JPH11106864A - Steel for machine structure excellent cold-workability and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a steel for machine structure having cold-workability equal to that of a low carbon steel and having characteristics of levels of medium and high carbon steels by heat treatment. SOLUTION: This steel for machine structure has the compsn. contg., by weight, 0.3 to 0.8% C, >0.15 to <0.30% Si, 0.05 to 0.6% Mn, 0.01 to 0.5% W, 0.005 to 0.10% Al, 0 to 0.5% Cu, 0 to 0.5% Ni, 0 to 0.2% Cr, 0 to 0.3% Mo, 0 to 0.05% Nb, 0 to 0.2% V, 0 to 0.2% Ti, 0 to 0.2% Zr, 0 to 0.5% Pb, 0 to 0.5% Bi, 0 to 0.1% Te, 0 to 0.1% Se, 0 to 0.01% Ca, <=0.03% P, <=0.05% S and the balance Fe with impurities, and has the structure composed of ferrite of 5 to 10 JIS size number by grain size, graphite having <=20 μm maximum diameter and cementite and having <=120 HV hardness. The steel is produced by applying hot working, cold working at 5 to 50% reduction of area, and annealing at 650 to 720 deg.C for 5 to 20 hr.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel material for machine structure having excellent cold workability and a method for producing the same.
The present invention relates to a steel material for machine structure which is excellent in cold workability and is used after being formed into various machine structure parts by cold working such as drawing, forging and rolling, and then subjected to heat treatment, and a method for producing the same. More specifically, among JIS carbon steel materials for machine structural use, S2
It has a cold workability of the spheroidized annealing material level of 0C, and
After the so-called "tempering treatment" of quenching and tempering, or after induction hardening as a surface hardening treatment, it is used for mechanical structures having medium- and high-carbon steel material level properties (strength, toughness, wear resistance, etc.) such as S35C to S58C. The present invention relates to a steel material and a method for manufacturing the same.

[0002]

2. Description of the Related Art In many cases, parts for machine structural use are manufactured by performing mechanical processing after hot forging, forming and then performing heat treatment such as quenching and tempering.

[0003] On the other hand, a method of working a steel material which is cold forged or rolled has a higher production efficiency, a higher material yield, and a higher dimensional accuracy than cutting. For this reason, from the viewpoint of cost rationalization and productivity improvement, in recent years, there has been an increasing demand for parts production by cold forming such as cold forging.

However, "cold forming" is an extremely intense working method. For this reason, high performance of tools and lubrication is strictly required. Further, regarding the workability of the workpiece, it is necessary that the deformation resistance is small and the deformability is large. Therefore, the development of a material steel excellent in cold workability is required.

Generally, as the C (carbon) content of steel increases, the deformation resistance increases and the deformability also deteriorates. Therefore, conventionally, most of cold working has been applied to steel materials having a relatively low C content and having a C content of 0.25% by weight or less. In addition, even in the case of steel having a C content of about 0.20% by weight, spheroidizing annealing may be performed before working in order to ensure cold workability.

[0006] Material steel which is subjected to heat treatment such as quenching and tempering after working to obtain high strength usually contains 0.3% by weight or more of C. In this case, it is necessary to perform spheroidizing annealing before working in order to ensure cold workability, but the C content of the steel material still ensuring cold workability is at most about 0.53% by weight. It has been difficult to apply a forming process such as cold forging to steel having the above C content.

[0007] Parts requiring wear resistance, such as gears, need to have high hardness, especially high surface hardness. for that reason,
When cold forging or rolling is applied as a forming means using low-C steel as a material, carburizing is performed after processing,
The wear resistance and the fatigue resistance are improved by increasing the C content of the surface portion that is the contact portion. However, carburization is a heat treatment that requires a long time at a high temperature, and therefore has low productivity. In addition, distortion due to quenching after carburization is likely to occur.

[0008] As surface hardening treatment for improving abrasion resistance similar to "carburizing", "induction hardening" and "flame hardening" are used.
Is known, and processing can be efficiently performed in a short time.
However, in order to impart not only wear resistance but also desired mechanical properties such as fatigue resistance, strength and toughness to the parts, it is necessary to include a certain amount of C in the base material (base steel). is necessary.

Under these circumstances, so-called "graphitized steel" has attracted attention as a steel excellent in cold workability even with a higher C content. Even if the steel has the same C content, if the carbon in the steel is graphitized, the hardness (strength) is lower than that when cementite is spheroidized by spheroidizing annealing and cold working. The property is improved.

