JPH0873938A - Production of high strength and high toughness steel pipe - Google Patents

Abstract

PURPOSE: To produce a high strength and high toughness steel pipe excellent in induction hardenability. CONSTITUTION: A steel contg. 0.35 to 0.55% C, 0.10 to 0.50% Si, 1.0 to 2.0% Mn, <=0.03% P, <=0.03% S and one or >= two kinds among <=1.0% Cr, <=0.2% V, 0.015 to 0.05% Nb, 0.010 to 0.05% Ti and 0.0010 to 0.0050% B, and the balance Fe with inevitable impurities is used to make a seamless steel pipe. At this time, it is subjected to final rolling at the finishing temp. of the Ar3 transformation point or above, is thereafter subjected to cold rolling at 10 to 70% reduction in area and is finished to a prescribed dimension. Next, it is subjected to heat treatment at 500 to 700 deg.C, is furthermore subjected to surface induction hardening in the temp. range of 900 to 1000 deg.C and is thereafter tempered. Thus, the convertion of the material for under-carriage member into hollow steel pipes and the lightening thereof are made possible.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high-strength and high-toughness steel pipe, which is suitable for a high-strength member used for the suspension of an automobile, among steel materials for machine structures used in automobiles and the like.

[0002]

2. Description of the Related Art At present, among the steel materials used in the drive trains and suspensions of automobiles, particularly steering shafts,
The drive shaft, etc., has a tensile strength of 80 kgf / mm ²
Steel bars such as S45C and S50C, which are carbon steel materials for machine structures specified in JIS G4051 of the class, are often used, and surface induction hardening and tempering are used. However, as automobiles become lighter, hollow steel pipes are strongly demanded for the steel materials used for the drive system and the suspension. When hollowing steering shafts, drive shafts, etc., compared to the current steel bars of mechanical strength carbon steel S45C, S50C with a tensile strength of 80 kgf / mm ² class, the hollow part has higher strength and higher toughness. Steel pipes are required, and the surface is induction hardened and tempered,
Not only the hardness as a surface induction-hardened member,
High torsional strength and good bending properties are required.

Therefore, the high-strength, high-toughness seamless steel pipe used for the suspension of automobiles is simply strengthened by subjecting the seamless steel pipe to hot quenching and tempering. Is not sufficient, and it has been attempted to cold work a seamless pipe by hot working, heat it after ensuring a certain dimensional accuracy, and then increase the strength and toughness. .

[0004]

However, if the seamless pipe is hot-worked by cold working and then heat-treated to increase the strength and the toughness, the steering shaft, the drive shaft, etc. S4 of carbon steel for machine structure with tensile strength of 80kgf / mm ² class used for
It is impossible to impart high strength and high toughness higher than the strength of steel bars such as 5C and S50C.

The present inventors have proposed as a method for producing a high-strength, high-toughness steel pipe having excellent material properties before induction hardening, by using C:
0.35 to 0.55%, Si: 0.10 to 0.50%,
Mn: 1.0 to 2.0%, P: 0.03% or less, S:
0.03% or less, Cr: 1.0% or less, V: 0.2%
Hereinafter, Nb: 0.015 to 0.05%, B: 0.001
Final rolling of a seamless steel pipe using a steel material containing one or more of 0 to 0.0050% and the balance being Fe and unavoidable impurities at the finishing temperature Ar ₃ transformation point or more during pipe making. After that, cold work with a cross-sectional reduction rate of 10 to 70% to finish to the specified dimensions, then 500 to 700 ° C
A patent application has already been filed as Japanese Patent Application No. 5-192885 for a method for manufacturing a high-strength and high-toughness steel pipe, which is characterized by performing heat treatment at the temperature of. Although the method disclosed in Japanese Patent Application No. 5-192885 can obtain a high-strength and high-toughness steel pipe, it has been found that the induction hardening characteristics in the post-process are insufficient.

An object of the present invention is to provide a method for producing a high-strength and high-toughness steel pipe which is suitable for a high-strength member used around the periphery of an automobile and which has excellent induction hardening properties.

[0007]

Means for Solving the Problems As a result of various tests and examinations of induction hardening conditions for achieving the above object, the inventors of the present invention conducted induction hardening at 900 to 1000 ° C. to obtain the current S45C, S50C, etc. It was confirmed that a high-strength and high-toughness steel pipe having more excellent torsional strength and bending characteristics than a steel bar can be produced, and the present invention was reached.

That is, according to the present invention, C: 0.35 to 0.
55%, Si: 0.10 to 0.50%, Mn: 1.0 to
2.0%, P: 0.03% or less, S: 0.03% or less, Cr: 1.0% or less, V: 0.2% or less, Nb:
0.015-0.05%, Ti: 0.010-0.05
%, B: 0.0010 to 0.0050% of one or more, and the balance is Fe and inevitable impurities. When a seamless steel pipe is manufactured, a finishing temperature Ar is used. After final rolling at ₃ or more transformation points, cold work is performed at a cross-section reduction rate of 10 to 70% to finish to a predetermined size, and then heat treatment is performed at a temperature of 500 to 700 ° C, and further 9
A method for producing a high-strength and high-toughness steel pipe, which comprises performing surface induction hardening in a temperature range of 00 to 1000 ° C. and then tempering.

