JPH0623420B2 - Mechanical structural steel with excellent cold workability - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a steel for machine structural use, which has less segregation of components and is excellent in cold workability and strength characteristics such as rolling and rotary bending fatigue.

Generally, as steel for machine structure, JIS G 4051 is used.
Carbon steel materials for machine structures (SC materials, S-CK materials, etc.) and JIS G 4102, 4103, 4104
4105, 4106 and other low alloy steels for machine structures (SCr, SCM, SMn, SNC, SNC)
M material, SMnC material, etc.) are known, and all of these steel materials are manufactured from killed steel ingots as described in “3. Manufacturing method” of JIS, respectively. In conventional steel for machine structural use, hot forging or semi-hot forging is performed in the as-rolled state, and then cold forging, cold ironing, sizing,
When plastic working such as coining is performed, there is a problem that cracking may occur due to poor cold plastic workability.

For this reason, in the conventional case, strength processing is performed during hot forging or semi-hot forging, and the degree of processing is reduced during subsequent plastic forming in cold such as cold forging. In some cases, measures have been taken such as reducing the heat treatment or performing softening treatment such as intermediate annealing.

However, in the case of performing strength processing during hot forging or semi-hot forging, there is a problem that the life of the hot forging die is significantly reduced, and the die cost increases, and the intermediate firing When the softening treatment such as anneal is performed, there is a problem that the treatment cost increases, the product price rises, and the productivity also decreases. Further, the C content is 0.3 to
Even if the 0.7% steel is subjected to the intermediate annealing, the improvement of the cold plastic workability is not so large, and there is a problem that the life of the plastic working die is short and the product may still crack. Was.

Therefore, the inventors of the present invention have conducted experiments and research aiming at solving such conventional problems and obtaining a steel excellent in plastic workability especially in the cold. It was found that the reason why the plastic workability was poor was that the C content itself was present, and at the same time, segregation, particularly microsegregation occurred. Of these, the C content cannot be changed in order to secure the strength and toughness of structural parts or products, but it is possible to prevent microsegregation by taking some measures. Therefore, the first conceivable thing is to perform a soaking treatment at 1200 to 1300 ° C. for a long time in order to eliminate such microsegregation. However, such a soaking treatment requires a furnace facility and therefore the production cost is high. However, there is a problem in that the yield is greatly reduced due to the generation of oxide scale, as well as the increase in the production efficiency and the reduction in the production efficiency. Also,
It is possible to reduce the microsegregation by reducing the unit weight of the steel ingot or the size of the continuous cast piece, but this is still insufficient to prevent the microsegregation, and there is a limit to improving the plastic workability in the cold. Had the problem that there is.

The present invention has been made to solve the above-mentioned conventional problems, and does not necessarily require intermediate annealing treatment or soaking that causes an increase in equipment cost and an increase in production man-hours. The purpose of the present invention is to obtain a steel for machine structural use which is excellent in plastic workability especially in the cold state without reducing the size of slab and slab.

That is, the steel for machine structural use according to the present invention, which is excellent in cold plastic workability, contains C: 0.3% to 0.7% by weight,
Si: 0.02-0.3%, Mn: 0.8% or less as a basic content component, and if necessary, in order to control hardenability and improve strength, Cr: 0.2- 0.5%, B:
0.0005 to 0.005% of 1 or 2 is contained, the balance is Fe and impurities, and P ≦ 0.02% in the impurities and 0.25 ≦ 1.5 Si (%) + 1.3. The feature is that Mn (%) ≦ 1.0 is regulated.

Next, the reason for limiting the component range (% by weight) of the steel for machine structure excellent in cold plastic workability according to the present invention will be described.

C is an element added to secure the strength required for mechanical structural parts or products, but if the content is 0.3% or less, it is difficult to secure the above strength, and there is also a problem with cold plastic workability. Is not preferable, and if over 0.7%, the toughness deteriorates, so the range was made over 0.3 to 0.7%.

