing.

Therefore, as described in detail below, in the steel for cold working machine structure 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 the ferrite grains is controlled to be 5 to 15 µm, have.

[2-1. Area ratio of proeutectoid ferrite: 30% or more and 70% or less]

By presenting a large amount of proeutectoid ferrite, aggregation and spheroidization of carbides such as cementite can be promoted during spheroidizing annealing, and as a result, the spheroidization degree of steel can be improved and the hardness of steel can be reduced. From this viewpoint, the area ratio of proeutectoid ferrite needs to be 30% or more. The area fraction of proeutectoid ferrite is preferably greater than 30%, more preferably greater than 35%, still more preferably greater than 40%. On the other hand, if a large amount of proeutectoid ferrite is to be present, the manufacturing time increases. Considering a realistic manufacturing time, it is necessary to suppress the area ratio of proeutectoid ferrite to 70% or less.

[2-2. Average particle size of ferrite grains: 5 to 15 μm]

By refining the average grain size of the ferrite crystal grains, aggregation and spheroidization of carbides such as cementite after spheroidization annealing can be promoted, and as a result, the spheroidization degree improvement and hardness reduction of steel can be realized. From such a viewpoint, it is necessary to control the average grain size of the ferrite grains to be 15 µm or less. Preferably it is 13 micrometers or less. On the other hand, in order to cause an increase in hardness when refinement|miniaturization too much, it is necessary to control to 5 micrometers or more. Preferably it is 7 micrometers or more.

The ferrite grains used herein refer to a boundary (also called a diagonal grain boundary) in which the crystal orientation difference (oblique angle) exceeds 15° as a crystal grain boundary as a result of a backscattering electron diffraction image (EBSP) analysis, and the A ferrite region surrounded by grain boundaries. In addition, the average particle diameter here means the average value of the diameter when the area of the area|region enclosed by the crystal grain boundary is converted into a circle, ie, average circle equivalent diameter.

The average particle diameter of the ferrite grains is measured using, for example, a field emission scanning electron microscope (FE-SEM) and an EBSP analyzer.

Furthermore, the steel for machine structural use for cold working which concerns on embodiment of this invention may have the following arbitrary metal structures as needed, whereby the characteristic of the steel after spheroidization annealing is further improved.

[2-3. The ratio of the area ratio of pearlite to the total area ratio of structures other than proeutectoid ferrite: 80% or less]

From the viewpoint of further improving the spheroidization degree of steel after spheroidization annealing, it is effective to reduce the proportion of pearlite in structures other than proeutectoid ferrite (hereinafter, sometimes referred to as "residual structures"). When there are too many ratios of pearlite in a residual structure, even after spheroidization annealing, rod-shaped carbide tends to exist and the spheroidization degree of steel tends to worsen. Preferably, the ratio of the area ratio of pearlite to the total area ratio of structures other than proeutectoid ferrite is 80% or less, and more preferably 70% or less.

Although bainite, martensite, austenite, etc. are mentioned as a structure other than pearlite in a remainder structure|tissue, When all are bainite, when improving the spheroidization degree of steel, it is more preferable. Specifically, when the ratio of the area ratio of pearlite in the remainder structure is 80% or less, it is more preferable that the ratio of the area ratio of bainite in the remainder structure is 20% or more, and the ratio of the area ratio of pearlite in the remainder structure is 70% When it is below, it is more preferable that the ratio of the area ratio of bainite in the remainder structure is 30 % or more.

<3. Hardness>

Furthermore, the steel for machine structural use for cold working which concerns on embodiment of this invention may have the following arbitrary hardnesses as needed, whereby the characteristic of the steel after spheroidization annealing is further improved.

[3. Hardness less than 300HV]

In achieving softening of steel after spheroidization annealing, it is effective to lower|hang the hardness of steel. Therefore, the hardness of steel is made into HV350 or less, Preferably it is made into HV300 or less. More preferably, it is HV290 or less.

<4. Manufacturing method>

In the manufacturing method of the steel for machine structural use for cold working which concerns on embodiment of this invention, processing and cooling after processing are performed using the steel material which satisfy|fills the above-mentioned chemical composition composition. Processing and cooling after processing are performed in two steps, respectively.

Specifically, the manufacturing method of the steel for machine structural use for cold working according to an embodiment of the present invention comprises:

(a) a step of performing pre-processing with a compression ratio of 20% or more and a holding time of 10 seconds or less;

(b) after the step (a), a step of performing finish processing at a compression ratio of 20% or more at a compression ratio of more than 800°C and 1050°C or less;

(c) after the step (b), a step of cooling to 750 ° C. or more and 840 ° C. or less 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./sec or more and less than 10° C./sec.

