JPH05253644A - Manufacture of shaft parts having excellently extruding workability and forgeability for automobile - Google Patents

Abstract

PURPOSE:To provide a manufacturing method of a shaft parts intergratively forming a head part having the diameter larger than a shaft part to the end part of the shaft part, such as rear axle, gear shaft, propeller shaft, particularly, a manufacturing method, by which chevron crack is not developed at the time of extrusion working and forging. CONSTITUTION:Molten metal having composition composed of 0.20-0.8wt.% C, 0.01-1.0wt.% Si, 0.3-2.0wt.% Mn, 0.05-1.0wt.% Cr, 0.005-0.050wt.% P, 0.005-0.050wt.% S and the balance Fe with inevitable impurities is continuously cast. At this time, a component concentration preventing treatment, in which the ratio C/C0 of C content (C) in the axial part of a cast slab to C content (C0) in molten steel in a ladle near the crater end completing the solidification of the molten steel in the inner part of a cast slab becomes 0.8-1.1, is applied. Successively, the formation of the shaft part by the extrusion working and formation of the head part by the forging are applied, and by integratively forming the head part having the diameter larger than the shaft part to the end of the shaft part, the development of the chevron crack is prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an automobile shaft component such as a rear axle shaft, a gear shaft, and a propeller shaft, which are integrally formed with a head having a diameter larger than that of the shaft portion. It relates to a manufacturing method.

[0002]

2. Description of the Related Art A shaft part of this type, for example, a rear axle shaft, is formed by extruding a round bar to form a shaft part having a predetermined diameter, and forging from the end face of the shaft part so that a predetermined region of the shaft part is formed. It is often manufactured in a process in which the head has a larger diameter than the shaft. A problem in this manufacturing is that a so-called chevron crack, which originates from the axial center of the round bar in the extrusion process and from the center of the head in the forging process, is frequently generated. In particular, extrusion processing has a higher surface reduction rate and colder than ordinary drawing processing, so chevron cracks are likely to occur, and minute cracks generated during extrusion processing grow in the next forging process. There are also things to do. This chevron crack is caused by macro and semi-macro segregation in the vicinity of the center of the slab, which is generated during solidification during continuous casting.

As a measure for preventing such center segregation, for example, 2
Although electromagnetic stirring in the subcooling zone has been attempted, it has not been possible to reduce even semi-macro segregation, and its effect is not sufficient. Also, the so-called in-line reduction method {iron and steel 60th (1974) No. 7 pages 875-884}, in which a large reduction is performed using a pair of rolls at the final stage of solidification of the slab, was also tried, but this method was also tried. However, there is a problem that cracking (hereinafter referred to as internal cracking) occurs at the solidified interface where C, Mn, P, S, etc. are segregated when the reduction in the cast area having a large unsolidified layer is insufficient. It was

In addition, in JP-A-49-121738, a light reduction is applied by a pair of rolls near the solidification tip of the cast slab,
A method of compensating the amount of solidification shrinkage of the portion by reduction is proposed, and Japanese Patent Application Laid-Open No. 52-54623 proposes a method of greatly reducing the vicinity of the solidification completion point of a cast piece by using a forging die. ..

However, in the case of light reduction by rolls, even if a plurality of pairs of rolls are applied to reduce the pressure by several mm / m, it is not possible to sufficiently prevent solidification shrinkage and bulging which occur between roll pitches. The effects of reducing segregation and preventing internal cracking are insufficient, and if the rolling position is not appropriate, there is a disadvantage of promoting the occurrence of center segregation.
On the other hand, the method of using a forging die to largely reduce the vicinity of the solidification completion point of the slab is less likely to crack the solidification interface as compared to large reduction with a roll as in the in-line reduction method, and it is possible to avoid negative segregation as much as possible. Although it has been clarified that it is possible and can improve to semi-macro segregation, internal cracking occurs if the reduction in the cast area of the large unsolidified layer is still insufficient, and even if the area of the small unsolidified layer is reduced. Since the effect cannot be obtained, it is the current situation to seek the optimum rolling reduction condition.