Regarding the above graphitized steel, for example, JP-A-7-3390, JP-A-7-138697 and JP-A-7-150293 are disclosed.

Among them, Japanese Patent Application Laid-Open No. 7-3390 discloses that
“Steel for machine structural use excellent in machinability and cold forgeability” has been proposed. However, the steel described in this publication contains 0.5 to 2.0% by weight of Si. For this reason, graphite is precipitated using the above-mentioned steel as a raw material steel, cold-formed into a desired part shape, and then quenched and tempered in order to impart desired properties to the part (so-called “heat treatment”). ) Was not always satisfactory in terms of toughness.

[0012] The "graphite composite free-cutting steel" proposed in Japanese Patent Application Laid-Open No. Hei 7-150293 also contains 0.5 to 2.0% by weight.
Of Si is contained. For this reason, graphite is precipitated using the above-mentioned steel as a raw material steel, cold-formed into a desired part shape, and then subjected to a tempering treatment to impart desired characteristics to the part, but the toughness is not improved. Then it was not always satisfactory.

Japanese Unexamined Patent Publication (Kokai) No. 7-138697 proposes "hypoeutectoid graphite-precipitated steel having excellent fatigue strength and cold workability". This steel is subjected to annealing for graphitization. It is necessary to perform a quenching process before performing. However, 0.
When normal quenching is performed on steel containing as much as 35 to 0.65% by weight of C, quenching cracking may occur.

Further, almost all of C in the steel is graphitized, and graphite in an amount substantially equal to the C content is precipitated.
For this reason, after cold forming into a desired component shape, when performing tempering treatment or induction hardening to impart desired characteristics to the component, it is necessary to sufficiently perform solid solution of graphite in austenite. , Heating is required for a long time.

That is, by graphitizing cementite, the cold workability of steel having a high C content is certainly improved. However, in order to obtain the required quenching hardness after cold forming into a desired shape and then quenching, C must be sufficiently solid-dissolved in austenite when heated to the austenite region. In the conventional case where C exists in the form of cementite, which is a compound with Fe, in steel, C easily re-dissolves. On the other hand, if almost all of the contained C is converted to graphite, a long-time heating is required for re-solid solution, which increases the cost. Conversely, if the heating for quenching is heating for a short time such as induction hardening, sufficient quenching hardness cannot be obtained, and desired characteristics cannot be imparted to the component.

Further, as is apparent from the description of the embodiment, the critical compression ratio in the cold forging of the steel is at most 66%, and parts requiring a high compression ratio exceeding this are required. For example, it is difficult to use this as a material steel for a part with a flange or the like.

[0017]

If processing methods such as cold forging, which have been frequently used for low-carbon steel materials, can be applied to medium- and high-carbon steel materials formed mainly by cutting methods, a great deal of work will be required. The yield can be improved. Alternatively, it is also conceivable to rationalize the manufacturing method of cold-working low-carbon steel materials and then carburizing for surface hardening to a method of cold-working medium- and high-carbon steel materials and induction hardening or batch furnace quenching. . For example, parts like the gears of a car
Usually, carburizing is performed after gear cutting, but in order to obtain a product with the same performance, the forming process can be changed from cutting to cold forging, and the surface hardening process can be switched from carburizing quenching to induction hardening or batch furnace quenching. it can.

An object of the present invention is to provide a steel material for a machine structure which is excellent in cold workability such as cold forging and rolling and has good heat treatment properties, particularly heat treatment properties in short-time heating such as induction hardening. And a method of manufacturing the same. A more specific object of the present invention is that the cold workability at the time of forming is equivalent to that of a spheroidized annealed S20C class low carbon steel material, and after tempering treatment or after induction hardening as surface hardening treatment. It is an object of the present invention to provide a steel material for a machine structure having properties (strength, toughness, wear resistance, etc.) of a medium / high carbon steel material such as S35C to S58C, and a method of manufacturing the same.

[0019]

The gist of the present invention is to provide the following (1) a steel material for a machine structure having excellent cold workability and (2):
In the method for producing a steel material for machine structural use having excellent cold workability.

