JP2002273527A - Method of manufacturing steel pipe for hydroforming having strain aging property - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of manufacturing a steel pipe which has excellent hydroformability and is cured by coating and baking treatment after hydroforming.. SOLUTION: A hot rolled or cold rolled steel sheet containing, by mass %, 0.01 to <0.05% C, <=1.0% Si, <=3.0% Mn, <=0.15% P. <=0.015% S, <=0.04% Al, 0.005 to 0.02% N and <=0.003% N in a solid solution state and consisting of the balance Fe and inevitable impurities is formed to a cylindrical shape and is welded in such a manner that the temperature of the areas exclusive of a joint is maintained at <100 deg.C. The formed pipe is cooled at a rate of >=5 deg.C/s down to ordinary temperature after welding.

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 Used as structural members for automobiles,
In order to manufacture hollow members with various cross-sectional shapes, a method has been adopted in which parts formed by pressing a steel plate are spot-welded to each other at the flange portion, which is a welding margin, but quality has been conventionally used. Therefore, improvement was required from both aspects of production efficiency. In addition, a higher impact absorbing ability at the time of collision is required for the hollow member for this structure, and higher strength is required. 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 formed member. 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 that the hydroforming property expressed by the tensile strength (TS) × expansion rate (under the condition of axial compression) of the steel pipe is 13000 MPa ·% or more, and the baking treatment is performed. The difference between the later tensile strength and the tensile strength of the steel pipe has a strain age hardening amount of 40 MPa or more.
In addition, in this invention, although there is a part described as a steel plate, this means including 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
Use semi-ultra low carbon steel with a range of 0.01 to less than 0.05%, contain an appropriate amount of solute N, and keep the temperature of parts other than this part below 100 ° C when electric resistance welding the joint It has been found that it is extremely effective to maintain the temperature as described above and to cool to room temperature after welding.

The present invention has been completed on the basis of the above findings. In terms of mass%, C: 0.01 to less than 0.05%, Si: 1.0% or less, Mn: 3.0% or less, P: 0.15% or less, S: 0.015% % Or less, Al: 0.04% or less, N: 0.005 to 0.02%, and 0.003% or more in solid solution state, with the balance being Fe and unavoidable impurities, hot-rolled or cold-rolled steel sheet formed into a cylindrical shape After the welding, welding is performed so that the temperature of the parts other than the joints is maintained at less than 100 ° C., and cooling from the welding to room temperature at a rate of 5 ° C./s or more is performed. This is a method for manufacturing a forming steel pipe.

[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. Here,% of the steel component means mass% unless otherwise specified. 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: 1.0% or less Si is an element useful for improving the strength of steel, and is added according to desired strength. However, if added over 1.0%,
The surface properties of the hot-rolled steel sheet or cold-rolled steel sheet used as the material for the steel pipe deteriorates, and eventually the surface properties of the steel pipe formed from the material deteriorates. Cracks develop and the steel pipe breaks
(Burst). For this reason, the Si content is set to a range of 1.0% 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%. Too much, and the expansion ratio 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: 0.04% 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 0.04%,
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, it is desirable that the content be contained in the range of 0.04% or less, preferably 0.02% 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 subtracting the amount of N obtained by chemically dissolving the base iron from the amount of N in the entire steel and 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.
It is particularly preferable to perform joining 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, it is extremely important to secure a predetermined amount or more of solid solution N contributing to strain age hardening, and therefore, in the above process, especially in the steel plate manufacturing stage,
It is effective to shorten the time of holding in the high temperature range (over 750 ° C).

Similarly, in the process of ERW welding and the manufacturing process of ERW steel pipes that are adjacent thereto, it is desirable to avoid unnecessary heating and to suppress the temperature rise and high temperature holding of the material steel plate and the steel pipe after welding. From this point of view, it is necessary to perform ERW welding so that the temperature of the portion excluding the seam portion is maintained at less than 100 ° C., and to cool at a rate of 5 ° C./s or more from ERW welding to normal temperature. . If both of these conditions are not satisfied, the strain received when the steel pipe is rolled into a tube will cause strain age hardening and increase the strength, and the ductility will decrease during hydroforming, making it easier to break (burst), It becomes impossible to secure solid solution N that contributes to strain aging after hydroforming. Here, the portion excluding the seam portion is a portion that is separated from the center of the seam portion by 10 degrees or more in the left and right directions in the pipe circumferential direction. Further, the cooling rate after the electric resistance welding refers to a cooling rate determined by measuring 10 seconds after welding. The ERW steel pipe is conveyed in the longitudinal direction while rolling the steel sheet, and the width ends of the steel sheet are abutted to each other and welding is advanced. Therefore, the welded portion is gradually sent downstream in the conveying direction. Therefore, the temperature may be measured.

