JPS62202026A - Production of high rupture toughness chain - Google Patents

Abstract

PURPOSE:To obtain a chain having excellent rupture toughness at a low temp. in a flash butt weld zone by subjecting a steel bar consisting of a low alloy steel to two times of hardening and tempering treatments in a heat treatment after chain making and further limiting the heating temp. in the respective heat treatments to a specific range. CONSTITUTION:The steel bar consisting of the low alloy steel is cut to a prescribed length and is formed to a toric shape; thereafter, the end face is subjected to flash butt welding and studs are press-fitted thereto to form the chain. The chain is further subjected to the heat treatment, by which the large- diameter chain is produced. The chain is subjected to the tempering treatment at 500-700 deg.C after two times of hardening including primary hardening by which the chain is cooled after heating to 900-1,050 deg.C and secondary hardening by which the chain is quickly cooled after heating to 850-950 deg.C as the heat treatment after the chain making. The high rupture toughness chain useful under sever conditions such as anchoring of an ocean structure, for example, rig for petroleum drilling is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for manufacturing a chain with excellent fracture toughness at a flash butt weld.

(Prior Art) In recent years, as the energy situation has changed, efforts to develop new energy resources have become active in various parts of the world. Under these circumstances, as onshore development resources have been depleted, attention has been focused on offshore oil fields, and development using oil drilling rigs has begun from the south to the North Sea, mainly near the continental region. It has come to be practiced in a wide range of areas.

As the number of offshore structures, such as the above-mentioned offshore oil drilling rigs, increases, the demand for large-diameter chains used to moor them continues to increase. The frequency of oil drilling in extremely cold regions with poor conditions is increasing.

On the other hand, chain breakage is directly linked to serious accidents such as rig collapse, so ensuring safety is an important issue, and from this perspective, efforts are being made to improve the fracture toughness of chains, especially when used in extremely cold regions. There is a need for a chain with excellent fracture toughness at low temperatures that can withstand high temperatures.

Incidentally, a large diameter chain is produced by cutting a hot-rolled steel bar to a predetermined length, forming it into an annular shape, flash-butt welding the end faces, and then press-fitting a stud. Furthermore, it is manufactured by subsequently performing heat treatment.

Therefore, in a chain, the flash hunt weld is the part with the lowest fracture toughness, and improving the fracture toughness of this weld is the key to manufacturing a high-toughness chain.

(Problems to be Solved by the Invention) Therefore, an object of the present invention is to provide a method for manufacturing a chain that has excellent fracture toughness at low temperatures in flash butt welds.

Specifically, the purpose of this invention is to have a tensile strength of 70 kgf/mm" or more and a fracture toughness COD of the flash butt welded part at -30°C of 0.12 m+w.
Fracture toughness C of Franschhat welds at -20℃ or above or tensile test 90 kgf/lsm'' or above
It is an object of the present invention to provide a method for manufacturing a chain with high strength and high fracture toughness having an OD of 0.12 m or more.

(Means for Solving the Problems) The present inventors have conducted various researches in order to realize a method of manufacturing a chain with high strength and excellent fracture toughness at low temperatures in the flash hat part. Continuing with this process, we found that in the heat treatment after chain making, we performed two quenching and tempering treatments.
Furthermore, it has been found that by limiting the heating temperature in each heat treatment to a predetermined range, the fracture toughness of the Furanoche buff He joint can be significantly improved.

This invention was made based on the above findings,
The gist of this is that after chain-making a steel bar made of low alloy steel, primary quenching = heating to 900-1050°C followed by cooling and secondary quenching:
After heating to 850-950℃ and quenching twice, 55℃
This is a method for manufacturing a chain with excellent fracture toughness at a flash butt weld, which is characterized by performing a tempering treatment at 0 to 700°C.

Next, the reason why the heat treatment conditions are limited as described above in this invention will be explained. Incidentally, the chain manufacturing process by hot rolling steel bars, cutting steel bars, bending, Franchebat welding, etc. is already well known to those skilled in the art, and this invention also uses conventional chain manufacturing processes. Therefore, there are no particular limitations, and the explanation will be omitted.

Conventionally, the heat treatment method for chains has been normalizing treatment or one-time quenching and tempering treatment. In particular, attempts have been made to improve the toughness of high-strength chains by employing double quenching and tempering treatments.

However, in double quenching and tempering treatments, coarse carbides generated in flash butt welds do not dissolve sufficiently at the normal quenching heating temperature, resulting in decreased hardenability of the weld and after quenching. The microstructure becomes a coarse bainite structure, and the hardness is also significantly lower than that of the fabric part. Therefore, a sufficient effect of improving fracture toughness cannot be expected.

Furthermore, in order to dissolve coarse carbides in the weld zone in the double quenching and tempering method, increasing the quenching heating temperature causes coarsening of crystal grains, which causes deterioration of fracture toughness.

In contrast, this invention employs two quenching processes: primary quenching (high temperature heating) to dissolve coarse carbides, followed by secondary quenching to generate fine-grained martensite and lI weave. By doing so, the microstructure and hardness distribution of the flash hunt welded area are made more uniform, and the fracture toughness is significantly improved.

First, in the secondary quenching, it is heated to 900 to 1050°C and then cooled. -The main purpose of the secondary quenching is to fully dissolve the coarse carbides that have formed in the flash butt weld, thereby restoring the hardenability of the weld.

