JP2002275535A - Method for producing steel pipe for hydroforming having strain aging property - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a steel pipe which has excellent hydroformability, and is hardened by coating/baking treatment after hydroforming. SOLUTION: A steel sheet having a composition containing, by mass, 0.01 to <0.05% C, <=0.01% Si, <=3.0% Mn, <=0.15% P, <=0.015% S, <=0.01% Al and N of 0.005 to 0.02%, also by >=0.003% in a solid solution state, and the balance Fe with inevitable impurities and subjected to hot rolling or cold rolling is formed into a cylindrical shape, and the joint parts are welded. The steel pipe after the welding is annealed at 600 to 750 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel pipe suitable for use as a structural member or undercarriage member of an automobile, and has excellent workability (hydroforming property) particularly in hydroforming and has strain aging. The present invention relates to a structural steel pipe.

[0002]

2. Description of the Related Art In order to manufacture hollow members having various cross-sectional shapes used as structural members of automobiles, conventionally, parts formed by pressing a steel plate are spot-welded to each other at a flange portion which is a welding margin. Bonding methods have been adopted, but improvements have been demanded in terms of both quality and production efficiency. In addition, the hollow member for this structure is required to have a higher shock absorbing ability at the time of collision, and is required to have higher strength. For this reason, it is becoming increasingly difficult to produce a member having no molding defects and having good shape and dimensional accuracy of a molded product by the conventional press molding method.

As a new molding method for solving such a problem, recently, a molding method by hydroforming has begun to attract attention. Hydroforming is a method of loading a steel pipe inside a mold, injecting a high-pressure liquid into the steel pipe, expanding the pipe, and performing plastic working along the mold. It is an excellent molding method that can be peeled off, and the strength and rigidity can be increased by a monocoque action due to the shape characteristics of the member after molding. By the way, as a steel pipe for hydroforming, generally, strength is easily obtained, and it is inexpensive.
In many cases, an electric resistance welded steel pipe made of low and medium carbon steel of 0.10 to 0.20% in mass% was used.

[0004]

However, even if hydroforming is performed on an ERW steel pipe having such a C content, there is a problem that a sufficient expansion ratio may not be obtained due to poor workability of the material. there were. In such a situation, in order to enhance the workability of the material of the electric resistance welded steel pipe itself, it is conceivable to use a very low carbon steel having a significantly reduced carbon content as the material. However, in the case of an electric resistance welded steel pipe made of extremely low carbon steel, although the hydroforming property is good, a new problem caused by welding occurs. The problem is that the ERW steel pipe made of ultra-low carbon steel is coarsened and softened by the welding heat during the production of the steel pipe, causing the crystal grains of the heat-affected zone to coarsen
When this is hydroformed, the deformation is concentrated locally in the same part, the high ductility of the material can not be sufficiently exhibited, it is easy to break, and also when the hydroformed member is welded with other members The same softening occurs, and sufficient strength of the welded portion cannot be obtained. As a result, the strength of the used portion of the product using the same cannot be sufficiently obtained. As described above, at present, there is no steel pipe capable of obtaining a sufficient pipe expansion ratio and hardly causing the welding heat affected zone to soften.

In view of these problems of the prior art, the present invention proposes a new steel pipe suitable for hydroforming. In particular, this invention is excellent in hydroforming property,
An object of the present invention is to propose a method for producing an electric resistance welded steel pipe, which hardly causes welding softening and hardens by a baking treatment after hydroforming, so-called strain aging.
The specific target properties of the steel pipe aimed at by the method of the present invention are as follows. The hydroforming property expressed by the tensile strength (TS) of the steel pipe × expansion ratio (under the condition of axial compression) is 13000 MPa ·% or more, and 170 ° C. The amount of strain age hardening expressed as the increase in tensile strength due to × 20 minute strain aging treatment (equivalent to baking treatment)
It is assumed that the pressure is 40 MPa or more. In the present invention, there is a portion described as “steel plate”, which means that it includes a steel strip.

