JP2000328189A - Steel for cold forging - Google Patents

Abstract

(57) [Abstract] [Problem] Workability of 70% without spheroidizing annealing or softening annealing
Provide cold forging steel that can perform the following cold forging and exhibit a fine-grained structure without causing coarsening and mixing of crystal grains even if reheated to austenite region of 950 ° C or less after forging. . [Solution] C: 0.15 to 0.30%, Mn: 0.60 to 1.30%, Si
≦ 0.50%, P ≦ 0.10%, S ≦ 0.10%, Cu ≦ 0.3%, Ni ≦ 0.1
5%, Cr ≦ 0.50%, Mo ≦ 0.15%, Nb ≦ 0.05%, Ti ≦ 0.10
%, B ≦ 0.010%, Al ≦ 0.100%, N ≦ 0.01%, Bi ≦ 0.50
%, Pb ≤ 0.50%, Te ≤ 0.10%, Ca ≤ 0.050%, and the balance consists of Fe and impurities, and the symbol of the element is C + 0.2Si + 0.5 (Mn + Cr + Mo) -1.10≤0 and 0.
5Al + 0.3Ti + 0.15Nb-N-0.02 ≧ 0 for cold forging steel.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel for cold forging, and more particularly, to cold forging without a spheroidizing annealing or a softening annealing when the working ratio during cold forging is 70% or less. In addition, the present invention relates to a steel for cold forging that does not cause coarsening of crystal grains or mixed grains even when reheated to an austenite region of 950 ° C. or less after cold forging.

[0002]

2. Description of the Related Art Mechanical structural parts are made of carbon steel or alloy steel for mechanical structure of JIS standard as raw material steel, and (a) rough-worked into a predetermined shape by hot forging, and then finished to a desired shape by machining. After quenching and tempering,
(B) After hot forging and quenching and tempering, it was general to finish to a desired shape by machining.

[0003] Recently, attempts have been made to reduce the cost by finishing by cold forging to a state very close to a desired shape, omitting machining, or reducing the machining allowance in machining. In the case of the cold forging method, spheroidizing annealing or softening annealing is generally performed before the working in order to facilitate forging. This spheroidizing annealing and softening annealing prevent the coarsening and mixing of crystal grains when a component processed by cold forging is reheated to the austenite region for quenching, and therefore, large distortion occurs in the component. This is a process that can be prevented.

However, since the above-mentioned spheroidizing annealing or softening annealing does not make a small contribution to the product cost, such a treatment can be omitted, and the parts after cold forging are reheated to the austenite region for quenching. In addition, there is a demand for the development of a steel for cold forging which does not cause coarsening of grains or mixing of grains.
In particular, the working ratio at the time of cold forging such as spindles, pinions and shafts of automobiles and industrial machines is at most 70.
In the case of parts of about%, there is a great demand for an inexpensive cold-forging steel having the above characteristics.

[0005]

DISCLOSURE OF THE INVENTION The present invention has been made in view of the above-mentioned circumstances, and an object of the present invention is to perform no spheroidizing annealing or softening annealing when the working ratio during cold forging is 70% or less. Can be cold forged, and after cold forging
An object of the present invention is to provide a cold-forging steel which exhibits a fine-grained structure without causing coarsening and mixing of crystal grains even when reheated to an austenite region of 0 ° C. or lower.

Here, the term "fine-grained structure" means a structure having an austenite grain size number of 5 or more and having no mixed grains. In addition, “mixed grain” complies with the provisions of JIS G 0551.

[0007]

The gist of the present invention resides in the following steel for cold forging.

That is, “in weight%, C: 0.15
0.30%, Mn: 0.60 to 1.30%, Si: 0.
50% or less, P: 0.10% or less, S: 0.10% or less, Cu: 0.3% or less, Ni: 0.15% or less, C
r: 0.50% or less, Mo: 0.15% or less, Nb:
0.05% or less, Ti: 0.10% or less, B: 0.01
0% or less, Al: 0.100% or less, N: 0.01% or less, Bi: 0.50% or less, Pb: 0.50% or less, T
e: 0.10% or less, Ca: 0.050% or less, the balance consists of Fe and unavoidable impurities, and the value of fn1 represented by the following formula is 0 or less, represented by the following formula. A steel for cold forging wherein the value of fn2 satisfies 0 or more.

