JPH108189A - Steel for induction hardening excellent in bendability and induction hardened part excellent in bendability using the same steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent brittle fracture in a steel even if excessive loads are applied by prescribing the contents of elements. SOLUTION: It is effective for preventing the generation of brittle fracture even if excessive loads are applied and for preventing the fracture by bending deformation to incorporate B into the steel. Namely, by the addition of B, its bendability after induction hardening treatment can remarkably be improved. Furthermore, by the incorporation of Mn, Cr, Ni, Mo, or the like, this effect can moreover be improved. For this purpose, the contents of the elements are prescribed as follows: by weight, 0.30 to 0.60% C, <=0.50% Si, 0.20 to 2.0% Mn, 0.0005 to 0.0050% B, <=0.020% N, <=0.1% To, also, the ratio of the contents of Ti to N: 3.42<=Ti/N<=8.0, and the balance Fe or the like. If required, one or >=two kinds among <=1.50% Ni, <=0.50% Mo, 0.50% V and <=2.0% Cr may be incorporated therein.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to steel materials and parts which are subjected to induction hardening such as steering parts (steering racks), power transmission shafts, automobile undercarriage parts and the like, and which are statically or dynamically applied. The present invention relates to an induction hardened steel having excellent bending characteristics, which does not break brittlely even when an excessive load is applied, and an induction hardened part using the steel.

[0002]

2. Description of the Related Art Conventionally, carbon steel, Cr-Mo steel, or V-added steel having a carbon content of about 0.4% to 0.5% has been used as a steel material for induction hardening. For example, a steering rack for an automobile includes S45C or S48C.
C is applied, and after the parts are manufactured by machining or the like, the required strength is obtained by performing induction hardening and tempering. The steering rack is an indispensable part of the steering mechanism. Reliability and strength characteristics are required.
The most important characteristic required of a steering rack is that it does not break brittlely even when an excessive load is applied statically or dynamically, and that it bends and deforms even when an excessive load is applied. Requires that the parts do not completely break apart.

[0003] At present, the manufacturing of a steering rack employs the steps of rolled steel → quenching and tempering → machining → induction hardening and tempering → finishing. In order to avoid the brittle fracture as described above as much as possible, without directly processing and induction hardening treatment of the rolled steel material, improve the toughness of the material by performing quenching and tempering treatment,
By processing the parts and performing induction hardening, brittle destruction is prevented.

However, with the recent mounting of high-power engines and the like, further improvement in strength of steering racks is required, and development of steel materials to achieve this is desired. In addition, quenching and tempering treatment is applied in the current manufacturing process, which causes a decrease in productivity and an increase in manufacturing costs. There is a demand for the development of steel that can be prevented. Also, in the induction hardening process, if the subsequent tempering process can be omitted, the manufacturing cost can be reduced, and the development of a material that can cope with these is desired.

[0005]

SUMMARY OF THE INVENTION The present invention relates to a part to be subjected to induction hardening such as a steering rack,
Even if an excessive load acts on the part, without providing brittle fracture, it is to provide an induction-quenched steel and an induction-quenched part capable of preventing fracture by bending deformation, and furthermore, Provided is a method for producing induction hardened steel and an induction hardened part capable of obtaining the same strength characteristics even if the conventional quenching and tempering treatment or the tempering treatment after the induction hardening treatment is omitted. Therefore, it is intended to reduce the manufacturing cost.

[0006]

SUMMARY OF THE INVENTION In order to solve the above problems, the present invention does not cause brittle fracture even when an overload is applied,
It has been found that it is effective to contain B in order to prevent breakage by bending deformation. Also, M
It has been found that this effect can be further improved by including n, Cr, Ni, Mo and the like. Further, the effective hardening depth t obtained by induction hardening and tempering
With respect to the relationship between the thickness and the component radius or thickness r, by setting the t / r to 0.3 or more and the surface hardness to 600 HV, the strength is equal to or more than that of the conventional quenched and tempered component. Was obtained.

[0007] The induction hardening steel having excellent bending properties according to the present invention has a content of alloying elements by mass% and C: 0.30%.
~ 0.60%, Si: ≤0.50%, Mn: 0.20
2.0%, B: 0.0005-0.0050%, N: ≦
0.020%, Ti ≦ 0.1%, the ratio of the contents of Ti and N is 3.42 ≦ Ti / N ≦ 8.0, and the balance F
e and inevitable impurities (claim 1). further,
Ni: ≦ 1.50%, Mo: ≦
One or more of 0.50%, V: ≤0.50%, and Cr: ≤2.0% can be contained (claim 2). Further, if necessary, Nb: ≦ 0.
20%, Zr: ≤0.10%, Ta: ≤0.20%, A
1: One or more of 0.10% can be contained (claim 3). Further, if necessary, S: ≦ 0.20%, Pb: ≦ 0.20%, B
i: ≦ 0.20%, Te: ≦ 0.10%, Ca: ≦ 0.
It may contain one or more of the compounds of the present invention (Claim 4).

