JPH01231A - Manufacturing method of high-tensile chain - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) 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 2,000 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. ing. The tensile strength of the chain is 70-100kgf
/--2, as absorbed energy by Charpy test, welding part vBo≧5kgfs+ or vE4゜≧4kg
Values such as fm are specified.

As a conventional technique for manufacturing a chain having such high strength and high toughness, for example, Japanese Patent Application Laid-Open No. 59-15977
As shown in No. 0 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 a large member is subjected to water quenching and tempering as described above, pit cracks may occur due to small defects on the surface of the steel material or defects in the shape of the welded part. was there. 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 interior 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 inside, thereby alleviating the tensile stress generated on the surface of the steel material and thereby preventing quench cracking. Based on the quenching method, we have found steel components that do not cause quench cracking in the relationship between Cl and Ms points.

The present inventors experimentally determined 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 Cjl and Ms points, and in the case of cold water quenching, Cjl and Ms points are closely related.
While cracking occurs when the amount is 0.28% or more and the Ms point is 370°C or less, in the case of hot water quenching, this is alleviated to 0.35% or more and 350°C or less, respectively. From now on, it will be possible to use carbon, which is the cheapest as an element that prevents quench cracking and improves hardenability, making it possible to reduce the cost of steel materials.

That is, the gist of the present invention is that C: 0.15-0.35%, Si: 0.15-0.
0.50%, Mn: 1.00-2.00%, Cr: 0
．． 50-1.20%, Mo + 0.10-0.60
%, acid-soluble A1: contained in the range of 0.010 to 0.060%, the remainder consisting of Fe and inevitable impurities, and under the following conditions (1) Ms point (martensite start temperature) ≧ 350
℃(2) A high-tensile chain made of steel satisfying Di≧1.5240+0.8 is quenched in hot water of 90℃ or higher, and then tempered at a predetermined temperature of Ac or lower. This is the manufacturing method.

However, Ms point ('c) -550-361x (%C) -
39x (Xlln)-20(%Cr)-17X(χN
5) -5x (XMo) + 30X (χAj) DM: Ideal critical diameter (in,) Based on the calculation formula determined by STH (calculated value 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 specified strength cannot be obtained, while if it exceeds 0.35%, it will not work at temperatures above 90℃. Hot water also tends to cause quench cracking in the base metal part, and residual Cl in the flash butt weld increases, leading to deterioration of the toughness of the part (therefore, the upper limit was set at 0.35%).

Sl: Si is used for strengthening and deoxidizing, and if it is less than 0.15%, these effects cannot be expected, and if it exceeds 0.50%, the toughness and ductility will decrease due to an increase in silicate inclusions, so the upper limit is set. was set at 0.50%.

Mn: Mn is inexpensive and causes relatively little deterioration in toughness, and is used as an element to improve hardenability. However, if it is less than 1%, a sufficient hardening effect cannot be obtained, and it is difficult to ensure strength. If it exceeds 2.00%, quench cracking tends 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 flash butt weld zone, and for this purpose, 0.5% or more is required.

On the other hand, the upper limit was set at 1.2% because it is easy to form oxides (which remain on the joint surface and are the main cause of reducing toughness).

Mo: No 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 welded part. In addition, this element has a high resistance to temper softening and significantly improves the toughness of welded joints in particular. For this purpose 0
．． 1% or more is necessary, and if it exceeds 0.60%, the above effect will be saturated, which will only lead to an increase in cost, so the upper limit was set at 0.660%.

Acid-soluble Al: Acid-soluble Al has the effect of preventing coarsening of austenite crystal grains during hardening of the chain and refining the structure after heat treatment, thereby improving toughness. In order to obtain this effect, o, oio% or more is required, and 0.
If Ni exceeds 0.060%, alumina-based inclusions will increase, leading to deterioration of toughness, so the upper limit was set at 0.060%.Addition of Ni is also effective in improving the toughness of welds, and when added, the steel Considering the cost increase, it is desirable to set it to 0.5% or less.

In addition, N is used to refine steel grains or strengthen precipitation, etc.
So-called Micr such as b, Ti, ν or Zr.

It is effective to use A11oy.

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 by hot water quenching.

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. 0-strand chains are manufactured by flash butt welding, but the welding surface is Since the alloying elements in the portion deviate, the Di value decreases significantly compared to the base metal portion.

Therefore, the required Di value of the chain must be determined by taking into account the amount of deviation of the welded part alloy. I found the following formula, so I decided as follows.