[0009]

In the present invention, the reasons for limiting the chemical composition of the raw material are as follows. C is an element necessary to obtain the strength of the material and the strength of the gear cutting portion after induction hardening, and 0.3
If less than 5%, the target hardness cannot be obtained, and 0.55%
If it exceeds 1.0, the hardness becomes high and the toughness deteriorates, so 0.3
It was set to 5 to 0.55%. Si is an important element for deoxidizing steel, and if it is less than 0.10%, deoxidation becomes insufficient, and if it exceeds 0.50%, the effect is saturated and the cost becomes high. It was set to 0.50%. Mn is the strength of the material,
It is an element important for ensuring ductility and toughness. If it is less than 1.0%, the desired mechanical properties cannot be obtained, and if it exceeds 2.0%, the toughness deteriorates. It was set to 0.0%.
P is an element that deteriorates the toughness after quenching, and if it exceeds 0.03%, the toughness decreases, so it was made 0.03% or less.
S is an element that forms MnS of non-metallic inclusions and deteriorates toughness. When it exceeds 0.03%, the tendency is remarkable, so S was made 0.03% or less.

Cr is an important element for obtaining the strength of the material, but it is expensive, so the content is made 1.0% or less. V is an element effective in securing the strength and toughness of the material, but since it is expensive, it was set to 0.2% or less. Nb is an element effective in improving the strength of the material and the toughness by refining the crystal grains.
If it is less than 5%, the effect is not sufficient, and 0.050
%, The toughness deteriorates, so 0.015 to 0.0
It was set to 50%. Ti is an element effective for improving the toughness by refining the crystal grains, and if it is less than 0.010%, the effect is not sufficient, and if it exceeds 0.050%, the toughness deteriorates, so that it is 0.010 to 0. It was set to 0.050%. B is an element effective in improving the hardenability of the induction hardened part and the toughness by strengthening the grain boundary. If it is less than 0.0010%, the effect is not sufficient, and if it exceeds 0.0050%, the toughness deteriorates. It was set to 0.0010 to 0.0050%.

The finishing temperature during hot pipe forming is set to the Ar ₃ transformation point or higher in order to make the rolling structure a grain size controlling structure. Further, the working degree during cold working is set to 10 to 70% because the predetermined dimensional accuracy cannot be obtained when the working ratio is less than 10%.
This is because if it exceeds 70%, cracking due to cold working may occur. Furthermore, the heat treatment temperature after cold working is 700 to 5
The reason why the temperature is set to 00 ° C. is that if the temperature exceeds 700 ° C., a predetermined strength cannot be obtained, and if the temperature is lower than 500 ° C., the strength becomes too high and the ductility and toughness deteriorate. Furthermore, the reason why the heating temperature is set to 900 to 1000 ° C. as the induction hardening condition is that if the temperature is less than 900 ° C., the carbide cannot be sufficiently solid-dissolved due to rapid heating for a short time, and a predetermined quenching hardness cannot be obtained.
This is because if the temperature exceeds 000 ° C., the strength becomes too high and the toughness of the hardened part deteriorates. The tempering temperature is not particularly specified, but is generally 150 to 200 ° C. Further, it is possible to change the quenching depth by changing the high frequency.

In the present invention, C: 0.35 to 0.
55%, Si: 0.10 to 0.50%, Mn: 1.0 to
2.0%, P: 0.03% or less, S: 0.03% or less, Cr: 1.0% or less, V: 0.2% or less, Nb:
0.015-0.05%, Ti: 0.010-0.05
0%, B: 0.0010 to 0.0050% of 1 type or 2 types or more are used, and the balance of Fe and unavoidable impurities is used.
Ductility and toughness are ensured, and since the seamless steel pipe is finally rolled at the finishing temperature Ar3 transformation point or higher during pipe manufacturing, a sized structure can be obtained. Moreover, after cold working in the range of 10 to 70% in cross-section reduction to finish to a predetermined size, 500
Since the heat treatment is performed at a temperature of up to 700 ° C, the seamless steel pipe is provided with a tensile strength of 80 kgf / mm ² class used for steering shafts, drive shafts and the like, and at the same time, toughness and ductility are secured. 900-
By performing surface induction hardening in the temperature range of 1000 ° C. and then tempering, it is possible to obtain a high-strength, high-toughness steel pipe having the required high twist strength and good twist characteristics.
Hollow steel pipes such as steering shafts and drive shafts can be made, and the weight of automobiles can be reduced.