The induction hardened part having excellent bending properties according to the present invention has a content of alloying elements by mass% and C: 0.30%.
~ 0.60%, Si: ≤0.50%, Mn: 0.20
2.0%, B: 0.0005-0.0050%, N: ≦
0.020%, Ti ≦ 0.1%, the ratio of the contents of Ti and N is 3.42 ≦ Ti / N ≦ 8.0, and the balance F
e and unavoidable impurities, according to JIS G 0559
T / r between the effective hardening depth t and the part diameter or thickness r obtained by the induction hardening and tempering treatment specified in
≧ 0.3 is satisfied, and the surface hardness is 600 HV or more (claim 5). Further, if necessary, in mass%, Ni: ≤ 1.50%, Mo: ≤ 0.50
%, V: ≦ 0.50%, and Cr: ≦ 2.0%.
Further, if necessary, in mass%, Nb: ≦ 0.20%,
Zr: ≦ 0.10%, Ta: ≦ 0.20%, Al: ≦
One or more of 0.10% can be contained (claim 7). If necessary, mass%
S: ≦ 0.20%, Pb: ≦ 0.20%, Bi: ≦
0.20%, Te: ≤0.10%, Ca: ≤0.05%
And at least one of them can be contained (claim 8).

[0009]

The reason for limiting each alloy element will be described below. C: 0.30 to 0.60% C is an essential element for obtaining the strength of steel materials and components, and contains at least 0.30% for performing induction hardening to obtain a surface hardness of 600 HV or more. It is necessary. However, even if the content exceeds 0.60%, the surface hardness is saturated and cracking during induction hardening becomes remarkable, so the upper limit of the C content is set to 0.60%.

Si: ≦ 0.50% Si is added as a deoxidizing agent, but if it is contained in excess of 0.5%, hot workability and machinability are reduced. It was defined as 0.50%.

Mn: 0.20 to 2.0% Mn is also added as a deoxidizing agent similarly to Si, but is an element that greatly improves hardenability. In order to obtain a predetermined hardened layer in induction hardening treatment. Must contain at least 0.20%. Also, in order to improve the bending characteristics with an increase in the amount of Mn, the content is preferably 0.5% or more. However, if the content exceeds 2.0%, the occurrence of quenching cracks becomes remarkable. 2.0% upper limit of content
Stipulated.

B: 0.0005% to 0.0050% B is an extremely important element in the present invention, and has an effect of significantly improving bending properties after induction hardening in addition to an effect of improving hardenability. To be added. In order to obtain this effect, it is necessary that the B content is at least 0.0005% or more. If the B content exceeds 0.0050%, cracking during hot working becomes remarkable. , B content is specified as an upper limit of 0.0050%.

N: ≦ 0.020% N combines with B in steel to form BN. However, if BN is formed, the hardenability and bending characteristics are reduced. It was specified as 0.020%.

Ti: ≦ 0.10% Ti bonds to N in steel to form TiN,
N is added to suppress the bonding of N to B and maintain the effect of B on hardenability and bending properties. On this occasion,
The amount of Ti added is determined according to the amount of N, and it is necessary to contain Ti at least 3.42 times the amount of N. However, even if the content exceeds 0.1%, the effect is saturated and large TiN is generated, which may cause a decrease in fatigue strength. Therefore, the upper limit of the Ti content is set to 0.10%. Stipulated.

[0015] As described above, in order to combine N in steel with Ti, as described above,
At least Ti / N needs to be 3.42 or more.
This is based on the assumption that Ti and N are bonded one-to-one.
This is a value determined from the weight ratio of Ti and N. Also, N
When the amount is 0.015% or more, if the ratio of Ti and N is increased, large TiN is generated and the fatigue characteristics are deteriorated. Also,
When Ti / N> 8.0, the tendency is remarkably recognized. Therefore, the ratio of Ti / N is set to 3.42 ≦ Ti / N ≦ 8.0.

T / r ≧ 0.3 Assuming that the effective hardening depth obtained after induction hardening treatment is t and the component radius or thickness is r, it was confirmed that the larger the t / r, the better the bending characteristics. . However, t /
When r is smaller than 0.3, a remarkable effect of improving bending characteristics cannot be obtained, so t / r is specified to be 0.3 or more.

It has been found that in order to improve the surface hardness and bending characteristics after induction hardening and tempering, it is important to make the structure a uniform martensitic structure.
In steel containing 0.3 to 0.6% of C, in order to obtain a uniform steel martensite structure during induction hardening,
It is required that the surface hardness be 600 HV or more.
Further, when the surface hardness is reduced, the surface hardness after induction hardening is set to 600 HV or more in order to deteriorate the wear resistance and the fatigue characteristics in addition to the deterioration of the bending characteristics.

