WO2002103069A1

WO2002103069A1 - Steel pipe having high formability and method for production thereof

Info

Publication number: WO2002103069A1
Application number: PCT/JP2001/005053
Authority: WO
Inventors: Takaaki Toyooka; Yoshikazu Kawabata; Akira Yorifuji; Masanori Nishimori; Motoaki Itadani; Takatoshi Okabe; Masatoshi Aratani
Original assignee: Kawasaki Steel Corporation
Priority date: 2000-01-28
Filing date: 2001-06-14
Publication date: 2002-12-27
Also published as: CA2403830C; CA2403830A1; EP1437422A4; CN1426489A; CN1234896C; EP1437422A1; BR0110441B1; BR0110441A

Abstract

A steel pipe having high formability which has an r value in the longitudinal direction of 1.2 or more, preferably 1.6 or more over the whole region of perimeter direction containing a seamed portion; and a method of producing the steel pipe which comprises heating a steel pipe prepared by the electric welding of steel hoops to an Ac1 temperature or higher and subjecting the heated pipe to a stretch reducing of a diameter reduction percentage of 30 % or more in a temperature region of 600 ° or higher and up to an Ac1 temperature, and optionally heating again the pipe during or after completion of cooling after said reducing to a temperature of 600 to 900 ° and holding it at the temperature for 1 second or longer. The steel pipe is a highly formable steel pipe wherein a portion having been molten or transformed by seam welding has a high r value in the axial direction thereof which is of the same degree as that of a portion having not been affected by welding and is excellent particularly in bending formability.

Description

Specification

High workability steel pipe and its manufacturing method

Technical field

The present invention relates to a steel pipe excellent in workability and a method for manufacturing the same. Background art

The application of ERW pipes to automotive parts is being studied for weight reduction and cost reduction. However, conventional ERW pipes did not provide sufficient workability. For example, bending is performed on undercarriage parts of automobiles. However, in conventional ERW pipes, there was a problem that the wall loss outside the bend was large, and if it was severe, the pipe was broken. In addition, even if it does not break, if the wall thickness is large, it is necessary to use a thick-walled material to satisfy the design stress.

To solve such a problem, it has been known that it is effective to improve the r value (Rankford value) in the tube axis direction as disclosed in, for example, Japanese Patent Application Laid-Open No. 55-56624. I have. However, as a method of increasing the r-value of a steel pipe, for example, as disclosed in Japanese Patent Application Laid-Open No. 6-41689, it is only known to increase the r-value of a steel strip used as a material of a steel pipe. is there. When manufacturing ERW pipes, there was a problem that the r-value of the portion that was melted or transformed by seam welding was reduced. Another problem is that it cannot be applied to steel sheets for which high r values are not obtained, such as hot-rolled steel sheets, high-strength steel sheets, low-carbon, medium-carbon, and high-carbon steel sheets.

Therefore, the present invention provides a steel pipe excellent in workability, particularly in bending workability, having a pipe axis r value as high as that of a part melted or transformed by seam welding, and a method of manufacturing the same. The purpose is to: Disclosure of the invention

The present inventors have considered that in order to solve the above-mentioned problems, it is necessary to process and heat-treat the ERW steel pipe in order to improve the r-value of the welded portion near the seam. We also studied a method of uniformly processing and heat treating the steel pipe obtained by electric resistance welding of a cold rolled steel strip having a high r value at all positions in the circumferential direction. In the course of this research, the ERW steel pipe was rolled at a temperature range of 600 ° C. or higher and Ac ₃ or lower at a diameter reduction ratio of 30% or more (hereinafter referred to as “method according to the present invention”). It was discovered that the r value in the longitudinal direction (in the direction of the pipe axis) was significantly improved to 1.2 or more at all positions in the circumferential direction including the seam portion, and further to 1.6 or more.

Furthermore, as a result of applying the method according to the present invention to ERW pipes using various steel plates as raw strips, it was found that a high r-value was obtained irrespective of the r-value of the original strip. In addition, according to the method of the present invention, the limitation of components conventionally used to obtain a high r value in a thin steel plate, It was found that it was not necessary to reduce the N and N contents and to add stabilizing elements such as Ti and Nb. Therefore, even when high-strength steel such as hot-rolled steel sheet, dual-phase steel, etc., and low-carbon steel, medium-carbon steel, and high-carbon steel, which were difficult to increase in r-value with steel strip, were used as material steel strips, An electric resistance welded steel pipe having an r value can be manufactured.

