JPH036206B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for manufacturing a chain for mooring an offshore oil drilling rig, which is used in icy areas such as the North Sea and the Arctic Ocean.

[Conventional technology]

Japanese Patent Publication No. 60-33178 and Japanese Patent Publication No. 61-7486 filed by the present applicant are known as methods for producing chains with tensile strength of 70 Kg/mm ² or more and excellent toughness. However, as oil drilling activities progress to ice-cold areas such as the North Sea and the Arctic Ocean, chains for mooring rigs are required to have a diameter of 70 mm or more and have excellent constant-temperature toughness. . For example, the tensile strength is 70Kgf/mm2 ^or more, and the yield strength is 46Kg.
f/mm ² or more, elongation rate of 17% or more, reduction of area of 50% or more, and shear py absorbed energy of -2.8 Kgfm at -60°C in the base metal and welded joint.

Currently, the scientific components of three types of chain materials with a tensile strength of 70 Kgf/mm ² or higher are KSBC70, JIS, Nippon Kaiji Kyokai Standards (NK), Norway (NV), and American Classification Society (AB), respectively.
It is defined as NVK3 steel and 3a steel, and 3
The seed chain is made of round steel made of these ingredients and heated to 800℃.
After being heated to the above-mentioned high temperature and processed into a link shape, it is generally manufactured by heat treatment such as normalizing and back-quenching. However, these three types of chains have low constant-temperature toughness and are used in sea areas with temperatures down to 0°C at most, and cannot be used at -60°C, which is the environmental temperature of icy waters. In contrast to this-
As a steel for chains that takes into account constant temperature toughness up to 60℃, for example, Japanese Patent Publication No. 51-24967 describes a low carbon high strength structural steel with good weldability, and the applicable temperature of this steel is -60℃. This has been taken into consideration. However, this steel has 2 hardenability improving elements.
It is a very expensive steel material that requires a large amount of Cr, up to 5%, and the toughness of the welded joint at -60°C is not described at all and is unknown. Further, although the steel material described in this invention has a small diameter of about 10 to 30 mm, it is generally known that as the diameter becomes thicker, the strength and toughness decrease significantly.

Therefore, when the present inventors conducted a flat butt welding experiment using round steel with a diameter of 78 mm having the same composition as that of the present invention, a large amount of Cr oxide was scattered on the joint surface, and the toughness of the welded part was significantly reduced. I found out. As described above, with the conventional technology, it has been particularly difficult to ensure the toughness of the welded portion.

[Problem that the invention seeks to solve]

The present invention relates to a method for manufacturing a chain used in icy areas, which has not been possible until now.
Specifically, it is a large diameter material with a diameter of 70 mm or more, a tensile strength of 70 Kgf/mm ² or more, a yield strength of 46 Kgf/mm ² or more,
The present invention relates to a method for producing a chain that has an elongation rate of 17% or more, a reduction rate of 50% or more, and a shear py absorbed energy of 2.8 Kgfm or more at -60°C in the base metal portion and welded joint.

Generally, chain links are joined by flash butt welding, but various defects as shown below occur on the welded surfaces, deteriorating tensile properties and toughness.

First, carbon and alloying elements move from the adjacent solid phase to the molten phase formed by flushing and become concentrated.However, as this molten phase is pushed out during upsetting, these elements are not present in the joint. hardenability decreases. In addition, various oxides such as Al, Si, Mn, and Cr, which are generated depending on the steel composition, are not completely discharged and remain partially on the welded surface, resulting in deterioration of tensile properties and toughness. In addition, the pressure bonding force during upset generates metal flow parallel to the joint surface, which also causes deterioration of toughness in the longitudinal direction. As described above, the mechanical properties of welded parts inevitably deteriorate due to various causes.

The present invention enables the production of large-diameter chains with excellent strength and low-temperature toughness by appropriately combining steel components and controlling the quenching and tempering temperature and cooling rate, even with conventional welding techniques. It provides a method of manufacturing at low cost.

[Means for solving problems]

The present invention has C: 0.15 to 0.35%, Si: 0.15 to 0.50%,
Mn: 1.00-2.00%, Cr: 0.50% or less, Mo: 0.10
~ 0.60%, acid-soluble Al in the range of 0.010 to 0.100%, and a hardenability satisfying the following formula (1). After hot bending and ring alignment by welding. , 3℃/s after heating from 900℃ to 1100℃
The present invention provides a method for manufacturing a large diameter, high strength chain with excellent low temperature toughness, characterized by cooling at the above rate, followed by heating to 550°C to 700°C, and then cooling at a rate of 0.1°C/s or more.

