JPH0924418A - Butt welded steel tube and its producing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a butt welded steel tube having high quality with no difference between a forge welded part and its base material by feeding a steel strip having a specified chemical constituent to a butt welding mill for a steel tube, shielding an open tube just before butt welding rolling with an inert gas and applying butt welding. SOLUTION: The chemical constituent of the steel strip 1 is composed of, by wt.%, 0.03-0.25C, 0.3-1.5 Mn, <=0.06 P, <=0.04 S, 0.003-0.3 Ti, 0.005-0.1 Al and the balance Fe with inevitable impurity. After the steel strip 1 is heated at 1300 deg.C with a heating furnace 2, it is formed into the open tube 1A with forming rolls 3 and 4. Successively, the both edge part of the steel strip are heated at 1400 deg.C with a plasma arc heating device 6 and the butt welded steel tube is produced by applying butt welding with a butt welding roll 5 while injecting N2 gas from the inert gas injection nozzle 7 on the both edge part just before butt welding rolling. In this case, the area ratio of Ti oxide existing on the butt welded surface and the flatness ratio are reduced to 0.03-3% and <=0.2 respectively, by specifying the concentration of oxygen to be <=10% during butt welding.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a forged steel pipe and a method for manufacturing the same, and more particularly to a forged steel pipe having high quality at a forged abutting portion and a preferable method for manufacturing the same.

[0002]

2. Description of the Related Art A general method for manufacturing a forged steel pipe is to heat a continuously supplied skelp to about 1200 to 1300 ° C. in a heating furnace, and to make the edge faces face each other by a forming roll provided subsequently. After forming a circular arc into an open tube, blow off high pressure air to both edge surfaces of the open tube, that is, the forged surface, to scale off the forged surface, then blow oxygen to the forged surface with a welding horn, and heat the oxidation. The forged surface is further heated to a high temperature of about 1350 to 1400 ° C, and then fed to a forged roll to apply lateral pressure to forge and complete a tubular body. The tubular body is then reduced in diameter to a predetermined dimension by a hot stretch reducer, cut into a regular size by a rotary hot saw, the outer diameter is determined by a sizer, and after being cooled, straightened by a straightener to straighten the product. For forged steel pipes, by adjusting the outer diameter drawing amount of the hot stretch reducer, it is possible to manufacture several kinds of outer diameter sizes from the skelps of the same size. In addition, it is a highly productive continuous steel pipe manufacturing method, and forged steel pipes with an outer diameter of 4 inches or less are manufactured at high speeds of 100 to 400 m / min.

[0003]

However, since the forged steel pipe manufactured by the above-described manufacturing process is forged by oxides such as iron scale being wrapped in the forged part,
The strength of the forged joint is significantly weaker than that of the base metal, and such forged steel pipe cannot be applied to high-grade steel pipes such as high-pressure pipes or oil well steel pipes, and gas pipes, water pipes, and electric pipes. It has been limited to steel pipes for applications that do not require strength such as.

It is an object of the present invention to provide a high quality forged steel pipe having no difference in material between the forged portion and the base material.

[0005]

The forged steel pipe according to claim 1 has C: 0.03 to 0.25%, Si: 0.3% or less, and Mn: 0.
3 to 1.5%, P: 0.06% or less, S: 0.04% or less, Ti:
0.003 to 0.3%, Al: 0.005% to 0.1%, and the balance Fe and unavoidable impurities.

The forged steel pipe according to claim 2 has a C: 0.03 to
0.25%, Si: 0.3% or less, Mn: 0.3 to 1.5%, P:
0.06% or less, S: 0.04% or less, Ti: 0.003 to 0.3%,
Al: 0.005% to 0.1%, the balance Fe and unavoidable impurities, and the area ratio of the Ti-based oxide present on the forged surface is 0.03 to 3%.