The following is a discussion by the present inventors on the reason why the steel pipe has a high r value even if the steel plate does not have a high r value.

When rolling is performed at a reduction ratio of 30% or more in the temperature range of 600 ° C or more and Ac ₃ or less, ideal <110> axis in the longitudinal direction and <111> ~ <110> axis in the radial direction are ideally parallel. Rolled texture is formed, and then recovered and recrystallized. This texture gives a high r-value. The texture obtained by rolling has an extremely large driving force because the crystal rotates due to processing strain. This texture is different from the texture created by recrystallization when a high r-value is obtained from a thin steel sheet, and is affected by the second phase and solid solution carbon. Peg. As a result, high r-values can be obtained in the steel pipe manufacturing stage even for strip steel types that were difficult to increase in the r-value in the steel plate manufacturing stage.

Also, the reason why a high r-value is not obtained even when diameter reduction rolling is performed at a low temperature is because the work hardening is large and ideal crystal rotation does not occur, or sufficient recovery and recrystallization are performed at a low temperature. This is because it does not occur. The reason why high r value cannot be obtained by recrystallization annealing after cold rolling and diameter reduction is that texture develops in cold rolling and recrystallization due to the effect of the second phase and solute carbon. This is because they do not.

In the field of manufacturing thin steel sheets, there is known a method for manufacturing high r-value steel sheets by rolling steel in a hot ferrite region. The method for producing a high r-value steel sheet is characterized in that a steel to which the amounts of C and N are reduced and to which stabilizer elements such as Ti and Nb are added is rolled at a low temperature and then recrystallized. This cold rolling is different from the hot rolling of the method according to the invention. In fact, when the ferrite zone plate rolling is performed at 600 ° C. or higher, the r-value is not only improved but rather significantly reduced. This is because, in sheet rolling in which the reduction is performed in the thickness direction, the strain direction is different from that in diameter reduction rolling of the steel pipe in which the reduction is performed in the circumferential direction, so that a texture advantageous for the r value does not develop.

As a result of further continued investigations, in the process according to the present invention, heating the electric resistance welded steel pipe before diameter reduction rolling than over且A _{C l} Temperature, partially or Overall Ru is Osutenaito transformation, seam hardened structure It was found that the difference in mechanical properties between the steel and the other parts became smaller, the wall thickness variation was significantly reduced, and the occurrence of wrinkles near the seam could be suppressed. The present invention has been made based on the above findings, and the gist is as follows.

(1) A high workability steel pipe having a longitudinal r value of 1.2 or more, more preferably 1.6 or more, in the entire circumferential direction including the seam portion.

(2) A steel pipe formed by electric resistance welding of a steel strip is shrunk in a temperature range of 600 or more and Ac ₃ or less. A method for producing a highly workable steel pipe, characterized in that a diameter reduction of 30% or more is performed.

(3) Immediately after the heating of _ACl temperature or higher, or after cooling and reheating, the steel pipe formed by ERW welding of the steel strip is reduced in diameter in the temperature range of 600 ° C or higher and Ac ₃ or lower. A method for producing a high workability steel pipe, characterized in that a diameter reduction of 30% or more is performed.

(4) The reduced diameter rolled steel pipe is subjected to a heat treatment during cooling after the rolling or after the completion of the cooling, and reheating, and holding at 600 ° C or more and 900 ° C or less for 1 second or more. The method for producing a high workability steel pipe according to (2) or (3). BRIEF DESCRIPTION OF THE FIGURES

Fig. 1 is a graph showing the relationship between the r value in the longitudinal direction and the diameter reduction ratio of the diameter-reduced rolled steel pipe.

