JP3419333B2 - Cold work steel excellent in induction hardenability, component for machine structure, and method of manufacturing the same - Google Patents

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 hardenability, a machine structural part, and a method for manufacturing the same. More specifically, it is a low-cost cold working steel that has low deformation resistance during cold working, is excellent in induction hardenability, and does not coarsen grains during induction hardening, and a machine that uses that steel as a base material. The present invention relates to a structural component and a method for manufacturing the same.

[0002]

2. Description of the Related Art Conventionally, mechanical structural parts, especially constant velocity joints, which are undercarriage parts of automobiles, are hot forged JIS medium carbon steel materials for mechanical structure (S45C and S4).
(8C etc.) is cut and shaped into a predetermined shape, then induction hardened, and if necessary, tempered. However, in the case of hot forging, the dimensional accuracy is poor, and therefore heavy cutting is required to form it into a predetermined shape, which results in an increase in the cost of cutting and an inevitable reduction in yield. Therefore, in recent years, cold forging has come to be adopted, which has high dimensional accuracy and therefore can reduce the cutting amount.

When performing the above-mentioned cold forging, the material to be processed is previously subjected to spheroidizing annealing in order to reduce the deformation resistance.
However, when the JIS medium-carbon steel material for mechanical structure described above is used, the deformation resistance is high even if the spheroidizing annealing treatment is performed, so the tool life of cold forging is short, and the deformability is low, so cold forging is performed. In some cases, the cracked parts may occur.

To solve such problems, various techniques have been proposed for improving the cold forgeability while ensuring the induction hardenability.

Japanese Patent Publication No. 1-38847 and Japanese Examined Patent Publication No. 2
No. 47536 discloses that in order to improve cold forgeability, the contents of Si and Mn are kept low, and C, B, Ti and, if necessary, Cr are added to ensure induction hardenability. Steel for forgings is disclosed. However, the steels proposed in the above publications contain 0.02 to 0.04 wt% of Ti, as is clear from the description in the examples. Therefore, it is difficult to say that cold forgeability is necessarily good because Ti carbonitride precipitates and does not soften sufficiently even if it is spheroidized and annealed due to its precipitation hardening, resulting in high deformation resistance.

Japanese Unexamined Patent Publication No. 5-59486 and Japanese Unexamined Patent Publication No. 9-
268344, JP-A-9-272946,
JP-A-9-287054 and JP-A-9-28705.
No. 5 gazette also discloses steel having both cold forgeability and induction hardenability. However, the steels proposed in the above publications also contain Ti as an essential element in order to fix N and secure a solid solution B. For this reason, even if it is made into a spheroidizing annealing, it may not be sufficiently softened and its deformation resistance may be high, resulting in poor cold forgeability.

Japanese Unexamined Patent Publication No. 2-145744 discloses that Ti
A "carbon steel for machine structural use which is excellent in cold forgeability and induction hardenability" is disclosed. However, induction hardening of the steel proposed in this publication may cause coarsening of crystal grains, or the desired induction hardening depth may not be obtained because B is not contained as an essential element. Further, in the case of a steel containing no B, since the content of alloying elements is higher than that of a steel containing B having the same hardenability, the deformation resistance during cold forging is high and the cold forgeability is poor. Sometimes. Furthermore, the scale produced by hot working or spheroidizing annealing does not easily fall off during the descaling process, making it difficult to avoid that descaling takes a long time or the process becomes complicated.

[0008]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has a small deformation resistance during cold working such as cold forging, is excellent in induction hardenability, and is rough in induction hardening. An object of the present invention is to provide a low-cost cold working steel that does not granulate, a machine structural component using the steel as a base material, and a manufacturing method thereof. Specifically, the deformation resistance during cold working is 10% or more lower than that of a carbon steel for JIS mechanical structure having an equivalent C content, and the upsetting ratio that is the limit of cracking as the deformability occurs. If the hardening depth is 75% or more and the Vickers hardness (Hv) is 400 when induction-hardened, then t / r is 0.3 or more, where t is the average radius of the induction-hardened part and r is the hardened part after induction hardening. That is, the target is that the austenite grain size of the quench hardened layer described later is JIS grain size number 7 or more. The hardened portion after induction hardening (quenching hardened layer) refers to a portion having Hv of 400 or more.

