JPH04314843A - Medium carbon steel material for machine parts excellent in induction hardenability - Google Patents

Abstract

PURPOSE:To obtain a steel material excellent in shearing property and cold workability and having high torsion strength after induction hardening by reducing Si content, adding Ti and B, and regulating respective contents of Cr and Mn. CONSTITUTION:The steel material is a medium carbon steel material for machine parts having a composition consisting of, by weight, 0.35-0.60% C, <=0.05% Si, >0.50-<1.70% Mn, <=0.020% P, <=0.035% S, >0.50-<1.00% Cr, 0.01-0.05% Ti, 0.0003-0.0050% B, <=0.005% N, <=0.002% 0, 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 medium-carbon steel material for mechanical parts that has excellent cold workability and induction hardenability even after hot rolling, and is particularly suitable for mechanical structural parts such as constant velocity joints for automobiles. Regarding steel.

0002

[Prior Art] Conventionally, constant velocity joints and other mechanical parts have been hardened and made of high carbon steel for wear resistance and workability during use.
It is used after being tempered to increase its surface hardness. In addition, due to environmental issues in recent years, especially the demand for reducing CO2 gas,
There is a demand for lighter weight automobile parts, and from this perspective, in the case of shaft parts, etc., there is a demand for improvement in torsional strength, which is the most important product characteristic. Torsional strength depends on the quenching depth in induction hardening, and mere addition of alloying elements improves hardenability but inhibits other properties.

[0003] In order to improve torsional strength, strong steels are being used instead of carbon steels. Hardenability and strength of tough steel are mainly improved by adding C, Si, Mn, Cr, etc., but because the amount of these elements added is relatively large, the deformation resistance is extremely high. Deformation during cold working is inhibited, and in the shear cutting process of as-rolled materials, the machining capacity for cutting increases, which is a significant obstacle in cost and productivity.

0004

[Problems to be Solved by the Invention] Therefore, the present invention solves the above-mentioned problems and provides medium carbon steel bar wire rods and steel materials for machine parts that have excellent shear cutting properties, high torsional strength, and excellent induction hardenability. The purpose is to provide

[0005]

[Means for Solving the Problems] Workability and hardenability are contradictory properties, and it has hitherto been considered difficult to have both of them. As a result of intensive studies, the present inventors have found that it is possible to improve the hardenability and torsional strength while maintaining the current level of workability. As a result of examining the effects of various alloying elements on as-rolled strength, which is related to shear cutting properties, and hardenability, which is related to torsional strength, we found that low Si,
The present invention was constructed by discovering that by adding B, Cr, or further Mo, the strength as rolled can be lowered and the hardenability can be improved.

That is, the present invention provides 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 1.70% by weight, P:
0.020% by weight or less, S: 0.035% by weight or less, Cr: more than 0.50% by weight and less than 1.00% by weight, Ti: 0
．． 01-0.05% by weight, B: 0.0003-0.0050% by weight, N:
A medium-carbon steel material for mechanical parts having excellent induction hardenability, containing 0.005% by weight or less O: 0.002% by weight or less, and the remainder consisting of Fe and unavoidable impurities.

[0007] A medium carbon steel material for mechanical parts having excellent induction hardenability, which further contains Mo: 0.05 to 0.50% by weight.

[0008]

[Action] C: An element that strongly affects 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 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 must be reduced to 0.0
1% by weight or less is preferable, but considering surface oxidation during heat treatment,
The content was set to 0.05% by weight or less.

[0009] Mn: An element that improves the hardenability of steel materials, and at the same time fixes S in the steel to prevent hot embrittlement. When added, but below 0.50% by weight, the hardenability The upper limit was set because there was no improvement effect and adding more than 1.7% by weight would reduce tool life during rolling. P: This element promotes quench cracking during quenching, so it is desirable to reduce it as much as possible, but 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 0.50% by weight in order to improve deformability during cold working. If 1.00% by weight or more is added, shear cutting performance will decrease, and from a cost perspective, the upper limit is set at 1.00% by weight.
shall be.

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 near the center of the shaft.

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 investigations on shear cutting performance, quenching depth of steel materials, 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 16: The steels of the present invention exhibit superior properties to conventional steels in all investigated items such as shear cutting properties and torsional strength.

[0016]No. 17, 18: Regarding the influence of the amount of C, when the amount of C is less than 0.35% by weight (No. 17), the torsional strength is insufficient, and when it is added in a large amount of 0.68% by weight (No. 18). The TS in the as-rolled state increases, leading to damage to the shear. No. 19: If Si is more than 0.05% by weight, surface oxidation will occur during heat treatment, making the shear blade more likely to be damaged.
There was a series of exchanges. A decrease in cold workability was also observed.

[0017]No. 20, 21: When the amount of Mn is less than the claimed range (No. 20), the shear cutting property is good, but the torsional strength is not increased compared to conventional steel. Added a large amount of No.
．． In case 21, when cutting with shear, leave it as rolled,
As TS increases, the shear becomes more likely to be damaged. No. 22, 23: P and S are each 0.020% by weight
, if more than 0.035% by weight is added or remains,
Due to grain boundary P segregation and increase in MnS inclusions, cracks occur on the steel side during shear cutting. No. 24, 25, 26: Cr (No. 24, 25),
Addition of Mo (No. 26) improves hardenability and thus increases torsional strength, but contributes to an increase in TS as rolled and causes shear damage.

[0018]No. 27: Adding a large amount of Ti means adding a large amount of Ti.
It forms N, becomes a starting point for crack initiation, and promotes brittle fracture. No. 28: Adding a large amount of B not only reduces toughness, but also segregates at grain boundaries, causing cracks in the steel material during shear cutting.

[0019]No. 29: Addition of a large amount of N promotes age hardening during heat treatment and reduces shear cutting properties. No. 30: When the amount of O was large, brittle fracture occurred in the steel material during shear cutting due to the increase in the amount of oxides.

[0020]

[Table 1]

[0021]

[Table 2]

[0022]

[Table 3]

[0023]

[Table 4]

[0024]

According to the present invention, high-strength medium carbon wire rod products can be produced without hindering productivity and cost, and the present invention has great utility in industry.

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 1.70% by weight P:
0.020% by weight or less, S: 0.035% by weight or less, Cr: more than 0.50% by weight and less than 1.00% by weight, Ti: 0
．． 01-0.05% by weight, B: 0.0003-0.0050% by weight, N:
A medium-carbon steel material for mechanical parts having excellent induction hardenability, containing 0.005% by weight or less O: 0.002% by weight or less, and the remainder consisting of Fe and unavoidable impurities. 2. The medium carbon steel material for mechanical parts having excellent induction hardenability according to claim 1, further containing Mo: 0.05 to 0.50% by weight.