JPS60255957A - Steel for cold forging - Google Patents

Abstract

PURPOSE:To improve the cold forgeability and dimensional accuracy of a steel contg. prescribed percentages of C, Si, Mn and Cr by heat treating the steel under prescribed conditions. CONSTITUTION:A steel consisting of, by weight, 0.05-0.3% C, 0.5-3% Si, 0.5- 2% Mn, 0.3-1.5% Cr and the balance Fe is refined. The steel is cooled from a temp. in the ferrite-austenite two-phase temp. range at the air cooling rate or higher to obtain a steel for cold forging having a fine structure consisting of ferrite and pearlite, ferrite, pearlite and bainite, or ferrite and bainite.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a cold forging steel suitable as a material for machine structural parts, particularly parts manufactured by cold forging.

(Prior Art) Conventionally, mechanical structural steels used for cold forging include carbon steel (SC), chromium steel (SCr), and chromium molybdenum steel (SCM).

There are nickel chromium molybdenum steels (SNCM), etc.
For example, inexpensive carbon steel is often used for the shafts of automobile steering and drive mechanisms. Cold forging has higher processing accuracy and better material yield than hot forging, so its proportion in forming materials has tended to increase in recent years. Generally, carbon steel subjected to cold forging is subjected to spheroidization heat treatment (SA treatment) in order to spheroidize cementite and give it high deformability.

Ru.

However, in the case of mechanical structural steel used for such conventional cold forging, A
After soaking for about 1 hour just above the A1 transformation point, it is necessary to continue cooling at a rate of about 10°C/hr to just below the A1 transformation point.
Since the time required for this process was more than 30 hours, there were problems in that productivity was low and energy consumption was enormous.

(Objective of the Invention) The present invention was made by focusing on these conventional problems, and does not require long-term heat treatment such as spheroidization heat treatment, has high deformability in cold, and can be The object of the present invention is to provide cold forging steel suitable as a material for parts manufactured by cold forging.

(Structure of the Invention) The cold forging steel according to the first invention of the present invention has, in weight%,
C: 0.05-0.3%, Si: 0.5-3%, Mn
: 0.5-2%, Cr: 0.3-1.5% as basic components, Ni: 2% or less, Mo+0.5 as necessary.
% or less, and if necessary,
As a grain refining element, AI: 0.1% or less, Nb: 0
．． 3% or less, Ta: 0.3% or less, Ti: 0.3% or less.

Contains one or more of the following: Zr: 0.1% or less, N: 0.03% or less, the remainder consisting of Fe and impurities, and has a cooling rate of air cooling or higher from the ferrite-austenite two-phase temperature range. and cooled with a fine grain size, more preferably grain size number 6 or higher, particularly preferably 7.
Fine ferrite of size 8 or above - pearlite, ferrite - pearlite - bainite, or ferrite -
It is characterized by having a bainite structure.

A cold forging steel according to a second invention that achieves the same purpose is:
In weight%, C: 0.05-0°, 3%, Si:
0.5-3%, Mn: 0.5-2%, Cr: 0.3
% to 1.5% as a basic component, and if necessary, Ni:2
% or less, one or two of Mo + 0.5% or less,
and if necessary, AI+0 as a grain refining element.
．． 1% or less, Nb: 0.3% or less, Ta: 0.3% or less, Ti: 0°3% or less, Zr: 0.1% or less, N: 0.
03% or less, and the remainder consists of Fe and impurities, and the Ar transformation point to Ar3 transformation point +1 during finish rolling in material rolling prior to cold forging.
A finer ferrite-pearlite than the above-mentioned heat-treated material is obtained by applying a rolling reduction of at least 20% or more in a temperature range of 00°C and then cooling with air cooling or a cooling rate higher than that.

It is characterized by having a ferrite-pearlite-bainite or ferrite-bainite structure, and is designed to further improve deformability.

Next, the composition range (weight%) of the steel for cold forging according to this invention
) will be explained below.

