JP2000144307A - Steel for cold working excellent in induction hardenability, parts of machine structure and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce steel excellent in cold workability after spheroidizing annealing and induction hardenability having graded grains without grain coarsening even if induction hardening is executed under the conditions of high temp. and long time, to provide parts for machine structure using it as a base material and to provide a method for producing the same. SOLUTION: The parts for machine structure contain (1) 0.40 to 0.6% C, >1.0 to 0.30% Si, 0.10 to 0.60% Mn, 0.0005 to 0.005% B, 0.005 to 0.05% Nb, 0.005 to 0.05% Ti, >0.050 to 0.10% Al, and the balance Fe with impurities, and, in the impurities, <=0.015% P, <=0.015% S, <=0.10% Cu, <=0.10% Ni, <=0.15% Cr, <=0.10% Mo, <=0.005% N and <=0.005% O are controlled. (2) The base material has the chemical compsn. of (1) and provided with a quench-hardening layer of spheroidized carbides and graded grains of >=5 JIS grain size number. (3) The steel is heated to >=1200 deg.C, and thereafter subjected to hot working and then subjected to spheroidizing annealing and cold working to be formed into a prescribed shape, and, after that, induction hardening is executed finally to produce the parts.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cold working steel excellent in induction hardening, a component for a machine structure, and a method for producing the same. More specifically, the deformation resistance at the time of cold working is small, the induction hardening property is excellent, and further, for example, at a higher temperature and longer time than before, such as a heating part surface temperature of 1150 ° C. and a holding time of 10 seconds. The present invention relates to a cold working steel which is not coarsened even if induction hardening is performed, that is, a fine grain, a machine structural part using the steel as a base material, and a manufacturing method thereof.

[0002]

2. Description of the Related Art Conventionally, components for mechanical structures, especially constant velocity joints, which are undercarriage components of automobiles, are manufactured by hot forging JIS medium carbon steel materials for mechanical structures (S45C and S4C).
8C), is formed into a predetermined shape, is induction hardened, and is further tempered if necessary.

[0003] However, in the case of hot forging, dimensional accuracy is inferior, so that heavy cutting is required to form a predetermined shape, so that the cost of cutting is increased and the yield is further reduced. Did not. Therefore, in recent years, cold forging, which has high dimensional accuracy and can therefore reduce the amount of cutting, has been adopted.

[0004] In the case of performing the above-mentioned cold forging, a workpiece is preliminarily subjected to spheroidizing annealing in order to reduce deformation resistance.
However, when the medium-carbon steel material for machine structure of JIS described above is used, since the deformation resistance is high even when the spheroidizing annealing is performed, the tool life is shortened when performing strong working during cold forging,
Further, since the deformability is low, cracks may occur in the cold-forged parts.

[0005] In recent years, induction hardening has been frequently performed under conditions of higher temperature and longer time than in the past in order to increase the strength of mechanical structural parts. But,
When induction hardening is performed under such conditions, the above-mentioned JIS
In the case of the medium-carbon steel material for machine structure described above, the particles are extremely coarsened.

To solve such a problem, a technique for improving cold forgeability while ensuring induction hardening is disclosed in Japanese Patent Publication No.
JP-A-38847, JP-B-2-47536, JP-A-5-59486, JP-A-9-268344, JP-A-9-272946, JP-A-9-287
No. 054, JP-A-9-287055, JP-A-2-145744, and the like.

However, in the steel proposed in Japanese Patent Publication No. 38847/1989, since the content of Al is small, the effect of improving the hardenability of B may not be obtained in some cases.

[0008] Japanese Patent Publication No. 2-47536,
The steel proposed in Japanese Patent No. 59486 has a low Al content, so that the effect of improving the hardenability of B may be difficult to obtain.
In addition, since Nb is not contained, crystal grains may be coarsened. Further, since the content of Si is low, the releasability of scale generated by heating before hot working may be poor.

JP-A-9-268344, JP-A-9-268344
In the steel disclosed in Japanese Patent No. 272946, the effect of improving the hardenability of B may be difficult to obtain because the content of Al is small, and the grains may be coarse because the steel does not contain Nb. Was.

