JPS6089551A - Chain steel having high strength and toughness - Google Patents

Abstract

PURPOSE:To improve the strength and toughness by adding prescribed percentages of C, Si, Mn, Cr, Mo, Sol.Al and N and regulating the amounts of P and S among inevitable impurities to prescribed values or below. CONSTITUTION:The titled steel consists of 0.15-0.3% C, 0.1-0.5% Si, 1.3-2.5% Mn, 0.5-1.5% Cr, 0.1-0.6% Mo, 0.01-0.06% Sol.Al, 0.003-0.02% N and the balance Fe with inevitable impurities including <=0.01% P and <=0.005% S and satisfies an equation C%+(Mn%+Cr%+2Mo%)/3>=1.25X10<-2>sigmaB+0.11 [where sigmaB is the tensile strength (kgf/mm.<2>) of a chain]. The steel has superior strength and toughness.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a copper phase for chains that exhibits high strength and excellent weld toughness even in low temperature environments such as -301C.

In recent years, with changes in the energy situation, efforts to develop new energy resources have become active in various parts of the world, and as onshore development resources are depleted, attention is also being focused on offshore oil fields. In recent years, development using oil drilling rigs has begun to take place in a wide range of areas, centered around the continent, from the south to the North Sea.

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. Recently, offshore structures such as rigs have started to become more and more dog-shaped, so the chains used to hold them have a diameter of 6o to 160mm.
However, because it is not possible to increase the diameter further due to loading weight restrictions, there is a need for something with even higher strength.

By the way, large-diameter chains are manufactured by cutting hot-rolled steel bars into predetermined lengths, forming them into annular shapes, and welding the end faces together, and then heat-treating them to obtain the required mechanical properties. It goes without saying that quenching and tempering the chain after it has been formed is the best way to obtain high strength and toughness.

However, for example, if the tensile strength is 90 kgf/mi
The mooring chains for the above rigs, etc., which require such high strength, require a high ■ Compared to plate materials that require hardenability, which can be completely cooled from the top and bottom, chains have problems such as it is difficult to cool them sufficiently, and it takes a long time to obtain the desired hardened structure. Therefore, the content of alloying components needed to increase the value had to be increased. However, increasing the content of hardenability-enhancing components such as carbon leads to deterioration of toughness.Moreover, as mentioned above, chains are flash butt welded after forming steel bars, but increasing the content of hardenability-improving components such as carbon leads to deterioration of toughness. There was a concern that a high content would cause a new problem in that there would be an increased risk of weld cracking.

For this reason, the present inventors first developed a low-C, high-Mn-based steel with a low C content to improve weldability and base metal toughness at 0°C. In addition, Cr and Mo were added to improve the hardenability of the steel for high-strength chains.'' (Japanese Patent Application No. 58-34833)

However, recently, the frequency of oil drilling in extremely cold regions with poor conditions has become more and more frequent. , for example, the tensile strength is 90 kgf/, (ii
In view of the above, there has been a demand for a high-strength chain steel with even better low-temperature toughness, with a Charpy impact absorption energy value of 5 kgf-m or more at the welded part at -30°C. Even with steel,
It has not been possible to fully meet the demands of these extremely low temperature environments.

From the above-mentioned viewpoints, the present inventors have developed the previously proposed chain steel material in order to realize a steel material for chains that has both high strength and high toughness and exhibits even better performance than conventional materials. As a result of continued research into the factors that hinder the further improvement of the low-temperature toughness of K, and the means to remove them, we have found that (a) currently, we have carried out refining with special attention to reducing the S and P contents; Also, when normal refining is adopted, p: Q, 04 to 0.015% (hereinafter, CH indicating the component proportion is in weight), S: 0.04 to 001%
It has been acknowledged that the limit is to obtain a steel that exhibits a value of about It was thought that the effect of improving the low-temperature toughness of the copper phase for chains was almost sufficient, and that even if the contents of P and S were lowered further north, no significant practical effect would appear.
If measures are taken to reduce the P content in steel to 0.010% or less, or to reduce the S content to 0.05% or less, or both, by special refining etc., surprisingly, (b) Even if P and S in chain steel are reduced as much as possible through special refining, etc., the low-temperature toughness of chain steel will be significantly improved. A trace amount of sulfide-based nonmetallic inclusions will inevitably remain, but when a specific amount of Ca or rare earth elements is added to the steel material, the morphology of the imaged nonmetallic inclusions changes (spheroidization). The adverse effect on toughness is extremely reduced, and the toughness of welded parts at low temperatures is significantly improved. When a specific amount of one or more of V, Nb, and T is added to chain steel that has been modified to form sulfide-based nonmetallic inclusions, the crystal grains of the steel material become image layers 11 and their The findings as shown in (a) were obtained that the toughness was further improved.