Hereinafter, each process is described in detail. In addition, the processing mentioned here may be arbitrary processing as long as the above-mentioned requirements are satisfied, For example, press working and rolling processing may be included in this. In addition, processes (c) and (d) may be respectively called 1st cooling and 2nd cooling.

[(a) A step of pre-processing with a compression ratio 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 grains, pre-processing with a compression ratio of 20% or more is performed. Preferably, the compression ratio is 30% or more. On the other hand, the compression ratio is calculated as follows.

<Compression rate in the case of performing press working (in this case, the compression rate is also referred to as a reduction rate)>

Compression rate (%)=(h1-h2)/h1×100

h1: height of steel before machining, h2: height of steel after machining

<Compression ratio in the case of obtaining a wire rod by rolling processing (in this case, the compression ratio is also referred to as area reduction ratio)>

Compression rate (%)=(S1-S2)/S1×100

S1: cross-sectional area of steel before machining, h2: cross-sectional area of steel after machining

The temperature at the time of pre-processing is preferably relatively low in order to increase the proportion of proeutectoid ferrite and to refine the ferrite grains.

In addition, the holding time from the pre-processing to the finishing process 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) After the step (a), the step of performing finish processing at a compression ratio of 20% or more at more than 800°C and 1050°C or less]

In order to increase the proportion of proeutectoid ferrite and to refine the ferrite grains, finish processing is performed at a compression ratio of 20% or more. Preferably, the compression ratio is 50% or more. In addition, the processing temperature is made into more than 800 degreeC and 1050 degrees C or less in order to make the average particle diameter of a ferrite crystal grain 5-15 micrometers. In order to refine|miniaturize a ferrite crystal grain, 1000 degrees C or less is preferable, and 950 degrees C or less is more preferable. On the other hand, in order to prevent excessive refinement|miniaturization of a ferrite crystal grain, 825 degreeC or more is preferable, and 850 degreeC or more is more preferable.

[First cooling: (c) After step (b), a step of cooling to 750°C or higher and 840°C or lower in 10 seconds or less]

In order to increase the proportion of proeutectoid ferrite and refine the ferrite grains, after finishing processing, it is rapidly cooled to a predetermined temperature (hereinafter, sometimes referred to as a first cooling stop temperature). The cooling time from the finishing temperature to the first cooling stop temperature is set to 10 seconds or less. Preferably it is 5 seconds or less, More preferably, it is set as 3 seconds or less.

In order to increase the proportion of proeutectoid ferrite and make the average grain size of ferrite grains 5 to 15 µm, the first cooling stop temperature is set to 750°C or more and 840°C or less. In order to increase the proportion of proeutectoid ferrite, 775° C. or higher is preferable. On the other hand, since the average particle diameter of a ferrite crystal grain becomes large easily when temperature is too high, 820 degrees C or less is preferable.

[Second cooling: (d) step (c), followed by cooling at an average cooling rate of 0.1°C/sec or more and less than 10°C/sec to 500°C or less]

Cooling from the first cooling stop temperature to 500°C or less 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 residual structure, and reduce hardness do. As a preferable average cooling rate, it is 1-3 degreeC/sec.

After the step (d), the cooling method in the temperature range of 500° C. or less is not particularly limited, for example, standing to cool may be used, or the average cooling rate of the second cooling is, for example, less than 1° C./sec. When it is slow, gas quenching etc. may be sufficient for time shortening.

As described above, the steel for cold working machine structure according to the embodiment of the present invention can be obtained. The steel for machine structural use for cold working according to the embodiment of the present invention is assumed to be subjected to spheroidizing annealing thereafter. In some cases, other processing (drawing processing, etc.) may be performed before spheroidizing annealing or after spheroidizing annealing.

The steel for machine structural use for cold working according to the embodiment of the present invention is then subjected to a spheroidizing annealing with relatively reduced time (for example, conventionally: spheroidizing annealing reduced to about 11 hours from about 15 hours) , it is excellent in spheroidization degree, and can be softened sufficiently. Further, in the embodiment of the present invention, a steel wire can be manufactured by performing one or more steps of annealing, spheroidizing annealing, wire drawing, rolling, and quenching and tempering on the steel material obtained under the above manufacturing conditions. The term "steel wire" as used herein refers to a linear steel material in which properties are adjusted by performing annealing, spheroidizing annealing, wire drawing, rolling, quenching and tempering, etc. on steel obtained under the above manufacturing conditions. , also includes linear steel materials that have undergone a process generally performed by secondary processing makers.