Fig. 2 is a graph showing the relationship between the r value in the longitudinal direction of the diameter-reduced rolled steel pipe and the temperature on the rolling exit side. Figure 3 is a graph showing the relationship between the thickness deviation of the seam of the reduced-diameter rolled steel pipe and the heating temperature before reduction rolling. BEST MODE FOR CARRYING OUT THE INVENTION

The high workability steel pipe according to the present invention has a longitudinal r value of 1.2 or more in the entire circumferential direction including the seam portion. The reason for this is that when the r-value is 1.2 or more, the bendability of the steel pipe is significantly improved. Since the bending property is further improved when the r value is 1.6 or more, a highly workable steel pipe having an r value of 1.6 or more is more preferable.

The above-mentioned high workability steel pipe shall be manufactured by subjecting a steel pipe with seam welded by ERW to rolling with a diameter reduction ratio of 30% or more in a temperature range of 600 ° C or more and Ac ₃ or less. Can be. The r value is affected by the reduction ratio and temperature of the reduction rolling.

Figure 1 shows an ERW steel pipe manufactured from a steel strip having the same composition as steel A in Table 1 by a conventional method, under the condition of an outlet temperature of 730 ° C, with reduced diameter reduction and rolling. Circumferential position of 4 is a graph showing the relationship between the r value in the longitudinal direction and the diameter reduction rate at 0 °, 90 °, 180 °, and 270 °. The seam position was set to 0 ° (the same applies hereinafter). From Fig. 1, it can be seen that an r value of 1.3 or more is obtained at a diameter reduction ratio of 30% or more, regardless of the circumferential position, and an r value of 1.6 or more is obtained at a diameter reduction ratio of 50% or more. .

Figure 2 shows that ERW pipes manufactured from strip steel having the same composition as steel A in Table 1 by the usual method were subjected to diameter-reduction rolling under various conditions of the exit temperature and the diameter reduction rate of 30%. 4 is a graph showing the relationship between the r value in the longitudinal direction and the outlet temperature at circumferential positions 0 °, 90 °, 180 °, and 270 ° of the manufactured steel pipe. Fig. 2 shows that an r value of 1.2 or more can be obtained at an outlet temperature of 600 ° C or more.

Based on these experimental results, the lower limit of the diameter reduction rolling temperature was set to 600 ° C, and the lower limit of the diameter reduction rate was set to 30%. The upper limit of the diameter reduction rolling temperature is the upper limit of the temperature range in which the steel structure contains ferrite, and the Ac ₃ temperature. The r-value does not improve even if diameter reduction rolling is performed on steel with a structure that does not contain ferrite. The AC 3 temperature is determined by the chemical composition of the steel pipe, and can be determined by experiment. Its range is generally below 900 ° C. In the present invention, there is no particular limitation on the second phase (phase other than ferrite) as long as the structure contains ferrite. For example, austenite may be in the second phase. More preferably, diameter reduction rolling is performed at a temperature at which ferrite becomes a main phase (a phase having a volume ratio of 50% or more).

The gist of the present invention is to reduce the diameter of a steel pipe in a temperature range that is a ferrite phase. From the viewpoint of improving the r-value, there is no particular limitation on the history before diameter reduction rolling. For example, the heating temperature before the diameter reduction rolling may be any of a temperature at which a single phase of austenite, a temperature of two phases of austenite and ferrite, a temperature of a single phase of ferrite, and the like. Further, before the diameter reduction rolling, rolling may be performed at a temperature at which the austenite single phase or the main phase is formed. Fig. 3 shows that ERW pipes manufactured from a steel strip having the same composition as steel A in Table 1 by the usual method were subjected to various heating temperatures and reduced under the conditions of a diameter reduction rate of 30% and a rolling temperature of 700. It is a graph which shows the relationship between the heating temperature of the steel pipe manufactured by diameter rolling, and a wall thickness deviation rate. From FIG. 3, it can be seen that heating before diameter reduction rolling is preferably equal to or higher than the Ac i temperature in order to suppress uneven thickness and wrinkling near the seam. The Ac i temperature is a temperature determined by the chemical composition of the steel pipe, and may be determined experimentally. Its temperature is above about 800 ° C. However, if the heating temperature is too high, there is a problem that the crystal grain size becomes too large and the surface is roughened during processing.

Cooling after heating does not need to be particularly limited. After the heating, for example, the steel may be cooled to a temperature at which the ferrite becomes a main phase and subjected to diameter reduction rolling, or may be cooled to room temperature, reheated and then subjected to diameter reduction rolling.

