JPH04314842A - Steel material for shaft parts excellent in induction hardenability - Google Patents

Abstract

PURPOSE:To obtain shaft parts excellent in shearing property and cold workability and having high induction hardenability and high torsion strength by reducing Si content, adding Ti and B, and regulating respective contents of Cr and Mn. CONSTITUTION:The steel is a steel for shaft parts having a composition consisting of, by weight, 0.35-0.60% C, <=0.05% Si, >0.50-<0.65% Mn, <=0.020% P, <=0.035% S, >0.30-<0.50% Cr, 0.01-0.05% Ti, 0.0003-0.0050% B, <=0.005% N, <=0.002% O, and the balance Fe with inevitable impurities and excellent in induction hardenability.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to a steel material for shaft parts that has excellent cold workability and induction hardenability even after hot rolling.
In particular, it relates to steel suitable for automobile drive shafts and constant velocity joints.

0002

2. Description of the Related Art Shaft parts such as drive shafts for automobiles are used after warm forging, followed by induction hardening and tempering to increase surface hardness. However, in recent years, there has been a tendency for parts to become more diverse, and there has been a demand for improved dimensional accuracy. A method is being taken to do so.

[0003] In recent years, there has been a demand for lighter automobile parts in order to reduce environmental issues, especially CO2 gas.In the case of shaft parts, etc., torsional strength, which is the most important product characteristic, has been required in order to reduce weight. Improvement is required. Generally, in order to increase torsional strength, a method of increasing the quench hardening depth by induction hardening is used. In order to increase the quench hardening depth, it is possible to change the induction hardening conditions, particularly to reduce the frequency of the high frequency induction heating device, or to increase the amount of alloying elements in the steel material. In order to lower the frequency of the high-frequency induction heating device, it is necessary to purchase a new and expensive high-frequency oscillator, which is not economical. On the other hand, when the alloying element is increased, the strength of the material increases, causing problems in cutting, cold working, etc. Furthermore, the quenching depth (X/r, X: depth of the quenching layer, r: radius of the shaft material)
It is not efficient to achieve high strength only by increasing
Even if r exceeds 0.5 and the quenching depth is increased, the strength does not increase significantly.

0004

Problems to be Solved by the Invention In order to satisfy the above-mentioned required characteristics, the problem cannot be solved by simply adding or increasing the hardenability improving element as in the conventional method. In other words, shear cuttability, cold workability, and hardenability are contradictory, and a technology that achieves both of these properties has not yet been established.

In order to solve the above-mentioned problems, the present invention provides a shaft member that has good shear cutting properties, a long tool life during cold working, and high torsional strength. The purpose is to provide a high-strength steel material for shaft parts with excellent machinability.

[0006]

[Means for solving the problems] The present invention provides C: 0.3
5 to 0.60 wt%, Si: 0.05 wt% or less, Mn: more than 0.50 wt% to less than 0.65 wt% P:
0.020% by weight or less, S: 0.035% by weight or less, Cr: more than 0.30% by weight and less than 0.50% by weight, Ti: 0
．． 01-0.05% by weight, B: 0.0003-0.0050% by weight, N:
This is a steel for shaft parts that contains 0.005% by weight or less of O: 0.002% by weight or less, and the remainder is Fe and unavoidable impurities, and has excellent induction hardenability. Furthermore, when Mo: 0.05 to 0.50% by weight is contained in addition to the above-mentioned components, a shaft component steel having even more suitable induction hardenability can be obtained.

[0007]

[Function] As a result of extensive studies, the inventors have found that the influence of chemical composition on as-rolled steel TS (tensile strength), which is involved in shear cutting properties, is as follows: C>Mn>Cr>Mo>Si> It was found that the order of hardenability was B>C>Mn>Mo<Cr>Si. Based on these findings, Si, which causes surface oxidation in the heat treatment process, can be removed from rolled material TS,
Since it has little effect on hardenability, it should be reduced as much as possible, and B.
Regarding Cr and Mo, they have a small influence on TS and can improve hardenability, and by actively utilizing them, the purpose can be achieved and the present invention has been constructed.

[0008] The reason for limiting the ingredients of the present invention will be described. C: It is an element that has the strongest influence on induction hardenability,
In order to secure the required torsional strength, it is necessary to increase the quench hardening depth, which requires 0.35% by weight or more, and to obtain good shear cutting properties and rolling properties, the upper limit is set to 0.60% by weight.
And so. Si: It has little effect on the hardenability of steel materials and has the effect of reducing tool life during rolling, so it is desirable to reduce it as much as possible, but considering surface oxidation during heat treatment, it is set to 0.05% by weight or less.

Mn: An element that improves the hardenability of steel, and at the same time fixes S in the steel to prevent hot embrittlement. If it is less than 0.50% by weight, there is no effect of improving hardenability. If it is 0.65% by weight or more, the life of the tool during rolling will be reduced, so the content is set to be less than 0.65% by weight. P: Since it is an element that promotes quench cracking during quenching, it is desirable to reduce it as much as possible, and it is allowed up to 0.02% by weight.