Since Si acts as a deoxidizing agent during the production of killed steel, and is an element effective in increasing the strength of the steel, it is preferable that
It is necessary to set the content to 02% or more, but if it is too large, the toughness is rather deteriorated and the plastic workability in the cold is deteriorated.
It should be 3% or less.

Mn acts as a deoxidizing agent and a desulfurizing agent during the production of killed steel, and there is an element effective for improving the hardenability of the steel and improving the strength, but since it impairs the cold workability, Mn.
It should be 8% or less.

And in the relation between Si and Mn, 0.25 ≦
1.5 · Si (%) + 1.3. By controlling Mn (%) ≤ 1.0, segregation of steel, especially microsegregation, is prevented, plastic workability in cold is remarkably improved, and rolling fatigue and rotational bending fatigue are improved in strength characteristics. It is possible to raise it. However, at the above value, such an effect that is less than 0.25 cannot be sufficiently obtained, and it becomes difficult to secure the strength and the good cleanliness.
It is necessary to set it to the above value. On the other hand, if the value of the above formula is larger than 1.0, it becomes difficult to prevent microsegregation and the plastic workability in the cold deteriorates rather. Therefore, it is necessary to set the value to 1.0 or less. is there.

Since P is an element that tends to cause segregation in steel ingots or continuous cast pieces, it prevents such segregation, especially microsegregation, improves the plastic workability in the cold, and prevents rolling fatigue and rotational bending fatigue. 0.02 to enhance strength properties
It must be less than or equal to%.

Cr and B are effective elements for controlling the hardenability of steel and increasing its strength, and Cr and B do not affect plastic workability in cold as much as Si and Mn. Therefore, one or two of these may be added depending on the purpose of use. In this case, if the content of Cr is less than 0.2%, the above-mentioned effect is not sufficient, and securing of strength cannot be expected so much.
If it exceeds 0.5%, the toughness deteriorates and the plastic workability in the cold deteriorates. Therefore, it is preferable to set it in the range of 0.2 to 05%. On the other hand, if B is less than 0.0005%, the above-mentioned effect is not sufficient, and securing strength cannot be expected so much.
Even if it exceeds 0.005%, the effect does not increase so much, so it is preferable to set it in the range of 0.0005 to 0.005%.

Hereinafter, description will be made with reference to examples.

Example 1 Steels having the chemical composition shown in Table 1 were melted by an electric furnace, then ingots were made into 2.5 ton steel ingots, and slabbing and product rolling were performed to prepare test materials having a diameter of 30 mm. During this rolling, N
o. Soaking was performed on a part of Nos. 7 and 9 under the condition of 1300 ° C. × 10 hr.

Next, in order to evaluate the cold formability of each of the test materials, cold plastic working was performed in a non-lubricated state to obtain the critical compressibility of each of the test materials. The results are shown in Table 2. In addition, No. 7,
With respect to a part of No. 9, the specimens were subjected to spheroidizing annealing, and then subjected to cold plastic working in the same non-lubricated state to determine the respective limit compressibility. The results are also shown in Table 2.

On the other hand, rolling life test pieces were prepared in order to evaluate the rolling life of each test material. Creating specimens in this case, subjected to tempering also induction hardening sintered against test piece of the diameter 30 mm, the average surface hardness H _R C45, the surface hardening depth 2.0mm
Was obtained. Then, using a cylindrical rolling life tester, a rolling life test was performed at a Hertzian stress of 600 kgf / mm ² , and a cumulative failure probability (B ₁₀ life) was obtained. The results are also shown in Table 2.

As is clear from Table 1 and Table 2, 1.5 · Si
(%) + 1.3 · Mn (%) is too small. 1
In the case of No. 2, the rolling life was short, and conversely it was too large.
9, No. It can be seen that in the case of 14, the critical compressibility is small and the cold plastic workability is deteriorated. In addition, the P content is too large No. In the case of 8, the limit compression rate and rolling life are inferior. On the other hand, in the case of the present invention, it is clear that the limit compression rate is high, the plastic workability in cold is excellent, and the rolling life is also excellent.

