JPS62290822A - Production of thick-walled steel pipe - Google Patents

Abstract

PURPOSE:To easily produce a thick-walled steel pipe having excellent surface characteristics by molding thick-walled steel products having a specific compsn. to a specific cold bent and worked steel pipe having a specified surface strain and quickly heating only the outside surface part thereof to a prescribed temp. and allowing the same to cool after holding for the prescribed time. CONSTITUTION:The steel products which consist, by weight, of <=<=0.12% C, <=0.50% Si, 1.0%-1.8% Mn, <=0.01% P, <=0.005% S, and <=0.1% Nb or V and has >=50mm thickness are prepd. The steel pipe is manufactured by cold bending of such steel products and the surface strain in this stage is maintained at 5-15%. The steel pipe is then quickly heated to 800-1,000 deg.C from the outside surface thereof to 1/4 the thickness and is held for 5-20sec according to the surface strain quantity; thereafter the steel pipe is allowed to cool. High-frequency induction heating is most adequate as a method for heating only the surface part of the thickness. The surface characteristics which are deteriorated by the cold working are restored and the thick-walled steel pipe having the excellent performance is easily produced by the above-mentioned method.

Description

[Detailed description of the invention] 3. Detailed description of the invention [Industrial application field] Unreleased 1! II relates to a method for manufacturing thick-walled steel pipes, and further includes 1.
The method is to restore the surface properties of cold-bent #4 pipes with a plate thickness of 50 mm or more to the state before bending, such as steel pipe members for supports of station buildings, mole rails, etc., or steel pipe members for jackets. The present invention provides a method for manufacturing thick-walled steel pipes.

[Conventional technology]

When a steel pipe is manufactured by cold bending, the part of the pipe with a large amount of strain, especially the surface layer, becomes brittle due to strain aging, resulting in a difference in toughness in the thickness direction.

For example, in Figures 2 and 3, the horizontal axis indicates the position in the thickness direction (
(Ft and Fc indicate the tension side surface and compression side surface, respectively)
The yield point (YS) (kg/mm') and elongation (Ei) (%) of JIS-5M4IC steel before and after bending in the thickness direction! Figure 2 shows that the yield point of the surface of the plate is abnormally high after processing compared to the center of the plate, and Figure 3 shows that the yield point of the surface of the plate is abnormally high after processing compared to the center of the plate. This shows that the elongation of the surface area has decreased significantly. Further, FIG. 4 shows the relationship between temperature and impact absorption energy (kg-rn) on the outer surface side and the inner surface side of the tube, and it can be seen that the surface has become extremely brittle after processing.

Conventional methods to prevent such embrittlement have been to either subject the entire steel pipe to long-term heat treatment (normalizing) after cold bending, or to perform hot bending. [Point m to be solved by the invention] The method of heat treating the entire steel pipe after cold bending requires large heat treatment equipment and heat treatment energy to heat the entire steel pipe, resulting in high cost and This has the disadvantage that the mechanical properties of the entire steel pipe, such as its strength, decrease because the properties of the parts where the properties do not change are also changed.

If thick-walled steel pipes are manufactured by hot bending, it is possible to obtain steel pipes with uniform properties in the thickness direction, but since the steel pipes are heated at high temperatures for long periods of time, the crystal grains become large and no improvement in strength can be expected. Also, low-temperature toughness is poor.

The present invention uses a simple method to restore the mechanical properties of only the surface of a raw pipe whose surface properties have deteriorated due to the cold bending process described above, and enables the easy production of thick-walled steel pipes with excellent surface properties at low cost. The purpose is to provide a person who can do the following.

[Means for resolving issues]

In order to achieve the above object, the present invention is an invention that takes measures to restore toughness by heat treating only the surface layer whose properties are particularly deteriorated by cold bending, and uses the following technical means as a means of solving the problem. It is something to do.

■As a material.

C: 0.12% or less Si: 0.50% by weight or less Mn: 1.0-1.8 weight 9% P: 0.01% by weight or less S: 0.005% by weight or less Nb, V: 1 or 2 types total amount 0.1% or less A steel material with a plate thickness of 50 mm or more is used.