[0010] S: An element that improves machinability, but it lowers cold workability, so the upper limit is set to 0.03.
The content was 5% by weight. Cr: is an element effective in improving hardenability, and is added in excess of 0.30% by weight in order to improve deformability during cold working. From the viewpoint of shear cutting performance and cost, the content should be less than 0.50% by weight.

Mo: Like Cr, it is an element that improves hardenability and is effective in softening resistance, and is required in an amount of 0.05% by weight or more. If it exceeds 0.50% by weight, cold workability and toughness near the shaft center will deteriorate. Ti: An element that combines with N to form TiN and exhibits the effect of improving the hardenability of B. Although it is actively added, the amount of 0.01
If the content is less than 0.05% by weight, the effect will be small, while if the content exceeds 0.05% by weight, a large amount of TiN will be formed, leading to deterioration of fatigue properties, so the content is limited to a range of 0.01 to 0.05% by weight.

[0012] B: An element that improves hardenability when added in a trace amount; if it is less than 0.0003% by weight, the effect is small, and if it is added in excess of 0.0050% by weight,
The hardenability effect of B is reduced. N: It is reduced as much as possible to improve the hardenability of B, but 0.00
If the content exceeds 5% by weight, this leads to the formation of coarse TiN and an increase in the amount of free solid solution N, leading to deterioration of fatigue properties and toughness of the shaft center.

O: Forms oxides and impedes cold workability, so the upper limit was set at 0.002% by weight.

[0014]

[Example] The steel of the present invention is shown in Examples together with comparative steel and conventional steel. A 5t steel ingot having the chemical composition shown in Tables 1 and 3 was melted and subjected to billet rolling and steel bar rolling to form a straight bar with a diameter of 25 mm. This steel bar was used as a raw material, and after shear cutting, it was subjected to serration processing by warm forging, cold working, and rolling, and then induction quenching and tempering.

[0015] Tables 2 and 4 show the results of an investigation of shear cutting performance, quenching depth of steel, torsional strength, and tool life during rolling. Note that the tool life was evaluated with the conventional steel (No. 31) as 1. No. 1 to 12: In the steel of the present invention, shear cutting property,
It shows superior properties compared to conventional steel in terms of tool life ratio and torsional strength.

[0016]No. 13,14: C content is 0.35% by weight
If the C content is less than 0.0, the torsional strength will be lower than that of conventional steel, and the C content will be less than 0.
If it is higher than 60% by weight, shear damage will occur frequently due to an increase in TS during as-rolling. No. 15: When the amount of Si is higher than 0.05% by weight, surface oxidation is promoted during heat treatment, leading to shear damage.

[0017]No. 16,17: For Mn, 0.5
When the content is 0% by weight or less, the shear cutting properties are good, but the torsional strength is not improved compared to conventional steel. A large amount of Mn causes an increase in TS of the as-rolled material and affects workability. No. 18,19: P and S are each 0.020% by weight
, or if it remains in excess of 0.035% by weight, cracks will occur on the steel material side during shear cutting due to grain boundary P segregation and the formation of MnS inclusions.

[0018]No. 20, 21, 22: Addition of Cr and Mo increases torsional strength, but increases as-rolled TS and reduces workability. No. 23: An increase in Ti increases the formation of TiN, which causes cracks to occur on the steel side, especially during shear cutting.

[0019]No. 24: A large amount of B segregates at grain boundaries,
This causes cracks to occur on the steel side when shearing. No. 25: A large amount of N forms coarse TiN, reduces the toughness and ductility of the steel material, and impedes workability and shear cutting performance.

[0020]No. 26: The formation of a large amount of oxides induces cracks in the steel material during shear cutting.

[0021]

[Table 1]

[0022]

[Table 2]

[0023]

[Table 3]

[0024]

[Table 4]

[0025]

[Effects of the Invention] The present invention has discovered an optimal composition system for a steel material for shaft parts that satisfies shear cutting, cold working, and torsional strength.
The aim was to establish a manufacturing technology that would not hinder productivity or cost. It can also be applied to general mechanical parts.

Claims (2)

[Claims] [Claim 1] C: 0.35 to 0.60% by weight,
Si: 0.05% by weight or less, Mn: more than 0.50% by weight and less than 0.65% by weight P:
0.020% by weight or less, S: 0.035% by weight or less, Cr: more than 0.30% by weight and less than 0.50% by weight, Ti: 0
．． 01-0.05% by weight, B: 0.0003-0.0050% by weight, N:
A steel for shaft parts containing 0.005% by weight or less of O: 0.002% by weight or less, with the balance consisting of Fe and unavoidable impurities, which has excellent induction hardenability. 2. The shaft component steel with excellent induction hardenability according to claim 1, further containing Mo: 0.05 to 0.50% by weight.