[0006]

Means for Solving the Problems In order to solve the above-mentioned problems, the inventors have repeated various studies on the composition of the steel pipe, the production method, and the like. As a result, the amount of C
It is extremely important to use semi-ultra low carbon steel with a range of 0.01 to less than 0.05%, to contain an appropriate amount of solute N, and to remove the distortion generated when the steel sheet is formed into a cylindrical shape by annealing after welding. Found to be effective.

The present invention has been completed on the basis of the above findings, and is expressed in terms of% by mass: C: 0.01 to less than 0.05%, Si: 0.01%
Mn: 3.0% or less, P: 0.15% or less, S: 0.015%
Hereafter, a hot-rolled or cold-rolled steel sheet containing 0.01% or less of Al, 0.005% to 0.02% of N, and 0.003% or more in a solid solution state and the balance of Fe and unavoidable impurities was formed into a cylindrical shape. After that, weld the seam and weld the steel pipe to 600-750.
This is a method for producing a steel pipe for hydroforming having strain aging, characterized by annealing at a temperature of ° C.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The reasons for limiting the steel composition in the present invention, the method of manufacturing a steel pipe, and the like will be described. C: 0.01 to less than 0.05% When the C content is increased, the strength of the steel is improved, but the formability is reduced. In particular, if the C content is 0.05% or more, the moldability is greatly reduced. On the other hand, if the content is less than 0.01%, the crystal grains of the heat affected zone at the time of steel pipe production become coarse, and when the hydroformed member is arc-welded as well, the crystal grains become coarse. This may cause a decrease in the strength of the used portion of the product used. Therefore, the amount of C is 0.01
The range is less than 0.05%.

Si: 0.01% or less Si is bonded to solute N in the steel when the steel sheet is welded and when it is reheated to a high temperature during annealing after welding according to the present invention. The amount of dissolved N is reduced, and the amount of strain age hardening is reduced. In order to exert the target strain age hardening characteristics, it is necessary to suppress the content to 0.01% or less.

Mn: 3.0% or less Mn is an element effective for improving the strength of a steel sheet and thus a hydroformed member without deteriorating the surface properties and weldability, but hardens when added over 3.0%. The expansion rate achievable during hydroforming is reduced. Therefore, the Mn content is limited to 3.0% or less.

P: 0.15% or less P is an element effective for improving the strength of steel. However, if it exceeds 0.15%, the weldability deteriorates. In particular, when the strengthening action by P is not so required, or when the C content is high and there is a concern that the weldability may be reduced, it is desirable to limit the content to 0.02% or less.

S: 0.015% or less S exists as nonmetallic inclusions in steel, and there is a possibility that the steel pipe may break (burst) during hydroforming by using this as a starting point. For this reason, the burst resistance is improved as the S content is lower, and the effect appears when the S content is 0.015% or less. In order to further improve the burst resistance, the content is preferably limited to 0.010% or less, more preferably 0.005% or less.

Al: 0.01% or less Al is necessary for deoxidizing steel and is a useful element for suppressing the coarsening of crystal grains. Therefore, the content of Al is desirably 0.005% or more. However, if it contains a large amount exceeding 0.01%,
The amount of N remaining in the solid solution state decreases, and the amount of strain age hardening decreases. For this reason, in order to sufficiently exhibit the effect of strain age hardening, the content is contained in the range of 0.01% or less.

N: 0.005 to 0.02%, and 0.003% or more as N in a solid solution state N is an element useful for strengthening steel without lowering formability (particularly ductility). Such an effect is
0.005% or more in terms of the amount (total N amount) and 0.1% in the solid solution state.
It is caused by containing 003% or more. On the other hand, 0.02
If N is contained in excess of%, cracks occur during slab production, making production difficult. Therefore, the N amount is 0.005 to 0.02
% And the solid solution state N are in the range of 0.003% or more. The amount of N in the solid solution state can be determined by a method of chemically dissolving base iron from the amount of N in the entire steel and subtracting the amount of N obtained by analyzing the extraction residue.

