JP2001303196A - Electric resistance welded steel tube for structural use, excellent in hydroformability and having strain aging characteristic, and its manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electric resistance welded steel tube excellent in formability at hydroforming and having strain aging characteristic. SOLUTION: A hot rolled or cold rolled hoop stock, which has a composition consisting of 0.01-<0.05% C, <=1.0% Si, <=3.0% Mn, <=0.15% P, <=0.015% S, <=0.04% Al, 0.005-0.02% (and >=0.003% in a state of solid solution) of N and the balance Fe with inevitable impurities and containing, if necessary, at least one kind selected from 0.005-0.040% Nb, 0.005-0.50% Ti, 0.005-0.020% B, 0.02-1.5% Cu, 0.02-1.0% Ni, 0.02-1.0% Cr, 0.02-1.0% Mo, 0.0020-0.02% Ca and 0.0020-0.02% REM, is formed into cylindrical shape and the resultant seam is subjected to electric resistance welding, followed by sizing at 0.3-10% drawing rate of outer peripheral length.

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 an electric resistance welded steel pipe for a structure.

[0002]

2. Description of the Related Art Used as structural members for automobiles,
Conventionally, to manufacture hollow members with various cross-sectional shapes,
A method has been adopted in which parts formed by pressing a steel plate are joined together by spot welding at a flange portion, which is a welding margin, but improvements have been demanded both in terms of quality and production efficiency. On the other hand, recently, structural hollow members have been required to have a higher shock absorbing ability at the time of collision, and the material has been further strengthened. 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.

[0003] As a new molding method for solving such a problem, a molding method by hydroforming has recently attracted attention. Hydroforming is a method of performing plastic working by injecting a high-pressure liquid into a steel pipe,
This is an excellent molding method that has the function of increasing the strength and rigidity while changing the cross-sectional dimensions of the steel pipe by pipe expansion and the like, while at the same time integrating the complicated shaped members. By the way, as a steel pipe provided for this hydroforming, generally,
Easy to obtain strength and inexpensive C: 0.20 to 0.10%
Among them, an electric resistance welded steel pipe made of low carbon steel was often 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 pipe expansion rate cannot be obtained due to poor workability of the material. On the other hand, in order to improve the workability of the material of the electric resistance welded steel pipe itself, it is conceivable to use ultra-low carbon steel whose carbon content is significantly reduced as the material. However, in the case of an ultra-low carbon electric resistance welded steel pipe, although the hydroforming property is good, another problem due to welding newly arises.
In other words, in ultra-low carbon ERW steel pipes, the heat of welding at the joints during the production of steel pipes causes the crystal grains in the heat-affected zone to coarsen and soften, and the deformation during pipe expansion is concentrated locally, resulting in a high material strength. The ductility cannot be sufficiently exhibited, and when the hydroformed member is welded to another member, the same softening occurs and the member cannot have sufficient strength. As described above, at present, a steel pipe that can sufficiently withstand hydroforming and does not cause softening of the weld heat affected zone has been proposed yet.

[0005] In view of the above problems of the prior art, the present invention proposes a new electric resistance welded steel pipe suitable for hydroforming. In particular, the present invention relates to an electric resistance welded steel pipe for structural use and a method for producing the same, which not only has excellent hydroforming properties but also does not cause welding softening, but also has a so-called strain aging property that is hardened by a paint baking treatment after hydroforming. The purpose is to propose. A specific target characteristic aimed at by the steel pipe of the present invention is that the hydroforming property expressed by (TS of steel pipe) × expansion rate (condition of axial compression) is 13000 MPa ·
%, And the difference between the tensile strength after the baking treatment and the tensile strength of the steel pipe has a strain age hardening amount of 40 MPa or more.

[0006]

Means for Solving the Problems In order to achieve the above object, the inventors have made various studies on the component composition of the ERW steel pipe, the production method, and the like. As a result, the amount of C is 0.01 to 0.
Use a semi-ultra-low carbon material with a range of less than 05%, contain a proper amount of solute N, form a pipe with electric resistance welding at the seam, and then draw a 0.3 to 10% sizing (diameter reduction). Was found to be extremely effective.
The present invention has been completed based on the above findings, and the gist configuration thereof is as follows.