Fn1 = C + 0.2Si + 0.5 (Mn + Cr + Mo) -1.10 fn2 = 0.5Al + 0.3Ti + 0.15Nb-N-0.02 ... Here, the element symbol in each formula is the symbol of the element. The content in% by weight is shown. ".

The present inventors have conducted various studies to solve the above-mentioned problems, and have obtained the following findings.

(A) In a part having a workability of 70% or less during cold forging, the value of fn1 represented by the above equation is set to 0 or less after adjusting the content of the constituent elements of the base steel. Then, cold forging into a desired shape can be performed without performing spheroidizing annealing or softening annealing.

(B) When the working ratio at the time of cold forging is 70% or less, if the value of fn2 represented by the above formula is set to 0 or more after adjusting the content of the constituent elements, To 9
Even when reheated to an austenite region of 50 ° C. or less, a fine-grained structure can be obtained without causing coarsening or mixing of crystal grains.

The present invention has been completed based on the above findings.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The requirements of the present invention will be described in detail below. In addition, "%" of the content of each element means "% by weight".

C: 0.15 to 0.30% C is an element effective for securing strength. But,
If the content is less than 0.15%, the effect of addition is poor. On the other hand, if it exceeds 0.30%, the cold forgeability decreases, so
Even if the value of fn1 represented by the above formula is set to 0 or less, spheroidizing annealing or softening annealing may be required before cold forging. Therefore, the C content is 0.15 to 0.3
0%.

Mn: 0.60 to 1.30% Mn is an element necessary for enhancing the deoxidation and hardenability and increasing the strength. For this purpose, the content of Mn must be 0.60 or more. is necessary. On the other hand, when the content exceeds 1.30%, the cold forgeability is reduced. Therefore, the Mn content is set to 0.60 to 1.30%.

Si: 0.50% or less Si need not be added. When added, it has the effect of increasing the deoxidizing action and hardenability to improve the strength. In order to surely obtain such an effect, the content of Si is preferably set to 0.05% or more. However, the content is 0.5
If it exceeds 0%, the cold forgeability is significantly reduced. Therefore, the content of Si is set to 0.50% or less.

P: 0.10% or less P may not be contained. If contained, it has the effect of increasing the fatigue strength. To ensure this effect, the P content is desirably 0.02% or more. But,
If the content exceeds 0.10%, the toughness is remarkably reduced and the fatigue strength is rather lowered.
Therefore, the P content is set to 0.10% or less.

S: 0.10% or less S may not be contained. If contained, it has the effect of enhancing machinability. In order to ensure this effect, S is set to 0.
The content is preferably at least 02%. However, if the content exceeds 0.10%, the impact characteristics deteriorate.
Therefore, the content of S is set to 0.10% or less.

Cu: 0.3% or less Cu need not be added. If added, it has the effect of increasing the hardenability and improving the strength. To ensure this effect, the Cu content is desirably 0.05% or more. However, when the Cu content exceeds 0.3%, the hot workability decreases. Therefore, the content of Cu is set to 0.3% or less.

Ni: 0.15% or less Ni may not be added. If added, it has the effect of increasing strength and impact resistance. In order to surely obtain such an effect, the Ni content is preferably set to 0.05% or more. However, when the content exceeds 0.15%, the machinability is significantly reduced. In addition, economic efficiency is impaired. Therefore, the content of Ni is set to 0.15% or less.

Cr: 0.50% or less Cr need not be added. If added, it has the effect of increasing strength and toughness. To ensure this effect, Cr
Is preferably 0.10% or more. However, if the content exceeds 0.50%, the cold forgeability is reduced. Therefore, the Cr content is reduced to 0.50%
It was as follows.

Mo: 0.15% or less Mo may not be added. If added, it has the effect of increasing the strength and impact resistance. To ensure this effect, Mo
Is desirably 0.05% or more. However, when the content exceeds 0.15%, the cold forgeability decreases. In addition, economic efficiency is impaired. Therefore, Mo
The content was 0.15% or less.

Nb: 0.05% or less Nb may not be added. If added, it forms a nitride together with N and has the effect of preventing austenite crystal grains from becoming coarse. In order to ensure this effect, Nb should be set to 0.1.
It is desirable that the content is 005% or more. But,
Even if Nb is contained in an amount exceeding 0.05%, the above-mentioned effect is saturated, so that the cost is only increased. Therefore,
The content of Nb was set to 0.05% or less.

