JP3464586B2 - Method for manufacturing high-strength steel pipe with excellent hydroformability - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hydroforming method in which a metal pipe is put into a split mold, an internal pressure is applied to the inside of the pipe, and the pipe is pushed in the axial direction to form the pipe into a predetermined shape. The present invention relates to a method for manufacturing a high-strength steel pipe having excellent workability. 2. Description of the Related Art In automobile parts and the like, products obtained by forming a metal pipe such as a steel pipe by a hydroforming method have begun to be adopted. As shown in the example of the T-shaped tube in FIG. 3, the molding method is as follows. The raw tube 1 is put into the split dies 4 and 5, a liquid is introduced into the raw tube 1 through the liquid introduction hole 8, and internal pressure is applied. This is a method in which a compressive load is applied in the tube axis direction by the pushing cylinders 6 and 7 to push the cylinder into a predetermined shape with a T-forming height h. [0003] In addition, examples of molding include a method in which the diameter of the raw tube 1 is partially enlarged, a method in which the diameter is enlarged to have various cross-sectional shapes, and the like. The obtained molded article 3 is lightweight and can be molded into a complicated shape. The base tube 1 is required to have a uniform thickness, and an electric resistance welded steel tube is often used as a raw steel tube. The ERW steel pipe is manufactured by forming a strip produced by hot rolling into a tubular shape using a cage roll or the like, and performing butt welding. [0004] The product formed by the hydroforming method must have strength and toughness that will not cause breakage or deformation in the use environment in various applications such as automobile parts. . On the other hand, from the viewpoint of workability, a thin-walled steel pipe that is easily formed by the indentation force and the internal pressure is required. [0005] However, if the strip is made thinner in the conventional method of manufacturing an electric resistance welded steel pipe, there is a problem that the butt welding cannot be performed due to the edge extension of the strip when it is formed into a tubular shape. No thin-walled electric resistance welded steel pipe has been obtained. Further, a thin-walled electric resistance welded steel pipe having sufficient strength and toughness after forming has not been obtained. Therefore, there is a need for a hydroforming steel pipe having high strength and excellent formability. The present invention relates to a hydroforming method in which a metal pipe is put into a split mold, an internal pressure is applied to the inside of the pipe, and the metal pipe is pushed in the pipe axis direction to form a predetermined shape. The hydroforming method has excellent workability, sufficient strength and toughness, And t suitable for hydroforming
/ D (t: wall thickness, D: outer diameter) to provide a thin steel pipe. SUMMARY OF THE INVENTION In order to achieve the above object, the present invention provides a hydroforming method in which a metal pipe is put into a split mold, an internal pressure is applied to the inside of the pipe, and the pipe is pushed in the axial direction to form a predetermined shape. The method for producing a steel pipe processed by the method described in (1) above, wherein C: 0.02 to 0.12% by weight%;
5% or less, Mn: 1.0-2.5%, P: 0.
An ERW steel pipe containing 1% or less, S: 0.01% or less, and the balance consisting of Fe and inevitable impurities is t / D (t: wall thickness, D:
A thin tube having an outer diameter of 0.7 to 1.2% is maintained at a temperature range of Ac ₁ or more and (Ac ₁ + 75 ° C.) or less for 15 seconds or more, and then 200 ° C. at a cooling rate of 10 ° C./sec or more. This is a method for producing a high-strength steel pipe excellent in hydroform workability, characterized by cooling to the following temperature. DETAILED DESCRIPTION OF THE INVENTION A steel pipe produced by the method of the present invention has a martensite and other low-temperature generated phases of 5 to 3 hours.