As mentioned above, although the manufacturing method of the steel for machine structural use for cold working which concerns on embodiment of this invention was described, those skilled in the art who understood the desired characteristic of the steel for machine structural use for cold working which concerns on embodiment of this invention made trial and error, The embodiment of this invention As a method of manufacturing steel for cold working machine structural use having desired properties according to

Example

Hereinafter, an Example is given and embodiment of this invention is described more concretely. The embodiments of the present invention are not limited by the following examples, and can be implemented with appropriate changes within a range that is consistent with the gist of the above and below, and they are all technical aspects of the embodiments of the present invention. included in the scope.

Example One

A test piece for a working former having a diameter of 10 mm x 15 mm was produced using steel having a chemical composition shown by the steel types A and D in Table 1. Press working and cooling were performed with the processed former tester under the conditions shown in Table 2 using the produced test piece for processed formers. Although not described in Table 2, cooling in a temperature range of 500° C. or less is performed near room temperature (25° C. to 40 degreeC), and when the average cooling rate at the time of 2nd cooling was less than 1 degreeC/sec, it was set as gas rapid cooling.

In Tables 1 and 2, and Tables 3 to 5 to be described later, the underlined numerical values indicate a departure from the scope of the embodiment of the present invention. In addition, in the column of carbon equivalent of Table 1, the value calculated by following formula (1) was described.

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

where [C], [Si], [Mn], [Ni], [Cr], [Mo] and [V] are, respectively, C, Si, Mn, Ni, Cr, Mo and The content of V is shown.

The test piece subjected to the working heat treatment test was cut along the central axis and divided into quarters to obtain four samples including longitudinal sections. One of them is a sample not subjected to spheroidizing annealing (hereinafter, it may be called a sample before spheroidizing annealing), and the other is a sample subjected to spheroidizing annealing (hereinafter, it is called a sample after spheroidizing annealing). did with The spheroidizing annealing was performed by putting the test piece into a vacuum sealed tube, respectively.

Spheroidizing annealing was performed under the following two conditions (SA1 and SA2).

SA1: After holding the crack at 760°C for 5 hours, it is cooled to 685°C at an average cooling rate of 13°C/hour, and then left to cool

SA2: After maintaining the crack at 750°C for 2 hours, it is cooled to 660°C at an average cooling rate of 13°C/hour, and then left to cool

SA1 was set as the condition shortened to spheroidizing annealing time: about 11 hours with respect to the spheroidizing annealing time in a prior art: about 15 hours. In addition, the spheroidizing annealing time here was made into the time which added all the cooling time until the crack holding time and standing to cool. In addition, compared with SA1, SA2 was made into the conditions performed at low temperature, supposing the delay of temperature tracking.

For the sample before spheroidizing annealing, resin embedding so that the longitudinal section can be observed, (1) area ratio of proeutectoid ferrite, (2) average grain size of ferrite grains, (3) area ratio of the total of structures other than proeutectoid ferrite The ratio of the area ratio of pearlite and the hardness before (4) spheroidizing annealing were measured.

In addition, the sample after spheroidization annealing was also embedded in resin so that a longitudinal cross-section could be observed similarly to the above, and the hardness and (6) spheroidization degree after (5) spheroidization annealing were measured.

In any of the measurements (1) to (6), the diameter of the test piece was set to 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 proeutectoid ferrite

With respect to the longitudinal section of the sample before spheroidization annealing, the tissue appeared by nital etching, and the D/4 position was viewed under an optical microscope at 400 times magnification (viewing area: 220 µm in width × 165 µm in length) and 1000 times (viewing area: 88 µm in width × 66 μm in length). With respect to the obtained photograph, 15 equally spaced vertical lines and 10 equally spaced horizontal lines are drawn in a lattice form, the scores of proeutectoid ferrite present on 150 intersections are measured, and the value obtained by dividing the score by 150 is the proeutectoid ferrite. It was set as the area ratio (%).

At this time, for a sample having an average particle diameter of ferrite grains of 10 μm or more, which will be described later, it is measured using a 400-times magnification photograph, and for a sample less than 5 μm, it is measured using a 1000-fold magnification photograph, and for a sample of 5 μm or more and less than 10 μm, Any photograph with a magnification of 400 times or 1000 times was appropriately selected and measured.