[0010] The steel proposed in Japanese Patent Application Laid-Open No. 9-287054 is difficult to avoid deterioration in cold workability due to the high content of Si.

The steel disclosed in Japanese Patent Application Laid-Open No. 9-287055 is difficult to avoid deterioration in cold workability due to the high Mn content.

In the technique disclosed in Japanese Patent Application Laid-Open No. 2-145744, Nb and Ti are not added in combination. Therefore, when the steel proposed in this publication is induction hardened, crystal grains may be coarsened in some cases. In particular, if induction hardening is performed at a higher temperature and for a longer period of time than in the past for the purpose of increasing the strength of the machine structural component, the grains become extremely coarse. Furthermore, since the steel proposed in the above publication does not contain B as an essential element, a desired induction hardening depth may not be obtained in some cases. In addition, in the case of steel containing no B, since the content of alloying elements is larger than that of steel containing B having the same hardenability, the deformation resistance during cold forging is increased, and the cold forgeability is increased. Sometimes it was inferior. In addition, the scale generated by the hot working or the spheroidizing annealing is hard to fall off in the descaling step, and it is difficult to avoid taking a long time for the descaling or complicating the step.

[0013]

SUMMARY OF THE INVENTION The present invention has been made in view of the above situation, and has a small deformation resistance at the time of cold working such as cold forging, is excellent in induction hardening properties, and is more than a conventional one. Provided is a low-cost type cold-working steel that exhibits fine-grained grains without being coarsened even when induction hardening is performed at high temperature and for a long time, a component for a machine structure using the steel as a base material, and a method of manufacturing the same. The purpose is to do. Specifically, the deformation resistance at the time of cold working is 10% or more lower than that of JIS carbon steel for machine structural use having the same C content, and the upsetting ratio at which cracking as a deformability occurs is limited. T / r is at least 0.3, where t is the hardening depth at which the Vickers hardness (Hv) becomes 400 when induction hardening is 85% or more, and r is the average diameter of the induction hardened part, and the heating part surface temperature is 1150. Even if induction hardening is performed under the condition that the holding time is 10 seconds at ℃, the hardened portion after induction hardening, that is,
The austenite grain size of the quench-hardened layer described below is JI
The target is to have an S particle size number of 5 or more and to be sized. The “quenched hardened layer”, which is a hardened portion after induction hardening, indicates a portion having an Hv of 400 or more. "Sizing" means that the particles are not different by 3 or more in particle size number.

[0014]

SUMMARY OF THE INVENTION The present invention provides a steel for cold working excellent in induction hardening property as shown in the following (1):
The gist of the present invention is a method for manufacturing a mechanical structure component shown in (2) and a mechanical structure component shown in (3).

(1) By weight%, C: 0.40 to 0.60
%, Si: more than 0.10% and 0.30% or less, Mn:
0.10 to 0.60%, B: 0.0005 to 0.005
%, Nb: 0.005 to 0.05%, Ti: 0.005
-0.05%, Al: More than 0.050% and 0.10% or less, with the balance being Fe and unavoidable impurities, P in the impurities is 0.015% or less, and S is 0.015% or less. , Cu is 0.10% or less, Ni is 0.10% or less, C
r is 0.15% or less; Mo is 0.10% or less;
005% or less and O is 0.005% or less. Cold working steel excellent in induction hardening property.

(2) A machine structural component having a base material having the chemical composition described in (1) above, a spheroidized carbide, and a quenched and hardened layer of austenite grain size of 5 or more according to JIS. .

(3) The steel material having the chemical composition as described in (1), which has been subjected to hot working after heating to 1200 ° C. or more and then spheroidized and annealed, is cold-worked and formed into a predetermined shape. Thereafter, induction hardening is performed, followed by a method for manufacturing a component for a machine structure.

The "hardened hardened layer" referred to in the above (2)
As described above, the term “grain size” means a portion having a difference of 3 or more in grain size number, which means a portion having Hv of 400 or more by quenching as described above.

