JPS60248867A - Chain having >=70kg/mm2 tensile strength and excellent toughness and its production - Google Patents

Abstract

PURPOSE:To produce inexpensively a large-diameter chain having excellent toughness and high tensile strength by forming a steel material made of the specified compsn. consisting of C, Si, Mn, Cr, P, S, Ca, Al, N and Fe then normalizing the steel material at an adequate temp. and allowing the same to cool. CONSTITUTION:The steel material having the compsn. consisting of 0.25-0.45% C, 0.15-0.55% Si, 1.00-1.90% Mn, <=0.50% Cr, <=0.040% P, <=0.005% S, 0.0010- 0.0080% Ca, 0.025-0.080% sol. Al, 0.0060-0.0200% sol. N, 2-6 Al/N ratio by weight and the balance substantially Fe with inevitable impurities is formed to a chain and is then subjected to a normalizing treatment by heating the same to the AC3 transformation point or above -950 deg.C; thereafter the chain is allowed to cool or is cooled preferably at about 0.1-1.0 deg.C/sec cooling rate of the surface layer from the heating temp. down to the A1 transformation point. The large- diameter and long-sized chain having >=70kg/mm.<2> tensile strength and excellent toughness is thus obtd. at the low cost of the steel material.

Description

Detailed Description of the Invention The present invention relates to a large-diameter high-tensile chain, especially a chain with a diameter of 70 mmφ to 1
60ff1mφ, tensile strength 70Kg/mm' or more, 0
The present invention relates to a chain with excellent tensile strength and toughness, and a method for manufacturing the same, which has characteristics in which the absorbed energy of the base material is 6 kg-m or more and the absorbed energy of the welded part is 5 kg-+n or more at °C.

The above-mentioned chain is known as a type 3 chain (SBC?0) in JIS G3105, and this standard is also stipulated in the shipbuilding standards of each country. Large-diameter 3-type chains with a diameter of 70 mm or more are conventionally manufactured using a quenching and tempering method, which requires extremely high heat treatment costs.Also, since the quenching and tempering method is performed using a patch method, it is difficult to manufacture long chains. It was difficult. On the other hand, due to the oil situation in recent years, there has been a strong demand for energy conservation and reduction in manufacturing costs, and with the increase in oil drilling projects and offshore oil stockpiling plans, there has been an urgent need for large diameter and long chains.

As a measure to save energy and manufacture long chains, manufacturing methods using normalizing heat treatment have been attempted, but in order to obtain the specified strength for large-diameter chains, it is necessary to increase the amount of added C, Mn, etc. Therefore, the toughness decreased and the above specifications could not be satisfied. In addition, chains are manufactured by forming round steel into an annular shape and flash-butt welding the end faces before normalizing, but the austenite grain size becomes coarse in the weld, and this effect remains even after normalizing. However, the toughness significantly decreased, making it difficult to manufacture large-diameter long chains by normalizing.

The present invention makes it possible to manufacture three types of large-diameter chains according to standards without using expensive alloying elements, that is, at a steel material cost as low as that of conventional steel.The features of the present invention are as follows: By adding A and N in combination, the coarsening of austenite crystals KM can be stopped, and the amount of S in the steel can be reduced, and Ca can be roughly added to achieve high toughness. be.

Adding a small amount of A-length (approximately 0.0IO% to O, O3O%) to steel materials forms A-cross-N by bonding with N remaining in the steel as an impurity, thereby refining grains and improving toughness. It is a well-known technique to improve the However, A
The effect of iN on preventing coarsening of crystal grains is at most 1000
This effect is exhibited only at heating temperatures below .degree. C., and when heated to temperatures exceeding this temperature, A31N dissolves in solid solution in austenite and loses its effect of inhibiting crystal grain coarsening.

Therefore, in the case of normalized high-strength chains, it is effective in improving the toughness of the base metal, but the conventional A and N addition methods cannot meet the standards after normalizing for large diameter chains. There wasn't. Furthermore, in cases where the temperature is locally raised to a melting temperature of 1500°C or higher, such as in a flash butt weld, grain refinement by A/N cannot be achieved, so the use of A/N has not been seen in the past. .