Therefore, the center segregation generated in the cast slab has not been dramatically improved, and the target value of the C concentration of the molten steel is lowered in order to reduce the concentrations of C, P, S and the like in the segregated portion, It is the actual situation that P and S are set to less than 0.005%, and depending on the type of steel and the application, diffusion annealing or the like is performed at the stage of the slab, which is a substantial increase in cost.

[0007]

SUMMARY OF THE INVENTION The present invention advantageously solves the above problems. Even when a continuous casting method is used, the formation of center segregation is reduced as much as possible, and therefore extrusion and forging are performed. It is an object of the present invention to propose an advantageous manufacturing method of an automobile shaft component which is excellent in extrudability and forgeability without causing chevron crack at times.

[0008]

That is, the present invention is
C: 0.20 to 0.8 wt% (hereinafter simply indicated as%), Si: 0.1 to
1.0%, Mn: 0.3 to 2.0%, Cr: 0.1 to 1.0%, P: 0.
Molten steel containing 005 to 0.050% and S: 0.005 to 0.050% with the balance being Fe and inevitable impurities is continuously cast. At that time, in the vicinity of the crater end where the molten steel inside the slab completes solidification, The ratio C / C ₀ of the C content (C) in the slab axial center to the C content (C ₀ ) of the molten steel in the ladle is 0.8 to 1.
Extrusion workability characterized by performing forging pressure processing that becomes 1, then extruding to form the shaft part, and then forging the shaft end to integrally form a head with a diameter larger than the shaft part And a method for manufacturing an automobile shaft component having excellent forgeability (first invention).

Further, the present invention is the same as the above-mentioned first invention, further containing Mo: 0.01 to 0.5% and V: 0.
005 to 0.050%, Ti: 0.002 to 0.050%, Nb: 0.005 to
0.050%, Al: 0.002-0.100% and B: 0.0002-0.00
It is a manufacturing method (second invention) of a shaft part for an automobile containing 30%.

[0010]

First, the reason why the composition of molten steel is limited to the above range in the present invention will be explained. C: 0.20-0.8% C content is mainly required strength of the product after quenching (surface hardness,
It depends on the effective hardening depth of quenching), but the higher the C concentration, the lower the extrudability and the more C
If it exceeds 0.8%, the problem of reduction of impact value occurs and it is not practical, so 0.8% is made the upper limit. On the other hand, if C is less than 0.20%, sufficient workability can be secured, so 0.20% was made the lower limit.

Si: 0.01 to 1.0% Si requires at least 0.01% as a deoxidizing agent.
On the other hand, Si has the effect of increasing the activity of C, and especially when it exceeds 1.0%, the formation of a decarburized layer becomes prominent, which causes deterioration in hardenability and fatigue strength, so the upper limit was made 1.0%.

Mn: 0.3-2.0% Mn not only acts as a deoxidizer like Si but also fixes S that causes embrittlement of steel, and further improves hardenability to improve strength and ductility. Although it is a useful element for increasing the content of steel, if its content is less than 0.3%, its effect is poor. On the other hand, if it exceeds 2.0%, it not only becomes expensive, but it also becomes a controlled cooling after hot rolling or a heat treatment process during processing. In this case, since it promotes the formation of micro martensite and impairs the workability especially in the cold, it was added in the range of 0.3 to 2.0%.

Cr: 0.05 to 1.0% Cr is an essential component for ensuring the hardenability, and also has the function of improving the deformability in cold extrusion,
It is necessary to contain 0.05% or more. On the other hand, if it exceeds 1.0%, the effect of improving the deformability is saturated and it is also an expensive element, so the upper limit is 1.0%.