The inventors of the present invention carry out plastic working by cold working such as cold forging after spheroidizing annealing, and then become a base material of a machine structural component manufactured by induction hardening at a higher temperature and longer time than before. The chemical composition of steel was investigated and examined. As a result, the following findings were obtained.

Niobium titanium carbonitride [NbTi (C
N)] is so coarse that it has no effect on preventing austenite grains from coarsening during induction hardening. However, when [NbTi (CN)] is miniaturized, a so-called “pinning action” is exhibited, so that coarsening of austenite grains can be prevented.

In order to make [NbTi (CN)] finer, the heating temperature at the time of hot working may be increased to cause a solid solution in the base material once, and then re-precipitated during the next working / cooling.

The prevention of coarsening of austenite grains by the fine [NbTi (CN)] is particularly exhibited when the Mn content of the steel is low, and when the surface temperature of the heating section is 11%.
Even if induction hardening is performed at a higher temperature and longer time than before, such as a holding time of 10 seconds at 50 ° C., coarse particles are not formed and fine particles are formed.

In the case of steel containing an appropriate amount of Si, Nb, Ti, Al and B while keeping the Mn content low, even if [NbTi (CN)] is fine, ordinary spheroidizing annealing is sufficient. Softens. Therefore, the deformation resistance at the time of cold working is lower than that of JIS carbon steel for machine structural use having the same C content, and the deformability is sufficiently large.

The steel in which the contents of Mn, Nb, Ti, Al and B are adjusted and the content of N as an impurity element is adjusted low has good induction hardenability.

The steel in which the contents of C, Mn, Nb, Ti, Al and B are adjusted and the content of N as an impurity element is adjusted low has a heating part surface temperature of 1150 ° C. and a holding time of 10 seconds. Even if induction hardening is performed at a higher temperature and longer time than in the conventional case, the above-mentioned t / r can easily satisfy 0.3 or more.

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

[0027]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Each requirement of the present invention will be described in detail below. In addition, “%” of the content of the chemical component means “% by weight”.

(A) Chemical composition of base steel C: 0.40 to 0.60% C is an element that affects the induction hardenability, and secures the hardness and depth of the quenched hardened layer to ensure the mechanical structure. It is an element effective for imparting desired mechanical properties to parts for use. But,
If the content is less than 0.40%, the effect of addition is poor. On the other hand, if the content exceeds 0.60%, even if it is annealed in a spheroid, it does not sufficiently soften, so that the cold workability may be deteriorated, the toughness may be deteriorated, and the occurrence of crazing may be caused. Therefore, the content of C is set to 0.40 to 0.60%.

Si: more than 0.10% and not more than 0.30% Si has the effect of stabilizing the deoxidation of steel and increasing the strength. In addition, the steel to which Si is added has a low melting point oxide such as firelite (Fe ₂ Si) during heating for hot working.
O ₄ ) is produced, so that if it is heated above its melting point (1173 ° C.), the descaling properties will be extremely good. However, when the content is 0.10% or less, the effect of addition is poor. On the other hand, when the content exceeds 0.30%, the deformation resistance at the time of cold working becomes large and the cold workability is reduced. Therefore, the content of Si exceeds 0.10% and exceeds 0.1%.
30% or less. Preferably, Si is 0.15%.
It is better to contain more than.

Mn: 0.10 to 0.60% Mn is an element effective for fixing S in steel to enhance hot workability and secure strength, and is contained at 0.10% or more. is necessary. On the other hand, when the content of Mn exceeds 0.60%, the deformation resistance increases and the cold workability deteriorates. Therefore, the content of Mn is set to 0.1.
It was set to 10 to 0.60%. The Mn content is 0.10
It is preferable to set it to 0.40%.

B: 0.0005 to 0.005% B is an element effective for securing good induction hardenability without impairing cold workability. However, if the content is less than 0.0005%, the effect of addition is poor. On the other hand, when the content exceeds 0.005%, not only the effect is saturated, but also grain boundary embrittlement may be caused. Therefore, the content of B is set to 0.0005 to 0.005%.