(1) C: 0.3-0.8% by weight%
Si: more than 0.15% and less than 0.30%;
05 to 0.6%, W: 0.01 to 0.5%, Al: 0.
005 to 0.10%, Cu: 0 to 0.5%, Ni: 0 to 0%
0.5%, Cr: 0 to 0.2%, Mo: 0 to 0.3%,
Nb: 0 to 0.05%, V: 0 to 0.2%, Ti: 0
0.2%, Zr: 0 to 0.2%, Pb: 0 to 0.5%,
Bi: 0 to 0.5%, Te: 0 to 0.1%, Se: 0
0.1%, Ca: 0 to 0.01%, P: 0.03% or less, S: 0.05% or less, and the balance is a chemical composition comprising Fe and unavoidable impurities. JIS
It is composed of ferrite having a particle size number of 5 to 10, graphite having a maximum diameter of 20 μm or less, and cementite, and having a hardness of Hv12.
A steel material for machine structure excellent in cold workability, characterized by being 0 or less.

(2) The steel having the chemical composition described in the above (1) is hot-worked, then cold-worked at a reduction of area of 5 to 50%, and then in a temperature range of 650 to 720 ° C. 5 to 2
After annealing for 0 hours, the grain size is JIS grain size number 5
A method for producing a steel material for machine structural use having excellent cold workability, characterized by having a structure comprising ferrite of No. 10, graphite having a maximum diameter of 20 μm or less, and cementite, and having a hardness of Hv 120 or less.

Here, the longest major axis length of each graphite particle is the "maximum diameter" of the graphite.

[0023]

BEST MODE FOR CARRYING OUT THE INVENTION In order to solve the above-mentioned problems, the present inventors have made various studies on the graphitization of C in steel, which may provide sufficient cold workability even with medium and high carbon steel materials. Was considered. As a result, the following findings were obtained.

The hardness decreases with the progress of graphitization. However, when graphitization proceeds excessively, the deformability deteriorates.

If the precipitated graphite particles are too large, they will be the starting points of crack generation during deformation.

In order not to cause cracks originating from the precipitated graphite particles, it is necessary to keep the maximum diameter of the graphite particles at 20 μm or less and to distribute them finely. As described above, the length of the longest major axis of each graphite particle is referred to as the “maximum diameter” of the graphite.

In order to enhance the cold workability of medium- and high-carbon steels and to enhance the heat treatment properties in short-time heating quenching such as quenching for tempering and induction quenching, C in the steel is partially removed. What is necessary is just to graphitize. That is, graphite having a maximum diameter of 20 μm or less may be uniformly and finely dispersed in grain boundaries and / or grains, and the structure may be made of ferrite, cementite, and graphite.

The precipitation of graphite occurs with ferrite grain boundaries as nuclei. When the ferrite crystal grains are controlled to an appropriate size, the precipitated graphite particles can be finely dispersed, and the deformation resistance of the steel can be suppressed.

If cold working with an appropriate reduction in area is performed after hot working, the lamellar cementite is separated and strain dislocations serving as nuclei for the precipitation of graphite are introduced. , And quickly precipitate.

Si is known as an element that promotes graphitization. However, when there are few nitrides or oxides serving as graphite precipitation nuclei, a large amount of Si causes the precipitated graphite to become coarse. I will. For this reason, it is necessary to strictly adjust the Si content.

If the hardness of the steel material is controlled to Hv 120 or less, the cold workability is remarkably improved, and the life of the mold can be remarkably improved.

The present invention has been completed based on the above findings.

Hereinafter, each requirement of the present invention will be described in detail. In addition, “%” of the component content means “% by weight”.

(A) Chemical composition of steel C: C is an element effective for improving the hardenability of steel and for improving the strength and hardness after tempering and induction hardening. However, if the C content is less than 0.3%, even if tempering is performed at a low temperature after quenching, desired characteristics (strength and toughness at S35C to S58C level) cannot be obtained. On the other hand, if it exceeds 0.8%, deterioration of toughness and occurrence of cracking are caused,
Graphite having a maximum diameter of more than 20 μm may be precipitated by the annealing treatment, and cracks may be generated from the graphite during cold working. Therefore, the C content is set to 0.3 to 0.8%.

Si: Si is an element effective for promoting deoxidation and graphitization. However, its content is 0.15
%, The effect of addition is poor. On the other hand, in steels with a small amount of nitrides and oxides serving as graphite precipitation nuclei, Si
If it is contained in an amount of 0% or more, the precipitated graphite becomes coarse, and cracks are generated from this as a starting point, and the cold workability is greatly deteriorated. Therefore, the content of Si is set to 0.15
% To less than 0.30%.