This invention was made based on the findings described in Section 2, and the copper phase for the chain was made of: C: 0.15 to 0.30%, Si: 0.10 to 050%.
.

Mn: 1.30-2.50%, Cr: 0.5
0-1.50%.

Mo: 0.10-060 Chi.

Sot, A+! : 0.0 1 0~0.0 6 0
%.

N: 0.003 to 0.020%, and if necessary, one or more of V, Nb, and Ti: 0.20% or less in total amount.

Ca': 0.010% or less.

Rare earth elements: Contains one or more of the following with a coefficient of 010, Fe and unavoidable impurities: Contains the remainder, and has the formula [however, σB, chain tensile strength kgf/+uf
)) and the content ratio of at least one of P and S in the inevitable impurities is P:0.01
Less than 0%.

S: By configuring the composition to satisfy the condition of 0.005% or less, it has the characteristics of providing high strength and excellent weld toughness at low temperatures (e.g. -30°C). It is.

Note that ordinary quenching and tempering is sufficient for the tempering treatment of the steel material for chains of this invention, but if quenching and tempering is performed after normalizing treatment or primary quenching treatment, flash/gut welding may occur. The coarse grains become finer.
This is a desirable method because it improves the toughness of some parts of the weld.

Additionally, since chains are normally used in seawater, corrosion may be a problem, but in such cases it is effective to add one or both of Cu and Ni to the steel material. be.

Next, in the #l material for chains of the present invention, the reason why the content ratio of each component element and the carbon equivalent represented by the formula are limited as described above will be explained.

a) C The C component has the effect of ensuring the hardenability of the steel material and maintaining its strength and toughness, but if its content is less than 0.101, the desired effect cannot be obtained, and on the other hand, If the content exceeds 0.30%, the toughness deteriorates and the probability of cracking in the weld increases. Therefore, the content was set at 0.10 to 0.30%.

b) 5i The S1 component has the effect of ensuring the strength of the copper material and also acts as a deoxidizing agent, but if its content is 0
．． If it is less than 101, the desired deoxidizing effect cannot be obtained, and nonmetallic inclusions in the steel material increase, resulting in deterioration of toughness. On the other hand, if the S1 content exceeds 0.50%, it will still cause toughness deterioration, so the S1 content should be reduced to 0.10%.
It was set at ~0.50%.

c) Mn Mn is an essential component to ensure the desired hardenability, but if its content is less than 130%, it will not be possible to ensure sufficient hardenability, while if it is included in excess of 250%, the toughness of the steel material will deteriorate. and deteriorates weldability, so 114n
The content was determined to be 130 to 2.50φ.

d) Cr The Cr component has the effect of improving the toughness of steel materials to some extent and increasing the hardenability, but when the content is 0.
If the content is less than 50%, it will be difficult to secure the desired hardenability for large diameter chains, while if the content exceeds 150%, the effect of improving toughness will be small and weldability will deteriorate. From 0.50 to 1.50%
It was determined that

e) M.