(1) The steel composition is represented by mass% (hereinafter simply referred to as%)
C: 0.01 to less than 0.05%, Si: 1.0% or less, Mn: 3.0
%, P: 0.15% or less, S: 0.015% or less, Al: 0.04%
%, N: 0.005 to 0.02%, and 0.003% in a solid solution state
The remaining steel is an ERW steel pipe consisting of a steel composition of Fe and unavoidable impurities. Hydroforming with tensile strength (MPa) × expansion ratio (%) of 13000 MPa ·% or more and strain of 10% An electric resistance welded structural steel pipe having excellent hydroforming properties and strain aging, characterized in that the amount of increase in tensile strength by strain aging treatment in which heat treatment is performed at 170 ° C. for 20 minutes is 40 MPa or more.

(2) In the above (1), in addition to the above components, the steel composition may be one or two selected from the following groups A to C:
An electric resistance welded steel pipe for structural use having excellent hydroforming properties and strain aging characteristics, characterized by containing at least one kind. Note A group: Nb: 0.005 to 0.040%, Ti: 0.005 to 0.50%,
B: one or more of 0.0005 to 0.020% Group B: Cu: 0.02 to 1.5%, Ni: 0.02 to 1.0%, Cr: 0.02
1.0%, Mo: one or more of 0.02 to 1.0% Group C: Ca: 0.0020 to 0.02%, REM: One or two of 0.0020 to 0.02%

(3) C: 0.01 to less than 0.05%, Si: 1.0%
Mn: 3.0% or less, P: 0.15% or less, S: 0.015%
In the following, Al: 0.04% or less, N: 0.005 to 0.02%, and 0.003% or more in a solid solution state.
Group and C containing at least one selected from the components, the balance is composed of Fe and unavoidable impurities, after forming a hot-rolled or cold-rolled strip material into a cylindrical shape, the seam portion is subjected to electric resistance welding, Then, a sizing of 0.3 to 10% is performed at a drawing ratio of an outer peripheral length, which is characterized by being excellent in hydroforming property and having a strain aging property. Note A group: Nb: 0.005 to 0.040%, Ti: 0.005 to 0.50%,
B: one or more of 0.0005 to 0.020% Group B: Cu: 0.02 to 1.5%, Ni: 0.02 to 1.0%, Cr: 0.02
1.0%, Mo: one or more of 0.02 to 1.0% Group C: Ca: 0.0020 to 0.02%, REM: One or two of 0.0020 to 0.02%

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The reasons for limiting steel components in the present invention, a method for manufacturing an electric resistance welded steel pipe, and the like will be described. C: 0.01 to less than 0.05% C contributes to the strengthening of the steel, but is an element that lowers the formability. In particular, when the C content is 0.05% or more, the decrease in the formability increases. On the other hand, if the content is less than 0.01%,
Crystal grains in the heat affected zone are coarsened by resistance welding during ERW pipe manufacturing, and crystal grains are similarly coarsened when arc-welding a hydroformed member, causing strength reduction and uneven deformation. Becomes Therefore, the amount of C is 0.01 to
The range is less than 0.05%.

Si: 1.0% or less Si is an element useful for strengthening steel, and is added according to desired strength. However, if it is added in excess of 1.0%, the surface properties of the hot-rolled or cold-rolled steel sheet used as the material for steel pipes deteriorate, and eventually the surface properties of the steel pipes remarkably deteriorate. Decreases burstiness. Therefore, it is added in a range of 1.0% or less.

Mn: 3.0% or less Mn is an effective element for improving the strength of a steel sheet and thus a hydroformed member without deteriorating the surface properties and weldability. To reduce the pipe expansion rate. Therefore, the Mn content is less than 3.0%.

