JPH01225750A - Tough and hard steel for machine structural use excellent in cold forgeability - Google Patents

Abstract

PURPOSE:To easily obtain a tough and hard steel for machine structural use excellent in cold forgeability by specifying a composition consisting of C, Si, Mn, Cr, Cu, P, S, O, and Fe. CONSTITUTION:This tough and hard steel for machine structural use has a composition consisting of, by weight, >0.45-0.60% C, <0.02% Si, 0.40-0.70% Mn, 0.35-1.20% Cr, <=0.02% Cu, <=0.01% P, <=0.01% S, <=0.002% O, and the balance essentially Fe and satisfying 5.0<=(1+4.1Mn)(1+2.33Cr)<=10, and this steel is excellent in all of various cold forging characteristics, such as low deformation resistance, high hardenability, and surface oxidation resistance.

Description

[Detailed Description of the Invention] (Field of Industrial Application) The present invention relates to a strong steel for mechanical structures with excellent cold forging properties, and is particularly suitable for application to mechanical structural parts such as constant velocity joint outerers for automobiles. It is something.

(Conventional technology) Conventionally, mechanical parts such as constant velocity joint outer,
Carburized case-hardened steel has been used, but recently, with improvements in steel quality, cold and hot forging processing technology, and induction hardening technology, strong steel for machine structures has come to be used. .

However, such strong steels for machine structures are mainly made of C, St.
, Mn and Cr are added to improve hardenability and strength, but because the amounts of these elements added are relatively large, the deformation resistance during cold forging as plastic working is extremely high. Therefore, problems tend to occur in terms of machine processing ability, surface lubrication, and mold life. In other words, when the deformation resistance increases, not only does it become necessary to increase the capacity of the machine, but it also causes insufficient strength of the lubricating film and shortens the life of the mold.

Regarding this point, various improvement measures have been proposed in the past. For example, "Plastic Working Spring Lecture J (1987゜5.
15-17 P, 301-302), Cr-added steel and Cr-B-added steel are disclosed as materials that do not impair hardenability (reduce deformation resistance).However, all of the above steels are basic. The amount of Si added is 0.06
%, the deformation resistance is still high as will be described later, and there is also the disadvantage that surface oxidation progresses during softening annealing.

In addition, in Japanese Patent Publication No. 61413744, St, Mn
+ By limiting the amount of Cr as well as S, P, N, and O, the deformation resistance is reduced and the deformability is improved.

However, even though the above-mentioned technique achieves low deformation resistance, the hardenability is poor and the prevention of surface oxidation during softening annealing is not sufficient.

(Problems to be Solved by the Invention) As mentioned above, there has been no known material that satisfies all of the characteristics of low deformation resistance, high hardenability, and surface oxidation resistance in strong steel for machine structures, and the development of such a material is difficult. was desired.

The present invention aims to advantageously solve the above-mentioned problems, and to propose a strong steel for machine structures that has all of the properties required of a steel for cold forging.

(Means for Solving the Problem) This type of steel material has a ferrite-pearlite structure that has high deformation resistance and poor deformability, so it is subjected to cold forging after carbide is spheroidized. After being subjected to such cold forging and machining, it is subjected to induction hardening.

As a result of careful study of the deformation resistance, induction hardenability, and surface properties of the spheroidized structure, the inventors found that the desired objective could be advantageously achieved by using the following component system. This knowledge led to the completion of this invention.

That is, this invention provides C: more than 0.45 to 0.60wt1
(hereinafter simply expressed as %), Si: less than 0.02%, M
n: 0.40-0.70%, Cr: 0.35-1.
20% and contains Mn and Cr in a range that satisfies the following formula (1), further Cu: 0.02%
Below, P: 0.01% or less, s: o, oi% or less, O: 0.002wtX or less, and the remainder is essentially Fe for mechanical structures with excellent cold forgeability. It is strong steel.

Note 5.0 ≦(1+4. I X Mnχ) (1+
2.33XCrX)≦10 (1) (Function) The reason why the component composition is limited to the above range in this invention will be explained.

C: More than 0.45 to 0.60% C is an element useful for increasing the strength of a member, and the surface hardness especially when induction hardened is determined by the C content.

At least 0.45% to satisfy such surface hardness.
Therefore, the lower limit was set to exceed 0.45%. On the other hand, C
If the amount is too large, the surface hardness after induction hardening will become excessive, and the deformation resistance during cold forging will increase, so the upper limit was set at 0.60%. A more preferable composition range is more than 0.50 to 0.60%.

Si: Less than 0.02% St is a harmful element that not only causes solid solution hardening and increases deformation resistance, but also promotes surface oxidation during the heat treatment process. Therefore, it is desirable to reduce the amount of Si mixed in as much as possible, but less than 0.02% is acceptable. Particularly preferably less than 0.01%.

Mn: 0.40 to 0.70% Mn is an effective element for improving hardenability, and can increase the strength of the member at relatively low cost. However, it forms a solid solution in the matrix in a spheroidized state, causing solid solution hardening like Si. Since at least 0.40% is required to ensure strength and prevent hot embrittlement due to S, the lower limit was set at 0.40%. On the other hand, adding too much Mn causes solid solution hardening, so the upper limit is 0.70.
%.

Cr: 0.35-1.20% Cr is a useful element that improves hardenability, and it also concentrates during the process of spheroidizing carbides and reduces the amount of solid solution in ferrite, so it does not solidify in the spheroidized structure. It has the advantage that melt hardening is smaller than that of Si and the like. It has also been found that the presence of Cr reduces re-dissolution of carbides due to undesired deformation and also effectively contributes to reducing strain aging due to C. At least 0.35% to fully exhibit this effect.
Since the above amount is required, the lower limit was set to 0.35%. On the other hand, when the amount of Cr added is increased, the diffusion rate of C decreases and the progress of spheroidization is inhibited, and the deformation resistance increases due to finer carbides, so the upper limit is 1.20%.
And so.