(2) Measurement of average grain size of ferrite grains

The average particle diameter of the ferrite crystal grains was measured using FE-SEM and EBSP analyzers.

With respect to the D/4 position of the longitudinal section of the sample before spheroidizing annealing, a backscattered electron diffraction image was obtained by FE-SEM. In the obtained image, using the EBSP analyzer, the boundary where the crystal orientation difference (oblique angle) exceeds 15°, that is, the diagonal grain boundary is used as the grain boundary to define “crystal grains”, and the average grain size of the grains in ferrite is determined decided. At that time, the measurement area was 200 µm × 200 µm, and the measurement step was performed at 0.4 µm intervals, and measurement points having a Confidence Index of 0.1 or less indicating the reliability of the measurement orientation 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 proeutectoid ferrite

With respect to the longitudinal section of the sample before spheroidization annealing, the tissue appeared by nital etching, and the D/4 position was viewed under an optical microscope at 400 times magnification (viewing area: 220 µm in width × 165 µm in length) and 1000 times (viewing area: 88 µm in width × 66 μm in length). With respect to the obtained photograph, 15 equally spaced vertical lines and 10 equally spaced horizontal lines were drawn in a grid shape, and the score A of proeutectoid ferrite present on 150 intersections was measured. Next, the score B of the pearlite present on 150 intersections is measured, and the value obtained by dividing the score B by the score (150-A) is the ratio of the area ratio of pearlite to the area ratio of the sum of tissues other than proeutectoid ferrite ( %).

At this time, for a sample having an average particle diameter of ferrite grains of 10 μm or more, which will be described later, it is measured using a 400-times magnification photograph, and for a sample less than 5 μm, it is measured using a 1000-fold magnification photograph, and for a sample of 5 μm or more and less than 10 μm, the magnification is used. Any photograph of 400 times or 1000 times was appropriately selected and measured.

(4) Measurement of hardness before spheroidizing annealing

About the longitudinal cross section of the sample before spheroidization annealing, using a Vickers hardness tester, 3-5 points|pieces were measured at D/4 position with a load of 1 kgf, and the average value (HV) was calculated|required.

(5) Measurement of hardness after spheroidizing annealing

About the longitudinal cross section of the sample after spheroidization annealing, using a Vickers hardness tester, 3-5 points|pieces were measured with the load of 1 kgf at D/4 position, and the average value (HV) was calculated|required.

Since it is known that hardness increases so that the carbon equivalent of a steel type is large, the determination criterion of hardness after spheroidizing annealing of a present Example was set according to the carbon equivalent (Ceq) of a steel type. Specifically, the hardness after SA1 was determined by whether the following formula (2) was satisfied.

(Hardness (HV)) < 97.3×Ceq+84 ...(2)

The case where the hardness after SA1 satisfies the formula (2) was considered the most favorable (double-circle), and the case where the said formula (2) was not satisfied was made into bad (x).

On the other hand, when the carbon equivalent is 0.70 or more, it is more preferable that the hardness after SA1 is HV150 or less.

In addition, since SA2 is an annealing condition in which it is difficult to soften at low temperature than SA1, about the hardness after SA2, the reference|standard (gentle standard) different from said Formula (2) was set. Specifically, the hardness after SA2 was determined by whether the following formula (3) was satisfied.

(Hardness (HV)) < 97.3×Ceq+98 ...(3)

The case where the hardness after SA2 satisfies the formula (3) was considered the most favorable (double-circle), and the case where the said formula (3) was not satisfied was made into bad (x).

On the other hand, 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 nodularity

After the spheroidization annealing, the tissue was formed by nital etching on the longitudinal section of the sample, and was observed at D/4 position using an optical microscope at a magnification of 400 times (viewing area: 220 µm in width × 165 µm in length). About the observed image, according to "the degree of spheroidization structure|tissue" described in JISG3509-2, the spheroidization degree 1-3 was determined. Judgment was made into the best (double-circle) when the spheroidization degree was No. 1, it was made into good (circle) when it was No. 2, and it was made into bad (x) when it was No. 3.

Table 3 shows the structure and hardness before spheroidization annealing, and the hardness and spheroidization degree after spheroidization annealing, evaluated in the manner of (1) to (6) above. On the other hand, about the comprehensive judgment after SA1, in the hardness and spheroidization degree after SA1, when all were (double-circle), it was set as the most favorable (double-circle). When (double-circle) and (circle) were mixed, it was set as favorable (○). When even one x was mixed, it was set as defective (x).