The Mo component is an extremely effective element for improving the toughness and ensuring hardenability of steel materials, but if the content is less than 0.10%, the above effects cannot be expected; on the other hand, if the content exceeds 0.60%. If the hardenability becomes excessively high, the inconvenient result of increasing the cost will be brought about. Therefore, the content is set as O, ], 0 to 060.

f) sot, Ae sol, and AQ acid components have the effect of adjusting the grain size of steel materials and making them finer in addition to deoxidizing, but if the content is 0.0
If the content is less than 10%, it will not be possible to obtain a sufficient grain refining effect, causing deterioration of toughness, while if the content exceeds 0.60%, alumina-based nonmetallic inclusions will increase in the steel, which will also cause deterioration of toughness. Therefore, the sot and AQ contents were determined to be between 0.010 and 0.060.

g) N The N component contains AQ, which is effective for adjusting grain size by combining AQ.
It has the effect of precipitating N, but its content is 0.003%.
If it is less than that, the above-mentioned effect will not be achieved sufficiently, and no m-graining effect can be expected. On the other hand, if the content exceeds 0.020, solid solution N increases and the toughness of the copper phase deteriorates, so the N content was set at 0.003 to 0.020%.

h) v, Nb, and T1 All of these components have the effect of precipitating carbides, carbonitrides, or nitrides in the steel, refining the crystal grains of the steel material, and improving the toughness. One or more types may be added if necessary, but the total content of these substances may be 0.
．． Even if it exceeds 20%, not only will the above effect not be obtained any further, but the cost of steel material will increase.
The total content of these elements was determined to be 0.020% or less.

It should be noted that these components also exhibit the effect of improving toughness with a very small amount of content, but in order to make the effect more noticeable, it is preferable that the total content is 0.034 or more.

1) Ca and rare earth elements These components have the effect of cleaning the steel, as well as changing the form of non-metallic inclusions and further improving the low-temperature toughness of the weld, so that they are more However, if the Ca content exceeds 0.001% or the total content of rare earth elements exceeds 0.10%, the toughness of 1 will deteriorate. Weight: O, O
' 1% or less, and rare earth element content 1do1o% or less. It should be noted that although both Ca and rare earth elements exhibit the effect of improving low-temperature toughness with extremely small amounts of content, in the case of Ca, a content of 0.0005 or more produces a more significant effect. , 0.00 for rare earth elements
The content is preferably 5% or more.

, ]) P P is an impurity that cannot be avoided in steel manufacturing, and S
It also has a detrimental effect on the toughness of steel, especially low-temperature toughness.
It is an element. However, special treatments during the steelmaking stage, such as double blowing and slag adjustment, lowered the P content to 0.01.
When the S content is lowered to 0% or less, the impact fracture transition temperature at high strength is lowered and low-temperature toughness is significantly increased. Therefore, the P content ratio is set to 0.0-10.
% or less.

k) s S, like P, is an unavoidable impurity in the production of steel materials, but it is an element that exerts a detrimental effect on the toughness of steel materials. Particularly in welded parts, the metal flow is almost perpendicular to the rolling direction of the preparation, so the sulfide-based nonmetallic inclusions extended in the rolling direction increase the toughness of the part.
You will be harmed. However, if the S content is reduced to 0.0O15% or less by special treatments such as VAD hot metal desulfurization, the low'7iFA toughness at high strength will increase significantly, even if the P content is normally smelted. Therefore, the S content was determined to be 5% or less for O and OO, since it shows excellent low-temperature toughness even if it has a value similar to that of copper.

In order to ensure a good impact absorption energy value for the welded part of the steel material for chains (r-1, the higher the strength of the steel material, the more alloying elements are added, ff1J), it is important to increase the value of the formula. According to the level σ1. [i.e., the tensile strength of the chain 1g17m+7)], the previous MQ formula is
B' +0.11) JJ needs to move up. That is, the value of the above formula is [1,2'5 X 10-2Xσ
B+0.11] If unmixed, the desired low-temperature toughness of the weld zone cannot be ensured.

Next, the present invention will be specifically explained using examples and comparing with comparative examples.

Example First, a 70-ton converter and a 3-ton electric furnace were used to melt steel with the composition shown in Table 1 by blowing twice and desulfurizing hot metal using VAD, and then hot rolling. A round steel bar with a diameter of 84 mm was obtained. Next, after cutting this, it was formed into a chain by hot bevelling, and then flash butt welded and organized.

Then, after deburring the welded part, inserting the stud, and then heating it to 900°C, holding it for 3.5 hours, and cooling it with water, followed by tempering at the temperature shown in Table 2. Processed and manufactured into chains.