C: 0.05 to 0.3% C is an effective element for ensuring strength as a mechanical structural steel, especially quenching hardness, and the lower limit was set to 0.05% in consideration of strength. Further, in order to obtain a sufficiently wide temperature range of the ferrite-austenite dual phase, the upper limit was set to 0.3%.

Si: 0.5 to 3% Si is an effective element for widening the temperature range of the ferrite-austenite dual phase, and therefore needs to be added in an amount of 0.5% or more. Further, although there are some elements that are effective in strengthening the ferrite structure, excessive addition causes an increase in deformation resistance and a decrease in toughness, so the upper limit was set at 3%.

Mn: 0.5-2% Mn is an effective element for deoxidizing and desulfurizing during melting, and is also an effective element for ensuring hardening hardness. In order to obtain sufficient deoxidation and desulfurization effects and at the same time improve hardenability, the lower limit was set to 0.5%. However, if added excessively, cold forgeability will be reduced, so the upper limit was set at 2%.

Cr: 0.3 to 1.5% Cr is an effective element for improving hardenability and preventing graphitization, and in order to obtain such effects, the lower limit was set to 0.3%. However, in consideration of the decrease in toughness, the upper limit was set at 1.5%.

One or two of Ni: 2% or less, Mo: 0.5% or less Ni and Mo are effective elements to further improve hardenability and strengthen the base during tempering after forging. Therefore, it is good to add more to the above basic components if necessary, but if the Ni content exceeds 2% and the Mo content exceeds 0.
If it exceeds 5%, the toughness will deteriorate, so Ni should be 2% or less.
Mo was set to 0.5% or less.

AQ: 0.1% or less, Nb: 0.3% or less, Ta: 0.
396 or less, Ti: 0.3% or less, Zr: o, i% or less, N: 0.03% or less Al, Nb, Ta, T (, Zr, N are carbon Af
The L content exceeds 0.1%, the Nb content exceeds 0.3%, the Ta content exceeds 0.396, and the TI content exceeds 0.3%.
If the Zr content exceeds 3% and the Zr content exceeds 0.1%, the carbonitrides become coarser, thereby reducing the crystal grain refining effect and causing a decrease in cold deformability, so it is added. In each case, it is necessary to meet the above-mentioned upper limit. In addition, if the N content exceeds 0.03%, the integrity of the steel ingot or slab will be damaged by N blowholes, so N
The upper limit was set at 0.03%.

In addition, in order to increase cold deformability and improve cold forgeability, [0]: 0.003% or less, S: 0.
It is also desirable to regulate it to 0.02% or less, if necessary.

Then, the steel whose composition has been adjusted in this manner is cooled from the ferrite-austenite dual phase temperature range by air cooling or at a higher rate to form a fine ferrite-pearlite, ferrite-pearlite-bainite, or ferrite-bainite structure. As a result, it is possible to obtain a mechanical structural steel with outstanding cold deformability, and this material can be used to make constant velocity joint parts such as outer races and housing shafts, and axle parts such as rear spindles and axle shafts into spherical shapes. It becomes possible to process by cold forging without applying heat treatment.

In addition, the steel whose composition has been adjusted as described above is subjected to a reduction rate of at least 20% in the temperature range of Ar transformation point to Ar3 transformation point + 100"C during finish rolling in material rolling prior to cold forging, and then air cooled or By cooling at the above cooling rate, it is possible to further improve the cold deformability by creating a ferrite-pearlite, ferrite-pearlite-bainite, or ferrite-heinite structure that is finer than that of the heat-treated material. It can be made into a mechanical structural steel with extremely excellent properties for cold forging.Here, during the finish rolling of the material prior to cold forging, Ar is applied in the temperature range from the transformation point to the Ar3 transformation point + 100°C. The reason why we decided to apply a rolling reduction rate of at least 20% was at Ar3 transformation point +100°C.
This is because even if rolled at a temperature higher than Ar, a fine structure cannot be obtained and cold forgeability will not be improved, and if rolled at a temperature lower than the Ar or transformation point, the effects of processing will remain. This is because cold forgeability similarly decreases, and furthermore, by setting the reduction ratio to 20% or more, it is possible to refine the structure, thereby improving cold forgeability. This is because a rolling reduction ratio smaller than this has almost no effect.

