JPH11256264A - Steel for cold forging - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel for cold forging suitable for forming a near-net shape in which the generation of cracking in a region with a severe working ratio is prevented, reduction in the yield caused by the increase of a cutting allowance is prevented, also, low O and low S are attained for the prevention of cracking caused by inclusions, and there is no deterioration in machinability therefor or there is no need of incorporating poisonous lead for supplementing this. SOLUTION: For realizing high cold forgeability, not only the content of S and the content of oxygen are reduced, but the number of oxide inclusions of certain dimensions or above present in the steel as shown by <=80 in the axis of abscissas, i.e., in the thick type (heavey) ones of >=8 μm of D series oxide inclusions based on the ASTM-D method, the number of the ones of 0.5 grade is made to <=10 pieces, also, the ones of >=1.0 grades are not allowed to contain, and furthermore, its hardness is made to <=90 HRB.

Description

DETAILED DESCRIPTION OF THE INVENTION

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cold forging steel suitable for near net shaping.

2. Description of the Related Art In recent years, the application of cold forging has been rapidly expanded, especially in parts for white moving vehicles, from the viewpoint of energy saving and improvement of yield as compared with hot forging due to near net shaping. However, if high cold forging is aimed at in order to further improve the yield, cracks may occur in a region where the working ratio is severe. Cracks generated during cold forging include cracks due to the limitations of the material itself and Al ₂ O ₃ , Mn.
There are cracks due to inclusions such as S. The cracks observed when the upsetting test is performed are shear cracks in the 45 ° direction in the former case with respect to the upsetting direction and cracks parallel to the upsetting direction in the latter case. In particular, in the latter case, the cracking occurs earlier than in the former case, so that the final processing rate is lowered, and the cutting allowance for that is increased and the yield must be reduced.

As means for solving this problem, a conventional example of technology includes a method of high cleanliness by low O and low S in order to reduce cracks caused by inclusions ("Special Steel", Vol. 29, No. 2, P.S.
31-32, "Special Steel" Vol. 29, No. 9, p. 17-1
9, p. 26-28, "Special Steel", Vol. 31, No. 12, p.
21 to 23, p. 32-34, p. 35-37, p. 3
8 to 39, "Special Steel", vol. 34, No. 3, p. 15-17,
P. 21 to 23, p. 29-30). However, even if the final processing rate is increased by this method and the cutting step is omitted, some parts requiring high surface accuracy require a polishing step. S exists as MnS in steel and is used as a free-cutting element.
Since S decreases (0.010% or less), the machinability deteriorates.

[0002] Therefore, in addition to high cleanliness, a method of finely dispersing lead which is a free-cutting element other than MnS (“Special steel” 2
Vol. 9, No. 2, p. 28-29, "Special Steel", Volume 31, 12
No., P. 16-18, p. 29-31, "special steel" 34
Vol. 3, No. 3, p. 31-32, "Special Steel" 35 volumes, 10
No., P. 48-39), but lead is a toxic substance, and its use is being reduced in consideration of recent environmental issues, and there is also a problem that it cannot be applied to parts requiring rolling fatigue.

[0003]

DISCLOSURE OF THE INVENTION The present invention is intended to prevent a decrease in yield due to an increase in cutting allowance without cracking in a region with a severe processing rate, and to reduce cracking due to inclusions. An object of the present invention is to provide a cold-forging steel suitable for near-net shaping by reducing low S, thereby eliminating machinability or eliminating toxic lead in order to compensate for this.

[0004]

Means for Solving the Problems The present inventor has analyzed the factors affecting cold forging cracks, particularly the effects of sulfide inclusions, oxide inclusions and hardness. Steel suitable for cold forging, ie SC, SC in JIS standard steel
In R, SCM, SUJ, etc., a material in which the amount of oxygen, the amount of S, and hardness were changed, and the size of the oxide-based inclusion was changed even with the same amount of oxygen, was used as a test material, and a critical upsetting test ( (Based on the Japan Society for Technology of Plasticity). In order to explain the relationship between inclusions and the marginal upsetting rate, it was decided to associate the results of the ASTM-D method with the marginal upsetting test results. As shown in Table 1, in the ASTM-D method, the width of the inclusion is thin (T
hin) and thick type (Heavy), graded according to the total length of inclusions observed in one field of view for A to C-based inclusions, and confirmed in one field of view for D-based oxide-based inclusions Grades according to the number of inclusions. This is done by performing the grading shown in Table 2 on the visual field confirmed by the optical microscope so that the visual fields do not overlap and the total visual field area is 18
It is evaluated at 0.5 mm ² . Therefore, as for the handling of the data, in order to weight each class, a coefficient for each class shown in Table 3 was multiplied, and the evaluation was made by the sum thereof. The results are described below.

