JPH0465884B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for manufacturing a high-strength chain for mooring offshore structures such as offshore oil drilling rigs. (Prior Art) Long chains with a total length of 100 to 2000 m manufactured using steel bars with a diameter of 70 to 160 mm are used for mooring offshore floating structures such as offshore oil drilling rigs and power generation equipment. The tensile strength of the chain is 70~
At 100Kgf/mm ² , welded part vEo≧5Kgf/m or as absorbed energy by Shapey test
A value such as vE _-20 ≧4Kgf/m is specified. As a conventional technique for manufacturing such a chain having high strength and high toughness, for example, Japanese Patent Application Laid-Open No. 59
As shown in No.-159770 or JP-A No. 49-29214, there was a method in which a low carbon steel bar was quenched in cold water and then tempered. (Problems to be Solved by the Invention) When water quenching and tempering a large member as described above, there is a drawback that quench cracking is likely to occur due to small defects on the surface of the steel material or defects in the shape of the welded part. It was hot. When quench cracking occurs, it is necessary to remanufacture the entire series of links, or to remove the broken links and manufacture new ones, resulting in a significant increase in cost and a significant decrease in productivity. (Means and actions to solve the problem) This quench cracking occurs because the cooling rate of the surface layer and the inside differ during quenching, creating a temperature difference, so martensite generated in the surface layer becomes plastic as the temperature decreases. This occurs because the interior expands due to martensitic transformation when the temperature decreases, and tensile stress acts on the surface layer. The present invention is a quenching method in which the steel is cooled in hot water to alleviate the difference in cooling rate between the surface layer and the interior, thereby alleviating the tensile stress generated on the surface of the steel material and making it difficult to generate quench cracks. Assuming that, the amount of C and
We have discovered steel components that do not cause quench cracking in relation to the Ms point. The present inventors conducted experiments to determine the relationship between the amount of C that affects quench cracking and the Ms point when quenching is performed in cold water (24°C) and hot water (94°C) as in the past.
Quench cracking is closely related to C content and Ms point.
In the case of cold water quenching, the C content is 0.28% or more and the Ms point is
While cracking occurs at temperatures below 370℃, in the case of hot water quenching, this is 0.35% or higher and
Temperature reduced to below 350℃. From now on, it will be possible to effectively use C, which is the cheapest element to prevent quench cracking and improve hardenability, and to reduce the cost of steel materials. That is, the gist of the present invention is as follows: C: 0.15-0.35%, Si: 0.15-0.50%, Mn:
1.00~2.00%, Ni: 0.10~0.50%, Cr: 0.50~1.20
%, Mo: 0.10~0.60%, acid soluble Al: 0.010~
Contains within the range of 0.060%, with the remainder consisting of Fe and unavoidable impurities, and is made of a steel material that satisfies the following conditions: (1) Ms point (martensite start temperature) ≧350℃ (2) Di≧1.524D+0.8 This is a method for producing a high-tensile chain, which is characterized in that the chain is quenched in hot water of 90°C or higher, and then tempered at a predetermined temperature of Ac ₁ or lower. However, Ms point (℃) = 550−361×(%C)−39×(%Mn)−
20 (% Cr) - 17 x (% Ni) - 5 x (% Mo) + 30 x (%
Al) Di: Ideal critical diameter (in.), calculated value based on the calculation formula specified by ASTM D: Indicates the diameter (in.) of the steel material. Next, the reason for determining the conditions of the present invention will be described. C: C is the best element to improve the hardenability of steel at low cost, but if the content is less than 0.15%, the required strength cannot be obtained, while if it exceeds 0.35%, it will not be able to be heated by hot water of 90℃ or higher. Also, quench cracking is likely to occur in the base metal and residual C in the flat butt weld.
If the amount increases, the toughness of the relevant part will deteriorate, so
It was set at 0.35%. Si: Si is used for strengthening and deoxidizing.
These effects cannot be expected if the content is less than 0.15%. Moreover, if it exceeds 0.50%, the toughness and ductility will decrease due to an increase in silicate inclusions, so the upper limit was set at 0.50%. Mn: Mn is inexpensive and has relatively little deterioration in toughness and is used as an element to improve hardenability.
If it is less than 2.00%, a sufficient hardening effect cannot be obtained and it is difficult to ensure strength, and if it exceeds 2.00%, quench cracking is likely to occur, so the upper limit was set at 2.00%. Cr: Cr is an element effective in reducing the activity of C in molten steel and reducing decarburization in the welded part of the flash butt, and for this purpose, 0.5% or more is required. On the other hand, the upper limit was set at 1.2% because oxides are likely to form and remain on the joint surface and become a major cause of deterioration of toughness. Ni: Ni is effective in improving the toughness of welds. However, if it is less than 0.10%, it will have little effect, and if it exceeds 0.50%, the cost will increase significantly. For this reason, Ni was set in the range of 0.10 to 0.50%. Mo: Mo is one of the elements whose rate of decrease is extremely small during flash butt welding, and is used to ensure the hardenability of the weld. In addition, this element has a high resistance to temper softening and significantly improves the toughness of welded joints in particular. For this we need more than 0.1% and 0.60
%, the above-mentioned effect becomes saturated and costs increase, so the upper limit was set at 0.60%. Acid-soluble Al: Acid-soluble Al prevents coarsening of austenite crystal grains during chain quenching,
It has the effect of improving toughness because it refines the structure after heat treatment. 0.010% to obtain this effect
If it exceeds 0.060%, alumina-based inclusions will increase, leading to deterioration of toughness, so the upper limit should be set.
It was set as 0.060%. Next, the reason why the Ms point is set to 350°C or higher is because, in relation to the C content, if the temperature is lower than this, quench cracking is likely to occur even when quenching in hot water. Di: When improving the toughness of large-diameter steel materials by heat treatment, it is necessary to use steel components that are sufficiently hardened to the inside. This chain is manufactured by flash butt welding, but most of the alloying elements including C are deviated from the welded surface, so the Di value is significantly lower than that of the base metal. Therefore, the required Di value of the chain must be determined by taking into account the amount of deviation of the weld alloy. The inventors of the present application researched the amount of dealloying in the weld zone and found the following formula between the Di value, the chain size, and the material of the welded parts, so this was determined. Di(in.)≧1.524D+0.8 (Example) Next, the present invention will be explained with reference to Examples. Table 1 shows the chemical components of the inventive steel and comparative steel produced in converter and experimental furnaces, the Ms point and Di value determined from these components, and the diameter (D) of the steel material determined by the Di value. Applicable sizes are also shown. Inventive steels 1 to 3 are rolled to 80 and 120 mm.
(3.1in., 4.7in.) round steel, all comparative steels are 80mm
(3.1in.) round steel. Next, add 1 m of these round bars.
It is cut into long pieces and heated to 970°C and then quenched in cold water (25°C), which was conventionally carried out, and hot water quenched (94°C) according to the present invention.
°C) to investigate the presence or absence of quench cracking. Grilled