Nb: 0.005 to 0.05% Nb combines with Ti to form [NbTi (CN)]. When this [NbTi (CN)] is finely precipitated, the temperature becomes higher than before. In addition, coarsening can be prevented even when induction hardening is performed for a long time. However, if the content is less than 0.005%, the desired effect cannot be obtained. On the other hand, if it exceeds 0.05%, it is inevitable to increase the deformation resistance, and coarse undissolved carbonitrides may remain to cause deterioration in cold workability. Therefore,
The content of Nb was set to 0.005 to 0.05%. In addition,
The upper limit of the Nb content is preferably set to 0.03%,
More preferably, it is 0.02%.

Ti: 0.005 to 0.05% Ti combines with Nb to form [NbTi (CN)]. If this [NbTi (CN)] is finely precipitated, the temperature becomes higher than before. In addition, coarsening can be prevented even when induction hardening is performed for a long time. However, if the content is less than 0.005%, the effect of addition is poor. On the other hand, if it exceeds 0.05%, it is inevitable to increase the deformation resistance, and coarse undissolved carbonitrides may remain to cause deterioration in cold workability. Therefore, the content of Ti is set to 0.005 to 0.05%. The upper limit of the Ti content is preferably set to 0.03%,
More preferably, it is 15%.

Al: more than 0.050% and 0.10% or less Al has a deoxidizing effect. Further, since the nitrides are generated to fix N in the steel, there is an effect of suppressing work hardening during cold working. It is also effective in securing the induction hardening property of B by fixing N in steel. But,
When the content is 0.050% or less, the effect of addition is poor.
On the other hand, if the content exceeds 0.10%, the deformability of the steel during cold working decreases. Therefore, the content of Al is set to more than 0.050% and 0.10% or less.

In the present invention, P, S, Cu, Ni, Cr, Mo, N and O as impurity elements are limited as follows.

P: 0.015% or less P reduces the deformability during cold working. In particular,
If the P content exceeds 0.015%, the deformability during cold working is significantly reduced. Therefore, the content of P as an impurity element is set to 0.015% or less.

S: 0.015% or less S also lowers the deformability during cold working. In particular, S
If the content exceeds 0.015%, the deformability during cold working is significantly reduced. Therefore, the content of S as an impurity element is set to 0.015% or less.

Cu: 0.10% or less Cu increases deformation resistance and deteriorates cold workability. In particular, when the content of Cu exceeds 0.10%, the cold workability significantly deteriorates. Therefore, the content of Cu as an impurity element is set to 0.10% or less. Note that C
It is preferable that the u content is regulated to 0.05% or less.

Ni: 0.10% or less Ni increases deformation resistance and deteriorates cold workability. Furthermore, it is difficult to remove scale after spheroidizing annealing.
In particular, when the Ni content exceeds 0.10%, the cold workability and the scale removability are significantly reduced. Therefore, the content of Ni as an impurity element is set to 0.10% or less. Note that the Ni content is preferably regulated to 0.05% or less.

Cr: 0.15% or less Cr also increases the deformation resistance and deteriorates the cold workability. Furthermore, it is difficult to remove scale after spheroidizing annealing.
In particular, when the content of Cr exceeds 0.15%, the cold workability and the scale removability are significantly reduced. Therefore, the Cr content as an impurity element is set to 0.15% or less. Note that the Cr content is preferably regulated to 0.10% or less.

Mo: 0.10% or less Mo increases deformation resistance and deteriorates cold workability. Furthermore, it becomes difficult to remove scale after spheroidizing annealing. In particular, if the Mo content exceeds 0.10%, the cold workability and the scale removability are significantly reduced.
Therefore, the Mo content as an impurity element is set to 0.10
% Or less. It is preferable that the Mo content be regulated to 0.05% or less.