The elements Ni, Mo, V, and Cr have the effect of improving the bending characteristics after induction hardening and also have the effect of improving the fatigue characteristics and wear resistance. Can be. However, even if it is contained in a large amount, the effect is saturated and the machinability is deteriorated. Therefore, Ni: ≦ 1.50% and Mo: ≦
0.50%, V: ≤0.50%, Cr: ≤2.0%.

The elements Nb, Zr, Ta, and Al have the effect of suppressing the growth of austenite crystal grains and maintaining the crystal grains finely, and contribute to the improvement of bending characteristics. Can be. However, when added in a large amount, a large crystallized substance is formed and the fatigue properties and workability are reduced. Therefore, the upper limit of each content is set to Nb: ≦ 0.20.
%, Zr: ≤0.10%, Ta: ≤0.20%, Al:
≤ 0.10%.

The elements S, Pb, Bi, Te, and Ca are added to improve machinability. However, if contained in a large amount, the bending characteristics are deteriorated.
20%, Pb: ≤ 0.20%, Bi: ≤ 0.20%, T
e: ≤ 0.10%, Ca: ≤ 0.05%.

[0021]

EXAMPLES Table 1 shows the chemical composition of the steel material according to the present invention and the comparative steel.
Shown in These steel materials are all produced by a conventional method, and are hot-rolled into billets after melting and casting, and then rolled into steel bars having a diameter of 30 mm.

[0022]

[Table 1]

The induction hardenability was evaluated from the rolled material with a diameter of 1 mm.
A 5 mm long, 200 mm long columnar test piece for induction hardening test was prepared by machining,
Induction hardening was performed at a frequency of 55 kHz, a frequency of 1.5 kHz, and a cooling time of 1.5 seconds, and a tempering treatment was performed at 150 ° C. for 2 hours. In addition, JI
The effective hardened layer depth was measured in accordance with SG0559, the ratio to the part radius was determined, and the surface hardness was measured.
Table 2 shows the measurement results.

[0024]

[Table 2]

In the evaluation of the bending characteristics, the same test piece as the test piece used in the above-mentioned induction hardening test was used, and an induction hardening and tempering treatment was performed under the same conditions. This test piece was subjected to a three-point bending test with a fulcrum interval of 150 mm, and the maximum deformation amount before fracture was measured at a load applied point at the fulcrum center. In addition, the load speed is two of a static load of 0.01 mm / min and an impact load of 50 mm / sec (3000 mm / min).
Performed the standard. The maximum deformation is measured by a dial gauge. Table 3 shows the measurement results.

[0026]

[Table 3]

Induction hardening and bending characteristics were evaluated using a real steering rack. In the manufacturing process of the steering rack, in the invention steel, rolling material → cutting → machining → induction hardening or tempering or tempering after induction hardening is omitted. In addition to the comparative product manufactured in the same process as described above, parts were manufactured using a material that had been subjected to the same process as in the prior art and subjected to quenching and tempering after rolling. Each was evaluated for hardness characteristics and bending characteristics after induction hardening. In the manufacture of the steering rack, the quenching and tempering conditions after rolling were as follows: quenching temperature: 870 ° C., holding: 30 minutes, cooling: water cooling, tempering temperature: 550 ° C., holding: 2 hours, cooling: cooling. is there. The induction hardening conditions were as follows: frequency: 20 kHz, output:
Moving quenching was performed at 11.5 kW, moving speed: 10 mm / sec, and cooling: water cooling. The tempering treatment after induction hardening is as follows: temperature: 160 ° C., holding: 2:00, cooling: standing cooling.

In order to evaluate the induction hardening property using a steering rack, the hardness of the section at a position 200 mm from the end was measured by hardness measurement in accordance with JIS G 0559, and the surface hardness, effective hardening depth, and part radius (thickness) were measured. The ratio with r was determined. For the evaluation of bending characteristics, a three-point bending test with a fulcrum interval of 300 mm was performed, and a load speed of 0.01 mm / min.
A bending test at mm / sec was performed to measure the maximum bending amount at the load point. Table 4 shows the results of the test using the actual product.