Mo: 0.01 to 0.5% Mo is a component effective for improving the hardenability and does not increase the deformation resistance during cold working, so 0.01% or more is contained. On the other hand, if it exceeds 0.5%, the deformation resistance increases, and since it is an expensive element, the upper limit is 0.5%.

V: 0.005 to 0.050%, Nb: 0.005 to 0.050% V and Nb each contribute effectively to the improvement of strength and have the effect of making the crystal grain size finer. However, if the content is less than 0.005%, the effect of addition is poor, while if it exceeds 0.050%, the effect reaches saturation, so the content was made 0.005 to 0.050% respectively.

Ti: 0.002 to 0.050% Ti, like Al, is a strong deoxidizing agent and, at the same time, has the function of making the crystal grain size fine and controlling the hardenability. However, if the content is less than 0.002%, the addition effect is poor,
On the other hand, if it exceeds 0.050%, the effect reaches saturation, so
It was made to contain in 0.002 to 0.050% of range.

Al: 0.002 to 0.100% Al is a strong deoxidizing agent and at the same time makes the crystal grain size fine,
It has the function of controlling hardenability. However, if the content is
If less than 0.002%, the effect of addition is poor, while 0.10%
If it exceeds 0%, not only the effect reaches saturation, but it also leads to an increase in alumina-based nonmetal oxides, so 0.002 to 0.
The content was set to be 100%.

B: 0.0002 to 0.0030% B is a component useful for improving the hardenability, but its content is 0.
If it is less than 0002%, its effect is poor, while 0.0030
Even if it exceeds%, the effect is saturated, and no further effect can be expected, so the content was made 0.0002 to 0.0030%.

Generally, a steel material which is hardened to give a predetermined strength in a final product is appropriately determined in an amount within the above-mentioned compositional range, including other quality requirements (for example, softening resistance during use of the product). Preferably.

P and S: 0.005 to 0.050% P and S should be suppressed as harmful elements as much as possible. Therefore, conventionally, in order to prevent quenching cracks caused by center segregation or internal cracking of cast slabs, the P and S concentrations in the segregated portion are suppressed to less than 0.005%. However, if the forging is performed according to the present invention, it is possible to reduce the central segregation and prevent the internal cracks at the same time, so it is not necessary to suppress the P and S concentrations to less than 0.005%. But if it exceeds 0.050%,
Since internal cracking occurs even after forging, and cracking occurs during cold working or quenching, the upper limit is 0.050%.

According to the present invention, in continuous casting of molten steel having the above-described preferable composition, the ladle is subjected to a component concentration preventing treatment near the crater end where the molten steel inside the slab completes solidification. The ratio C / C ₀ of the C content (C) in the slab axial center to the C content (C ₀ ) of the medium molten steel is 0.
Control to 8 to 1.1. Forging treatment is particularly advantageous here as a component thickening prevention treatment, but the present invention is not limited to this, and the C / C ₀ ratio is 0.8.
Other means may be used as long as it can be controlled to ~ 1.1.

The reason why the C / C ₀ ratio can be controlled by the above forging process will be described below. That is, at the final stage of solidification of the internal molten steel, molten steel with a high concentration of C exists near the crater end, so if it solidifies as it is, central segregation will occur, but if forging processing is performed before solidification, As a result of the C-rich molten steel being extruded upward, the C concentration in the central portion does not rise so much. Therefore, by adjusting the timing of forging according to the degree of enrichment of C, the C content in the slab axial center can be adjusted.

Here, the upper limit of the C / C ₀ ratio is 1.1,
This is because if it exceeds 1.1, chevron cracks will occur. On the other hand, the lower limit of the C / C ₀ ratio is set to 0.8 because the tensile strength of the product after quenching and tempering is lowered, and the final product has inconvenience in use. This is C / C _{0 at the} center of the slab
Negative segregation with a ratio of less than 0.8 results in C and Mn required for strength assurance.
This is because the content of can not be maintained.