Mn: Mn is an element effective for improving hardenability and strength. However, if the content is less than 0.05%, the above effects cannot be expected. On the other hand, if it exceeds 0.6%, graphitization is inhibited. Therefore, the content of Mn is set to 0.05 to 0.6%.

W: W has the effect of increasing the hardenability and strength of steel. However, if the content is less than 0.01%, the effect of addition is poor. On the other hand, if the content exceeds 0.5%, carbides are stabilized and the progress of graphitization is extremely slowed.
Therefore, the content of W is set to 0.01 to 0.5%.

Al: Al has an effect of promoting graphitization. Further, Al is an element effective for stabilizing deoxidation.
However, if the content is less than 0.005%, the effect of addition is poor. On the other hand, if it exceeds 0.10%, not only the above effects are saturated, but also the toughness is reduced. Therefore, Al
The content was 0.005 to 0.10%.

Cu: Cu need not be added. When added, it has the effect of promoting graphitization. Furthermore, it has the effect of increasing the hardenability of steel and increasing the strength after tempering treatment. In order to ensure such an effect, Cu is 0.01
% Is preferable. However, when the content exceeds 0.5%, the hot workability and the cold workability are reduced and the toughness is deteriorated. Therefore, the content of Cu is set to 0 to 0.5%. Note that Cu is preferably contained at 0.05% or more. A more preferred lower limit of the Cu content is 0.10%. The Cu content is more preferably 0.15% or more. On the other hand, steel using scrap as a melting material (for example, steel melted in an industrial-scale electric furnace)
In some cases, about 0.1% of Cu may be contained as an impurity, and Cu as the impurity also has the above-described action.

Ni: Ni may not be added. If added, it promotes graphitization, improves the hardenability of steel,
Further, it has the effect of increasing the strength after the tempering treatment. In order to reliably obtain such effects, it is preferable that the content of Ni be 0.05% or more. However, if the content exceeds 0.5%, the above effect is saturated and the cost is increased. Therefore, the Ni content was set to 0 to 0.5%. It is preferable that Ni is contained at 0.10% or more.
A more preferred lower limit of the Ni content is 0.15%.
On the other hand, in the case of steel using scrap as a raw material for melting (eg, steel melted in an industrial-scale electric furnace), 0.1%
Although a certain amount of Ni may be contained as an impurity, Ni as the impurity also has the above-mentioned action.

Cr: Cr need not be added. If added, it has the effect of increasing the hardenability and strength of steel. To ensure this effect, it is preferable that the content of Cr is 0.01% or more. However, if the content exceeds 0.2%, the carbide is stabilized and the precipitation of graphite becomes extremely slow. Therefore, the Cr content was set to 0 to 0.2%.

Mo: Mo may not be added. If added, it has the effect of increasing the hardenability and strength of steel. To ensure this effect, it is preferable that the content of Mo be 0.01% or more. However, when the content exceeds 0.3%, carbides are stabilized and the progress of graphitization is extremely slowed. Therefore, the content of Mo is set to 0 to 0.3%.

Nb: Nb may not be added. If added, it forms fine carbides and suppresses the growth of austenite grains during heating for induction quenching after cold working or for quenching in a batch furnace, so it is an element effective for reducing quenching strain and ensuring toughness. To ensure this effect, Nb
Is preferably 0.003% or more. However, if the content exceeds 0.05%, the effect of suppressing the growth of austenite grains is saturated and not only the cost is increased, but also the cold workability is greatly reduced by precipitation strengthening. Therefore, the content of Nb is set to 0 to 0.05%.

V: V may not be added. When added, fine carbides are formed, which serve as graphite precipitation nuclei, and thus have the effect of promoting graphitization. Further, it has the effect of increasing the strength after the tempering treatment. In order to surely obtain the above effects, it is preferable that the content of V is 0.01% or more. However, when the content exceeds 0.2%, toughness is reduced. Therefore, the content of V is set to 0 to 0.2%.

Ti: Ti need not be added. When added, fine carbides are formed, which serve as graphite precipitation nuclei, and thus have the effect of promoting graphitization. Further, it has the effect of increasing the strength after the tempering treatment. In order to ensure the above effects, it is preferable that the content of Ti be 0.01% or more. However, when the content exceeds 0.2%, toughness is reduced. Therefore, the content of Ti is set to 0 to 0.2.
%.

