JP2000219933A - High strength steel tube excellent in liquid pressure formability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength steel tube exhibiting excellent formability in the case of being used for liq. pressure forming. SOLUTION: This steel tube comprises, by mass, 0.05 to 0.3% C, 0.5 to 3.0% Si, 0.5 to 2.5% Mn, and the balance Fe with inevitable impurities, in which the product between tensile strength and elongation by tubular test pieces is >=15,000 [MPa.%], additionally having a retained austenitic phase of >=2.5% by volume, and also, the concn. of C in the retained austenite phase is >=5 times the average value in the whole. Furthermore, the tube contains one or >=two kinds among Cr, Ni, Mo, Cu, Ti and Nb by <=3% in total by mass.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength steel pipe which exhibits excellent formability when used in hydroforming.

[0002]

2. Description of the Related Art After holding a metal tube in a predetermined mold, the inside of the tube is pushed in by filling a liquid therein and controlling the pressure of the liquid to a required value or in addition thereto. A processing method of bulging into a desired shape by performing the process is known as hydraulic bulging or hydroforming.

Conventionally, this processing method has been used exclusively for the production of butt-welded joints for piping and connection members for bicycle frames. Recently, however, a shape having a closed cross section by combining a plurality of parts obtained by bending a plate material or the like has been used. Application to automobile parts has been studied, mainly focusing on attempts to replace parts and the like made from tubing by the processing method.

As a result, in the case of welding joints and bicycle frame members, it is common to use the same material pipe as the pipe to be connected, so that the material of the bare pipe, which has hardly been studied, may be a problem. It has become

[0005] By the way, a typical element required for automobile parts is light weight and high rigidity. To meet this requirement, the material for the parts, that is, the raw pipe for processing, is a thin-walled pipe made of high-strength steel. desirable. However, in general, since high strength of steel involves deterioration of ductility, using high strength steel
It often leads to deterioration of the bulging formability due to the liquid pressure of the tube.
Therefore, in order to obtain a steel pipe excellent in swelling formability by hydraulic pressure even among steel pipes of the same strength, it is advantageous to produce a steel pipe using steel excellent in ductility.

One of such high strength and high ductility steels is
And the residual austenite phase (hereinafter γ _RTransformation induced plasticity
So-called TRIP steel that utilizes the properties (hereinafter referred to as TRIP)
Known, methods for producing steel plates and pipes have been proposed.
You.

For example, Japanese Patent Application Laid-Open No. 61-157625 discloses a manufacturing method for obtaining a TRIP thin steel sheet. JP-A-6-88129 describes a method of manufacturing a high-strength TRIP steel pipe with improved bending characteristics. On the other hand, Japanese Patent Application Laid-Open No. Hei 10-88278 discloses an ERW pipe excellent in hydraulic bulge formability by forming a two-layer composite of a ferrite phase and a bainite phase as a similar example, but not a TRIP steel pipe. The method is disclosed in
No. 58163 describes a method for producing a steel pipe having excellent wear resistance by forming a composite structure of a ferrite phase and a martensite phase.

[0008]

SUMMARY OF THE INVENTION The present inventors have considered that TRIP steel pipes are suitable for hydroforming and have considered chemical components and "product of strength and elongation" (hereinafter referred to as strength-ductility balance). , and "γ volume percentage of _R" (hereinafter, V _g) was hydroforming test using a plurality of TRIP steel pipe of different. As a result, in addition to the importance of having a high strength-ductility balance, it has been found that even when V _g is almost the same, the hydraulic formability may be significantly different. This means
"The more V _g increases expression of TRIP phenomenon, whereby a greater ductility is obtained resulting in high moldability" liquid metrics alone tubes TRIP steel which has been used in molding the field of conventional steel sheets and It seems that the press formability means that it cannot be measured. That is, in the case of hydraulic forming of a pipe, not only the magnitude of V _g but also the characteristics of γ _R itself are considered to be involved in the formability. No examples have been found for high-strength steel pipes.

Japanese Patent Application Laid-Open No. 61-157625 discloses that γ _R
No studies have been made on the properties of the steel pipe, and there is no description regarding use as a steel pipe. Invention of JP-A-6-88129 Patent Publication, although those proposed to secure a predetermined amount or more of V _g in order to improve the bending properties by reducing the residual stress, examination of the characteristics of gamma _R Is not done at all. Also in JP-A-10-88278 and JP 6-158163 not descriptions made about the gamma _R. There is no example referring to the strength-ductility balance in a tubular specimen.

Accordingly, an object of the present invention is to provide a high-strength steel pipe which exhibits excellent formability when used in hydroforming.