[Table] All of the steels of the present invention and comparative steels have a martensitic structure in the surface layer due to surface hardening.
In steels with a Di value of 5 in. or more, a martensitic structure is formed up to 1/2R (middle part). Table 2 shows the results of quench cracking.

【table】

[Table] Invention steels 2 and 3 and comparison steels 5, 7, and 8 are 80;
Although cracks occurred during cold water quenching for both sizes of 120 mm, steels 2, 3, and 7 had C contents and Ms points well within the range of the present invention, and cracks were avoided by cooling with hot water. However, the Ms point and C content of Steel 5 and Steel 8 are outside the scope of the present invention, and cracking cannot be avoided even by cooling with hot water. Next, Steel 1 to Steel 7 are cut into predetermined lengths according to the chain size, heated to 1050℃ in a gas-fired heating furnace, bent into elliptical links, and joined by flash butt welding to form the links. The chain was quenched and tempered. The quenching and tempering treatment is heated to 970℃ in a continuous furnace.
The material was introduced into a hot water cooling layer adjusted to 90°C or higher for quenching, then transferred to a tempering furnace and tempered at 670°C for about 40 to 70 minutes, and the material properties were investigated. A JIS No. 14 tensile test piece and a JIS No. 4 impact test piece were taken from the 2/3R (R: radius) portion of the link and subjected to mechanical testing. Table 3 shows the mechanical test results. Invention steel 1 has a Di value of 7 in., so the diameter is 103 mm.
The following links should be applied to 80, 100 and 120mm to investigate size effects

Three sizes of links were manufactured as shown in [Table]. 80 and 100mm
Good strength and toughness were obtained for both links, but at 120 mm, quenching was insufficient and the strength and toughness significantly deteriorated. Steel 2 also has almost the same Di value as Steel 1, but
Compared to Steel 1, the C content is increased and the Ni content is significantly reduced. Steel 2 also shows good toughness with 80mm links, but like Steel 1, quenching is insufficient and the toughness deteriorates when it comes to 120mm links. Compared to Steel 2, Steel 3 has a higher Mo content and a Di value of 9in.
It is applicable to up to 136mm links.
Both the strength and toughness of the 120mm link made of this steel are good. As described above, even if the elemental composition falls within the scope of the present invention, the material cannot satisfy the required values unless it has a Di value commensurate with the link size. Comparative steel 4, steel 6 and steel 7 have a low Di value of 80mm
Even the links lack toughness. Steel 5 uses a large amount of Mn and Cr and has a Di value raised to about 1 or 2 of the steel of the present invention, and can be applied up to 91 mm from the Di value, but since it does not use Mo, it can be used even with an 80 mm link. Low toughness. (Effects of the invention) As described above, the present invention improves the quenching method and discovers the relationship between quench cracking, C amount, and Ms point in this quenching method, and makes it possible to use C, which is inexpensive and has high hardenability, and is effective. This has made it possible to reduce the cost of steel materials for large diameter chains for offshore structures, and the effect is significant.

Claims (1)

[Claims] 1 C: 0.15-0.35% Si: 0.15-0.50% Mn: 1.00-2.00% Cr: 0.50-1.20% Ni: 0.10-0.50% Mo: 0.10-0.60% Acid-soluble Al: 0.010-0.060 %, the balance being Fe and unavoidable impurities, and satisfying the following conditions: (1) Ms point (martensite starting temperature) ≧350℃ (2) Di≧1.524D+0.8. A method for producing a high-tensile chain, characterized by quenching in the above hot water and then tempering. However, Ms point (℃) = 550−361×(%C)−39×(%Mn)−
20×(%Cr)−17×(%Ni)−5×(%Mo)+30×
(%Al) Di: Ideal critical diameter (in.), calculated value based on calculations specified by ASTM D: Indicates the diameter (in.) of the steel material.