N: 0.005% or less N increases the deformation resistance and degrades the cold workability. Furthermore, since it easily combines with B to form BN,
The effect of improving the induction hardening property of B cannot be secured. In particular, when the content of N exceeds 0.005%, the cold workability significantly decreases and the effect of improving the induction hardening property of B becomes difficult to obtain. Therefore, the N content as an impurity element is set to 0.005% or less. Note that the N content is 0.1.
004% or less is preferable and 0.003%
The following is more preferable.

O (oxygen): 0.005% or less O forms an oxide and reduces the deformability during cold working. In particular, if the O content exceeds 0.005%, the deformability during cold working is significantly reduced. Therefore, the content of O as an impurity element is set to 0.005% or less.

(B) Hot working Steel having the chemical composition described in (A) above, in which Nb and Ti are added in a complex manner, has coarse [NbTi (C
N)] exists. This coarse [NbTi
(CN)] is a starting point of work cracking in the subsequent cold working, and has no effect on preventing austenite grains from being coarsened during induction hardening.

However, if the heating temperature at the time of hot working such as hot rolling is set to a high temperature of 1200 ° C. or more, [NbTi (CN)] is once dissolved in the base material, and the next working / cooling is performed. Sometimes, the fine particles are reprecipitated, so that a so-called “pinning action” can be exhibited, so that the austenite grains can be prevented from becoming coarse. Therefore, the heating temperature for hot working is set to 12
The temperature was set to 00 ° C. or higher. The upper limit of the heating temperature does not need to be particularly specified, but is preferably set to 1350 ° C. in order to suppress the energy cost for heating, further suppress the scale loss, and increase the yield.

(C) Spheroidizing Annealing The steel having the chemical composition described in the above (A) is the same as the above (B)
After being heated under the conditions described in (1), the workpiece is hot-worked, and further subjected to spheroidizing annealing to reduce deformation resistance during cold working. The spheroidizing annealing is not particularly limited, and may be performed by a usual method.

(D) Cold working The steel material having the chemical composition as described in (A), which has been subjected to spheroidizing annealing after hot working, is subjected to cold working such as cold forging and has a mechanical structure having a predetermined shape. Molded into parts. The cold working method is not particularly limited, and may be performed by a usual method.

The hardened portion (hardened hardened layer) of the machine structural component formed into a predetermined shape by cold working after induction hardening has a stable austenitic crystal grain size of JIS grain size number 5 or more, which will be described later. In order to ensure the grain structure, the cold working is performed so that the working amount in the part of the workpiece to which the largest working is applied is 2.5 or less with the equivalent strain represented by the following equation (a). It is more preferable to perform the process so that the equivalent strain is 2.0 or less.

[0049] ^{_{ε = {(ε 1 2 +}} ε 2 2 + ε 3 2) × 2/3} where ^{1/2 ···· (a), (a} ) 1 ε in the expression, ε _2, ε ₃ is the main This is the logarithmic distortion of the direction.

(E) Induction hardening A steel material having the chemical composition described in (A) above, subjected to spheroidizing annealing after hot working, and then cold worked to form a predetermined shape is subjected to induction hardening. Alternatively, if necessary, tempering is performed after induction hardening to obtain a mechanical structural component having desired mechanical properties.

When the austenite grain size in the quenched and hardened layer of the machine structural component is less than 5 in the JIS grain size number, or when the grain size is not uniform, that is, when the grain size is mixed, the heat treatment strain is generated and the toughness is reduced. However, variations in hardness (strength) occur, and in particular, when the strength of the mechanical structure component is high, the decrease in toughness is remarkable. Therefore, the quenched and hardened layer of the machine structural component was defined to have an austenite crystal grain size of JIS grain size number 5 or more.
The JIS particle size number is preferably 6 or more.
Since the smaller the austenite crystal grain, that is, the larger the JIS grain size number, the higher the toughness, the upper limit of the JIS grain size number need not be set.

The steel according to the present invention having the chemical composition described in the above (A) can be used under normal conditions, that is, when the surface temperature of the heating section is 9%.
In the induction hardening under the condition of about 50 ° C. and the holding time of about several seconds, the crystal grains are not coarsened, and fine grains having an austenite crystal grain size of JIS grain size number 5 or more can be obtained. Furthermore, even if induction hardening is carried out at a higher temperature and longer time than before, such as a heating part surface temperature of 1150 ° C. and a holding time of 10 seconds, fine austenite grains having a JIS grain size number of 5 or more can be obtained. It has been adjusted as follows. For this reason, the method of induction hardening is not particularly limited.