Ti: 0.10% or less Ti need not be added. If added, it has the effect of forming carbonitrides with C and N and preventing the austenite crystal grains from becoming coarse. To ensure this effect, Ti
Is preferably 0.005% or more. However, when the content exceeds 0.10%, the hot workability decreases. Therefore, the content of Ti is set to 0.10% or less.

B: 0.010% or less B may not be added. If added, it has the effect of increasing the hardenability and improving the strength. In order to surely obtain this effect, the content of B is desirably 0.0005% or more. On the other hand, even if B is contained in an amount exceeding 0.010%, the above-mentioned effect is saturated, so that economic efficiency is only lost. Therefore, the B content is set to 0.010% or less.

Al: 0.100% or less Al may not be added. If added, they become nitrides or oxides, and have the effect of preventing the austenite crystal grains from becoming coarse. To ensure this effect, the Al content is preferably set to 0.010% or more. On the other hand, if the content exceeds 0.100%, the nitrides and oxides are agglomerated and coarse, so that the austenite crystal grains may be rather coarsened. Therefore, the Al content is 0.100
% Or less.

N: 0.01% or less N may not be contained. If it is contained, it has an effect of forming nitride and preventing coarsening of austenite crystal grains. To ensure this effect, it is desirable that the content of N be 0.0020% or more. However, if the content exceeds 0.01%, the cold forgeability is reduced. Therefore, even if the value of fn1 represented by the above formula is set to 0 or less, spheroidizing annealing and Softening annealing may be required. Therefore, the content of N is set to 0.01% or less.

Bi: 0.50% or less Bi may not be added. If added, it has the effect of improving machinability. To ensure this effect, it is desirable that the content of Bi be 0.01% or more. However, when the content exceeds 0.50%, the hot workability decreases. Therefore, the Bi content is set to 0.50% or less.

Pb: 0.50% or less Pb may not be added. If added, it has the effect of improving machinability. To ensure this effect, it is desirable that the content of Pb be 0.05% or more. However, when the content exceeds 0.50%, the hot workability decreases. Therefore, the content of Pb is set to 0.50% or less.

Te: 0.10% or less Te need not be added. If added, it has the effect of improving machinability. To ensure this effect, the content of Te is preferably 0.005% or more. However, when the content exceeds 0.10%, the hot workability decreases. Therefore, the content of Te is set to 0.10% or less.

Ca: 0.050% or less Ca may not be added. If added, it has the effect of improving machinability. In order to surely obtain this effect, the content of Ca is preferably set to 0.0005% or more. However, even if Ca is contained in an amount exceeding 0.050%, the effect is saturated and the economic efficiency is only lost. Therefore, the content of Ca is set to 0.050% or less.

Fn1: 0 or less When the chemical composition of the base steel satisfies the above condition of the content of C to Ca and the value of fn1 represented by the above formula is 0 or less, the workability during cold forging is reduced. In the case of 70% or less, cold forging into a desired shape can be performed without performing spheroidizing annealing or softening annealing. Therefore, the value of fn1 represented by the above equation is set to 0 or less. The lower limit of fn1 is such that the content of C is 0.15% and the content of Mn is 0.60%.
% May be -0.65 when Si, Cr and Mo are not contained (that is, the content of these elements is 0).

Fn2: 0 or more When the chemical composition of the base steel satisfies the above condition of the content of C to Ca and the value of fn2 represented by the above formula is 0 or more, the workability during cold forging is reduced. In a 70% or less part, even if reheated to an austenite region of 950 ° C. or less for quenching, a fine-grained structure can be obtained without causing coarsening or mixing of crystal grains. Therefore, the value of fn2 represented by the above equation is set to 0 or more. Note that the upper limit of fn2 is 0.1% when the Al content is 0.100%, the Ti content is 0.10%, the Nb content is 0.05%, and the N content is close to 0. 0675 may be used.

Hereinafter, the present invention will be described with reference to examples.

[0036]

EXAMPLE Steel having the chemical composition shown in Tables 1 and 2 was used for 15 steels.
0 kg vacuum furnace was produced. Steels 1 to 15 in Table 1 are steels of the present invention whose chemical composition is within the range specified by the present invention, and steels 16 to 27 in Table 2 are any of the components out of the range of the content specified by the present invention. It is a steel of a comparative example. Among the steels of the comparative example, steels 26 and 27 are steels corresponding to SCr440 and SCM440 specified by JIS, respectively.
In addition, "-" of the content of each element indicates that the content is below the lower limit of analysis, and was set to "0 (zero)" when calculating the values of fn1 and fn2.