P: 0.15% or less P is an element effective for improving the strength, but when added in excess of 0.15%, the weldability is significantly deteriorated. 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 the steel, and there is a possibility that this may be a starting point to break the steel pipe during hydroforming. For this reason, the burst resistance is improved as the S content is lower, and the effect can be obtained if the S content is 0.015% or less. In order to further improve the burst resistance, preferably 0.010
%, 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 addition of 0.005% or more is desired. However, when a large amount is added exceeding 0.04%, the amount of N remaining in a solid solution state decreases, and the amount of strain age hardening decreases. Therefore, in order to exert strain age hardening, it is added in a 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 attained by 0.005% or more in N amount (total N amount) and 0.005% in N in solid solution state.
It must be present at 3% or more. On the other hand, if N is contained in excess of 0.02%, it is difficult to produce a sound slab with the current steelmaking technology. 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 and analyzing the extraction residue.

Nb: 0.005 to 0.040%, Ti: 0.005 to 0.50
%, B: 0.0005 to 0.020% Nb, Ti and B are all useful elements for refining crystal grains. Such effects are obtained when Nb: 0.005% or more and Ti:
Appears when 0.005% or more and B: 0.0005% or more are added.
However, the effects were as follows: Nb: 0.040%, Ti: 0.50%, B:
Addition of more than 0.020% not only saturates, but also increases the hot deformation resistance of steel and impairs manufacturability. Therefore, these elements are added in the above range.

Cu: 0.02-1.5%, Ni: 0.02-1.0%, C
r: 0.02 to 1.0%, Mo: 0.02 to 1.0% Cu, Ni, Cr and Mo are useful elements for improving the strength without impairing the ductility of the steel pipe. All of these effects can be obtained with the addition of 0.02% or more. However, the effect is saturated even if the addition exceeds 1.5% with Cu and 1.0% with Ni, Cr and Mo. Decreases workability and cold workability. Therefore, these elements are added in the above range.

Ca: 0.0020-0.02%, REM: 0.0020-0.02
% Ca and REM are elements useful for making the form of nonmetallic inclusions mainly composed of S in steel into a sphere, reducing the notch action, and improving the burst resistance. These effects are:
Both Ca and REM can be obtained by adding 0.0020% or more, but 0.02%
%, The effect tends to be saturated or slightly reduced. Therefore, these elements are added in the above range. When both Ca and REM are used together,
It is preferable to add in the range of 0.03% or less.

Further, the steel pipe of the present invention has a tensile strength (MPa) ×
The expansion ratio (%) shall be 13000 MPa ·% or more.
If the tensile strength of the steel pipe is low, a high shock absorbing capacity cannot be obtained, and if the pipe expansion rate is low, the shape that can be formed by hydroforming is limited. In the present invention,
It is necessary that these two properties be balanced, so the tensile strength (MPa) x expansion ratio (%) is calculated as 13000 MPa
% Or more. The tensile strength is preferably 350 MPa or more, and the pipe expansion ratio is preferably 28% or more. Here, the pipe expansion ratio is defined as a steel pipe having an outer diameter do with a deformed portion length lc = 2do.
A liquid is supplied from the pipe end to the inner surface of the pipe to apply a hydraulic pressure, and a free bulge is deformed in a circular cross section.
From x, it is defined as (dmax-do) / do × 100. This expansion ratio is measured by a free bulge test. This free bulge test is, for example, shown in FIGS.
Can be carried out by expanding the dies 2a and 2b shown in FIG. 1 using a hydroforming apparatus having the configuration shown in FIG.

FIG. 1 is a perspective view of a mold, and FIG. 2 is a sectional view of the mold. Each of the molds 2a and 2b has a steel pipe holding portion 3 formed of a semi-cylindrical surface having a diameter substantially equal to the outer diameter do of the steel pipe at both ends in the length direction. d
c has a deformed portion 6 comprising a semi-cylindrical deformed portion 4 and a tapered deformed portion 5 having an inclination angle θ = 45 °, and the length lc of the deformed portion 6 is twice as long as do. It comprises a mold 2a and a lower mold 2b. The diameter dc of the semi-cylindrical deformed portion 4 may be about twice 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, when 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 through the shaft pressing cylinder 7a, a hydraulic pressure P is applied, and a circular cross-section free bulge deforms and bursts. The maximum outer diameter dmax is measured. Note that FIG.
Reference numerals 8 and 9 denote a mold holder and an outer ring for holding the molds 2a and 2b with the steel tube sandwiched therebetween.