The torsional strength of the steel material subjected to induction hardening after cold working is 400 in Hv as the induction hardening depth.
If t / r is less than 0.3, the torsional strength is small, depending on the hardening depth. Therefore, if the component to be induction hardened is large, that is, if r is large,
When induction hardening is performed under normal conditions, t / r of 0.3 or more may not be obtained. In such a case, in order to secure a value of 0.3 or more in t / r, for example, as described above, the temperature of the heating unit is higher than that of the related art such as 1150 ° C. and holding time of 10 seconds. Induction hardening under long-term conditions becomes necessary. In the steel according to the present invention having the chemical composition described in (A), even in such a case, the crystal grains do not become coarse.

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

[0055]

EXAMPLES Steels having the chemical compositions shown in Tables 1 and 2 were melted by a conventional method using a test furnace. Steels A to N in Table 1 are steels of the examples of the present invention whose chemical composition is within the range of the content specified in the present invention, and steels a to r in Table 1 are steels having a content in which any of the components is specified in the present invention. It is a steel of a comparative example out of the range. Among the steels of the comparative examples, steel p, steel q and steel r are S40C, S50C and S58C of JIS standard, respectively.
Is the steel equivalent to

[0056]

[Table 1]

[0057]

[Table 2]

Next, these steels were formed into billets by a usual method, and then heated to 1100 ° C. or 1250 ° C. and hot forged to form round bars having a diameter of 65 mm. After this, C
Spheroidizing annealing was performed according to a usual method according to the content.

The diameter obtained as described above is 65 mm.
From the R / 2 part of the round bar (R is the radius of the round bar), the diameter is 15
mm and a length of 22.5 mm are cut out of a test piece for cold working, and subjected to a cold (room temperature) constrained upsetting test by a normal method using a 500-t high-speed press machine to determine the upsetting rate at which cracking occurs. It was measured. In addition, the upsetting rate is 85%, and the upsetting test is performed five times for each condition, and the minimum processing rate (upsetting rate) in which cracks occur in three or more of the five test pieces is set as a limit. It was evaluated as an incorporation rate. When the upsetting rate was 85% and three or more pieces did not crack, the test was terminated there.

Further, the deformation resistance was measured when the upsetting rate was 60% or less (the equivalent strain at the center of the test piece subjected to the largest processing was 1.5) which was not more than the limit upsetting rate of all steels. In addition, as shown in FIG. 1, the deformation resistance is arranged by the content of C, and S40C, S50C and S40 of JIS standard are arranged.
The straight line obtained from the deformation resistance of steel p, steel q and steel r corresponding to 58C is defined as the deformation resistance of JIS mechanical structural steel, and steels A to
The deformation resistance of the steel of the present invention of N and the steel of the comparative example of the steels a to o were compared.

Further, a test piece having a diameter of 63 mm and a length of 50 mm was cut out from the above-mentioned round bar having a diameter of 65 mm, and was cold-extruded forward to a diameter of 40 mm by a normal method. 1.3)) was carried out with the equivalent strain of the surface portion of the test piece on which large processing is applied, that is, the outermost layer of the test piece. A test piece having a length of 50 mm was sampled from a cold extruded piece having a diameter of 40 mm, and subjected to induction hardening. In the case of high frequency heating, the average heating rate is 200 ° C
/ Sec, and were performed under the following two conditions. That is, frequency 2
General high-frequency heating conditions of 0 kHz, heating section surface temperature of 950 ° C., and holding time of 2 seconds, and a frequency of 20 kHz for increasing the curing depth to increase the strength and the heating section surface temperature of 1
This is a high-frequency, high-temperature heating condition of 150 ° C. and a holding time of 10 seconds at a high temperature for a long time. Note that water was used as a cooling medium.