[0011]

The present inventors have SUMMARY OF THE INVENTION can review and strength ductility balance in the tube state, gamma _R having any characteristics whether it is effective for hydroformed property detail required strength clarified ductility balance amount, leading to further added V _g and gamma _R characteristics finds to exhibit excellent hydroformed properties if they are appropriately combined.

That is, according to the present invention, (1) in terms of mass percentage, C: 0.05 to 0.3%, Si: 0.5 to 3.0%
%, Mn: 0.5 to 2.5%, the balance being Fe and inevitable impurities, the product of tensile strength and elongation of the tubular test piece being 15000 [MPa ·%] or more, and the volume A high-strength steel pipe excellent in hydraulic formability, having a retained austenite phase of 2.5% or more in percentage and having a C concentration in the retained austenite phase of 5 times or more of the overall average value.

And (2) In addition to the above, Cr, N
A high-strength steel pipe excellent in hydraulic formability, comprising one or more of i, Mo, Cu, Ti, and Nb in a total of 3% or less in terms of mass percentage. It is the gist.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An experiment which led to the discovery of the present invention will be described. First, C: 0.1 in mass percentage (hereinafter,%).
15%, Si: 1.95%, Mn: 1.70%,
A steel having a balance of Fe and inevitable impurities was formed into a 1.6 mm cold-rolled steel sheet based on a conventional method. After maintaining this at 830 ° C. for 60 seconds, it is cooled down to 700 ° C. at 10 ° C./sec.
Rapidly cool to 00 ° C at 65 ° C / sec.
The temperature was maintained for 秒 間 seconds to 1000 seconds, and then air-cooled. Then cut to a predetermined size, the introduction of strain to produce a large number of different short steel pipe V _g and gamma _R as the only uniform possible in the circumferential direction. The finished dimension of the steel pipe was 60.5φ × 300 mm, and the seam was welded by laser. A plurality of steel tubes heat-treated under the same conditions were prepared, and a tubular test piece (JIS
Tensile strength and elongation measurements of 11 test piece No.) Determination of V _g, gamma investigation of characteristics of _R, and was subjected to evaluation of formability.

The hydraulic forming test was performed using an apparatus schematically showing a sectional view of the main part in FIG. The mold shape is a T-shaped joint consisting of an upper mold 1 and a lower mold 4, and a steel pipe (showing a cross section 2 of the steel pipe before forming) is set so that the welding line forms 180 ° with the bulging direction. The pushing cylinder 5 was pushed in to start molding. Shaft pushing amount is 1 on both sides
0 mm, the internal pressure is increased in proportion to the amount of shaft pushing,
The steel pipe was formed so that the cross section 3 of the formed steel pipe was formed. The final load internal pressure was increased stepwise at regular intervals until the pipe burst.

At a pressure one step lower than the burst internal pressure
Maximum perimeter L of the bulge of the formed tube _maxAnd ask for it
Perimeter L before molding₀Divided by (L_max/ L₀) With each
The formability of each tube was evaluated.

The strength-ductility balance is 19500-2600
0 MPa ·%, and no correlation was observed with L _max / L ₀ . On the other hand, (L _max / L ₀₎ is generally increases in proportion to V _g, be substantially the same tube is V _g L _max / L ₀
Can vary widely, up to a factor of about two.

The present inventors have investigated in detail why such a difference occurs. As a result, it has been found that the magnitude of the C concentration (hereinafter, C _g ) concentrated in γ _R has an extremely strong correlation with the magnitude of L _max / L ₀ . As a result of further intensive research, it was found that the strength-ductility balance in a tubular state was 1500.
0 MPa ·% or more, and C _g is the average C of the entire steel.
When the concentration is 5 times or more of the concentration (hereinafter C ₀ ) and V _g is 2.5 or more by volume percentage (hereinafter, vol%), the TRIP steel pipe has excellent hydraulic formability. And completed the present invention.

The reasons for limiting the present invention will be described below.

First, as for the chemical composition of the steel material, C is an essential element for allowing γ _R to exist stably at room temperature. γ _R
Is set to 2.5 vol% or more, 0.05% or more is required, so this is set as the lower limit. On the other hand, if it exceeds 0.3%, the weldability deteriorates, so this is made the upper limit.

[0021] Si is effective as a deoxidizing element and also contributes to strengthening of steel. However, if the content exceeds 3.0%, rolling becomes difficult, so this is made the upper limit. On the other hand, this element has an effect of promoting the enrichment of C in an austenite phase (hereinafter, γ). The effect becomes significant at 0.5% or more, so 0.5% is made the lower limit.