Then, when soaking is performed on the steel of the present invention (No. 7), the compression limit rate and rolling life are considerably improved, whereas the same soaking is performed on the conventional steel (No. 9). In this case, the critical compressibility was improved, but the rolling life was not significantly improved.

Example 2 Steel having the chemical composition shown in Table 1 was ingot-cast, and then slab rolling and product rolling were performed to manufacture a steel bar having a diameter d ₁ = 50 mm.
Next, a test material 1 shown in FIG. 1 having a height H = 26 mm was prepared. In addition, the sample material No. In 7, 9 part of it is 1
Soaking was performed under the conditions of 300 ° C. × 10 hr, a steel bar having a diameter d ₁ = 50 mm was manufactured, and then a test material 1 having a height H = 26 mm was manufactured. Next, using a forging press having a punch with a diameter of 30 mm, the shape shown in FIG. 2 is subjected to semi-hot forging at 850 ° C. to make a hole 2 with a diameter d ₂ = 30 mm, and then as shown in FIG. Then, the outer diameter portion was squeezed and cold-ioned, and the limit cold-working rate at the time when cracks occurred in each test material was obtained. The limit cold working rate here is the area S of the upper end surface in the state of FIG.
The ratio of ₀ to the area S ₁ of the upper end surface in the state of FIG. I asked for. The results are shown in Table 3.

As is clear from the results shown in Table 3, all of the steels of the present invention have a high limit cold workability and are excellent in plastic workability in cold work. It was also clarified that the critical cold working rate was further improved when soaking was performed. In addition, No. Although No. 1 also resulted in a high limit cold working rate, it is not preferable because the rolling life is short as described above.

As described above, according to the steel for machine structural use of the present invention, in weight%, C: exceeds 0.3 to 0.7%, Si: 0.
02-0.3%, Mn: 0.8% or less, C if necessary
r: 0.2 to 0.5%, B: 0.0005 to 0.005
% Of 1 or 2 with the balance Fe and impurities, P ≦ 0.02% in the impurities and 0.25
≦ 1.5 · Si (%) + 1.3. Because Mn (%) ≤ 1.0 is regulated, it is suitable for machine structures with excellent cold workability and strength characteristics such as rolling and rotational bending fatigue without intermediate annealing or soaking. Steel can be obtained, and various mechanical structural parts and products such as constant velocity joins and outer rings, racks, connecting rods, gears, and rear spindles can be produced with high accuracy and without causing defects such as cracks.
Moreover, it is possible to manufacture without significantly reducing the mold life, and it is not necessary to perform intermediate annealing or soaking, so that it is possible to realize a reduction in equipment cost and man-hours. In addition, it has a great effect that the price of structural parts can be reduced and the delivery time can be shortened.

[Brief description of drawings]

1, 2, and 3 are a side view of each material, a side view after semi-hot forging, and a side view after cold ironing showing how plastic working is performed in an embodiment of the present invention. .

Claims (2)

[Claims] 1. C: Excess of 0.3 to 0.7% by weight, S
i: 0.02-0.3%, Mn: 0.8% or less, balance F
e and impurities, P ≦ 0.0 in the impurities
Steel for machine structural use, which is excellent in cold plastic workability, characterized in that it is 2% and 0.25 ≦ 1.5 · Si (%) + 1.3 · Mn (%) ≦ 1.0. . 2. C: over 0.3 to 0.7% in weight%, S
i: 0.02-0.3%, Mn: 0.8% or less, and Cr: 0.2-0.5%, B: 0.0005-0.00
5% of 1 type or 2 types, balance Fe and impurities, P ≦ 0.02% in the impurities and 0.25 ≦ 1.5 · Si (%) + 1.3 · Mn (%) ≦ A steel for machine structural use that is excellent in cold plastic workability, characterized by being regulated to 1.0.