Zr: Zr may not be added. When added, fine carbides are formed, which serve as graphite precipitation nuclei, and thus have the effect of promoting graphitization. Further, it has the effect of increasing the strength after the tempering treatment. To ensure the above effects, it is preferable that the content of Zr be 0.01% or more. However, when the content exceeds 0.2%, toughness is reduced. Therefore, the content of Zr is set to 0 to 0.2.
%.

Pb: Pb may not be added. If added, it has the effect of enhancing machinability by the effect of liquid lubrication.
In order to surely obtain this effect, the content of Pb is preferably set to 0.1% or more. On the other hand, Pb inhibits graphitization, and particularly when its content exceeds 0.5%, causes a significant delay in graphite precipitation. Further, a reduction in hot workability and toughness occurs. Therefore, the content of Pb is set to 0 to 0.5%.

Bi: Bi may not be added. If added, it has the effect of enhancing machinability by the effect of liquid lubrication.
To ensure this effect, the content of Bi is preferably set to 0.1% or more. On the other hand, Bi inhibits graphitization, and particularly when its content exceeds 0.5%, causes significant delay of graphite deposition. Further, the hot workability and toughness are reduced. Therefore, the content of Bi is set to 0 to 0.5%.

Te: Te need not be added. If added, MnTe is formed together with Mn, and this acts as a chip breaker at the time of cutting, and thus has an effect of improving machinability. To ensure this effect, Te is 0.05
% Is preferable. On the other hand, the addition of a large amount of Te lowers the hot workability, and further lowers the toughness. In particular, when the content exceeds 0.1%, a remarkable decrease in hot workability and toughness occurs. Therefore, the content of Te is set to 0 to 0.1%.

Se: Se need not be added. If added, MnSe is formed together with Mn, which acts as a chip breaker at the time of cutting, and thus has an effect of improving machinability. To ensure this effect, Se is 0.05
% Is preferable. On the other hand, the addition of a large amount of Se causes a reduction in hot workability and further a reduction in toughness. In particular, when the content exceeds 0.1%, hot workability and toughness are significantly reduced. Therefore, the content of Se is set to 0 to 0.1%.

Ca: Ca may not be added. If added, it has the effect of enhancing machinability. Further, since it is dispersed in the form of oxides in steel to form graphite precipitation nuclei, it has an effect of promoting graphitization. To ensure these effects, C
a is preferably 0.002% or more.
On the other hand, if the content exceeds 0.01%, coarse inclusions are formed, and the toughness and the fatigue strength are reduced. Therefore, C
The content of a was set to 0 to 0.01%.

P: P segregates at the grain boundaries and causes remarkable deterioration of cold workability, and at the same time, suppresses the movement of the C element to inhibit the precipitation of graphite. In particular, the content is 0.03
%, A large deterioration in cold workability and a remarkable delay in graphite deposition are caused. Therefore, the upper limit of the P content is set to 0.0
3%. The upper limit of the P content is preferably set to 0.025%. On the other hand, if the P content is extremely reduced, the steelmaking cost increases, so that the cost of the raw steel increases. Therefore, the lower limit of the P content is desirably about 0.005%.

S: Like P, S inhibits graphitization. Furthermore, while deteriorating toughness, inclusions are formed and cold workability is degraded. In particular, the content is 0.1.
If it exceeds 05%, the precipitation of graphite is significantly delayed, and the toughness and the cold workability are greatly deteriorated. Therefore, the upper limit of the S content is set to 0.05%. The upper limit of the S content is 0.1.
It is preferably set to 025%. On the other hand, if the S content is extremely reduced, the steelmaking cost increases, so that the cost of the raw steel increases. Therefore, the lower limit of the S content is 0.0
It is desirable to set it to about 03%.

(B) Structure and Hardness of Steel Material As will be described in detail in Examples below, the steel material has a predetermined chemical composition, and the structure has a crystal grain size of JIS grain size number 5 to 10.
When the steel material is made of ferrite, graphite having a maximum diameter of 20 μm or less, and cementite, and has a hardness of Hv 120 or less, excellent cold workability can be exhibited even if the steel material is a medium or high carbon steel material. Further, the heat treatment property in short-time heat quenching such as quenching for heat treatment and induction quenching is also improved.