In hydroforming, there are a case where both ends of the tube are fixed and a case where a compressive force is applied from both ends of the tube (referred to as axial compression). Generally, tube end compression has a higher expansion ratio. In the present invention, a compressive force is appropriately applied from both ends of the pipe so as to obtain a high expansion rate. 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, the steel pipe of the present invention has a characteristic that the tensile strength is increased by 40 MPa or more by a strain aging treatment of performing a heat treatment at 170 ° C. for 20 minutes after hydroforming with a strain amount of 10%. Here, in the hydroforming with a distortion amount of 10%, the semi-cylindrical deformed portion 4 having a diameter dc of 1.1 times the outer diameter do of the steel pipe is used in the mold shown in FIG. This is performed by performing hydroforming until the deformation portion 6 is formed. Further, the heat treatment at 170 ° C. for 20 minutes corresponds to a paint baking treatment of a molded part. Therefore, tensile strength is 40 MPa by strain aging treatment.
By having the above-mentioned characteristic of increasing, the molded part is strengthened by the coating baking treatment after molding by the hydroform, and has high collision resistance.

Next, a method for manufacturing the steel pipe of 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 made into a hot-rolled steel sheet by hot rolling, or further into a cold-rolled steel sheet by cold rolling and annealing. Using the hot-rolled steel sheet or cold-rolled steel sheet thus obtained as a raw material, it is formed into a substantially cylindrical shape by roll forming or bending, and a seam portion in which both width end portions are butted together is subjected to electric resistance welding. Join. Here, it is necessary to secure 0.003% or more of solute N in a hot rolled steel sheet or a cold rolled steel sheet which is a material for pipe making. Such a hot-rolled steel sheet, in the hot rolling step of the steel slab according to the above component composition, 0.5 hours after the completion of hot rolling
It can be manufactured by starting cooling within seconds, cooling to about 650 ° C. or less at a cooling rate of about 40 ° C./s or more, and winding it at a temperature not more 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.

Next to the electric resistance welding, at the drawing ratio of the outer peripheral length,
Sizing 0.3 to 10%. The purpose of sizing is to obtain sufficient shape accuracy and to ensure uniformity of material deformation in order to subject the ERW steel pipe to hydroforming. In order to achieve such an object, a drawing ratio of at least 0.3% is required. However, if the drawing ratio exceeds 10%, work hardening becomes remarkable, leading to a reduction in ductility and a reduction in a pipe expansion ratio. Therefore, after the electric resistance welding, the sizing should be performed at a reduction ratio of the outer peripheral length of 0.3 to 10%. In the present invention, it is extremely important to secure a predetermined amount of solid solution N contributing to strain age hardening. Therefore, in the above process, particularly in the steel plate manufacturing stage, the steel stays in a high temperature region (750 ° C. or higher). It is effective to shorten the time. Similarly, when manufacturing a steel pipe, it is desirable to suppress unnecessary heating as much as possible in order to secure a sufficient amount of dissolved N.

[0026]

EXAMPLES Slabs were produced by melting steels having the chemical components shown in Table 1. After heating this slab to 1220 ° C, hot rolling
A hot-rolled steel sheet of 2.0 mm, or a sheet thickness of 2.0 mm by a process of pickling, cold rolling and continuous annealing following hot rolling.
mm cold-rolled steel sheet. Here, after the completion of hot rolling, 0.
The cooling was started within 5 seconds, cooled to about 650 ° C. or less at a cooling rate of about 40 ° C./s or more, and wound at a temperature of not more than the cooling end temperature and a temperature of 400 ° C. or more. For cold-rolled steel sheets, the heating temperature during annealing was set to 750 ° C or less. After forming these hot-rolled steel sheets or cold-rolled steel sheets into a cylindrical shape, their joints are subjected to electric resistance welding to form a steel pipe having a diameter of 63.5 mm and a wall thickness of 2.0 mm, and then a sizing of 1.2% in outer peripheral length drawing ratio. Was done.

