JPH0772324B2 - Steel for low yield ratio chains - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention relates to a steel material for chains, in particular, high strength, high toughness, and a low yield ratio, which facilitates fixing of studs in the chain manufacturing process. The present invention relates to a steel material for chains that can be used with studs.

(Prior Art) In recent years, along with changes in energy circumstances, the movement to develop new energy resources has become active in various parts of the world. Under these circumstances, as land-based development resources have been depleted, attention has focused on offshore oil fields, and development using oil drilling rigs has focused on the area near the continental shelf from the south to the north sea. It has come to be performed in a wide range of areas up to.

Along with the increase in offshore structures represented by the above-mentioned offshore oil drilling rigs, the demand for large-diameter chains used to moor them is also increasing. Since offshore structures such as the above have begun to show a tendency toward larger sizes, thicker chains with a diameter of 60 to 160 mm have been used for mooring them. In addition, since it is impossible to increase the diameter further from the viewpoint of the cumulative weight limitation, for example, a tensile strength of 90 kgf / mm ² or higher has been demanded.

Furthermore, recently, the frequency of oil drilling in extremely cold regions where the conditions are poor has been increasing more and more, and along with this, steel materials for chains that exhibit sufficient toughness even in lower temperature environments have been developed. It has come to be expected.

In addition, the stud loosening has been regarded as a serious problem due to the recent analysis results of the cause of breakage of the mooring chain. That is, it was clarified that when the stud is loose, bending stress is applied to the chain link when the chain is used, which causes fatigue failure. From this viewpoint, various means for preventing loosening of studs have been studied and implemented in the manufacture of chains in recent years. Among these, the most practical method, which is generally used at present, is to fix the studs by subjecting the chain to chain-forming and heat treatment, then applying a predetermined tensile load to the chain and plastically deforming the chain. To do. In order to perform this method easily and reliably, for example, a yield ratio of 92
The demand for a low yield ratio of less than 100% has been added.
That is, in the case of a steel material having a low yield ratio, the chain is easily plastically deformed by a predetermined tensile load, and the stud can be securely fixed.

(Problems to be solved by the invention) By the way, a large-diameter chain is manufactured by cutting a hot-rolled steel bar into a predetermined length and forming it into an annular shape, and then flash-butt welding the end face to the stud. Needless to say, it is usual to obtain the required mechanical properties by heat treatment, and as a means to obtain high strength and high toughness, quenching and tempering treatment in the state of being formed into a chain is the optimum. is there.

However, for example, in order to obtain high strength with a tensile strength of 90 kgf / mm ² or more, and to obtain sufficient low temperature toughness, high quenchability is imparted to the steel material, and the chain is completely heat treated, that is, martensite. It was necessary to perform a tempering treatment after forming the site, and therefore the yield ratio was inevitably high. That is, in general, it must be said that high strength and low yield ratio are contradictory characteristics.

Therefore, an object of the present invention is to provide a steel material for a chain having both high strength and high toughness and a low yield ratio.

Specifically, the object of the present invention is that the tensile strength is 90 kgf / mm ² or more, the low temperature toughness is excellent, and the yield ratio is 92.
% Steel or less to provide a steel material for tuning.

(Means for Solving the Problem) Here, the inventors of the present invention continued various studies to realize a steel material for chains having both high strength and high toughness and a low yield ratio, and found that the C content was increased. The Cr content and the Cr content have a great influence on the yield ratio after quenching and tempering of the steel material, and the yield ratio decreases as the content of both components increases. In particular,
It has been found that when the contents of both components satisfy the following equation, a remarkable low yield ratio can be realized.

8.5 × C (%) + Cr (%) ≧ 2.85 (%) Further, the present inventors have the following knowledge based on abundant research and manufacturing results on steel materials for chains: (a) C content Since increasing the amount causes increase in susceptibility to quenching cracks and deterioration in toughness, it is necessary to keep the C content at 0.35% or less.

(B) Mn, Mo in order to provide the steel with sufficient hardenability
Is essential, and especially addition of Mo is very effective in improving toughness.

(C) By adding V, Nb, and Ti to the steel material, the crystal grains of the steel material are refined and the toughness is further improved.

(D) Addition of Ca or rare earth element (REM) to the steel material
The morphology of sulfide-based non-metallic inclusions is changed (spheroidized), which further improves toughness.