From each chain produced in this way, the following test pieces were collected: a tensile test piece with a diameter (D) of 14φ and a gauge length of 5D, and an impact test piece: JI No. 84 Charpy test piece. Let's investigate its mechanical properties.

The obtained results are also shown in Table 2.

From the results shown in Table 2, it is clear that non-inventive steel Control ~ 12, in which the amount of alloying elements added was adjusted to a predetermined value according to the tensile strength, and sufficient dephosphorization and desulfurization were performed, achieved high strength. It has a shock absorption energy value of 5 kg at -30℃.
: It is clear that a good weld zone toughness of more than r-1Ti has been obtained.On the other hand, in comparison steel materials 13 to 16 whose compositional content is outside the range of the present invention, the impact strength at low temperatures of the weld zone is It is clear that the characteristics have deteriorated.

Further, steel materials of the present invention 5 to which Ca or rare earth elements are added
7. And, in the unexploded E) J steel materials 8 to 2 to which V, Nb, or T1 were added, the impact absorption energy value at 130°C of the welded part was 1 to 2 k compared to those to which these elements were not added.
gr-m, and the effect of its addition was also confirmed.

As mentioned above, according to the present invention, extremely high strength and
It is possible to obtain a copper phase for large-diameter chains at a low cost that has excellent toughness even at low temperatures, and it is expected to play an extremely useful role in resource development under harsh conditions. Industrially useful effects are brought about.

Applicant: Sumitomo Metal Works Co., Ltd. Agent Tomi 1) Kazuo Haga 1 person

Claims (1)

[Claims] (1) C: 0.15 to 0.30. Si: 0.10-0.50%. Mn: 1.30-2.50. Cr: 0.50-1.50% + Mo: 0.10-0.60%. soLM: 0.010-0.060%. N: 0.003-0.020%. Fe and unavoidable impurities: consisting of the remainder 9, and the formula [However, thin; chain tension 6 insects vgf/mff
1)] and the content ratio of at least one of P and S among the inevitable impurities 9 is P:0.
．． 010% or less. S:0. A high-strength, high-toughness steel for chains, characterized by having a chemical composition (weight %) that satisfies the condition that the content is O05 or less. (2) C: 0. :15~0.30 person. Si: 0.10-050 candy. Mn: 1.30-25Q%. Cr°050~Co50%. Mo: 0.10-0.60%. sob-M: 0.010-0.060%. Contains N 0003 to 0.020%, and also contains one or more of V, 1-sushi, and Ti: 0.20% or less in total amount, Fe and unavoidable impurities! The content ratio of at least one of P and S in the inevitable impurities is P. :0010 rice cake or less. S: A high-strength, high-toughness steel for chains characterized by having a composition (with a weight of 918% or more) that also satisfies the condition of 0,005% or less. (3) C: 0.15-030 rice cake. Si: 0.10-050 Rice cake. Mn1.30-2.50%. Cr: 0.50 to 1.50%. Mo: 0.10-0.60%. 5ol-A1. : 0.010-0060%. N', 0. Contains OO3 to 0.020%, and further Ca: 0010% or less. It also contains one or more of rare earth elements, O1O or less, and is composed of Fe and unavoidable impurities and the rest, and has the formula [where σB is the tensile strength of the chain kgf/m+f]
f1) : ] and the content ratio of at least one of P and S in the inevitable impurities or P:0
010% or less. A high-strength, high-toughness steel for chains, characterized by having a chemical composition (in weight %) that satisfies the condition of 8.0005% or less. (4) C: 0.15-0.30%. Sl, 0] 0-050 section. 1vin,' 1.30-250%. Cr: 050-1.50%. lt, 4o:o, 10-060%. sot, AQ: 0010-0060%. N: 0. OO3 to 0.020%, and one or more of V, Nb and T]: 020% or less in total amount. In addition to this, Ca: 0.010% or less. Rare earth element Contains one or more of the following 010, consisting of Fe, unavoidable impurities, and residue, and having the formula [however, σB chain tensile strength l<gf/m+
A)] and the content ratio of at least one of P and S in the inevitable impurities is P:00
Less than 10 inches. S:0. A high-strength, high-toughness steel for chains, characterized in that it has a composition (in weight % or more) that satisfies the condition of OO, 5% or less.