[0005]

[Table 1]

[0006]

[Table 2]

[0007]

[Table 3]

[0008] When the hardness exceeds HRB90, A-type inclusions such as MnS, which are sulfide-based inclusions, exert an influence.
eavy), a spherical oxide of 8 mm or more was found to have a significant effect. In particular, in the latter case, the D type thick type (Hea
When the grade 0.5 of the oxide-based inclusions of vy) does not meet the grade of 0.5 or less and the grade of 1.0 or more, as shown in the horizontal axis of FIG. It was found to be improved. Further, it was recognized that this restriction was obtained by suppressing the oxygen content to 10 ppm or less, as a remarkable tendency.

[0009] It is considered that the cracks caused by inclusions cause the concentration of stress on the inclusions due to the deformation in the cold forging, and become the starting point of the destruction. The existence state of sulfide inclusions and oxide inclusions in steel indicates that the former extends along the rolling direction, whereas the latter does not. This is due to the difference in deformability, the former having higher deformability. In a region having a low hardness, that is, at an HRB of 90 or less, the matrix itself has a high deformability, and the sulfide-based inclusion has a deformability as compared with the oxide-based inclusion. On the other hand, oxide inclusions have little deformability, so that stress concentration tends to act. On the other hand, in a region having high hardness, that is, in a region exceeding HRB90, the matrix itself has a low deformability, so that the sulfide-based inclusions existing in a state of being expanded in the steel are more likely to act as stress concentration. Oxide-based inclusions also act as stress concentrations, but sulfide-based inclusions that have been expanded in shape have a greater effect on stress concentration, and therefore have less effect.

In other words, when the HRB is 90 or less, the influence of the sulfide-based inclusions can be considered to be small. Therefore, the key is to reduce the number of oxide-based inclusions. Among them, oxide-based inclusions to be reduced are large in size. Smaller ones can relieve stress concentration due to the high deformability of the matrix, while larger ones cannot relieve stress concentration. ASTM is the size limit
This can be said to be at the boundary between the thin type (Thin) and the thick type (Heavy) of the D-based oxide-based inclusions of the -D method. If there are many inclusions having a large size, they not only act as stress concentration, but also easily lead to crack propagation. Therefore, reducing the number is an extremely effective method for realizing high cold forgeability.

On the other hand, cold forging of a material having a hardness exceeding HRB 90 is not used for high cold forging because of high deformation resistance.

From the results of the above studies, the means for solving the problem of the present invention is not merely to reduce the S content and the oxygen content in order to realize high cold forging, but to exist in steel. Limiting the number of oxide-based inclusions larger than the size,
That is, the D-type oxide-based inclusions based on the ASTM-D method are set so that the thickness of the D-type oxide inclusions (Heavy) of 8 μm or more is 10 or less and the hardness is not more than 1.0 and the hardness is HRB90. It is as follows.

The use of the cold forged steel described above makes it possible to increase the final working ratio of the cold forging, thereby approaching the near net shape, reducing the cutting allowance or eliminating the cutting, thereby contributing to cost reduction. In addition, for components that require a polishing step, even if the amount of S is increased, sufficient coverage can be obtained by reducing the D type thick type (Heavy), so that there is an advantage that machinability can be added. It is possible to extend the life of the tool.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described with reference to examples comparing the developed steel of the present invention and a comparative steel.
The developed steel of the present invention is smelted in an electric furnace, subjected to out-of-furnace refining (LF), and then smelted by vacuum degassing (RH). A slab was produced. At that time, the total oxygen content, the size of the oxide-based inclusions, and the size of the sulfide-based inclusions were controlled by changing the LF-RH time, the type of refractory, and the casting temperature. A steel slab obtained by dissolving and rolling from these steel slabs was normalized at 900 ° C. × 1 hr. The heat treatment was performed so that the ferrite grain size and the ferrite area ratio were as uniform as possible and the hardness was uniform. The hardness at this time is HRB90 or less in the present invention.
From these rolled materials, the No. 1 restraint type upsetting test piece (φ14m
mx 21 mm length). Tables 4 and 5 show the results of the ASTM-D method for evaluating the chemical components, hardness, and inclusions, respectively.