[0022] Mn is an element necessary to ensure high strength and γ _R. However, if the content is less than 0.5%, a sufficient effect cannot be obtained. On the other hand, if the content exceeds 2.5%, no improvement in the material is observed, and it may cause welding defects.

Among the other optional elements, Cr, Ni,
And Mo are effective elements for securing V _g because they contribute to stabilization of γ, and are therefore 0.06% and 0.06%, respectively.
It is preferable to add 8% and 0.13% or more.
Since Cu can be used for adjusting the strength in addition to assisting the concentration of C into γ, it is preferable to add 0.07% or more. Since Ti and Nb are effective for adjusting the strength through the formation of carbide, it is preferable to add 0.01% or more of both. However, the addition of these elements not only increases the production cost, but excessive addition leads to a decrease in ductility, so the total upper limit is set to 3%.

As described in the embodiment, C _g / C ₀ is 5
If it is less than 5, excellent hydraulic formability cannot be obtained, so the lower limit of the C concentration in the residual γ is set to 5 times the overall average. Further, the strength-ductility balance by the tubular test piece is less than 15,000, or V
_{When the g} is less than 2.5%, even if C _g / C ₀ is 5 or more, excellent hydraulic formability cannot be obtained, so each value is set as the lower limit.

On the other hand, as the strength-ductility balance is larger, molding becomes easier, so the upper limit is not particularly defined.

V _g and C _g are not independently controllable quantities but are affected by the other. Excessively increasing either one leads to lowering the other, which is not desirable.
However, the upper limit depends on the chemical composition of the steel and the heat treatment conditions, and is not particularly limited since the manufacturer may set the upper limit in accordance with his or her own steel making ability or heat treatment ability.

The conditions for producing the steel pipe according to the present invention may be determined according to a general method for producing TRIP steel, and are not particularly limited. Hot rolling may be performed either directly after continuous casting or after cooling and reheating. The finish temperature of the finish rolling (hereinafter, FT) is desirably set to _three or more Ar points if possible in order to prevent the steel sheet surface layer from being subjected to shear deformation. When a steel pipe is manufactured from a hot-rolled material, the temperature history from the end of rolling to the winding is appropriately controlled so that _Vg and _Cg become predetermined amounts.

When a steel pipe is manufactured using a cold-rolled steel sheet, cold rolling is performed after pickling. The cold rolling reduction is not particularly limited as long as it is set in consideration of the capacity and workability of the equipment. Desirably, it is 50 to 90%.

The cold rolled sheet is heat-treated to control V _g and C _g .
First heated and held at least Ac ₁ point and then hold mixture was cooled to below a suitable temperature Ms point. Many combinations of the temperature, the holding time, and the heating / cooling speed in this process are conceivable, and the most efficient one may be selected from these in consideration of the capacity of the manufacturing equipment.

After cutting the steel pipe blank into a predetermined size, the steel pipe is manufactured by seam welding. At this time, it is desirable to form the tube so as not to give local deformation so as not to impair the subsequent formability as much as possible. Seam welding may be performed by any method.

On the other hand, after the steel pipe is manufactured from the original sheet, heat treatment is performed on the entire pipe, and the structure may be TRIP steel. Further, a method in which a steel pipe is manufactured by seamless rolling and then heat-treated is also possible.

The reason why C _g affects the hydraulic formability is not always clear. In general, Ms point C _g and a high gamma _R is known to increase thermodynamic stability of reduced gamma _R. This means that TRIP in γ _R having a high C _g is easily expressed in a high strain region. The hydroforming of the tube is assumed that indicates the moldability of the steel pipe is excellent with a gamma _R of high C _g illustrating the TRIP at high strain range since distortion of a wide range with respect to the substrate tube are loaded.

[0033]

EXAMPLES Examples of the present invention will be described together with comparative examples.

Example 1 C: 0.18 by mass percentage
%, Si: 1.98%, Mn: 1.82%, P: 0.0
A slab containing 1%, S: 0.002%, and Al: 0.030% was heated and hot-rolled to 2.6 mm. that time,
The FT, the winding temperature (hereinafter, CT), and the cooling rate between them were performed in plural combinations. After pickling these steel plates, a 60.5 mmφ steel pipe was produced. The seam method was TIG welding. The tensile strength and elongation were measured with a JIS No. 11 tensile test piece to determine the strength-ductility balance of these steel pipes, and the formability was evaluated by the same method as described above. V _g and C _g were also measured. Cut out a portion from the non-welded portion of the steel pipe, the sample subjected to mechanical cutting and chemical polishing, a ferrite phase with an X-ray (K _alpha line Mo) (hereinafter, alpha) of (200) plane and (211) plane,
The diffraction intensity (integral value) of the (220) plane and the (311) plane of γ _R was measured, and V _g (vol%) was obtained by averaging the values obtained from the following four equations.