[0009]

Means for Solving the Problems The gist of the present invention is to provide cold working steel excellent in induction hardenability as shown in (1) and (2) below, and machine structural parts and (4) as shown in (3). The method for manufacturing a machine structural component as shown in FIG.

(1) C: 0.40 to 0.60 in% by weight
%, Si: 0 to 0.40%, Mn: 0.10 to 0.60
%, B: 0.0005 to 0.005%, Nb: 0.00
5 to 0.05%, Al: 0.015 to 0.10%, further Pb: 0 to 0.30%, Bi: 0 to 0.10.
%, Te: 0 to 0.10%, the balance consisting of 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, N
i is 0.10% or less, Cr is 0.15% or less, Mo is 0.10% or less, N is 0.005% or less, and O is 0.00.
Steel for cold work with excellent induction hardenability of 5% or less.

(2) C: 0.40 to 0.60 in% by weight
%, Si: 0 to 0.40%, Mn: 0.10 to 0.60
%, B: 0.0005 to 0.005%, Nb: 0.00
5 to 0.03%, Al: 0.015 to 0.10%, the balance consisting of 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, Cr is 0.1
5% or less, Mo is 0.10% or less, N is 0.005% or less, and O is 0.005% or less. Cold working steel excellent in induction hardenability.

(3) A machine structural part in which the base material has the chemical composition described in (1) or (2) above, and is provided with spheroidized carbide and a quench hardened layer on the outer peripheral portion.

(4) A steel material having the chemical composition described in (1) or (2) that has been spheroidized and annealed after hot working is cold worked into a predetermined shape, and then induction hardened. And a method for manufacturing a machine structural component.

Incidentally, the quench-hardened layer referred to in the above (3) refers to a portion which has reached Hv 400 or higher by quenching as described above.

The inventors of the present invention investigated the chemical composition of steel which is a base material of machine structural parts manufactured by plastic working by cold working such as cold forging after spheroidizing annealing and then induction hardening. Study was carried out. As a result, the following findings were obtained.

A steel containing a proper amount of Nb, Al and B but not containing Ti but suppressing the Mn content to a low level is sufficiently softened by ordinary spheroidizing annealing. Therefore, the deformation resistance during cold working is low and the deformability is sufficiently large as compared with the carbon steel for JIS mechanical structure having the same C content.

Steel in which proper contents of Mn, Nb, Al and B are selected and the content of N as an impurity element is limited has good induction hardenability even if Ti is not contained. In addition, the crystal grains are not coarsened by the usual induction hardening, and the austenite grain size of JIS grain number 7 or more can be obtained.

Steels in which the proper contents of C, Mn, Nb, Al and B are selected and the content of N as an impurity element is limited, the above-mentioned t / r is easily 0.3 or more by induction hardening. Can be met. Moreover, the crystal grains do not become coarse during normal induction hardening, and JIS grain size number 7
The above austenite grain size is obtained.

In the steel to which Ti is added from the viewpoint of N fixation,
Coarse TiN is dispersed in the steel, reducing the rolling life of parts.

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

[0021]

BEST MODE FOR CARRYING OUT THE INVENTION Each requirement of the present invention will be described in detail below. In addition, "%" of the content of a chemical component means "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 quench hardened layer to secure the mechanical structure. It is an element effective in 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%, it may not be sufficiently softened even by spheroidizing annealing, and the cold workability may be deteriorated, the toughness may be deteriorated, and quench cracking may occur. Therefore, the content of C is set to 0.40 to 0.60%.

Si: 0 to 0.40% Si may not be added. If added, it has the effect of stabilizing the deoxidation of steel and increasing the strength. In order to reliably obtain this effect, the Si content is preferably 0.05% or more. In addition, the steel to which Si is added is firelite (Fe ₂ S) which is a low melting point oxide during heating for hot working.
Since iO ₄ ) is produced, if it is heated above its melting point (1173 ° C.), the descaling property becomes extremely good. This effect is particularly great when the Si content exceeds 0.15%. However, if its content exceeds 0.40%, the deformation resistance at the time of cold working becomes large, resulting in deterioration of cold workability. Therefore, the Si content is 0 to 0.40%.
And

Mn: 0.10 to 0.60% Mn is an element effective for fixing S in steel to improve hot workability and ensure strength, and is contained in an amount of 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 Mn content is set to 0.
It was set to 10 to 0.60%. The Mn content is 0.10.
It is preferable to be 0.40%.