(A cold bent steel pipe with a surface strain of 5 to 15% is formed.

■The distance from the outer surface of the processed steel pipe to 1/4 plate thickness is 800
Rapid heating to a temperature of -1000°C.

(2) Next, the temperature is maintained for 5 to 20 seconds depending on the amount of surface strain, and then allowed to cool.

[Effect]

Since strain aging embrittlement occurs in the surface layer of a cold-bent steel pipe depending on the degree of bending, the present invention restores the toughness of this portion by subjecting only the surface layer of the pipe to short-term heat treatment. It is.

FIG. 1 schematically shows a time pattern of heat treatment of the surface of a steel pipe according to the present invention. It is preferable to apply this heat treatment to both the inner and outer surfaces of the steel pipe, but in some cases it may be effective to apply it only to the outer surface that is particularly necessary for the structure.

The reason for limiting the steel pipe material composition, bending strain, and heat treatment conditions will be explained below.

The material targeted by this invention is JIS-SM41C150C
The material has a tensile strength of 41 to 59 kg/mm' corresponding to 153C.

The respective component definitions of C, Si, Mn, P, S, Nb, and V define such materials.

C is an element that strongly causes strain-aging embrittlement, and although it is better to add a small amount of C, the L limit is set to 0.12% by weight in order to ensure the strength of the material.

Si is necessary for deoxidizing steel, but as the amount added increases, the toughness of the material decreases and the weldability also deteriorates, so the upper limit is set at 0.50% by weight and 41%.

Mn also has the same effect as Si, and requires more than 1.0% by weight to ensure the strength of the material, while 1.8% by weight
If it exceeds 1.0, the toughness of the material will decrease, so the upper limit is set at 1.
8 t'R state%.

Like C, P causes strain aging embrittlement, so the limit is 0.
01 ball is 1%.

S tends to segregate in steel, and the presence of a segregated layer increases strain-age embrittlement, worsens the toughness of the material, and has poor weldability. Therefore, it is better to keep the S content as low as possible. ” 2nd limit O, OO5 @ g ':! Set it to 6.

Nb and ■ are added to ensure the strength of the material, but if the amount added is too large, the toughness and elongation of the material will decrease. Since the effects of Nb and V are almost the same, the upper limit of their addition amount is 0.1% by weight in total of one or both.

The present invention targets thick-walled ln4 pipes with relatively large diameters, and is intended to recover the deterioration of surface quality caused by bending, so thin plates are not applicable. . On the other hand, there is no limit to the plate thickness, but it is naturally within the turning limit for bending steel pipes.

In addition, from the perspective of surface strain of the plate, it is limited to 5 to 15%.
The surface strain is given as a function of the plate thickness and the bending radius, ε=t/(2R+t) where ε - the surface strain of the steel pipe due to bending D - the radius of curvature at the center of the steel pipe's thickness 7 - the plate thickness.

High-frequency induction heating is the most suitable method for heating only the surface portion of the plate according to the present invention. The heating temperature is determined by the frequency of the heating source and the spatial position ri between the heating source and the body to be heated.
Easily place 9 depending on the relationship and heating time! 3J to the value of
I can mourn. ? ,! The temperature is set at 800 to 1000°C to recover the deterioration in properties due to processing strain, and the penetration depth is 1-minute if the depth is 1/4 of the thickness of the plate, that is, the center between the surface of the plate and the neutral plane. effect can be obtained.

The surface area is then held at 800-1000'0 for a short time.

Cold pressurization caused by strain aging and embrittlement! Toughness and ductility are restored by recrystallizing the surface layer of the 4-tube by heating and converting the branched black structure in this area into fine ferrite.
In order to restore the toughness of a material that has undergone pre-strain of 0 to 15 degrees, it is necessary to heat the material to a temperature of 800°C or higher.
If heating exceeds 1000''C, the microstructure becomes a coarsened bainitic structure and the toughness is not recovered.