In the above case, even in the case of a medium or high carbon steel material, the critical upsetting ratio (critical compression ratio) in the double-ended restraint upsetting test at room temperature (room temperature) is 85% or more, and
Cold workability (deformability and deformation resistance) equivalent to that of a low-carbon steel material of the S20C class, in which the deformation resistance at an upsetting ratio (compression ratio) of 50% in the upsetting test is 600 MPa or less, is exhibited.

That is, regarding the structure, when the ferrite has a grain size exceeding JIS grain size number 10, the deformation resistance of the steel material becomes extremely large, and when the ferrite grain size falls below 5 according to JIS grain size, a "nucleus" for graphite precipitation is formed. As the size decreases, the size of graphite to be deposited increases, resulting in remarkable deterioration of cold workability.

With respect to graphite, if there is a graphite having a maximum diameter exceeding 20 μm, the critical upsetting ratio of the steel material is reduced, and the deformability is reduced. The lower limit of the maximum diameter of the graphite need not be particularly defined. Note that graphite that can be easily distinguished from cementite by observation with an optical microscope at a magnification of 500 times has a maximum diameter of 0.5 μm or more.

If the steel material is annealed for a long time so that the structure of the steel material does not contain cementite, the graphite becomes coarse and some of them have a maximum diameter exceeding 20 μm. For this reason, the critical upsetting ratio of the steel material is reduced, and the deformability is reduced. Further, such long-time annealing results in energy loss and lowers production efficiency. Therefore, it is necessary that the structure of the steel material contains cementite. In other words, C in steel needs to be partially graphitized.

The above-mentioned partial graphitization of C in steel means that the sum of the area occupied by cementite and graphite in the microstructure is 10%.
0% (hereinafter, cementite (%) + graphite (%) = 100
%), The ratio of the area occupied by graphite is 5
９０90%. If the proportion of the area occupied by graphite is less than 5%, the graphitization is not sufficient, so that the deformation resistance of the steel material increases and the cold workability is low. On the other hand, when the ratio of the area occupied by graphite exceeds 90%, the critical upsetting ratio of the steel material becomes small, and the deformability decreases.
Further, in short-time heating such as induction hardening, a sufficient amount of solid solution C cannot be secured, so that sufficient quenching hardness cannot be obtained.

By the way, the ratio of the area occupied by graphite when the cementite (%) + graphite (%) = 100% in the microstructure can be determined, for example, by observing 10 random visual fields with an optical microscope at 500 × magnification. What is necessary is just to obtain | require by performing image analysis about what has a maximum diameter of 0.5 micrometer or more which can distinguish graphite and cementite. In addition, the existence form of cementite is not particularly limited.

If the hardness after graphitization exceeds 120 in Hv, the deterioration of cold workability becomes large. Therefore, the life of the mold is greatly reduced, and the manufacturing cost is increased even by the cold forming method. Therefore, the hardness is Hv
To 120 or less. The lower limit of the hardness does not need to be particularly limited.

For the reasons described above, in the present invention, the structure and hardness of the steel material are defined as described above.

(C) Cold Working Cold working is performed to make the structure of the hot worked steel material into a desired structure by annealing.

When the reduction in area of the cold working performed before the annealing for graphitization is less than 5%, the laminar cementite is cut off by working and the introduction of strain dislocations serving as graphite precipitation nuclei is sufficient. Not done. Therefore, it is difficult to promote the graphitization and to make the graphite particles and crystal grains fine. On the other hand, even if cold working is performed with a reduction in area of more than 50%, the effect of refining graphite particles and crystal grains is saturated, and large power equipment is required for processing. The cost increases. Therefore, the area reduction rate of cold working performed after hot working is 5 to 50%.
It was specified.

The hot working may be performed by a normal method such as rolling or forging, and the cold working performed after the hot working may be performed by a normal method such as cold drawing.

(D) Annealing Annealing is an indispensable treatment for making the steel having the chemical composition described in (A) into a desired structure. That is, after the cold working of the above (C), annealing is performed for 5 to 20 hours in a temperature range of 650 to 720 ° C., and the steel material structure has a grain size of ferrite of JIS grain size number 5 to 10 and a maximum diameter of Desired structure composed of graphite of 20 μm or less and cementite, which is also cementite (%) + graphite (%) = 1
When it is set to 00%, a structure in which the area ratio of graphite is 5 to 90% can be obtained.