Di≧0.035D+0.35 (1) where Di: hardenability of the steel material (in.), D: diameter of the steel bar (mm) [Function] Next, the reason for determining the conditions of the present invention will be described.

(1) C is the best low-cost element to improve the hardenability of steel, but if the content is less than 0.15%, the required strength cannot be obtained, while if it exceeds 0.35%, cracks will occur during quenching. The upper limit was set at 0.35% because this would lead to deterioration of the toughness of the flat butt weld joint.

(2) Si is used both as a reinforcing element and as a deoxidizing element, and if it is less than 0.15%, both effects cannot be expected. Further, 0.50% is sufficient for strengthening and deoxidizing effects, and if it exceeds 0.50%, the toughness and ductility decrease due to an increase in silicate inclusions, so the upper limit was set at 0.50%.

(3) Mn is used as an element to improve hardenability with less deterioration of toughness, but if it is less than 1%, sufficient hardening effect cannot be obtained and it is difficult to ensure strength.
Moreover, if it exceeds 2.0%, quench cracking will occur, so the upper limit was set at 2.0%.

(4) Cr is an effective element for reducing the activity of C in molten steel and reducing decarburization during flash butt welding, but on the other hand, it tends to form oxides, which remain on the joint surface and reduce toughness. The upper limit was set at 0.50% as this is the main cause of decrease. Furthermore, even if Cr is not used, the required welding material quality can be obtained by combining C, Mn, etc., so the lower limit is 0.
%.

(5) Mo is one of the elements that decreases at a very low rate during flash butt welding, and is used to ensure the hardenability of the weld. Furthermore, since the softening resistance increases during tempering of the chain, high temperature tempering becomes possible and toughness and ductility are improved. The temperature reached by the chain in the icy area is −60
℃, and in order to obtain the specified impact value at the relevant temperature, 0.10% or more is required, and if it exceeds 0.60%, the above effect will be saturated, leading to an increase in cost.
The required amount of Mo was 0.10 to 0.60%.

(6) Acid-soluble Al has the effect of preventing coarsening of austenite crystal grains during quenching of the chain and refining the structure after heat treatment, thereby improving toughness. 0.010% to get this effect
The upper limit was set at 0.100% because a higher content is necessary, and if it exceeds 0.100%, alumina-based inclusions will increase, leading to deterioration of toughness.

(7) The reason for setting the Di value to Di≧0.035D+0.35 is as follows. The strength and low-temperature toughness of the base without welding can sufficiently satisfy the required values even with a steel material with a small Di. However, in the flash butt weld, hardenability is significantly reduced due to the redistribution of C and alloying elements from the solid phase to the liquid phase, resulting in a reduction in these elements. In order to improve this, it is necessary to increase the composition of the steel material in advance, and this formula was established after numerous experiments.

(8) The quenching temperature was set at 900℃ to 1100℃.
If the retention time is less than that, continuous heat treatment with a short retention time will not sufficiently austenitize, resulting in insufficient strength and toughness. In addition, the higher the heating temperature, the more the diffusion of the alloying elements reduced by welding is promoted, and the material quality of the welded part is improved, but if it exceeds 1100℃, the austenite crystal grain size will become coarser, and the low-temperature toughness in particular will deteriorate. It is. Furthermore, the cooling rate after quenching is set to 3°C/sec or more, so that even if the diameter of the chain is large enough to be covered by the present invention, it can be sufficiently quenched to 2/3R (R: radius of the chain material). This is to allow for the entry of

(9) The tempering temperature was set at 550℃ to 700℃.
If it is less than 700°C, the strength becomes too high and the desired low-temperature toughness cannot be obtained, and if it exceeds 700°C, it becomes austenite again and the quenching effect disappears. In addition, to avoid embrittlement after tempering, it is usually
Fast cooling rate of about ℃/s (equivalent to water cooling rate)
However, in the case of the steel of the present invention, embrittlement can be avoided even at an air cooling rate of 0.1°C/s, which corresponds to the air cooling rate for large-diameter materials, so the cooling rate after tempering was set to 0.1°C/s or more.

FIG. 1 is a block diagram showing the manufacturing process according to the present invention, and FIG. 2 is an explanatory diagram showing an example of a manufacturing apparatus that serializes quenching and tempering. 1 is a continuous quenching furnace, 2 is a continuous tempering furnace, and 3 is a continuous chain. 4 indicates a first cooling tank, and 5 indicates a second cooling tank. Note that the welding method for chain making is not particularly limited, but the flash butt method is common.

〔Example〕

Next, the present invention will be explained with reference to examples.