After the induction hardening, the hardening depth (ie, the surface hardness and Hv of 400) obtained by the usual method (ie,
The depth (t) of the hardened hardened layer was measured. Next, tempering was performed at 150 ° C. for 30 minutes using an electric furnace, and the hardened portion after induction hardening, that is, the austenite grain size of the hardened hardened layer was measured by a usual method.

Tables 3 and 4 summarize the above test results. In the present embodiment, r is the radius of a test piece having a diameter of 40 mm, that is, 20 mm.

[0064]

[Table 3]

[0065]

[Table 4]

Table 3 shows that steels A to N of the present invention having a chemical composition within the range of the content specified in the present invention were used as base materials and were heated at 1250 ° C. during hot forging (test number). 1
14 to 14) show that the deformation resistance at an upsetting rate of 60% (average equivalent strain of each part of the test piece: 1.0) is 10% or more lower than that of JIS carbon steel for machine structural use having the same C content, The upsetting rate at which the cracking occurs is 85% or more. In addition, even if induction hardening is performed at a temperature of 1150 ° C. and a holding time of 10 seconds at a higher temperature and longer time than before in the case where t / r is 0.3 or more, the austenite grain size of the hardened hardened layer is JIS. The particle size is 5 or more, and the particles are sized.

From Table 4, it can be seen that even when the steel of the present invention has a chemical composition within the range specified by the present invention, the heating temperature during hot forging is 1100 ° C., which is lower than the range specified in the present invention. (Test Nos. 15 and 16) show that the limit upsetting rate is 85
% Has not been reached.

When the steel of the comparative example is used as a base material,
(A) The reduction in deformation resistance is less than 10% for JIS carbon steel for machine structural use with the same C content, (b) the limit upsetting rate is less than 85%, (c) induction hardening (D) the austenitic crystal grain size of the hardened part after induction hardening, that is, the quenched hardened layer is less than JIS grain size number 5, or the austenite grain size is JIS grain size number 5 Any of the above is a mixed particle. For this reason, cold workability and induction hardening are not compatible.

[0069]

The steel of the present invention is excellent in cold workability and induction hardening after spheroidizing annealing, and has a heating part surface temperature of 115.
Even if induction hardening is performed at a high temperature and a long time such as 0 ° C. and a holding time of 10 seconds, coarse particles are not formed and fine grains are exhibited.
It can be used as a base material for parts for machine structure, especially for constant velocity joints which are suspension parts for automobiles.
This machine structural part can be manufactured relatively easily by the method according to the invention.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between deformation resistance and C content.

 ────────────────────────────────────────────────── ─── Continued on the front page F term (reference) 4K032 AA01 AA02 AA05 AA06 AA16 AA22 AA31 AA35 BA02 CA03 CF00 4K042 AA22 BA05 BA14 CA02 CA05 CA06 CA08 CA09 CA10 CA12 DA01 DB01

Claims (3)

[Claims] (1) C: 0.40 to 0.60% by weight, S
i: more than 0.10% and 0.30% or less, Mn: 0.10
~ 0.60%, B: 0.0005-0.005%, N
b: 0.005 to 0.05%, Ti: 0.005 to 0.
Al content: more than 0.050% and 0.10% or less, with the balance being Fe and unavoidable impurities, P in the impurities is 0.015% or less, S is 0.015% or less, Cu
Is 0.10% or less, Ni is 0.10% or less, and Cr is 0.1% or less.
15% or less, Mo is 0.10% or less, N is 0.005%
Hereinafter, O (oxygen) is 0.005% or less, a steel for cold working excellent in induction hardening property. 2. A base material having the chemical composition according to claim 1,
Spheroidized carbide and austenite grain size are JIS
A machine structural component having a sized hardened layer with a grain size of 5 or more. 3. Hot working after heating to 1200 ° C. or higher,
2. A method for manufacturing a component for a machine structure, comprising: subjecting a steel material having the chemical composition according to claim 1 subjected to spheroidizing annealing to cold working to form a predetermined shape, followed by induction hardening.