[0013]

【Example】

Example 1 Steels having the chemical composition shown in Table 1 and melted in vacuum (steel type No. 1 to
No. 11 of the present invention is steel type No. 11 12 to 16 are comparative steels) and then hot rolled to an outer diameter of 31.8 mm and a wall thickness of 8.0 mm (however, finishing at 830 ° C. above the Ar ₃ transformation point).
No seamless steel pipe, outer diameter 23.0 mm, wall thickness 6.0
After cold drawing to 46 mm (area reduction rate 46%),
Tensile strength 85 in the tensile test specified in JIS Z2241
About the heat treatment under the condition that the strength is kgf / mm ^2, the maximum heating temperature is 850 to 850 at the frequency of 20 kHz.
After quenching at 1050 ° C and then tempering at 170 ° C for 1 hour, the test piece of the seamless steel tube was fixed at one end and a torsion load was applied to the other end to confirm the torsional strength and the presence of cracks. investigated. The results are shown in Table 2. The steel grade No. The maximum heating temperature of 920 ° C. and 980 ° C. for the second invention steel heat-treated at 650 ° C.
Example of the present invention is the case of quenching with
o. Regarding the steel of the present invention No. 2 heat-treated at 800 ° C. and 400 ° C., when the quenching was performed at the maximum heating temperature of 950 ° C. and steel type No. The same test was carried out on the case where the steel of the present invention No. 2 which was heat-treated at 650 ° C. was subjected to quenching at the maximum heating temperature of 1050 ° C. as a comparative example. The results are also shown in Table 2.

[0014]

[Table 1]

[0015]

[Table 2]

As shown in Table 2, in each of the examples of the present invention, the torsional strength was high, and cracks were not generated in the torsion test at all, but in the comparative example, the torsional strength was low, and cracks were generated in the torsion test. , Ductility,
It had poor toughness.

Example 2 Among steels having chemical compositions shown in Table 1, steel type No. No. 2 steel of the present invention and steel type No. Outer diameter 23.0 made from 12 comparative steels
As to the steel pipes 1 and 2 of the present invention having a thickness of 6.0 mm and a thickness of 6.0 mm, and a comparative steel pipe, as shown in FIG. 1, a steel pipe 1 having a length of 300 mm has a width of 2 mm, a depth of 2 mm, and a bottom R of 1 m.
After processing the notch 2 of m, induction hardening at 100 kHz at that portion, and tempering at 170 ° C. for 1 hour, then, as shown in FIG. 2, with the notch 2 facing downward, 75 mm apart from the center of the steel pipe 1 on both sides. Support position 3, put it on 3, 3,
Press the metal fitting from the top to the central part and push in 15mm
The bending test of pushing and bending was performed twice, and the bending absorbed energy was obtained. The results are shown in Table 3 together with the heat treatment temperature and tensile strength of the raw steel pipe. 3 shows the hardness distribution from the notch position of the test material, FIG. 4 (a) shows a three-point bending chart example of the steel pipe of the present invention, and FIG. 4 (b) shows a three-point bending chart example of the comparative steel pipe. Show.

[0018]

[Table 3]

As shown in FIG. 3, steel type No. The steel pipe No. 2 of the present invention is steel type No. Quenching characteristics are superior to those of the 12 comparative steel pipes. Moreover, as shown in FIG. Two
The steel pipe of the present invention of No. Compared with No. 12 comparative steel pipe, the crack development is small and the toughness is remarkably excellent. Further, as shown in Table 3, steel type No. No. 2 steel pipe of the present invention
Steel type No. Compared with No. 12 comparative steel pipe, the absorbed energy is 1.5 to 1.7 times, which is a good bending property.

[0020]

As described above, according to the method of the present invention, high strength, high ductility, and high toughness can be achieved, and particularly, the existing S45 of the high strength member used for the suspension of automobiles is used.
Compared with C bar steel, it becomes possible to make a hollow steel tube, and it is possible to reduce the weight of the automobile.

[Brief description of drawings]

1A and 1B show the shape of a notched bending test piece, where FIG. 1A is a front view and FIG. 1B is a side view.

FIG. 2 is an explanatory diagram of a bending test method.

FIG. 3 is a graph showing the relationship between the distance from the notch position and hardness of the steel pipes 1 and 2 of the present invention and the comparative steel pipe in Example 2.

FIG. 4 shows an example of a three-point bending chart of the present invention steel pipe 2 and a comparative steel pipe in Example 2, where (a) is the case of the present invention pipe 2 and (b) is the case of the comparative steel pipe.

[Explanation of symbols]

 1 Steel pipe 2 Notch 3 Support

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication C22C 38/38

Claims (1)

[Claims] 1. C: 0.35-0.55%, Si: 0.
10 to 0.50%, Mn: 1.0 to 2.0%, P: 0.
03% or less, S: 0.03% or less, Cr: 1.0% or less, V: 0.2% or less, Nb: 0.015 to 0.05
%, Ti: 0.010 to 0.05%, B: 0.0010
Of 0.0050% to 0.0050%, the final rolling at a finishing temperature of Ar ₃ transformation point or more when producing a seamless steel pipe using a steel material containing the balance of Fe and unavoidable impurities After that, cold work is performed at a cross-sectional reduction rate of 10 to 70% to finish to a predetermined size, and then 500 to 700.
A method for producing a high-strength and high-toughness steel pipe, which comprises performing heat treatment at a temperature of ℃, and further performing surface induction hardening at a temperature range of 900 to 1000 ℃, followed by tempering.