JP2000309853A - Steel tube excellent in workability on low-strain forming system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel tube excellent in workability on a low-strain forming system and capable of sufficiently withstanding high-degree complicated forming, by carrying out forming by the use of a specific component system on a die bending system at tube forming. SOLUTION: The steel tube has a composition consisting of, by weight, 0.05-0.20% C, 0.5-2.0% Si, 0.5-2.5% Mn, <=0.005% S, <=0.15% P, 0.005-0.10% Al, and the balance iron with inevitable impurities and containing, if necessary, 0.0002-0.0020% Ca and also has a conjugated structure consisting of >=5% austenite metastable at ordinary temperature and the balance martensite, bainite, and ferrite. In the manufacture of this steel tube, the central part of a steel strip is bent after the bending of its edge to a prescribed curvature, and then the steel strip is worked into a tube by using a die having a caliber of a prescribed shape, followed by welding. By this method, the steel tube, having high ductility and capable of withstanding complicated working such as hydroforming, can be manufactured.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel pipe excellent in workability by a low distortion forming method.

[0002]

2. Description of the Related Art Hydroforming technology is one of the core technologies of next-generation automobile production that is being actively pursued. The purpose of this is to reduce the weight of automobiles and reduce costs by reducing the number of parts. Under such a background, the application of underbody parts and body parts made of steel pipe and formed by hydroforming is increasing. A part formed by hydroforming has a complicated shape, and it is important that a steel pipe used for the part has good ductility to withstand the processing. Until now, many researches and developments have been made to improve ductility while maintaining the same strength, that is, to improve strength-ductility balance.

[0003] In the past, what has drastically improved the strength-ductility balance is a so-called duplex stainless steel having a mixed structure of ferrite and martensite. For example, JP-A-51-12317 discloses a technique of duplex stainless steel obtained by hot-rolling followed by rapid cooling, and JP-B-57-45454 discloses a technique obtained by continuous annealing. At present, duplex stainless steels, which take advantage of their excellent properties, are attracting attention as applications for lightening materials for automobiles and the like.

[0004]

It is important that a steel pipe for hydroforming has good ductility to withstand complicated processing. The present invention provides a steel pipe having excellent workability and a method for producing the same. Although the materials obtained by the duplex stainless steel and the continuous annealing in the above-described conventional technology are very excellent in workability, they are still insufficient in ductility as a material for hydroform requiring high workability. Therefore, good ductility that can withstand complicated processing is required. For this purpose, it is important to increase the ductility of the raw material itself and to reduce the forming distortion when forming into a steel pipe. The former is a fundamental issue, but the latter is also important. In a conventional roll forming method such as an electric resistance welded tube, a molding distortion during molding is large, and a residual distortion remains even after molding in the tube, thereby greatly reducing ductility. These issues need to be solved.

[0005]

Means for Solving the Problems In weight%, C: 0.05
-0.20%, Si: 0.5-2.0%, Mn: 0.5
2.5%, S: 0.005% or less, P: 0.15% or less, Al: 0.005 to 0.10%
And 0.0020 to 0.0020%, the balance being iron and unavoidable impurities, the metastable austenite at room temperature being 5% or more and the balance being martensite, bainite and ferrite having a composite structure comprising:
After bending the edge of the steel strip to a predetermined curvature, bend the center,
Finally, it is an object of the present invention to provide a manufacturing method characterized by processing and welding a tube with a mold having a hole having a predetermined shape.

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION The steel pipe excellent in workability of the present invention and a method for manufacturing the same will be described in detail below. First, the reasons for limiting the chemical components will be described. C is an important element for the generation of a retained austenite phase, and if it is less than 0.05%, a sufficient amount of retained austenite cannot be obtained, so that good workability cannot be exhibited. However, when C exceeds 0.20%, the maximum hardness of the welded portion becomes too high as compared with the base metal, which is not preferable for severe processing such as hydroforming. Therefore, the upper limit of C is set to 0.
20%.

[0007] Si has the effect of accelerating the enrichment of C in austenite and facilitating the formation of retained austenite, and it is necessary to add 0.5% or more of Si. However, excessive addition of Si leads to embrittlement of steel and deteriorates the balance between strength and ductility. Further, Si-based reaction products are generated in the welded portion, deteriorating the characteristics of the welded portion. Therefore, the upper limit of the addition of Si is set to 2.0%.

Mn is an indispensable element for generating retained austenite by bainite transformation because it shifts the nose of ferrite-pearlite transformation toward a longer time. Moreover, like C, it is an austenite stabilizing element and is necessary for obtaining an excellent balance between strength and ductility. 0.5
%, Sufficient austenite cannot be secured.
On the other hand, if added excessively, the ductility of the steel sheet deteriorates. Therefore, the upper limit of Mn addition is set to 2.5%.

S is an element that forms MnS and reduces toughness and press workability. In particular, when the strength is high, the influence of deterioration in toughness and press workability is likely to be strong.
Therefore, it must be as low as possible. So S
Is set to 0.005% or less. P is effective for strengthening the steel, but if added in excess of 0.15%, the weldability is impaired. Further, if necessary, 0.0002
When it is contained in an amount of up to 0.0020%, the form of inclusions such as MnS is changed, and workability and ductility are improved. If it is less than 0.0002%, there is no effect, and if it exceeds 0.0020%, the effect is not less than that.