At 0%, t / D (t: wall thickness, having a structure consisting of a ferrite phase and suitable for hydroforming)
D: Outer diameter) is a high-strength thin-walled steel pipe having an outer diameter of 0.7 to 1.2%. For this reason, while being excellent in hydroform processability, the product after processing has sufficient strength and toughness. The reasons for limiting the steel components in the present invention are as follows. C is an element that strongly influences the structure of steel. If it is less than 0.02%, it becomes difficult to obtain the above-mentioned amount of martensite and other low-temperature formed phases, and if it exceeds 0.12%, The strength becomes too high and the moldability deteriorates. Therefore, it was set to 0.02 to 0.12%. [0010] Si is a solid solution strengthening element and not only makes it possible to adjust the strength of steel, but also suppresses the formation of carbides and facilitates the formation of a martensite structure by discharging solid solution C in ferrite into austenite. To be added. However, as described later, Ac ₁ or more and (Ac ₁ +
The solid solution C in ferrite is easily discharged into austenite during annealing in a temperature range of 75 ° C. or less,
It is not particularly necessary to increase the amount added. On the other hand, if it exceeds 1.5%, the strength becomes too high, and the moldability deteriorates. Therefore, it is set to 1.5% or less. Mn, like Si, is a solid solution strengthening element and is effective in adjusting the strength. It is also an austenite stabilizing element that facilitates the formation of martensite,
1.0% or more is added because it contributes to strength improvement. However, if it exceeds 2.5%, the moldability is reduced. Therefore, it was set to 1.0 to 2.5%. P is added in order to adjust the strength without significantly affecting martensite formation.
If added in excess of 1%, the moldability deteriorates.
% Or less. If S is added in excess of 0.01%, the moldability deteriorates, so S is set to 0.01% or less. [0013] Steel of such a composition is cast, and the resulting hot slab is subjected to hot rolling after being directly or heated, or by reheating the cold slab. Hot rolling can be performed in either a normal hot rolling process or a continuous hot rolling process in which sheet bars are joined and rolled before finish rolling. The conditions of the hot rolling are not particularly limited, because the conditions of the hot rolling hardly affect the final material. Further, there is almost no change in the finally obtained material regardless of whether the hot rolling end temperature is in the austenite region or the ferrite region. The strip obtained by hot rolling is formed into a tubular shape by a usual method and subjected to electric resistance welding. Then, a thin steel pipe is formed by cold working. As cold working means, cold drawing and pilger rolling are known, but any means is available.
Cold drawing generally performed may be used. Since the ERW steel pipe without uneven thickness is cold-worked, a steel pipe with uniform thickness without uneven thickness can also be obtained. In the case of an electric resistance welded steel pipe made of the material of the above-mentioned components, the limit of thickness reduction is about t / D = 2.5% in the ordinary production technique, but the hydroforming method is performed by cold working such as cold drawing. /D=0.7-1.2% suitable for processing by
Thin to the extent. At that time, as a cold working rate, in order to obtain sufficient strength and toughness by heat treatment in the subsequent process,
It is preferred to be 50% or more. Here, the cold working rate is represented by {(cross-sectional area before working) − (cross-sectional area after working)} / (cross-sectional area before working) × 100 (%). [0016] The annealing after the cold working is at least Ac ₁ and (Ac
₍₁ + 75 ° C.) or lower, the temperature below Ac ₁ results in less than 5% of the resulting martensite and other low temperature formed phases, and the temperature above Ac ₁ + 75 ° C. This is because the amount of the site and other low-temperature generated phases exceeds 30%. The reason for holding for 15 seconds or more in the above temperature range is as follows.
If the holding time is shorter than this, there is no sufficient diffusion of solid solution C from the ferrite phase to the austenite phase, and ferrite without precipitates, carbides, inclusions, etc. in the grains cannot be obtained. This is because the properties and the strength and toughness after processing cannot be obtained. The reason why the cooling rate after annealing is set to 10 ° C./second or more is that if the cooling rate is lower than this, pearlite or carbide precipitates during cooling, and a required amount of martensite cannot be obtained. . Also, at the above cooling rate, 2
The reason why the cooling is performed to 00 ° C. or less is that when the cooling is completed at a temperature exceeding 200 ° C., the strength becomes insufficient. The temperature pattern of the heat treatment is as shown in FIG. In such a heat treatment, as in the example shown in FIG. 2, the steel pipe 2 after cold working is moved and held in the induction coil 9 while being moved in the pipe axis direction indicated by the arrow, and cooled by the cooling device 10. It can be carried out. EXAMPLES Steels having various chemical compositions shown in Table 1 were cast, hot-rolled, and subjected to high-frequency electric resistance welding to an outer diameter of 60.5 mm and a wall thickness of 1.5 mm (t / D = 2.5%). ). This was cold drawn to an outer diameter of 60.5 mm and a wall thickness of 0.6
mm (t / D = 1.0%). The cold working ratio at this time is 59.4%. Then, heat treatment was performed under various conditions using the equipment shown in FIG. 2, the amount of martensite and the maximum strength were measured, and the T-forming height h shown in FIG. 3 was measured as hydroform workability. Table 2 shows the results.
