JPH03226548A - Steel for cold forging - Google Patents

Abstract

PURPOSE:To obtain a steel having corrosion resistance and wear resistance as well as excellent in cold forgeability by using a Cr-Mo-V series alloy steel of medium carbon medium chromium as a steel for cold forging. CONSTITUTION:At the time of manufacturing a valve lifter of an automobile engine, as the stock, a Cr-Mo-V series alloy steel of medium carbon medium chromium contg., by weight, 0.70 to 0.95% C, 0.50 to 1.00% Si, 0.30 to 0.80% Mn, 6.0 to 8.0% Cr, 0.50 to 1.20% Mo and 0.20 to 0.50% V or furthermore contg. 0.03 to 0.07% Nb is used. A valve lifter can be manufactured at a low cost by a cold forging method without depending on a conventional high cost machining method because of its excellent cold forgeability.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to steel for cold forging.

[Conventional technology]

Conventionally, 5U5440C steel has been widely used in various applications requiring corrosion resistance and wear resistance, including automobile engines. Further, valve lifters for automobile engines require wear resistance and corrosion resistance, but in this application, wear resistance is more important than corrosion resistance, and 5KDII, a type of cold work tool steel, is used.

5KDII for this purpose has conventionally been manufactured by cutting, but in recent years attempts have been made to manufacture it by cold forging instead of cutting in order to reduce processing costs.

There are many examples in which a certain level of corrosion resistance is required, such as the above-mentioned automobile parts.

[Problem to be solved by the invention]

In the 5KDII, giant carbides of type M and C are formed structurally, but these giant carbides are already formed during the normal solidification process and remain even after the subsequent hot working.

These giant carbides did not pose a particular problem during conventional cutting, but rather had the effect of improving wear resistance. but,
In cold forging, defects that lead to cracking occur from these giant carbides, which poses a major problem in terms of the processing process and product reliability.

This problem can be prevented to some extent by performing intermediate annealing once or twice in a series of cold forging steps, but
There is a disadvantage that the cost advantage of cold forging is reduced due to the increase in man-hours. However, until now 5KDI
Since there was no material to replace I, 5KDII had no choice but to be used, giving priority to wear resistance.

The present invention has corrosion resistance sufficient for normal use,
The objective is to improve cold workability (cold forgeability) while maintaining wear resistance, provide an inexpensive material with excellent cold workability and wear resistance, and solve the above-mentioned conventional problems. In particular, it is an object of the present invention to provide a steel that can change the manufacturing process of valve lifters from conventional cutting to cold forging, and that can provide sufficient wear resistance as a material for valve lifters.

[Means for Solving the Problems] The present invention has a weight ratio of C0.70 to 0.95% and Si0.
50-1.00%, Mn 0.30-0.80%, Cr
6.0-8.0%, M.

For cold forging, containing 0.50 to 1.20%, V 0.20 to 0.50%, or further Nb 0.03 to 0.07%, and consisting of the remainder of Fe and inevitable impurities. It is steel.

As a steel with similar composition to the steel of the present invention,
No. 770, G 0.45-0.80%, Cr 4.0-
8.0%, M.

0.5-2.0%, ■0.1-1.5%, N 0.01
``Tool steel for sputtering bottles with excellent toughness'' containing 0.2~1.5% or even WO01~1.5% and the balance Fe is also disclosed in Japanese Patent Application Laid-Open No. 1-176054.
7-1.5%, Si0.7-2.0%, Cr 7.0-
11.0%, Mo 1.3-3.0%, or even N
Go, Zr, and T in specific % ranges with i1.5% or less
i.

A "cold work tool steel" containing one or more of Nb and W, singly or in combination, and used for dies and punches, punches, cutting dies, and forming dies, the remainder of which is Fe, is disclosed.

In addition, it was developed as a structural steel with emphasis on cold workability.
No. 4-3123, JP-A-59-13048 and JP-A-60-255957, containing up to 1.5% Cr;
``Structural steel with excellent cold workability'', ``strong steel for cold forging'', and ``steel for cold forging'' are disclosed respectively, and Japanese Patent Publications No. 64-455, No. 62-54394 and Japanese Patent Publication No. 60-255960 discloses a high-strength martensitic stainless steel for cold heading containing 13-19% Cr.

The steel of the present invention differs in composition and use from the above-mentioned steels with similar compositions, and is different from the above-mentioned low-Cr steel, which emphasizes cold workability, and low-C-high Cr steel, as well as medium-C to medium-C steel. Cr is used to improve wear resistance and cold forgeability without significantly impairing corrosion resistance.

[Effect]

Next, the reason for limiting the components of the present invention will be explained.

(:, : 0.70 to 0.95% C is an element that solidly dissolves in the matrix to increase hardness and forms carbides to improve wear resistance.

In this composition, if it is less than 0.70%, hardness and carbide formation are insufficient. Therefore, the lower limit is 0.70%
And so. When C is added in large amounts, it forms M, C, type giant carbides. If added in excess of 0.95%, the formation of giant carbides becomes noticeable and impedes cold workability, so the upper limit was set at 0.95%.

Si 0.50-1.00% Si is added as a deoxidizing agent, but it also has the effect of improving tempering softening resistance. If it is less than 0.50%, the effect will be small, and if it is added in excess of 1,00%, hot workability and cold workability will be inhibited, so the content was set at 0.50 to 1.00%.