A method of manufacturing a forged steel pipe according to claim 3 is
C: 0.03 to 0.25%, Si: 0.3% or less, Mn: 0.3 to 1.
5%, P: 0.06% or less, S: 0.04% or less, Ti: 0.003
~ 0.3%, Al: 0.005% ~ 0.1%, the remaining steel strip consisting of Fe and unavoidable impurities is fed to the forged steel pipe mill,
The open pipe just before the forging roll is shielded with an inert gas and the forging is performed while keeping the oxygen concentration of the edge surface to be forged at 10% or less.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, the reasons for limiting the content of components of the steel strip as a raw material are as follows. C C is an element that can most easily secure the room temperature strength of the steel pipe, but if it is less than 0.03%, the desired strength cannot be secured, so it was set to 0.03 or more. On the other hand, if it exceeds 0.25%, the proper forging temperature range becomes extremely narrow and the quality of the forged portion becomes unstable, so the upper limit was made 0.25%.

Si Si is an element that inhibits forgeability, and if it exceeds 0.3%, the strength of the forged part is extremely lowered due to the interaction with Mn, so the upper limit was made 0.3% or less.

Mn Mn is an element which, like C, enhances the room temperature strength of the steel pipe.
In order to obtain the desired strength, it is necessary to set it to 0.3% or more. On the other hand, the excessive addition of Mn promotes the segregation of S in the central segregation zone of the steel strip, which deteriorates the workability of the central segregation zone, and the proper forging welding temperature range becomes extremely narrow, resulting in the quality of the forged portion. Is not stable, so its upper limit is 1.5
%.

P P is an element that inhibits the forgeability, and if it exceeds 0.06%, the strength of the forged part decreases, so its upper limit was made 0.06%.

S S is an element that inhibits the forgeability, and if the content exceeds 0.04%, the forged part deteriorates, so the upper limit was made 0.04%.

Al Al is a deoxidizing element, but if it is less than 0.005%, the deoxidation of the steel is not sufficient, so the content is made 0.005% or more. On the other hand, 0.
If it exceeds 1%, Al ₂ O ₃ will increase in the steel and the quality will deteriorate, so the upper limit was made 0.1%.

Ti Ti is an element capable of refining the material crystal grains by using fine Ti oxides existing in the steel as precipitation nuclei to enhance the strength and toughness of the material. By adding an appropriate amount of Ti to the forged steel pipe material, a fine Ti oxide having a size of 1 μm or less is generated in the forged portion during forging, the structure of the forged portion is refined, and the seam strength is improved. Figure 1 shows the relationship between the Ti content and the seam 90 ° flatness test results.
In the range of 0.3%, the flatness ratio (ratio H / D between flat height H when seam cracks occur and outer diameter D of the mother pipe before flat test) is 0.2.
The following shows excellent forging quality. Fig. 2 shows the relationship between the area ratio of the Ti-based oxide existing on the forged surface and the results of the 90 ° flatness test at the seam portion.
In the range of 3% to 3%, the flatness ratio becomes 0.2 or less and excellent forge welding quality is exhibited. That is, when the amount of Ti is less than 0.003% or the area ratio of the Ti-based oxide is less than 0.03%, the number of Ti-based oxides generated in the forged portion is small and the strength of the forged portion does not improve. Decided. On the other hand, when the Ti content exceeds 0.3% or the area ratio of the Ti-based oxide is 3%.
When the value exceeds the above range, the number of Ti-based oxides formed in the forged-welded portion becomes too large and the Ti-based oxide becomes harmful in obtaining the strength of the forged-welded portion, so the upper limit was determined.

Next, the reasons for limiting the pipe making conditions are as follows. Fig. 3 shows the relationship between the oxygen concentration on the edge surface during forging welding and the 90 ° flatness test result of the seam portion. When the oxygen concentration is 10% or less, the flatness ratio is 0.2 or less, indicating excellent forging welding quality.
That is, the open pipe just before the forging roll is shielded with an inert gas and the forging is performed while keeping the oxygen concentration of the edge surface to be forged at 10% or less, when the oxygen concentration exceeds 10%, it is harmful to the forged portion. This is because oxide is generated and the strength of the forged portion is reduced. As a method of shielding the open pipe with an inert gas, a method in which the entire open pipe including the forging roll is surrounded by a shield box, or a local shielding method in which an inert gas injection nozzle is arranged toward the edge surface to be forged, etc. Is preferred.