Heating at less than 900° C. does not sufficiently dissolve coarse carbides. Furthermore, even if heated above 1050°C, no improvement in the solid solution effect can be expected; on the contrary, it will cause coarsening of crystal grains and cause defects such as burning, and furthermore, it will shorten the life of the heat treatment furnace. , 1050°C or less. Cooling after heating may be done by standing or quenching depending on the purpose of secondary quenching, but it is possible to prevent the formation of coarse carbides during cooling or to change the microstructure to a low-temperature transformed structure. Rapid cooling is preferred for reasons such as speeding up the austenite transformation.

In the secondary quenching, the material is heated to 850 to 950°C and then rapidly cooled. The purpose of secondary quenching is to change the microstructure to martensite by heating to an austenite region and then rapidly cooling.

Heating below 850° C. will result in insufficient transformation to the austenite phase, and the microstructure after quenching will become non-uniform, causing deterioration in toughness. Furthermore, heating above 950° C. causes coarse grains and causes deterioration of toughness. For cooling after heating, rapid cooling such as water cooling or oil cooling is required in order to make the microstructure into a martensitic 1JI weave.

After the above-mentioned double quenching, a tempering treatment is performed at 550 to 700°C. The purpose of the tempering treatment is to change the martensitic structure obtained by the secondary quenching into a tempered martensitic structure with excellent toughness, and to adjust the strength to a desired level. Tempering at a temperature lower than 550°C may cause temper brittleness and cause deterioration of toughness.

Furthermore, when tempering exceeds 700°C, transformation to an austenite phase occurs, which causes toughness deterioration.
The tempering temperature is set at a box surface of 550 to 700°C, and the temperature is adjusted within this range according to the desired strength.

In addition, the low alloy steel in this invention is a material with a diameter of 60 to 1
It is necessary to have sufficient hardenability for a large diameter chain of 60 sun, and specifically, it is preferable to have the following chemical composition.

C: 0.10-0.30%, Si: 0.05-0.
50%, Mn: 0.50-2.50%, Sol, AQ+
0.010-0.060%, N: 0.003-0.0
20%, and further contains Cr: 0.5-3.0%, Mo
:0.05-0.70%, Ni:0.5-3.0%, C
u: 0.10 to 0.50%, V: O, O,: + to 0.2
0%, Nb:O, 01-0.10%, Ti: 0.005
~0.05%, Ca: 0.010% or less, REM: 0.
Contains 10% or less of one or more types, with the remainder being Fe and unavoidable impurities.

Next, the present invention will be explained in more detail through Examples and in comparison with Comparative Examples.

Example First, steel having the composition shown in Table 1 was melted using a 3-ton electric furnace, and then hot-rolled to a diameter of 85 mm.
A round steel bar was obtained. A rope has a tensile strength of 70 kgf/mm
” class, Btail, tensile strength 90kgf/IIIT@
It is a Z-class thing. Further, C steel is a steel equivalent to SR1 near 0 specified in JIS G3105. Next, this was cut, formed into a chain by hot bending, and then flash butt welded and organized. Then, after deburring the welded part, the stand was inserted into the 0th and 2nd parts.
A studded chain was manufactured by applying heat treatment as shown in the table.

The following test pieces were obtained from each chain manufactured in this way: Tensile test pieces: Diameter (D) was 14 mm and gauge length was 5.
COO test piece from the base metal part of D: 25” x 5 according to B55762
0'' was taken from the flash butt weld and used for testing.

The COD test was carried out at -30°C for A and C steels, and -22°C for B steels.
It was carried out at 0°C. The results obtained are summarized in Table 3.

First, if we look at steel B which has been adjusted to a tensile strength of 95 kgf/++m, we will see that in the case of normal single quenching and tempering (B1)
, the COO value is as low as 0.06 mm, whereas the method according to this invention (83-B5.87-B8) has a COO value as low as 0.06 mm.
The value has been significantly improved to 0.15-0.19mm,
The effect was confirmed.

Note that FIGS. 1 and 2 show the influence of the primary quenching heating temperature and the secondary quenching heating temperature on the COO value, respectively, for Steel B. - When the secondary quenching heating temperature is less than 900℃ and the secondary quenching heating temperature exceeds 950℃, CO
O value is decreasing. A similar tendency was observed for A steel.

Next, A was adjusted to have a tensile strength of 75 kgf/1m
Similarly, for steel, normal one-time quenching and tempering (A1)
``The COO value is as low as 0.07 mm, whereas in the method (A2) according to this invention, the COO value is 0.18n+w.''
This was a significant improvement, and its effectiveness was confirmed.

Note that C steel does not have sufficient hardenability, and the COO value after normal single quenching and tempering (CI) is as low as 0.04 mm.

Table 1 Table 2 *: Indicates that it is outside the scope of the present invention.

Table 3 (Effects of the Invention) As described above, the present invention provides an extremely safe large-diameter chain that has both high strength and excellent fracture toughness of flash butt welds that are exhibited even at low temperatures. It can be manufactured and is expected to play an extremely useful role in resource development under harsh conditions, resulting in industrially useful effects.

[Brief explanation of drawings]

Figures 1 and 2 are COO for steel B, respectively.
It is a graph showing the influence of primary quenching heating temperature and secondary quenching heating temperature on the value.

Claims (1)

[Claims] After chain-making a steel bar made of low-alloy steel, primary quenching: heating to 900-1050°C followed by cooling and secondary quenching:
A method for producing a chain with high fracture toughness, which comprises quenching twice by heating to 850 to 950°C and then rapidly cooling, followed by tempering at 550 to 700°C.