[0029]

[Table 4]

As shown in Table 2, the surface hardness after induction hardening and after tempering corresponded well with the C content. Comparative steel No. 15 has a low C content of 0.25%,
It can be seen that a surface hardness of 600 HV cannot be obtained, and it is necessary to contain C more than this in order to obtain a predetermined hardness. Invention Steel No. 1 has a C content of 0.31%, which is close to the lower limit, but it is possible to obtain a hardness of 600 HV or more. In order to stably obtain a hardness of 600 HV or more, 0.30% or more is required. It is necessary to set the C amount. Further, comparing the ratio t / r between the effective hardening depth and the radius of the test piece, all invention steels obtained 0.3 or more,
Even when the treatment is carried out under the same conditions, the effective hardening depth of each of the comparative steels is shallow, and a predetermined t / r cannot be obtained. Comparative steel N
o. 17 is an example in which B was added, but sufficient hardenability was not obtained because Ti / N was as low as 0.4, and the effective hardening depth was shallow. As described above, it is possible to improve the effective hardening depth by containing predetermined B as in the case of the invention steel and defining the ratio of Ti and N.

Table 3 shows the bending properties of the test pieces subjected to the induction hardening and tempering treatment. In both the case where the load speed is low and the case where the load speed is high, the deformation amount of the invention steel is larger and the bending properties are excellent. It was confirmed that. In addition, Cr, Ni, M
The bending properties tend to be further improved by adding an element such as o. Further, as described in Table 2, the effective hardening depth of the comparative steel is shallower and t / r is smaller than that of the developed steel, so that the bending characteristics are lower.

Table 4 shows the results of an investigation on the induction hardening characteristics and bending characteristics of the actual steering rack. Comparative steel N
o. Comparing the same material as in 11 or 12 under the manufacturing condition shows that the bending property is improved by quenching and tempering the rolled material as in the related art. The tempering treatment after induction hardening improves the bending characteristics. All of the inventive steels were manufactured without rolling and quenching and tempering after rolling, but it was confirmed that they exhibited much superior bending properties as compared with conventional process products. Further, in the comparative steel, the t / r did not satisfy 0.3, and the bending characteristics were lowered. However, it is recognized that the larger the t / r, the better the bending characteristics.

[0033]

According to the above embodiments, the present invention can improve the strength as an induction hardened component, and can omit the quenching and tempering process and the tempering process after the induction hardening. Industrial effects such as improvement of productivity and energy saving by omitting heat treatment are extremely remarkable.

Claims (8)

[Claims] 1. The method according to claim 1, wherein the content of the alloy element is% by mass and C:
0.30 to 0.60%, Si: ≤0.50%, Mn:
0.20 to 2.0%, B: 0.0005 to 0.0050
%, N: ≦ 0.020%, Ti ≦ 0.1%, and T
An induction hardening steel excellent in bending properties, characterized in that the ratio of i and N contents is 3.42 ≦ Ti / N ≦ 8.0 and the balance is Fe and unavoidable impurities. 2. Ni: ≦ 1.50 in mass%.
%, Mo: ≤ 0.50%, V: ≤ 0.50%, Cr: ≤
The steel for induction hardening excellent in bending properties according to claim 1, characterized by containing one or more of 2.0%. 3. Further, in mass%, Nb: ≦ 0.20
%, Zr: ≤0.10%, Ta: ≤0.20%, Al:
The steel for induction hardening excellent in bending properties according to claim 1 or 2, characterized by containing one or more of ≦ 0.10%. 4. Further, in mass%, S: ≦ 0.20
%, Pb: ≤0.20%, Bi: ≤0.20%, Te:
≦ 0.10%, Ca: ≦ 0.05%, wherein one or more of the following are contained: Excellent induction hardening steel. 5. The method according to claim 1, wherein the content of the alloy element is% by mass and C:
0.30 to 0.60%, Si: ≤0.50%, Mn:
0.20 to 2.0%, B: 0.0005 to 0.0050
%, N: ≦ 0.020%, Ti ≦ 0.1%, and T
The content ratio of i and N is 3.42 ≦ Ti / N ≦ 8.0, and the balance consists of Fe and unavoidable impurities.
The difference between the effective hardening depth t after induction hardening and tempering treatment specified in G 0559 and the part diameter or thickness r is t /
An induction hardened part having excellent bending characteristics, wherein r ≧ 0.3 is satisfied and the surface hardness is 600 Hv or more. 6. Further, in mass%, Ni: ≦ 1.50
%, Mo: ≤ 0.50%, V: ≤ 0.50%, Cr: ≤
The induction hardened component having excellent bending properties according to claim 5, characterized in that it contains one or more of 2.0%. 7. Further, in mass%, Nb: ≦ 0.20
%, Zr: ≤0.10%, Ta: ≤0.20%, Al:
The induction hardened part having excellent bending properties according to claim 5 or 6, characterized by containing one or more of ≦ 0.10%. 8. Further, in mass%, S: ≦ 0.20
%, Pb: ≤0.20%, Bi: ≤0.20%, Te:
≦ 0.10%, Ca: ≦ 0.05%, wherein one or more of the following are contained: The bending characteristic according to claim 5, claim 6, or claim 7. Excellent induction hardened parts.