[0015]

[Table 4]

[0016]

[Table 5]

Comparative steels 1-1 to 1-4 are D-type thick (He
avy) having a grade of 0.5 or more and a grade of 1.
Including 0 or more. On the other hand, the developed steels 1-1 to 1-6 satisfy the scope of the claims. Especially developed steel 1-4 is comparative steel 1
Despite the high oxygen content compared to -2 and 1-3,
eave is small.

The results of the limit upsetting test (based on the Japan Society for Technology of Plasticity) are shown below. FIG. 2 shows the relationship between the marginal upsetting ratio and the D-system Heavy. In addition, the horizontal axis is evaluated by the sum total obtained by multiplying each class by a coefficient (Table 3). D type Heav
As y is smaller, the critical upsetting ratio is improved. However, in the case of the developed steel within the scope of the present invention, the effect of improving the critical upsetting ratio is high, and the material is suitable for high cold forging. .

[0019]

[Table 6]

Further, S55C, a JIS standard steel, was refined in the same manner as described above, and a steel piece was prepared and rolled. Normalize these rolled materials (900 ° C × lhr),
Further, No. 1 restraint type upsetting test pieces were prepared from three times of spheroidizing annealing (maximum temperature 740 ° C.). As for the hardness, the former is not higher than HRB 90 and does not satisfy the hardness of the present invention, and the latter is satisfied. Tables 6 and 7 show the chemical components and the results of the ASTM-D method. Comparative steel 2-1 to 2
-4 is D-type Heavy having a grade of 0.5 or more, and also includes a grade of 1.0 or more. Comparative steel 2-4 is
It is a conventional steel for cold forging, and the amount of S is suppressed. On the other hand, the developed steels 2-1 to 2-5 satisfy the claims. In particular, although the developed steel 2-1 has a higher oxygen content than the comparative steel 2-4, the D-based heavy is small.

[0021]

[Table 7]

FIG. 3 shows the relationship between the D-type Heavy of the normalizing material and the critical upsetting ratio. Even if the hardness is HRB90 or more and the D-based Heavy is within the range of the present invention, there is no effect on the improvement of the critical upsetting ratio. As shown in FIG. 4, there is a correlation as shown in the relationship with the A-type inclusions, and it can be seen that the A-type affects the critical upsetting ratio more than the D-type. On the other hand, in the spheroidizing annealing which satisfies the hardness of the present invention three times, the critical upsetting ratio is greatly improved by setting the D-type Heavy in the scope of the claims as shown in FIG. Especially developed steel 2-1 and 2-
No. 2 is also a material suitable for cutting because it has a higher critical upsetting ratio than the comparative steel 2-4 which has been conventionally used for cold forging with a reduced amount of S.

[0023]

As described above, the steel for cold forging according to the present invention has a thickness of 8 μm of the inclusion D type thick die based on the ASTM-D method.
Oxide-based inclusions of m or more have a grade of 0.5 or less and do not include a grade of 1.0 or more and have a hardness of HRB90 or less. In addition, to prevent a decrease in yield due to an increase in cutting allowance, and to prevent deterioration of machinability by reducing O and S to prevent cracks due to inclusions, or to reduce deterioration of machinability. A cold-forging steel suitable for near-net-shaping that eliminates the inclusion of toxic lead in the steel component that has a negative impact on the environment to compensate for it is obtained, and as a result, cutting costs are reduced and production costs are reduced. It has excellent effects such as being able to achieve the following.

[Brief description of the drawings]

FIG. 1 is a diagram showing the effect of D-series Heavy on critical upsetting ratio.

FIG. 2 is a diagram showing an influence of a D heavy on a critical upsetting ratio.

FIG. 3 is a diagram showing a relationship between a D heavy of normalizing material and a critical upsetting ratio.

FIG. 4 is a diagram showing the influence of A-based inclusions on the critical upsetting ratio.

FIG. 5: Critical upsetting ratio of steel satisfying hardness according to the present invention and D system H
It is a figure showing the influence of eavy.

Claims (1)

[Claims] 1. A steel used for cold forging, comprising: AST
Inclusion D-based thick type oxide inclusions of 8 μm or more based on the MD method have a grade of 0.5 or less and a grade of 0.5 or less.
A steel excellent in cold workability, characterized in that hardness is not more than 0 and HRB is 90 or less.