Mn: 0.30-0.80% Mn has the effect of deoxidizing agent like Si, but also has the effect of improving hardenability. The 0.30% powder droplet only has an effect as a deoxidizing agent and has little effect on hardenability. Moreover, if added in excess of 0.80%, cold workability and corrosion resistance will be impaired, so the content was set at 0.30 to 0.80%.

Cr: 6.0-8.0% Cr dissolves in the matrix to improve strength, and reacts with C to form carbide. In this case, depending on the amount of C, huge M, C, type carbides are formed.

Giant carbides of M and C are harmful during cold working, but C
When r is less than 6.0%, the amount of the additive itself produced is small, resulting in poor wear resistance. Furthermore, if Cr is added in an amount exceeding 8.0%, a large amount of giant carbides of M and C will be formed, impeding cold workability. Therefore, the range of Cr is 6.0 to 8.0.
%.

Mo: 0.50-1.20% Mo, like Cr, forms a solid solution in the matrix to strengthen it and improve corrosion resistance. If it is less than 0.50%, the effect is small, and if it is added in excess of 1.20%, the effect commensurate with the amount added cannot be obtained. Therefore, it was set at 0.50 to 1.20%.

V: 0.20-0.50%, Nb: 0.03-0.
07% ■ forms fine VC carbides and improves strength. If it is less than 0.20%, the effect is small, and 0.50%
If it is added in excess of this amount, it becomes a huge VC carbide and deteriorates cold workability, so the content was set at 0.20 to 0.50%. In addition, Nb can be added in combination because it has the same effect as ■, but it has a lower solid solubility compared to ■.
It is necessary to add approximately 177% of NbC, and if it is less than 0.03%, the effect of addition will be small, and if it is added in an amount greater than 0.07%, a huge amount of NbC will be generated and the cold workability will be inhibited.

〔Example〕

Next, examples of the present invention will be described.

Table 1 shows examples of the steel of the present invention and comparative steel (SKDll
) chemical components and their evaluation results are summarized below.

Examples N011 to 8 of the present invention and comparative steel 5KDII were melted (20 kg) in vacuum, cast, and forged to a diameter of 15 mm.
It was made into a steel bar with a thickness of mm, and was annealed to prepare a test material.

Cold workability was evaluated using the cold upsetting test method (temporary standard: cf) of the Cold Forging Subcommittee of the Japan Society of Plasticity Working.

Plasticity and Processing Vol. 1.22 No. 241 P138), and the maximum upsetting rates in Table 1 are as shown in Figure 1 (Relationship of Cumulative Crack Occurrence Rate to Upsetting Rate for Inventive Steel N018 and Comparative Steel 5KDII) This is the maximum upsetting rate without cracking. From Table 1, each example steel of the present invention has a maximum upsetting rate of 5 of the conventional 5KDIl steel.
It can be seen that this is a significant improvement of at least 71% compared to 5%.

Figure 2 shows Example No. 007 of the present invention and comparative steel (SKDII).
) shows a photograph of the microstructure in an annealed state, and Table 1 shows the results of microstructure evaluation based on similar observation. From Table 1 and Figure 2, no giant carbides were observed in the steel of the present invention.
From this, it can be understood that the steel of the present invention is superior to 5KDII in cold workability.

The hardness values in Table 1 are 1030℃ air-cooled quenching, 200℃X1
The hardness of the quenched and tempered steel of the present invention is 5KDII.
It can be seen that they are almost equivalent.

Figure 3 summarizes the outline of the Okoshi type wear test on heat-treated materials under the same conditions as the hardness test described above, and the results for the invention steels No. 001, No. 8, and 5KDII. The results for other steels of the present invention are summarized below. Here, A- or A- indicates that it is slightly inferior to A. This shows that the steel of the present invention has wear resistance properties that are slightly inferior to 5KDII.

〔Effect of the invention〕

As mentioned above, the present invention has C0.70 to 0.95%,
Si 0.50-1.00%, Mn 0.30-0.
80%, Cr 6.0-8.0%, Mo 0.50-1
．． 20%, V 0.20~0o50% or more N
b 0.03-0.07%, the balance consisting of Fe,
Although it is slightly inferior to conventional 5KDII, it has practically sufficient wear resistance and significantly improves cold workability. Furthermore, the material costs are almost the same; for example, when manufacturing a pulp lifter using the steel of the present invention, a cold forging method can be used instead of the conventional cutting method, which reduces the total cost and is extremely effective. big.

[Brief explanation of drawings]

Figure 1 shows Example No. 8 of the present invention and comparative steel 5KD1.
1 is a diagram showing an example of a cold upsetting test result for Example No. 1.8, and a diagram showing an Okoshi type wear test result and test summary for Example No. 1.8 and comparative steel 5KDI 1. FIG. Fig. Zheng') So1 Invention steel P= f), 7 (×kojsu) S K 'J,'), 1 2rX: ochiJ

Claims (1)

[Claims] 1. C0.70-0.95%, Si0.50-0.50% by weight
1.00%, Mn0.30~0.80%, Cr6.0~
8.0%, Mo0.50-1.20%V0.20-0.
A steel for cold forging characterized by comprising 50% Fe, the balance Fe and unavoidable impurities. 2 Weight ratio: C0.70~0.95%, Si0.50~
1.00%, Mn0.30~0.80%, Cr6.0~
8.0%, Mo0.50-1.20%, V0.20-0
．． 50% Nb, 0.03 to 0.07% Nb, the balance Fe and unavoidable impurities.