JPH07116728A - Production of high toughness butt welded steel tube - Google Patents

Abstract

PURPOSE:To effectively produce a steel tube with excellent toughness and stable quality in high productivity by further hot drawing it in a specific temperature with a specific reduction rate of area after hot drawing. CONSTITUTION:A steel strip of a prescribed width is continuously fed into a heating furnace, after it is heated in about 1100 to 1300 deg.C, it is formed in a forming stand, oxygen blows on it just before forge welding, and it is forge- welded. It is hot drawn with a stretch reducer, and it is cut in a regular dimension with a cutter. At the time point of the steel tube temperature reaching between <=Ac3+100 deg.C and >=Ac3, it is hot drawn further with >=10% reduction rate of area with the stretch reducer. Austenite crystal granule is not made small in >=Ac3+100 deg.C, and the machining strain is left after drawing in <=Ac3. Further, when the reduction rate of area is made large, the austenite crystal granule is made small, and the reduction rate of area needs >=10% in order to increase the toughness.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a high toughness forged steel pipe.

[0002]

2. Description of the Related Art Generally, forged steel pipes are manufactured as shown in FIG.
A steel strip with a specified width is continuously inserted into the heating furnace,
After heating to ℃ ~ 1300 ℃, it is molded in a molding stand, oxygen blow is performed just before forging and forging is performed. After that, hot drawing is performed with a stretch reducer, after which the piece is cut to a fixed length, the thickness of the outer diameter is reduced to a predetermined value using a standard machine, and the piece is air-cooled. This method has the highest efficiency and high productivity as a method for producing a steel pipe and is generally widely used. However, in order to improve the toughness and the quality of the abutting portion, Japanese Patent Application No. 56-18364 is used. As described in Japanese Patent Application No. 58-123853 or Japanese Patent Application No. 58-123853, high frequency resistance welding is performed at a relatively low temperature, then bead removal cutting is performed, heating is performed again at about 900 ° C to 1100 ° C, and then squeezed by a stretch reducer There is.

[0003]

Generally, in the method for producing a forged steel pipe, a steel strip having a predetermined width is continuously inserted into a heating furnace, heated to about 1200 ° C to 1300 ° C, and then formed on a forming stand. Immediately before the forging, oxygen is blown and the forging is performed. After that, hot drawing is performed with a stretch reducer, after which the piece is cut to a fixed length, the thickness of the outer diameter is reduced to a predetermined value using a standard machine, and the piece is air-cooled. That is, since the hot drawing is performed at about 1200 ° C to 1300 ° C, it is recrystallized after the hot drawing, the austenite crystal grains become large, and the toughness is generally poor.

In order to solve this problem, it is necessary to perform hot drawing at a steel pipe temperature of Ac ₃ + 100 ° C. or lower and Ac ₃ or higher, but one method is high frequency resistance welding at a relatively low temperature. There is a method in which bead removal cutting is performed, heating is performed again at about 900 ° C. to 1100 ° C., and then squeezing is performed with a stretch reducer. As a result, the steel pipe temperature is A
Although hot drawing can be performed at c ₃ + 100 ° C or lower,
Due to the high frequency welding, the edge of the steel strip is locally heated, the weld abutment rises, that is, a bead is generated, which causes a new problem that it has to be removed by cutting at a high temperature, resulting in a high efficiency. , Greatly impedes productivity. The present invention is intended to solve the problems in such a method for manufacturing a forged steel pipe.

[0005]

[Means for Solving the Problems] In order to achieve the above object, a steel strip having a predetermined width is heated and hot-formed, and then forged and welded.
In the method for manufacturing a forge welding steel pipes performing hot stop, when the steel pipe temperature after squeezing hot becomes Ac ₃ + 100 ° C. or less and Ac ₃ or more, further performing hot stop at reduction rate of more than 10% The gist is a method of manufacturing a characteristic high toughness forged steel pipe.