B: 0.0005 to 0.005% B is an element effective for ensuring 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 occur. Therefore, the content of B is set to 0.0005 to 0.005%.

Nb: 0.005 to 0.05% Nb is an element effective for ensuring good induction hardenability without significantly impairing cold workability. Furthermore,
It is also effective in preventing the coarsening of crystal grains during induction hardening.
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 unavoidable to increase the deformation resistance, and coarse undissolved carbonitrides may remain to cause deterioration of cold workability. Therefore, the content of Nb is set to 0.005 to 0.05%. The upper limit of the Nb content is preferably 0.03%, and more preferably 0.02%. A more preferable upper limit of the Nb content is 0.015%.

Al: 0.015 to 0.10% Al has a deoxidizing effect. Further, since N is generated to fix N in the steel, it has an action of suppressing work hardening during cold working. Further, fixing N in the steel is also effective in securing the effect of improving the induction hardenability of B. But,
If the content is less than 0.015%, the above effect cannot be obtained reliably. On the other hand, if the content exceeds 0.10%, the deformability of steel during cold working decreases. Therefore,
The content of Al is set to 0.015 to 0.10%. In addition,
In order to secure the effect of improving the induction hardenability of B, the Al content is preferably 0.03% or more, and 0.05%
It is more preferable that the content be exceeded.

The base steel may further contain the following elements.

Pb: 0 to 0.30% Pb may not be added. If added, it has the effect of improving the machinability after cold working. In order to surely obtain this effect, the Pb content is preferably 0.10% or more. However, if the content exceeds 0.30%, the deformability during cold working deteriorates. Therefore, the Pb content is set to 0 to 0.30%.

Bi: 0 to 0.10% Bi need not be added. If added, it has the effect of improving the machinability after cold working. In order to reliably obtain this effect, the Bi content is preferably 0.05% or more. However, if the content exceeds 0.10%, the deformability during cold working deteriorates. Therefore, the Bi content is set to 0 to 0.10%.

Te: 0 to 0.10% Te may not be added. If added, it has the effect of improving the machinability after cold working. In order to reliably obtain this effect, the content of Te is preferably 0.05% or more. However, if the content exceeds 0.10%, the deformability during cold working deteriorates. Therefore, the content of Te is set to 0 to 0.10%.

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
When the content of Cr exceeds 0.015%, the deformability during cold working remarkably decreases. Therefore, the content of S as an impurity element is set to 0.015% or less.

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

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

Cr: 0.15% or less Cr also increases the deformation resistance and deteriorates the cold workability. Furthermore, it makes scale removal difficult after spheroidizing annealing.
In particular, if the Cr content 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. The Cr content is preferably 0.10% or less.

Mo: 0.10% or less Mo increases the deformation resistance and deteriorates the cold workability. Furthermore, it makes scale removal difficult after spheroidizing annealing.
In particular, when 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. The Mo content is preferably 0.05% or less.

N: 0.005% or less N increases the deformation resistance and deteriorates the cold workability. Furthermore, since it easily binds to B to form BN,
The effect of improving induction hardenability of B cannot be secured. In particular, when the content of N exceeds 0.005%, the cold workability is significantly deteriorated and it becomes difficult to obtain the effect of improving the induction hardenability of B. Therefore, the N content as an impurity element is set to 0.005% or less. The N content is 0.
It is preferably 004% or less, and more preferably 0.003% or less.

O: 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 remarkably decreases. Therefore, the content of O as an impurity element is set to 0.005% or less.

(B) Spheroidizing Annealing Steel having the chemical composition described in (A) above is hot-worked and then subjected to spheroidizing annealing in order to reduce the deformation resistance during cold working. This spheroidizing annealing is not particularly specified and may be performed by a usual method.