Therefore, in the present invention, C in the axial center of the cast slab to be controlled by a component concentration preventing treatment such as forging.
The / C ₀ ratio is limited to the range of 0.8 to 1.1. In addition,
As a preferred forging processing method, the inventors have previously described in JP-A-60
There is a continuous forging method disclosed in Japanese Patent Publication No. 82257.

[0025]

EXAMPLES Molten steel having the chemical composition shown in Table 1 (symbols A to
F) is continuously cast in a 400 x 560 mm mold, and the C / C ₀ ratio of the slab axial center is near the crater end where the molten steel inside the slab completes the solidification with respect to the slab being drawn. 0.85 to 1.
A bloom was manufactured by continuously performing forging processing with a target of 0. Then, it was hot rolled into a 150 x 150 mm billet by a billet mill. Further, it was hot rolled into a round bar of 34 mmφ with a steel bar mill. Then this round bar is extruded once
A shaft portion having a diameter of 24 mm was formed, and then the end surface of the shaft portion was hot forged to form a head portion having a diameter of 90 mm.

[0026]

[Table 1]

In the comparative example, a shaft component was similarly processed after the continuous casting, without performing the forging process, as in the conventional process. Further, the molten steel heating degree at the time of tapping is all cast in the range of 20 to 30 ° C., and the hot rolling temperature from the slab rolling to the bar rolling should be the same temperature history in both Examples and Comparative Examples of the present invention. did. About the shaft parts thus obtained,
The presence or absence of chevron cracks was examined by ultrasonic flaw detection. Furthermore, the drawing of the product after quenching and tempering was also investigated. As the result is shown in Table 2, no chevron crack was generated in the shaft component obtained according to the present invention.

[0028]

[Table 2]

[0029]

As described above, according to the present invention, the component concentration preventing treatment is continuously applied during the continuous casting, and C / C _{0 of the} slab axial center portion is obtained.
By controlling the ratio, the shaft part can be manufactured without causing chevron cracks. Further, by performing the forging process, chevron cracks can be prevented without suppressing the P and / or S concentration to less than 0.005%, so that the refining cost of steelmaking can be reduced.

Claims (2)

[Claims] 1. C: 0.20-0.8 wt%, Si: 0.01-1.0 wt%, Mn: 0.3-2.0 wt%, Cr: 0.05-1.0 wt%, P: 0.005-0.050 wt% and S: 0.005-0.050. Molten steel containing wt% and the balance of Fe and unavoidable impurities is continuously cast. At that time, the C content of the molten steel in the ladle near the crater end where the molten steel inside the slab completes solidification ( C
Ratio of C content (C) in the axial center of the slab to ( ₀ )
A component thickening prevention treatment that makes C ₀ 0.8 to 1.1 is performed, then the shaft part is formed by extrusion and the head part is formed by forging, and a head with a larger diameter than the shaft part is attached to the end of the shaft part. A method for manufacturing an automobile shaft component having excellent extrusion processability and forgeability, which is characterized by being integrally molded. 2. C: 0.20-0.8 wt%, Si: 0.01-1.0 wt%, Mn: 0.3-2.0 wt%, Cr: 0.05-1.0 wt%, P: 0.005-0.050 wt% and S: 0.005-0.050. including wt%, Mo: 0.01-0.5 wt%, V: 0.005-0.050 wt%, Ti: 0.002-0.050 wt%, Nb: 0.005-0.050 wt%, Al: 0.002-0.100 wt% and B: 0.0002- Molten steel containing at least one selected from 0.0030 wt% and the balance being Fe and unavoidable impurities is continuously cast. At that time, in the vicinity of the crater end where the molten steel inside the slab completes solidification, The ratio C / C ₀ of the C content (C) in the slab axis to the C content (C ₀ ) of the molten steel in the ladle is 0.8 to 1.1. Extrudability and forgeability, characterized by forming a shaft and a head by forging, and integrally forming a head with a larger diameter than the shaft at the end of the shaft Excellent production method for shaft parts automobiles.