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 manufactured in a converter and experimental furnace, the Ms point and Di value determined from these components, and the applicable size of the steel material diameter (D) determined by the Di value. Also shown.

Steels 1 to 4 of the present invention are rolled to 80.120 mm (3, 1
All comparative steels are 80m round steel.
It was made of round steel with a diameter of 3.1 inches (3.1 inches). Next, these round steels were cut into 1 m lengths and heated to 970°C, followed by conventional cold water quenching (25°C) and hot water quenching (94°C) according to the present invention.
The presence or absence of quench cracking was investigated. For Yakini & II weaving, both the invention steel and comparative steel have a martensitic structure in the surface layer due to double quenching, and in steels with a Di value of 5 inches or more, a martensitic structure is formed up to 1/2 R (middle part). are doing.

Table 2 shows the results of quench cracking.

Table 2 Invention $12.3.4 and comparative steel 6.8.9 are 110
．． Cracks occurred due to co-cooled water quenching at 120 am, but steel 2.3.4. The CM and Ms points of samples 8 and 8 are within the scope of the present invention, and the occurrence of cracks can be avoided by cooling with hot water. However, the Ms point and Cl of Steel 6 and Steel 9 are outside the scope of the present invention, and cracking cannot be avoided even by cooling with hot water.

Next, steel 1 to rope 8 are cut to a predetermined length according to the chain size, heated to 1050°C in a gas-fired heating furnace, bent into oval links, and joined by flash butt welding. The chain was quenched and tempered.

The quenching and tempering treatment is heated to 970℃ in a continuous furnace, and then heated to 90℃
The material was introduced into the hot water cooling tank adjusted above and quenched, then transferred to a tempering furnace and tempered at 670° C. for about 40 to 70 minutes, and the material properties were investigated.

The test piece is JIS1 from 2/3R (R: radius) part of the link.
A No. 4 tensile test piece and a JIS No. d impact test piece were taken, and the mechanical test result was 41.

Table 3 shows the mechanical test results.

The present invention 5Ilt has a Di value of 7 inches, so the diameter is 1
03■--Three sizes of links, 80.100 and 120■1, were manufactured to investigate size effects, as applicable to the following links. 80 and 100In
') Good toughness was obtained for both the steel and steel, but at 120 am, quenching was insufficient and the strength and toughness significantly deteriorated.

Steel 2 is almost the same as Steel 1 and has an i value, but compared to Steel 1, it has a higher Cl and a significantly reduced Ni content. Steel 2 also shows good toughness with 80 dragon links, but with 12
If it becomes a zero dragon link, like Steel 1, quenching will be insufficient and the toughness will deteriorate.

Steel 3 has a higher Mo content than Steel 2, and has a Di value of 9 inches.

It is applicable to up to 136 In links.

Both the strength and toughness of the 120m wood-like link are good.

Steel 4 has lower C and Cr content than Steel 3, and also has N
By not using i and lowering the Di value to the same level as steel l and steel 2, an 80 l link can satisfy both strength and toughness, but
Toughness is particularly deteriorated in 100g and 120g.

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 5, t14'l and w484;! Di4a strength is low, and even an 80m link lacks toughness.

Steel 6 uses a large amount of Mn and C, increasing the old value to about 1.2 and 4 of the steel of the present invention, and is applicable to Di values up to 91■, but it does not use Mo. Because there is no 80 dragon link, the toughness is low.

(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 enables the use of inexpensive and highly hardenable C. This has made it possible to reduce the cost of steel materials for large-diameter chains for offshore structures, and the effect is significant.

Patent applicant: Nippon Steel Corporation

Claims (1)

[Claims] C: 0.15-0.35% Si: 0.15-0.50% Mn: 1.00-2.00% Cr: 0.50-1.20% Mo: 0. Contains 10-0.60% acid-soluble Al: 0.010-0.060%, with the remainder consisting of Fe and unavoidable impurities, and under the following conditions (1) Ms point (martensite starting temperature) ≧ 350°C (
2) A method for manufacturing a high-tensile chain, which comprises quenching a chain made of steel satisfying Di≧1.524D+0.8 in hot water at 90° C. or higher, and then tempering the chain. However, Ms point (°C) = 550-361 x (%C) -39
×(%Mn)−20×(%Cr)−17×(%Ni)−
5×(%Mo)+30×(%Al) Di: Ideal critical diameter (in.) Calculated value based on the calculation specified by ASTM D: Indicates the diameter (in.) of the steel material.