If the annealing temperature is lower than 650 ° C., graphite does not easily precipitate, and the graphitization process takes an extremely long time, increasing the cost. On the other hand, the annealing temperature is 720 ° C
If the ratio exceeds 1, reverse transformation to austenite precedes graphite precipitation, and a desired structure cannot be obtained. Therefore, it is necessary to perform annealing for graphitization in a temperature range of 650 to 720 ° C.

Even in the above temperature range, if the annealing time is less than 5 hours, the graphitization is not sufficient and the cold workability is poor. On the other hand, if the annealing time exceeds 20 hours, the graphite becomes coarse and the maximum diameter exceeds 20 μm, or the graphite when the above-mentioned cementite (%) + graphite (%) = 100% is set. Is 9
Since it exceeds 0%, the critical upsetting ratio of the steel material is reduced, and the deformability is reduced. It also results in energy loss. In addition, if the area ratio of graphite exceeds 90%, a sufficient amount of solid solution C cannot be secured by short-time heating such as induction hardening, so that sufficient quench hardness cannot be obtained. . For this reason, the annealing time in the above temperature range is specified as 5 to 20 hours. The upper limit of the annealing time is preferably about 10 hours from the viewpoint of economy.

Under the manufacturing conditions described above, the "steel material for machine structure excellent in cold workability" of the present invention can be obtained. This steel material is subjected to cold working and heat treatment described below, and becomes a final product such as a machine component.

(E) Cold work The steel material given a desired structure by annealing is subjected to cold work such as cold forging or the like to be formed into a predetermined machine structural part. The cold forming method is not particularly limited, and may be performed by an ordinary method.

(F) Heat Treatment Heat treatment such as tempering treatment (quenching and tempering) and induction quenching are indispensable treatments for imparting the properties required as products to cold-formed parts for machine structures. is there. However, this processing method is not particularly defined, and may be performed by a normal method.

Hereinafter, the present invention will be described with reference to examples.

[0073]

【Example】

(Example 1) Steel having the chemical composition shown in Tables 1 and 2 was
A 3t test furnace was melted by an ordinary method. Steels A to J in Table 1 are steels subject to the present invention (hereinafter referred to as “steel of the present invention”), and steels K to Q in Table 2 are comparisons in which one of the components is out of the range of the content specified in the present invention. It is steel. The steel P and steel Q in the comparative steels are JIS S4
5C and S53C.

[0074]

[Table 1]

[0075]

[Table 2]

Next, these steels were slab-rolled by an ordinary method to form billets of 180 mm square, and then hot-rolled by an ordinary method to obtain bar steels having a diameter of 40 mm. It should be noted that air cooling was performed after hot rolling.

The rolled steel bar is pickled and lubricated by a usual method, and is drawn in a cold (room temperature) to a diameter of 33.5 mm (reduction ratio of 29.9%) using a draw bench. Thus, annealing was performed under the conditions shown in Tables 3 and 4.

The thus-annealed diameter of 33.5 m
A cylindrical test piece having a diameter of 10 mm and a length of 15 mm was cut out from a round bar having a diameter of 10 m and subjected to an upsetting test at both ends using a 500-t high-speed press in a normal manner. Was measured. In addition, three upsetting tests were performed for each condition.

The deformation resistance was represented by an average value of three deformation resistances when the upsetting ratio (compression ratio) in the upsetting test was 50%. The deformability was evaluated based on the maximum working rate (reduction rate) in which cracks did not occur in all three test pieces in the above-mentioned upsetting test, which was taken as the critical upsetting rate.

Further, a test piece having a diameter of 33.5 mm and a thickness of 20 mm was cut out from a 33.5 mm diameter rod after annealing, and ten visual fields were randomly observed with an optical microscope (magnification: 500 times) to examine the structure. (Determination of phase, measurement of ferrite grain size and graphite size, cementite (%) + graphite (%)
= 100% measurement of the area ratio of graphite). Further, the hardness of the central portion was measured by using a micro Vickers hardness tester using the test piece.

Tables 3 and 4 also show the results of the investigation. According to these tables, a steel material having the chemical composition specified in the present invention and subjected to the process of “cold drawing-annealing” after hot rolling under the conditions specified in the present invention (the test in Table 3) Number 1 ~
In the case of (30), since it has a specified structure, the cold workability is equivalent to that of S20C (the critical compression ratio in the both-end restraint upsetting test at room temperature is 85% or more, and the deformation resistance at the upsetting ratio of 50% is 600 MPa or less). On the other hand, even in the case of the steels of the present invention as shown in Test Nos. 45 to 50 in Table 4, the annealing conditions deviate from the conditions specified in the present invention, and therefore the structure deviates from the one specified in the present invention (Test No. 45 , 48 and 49), when the graphitization was insufficient and the hardness was high (test numbers 46, 47 and 50), the cold workability was poor.