However, as a result of various studies, the present inventors found that N and A in steel
It has been found that the toughness of large-diameter high-strength chains after normalizing can be improved by increasing the amount of carbon added and increasing the amount of A/N. Furthermore, by combining this with a reduction in Sl, the toughness can be further improved, especially when the amount of S is reduced to 0.00.
It has been found that a significant effect can be obtained by reducing the value to 51 or less and adding Ca. Next, we examined the toughness of the Franchebutt weld and found that if the 41' weld is left in contact, the toughness deteriorates significantly due to the coarsening of crystals and the generation of a rapidly cooled structure due to cooling after welding, but it is possible to By reducing the amount of S and reheating the welded part to about 850°C and normalizing it, the toughness was able to be significantly recovered. This is because AiN and N dissolved in the steel during the normalizing treatment re-precipitate as AiN.
The effect of controlling the shape of inclusions by addition of Ca and Ca synergistically restores toughness. At this time, if the large amount of N added is not sufficiently precipitated as AiN and remains as a solid solution, it will actually impair the toughness.
In order to precipitate as -N, it is necessary to add a slightly excessive amount of A, but if it is too large, the toughness will also deteriorate, so the ratio of AN to N should be between 2 and 6, which is appropriate.

The present invention was made based on the above new findings, and includes: (1) C: 0.25-0.45%, Si: 0
．． 15-0.55%, Mn: 1.00-1.90%
, Cr: 0.50% or less, P: 0.040%
Below, S: 0.005% or less, Ca: o, 0010
-0.0080y6, acid soluble A-0.025-00
80%, acid soluble N: 0.00130-0.0200χ
and the weight ratio of A and N is adjusted so that the weight ratio A/N is 2 to 6, and the remainder consists essentially of Fe and unavoidable impurities. Chain with excellent toughness of OKg#nm' or more, (2) C: 0.25 to 0.45
%, Si:0. ]5500.55%, Mn:1
．． 00-1. H%, Cr: 0.50% or less, P:
0.040% or less, S: 0.005XJu or less, C
a:0. o01cl~0.0080%, acid soluble
0.025-0.080%, acid soluble N: 0.0060
- A steel material having a composition of C', 0200χ, A cross 7'N: 2 to 6, the remainder being substantially Fe and unavoidable impurities was formed into a chain and heated to a temperature of Ac3 transformation point or higher and 950°C or lower. Tensile strength 7 characterized by cooling afterwards
The gist of the present invention is a method for manufacturing a chain with excellent toughness of 0 kg#nm' or more.

The reasons for limiting the range of components of the present invention will be described below.

Cwo 0.25-0.45i amount! (hereinafter simply written as z)
The reason for this is that if it is less than 0.25%, the required tensile strength can be increased by air cooling or similar cooling. OKg/mm
It is difficult to maintain a content of 0.45% or more, and if the content exceeds 0.45%, the toughness deteriorates significantly and the weldability deteriorates.

Sl is used as a deoxidizing agent in addition to ensuring tensile strength, and if it is less than 0.15%, it will not be possible to ensure strength and prevent deoxidation. In addition, if it exceeds 0.55%, the toughness deteriorates, so the content is set at 0.55% or less.

Mn is used as a reinforcing element with little deterioration in toughness, but if it is less than 1.0%, the required tensile strength cannot be obtained, so it should be 1.0% or more. If it becomes too large, bainite or martensite will be generated even by air cooling or other cooling, resulting in a significant decrease in toughness and further deterioration in weldability.1.
It was set to 90X or less.

Cr is used along with Mn as an element that causes less deterioration in toughness, but if it exceeds 0.5%, the toughness deteriorates like Mn, so it was set to 0.50X or less.

P is 0.040! If the content exceeds this amount, it causes embrittlement and co-degrades strength and toughness, so the content was set at 0.040% or less.