Next, a method for manufacturing a steel pipe according to the present invention will be described. After smelting a steel according to the above-described composition, the slab is formed by a continuous casting method or an ingot-bulking method. The slab is formed into a hot-rolled steel sheet by hot rolling or a cold-rolled steel sheet through a process of cold rolling and annealing. The hot-rolled steel sheet or the cold-rolled steel sheet obtained in this manner is used as a material, formed into a substantially cylindrical shape by roll forming, and a joint formed by abutting both width end portions is welded.
In particular, it is preferable to join by electric resistance welding. Here, it is important to secure 0.003% or more of solute N at the stage of the hot rolled steel plate or the cold rolled steel plate as the material of the steel pipe. Such a hot-rolled steel sheet starts cooling within 0.5 seconds after the completion of hot finish rolling in a hot rolling step of a steel slab according to the above-mentioned composition, and is cooled at a cooling rate of about 40 ° C./s or more at about 650 ° C.
It can be manufactured by cooling to a temperature of not more than ° C. and winding at a temperature not higher than the cooling end temperature. Further, by cold-rolling this hot-rolled steel sheet and then setting the heating temperature during annealing to 750 ° C. or lower, a cold-rolled steel sheet containing 0.003% or more of solute N can be manufactured. In the present invention, since it is extremely important to ensure a predetermined amount or more of solid solution N that contributes to strain age hardening,
It is effective to shorten the time for maintaining the steel sheet in a high temperature range (above 750 ° C.), especially in the steel plate manufacturing stage in the above process.

Next, the hot-rolled steel sheet or the cold-rolled steel sheet manufactured in the above process is formed into a cylindrical shape, and the seam portion is welded by electric resistance welding to form a steel pipe.
Annealing at a temperature of 750750 ° C. produces the ERW pipe of the present invention. Here, the steel pipe after welding is heated to 600-750 ° C.
Annealing at the temperature is to remove the strain generated during the forming into a cylindrical shape.If the annealing temperature is less than 600 ° C, the strain is not sufficiently removed, so the ERW steel pipe is left as it is for a long time. When left to stand, tensile strength increases due to strain age hardening, and ductility during hydroforming decreases. On the other hand, if the temperature exceeds 750 ° C., it becomes difficult to secure an amount of solid solution N which contributes to strain age hardening. Therefore, annealing after welding is performed in a temperature range of 600 to 750 ° C. In this annealing, shortening the annealing time within one hour is advantageous in securing solid solution N. For the above annealing, a long tens of ERW steel pipes are charged into a heating furnace in units of tens and heated to a predetermined temperature, or an inline induction heating coil is installed downstream of the ERW weld in the ERW steel pipe production line. Then, it is heated to a predetermined temperature, transported to a cooling bed, and cooled.

The characteristics of the ERW steel pipe manufactured by the above method are as follows.
Hydroforming properties with tensile strength (MPa) × expansion ratio (%) of 13000 MPa ·% or more, and the same treatment as after baking treatment (after hydroforming with expansion ratio of 10%, heat treatment at 170 ° C for 20 minutes) The difference from the tensile strength of the previous steel pipe has a strain age hardening amount of 40 MPa or more. If the tensile strength of the steel pipe is low, a high impact absorption capacity cannot be obtained, and if the pipe expansion ratio is low, the shape that can be formed by hydroforming is limited. Since it is important that these two properties are balanced, the tensile strength (MPa) × expansion rate (%) should be 13000 MPa ·% or more. After satisfying the above balance, it is desired that the tensile strength is preferably 350 MPa or more, and the breaking (burst) critical expansion ratio is preferably 10% or more, more preferably 28% or more. Here, the expansion ratio is defined as the maximum value when a steel pipe having an outer diameter do is set to a deformed part length lc = 2do, liquid is supplied from the pipe end to the inner surface of the pipe, liquid pressure is applied, a free bulge is deformed in a circular cross section, and burst occurs. From the outer diameter dmax, (dmax-do) / do
× 100 is defined. The expansion ratio is measured by a free bulge test.