Table 1 shows the steel of the present invention tapped in a converter and used for testing, and KSBC7 for JIS Class 3 chain as a comparison material.
In addition to the chemical composition of 0 steel, the chemical composition of steel disclosed in Japanese Patent Publication No. 51-24967, which is an example of the prior art (the chemical composition is taken from the examples), is also described. The net of the present invention is intended for application to chains for use in icy waters, but when compared with steel for Type 3 chains, both elements Cr and Mo are increased. However, compared to the JP-B-24967 steel, which was intended to be applied to the same ice-water chain, the steel of the present invention has
It is characterized by significantly lower Cr content and the use of a small amount of Mo. In addition, the steel of the present invention contains carbonitride-forming elements such as V, Ti, and
Nb etc. may be added.

Next, steels 1 and 2 of the present invention were used to create a diameter of 78 mm and
A 120 mm steel bar was produced by hot rolling. The steel for Class 3 chain was similarly rolled to a size of 78 mm. This was cut into lengths of 1 meter each and heated in a gas heating furnace at 950°C for 45 minutes, then immediately bent into an annular shape and the ends were joined by flash butt welding to form a ring (link). In order to prevent deformation, studs were fitted in the chain to form a ring, and the chain was connected to form a chain of 150 meters under the conditions shown in Table 2. Hardening back treatment was performed using a heat treatment furnace. On the other hand, according to the detailed description of the invention, the JP-B 51-24967 steel was heat treated as quenched (water-cooled,
Oil-cooling) and normalizing (air-cooling) treatments were carried out.

Next, a JIS No. 14A tensile test piece and a JIS No. 4 impact test piece were taken from the 2/3R (R: radius of the link material) position of the chain link after heat treatment, and the material properties were investigated. Table 3 shows the survey results.

In the case of A and B manufactured by the method of the present invention using the steel 1 of the present invention, quenching was performed by hot water cooling with an average temperature of 70°C, so in the case of other methods of the present invention, the average temperature was 25°C. Although the strength is slightly lower than that of Link C manufactured by water quenching, it fully satisfies the required values as well as the impact value. Note that the link manufactured by method C has the best strength and toughness values because it has been sufficiently quenched. Inventive steel 1 is used in D and E, but D is the case where the steel was air cooled (0.3°C/s) after quenching, and as a result, the tensile strength was not sufficiently quenched and the predetermined value could not be obtained. It also has a low impact value. E is a case where the tempering temperature is too high, and some of the austenite that has been solid-dissolved changes into a martensitic structure by the subsequent rapid cooling, so the strength is increased but the required value of the Shapey impact value cannot be satisfied. F is invention steel 2
This is a case in which a 120 mm steel bar manufactured from a 120 mm steel bar is manufactured by the method of the present invention, and the required values for both strength and impact value can be satisfied. On the other hand, with the current KSBC70 steel for Class 3 chains, the tensile properties and toughness of the base metal part can satisfy the required values due to the H condition, but the impact value of the welded joint is low and the required values cannot be met. Tokko Akira
According to the detailed description of the invention, 51-24967 steel can satisfy the required values for chains for use in icy areas in terms of strength and reduction rate in the 30mm size by water or oil cooling after heating, but the elongation rate is insufficient. In addition, the impact value of the base metal at -60°C satisfies the required value for a chain for use in icy waters, but it is unknown because the impact characteristics of the important welded joint are not described in the chain, and as mentioned above, the impact value of the base metal is As the steel becomes thicker, the strength and toughness (particularly the welded joints and toughness) decrease, so it is expected that the steel cannot be applied as is to chains for use in icy areas.

〔Effect of the invention〕

As described above, the present invention provides a method of manufacturing at low cost a chain for use in icy waters that requires large diameter and high strength and toughness, which has not been possible until now, and its effects are extremely large.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing the manufacturing process according to the present invention, Fig. 2 is an explanatory diagram showing an example of a manufacturing apparatus that serializes quenching and tempering, and Fig. 3 shows the sampling position of the test piece shown in the example. It is an explanatory diagram.

【table】

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Claims (1)

[Claims] 1 C: 0.15-0.35% by weight Si: 0.15-0.50% by weight Mn: 1.00-2.00% by weight Cr: 0.50% by weight or less Mo: 0.10-0.60% by weight Acid-soluble Al: 0.010-0.100% by weight A steel bar with a composition in which the content is within the range of /s or more, and continue to cool down to 550
A method for manufacturing a large-diameter, high-strength chain with excellent low-temperature toughness, which comprises heating the chain to 700°C and then cooling it at a rate of 0.1°C/s or more. Di≧0.035D＋0.35 …(1) However, Di: Hardenability of steel material (in.), D: Diameter of steel bar (mm)