[0016]

EXAMPLES Steel strips having the chemical composition according to the present invention and steel strips for comparison thereof were produced, and then these steel strips were passed through an apparatus shown in FIG. 4 to produce forged steel pipes. That is, after heating the steel strip 1 to 1300 ° C. in the heating furnace 2, the forming roll 3,
4, the open pipe 1A is formed, then both edges of the steel strip are heated to 1400 ° C. by the plasma arc heating device 6, and the inert gas injection nozzle 7 injects N ₂ gas to both edges immediately before the forging roll. While forging with the forging roll 5, the tube body 1B was formed, and then a forged steel tube was manufactured through a series of steps such as a hot stretch reducer.

The product seam 90 ° flatness test was performed to examine the seam flatness strength. The results are shown in Table 1. As can be seen from Table 1, according to the method of the present invention, the Ti addition amount of the material is 0.00
The area ratio of the Ti-based oxide existing on the forged surface is set to 3% to 0.3% and the oxygen concentration during forged welding is set to 10% or less.
The flatness ratio in the flatness test was 0.03 to 3%, and the flatness test was 0.2 or less, indicating excellent forged welding quality. In the conventional steel, the flatness was 0.5 and seam cracking occurred.

[Table 1]

[0018]

As described above, according to the present invention, it is possible to provide a high-quality forged steel pipe having no difference in material between the forged portion and the base material.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a Ti addition amount of a material and a 90 ° flatness test result of a seam portion.

FIG. 2 is a diagram showing the relationship between the area ratio of Ti-based oxide existing on the forged surface and the 90 ° flatness test result of the seam portion.

FIG. 3 is a diagram showing the relationship between oxygen concentration during forge welding and the 90 ° flatness test results of seams.

FIG. 4 is a schematic view showing an example of a forged steel pipe manufacturing apparatus.

[Explanation of symbols]

 1 Steel Strip 1A Open Tube 1B Tube 2 Heating Furnace 3, 4 Forming Roll 5 Forging Roll 6 Plasma Arc Heating Device 7 Inert Gas Injection Nozzle

 ─────────────────────────────────────────────────── --- Continuation of the front page (72) Inventor Takaaki Toyooka 1-1, Kawasaki-cho, Handa-shi, Aichi Kawasaki Steel Co., Ltd. Chita Works (72) Inventor Osamu Furu-kun, 1-1, Kawasaki-cho, Handa-shi, Aichi Kawasaki Steel Co., Ltd.Chita Works

Claims (3)

[Claims] 1. C: 0.03 to 0.25%, Si: 0.3% or less,
Mn: 0.3 to 1.5%, P: 0.06% or less, S: 0.04% or less, Ti: 0.003 to 0.3%, Al: 0.005% to 0.1%, and the balance Fe and inevitable impurities are characterized by the forged steel pipe. . 2. C: 0.03 to 0.25%, Si: 0.3% or less,
Mn: 0.3 to 1.5%, P: 0.06% or less, S: 0.04% or less, Ti: 0.003 to 0.3%, Al: 0.005% to 0.1%, balance Fe and inevitable impurities, and Ti existing on the forged surface A forged steel pipe characterized in that the area ratio of the oxide system is 0.03 to 3%. 3. C: 0.03 to 0.25%, Si: 0.3% or less,
Mn: 0.3 to 1.5%, P: 0.06% or less, S: 0.04% or less, Ti: 0.003 to 0.3%, Al: 0.005% to 0.1%, and a steel strip containing the balance Fe and unavoidable impurities in a forged steel pipe mill. The oxygen concentration of the edge surface to be forged is 10% by feeding and shielding the open pipe just before the forging roll with inert gas.
% Forged steel pipe is forged while maintaining the same.