Therefore, the appropriate heating temperature range is goo-iooo''c.

The time for maintaining the temperature at 800 to 1000°C is 5 to 20 seconds depending on the tube diameter and plate thickness, in other words, depending on the value of surface strain. According to actual examples, when the surface strain is 5 to 6%, a holding time of 5 seconds is effective, when the surface strain is 10%, the surface texture is recovered to the same level as the raw material in 10 seconds, and when the surface strain is 15%, the surface texture is recovered to the same level as the raw material.
Therefore, the holding time should be 5 to 20 seconds depending on the tube diameter and plate thickness, and after holding, it is sufficient to allow the tube to cool naturally.

Next, the present invention will be specifically described in detail with reference to examples, but the present invention is not limited to the following examples.

〔Example〕

Example 1 Tensile strength: 44 kg/mrrf, plate thickness: 130
A thick inner steel pipe material was produced by cold working a steel plate having the following M1 composition.

C: 0.08% by weight Si: 0.25% by weight Mn + 1.35ffl Presentation % P: 0.007% by weight S: 0.002% by weight -1% Nb: 0.025% by weight Bend onto the material at 5 m When a surface strain of 10% was applied due to processing, it was observed that the absorbed energy at 0°C at the outermost surface decreased by 12 kgm and the transition temperature shifted 30°C to the higher temperature side compared to the raw material. However, the central part of the plate thickness shows almost no change.

This n4/i? High-frequency induction heating is applied to the surface from the outside to 1000℃, and the temperature 30mm from the surface is 80℃.
It was heated to 0°C, held for 10 seconds, and then allowed to cool. When the absorbed energy at the outermost surface of the steel pipe was determined at 311, it was confirmed that it had recovered to the raw material level.

Example 2 A thick inner steel pipe was manufactured by cold working a steel plate having a tensile strength of 53 kg/mm' and a plate thickness of 80 mm and having the following composition.

C: 0.12 weight 5'l: Ri 6 Si: 0.30 weight % Mn: 1.5 weight % P: 0.009 weight % S+0.003 weight % Nb: 0.03 weight % V: 0. 06 Overlay % The absorbed energy of the material at 0° C. was 20 kgm.

When a surface strain of 6% was applied to the above material by bending, the absorbed energy at 0°C at the outermost surface decreased by 7 kgm, and the transition temperature shifted to the higher temperature side by 15°C.

This steel pipe is heated by high frequency induction from the outside, and the surface is
When the temperature was raised to 800° C. at a depth of 20 mm from the surface, held for 5 seconds, and then allowed to cool, the absorbed energy at the outermost surface recovered to the level of the material.

〔Effect of the invention〕

Since the method for manufacturing thick-walled steel pipes of the present invention is configured as described above, it is possible to recover the deterioration of the surface quality caused by cold working, and to produce thick-walled steel pipes that have excellent performance as supports for structures, etc. It became possible to manufacture steel pipes cheaply and easily.

[Brief explanation of the drawings] Figure 1 is a diagram schematically showing the heat treatment pattern of the steel pipe surface of the present invention, and Figures 2 and 3 are JIS-3M4IC, respectively.
A graph showing the distribution of yield point and elongation in the thickness direction of steel before and after bending, and FIG. 4 is a graph showing the relationship between temperature and impact absorption energy before and after bending.

Claims (1)

[Claims] 1 C: 0.12% by weight or less Si: 0.50% by weight or less Mn: 1.0 to 1.8% by weight P: 0.01% by weight or less S: 0.005% by weight or less Nb, V: type 1 or type 2 with a total of 0.1% by weight or less, using a steel material with a plate thickness of 50 mm or more, with a surface strain of 5 to 15
A cold-bent steel pipe of 50% was formed, and the area from the outer surface of the steel pipe to 1/4 of the plate thickness was rapidly heated to a temperature of 800 to 1000°C, and held for 5 to 20 seconds depending on the amount of surface strain. A method for producing a thick-walled steel pipe, which is characterized by allowing it to cool afterwards.