In the results of Table 3, all of the remaining structures were bainite except for pearlite.

From the results of Table 3, it can be considered as follows. Test No. in Table 3. All of 1-1 to 1-4, 1-9, and 1-10 are examples satisfying all of the requirements prescribed in the embodiment of the present invention, and after SA1 in which the spheroidization annealing time is shorter than before, hardness and spheroidization degree were all good or the best. In particular, test No. 1-1 to 1-2 are test No. 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 at the time of the second cooling is in a preferable range (1 to 3°C). /sec), and as a result, desirable requirements (proeutectoid ferrite area ratio more than 40% and residual structure pearlite area ratio 80% or less) were satisfied, so the spheroidization degree after SA1 is the best, and the best in overall judgment became

On the other hand, test No. in Table 3 1-5 to 1-8 are examples which do not satisfy|fill the requirements prescribed|regulated by embodiment of this invention, and the hardness or spheroidization degree after SA1 was unsatisfactory.

test no. In 1-5, since the finishing temperature was high as 1200 degreeC, the average particle diameter of a ferrite crystal grain became more than 15 micrometers, and the spheroidization degree after SA1 was unsatisfactory.

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

test no. In 1-7, since the average cooling rate of the second cooling was as fast as 10°C/sec, the area ratio of proeutectoid ferrite was less than 30%, and the hardness after SA1 was poor.

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

In addition, test No. in Table 3 By satisfying all of the requirements prescribed in the embodiment of the present invention such as 1-1 to 1-4, 1-9 and 1-10, spheroidizing annealing at low temperature assuming a delay in temperature tracking compared to SA1 It was found that even after SA2 was sufficiently softened.

Example 2

Using the steel of the chemical composition shown by the steel types B and C of Table 1, rolling processing and cooling were performed on the conditions of Table 4 in the rolling line of an actual machine. On the other hand, in the rolling line of the actual machine, a heating furnace, a bathing mill, an intermediate heat rolling mill, an intermediate water cooling zone, a block mill rolling mill, a sizing mill rolling mill, a product water cooling zone, a cooling conveyor, and a three-dimensional warehouse are in this order was connected to, the pre-processing was performed by a block mill rolling mill, the finishing processing was performed by a sizing mill rolling mill, and the first cooling and the second cooling were performed by a cooling conveyor. Although not shown in Table 4, cooling in the temperature range of 500 degrees C or less was cooled at the average cooling rate at the time of 2nd cooling up to about 400 degreeC, and it was set to stand to cool after that. A sample was cut out from the obtained rolling material, one of them was set as the sample which did not perform spheroidizing annealing, and the other was set as the sample which gave the spheroidizing annealing.

Spheroidizing annealing was performed under the following two conditions (SA3 and SA4). SA3 was made under the condition that the spheroidizing annealing time: about 15 hours in the prior art was shortened to spheroidizing annealing time: about 9 hours. In addition, compared with SA3, SA4 was made into the conditions performed at low temperature, supposing the delay of temperature tracking.

SA3: After maintaining the crack at 770°C for 2 hours, it is cooled to 685°C at an average cooling rate of 13°C/hour, and then left to cool

SA4: After maintaining the crack at 750°C for 2 hours, it is cooled to 660°C at an average cooling rate of 13°C/hour, and then left to cool.

As in Example 1, (1) area ratio of proeutectoid ferrite, (2) average grain size of ferrite grains, (3) ratio of area ratio of pearlite to the total area ratio of structures other than proeutectoid ferrite, (4) spheroidization The hardness before annealing, (5) spheroidization hardness after annealing, and (6) spheroidization degree were measured and evaluated. On the other hand, as judgment of hardness after spheroidizing annealing, with respect to hardness after SA3, the case where the above formula (2) is satisfied is regarded as the best (double-circle), and the case where the above expression (2) is not satisfied is regarded as bad (x). did. Moreover, when the carbon equivalent is 0.70 or more, it is more preferable that the hardness after SA3 is HV150 or less. With respect to the hardness after SA4, the case where the formula (3) was satisfied was regarded as the most favorable (double-circle), and the case where the expression (3) was not satisfied was regarded as poor (x). On the other hand, when the carbon equivalent is 0.70 or more, it is more preferable that the hardness after SA4 is HV165 or less.

A result is shown in Table 5.

In the results of Table 5, all of the remaining structures were bainite except for pearlite.