This invention was made based on the above findings,
% By weight, C: 0.35% or less, Si: 0.05 to 0.50%, Mn: 0.50 to 2.50%, P: 0.040% or less, S: 0.040% or less, Cr: 5.00% or less, Mo: 0.30 to 0.80%, sol .Al: 0.010 to 0.060%, N: 0.003 to 0.020%, and 8.5 x C (%) + Cr (%) ≥ 2.85 (%) Furthermore, if necessary, one or more of V, Nb, and Ti are added together. The content is 0.20% or less, and / or one or more of Ca: 0.010% or less and REM: 0.10% or less, and the steel composition is composed of a composition satisfying the condition that the balance is Fe and inevitable impurities. Tensile strength 90kgf / mm ² or higher, yield ratio 92% or less, high strength, high toughness and low yield ratio, and can easily fix studs in the chain manufacturing process. Low yield ratio chain steel, especially for chains with studs. It is a steel material.

Incidentally, since the chain is usually used in seawater, there may be a problem of corrosion, but in such a case, it is effective to add Cu to the steel material.

A typical example of the above-mentioned "chain" is a chain with studs, but it goes without saying that it may be a general ordinary chain.

(Operation) Next, the reason why the content ratio of each component element in the steel material for chains of the present invention is limited as described above will be described.

C (Carbon): The C component has an action of ensuring the hardenability of the steel material and maintaining the strength and toughness, but further increasing the content increases the tempering temperature for obtaining the predetermined strength. This makes it possible to reduce the yield ratio. However, 0.35%
If the content exceeds 0.35, the toughness deteriorates and the probability of cracking in the weld increases, so its content should be 0.35.
Defined to be less than or equal to%. Preferably, it is 0.15 to 0.30%.

Si (Si): The Si component not only secures the strength of the steel material but also acts as a deoxidizer, but its content is 0.05.
If it is less than%, the desired effect on the deoxidizing action cannot be obtained, and nonmetallic inclusions in the steel material increase, leading to deterioration of toughness.
On the other hand, if the content of Si exceeds 0.50%, the toughness deteriorates. Therefore, the Si content is set to 0.05 to 0.50%. Preferably, it is 0.15 to 0.40%.

Mn (manganese): Mn is an essential component for ensuring the desired hardenability, but if its content is less than 0.50%, sufficient hardenability cannot be secured, while if it exceeds 2.50%, Since it will deteriorate the toughness and weldability of the steel material, the Mn content is
It was defined as 0.50 to 2.50%.

P (phosphorus): P is an unavoidable impurity in steel production,
If the content exceeds 0.040%, the toughness deteriorates beyond the allowable limit, so the content of P was set to 0.040%. When the P content is decreased to 0.020% or less, the shock wave front transition temperature at high strength is lowered and the low temperature toughness is remarkably increased.
It is also effective to set it to 020% or less.

S (sulfur): S is an unavoidable impurity in steel production like P, but if the content exceeds 0.040%, the toughness will deteriorate beyond the allowable limit, so the S content. Is 0.04
It was set at 0% or less. If the S content is reduced to 0.010% or less by a special treatment such as a hot metal desulfurization method, the low temperature toughness at high strength remarkably increases.
It is also effective to set the content ratio to 0.010% or less.

Cr (Chromium): The Cr component improves the toughness of steel materials to some extent, and
It has the effect of increasing hardenability, but further increasing its content has the effect of lowering the yield ratio.

However, even if the content exceeds 5.00%, the yield ratio lowering effect is small, and the weldability deteriorates. Therefore, the content is defined as 5.00% or less. The lower limit is over 1.50% to ensure high strength and low yield ratio.
It is preferably more than 1.50% and not more than 3.50%.

Mo (molybdenum): The Mo component is an extremely effective element for improving the toughness and hardenability of steel materials, but if the content is less than 0.30%, the above effect cannot be expected, while 0.80% If it is contained in excess of 10%, the hardenability becomes excessively large, resulting in an inconvenient result that the cost increases, so the content was set to 0.30 to 0.80%. Preferably 0.40 to 0.70%
Is.

sol.Al (oxidizing soluble aluminum): The sol.Al component has the function of adjusting the crystal grain size of the steel and fine sulphurization in addition to the deoxidizing effect, but if the content is less than 0.010%, a sufficient fine sulphurization effect is obtained. Since it cannot be obtained, it causes deterioration of toughness, and on the other hand, if the content exceeds 0.060%, alumina-based non-metallic inclusions in the steel material increase and also cause deterioration of toughness. The amount was set to 0.010 to 0.060%.