(C) Cold Working Steel material having the chemical composition described in (A), which has been spheroidized and annealed after hot working, is subjected to cold working such as cold forging and has a mechanical structure of a predetermined shape. It is molded into parts. The cold working method is not particularly limited, and may be a normal method.

It is to be noted that the hardened portion after induction hardening of the machine structural component formed into a predetermined shape by cold working is stable at J
In order to ensure the austenite grain size of IS grain size number 7 or more, the cold working is performed by the equivalent strain represented by the following formula (a) in the amount of work in the part to which the largest work is applied in the workpiece. The strain is preferably set to be 0.5 or less, and more preferably 2.0 or less at the equivalent strain.

Ε = {(ε ₁ ² + ε ₂ ² + ε ₃ ² ) × 2/3} ^1/2 (a) where ε ₁ , ε ₂ and ε ₃ in the equation (a) are mainly It is the logarithmic distortion of the direction.

(D) Induction quenching A steel material having the chemical composition described in (A) above, which has been spheroidized after hot working and then cold worked into a predetermined shape, is induction hardened. Alternatively, or if necessary, tempering is performed after induction hardening to finish a mechanical structural component having desired mechanical properties. The induction hardening method is not particularly limited and may be a normal method.

The torsional strength of the steel material induction hardened after cold working is 400 Hv as the induction hardening depth.
Depending on the above hardening depth, when the t / r is less than 0.3, the torsional strength is small, but the steel according to the present invention having the chemical composition described in (A) above has a strength of 0. A t / r of 3 or more can be easily achieved. When t / r exceeds 0.6, the improvement in torsional strength is saturated or, conversely, is reduced, and further, quenching cracks easily occur. Therefore, induction hardening may be carried out so that the value of t / r becomes 0.3 to 0.6. For this purpose, preliminary experiments were conducted with different steel types and induction hardening conditions, and based on the results. It may be induction hardened.

Further, in the steel according to the present invention having the chemical composition described in the above (A), the crystal grains are not coarsened by ordinary induction hardening, and the austenite grain size of JIS grain size number 7 or more is obtained. It has been adjusted as follows.
Therefore, heat treatment distortion, hardness variation, strength reduction, etc. due to crystal grain coarsening do not occur.

The present invention will be described below with reference to examples.

[0049]

EXAMPLES Steels having the chemical compositions shown in Tables 1 and 2 were melted by a usual method using a test furnace. Steels A to V in Table 1 are examples of the present invention whose chemical composition is within the range specified in the present invention, and steels a to t in Table 2 are comparisons in which any of the components is out of the range of the content specified in the present invention. Here is an example. Among the steels of the comparative examples, steel r, steel s, and steel t are steels corresponding to JIS standards S40C, S50C, and S58C, respectively.

[0050]

[Table 1]

[0051]

[Table 2]

Then, these steels were made into billets by a usual method, heated to 1200 ° C., and then 1200 to
Hot forging was performed at a temperature of 950 ° C. to obtain a round bar having a diameter of 30 mm. After that, spheroidizing annealing was performed by a usual method depending on the C content.

The diameter obtained as described above is 30 mm.
A test piece for cold working having a diameter of 15 mm and a length of 22.5 mm was prepared from the round bar, and a cold (room temperature) constrained upsetting test was conducted by a normal method using a 500 t high-speed press.
The upsetting rate, which is the limit at which cracking occurs, was measured. Up to 75% upsetting rate, 5 upsetting tests were conducted under each condition, and the minimum processing rate (upsetting rate) at which 3 or more of 5 test pieces cracked was set to the limit. It was evaluated as the inclusion rate. The test was terminated when the upsetting rate was 75% and three or more cracks did not occur.

Further, the deformation resistance in the case of an upsetting ratio of 60% which is less than the limit upsetting ratio of all steel types (the equivalent strain at the center of the test piece to which the largest work is applied is 1.5) was measured. In addition, as shown in FIG. 1, the deformation resistance is sorted by the content of C, and a straight line obtained from the deformation resistance of steel r, steel s, and steel t corresponding to JIS standards S40C, S50C, and S58C is a JIS mechanical structure. Deformation resistance of steel for steel, steel A
~ V of the invention steels and steels aq of the comparative steels were compared with the deformation resistance.