Reducing S tends to reduce the amount of drawn inclusions and improve toughness, but in particular, by reducing Si to 0.005% or less and combining it with the fine grain effect of A-FN, toughness is significantly improved. The upper limit of S was set at 0.005% to satisfy the standard values for the toughness of the base metal and welded part of high-tensile chain.

In the steel of the present invention, Ca acts with S in a composite form with Ai to generate non-plastic sulfides and reduce the amount of elongated inclusions.
It contributes to improving toughness and is added as a sulfide shape control element.Since the effect appears from Ca/S = about 0.2, it should be set at 0.0010% or more, and the amount that can be added in normal converter operation is Since it is 0.0080X, the upper limit was set to 0.0080%.

A rare earth element (REM) or Ce may be used instead of Ca.

Acid soluble A is 0.025% - 0,080X. Tano combines with N and has a grain refining effect, and for steel materials subjected to normal heat treatment, it is generally less than 0.025%. However, it is not sufficient to improve the toughness of large diameter chains, especially when improving the toughness of flash butt welds by normalizing. On the other hand, if it exceeds 0.080%, an excessive amount of alumina-based inclusions will be formed in the steel and at the same time a large amount of alumina-based inclusions will be generated, resulting in deterioration of toughness.

Normal converter steel has a N content of 40 to 60 unless specially controlled.
It contains approximately ppm and is used as is in ordinary heat-treated steel materials. In the steel according to the present invention, acid-soluble N is 0.
0060$ to 0.020% is o, ooe
If it is less than 0.020 oz, the grain refining effect of AiN will not be sufficient, and 0.020! This is because the refinement effect reaches saturation at higher temperatures. An/N2~6 was set because Ai is less than 2.
This is because solid solute N that does not combine with the steel increases, impairing toughness.

On the other hand, if it exceeds 6, solid solution AM that does not precipitate as A-N increases, leading to a decrease in toughness.

In addition, in the chain manufacturing method, after forming the chain,
The purpose of heating above point c3 is to carry out standard treatment to redissolve the component elements into solid solution and refine the crystal grains that have become coarse due to thermal open forming and flash butt welding, and the upper limit temperature is set at 950°C. ℃ because at temperatures exceeding this temperature, austenite crystal grains become coarse and toughness deteriorates.

Cooling after heating to a temperature above the Ac3 transformation point and below 850°C means cooling at room temperature and cooling to the level of cooling using a normal blower. In this case, the surface layer from the heating temperature to the A1 transformation point The cooling rate is 0.1'C/sec ~
Approximately 1.0'C/sec is within the appropriate cooling rate range.

In this case, if the cooling rate is less than 0.1°C/see, the crystal grains will grow and coarsen during cooling, reducing both strength and toughness.
．． An effective cooling rate of 1°C/secC or higher was defined as an effective cooling rate.

When the temperature exceeds 1.0°C/sec, a large amount of martensite or bainite structure appears and the toughness decreases significantly.
The upper limit of the cooling rate is preferably 1.0° C./sec or less. Note that the cooling rate is from 0.1°C/sec'c to 1.0°C/sec'c.
In the range of °C/sec, the strength can be increased without deteriorating the toughness, so the faster the cooling rate, the better.

Next, examples of the present invention will be described.

Table 1 shows the chemical compositions of test steels A, B, and C. A shows the chemical composition of the chain according to the present invention, and B and C show the chemical composition of the conventional chain. Test steel A is a low S and Ca containing steel with increased amounts of A and N, and test steel B is a conventionally used SBC? 0 and C are 5AEI330 steels, which are currently quenched and tempered and used as steel for short Class 3 chains. did.

Figure 1 shows the manufacturing process of a chain using the sample steel. The above-mentioned test steel was forged into a 100 mmφ copper rod, formed into a chain by hot bending, and then flash butt welded and arranged. Thereafter, the ring was collected and used as a test piece, which was then heat treated.

Figure 2 shows the method of heat treatment of the test piece. AM according to the invention
chain and conventional B and C steel chains at 820°C.
Standardization was carried out after heating. In this case, the cooling rate of the steel surface is from 830°C to the A1 transformation point (6
It was 0.5-0.6°C/see up to 20°C). Table 2 shows the mechanical properties.