From the obtained ERW pipe, a tensile test piece (JIS No. 12 test piece) was sampled in the longitudinal direction, and the tensile strength of the steel pipe material was determined. In addition, the ERW steel pipe was cut into a length of 500 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 mold dimensions are as follows: lc in FIG.
127.0 mm, dc is 127.0 mm, rd is 5 mm, and lo is
550 mm and θ were 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. Also, using a sized ERW steel pipe, a hydroforming process is performed with a strain amount of 10%, and then a heat treatment equivalent to a paint baking process is performed at 170 ° C. for 20 minutes, and the tensile strength (TS) after each process is completed.
Was measured by cutting out a JIS No. 12 tensile test piece from a deformed portion of a steel pipe.

[0028]

[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, 1300 MPa ·% or more is obtained as the value of material strength × expansion ratio, and the difference between TS (D) after heat treatment equivalent to baking treatment and TS (B) of the steel pipe is 114 MPa or more, and heat treatment equivalent to baking treatment. TS after (D) 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 examples in which the chemical components are not appropriate have the disadvantage that either the hydroforming property is inferior or the strain age hardening amount is small, and the performance of the hydroforming member as a structural member is lacking.

[0030]

[Table 2]

Further, regarding the steel pipe of pipe No. 1 in Table 1,
TS of ERW pipe, expansion test result, strain amount 10% when squeezing rate at sizing was changed between 0.1 and 12%
TS after hydroforming and T after paint baking heat treatment
Table 3 shows S and the amount of strain age hardening. From Table 3, when the squeezing rate at the time of sizing is in the range of 0.3 to 10.0%,
It can be seen that a TS × expansion ratio of 13000 MPa ·% or more can be obtained.

[0032]

[Table 3]

[0033]

As described above, according to the present invention,
It is possible to provide an electric resistance welded steel pipe for a structure that has excellent hydroforming properties 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 a configuration of a hydroforming apparatus used for a free bulge test.

Claims (3)

[Claims] The steel composition is as follows: C: 0.01 to less than 0.05% by mass, 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, the rest is an ERW steel pipe composed of steel composition of Fe and unavoidable impurities. Tensile strength (MPa) x expansion ratio (%)
Excellent in hydroforming properties, characterized in that the tensile strength increase by strain aging treatment is 40 MPa or more after heat forming at 170 ° C for 20 minutes after hydroforming with a strain amount of 10% or more and 13,000 MPa ·% or more. Structural ERW steel pipe with strain aging. 2. The steel according to claim 1, wherein the steel composition contains, in addition to the above components, one or more members selected from the following groups A to C, which are excellent in hydroforming properties and strain aging. ERW steel pipe for structural use. Note A group: Nb: 0.005 to 0.040%, Ti: 0.005 to 0.50%,
B: one or more of 0.0005 to 0.020% Group B: Cu: 0.02 to 1.5%, Ni: 0.02 to 1.0%, Cr: 0.02
1.0%, Mo: one or more of 0.02 to 1.0% Group C: Ca: 0.0020 to 0.02%, REM: One or two of 0.0020 to 0.02% 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 a solid solution state, and if necessary, at least one selected from the following components of Group A, Group B and Group C, with the balance being Fe and unavoidable impurities, hot rolling or cold rolling. After forming the strip-shaped material into a cylindrical shape, the seam is welded by electric resistance, and then sizing of 0.3 to 10% is performed at the reduction ratio of the outer peripheral length. Excellent hydroforming property and strain aging For manufacturing electric resistance welded steel pipes. Note A group: Nb: 0.005 to 0.040%, Ti: 0.005 to 0.50%,
B: One or more of 0.0005 to 0.020% Group B: Cu: 0.02 to 1.5%, Ni: 0.02 to 1.0%, Cr: 0.02
1.0%, Mo: one or more of 0.02 to 1.0% Group C: Ca: 0.0020 to 0.02%, REM: One or two of 0.0020 to 0.02%