It is shown in Table 7. In Tables 2 to 6, the steel types are all components within the range of the present invention. However, in Comparative Examples in which the heat treatment conditions were out of the range of the present invention, a high-strength steel pipe was produced because of insufficient martensite. No excellent hydroform workability is obtained because the yield ratio increases and the work hardening property deteriorates. On the other hand, the examples of the present invention also satisfy the heat treatment conditions, and all have high strength and excellent hydroform workability. Table 7 shows the results for steel types whose components are out of the range of the present invention. Steel type No. 6 has insufficient martensite, so that high strength cannot be obtained. Steel type No. 7 has no martensite. Excessive strength is too high, and none of them has obtained excellent hydroform processability. Steel type No. 8 is T
Although high strength was obtained by precipitation strengthening by adding i, high strength was obtained, but excellent ductility was inferior and excellent hydroform workability was not obtained. [Table 1] [Table 2] [Table 3] [Table 4] [Table 5] [Table 6] [Table 7] According to the method of the present invention, a high-strength thin steel pipe having a uniform thickness of t / D (t: wall thickness, D: outer diameter) suitable for hydroforming is obtained. It has hydroform workability and has sufficient strength and toughness. Therefore, it is effective for the production of various molded articles such as automobile parts by the hydroforming method.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an explanatory view showing a heat treatment pattern in the method of the present invention. FIG. 2 is an explanatory view of a heat treatment step in the method of the present invention. FIG. 3 is a cross-sectional view showing an example of a hydroforming method to which the present invention is applied. [Description of Signs] 1 ... raw pipe 2 ... steel pipe 3 ... molded product 4, 5 ... split mold 6, 7 ... cylinder 8 ... liquid introduction hole 9 ... induction coil 10 ... cooling device

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI // B21D 26/02 B21D 26/02 C (72) Inventor Takeo Nagao 2-6-3 Otemachi, Chiyoda-ku, Tokyo New Japan (72) Inventor Hiroyuki Tanahashi 20-1 Shintomi, Futtsu City, Chiba Prefecture Nippon Steel Corporation Technology Development Division (56) References JP-A-6-220534 (JP, A) JP-A Heihei 6-158163 (JP, A) JP-A-64-17820 (JP, A) JP-A-64-4424 (JP, A) JP-A-7-188748 (JP, A) JP-B-57-17932 (JP, A) B1) Tube forming-secondary processing of tube material and product design-edited by Japan Society for Technology of Plasticity, Japan, Corona Co., Ltd., October 30, 1992, p. 65-80 (58) Field surveyed (Int.Cl. ⁷ , DB name) C22C 38/00-3/60 C21D 8/00-8/10, 9/08 B21D 26/02 B21C 37/00-37 / 08

Claims (1)

(1) A method for manufacturing a steel pipe processed by a hydroforming method in which a metal pipe is put into a split mold, an internal pressure is applied to the inside of the pipe, and the pipe is pushed in the axial direction to form a predetermined shape. Oh, weight%
C: 0.02 to 0.12%, Si: 1.5%
Hereinafter, Mn: 1.0 to 2.5%, P: 0.1%
Hereafter, an ERW steel pipe containing S: 0.01% or less, with the balance being Fe and unavoidable impurities, is t / D (t: wall thickness, D:
A thin tube having an outer diameter of 0.7 to 1.2% is maintained at a temperature range of Ac ₁ or more and (Ac ₁ + 75 ° C.) or less for 15 seconds or more, and then 200 ° C. at a cooling rate of 10 ° C./sec or more. A method for producing a high-strength steel pipe excellent in hydroform workability, characterized by cooling to the following temperature.