From the results of Table 5, it can be considered as follows. Table 5 No. All of 2-2 are examples which satisfy|fill all the requirements prescribed|regulated by embodiment of this invention, and the hardness and spheroidization degree after SA3 were all the best or favorable.

On the other hand, in Table 5, No. In 2-1, the cooling stop temperature of the first cooling is more than 840 ° C., the area ratio of proeutectoid ferrite is less than 30%, and the average grain size of ferrite grains is more than 15 μm, and the hardness and spheroidization degree after SA3 are poor. It was.

The steel for machine structural use for cold working according to an embodiment of the present invention is suitable for a raw material of various parts manufactured by cold working such as cold forging, cold forming, or cold rolling. Although the form of the steel is not specifically limited, For example, it can be set as rolled materials, such as a wire rod or a steel bar.

The parts include, for example, automobile parts and construction machine parts, specifically, bolts, screws, nuts, sockets, ball joints, inner tubes, torsion bars, clutch cases, cages, housings, hubs, Cover, case, cradle, tappet, saddle, valve, inner case, clutch, sleeve, outer race, sprocket, stator, anvil, spider, rocker arm, body, flange, drum, joint, connector, pulley, bracket, These include yokes, mouthpieces, valve lifters, spark plugs, pinion gears, steering shafts and common rails. The steel for machine structural use for cold working according to an embodiment of the present invention is industrially useful as a steel for mechanical structural use suitably used as a material for the parts, and after spheroidizing and annealing, when manufactured into the various parts at room temperature and heat generation region , the deformation resistance is low, and excellent cold workability can be exhibited.

This application is accompanied by a priority claim based on the Japanese Patent Application, Japanese Patent Application No. 2019-016219, filed on January 31, 2019, and Japanese Patent Application, Japanese Patent Application, and Japanese Patent Application, Japanese Patent Application, No. 2019-211181, dated November 22, 2019 as basic applications. Japanese Patent Application No. 2019-016219 and Japanese Patent Application No. 2019-211181 are incorporated herein by reference.

Claims (7)

C: 0.32 to 0.44 mass %;
Si: 0.15-0.35 mass %;
Mn: 0.55-0.95 mass %;
P: 0.030 mass % or less;
S: 0.030 mass % or less;
Cr: 0.85 to 1.25 mass%;
Mo: 0.15-0.35 mass %;
Al: 0.01 to 0.1 mass%;
balance: consisting of iron and inevitable impurities,
The area ratio of proeutectoid ferrite is more than 35% and not more than 70%,
Steel for cold working mechanical structure with an average grain size of ferrite grains of 5 to 15 μm. The method of claim 1,
The steel for cold working, wherein the ratio of the area ratio of pearlite to the total area ratio of structures other than the proeutectoid ferrite is 80% or less. The method of claim 1,
Mechanical structural steel for cold working, with a hardness of HV300 or less. 4. The method according to any one of claims 1 to 3,
Steel for cold working machine structure further comprising at least one of the following (a) to (c).
(a) at least one selected from the group consisting of Cu: 0.25 mass% or less (0 mass% is not included), and Ni: 0.25 mass% or less (0 mass% is not included).
(b) 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 not included) ) at least one selected from the group consisting of
(c) 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 mass % or less (0 mass % is not included), and REM: 0.05 mass % or less (0 mass % is not included) at least one selected from the group consisting of Preparing a steel of the chemical composition according to claim 1,
(a) a step of pre-processing with a compression ratio of 20% or more and a holding time of 10 seconds or less;
(b) after the step (a), a step of performing finish processing at a compression ratio of 20% or more at more than 800°C and 1050°C or less;
(c) after the step (b), a step of cooling to 750 ° C. or more and 840 ° C. or less in 10 seconds or less;
(d) after the step (c), cooling to 500° C. or less at an average cooling rate of 0.1° C./sec or more and less than 10° C./sec. Prepare a steel of the chemical composition according to claim 4,
(a) a step of pre-processing with a compression ratio of 20% or more and a holding time of 10 seconds or less;
(b) after the step (a), a step of performing finish processing at a compression ratio of 20% or more at more than 800°C and 1050°C or less;
(c) after the step (b), a step of cooling to 750 ° C. or more and 840 ° C. or less in 10 seconds or less;
(d) after the step (c), cooling to 500° C. or less at an average cooling rate of 0.1° C./sec or more and less than 10° C./sec. A method for manufacturing a steel wire in which at least one of annealing, spheroidizing annealing, wire drawing, rolling, and quenching and tempering is performed on the steel for cold working machine structure manufactured by the method of claim 5 or 6.