This free bulge test can be carried out, for example, by expanding the dies 2a and 2b shown in FIGS. 1 and 2 using a hydroforming apparatus having the structure shown in FIG. FIG. 1 is a perspective view of a mold, and FIG. 2 is a sectional view of the mold. In the figure, 1 is a steel pipe. Each of the upper mold 2a and the lower mold 2b has a steel pipe holding portion 3 which is formed at substantially both ends in the longitudinal direction and which is formed by substantially half of a cylindrical hollow surface having a diameter substantially equal to the outer diameter do of the steel pipe. In the center part in the direction, there is a deformed part 6 consisting of a deformed part 4 constituted by substantially half of the hollow surface of the cylinder having a diameter dc and a tapered deformed part 5 having an inclination angle θ = 45 °. The length lc is twice as long as do. The diameter dc of the deformed portion 4 is twice as large as the outer diameter do of the steel pipe. As shown in FIG. 3, the steel pipe 1 is sandwiched between the upper mold 2a and the lower mold 2b such that the steel pipe 1 fits into the steel pipe holding portion 3 of each mold. In this state, a liquid such as water is supplied from both ends of the steel pipe 1 to the inner surface side of the steel pipe 1 via the shaft pushing cylinder 7a.
A hydraulic pressure P is applied to the inner surface of the steel pipe 1, and the maximum outer diameter dmax when bursting by free bulge deformation in a circular cross section is measured.
In addition, 8 and 9 in FIG. 3 are mold holders for holding the molds 2a and 2b with the steel tube sandwiched therebetween, respectively.
It is an outer ring.

In hydroforming, there are a case where the both ends of the pipe are fixed, and a case where the axial pushing cylinder 7a is pushed in a direction for compressing the steel pipe and a compressive force is applied from both ends of the pipe (referred to as axial compression). In general, it is possible to obtain a high expansion ratio by applying axial compression, and in the present invention, in order to obtain a high expansion ratio, a compressive force is appropriately applied from both ends of the pipe. In FIG. 3, the compression force can be applied by applying a compression force F to the shaft pressing cylinders 7a and 7b in the axial direction.

Further, after hydroforming at a pipe expansion ratio of 10%, a strain aging treatment of performing a heat treatment at 170 ° C. for 20 minutes is performed. Here, in the hydroforming with a pipe expansion ratio of 10%, in the mold shown in FIG. 2, the deformed portion 4 having a diameter dc of 1.1 times the outer diameter do of the steel pipe is used, and the steel pipe is transferred to the deformed portion 6 of the mold. It is carried out by performing hydroforming until it conforms. Further, the heat treatment at 170 ° C. for 20 minutes corresponds to a paint baking treatment of a molded part. Therefore, by having the above-mentioned characteristic that the tensile strength is increased by 40 MPa or more due to the strain aging treatment, the molded part is strengthened by the paint baking treatment after molding by hydroforming, so that the molded part has a high shock absorbing ability. It becomes.

[0021]

EXAMPLE A steel slab composed of various chemical components was heated to 1220 ° C. and then hot-rolled into a hot-rolled steel sheet having a thickness of 2.0 mm, or pickled and cooled following hot rolling. A cold-rolled steel sheet having a thickness of 2.0 mm by a process of cold rolling and continuous annealing is used. Here, in the hot rolling, cooling is started within 0.5 seconds after the end of the rolling, cooled to 650 ° C. or less at a cooling rate of 40 ° C./s or more, and cooled to a temperature lower than the cooling end temperature and
The film was wound at a temperature of 400 ° C. or more. For cold-rolled steel sheets, the heating temperature during annealing was set to 750 ° C or less. Table 1 shows the component composition of the obtained hot-rolled steel sheet or cold-rolled steel sheet. These hot rolled steel sheets or cold rolled steel sheets are cut to an outside diameter of 6
A 3.5 mm steel pipe was manufactured, and then this steel pipe was subjected to annealing heat treatment under the conditions shown in Table 1.