The present invention will be described in detail below. FIG. 1 shows the manufacturing process of the present invention. In the conventional process, as shown in FIG. 2, a steel strip having a predetermined width is continuously inserted into the heating furnace, and about 12
After heating to 00 ℃ ~ 1300 ℃, molded on the molding stand,
Immediately before the forging, oxygen is blown and the forging is performed. After that, hot drawing is performed with a stretch reducer, after which the piece is cut to a fixed length, the thickness of the outer diameter is reduced to a predetermined value using a standard machine, and the piece is air-cooled. in this way,
As described above, the austenite crystal grains are large and the toughness is low.

In the present invention shown in FIG. 1, a steel strip having a predetermined width is continuously inserted into a heating furnace, heated to about 1100 ° C. to 1300 ° C., molded by a molding stand, and oxygen blown immediately before forging. , Forge. At that time, in order to lower the heating temperature, the forged part may be heated by irradiating the forged part with a laser or the like during the forging. After forging with a forging roll and then hot-drawing with a stretch reducer, after cutting to a standard length with a cutting machine, when the steel pipe temperature is below Ac ₃ + 100 ° C and above Ac ₃ , the surface reduction rate is 10% or more. Stretch reducer for hot squeezing.

Usually, when heated to 1100 ° C. to 1300 ° C., the temperature is about 900 ° C. to 1000 ° C. before and after the standard machine, so that even if hot drawing is performed before the standard machine, austenite crystal grains are generated. It becomes large and the toughness decreases. Therefore, after the normal position of the standard machine, the steel pipe temperature was Ac ₃ +
A stretch reducer is arranged at a position of 100 ° C or lower and Ac ₃ or higher, and hot drawing is performed. The standard machine may be placed after this stretch reducer or may also serve as the stretch reducer.

Steel pipe temperature is Ac ₃ + 100 ° C. or less and Ac
_The reason why the hot drawing is performed at ₃ or more is that the austenite crystal grains do not become small at Ac ₃ + 100 ° C. or more, and the processing strain remains after the drawing at Ac ₃ or less and the workability is deteriorated. However, when the steel pipe temperature is Ac ₃ + 100 ° C. or lower, hot reduction is performed by a stretch reducer, but the area reduction rate is important. Generally, as the area reduction ratio increases, the austenite crystal grains also decrease. Therefore, in order to increase the toughness, the surface reduction rate needs to be 10% or more.

On the other hand, in the process of the present invention, there are two hot reductions by the stretch reducer, the former stage and the latter stage. This is because when the drawing temperature becomes low, the drawing force becomes large and it becomes difficult to draw. This is to reduce the area. For this reason, it is important to arrange the stretch reducers in two stages, but as a result, depending on the temperature, they may be continuous.

[0011]

EXAMPLE A material having a size of 50 A and an Ac ₃ of 800 ° C. was compared between the conventional method and the method of the present invention in Table 1. No. 1 and No. No. 2 is a general conventional method, but in both cases, the austenite grain size is large (grain size number is small) and the impact value is poor. No. 3 and No. No. 4 is the method of the present invention, but the austenite grain size is small (grain size number is large) and the impact value is also good. No. No. 5 is No. 5 when the drawing temperature is high. No. 6 is a case where the surface reduction rate is low, but the impact value is bad in each case.

[0012]

[Table 1]

[0013]

When a forged welded steel pipe is manufactured by the conventional manufacturing method, hot drawing is performed at a high temperature of 1200 ° C to 1300 ° C, so that the austenite grain size is large and the toughness is poor, so that the pipe is extremely small. It could only be used for some purposes.
By applying the method of the present invention, it has become possible to manufacture a steel pipe having good toughness and stable quality while maintaining high efficiency and high productivity.

[Brief description of drawings]

FIG. 1 is a diagram showing a manufacturing process of the present invention.

FIG. 2 is a diagram showing a conventional manufacturing process.

Claims (1)

[Claims] 1. A method of manufacturing a forged steel pipe, comprising heating a steel strip of a predetermined width to perform hot forming, and then forging and hot drawing. In the method, the steel pipe temperature after hot drawing is Ac ₃ + 100 ° C. or less and Ac. _A method for producing a high toughness forged steel pipe, wherein hot drawing is further performed at a surface reduction rate of 10% or more when the number becomes ₃ or more.