Next, metastable austenite is formed at room temperature from the above components, and this improves ductility and workability. However, metastable austenite at room temperature
% Or more. This amount varies greatly depending on the hot rolling conditions of the steel sheet and the continuous annealing conditions after cold rolling. That is, conditions such as a cooling rate in hot rolling, a winding temperature, an annealing temperature in continuous annealing after cold rolling, a cooling rate, and the like. By optimizing them, the metastable austenite at room temperature is made 5% or more. The lower limit of the amount of austenite is set to 5%. If it is less than 5%, the ductility is still insufficient as a material for hydroforms requiring high workability, and good ductility and workability that can withstand complicated working. No characteristics can be obtained. The structure other than austenite is a composite structure composed of martensite and bainite and ferrite.

Next, a method of manufacturing a steel pipe will be described. Conventionally, a steel pipe used for such an application has been a so-called ERW steel pipe formed continuously from a steel strip by roll forming and subjected to ERW welding at a high frequency. However, many steel pipes used for hydroforming have a small thickness, and as shown in FIG. 4, the edges are elongated in the middle of roll forming, and they become edge buckling before welding.
Further, the distortion of the buckling and the distortion during the roll forming are large, and the workability after the steel pipe is formed is deteriorated.

The solution is to bend the edge of the steel strip to a predetermined curvature, then bend the center, and finally process and weld the pipe with a mold having a hole of a predetermined shape. This is a characteristic manufacturing method. As shown in FIG. 1, first, edge bend molding is performed. The steel plate 1 is inserted between the upper edge bend die 2 and the lower edge bend die 3, and both the die are pressed to form only the edge. This is because the edge is difficult to be formed thereafter, and must be formed first.

Next, center bend molding is performed as shown in FIG. The steel plate 1 after the edge bend is inserted between the upper center bend die 5 and the lower center bend die 6, and is sandwiched between the two dies to form the steel plate center. Furthermore,
Upper and lower dies 8, 9 having the final shape as shown in FIG.
The steel plate 4 after the center bend is inserted into the inside and pressed down to form a final shape. In this method, the edge buckling does not occur as in the roll forming, and since the bending is performed only, the distortion is very small. As in the present invention, by forming a specific component system and a bending method using a metal mold in pipe forming, a steel pipe excellent in workability that can sufficiently withstand sophisticated and complicated forming can be obtained.

[0014]

EXAMPLES Table 1 shows the size φ6 of the steel of the present invention and the comparative steel.
An example of 3.5 × T2.0 mm is shown. As is clear from the table, in the steel pipe manufactured according to the present invention, the amount of retained austenite is large, and since the pipe is manufactured by the bending method using a mold in the pipe forming, the residual strain is small, and the strength is high. EL and uniform elongation are very high. Evaluation is as follows: TS ≧ 600 N /
mm ² and EL ≧ 39% were defined as Δ. The steel according to the present invention is rated as Δ, and the comparative steel is rated as ×.

[0015]

[Table 1]

[0016]

The steel pipe produced by the method of the present invention is characterized by having metastable austenite of 5% or more and a composite structure composed of martensite, bainite and ferrite, and has excellent workability. In the future, there is an increasing demand for steel pipes having high ductility that can withstand complicated processing such as hydroforming. Therefore, the effect of the steel pipe manufactured according to the present invention is extremely large.

[Brief description of the drawings]

FIG. 1 is a diagram showing edge bend forming which is a manufacturing process of the present invention;

FIG. 2 is a view showing a center bend forming which is a manufacturing process of the present invention;

FIG. 3 is a diagram showing a final shape of the manufacturing process according to the present invention;

FIG. 4 is a view showing a conventional manufacturing process.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Steel plate 2 Edge bend upper die 3 Edge bend lower die 4 Steel plate after center bend 5 Center bend upper die 6 Center bend lower die 7 Final tube shape 8 Final upper die 9 Final lower die 10 Steel strip 11 Forming roll 12 Edge buckling

 ──────────────────────────────────────────────────続 き Continued on front page (72) Inventor Shinya Sakamoto 1 Kimitsu, Kimitsu-shi, Chiba F-term in Nippon Steel Corporation Kimitsu Works (reference) 4E028 CA04 CA13 CA16 CB01 CB04 CB06

Claims (1)

[Claims] C: 0.05 to 0.20%; Si: 0.5 to 2.0%; Mn: 0.5 to 2.5%; S: 0.005% or less; P: 0.15% or less, Al: 0.005 to 0.10% If necessary, Ca contains 0.0002 to 0.0020%, the balance being iron and unavoidable impurities, and metastable austenite at normal temperature In a steel pipe having a composite structure composed of martensite, bainite, and ferrite with 5% or more and the remainder, after bending the edge of the steel strip to a predetermined curvature, bending the center part, finally having a hole shape of a predetermined shape. A steel pipe with excellent workability characterized by being processed into a pipe by a mold and welded.