A test piece having a diameter of 28 mm and a length of 40 mm was cut out from the above-mentioned round bar having a diameter of 30 mm and cold-extruded by a conventional method to a diameter of 17.7 mm (area reduction ratio of 60%). (The equivalent strain of the test piece side surface portion, which is the portion to which the largest processing is applied, that is, the outermost layer portion of the test piece is 1.3). A specimen with a length of 50 mm was sampled from the cold-extruded specimen with a diameter of 17.7 mm, and induction hardening was performed at a frequency of 20 kHz on the specimen. The thickness (that is, the depth of the quench-hardened layer) t was measured. Then, using an electric furnace, tempering is performed at 150 ° C. for 30 minutes, and the hardened portion after induction hardening is subjected to a conventional method.
That is, the austenite grain size of the quench hardened layer was measured.

Tables 3 and 4 collectively show the above test results. In this example, r is the radius of a test piece having a diameter of 17.7 mm, that is, 8.85 mm.

[0057]

[Table 3]

[0058]

[Table 4]

From Tables 3 and 4, the steels A to V having the chemical composition within the range specified in the present invention as the base steels are the carbon steels for JIS mechanical structures having the same C content. On the other hand, the deformation resistance at the time of upsetting is lower by 10% or more, and the limit upsetting rate at which cracking occurs as the deformability is 75% or more. Moreover, the t / r at the time of induction hardening is 0.3 or more, and the austenite grain size of the hardened part after the induction hardening, that is, the quench hardening layer is JIS grain size number 7 or more, which satisfies the target performance.

On the other hand, when the steel of the comparative example is used as the base material, (a) the reduction margin of the deformation resistance is less than 10% with respect to the carbon steel for JIS mechanical structure having the same C content, (B)
The critical upsetting ratio is less than 75%, (c) t / r when induction hardening is less than 0.3, (d) the hardened part after induction hardening, that is, the austenite grain size of the hardening layer. Is less than JIS grain size number 7, which corresponds to any one or more of. Therefore, cold workability and induction hardenability are not compatible.

[0061]

INDUSTRIAL APPLICABILITY The steel of the present invention is excellent in cold workability and induction hardenability after spheroidizing annealing, and does not coarsen grains by induction hardening. It can be used as a base material for parts such as constant velocity joints. This mechanical structural component can be manufactured relatively easily by the method of the present invention.

[Brief description of drawings]

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

Claims (4)

(57) [Claims] 1. By weight%, C: 0.40 to 0.60%, S
i: 0 to 0.40%, Mn: 0.10 to 0.60%,
B: 0.0005-0.005%, Nb: 0.005-
0.05%, Al: 0.015 to 0.10%, Pb: 0 to 0.30%, Bi: 0 to 0.10%, T
e: 0 to 0.10%, the balance consisting of Fe and inevitable 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, Cr 0.15% or less, Mo 0.10%
Hereinafter, N is 0.005% or less and O is 0.005% or less, which is a steel for cold working excellent in induction hardenability. 2. C .: 0.40 to 0.60% by weight, S:
i: 0 to 0.40%, Mn: 0.10 to 0.60%,
B: 0.0005-0.005%, Nb: 0.005-
0.03%, containing Al: 0.015 to 0.10%,
The balance consists of Fe and unavoidable impurities. P in the impurities is 0.015% or less, S is 0.015% or less, and Cu is 0.
10% or less, Ni 0.10% or less, Cr 0.15%
Hereinafter, Mo is 0.10% or less, N is 0.005% or less,
O is 0.005% or less, which is a steel for cold working excellent in induction hardenability. 3. A machine structural component in which a base material has the chemical composition according to claim 1 or 2, and is provided with a spheroidized carbide and a quench hardened layer on the outer peripheral portion. 4. A steel material having the chemical composition according to claim 1, which is spheroidized and annealed after hot working, is cold worked into a predetermined shape, and then induction hardened. Manufacturing method of machine structural parts.