Further, in the present invention, it is more preferable that the steel pipe after the diameter-reduction rolling is subjected to a heat treatment for holding the steel pipe at a temperature of 600 ° C. to 900 ° C. for 1 second or more.

In the present invention, 600 ^e C than small work hardening since the contraction径圧rolling in, sufficient workability can be obtained as it is. By performing heat treatment at a certain temperature for a certain time after the diameter reduction rolling, elongation and r-value are further improved. This effect is manifested by holding at 600 ° C or higher for 1 second or longer. However, when the holding temperature exceeds 900 ° C, the structure transforms to an austenitic single phase, the texture becomes random, and the r-value decreases. Therefore, the heat treatment is preferably performed under the conditions of a holding temperature of 600 ° C. or more and 900 or less and a holding time of 1 second or more. The heat treatment may be performed during cooling after the diameter reduction rolling, or may be performed by reheating the steel pipe after the completion of the cooling. Example

A hot-rolled steel sheet having the chemical composition shown in Table 1 was formed into an ERW steel pipe by a normal method, and was subjected to diameter reduction rolling under the conditions shown in Table 2. Heating before diameter reduction rolling reaches the temperature shown in Table 2, Performed without holding or for 1-600 seconds. The circumferential position of the obtained steel pipe is 0 °, 90 °, 180 °. The JIS 12 No. A tensile test specimen was sampled from 270 °, subjected to a tensile test nominal strain 6% to 7% Paste strain gauges gauge length 2 mm, width direction to the true strain _{£ L} in the longitudinal Direction The true distortion _{E w} of the sample was measured, and the r value was calculated from the slope P. Ie

P — ε L / ε w

r value = ρ / (-1-ρ).

In addition, the thickness ts of the seam part and the average thickness tb of other parts were measured, and the wall thickness deviation η was calculated. Ie

Uneven thickness ratio J?% = (Ts- tb) / tb X 100%

In addition, a 50x magnified image near the seam in a cross section perpendicular to the steel pipe axis was observed to determine whether wrinkles had occurred.

Table 3 shows the results together with the tensile strength (TS) and elongation (E1).

In the example of the present invention, the r value reached 1.2 or more at any circumferential position, whereas in the comparative example, the r value was less than 1.2. Further, those having a heating temperature of Ac 1 or higher have a small wall thickness unevenness and do not have wrinkles. Industrial applicability

ADVANTAGE OF THE INVENTION According to this invention, the r value of the whole circumferential direction area | region including the seam part of a steel pipe is high, and the high workability steel pipe with favorable shape can be provided. Bending of steel pipes ゃ The limits of pipe expansion are greatly improved, and the process can be omitted and the weight can be reduced by integral molding. Furthermore, it was difficult to increase the r-value by the conventional manufacturing method of simply welding electric steel sheets to high-strength steels such as hot-rolled steel sheets and dual-phase steels, as well as low-carbon steel, medium-carbon steel, and high-carbon steel. High r-values can be obtained even with ERW steel pipes used as the material. Expanding the scope of bending of steel pipes is important for industrial development.

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* Effective diameter reduction: 600% or more and Ac ₃ or less *** Cooling and rolling after re-ripening (otherwise rolling immediately after I && heat) Table 3

Thickness = 1.6mm

Claims

The scope of the claims

1. A high workability steel pipe with a longitudinal r value of 1.2 or more in the entire circumferential direction including the seam.

2. A method for producing highly workable steel pipes in which a steel pipe formed by electrode welding a steel strip is rolled with a diameter reduction ratio of 30% or more in a temperature range of 600 or more and Ac ₃ or less.

3. In claim 2, the steel pipe formed by electric resistance welding of the steel strip is heated immediately after the heating of AC ₁ temperature or more, or cooled and reheated, and shrunk in the temperature range of 600 or more and Ac ₃ or less. A method for manufacturing high-workability steel pipes with diameter reduction of 30% or more.

4. The reduced diameter rolled steel pipe is subjected to a heat treatment during cooling after the rolling or after the completion of the cooling, and is reheated and held at 600 ° C or more and 900 ° C or less for 1 second or more. 4. The method for producing a high workability steel pipe according to claim 2 or 3.