(Example 1) Steel having the chemical composition shown in Table W41 was melted and ingot-formed, and a round bar with a diameter of 32 am was manufactured by forging.

Next, each round bar was normalized under the conditions of heating and air cooling at 925°C for 1 hour, and then the 6-temperature normalized material was
After turning, the specimens were heated and held for 1 hour at the soaking temperature shown in Table 1, and then air-cooled. Next, from each treated material, a cylindrical grooved compression test piece 1 (D=14+u, H=21 intestine) having conical holes 1a on the upper and lower bottom surfaces as shown in FIGS. 1 to 3 was prepared. θ；30°
, d, = 0.8 mm, R = 0.15 intestinal meal, α =
120°, d2 = 0.57 mm), and a compression test was performed using an Amsler tester with upper and lower end face constraint dies attached.

Then, during the compression test, the deformability was evaluated by setting the compression rate at which a crack occurred in the V-groove 1b as the crack occurrence limit. The results are shown in Table 1 along with the results for the spheroidized heat-treated carbon steels (SA) of 540C and 548C.

/ As shown in Table 1, all steel types whose chemical composition falls within the scope of the present invention are finely refined by air cooling from the ferrite-austenite dual phase temperature range.

It has a ferrite-pearlite structure and is 540C.

It showed deformability exceeding that of 548C. In addition, No. 5 has too much Si content and No. 5 has too much Cr content.
7 had a lower deformability than this.

(Example 2) Next, among the steel types shown in Table 1, No. l (540C)
, No. 2 (S48C) and No. 3-5.8-1
0, the normalized material under the same conditions as Example 1 was
5 After processing into intestine, lhr at the quenching temperature shown in Table 2.
After heating, quenching is performed by oil cooling, and then No. 3
~5. For steels of 8 to 10, tempering is performed by heating to 170°C for lhr and then cooling in air, No, l, No,
For No. 2, tempering was performed by heating to 600° C. for 1 hour and then rapidly cooling, and then hardness measurement, tensile test, and impact test were performed. At this time, the hardness measurement was performed using the Rockwell C scale, the tensile test was performed using a JIS No. 4 test piece (reduced size), and the impact test was performed using a JIS No. 3 Charpy test piece. These results are No. 1 (34
The results are also shown in Table 2 along with the results for the tempered materials No. 0C), No. 2 (348C).

In this example, since the quenching temperature is in the two-phase temperature range, the quenching structure becomes fine ferrite-martensite or bainite, and steel type No. 3 according to the present invention
．． At 4.8 to 1o, both tensile strength and impact value are S4. O
C,548C tempered material. However, Si is 3%
For No. 5, the impact value is 540C, 34
It was lower than 8C tempered material.

(Example 3) Next, the No. 8 to 1O steel according to the present invention shown in Table 1 was subjected to salt bath treatment in the ferrite-austenite dual phase temperature range shown in Table 3, and A compression test was conducted using the same test piece as in the case above to measure the cracking limit. The results are shown in Table 3.

As shown in Table 3, the three steel types (Nos. 6 to 8) of this invention became fine ferrite-bainite structures through the treatment in this example, and all of them had deformability equal to or higher than that of the SA-treated carbon steel. Indicated.

(Example 4) Next, No. 4 according to this invention shown in Table 1.

10 steel diameter 27.1mm, 29.9mm, 32.3a
After rolling a round bar with a diameter of 25 mm at the rolling temperature and reduction ratio shown in Table 4, test pieces similar to those in Example 1 were prepared and subjected to a compression test to detect cracks. The generation limit was measured. The results are also shown in Table 4.