By the way, Mn (!: Cr) are both elements that improve hardenability, but it has been found that simply satisfying the above range does not necessarily result in sufficiently satisfactory induction hardening.Therefore, stable induction hardening cannot be achieved. The relationship between the amounts of Mn and Cr added and the hardening depth was investigated. The results are summarized and shown in Figure 1.

That is, in order to obtain satisfactory hardenability, (1+4.
It was found that the parameter shown by I x Mn″t) (1+2.33×Cr%) needs to be 5 or more. However, if the above parameter exceeds 10, there is no problem with hardenability. However, since this causes an increase in deformation resistance, it is necessary to set it to 10 or less.

Other ingredients are as follows.

Cu: 0.02% or less Cu is a harmful element that not only deteriorates hot workability and product quality but also increases deformation resistance during cold forging, but within a range of 0.02% or less acceptable.

P: 0.01% or less P is normally contained as an impurity at about 0.015% or more, but by reducing it as much as possible, deformation resistance is lowered, cracking during cold forging is prevented, and it is also possible to prevent induction hardening. Since it improves the toughness of the parts, it was decided to limit it to 0.01% or less.

S: 0.01% or less S forms inclusions that extend in the rolling direction as sulfides, which not only serve as starting points for cracks during cold forging, but also promote the propagation of cracks, and even cause damage to induction hardened parts. Since it deteriorates toughness, it is desirable to reduce it as much as possible, and therefore the allowable upper limit was set at 0.01%.

0: 0.002% or less O combines with Mn, AI, and even trace amounts of Si to form oxides, which not only causes deterioration of ductility during cold forging, but also deteriorates the fatigue resistance of mechanical parts. Since it causes deterioration, the upper limit of tolerance is set at 0.002%.

In addition, AI can be contained in an amount that is used as a deoxidizing agent during the forging process, but if it is contained in a large amount, it will not only form underground, but also cause solid solution hardening and reduce deformation resistance during cold forging. It is desirable to minimize the amount as much as possible. Furthermore, N, V, Mo.

It is also desirable to reduce the inclusion of Ni and the like as much as possible.

Next, a general manufacturing method for this invention steel will be described.

It is melted in a converter or electric furnace, deoxidized and fine-tuned by out-of-furnace refining, and then continuously cast or ingot-formed. After that, it is rolled into a steel billet and then rolled into a product, mainly into a steel bar or wire rod. In addition, in the electric furnace, Si+S+Cu, NitMo, V, As,
Since there is a large risk of contamination with impurities such as Sn, and R selection of the scrap is required, melting in a converter is advantageous.

(Example) Example I C: 0.51-0.55%, St: less than 0.01%, Mn: 0.45-0.50%, Cr: 0.35
In addition to ~0.42%, steel materials containing P, S, Cu, and 0 in various ranges shown in Table 1 were subjected to spheroidization treatment, and the effects of each of the above elements on cold forgeability were investigated. investigated.

Cold forgeability was evaluated by machining test specimens for compression tests, determining the compression ratio at which cracks occur in 50% of the test specimens as the critical compression ratio, and determining the magnitude of this critical compression ratio.

The survey results are also listed in Table 1.

Test speed: 40 cps As is clear from the same table, a high critical compressibility was obtained only when the component composition of the present invention was satisfied.

Example 2 Steel materials having various compositions shown in Table 2 were subjected to spheroidization treatment and then subjected to the following test.

(]) We investigated cleanliness such as internal oxidation of the surface.

(2) A test piece for a compression test was machined, and the deformation resistance was measured by a pressure test.

(3) After machining a 30 mmφ test piece, it was induction hardened at 15 Hz, then tempered at 200°C for 140 m1n, and the hardness distribution from the surface was measured. The induction hardenability was evaluated using the distance from the surface as the effective hardening depth.

The results of each survey are also listed in Table 2.

As is clear from the same table, the surface cleanliness was such that internal oxidation did not occur due to the component composition of the present invention, particularly Si: less than 0.01%, and a clean surface condition was exhibited.

Next, the deformation resistance Rm is the load P obtained by the compression test.
(kgf) using the following formula.

Ho: Height of the sample before deformation do: Diameter of the sample before deformation H: Height of the sample after deformation All specimens that satisfy the appropriate range of this invention have low deformation resistance. Note that some of the materials outside the appropriate range have low deformation resistance, but are inferior in induction hardenability, which will be described later.

Next, regarding induction hardenability, the composition within the composition range of this application showed a hardening depth of 3.9 m or more. On the other hand, there are materials that exhibit good hardenability even outside the appropriate range, but these are all materials with high deformation resistance.

(Effects of the Invention) Thus, according to the present invention, it is possible to easily obtain a steel material that has low deformation resistance, excellent induction hardenability, and does not undergo internal oxidation on the surface during heat treatment processes such as spheroidizing annealing, and is suitable for industrial use. This greatly contributes to the production of consistently high-quality mechanical parts.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the influence of Mn and Cr on hardenability.

Claims (1)

[Claims] 1. C: more than 0.45 to 0.60 wt%, Si: less than 0.02 wt%, Mn: 0.40 to 0.70 wt%, Cr: 0.35 to 1.20 wt%. And, the amount of Mn and Cr is contained in a range that satisfies the following formula (1), furthermore, Cu: 0.02wt% or less, P: 0.01wt% or less, S: 0.01wt% or less, O: 0.002wt % or less, and the balance is substantially Fe, and has excellent cold forgeability. 5.0≦(1+4.1×Mn%)(1+2.33×Cr
%)≦10...(1)