[0037]

[Table 1]

[0038]

[Table 2]

Next, these steels were formed into a round bar having a diameter of 30 mm by ordinary hot forging, and then a test piece having a diameter of 20 mm and a height of 30 mm was prepared, and subjected to cold forging at room temperature by an ordinary method. went. The working ratio is 10%, 50%, 7%.
Three types of 0% were used, and five forgings were performed for each degree of working.

Of the test pieces subjected to the cold forging, each of the test pieces having a workability of 70% was visually inspected for forging cracks, and the cold forgeability was evaluated.

Next, all the test pieces subjected to the cold forging were heated at 950 ° C. for 30 minutes, then water-cooled and quenched, and the austenite grain size in the cross section was measured. Was investigated.

Table 3 summarizes the results of the above-mentioned investigation. In the column of "cold forgeability" in this table, a case where no forging crack was recognized in any of the five test specimens having a workability of 70% by visual inspection was evaluated as "good". The case where forging crack was recognized in the individual was evaluated as x. In the column of “fine-grained structure”, 3% of 10%, 50% and 70%
Each of the five test specimens cold forged at a different degree of processing had a fine-grained structure in each of the five test specimens, and x when at least one of the test specimens did not have a fine-grained microstructure. And

[0043]

[Table 3]

As can be seen from Table 3, in the case of the steel of the present invention, forging cracks did not occur even when cold forging was performed at a working ratio of 70% without spheroidizing annealing or softening annealing, and at 950 ° C. It is clear that even if re-heated, it exhibits a fine-grained structure and is excellent in cold forgeability and so-called "coarse-graining resistance".

On the other hand, in the case of the steel of the comparative example, at least one of the cold forgeability and the so-called “coarse grain resistance” is inferior. That is, C content, Si content,
Steel (steel 16 to 20, steel 22) in which any of the Mn content, the Cr content, the Mo content, and the value of fn1 represented by the formula is out of the range specified in the present invention is inferior in cold forgeability. ing. Any one of the Al content and the value of fn2 represented by the formula is out of the range specified in the present invention (steel 21 and steel 2).
3) is inferior in "coarsening resistance". Steel 24 is S
Since the i content and the value of fn1 are out of the ranges specified in the present invention, the cold forgeability is inferior. Furthermore, steel 25 has f
Since both the value of n1 and the value of fn2 are out of the range specified in the present invention, both the cold forgeability and the “coarse grain resistance” are inferior. Also, SCr44 specified by JIS
In steels 26 and 27 corresponding to 0 and SCM440, the C content, the Cr content, the value of fn1, and the value of fn2 are out of the ranges specified in the present invention. Chemical properties "are inferior.

[0046]

The cold forging steel of the present invention can be cold forged without performing spheroidizing annealing or softening annealing when the working ratio during cold forging is 70% or less. Use as raw material for spindles, pinions, shafts, etc. of automobiles and industrial machines, because even after re-heating to an austenite region of 950 ° C or less after hot forging, crystal grains are not coarsened or mixed. Can be.

Claims (1)

[Claims] (1) C: 0.15 to 0.30% by weight, M
n: 0.60 to 1.30%, Si: 0.50% or less,
P: 0.10% or less, S: 0.10% or less, Cu: 0.
3% or less, Ni: 0.15% or less, Cr: 0.50% or less, Mo: 0.15% or less, Nb: 0.05% or less, T
i: 0.10% or less, B: 0.010% or less, Al:
0.100% or less, N: 0.01% or less, Bi: 0.5
0% or less, Pb: 0.50% or less, Te: 0.10% or less, Ca: 0.050% or less, the balance being Fe and inevitable impurities, and further represented by f represented by the following formula:
The value of n1 is 0 or less, and the value of fn2 represented by the following formula is 0
Steel for cold forging satisfying the above. fn1 = C + 0.2Si + 0.5 (Mn + Cr + Mo) -1.10 fn2 = 0.5Al + 0.3Ti + 0.15Nb-N-0.02 ... Here, the element symbol in each formula is the weight% of the element. Is shown.