N (nitrogen): The N component has a function of binding Al and precipitating AlN which is effective for adjusting the crystal viscosity, but if the content thereof is less than 0.003%, the above function is not sufficient and the fine sulphurization effect is exerted. I can't expect. On the other hand, if the content exceeds 0.020%, the solid solution N increases and the toughness of the steel material deteriorates. Therefore, the N content was set to 0.003 to 0.020%.

8.5 × C (%) + Cr (%) value: According to the present invention, as is clear from the graphs of Examples and the attached drawings described later, a yield ratio of 92% or less is realized. Is effective to increase the content of C component or Cr component, especially 8.5 × C (%) + Cr
It is necessary to set the value of (%) to 2.85 (%) or more.

Regarding the effect of increasing the content of C component and Cr component, both increase the tempering softening resistance of the steel material, and thus it becomes possible to increase the tempering temperature for obtaining a predetermined strength,
It is considered that this has an advantageous effect on lowering the yield ratio.
However, there are many unclear points about the detailed mechanism.

By the way, the content of C component or Cr component is 8.5 × C
If (%) + Cr (%) ≥ 2.85 (%) is satisfied, it is not necessary to set the lower limit of each content from the viewpoint of lowering the yield ratio, but from the viewpoint of ensuring the hardenability of steel materials, When the steel of the present invention is applied to the diameter chain, it is preferable in practice that the C content be 0.15% or more and the Cr content be 0.80% or more.

V, Nb, Ti (vanadium, niobium, titanium): All of these components have the effect of precipitating carbides, carbonitrides or nitrides in the steel to refine the crystal grains of the steel and improve the toughness. Therefore, one or more kinds of additives are added if necessary, but the total content of these is
If the content exceeds 0.20%, not only the above effect cannot be obtained, but also the steel material cost will be increased, so the total content of these elements was set to 0.20% or less. It should be noted that these components exert the toughness improving effect with a very small content, but in order to make the effect more remarkable, the total content is preferably 0.03% or more.

Ca (calcium) and rare earth elements (REM): These components, together with the action of making the steel cleaner,
Since it has the effect of further improving the low temperature toughness of the weld by changing the form of non-metallic inclusions, it is necessary to add one or more if necessary, but the Ca content exceeds 0.010% or the total content of rare earth elements is increased. If the content exceeds 0.10%, the toughness deteriorates. Therefore, the Ca content is set to 0.01% or less and the rare earth element content is set to 0.10% or less.
It should be noted that any of the components of Ca or rare earth elements exerts a low temperature toughness improving effect with a very small amount of content, but in order to obtain a more remarkable effect Ca is 0.0005% or more,
In the case of rare earth elements, the content is preferably 0.005% or more.

Next, the present invention will be specifically described with reference to Examples and comparison with Comparative Examples.

Example 1 FIG. 1 of the accompanying drawings shows that a steel bar having a diameter of 84 mm obtained by hot rolling or hot forging each of the sample steels having the composition shown in Table 1 is drawn by quenching and tempering under normal conditions. Strength 100k
FIG. 6 is a diagram showing the relationship between the C component and Cr component contents and the yield ratio when adjusted to gf / mm ² .

As is clear from the diagram, 8.5 x C (%) + Cr
By satisfying (%) ≥ 2.85 (%), the yield ratios are all below 92%. In the figure, the yield ratio is shown, and the number with ○ is the steel type number in Table 1.

Example 2 In this example, a 70 ton converter or a 3 ton electric furnace is used.
After the steel having the composition as shown in Table 2 was melted, a round bar steel with a diameter of 76 mm was obtained by hot rolling. Next, after cutting this, it was formed into a chain by hot bending and subjected to flash butt welding to form a ring. After deburring the weld, insert the stud, then heat it to 900 ° C, hold it for 30 minutes, and then cool it with water.Take it so that the tensile strength will be approximately 95 kgf / mm ² . By tempering at the temperatures shown in Table 3, studded chains were manufactured.

The following test pieces from each chain manufactured in this way: Tensile test piece: Diameter (D) 14mm, gauge length 5D from the base material, Impact test piece: JIS No. 4 Charpy test piece Each was sampled from the weld and the mechanical properties were investigated. The obtained results are also shown in Table 3.

Furthermore, a tensile load of 610 tons was applied to the chains manufactured from the steels 1, 2 and 17 in Table 2, and the fixing state of the studs after unloading was investigated. The results obtained are shown in Table 4.

As is clear from the results shown in Table 3, 8.5 × C
(%) + Cr (%) value of the invention steel of 2.85 (%) or more 1 ~
Both 16 and Comparative Steel 19 have a low yield ratio of 92% or less. On the other hand, Comparative Steels 17 and 18 are 8.5 × C
The value of (%) + Cr (%) is out of the range of the present invention,
The yield ratio is as high as 93% or higher.