I _{(220) γ} / ₍ 1.35 × I _{(200) α} + I _{(220) γ} ) I _{(220) γ} / ₍ 0.70 × I _{(211) α} + I _{(220) γ} ) I _{(311 ) γ} / ₍ 1.50 × I _{(200) α} + I _{(311) γ} ) I _{(311) γ} / ₍ 0.78 × I _{(211) α} + I _{(311) γ} ) where I _{(200) α} , I _{(211) α} , I _{(220) γ} , and I
_{(311) γ} is the (200) plane of α and (21) of α, respectively.
1) plane, a diffraction intensity of gamma (220) plane of _R, and gamma of the _R (311) plane.

The C _g was determined by precisely measuring the lattice constant a _γ of γ _R using the K _α ray of Cu, and substituting it into the following equation.

That is, C _g (%) = (a _γ -3.572) /0.033. Units where a _gamma are Å (Angstroms).

Table 1 shows the strength-ductility balance, V _g , C _g , and C _g.
/ C ₀ and L _max / L ₀ which is a formability index.

No. 1, 4, 5, 7, and 10 are outside the scope of the present invention. In particular, no. Reference numeral 10 denotes a tube which was reheated to 500 ° C. after pipe formation to decompose γ _R and almost eliminate V _g . Thus, if the steel pipe is within the scope of the present invention, _Lmax /
L ₀ was found to have 1.5 or more high moldability.

[0040]

[Table 1]

Example 2 A steel slab having the main chemical components shown in Table 2 was processed according to a conventional method to obtain a 1.6 mm cold-rolled steel sheet. These were processed to produce a 60.5φ steel pipe. Laser was used for seam welding. The obtained steel pipe was heat-treated using a bright annealing furnace, and TRIP steel pipes having different strength-ductility balance, V _g and C _g were separately formed. Thereafter, the steel pipe was cut into a length of 300 mm, and a metal coating was uniformly applied to the outer surface of the steel pipe in order to remove the influence of the scale generated during the heat treatment, and was subjected to a forming test.

The molding test was performed using an apparatus schematically showing a sectional view of the main part in FIG. Mold shape is upper mold 1
And a lower mold 4 (simple (peripheral method)), and the axial pushing amount of the shaft pushing cylinder 5 is 15 on both sides.
mm. The other molding conditions and the measurement of V _g and C _g are the same as in Example 1.

FIG. 3 shows the results of the molding test using V _g and C _g / C ₀ as coordinate axes. The solid circles (）) indicate that the strength-ductility balance is 15000 MPa ·% or more.
Represents that it is less than 15000 MPa ·%.
The numerical value given to each symbol together with the steel name is L _max / L ₀ . Thus, it became clear that the steel pipe within the range of the present invention has high formability with L _max / L ₀ of 2.0 or more.

[0044]

[Table 2]

[0045]

According to the present invention, T having excellent hydraulic formability is obtained.
A RIP steel high strength steel pipe can be obtained.

[Brief description of the drawings]

FIG. 1 is a schematic view showing a main part of a hydraulic forming apparatus used for a forming test of a T-shaped joint.

FIG. 2 is a schematic view showing a main part of a hydraulic forming apparatus used in a simple expansion type forming test.

FIG. 3 is a graph showing a formability index L _max / L ₀ as V _g and C _g / C ₀ as a coordinate axis.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Upper die 2 Section of steel pipe before forming 3 Section of steel pipe after forming 4 Lower die 5 Cylinder for axial pushing

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Nobuhiro Fujita 20-1 Shintomi, Futtsu City Nippon Steel Corporation Technology Development Division (72) Inventor Tatsuo Yokoi 20-1 Shintomi, Futtsu City Nippon Steel Corporation Technology Development Division (72) Koji Kishida, Inventor 20-1 Shintomi, Futtsu City Nippon Steel Corporation Technology Development Division

Claims (2)

[Claims] 1. C: 0.05 to 0.3 in mass percentage
%, Si: 0.5 to 3.0%, Mn: 0.5 to 2.5%
And the balance is composed of Fe and inevitable impurities, and the product of tensile strength and elongation by the tubular test piece is 15000.
[MPa ·%] or more, and 2.5% by volume
%. A high-strength steel pipe excellent in hydraulic formability, having a retained austenite phase of at least 5% and a C concentration in the retained austenite phase being at least 5 times the overall average value. 2. The hydraulic formability according to claim 1, wherein one or more of Cr, Ni, Mo, Cu, Ti, and Nb are contained in a total mass percentage of 3% or less. Excellent high strength steel pipe.