As a specific method for satisfying the conditions at the time of manufacturing such an electric resistance welded steel pipe, for example, a high frequency welding machine having a frequency of 50 kHz or more is used, and a magnetic material such as a ferrite core or silicon steel is disposed on the inner surface of the pipe at a welding point. In addition, the welding speed is increased to 20
If the electric resistance welding is performed at m / min or more, the temperature of the material at the site excluding the seam can be suppressed to less than 100 ° C. Further, if the entire steel pipe is immersed in a cooling box (length 5 to 10 m) storing cooling water maintained at a normal temperature or lower by a cooling tower immediately after the electric resistance welding and cooled, the temperature is 5 ° C./s or more. Cooling rate can be secured.

The characteristics of the ERW steel pipe manufactured by the above method are as follows.
The same as the hydroforming properties with tensile strength (MPa) x expansion ratio (%) of 13000 MPa ·% or more, and the same as the tensile strength 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 steel pipe before the treatment 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 be balanced, the tensile strength (MPa) × expansion ratio (%) 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 by 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 the hydroforming, there are a case where both ends of the pipe are fixed, and a case where the axial pressing cylinder 7a is pushed in a direction of compressing the steel pipe 1 and a compressive force is applied from both ends of the pipe (referred to as axial compression). However, in general, a higher expansion ratio can be obtained 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. The load of this compression force is shown in FIG.
In the above, it can be carried out by applying a compressive force F to the shaft pushing cylinders 7a and 7b in the axial direction.

Further, after hydroforming with an 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.

[0022]

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 hot rolling, cooling is started within 0.5 seconds after the end of rolling, cooled to 650 ° C or lower at a cooling rate of 40 ° C / s or higher, and cooled to a temperature of 400 ° C or higher at a temperature lower than the cooling end temperature. Wound at temperature. 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 provided with a ferrite core magnetic material on the inner surface of the pipe at a welding point by a high-frequency welding machine having a frequency of 350 kHz, and are placed at a position 2 m downstream from the electric resistance welding position. Using a facility equipped with a 10 m cooling box (cooling water is kept at 35 ° C or lower by passing through a cooling tower), an ERW steel pipe with an outer diameter of 63.5 mm is manufactured at a welding speed of 80 m / min. did. At this time, the temperature (maximum temperature) at the part excluding the seam and
Table 1 also shows the cooling rate after welding at the position m downstream (after 10 seconds). In Table 1, an example in which the temperature other than the seam portion is out of the range of the invention is a high-frequency welding machine having a frequency of 20 kHz, manufactured without disposing a ferrite core magnetic material on the inner surface of a welding point pipe, and after welding. An example in which the cooling rate is out of the range of the invention is a comparative example manufactured using a high-frequency welding machine having a frequency of 20 kHz without using a cooling box.

From the obtained ERW steel pipe, a tensile test piece (No. 12B test piece according to JISZ2201) was 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 die dimensions in FIG. 2 are 127 mm for lc, 127 mm for dc, and rd in FIG.
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. Further, the ERW steel pipe is subjected to hydroforming at an expansion ratio of 10%, and then subjected to a heat treatment equivalent to a coating baking treatment at 170 ° C. for 20 minutes. A No. 12B test piece was cut out according to and measured.

[0025]

[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.

[0027]

[Table 2]

[0028]

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 (72) Inventor Shinjiro Kaneko 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Inside the Technical Research Institute of Kawasaki Steel (72) Inventor Akio Tosaka 1-Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Kawasaki Steel Corporation Chiba Works

Claims (1)

[Claims] 1. In mass% C: 0.01 to less than 0.05%, Si: 1.0% or less, Mn: 3.0% or less, P: 0.15% or less, S: 0.015% or less, Al: 0.04% or less, N: 0.005 or less After hot-rolled or cold-rolled steel sheet containing 0.02% and 0.003% or more in solid solution, and the balance consists of Fe and unavoidable impurities, the temperature except for the joint is 100 ° C. A method for producing a steel pipe for hydroforming with strain aging, characterized in that the pipe is welded so as to be maintained at a temperature of less than 5 ° C. and cooled from room temperature to a room temperature at a rate of 5 ° C./s or more.