Furthermore, the steel materials 3 to 1 of the present invention to which V, Nb or Ti is added
4, in the steel materials 8 and 9 to 16 of the present invention to which Ca or a rare earth element is added, the Charpy absorbed energy at -20 ° C of the welded portion is increased by about 1 to 3 kgf-m as compared with other steels, and the effect Was confirmed.

On the other hand, as is clear from the results shown in Table 4, the steels 1 and 2 of the present invention having a low yield ratio have no stud loosening after a predetermined tensile load is applied, and its effect is confirmed. Was done.

In Table 2, Comparative Steel 20 is SBC 70 stipulated in JIS G3105, which is normally used for chains with a tensile strength of 70 kgf / mm ^2.
Although it is an equivalent steel, this steel type has a low content of alloying elements and could not achieve a high tensile strength of 90 kgf / mm ² or more at normal tempering temperatures. Furthermore, the toughness of the welded part is also significantly reduced.

(Effect of the Invention) As described above, according to the present invention, extremely high strength,
Combined with excellent toughness even at low temperatures,
In addition, it is possible to obtain a steel material for large diameter chains that has a low yield ratio and that can easily fix studs in the chain manufacturing process at low cost, and plays an extremely useful role in resource development under severe conditions. Industrially useful effects such as the expectation of fulfillment can be brought about.

[Brief description of drawings]

FIG. 1 is a graph showing the criticality of the effect of the formula: 8.5 × C (%) + Cr (%) on the yield ratio.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kazuhiko Nishida 5-15 Kitahama, Higashi-ku, Osaka City, Osaka Prefecture Sumitomo Metal Industries, Ltd. (56) References JP-A-60-89551 (JP, A) JP-A-59 -159972 (JP, A) JP-A-59-159970 (JP, A)

Claims (4)

[Claims] 1. By weight%, C: 0.35% or less, Si: 0.05 to 0.50%, Mn: 0.50 to 2.50%, P: 0.040% or less, S: 0.040% or less, Cr: more than 1.5% and 5.00% or less, Mo: 0.30-0.80%, sol.Al:0.010-0.060%, N: 0.003-
0.020% and 8.5 x C (%) + Cr (%) ≥ 2.85 (%) Tensile strength 90 kg, characterized by having a composition consisting of balance Fe and unavoidable impurities
Steel material for chains with high yield and low yield ratio of f / mm ² or more and yield ratio of 90% or less. 2. By weight%, C: 0.35% or less, Si: 0.05 to 0.50%, Mn: 0.50 to 2.50%, P: 0.040% or less, S: 0.040% or less, Cr: more than 1.5% and 5.00% or less, Mo: 0.30-0.80%, sol.Al:0.010-0.060%, N: 0.003-
0.020%, 0.20% or less in total of one or more of V, Nb, and Ti, and 8.5 x C (%) + Cr (%) ≥ 2.85 (%) with the balance being Fe and inevitable impurities. 90 kg tensile strength
Steel material for chains with high yield and low yield ratio of f / mm ² or more and yield ratio of 90% or less. 3. By weight%, C: 0.35% or less, Si: 0.05 to 0.50%, Mn: 0.50 to 2.50%, P: 0.040% or less, S: 0.040% or less, Cr: more than 1.5% and 5.00% or less, Mo: 0.30-0.80%, sol.Al:0.010-0.060%, N: 0.003-
One or more of 0.020%, Ca: 0.010% or less and REM: 0.10 or less, and 8.5 x C (%) + Cr (%) ≥ 2.85 (%) The balance Fe and inevitable impurities. And tensile strength 90kg
Steel material for chains with high yield and low yield ratio of f / mm ² or more and yield ratio of 90% or less. 4. By weight%, C: 0.35% or less, Si: 0.05 to 0.50%, Mn: 0.50 to 2.50%, P: 0.040% or less, S: 0.040% or less, Cr: more than 1.5% and 5.00% or less, Mo: 0.30-0.80%, sol.Al:0.010-0.060%, N: 0.003-
0.020%, V, Nb, and Ti total of 0.20% or less, Ca: 0.010% or less and REM: 0.10 or less, and 8.5 × C (%) + Cr (%) ≧ 2.85 (%) Tensile strength 90kg, characterized by having a composition consisting of balance Fe and unavoidable impurities
Steel material for chains with high yield and low yield ratio of f / mm ² or more and yield ratio of 90% or less.