[0082]

[Table 3]

[0083]

[Table 4]

(Example 2) The steel D of the present invention, having a diameter of 33.5 of Test Nos. 11 and 48 described in Tables 3 and 4, was used.
mm from the round bar after annealing, 25mm in diameter x 70mm in length
The heat-treated material was cut out and subjected to the following heat treatments (a) and (b).

A heat-treated material having a diameter of 25 mm and a length of 70 mm was cut out from the annealed round bar having a diameter of 33.5 mm and having the test numbers 11 and 36 described in Tables 3 and 4 of the steel D of the present invention. The following heat treatments (a) and (b) were performed.

(A) Using a batch type electric furnace, 900
A quenching treatment of heating to 30 ° C and holding for 30 minutes followed by water cooling (B) Induction quenching treatment in which water is cooled by holding at 900 ° C. for 10 seconds using an induction quenching device having an output of 20 kW and a frequency of 100 kHz.

Next, the hardened round bar was cut with a micro cutter while spraying a cooling liquid to avoid heating, thereby producing a hardness test piece having a diameter of 25 mm and a thickness of 20 mm.

Thereafter, the hardness at a position 2 mm from the outer surface of the hardness test piece was measured using a Vickers hardness tester.

Table 5 shows the test results. From Table 5, it is clear that even in the case of the steel of the present invention, if the structure of the steel is out of the range specified in the present invention, the heat treatment characteristics will be inferior.

That is, in the case of using a heat-treated material cut out from the round bar of Test No. 11 and having a structure after annealing which is defined by the present invention (Test Nos. 51 and 52), Hardness exceeding Hv600 was obtained by quenching (b). On the other hand, the structure after annealing deviated from that specified in the present invention, and was 1% without cementite.
In the case of using a heat-treated material cut out from a round bar having a test number of 48 and having been graphitized to 00% (test numbers 53 and 54), the hardness obtained by the quenching treatment of (a) and (b) is H.
Below v500, it is clear that there is a problem in heat treatment properties.

[0091]

[Table 5]

[0092]

The steel material for machine structural use according to the present invention is excellent in cold workability such as drawing, forging, and rolling, and easily at a medium or high carbon steel level by heat treatment such as tempering treatment or induction hardening. Characteristics (strength, toughness, wear resistance, etc.) can be obtained. For this reason, if this machine structural steel material is applied as a material for a component having a particularly complicated shape such as a component of various machines or automobiles, it is possible to rationalize the manufacturing process and improve the manufacturing yield. The steel for machine structural use can be manufactured relatively easily by the method of the present invention.

Claims (2)

[Claims] C .: 0.3 to 0.8% by weight, Si:
More than 0.15% and less than 0.30%, Mn: 0.05 to
0.6%, W: 0.01 to 0.5%, Al: 0.005
-0.10%, Cu: 0-0.5%, Ni: 0-0.5
%, Cr: 0 to 0.2%, Mo: 0 to 0.3%, Nb:
0 to 0.05%, V: 0 to 0.2%, Ti: 0 to 0.2
%, Zr: 0 to 0.2%, Pb: 0 to 0.5%, Bi:
0 to 0.5%, Te: 0 to 0.1%, Se: 0 to 0.1
%, Ca: 0 to 0.01%, P: 0.03% or less, S:
0.05% or less, the balance is a chemical composition comprising Fe and unavoidable impurities, and the structure is composed of ferrite having a crystal grain size of JIS particle size number 5 to 10, graphite having a maximum diameter of 20 μm or less, and cementite, and having a hardness of A steel material for machine structure excellent in cold workability, having a Hv of 120 or less. 2. A steel having the chemical composition according to claim 1,
After hot working, cold working is performed at a surface reduction rate of 5 to 50%, and then annealing is performed at a temperature range of 650 to 720 ° C. for 5 to 20 hours. A method for producing a steel material for machine structural use having excellent cold workability, characterized by having a structure comprising ferrite, graphite having a maximum diameter of 20 μm or less, and cementite, and having a hardness of Hv 120 or less.