From these ERW steel pipes, tensile test pieces (No. 12B test pieces in accordance with JISZ2201) were sampled in the longitudinal direction, and the tensile strength of the material was determined. In addition, 50 ERW steel pipes
It was cut to a length of 0 mm to obtain a test piece for hydroforming. As described with reference to FIGS. 1 to 3, water was supplied from both ends of the test body, the bulge was deformed in a circular cross-section, and the tube expansion ratio when bursting was measured. Here, the dimensions of the mold are as follows: lc in FIG. 2 is 127 mm, dc is 127 mm,
rd was 7 mm, lo was 550 mm, and θ was 45 °. The characteristics of each electric resistance welded steel pipe are represented not only by the expansion rate but also by TS × expansion rate in consideration of the balance with the strength TS of the steel pipe. The ERW steel pipe was subjected to hydroforming at an expansion ratio of 10%, and then subjected to a heat treatment equivalent to a paint baking treatment at 170 ° C. for 20 minutes.
12B based on JISZ2201 from the deformed part of the steel pipe
No. test pieces were cut out and measured.

[0023]

[Table 1]

The results obtained are shown in Table 2. From Tables 1 and 2, it can be seen that the ERW steel pipe according to the present invention has a high TS × expansion ratio, excellent hydroforming properties, and a large amount of strain age hardening. That is, in the invention example, a value of 13000 MPa ·% or more is obtained as a value of material strength × expansion ratio, and a difference between TS (D) after heat treatment and TS (B) of the steel pipe is 114 MPa or more, equivalent to baking. TS (D) after heat treatment and TS after 10% hydroforming
A large amount of strain age hardening with a difference from (C) of 58 MPa or more is obtained. On the other hand, the comparative example has a disadvantage that either the hydroforming property is inferior or the amount of strain age hardening is small, and the performance of the hydroforming member as a structural member is lacking.

[0025]

[Table 2]

[0026]

As described above, according to the present invention,
It becomes possible to provide an electric resistance welded steel pipe which is excellent in hydroforming property and has a large amount of strain age hardening.
Therefore, the present invention greatly contributes to high quality and stable production of a structural member manufactured by performing a paint baking process after hydroforming.

[Brief description of the drawings]

FIG. 1 is a perspective view showing a mold used for a free bulge test.

FIG. 2 is a sectional view showing a mold used for a free bulge test.

FIG. 3 is a sectional view showing an example of the configuration of a hydroforming apparatus used for a free bulge test.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) // C22C 38/00 301 C22C 38/00 301A 38/06 38/06 B23K 101: 06 B23K 101: 06 ( 72) Inventor Shinjiro Kaneko 1 Kawasaki-cho, Chuo-ku, Chiba-city, Chiba Pref. In the Technical Research Institute of Kawasaki Steel Co., Ltd. F term (reference) 4K042 AA06 AA24 BA01 BA05 CA01 DA03 DC02

Claims (1)

[Claims] 1. In mass%, C: 0.01 to less than 0.05%, Si: 0.01% or less, Mn: 3.0% or less, P: 0.15% or less, S: 0.015% or less, Al: 0.01% or less, N: 0.005 After forming a hot-rolled or cold-rolled steel sheet into a cylindrical shape, containing 0.02% or more and 0.003% or more in the solid solution state, with the balance being Fe and unavoidable impurities, the seam is welded, and A method for producing a steel pipe for hydroforming with strain aging, characterized by annealing a steel pipe at a temperature of 600 to 750 ° C.