Chain A according to the present invention has superior tensile strength and absorbed energy compared to chains B and C manufactured using conventional component systems, and can be subjected to quenching and tempering, which are conventional heat treatment methods. Standard values for Type 3 chains (tensile strength ≧70 kg/mm', elongation ≧17.%)
, reduction of area ≧40%, absorbed energy of the matrix at 0°C ≧6 kg-m, absorption capacity of the weld zone ≧5 kg-m
It was found that m) was fully satisfied.

Figure 3 shows the microstructure and grain size;
All structures are ferrite/pearlite structures. The ferrite crystal grain size of the chain according to the present invention becomes a fine grain with JIS grain size No. 8 or more due to the A pattern and the increased amount of N, whereas the grain size of the chain with the conventional component system is JIS grain size No. 7 or more. The grains are coarse in comparison.

Figure 4 shows typical inclusion forms and cleanliness of each chain. Chains B and C contain many elongated inclusions and have poor cleanliness, but chain A according to the present invention has almost no elongated inclusions due to the reduction in the amount of S and the addition of Ca. It is not grained and granulated.

Cleanliness C has been confirmed to be better than conventional chains.

As described above, the present invention has invented a chain and a method for manufacturing the chain that have excellent toughness with a tensile strength of 70 kg/mm' or more by combining fine grain size and high cleanliness, and has achieved a large scale that could not be manufactured conventionally. Since it has made it possible to manufacture long chains with high tension and diameter, its effects are extremely large.

[Brief explanation of the drawing]

Figure 1 shows the manufacturing process of the chain, Figure 2 shows the standard treatment method for test pieces, Figure 3 shows micrographs (X100) showing the structures of the steel of the invention and conventional steel, and Figure 4 shows the invention of the invention. Micrograph diagram showing the shape of inclusions in steel and conventional steel (X100
), FIG. 5 is a graph showing the cooling rate of the steel of the present invention (100 mmφ chain). Patent applicant Representative patent attorney Tomoyuki Yafuki (and 1 other person) Figure 1 Lead 2riJ! Hanba 9 Procedural amendment (method) % formula % 1. Indication of the case 1982 Patent Application No. 192584 2. Name of the invention A chain with excellent toughness having a tensile strength of 70 kg/mm' or more and its manufacturing method 3. Amendment Relationship with the case of a person who does
65) Nippon Steel Corporation 4, Agent 5, Date of amendment order: February 6, 1985 7, Contents of amendment (1) "Figure 4 is . . . photographic diagram (X 100),] is corrected to ``Figure 4 is a sketch diagram showing the shape of inclusions in the steel of the present invention (A) and the conventional steels (B, C).'' (2) Figure 4 of the attached drawings is amended as shown in the attached sheet.

Claims (1)

[Claims] (+) C: 0.25 to 0.45X, Si: 0.
15-0.55%, Mn: 1.00-1.90%
, Cr: o, 50X or less, P: 0.040X or less, S: 0.005% or less, Ca: o, 0010-0
．． 0080%, M soluble A cross: 0.025~o,
oao%, acid soluble N: o, ooeo-o, o2oox
The weight ratio AfL/N of AA and N is adjusted to be 2 to 6, and the remainder is substantially Fe and unavoidable impurities.The tensile strength is 70 K g /
A chain with excellent toughness exceeding m rn'. (2) C: 0.25-0.45%, Si: 0
．． 15-0.551, Mn:i, oo~1.80X
, Cr: 0.5 (H: below, P: 0.040X
Below, S: 0.005% or less, Ca: o, 0010
~0.0080%, acid soluble Ai: 0.025~
0.080%, acid soluble N: 0.0OeO-0,020
0%, Ai/N: 2 to 6, and the balance is formed into a chain by forming a steel material having a composition of substantially Fe and unavoidable impurities, heating it to a temperature above the Ac3 transformation point and below 950°C, and then allowing it to cool. Characteristic tensile strength 70Kg/mtn
A method for manufacturing chains with superior toughness.