/'' As shown in Table 4, A during finish rolling in material rolling.
r, 20 in the temperature range of transformation point to Ar3 transformation point + 100°C
It was confirmed that when a rolling reduction of % or more is applied, the crack generation limit in subsequent cold plastic working increases, and cold plastic workability can be improved.

(Effects of the Invention) As explained above, the cold forging steel according to the present invention has C: 0.05 to 0.3%, Si+0.5% by weight.
~3%, Mn: 0.5-2%.

The basic component is Cr: 0.3% to 1.5%, and if necessary, one or two of Ni + 2% or less, Mo + 0.5% or less, and Al: O, 1% or less, Nb: 0.3% or less, Ta: 0.3% or less, Ti: 0
．． 3% or less, Zr: 0.1% or less, N: 0.03% or less, and the remainder consists of Fe and impurities, and is air-cooled from the ferrite-austenite two-phase temperature range or By performing cooling at a cooling rate higher than that, or during finish rolling in material rolling prior to cold forging, Ar transformation point ~ Ar3 transformation point + 10
A microstructure of ferrite-pearlite, ferrite-pearlite-bainite, or ferrite-bainite can be formed by applying a reduction rate of at least 20% or more in a temperature range of 0°C and then cooling with air or a cooling rate higher than that. Because of this, high deformability in cold conditions can be obtained, and therefore the deformability in cold conditions is extremely good without the need for conventional spheroidization heat treatment, which takes a long time and requires a huge amount of energy. It has excellent cold forging properties, making it easy to cold forge constant velocity joint parts such as outer races and housing shafts, and axle parts such as rear spindles and axle shafts, with excellent dimensional accuracy. A very excellent effect can be obtained in that the scope of application of cold forging, which has a high yield and good yield, to various mechanical structural parts can be significantly expanded.

[Brief explanation of drawings]

FIG. 1W42 and FIG. 3 are an overall explanatory diagram, an enlarged explanatory diagram of the V@ part, and an enlarged explanatory diagram of the center hole, respectively, of the compression test piece used in the embodiment of the present invention. Patent applicant Nissan Motor Co., Ltd. Patent applicant Daido Steel Co., Ltd. Representative patent attorney Yutaka Oshio 1st 2nd] 3rd

Claims (6)

[Claims] (1) C: 0.05 to 0.3% by weight. Si: 0.5-3%, Mn: 0.5-2%. Cr: 0.3% to 1.5% as a basic component, the balance consisting of Fe and impurities, and is cooled from the ferrite-austenite two-phase temperature range with air cooling or a cooling rate higher than that to produce fine ferrite-pearlite, ferrite- A steel for cold forging characterized by having a pearlite-bainite or ferrite-bainite structure. (2) The balance Fe is 2% or less by weight, Ni: 2% or less, Mo
The steel for cold forging according to claim (1'), containing one or two of: 0.5% or less. (3) Remaining Fe is % by weight, Al: 0.1% or less, N
b: 0.3% or less, Ta: 0.3% or less. Ti: 0.3% or less, Zr: 0.1% or less, N: 0.0
The steel for cold forging according to claim (1) or (2), which contains one or more of 3% or less. (4) C: 0.05 to 0.3% by weight. Si: 0.5-3%, Mn: 0.5-2%. Cr: 0.3 to 1.5% as a basic component, with the balance consisting of Fe and impurities, and during finish rolling in material rolling prior to cold forging, Ar, transformation point ~ Ar3 transformation point + lOO℃
After applying a reduction rate of at least 20% in a temperature range of Steel for cold forging. (5) The balance Fe is % by weight, Ni: 2% or less, Mo
The steel for cold forging according to claim (4), which contains one or two of the following: +0.5% or less. (6) Remaining Fe is 1% by weight, Al: O, 1% or less, N
b: 0.3% or less, Ta: 0.3% or less. Ti: 0.3% or less, Zr: 0.1% or less, N: 0.0
Cold forging steel according to claim (4) or (5) containing one or two of 3% or less