EP3418411B1

EP3418411B1 - Steel useful as a material for chains

Info

Publication number: EP3418411B1
Application number: EP16890565.1A
Authority: EP
Inventors: Shinya Teramoto; Shingo Yamasaki; Atsushi Monden
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2016-02-19
Filing date: 2016-02-19
Publication date: 2020-11-04
Anticipated expiration: 2036-02-19
Also published as: JP6590000B2; KR102183900B1; EP3418411A1; KR20180099873A; CN108603260B; US20190032178A1; JPWO2017141424A1; CN108603260A; EP3418411A4; WO2017141424A1

Description

[Technical Field of the Invention]

The present invention relates to steel having a high strength and an excellent low-temperature toughness after quenching and tempering.

[Related Art]

Recently, in response to changes in energy situation, active efforts have been made across the globe in order to develop new energy sources. In such circumstances, offshore oil fields have drawn attentions as sources developed onshore have been depleted, and development using oil-drilling rigs has been conducted across a broad range of regions, mainly, continental shelves. In particular, recently, the number of marine structures represented by offshore oil-drilling rigs that are operated in the depths of the sea has been rising, and, in order to prevent damage to drilling rigs by large-scale hurricanes, there has been a demand for increasing the strength of chains for mooring drilling rigs. Broken chains lead directly to serious accidents such as the collapse of rigs. In order to ensure safety which is a vital object, an increase in both the strength and toughness of chains has been pursued. Specifically, there has been a demand for chains having a tensile strength of 1,200 MPa or more and a Charpy impact value at -20°C of 75 J/cm² or more.
Such chains are manufactured by cutting a hot rolled steel bar having a diameter of ϕ 50 mm or more to a predetermined length, forming the steel bar to an annular shape, and welding butted end surfaces through flash butt welding. After flash butt welding, there are cases where a stud is press-fitted into the center of the annular chain. After that, the chain is quenched and tempered, thereby imparting a high strength and a high toughness to the chain.
Patent Documents 1 to 6 and the like can be exemplified as invention examples of steel for a high strength and high toughness chain. However, all of the documents aim to provide a chain having a tensile strength of 800 MPa to 1,000 MPa and do not study a case where the strength of steel is set to 1,200 MPa or more. In recent years, although an additional increase in strength has been demanded for chains, it is known that an increase in the strength of steel generally degrades the toughness of steel and thus decreases the impact value of steel. When the strength of the steel proposed by the above described documents is set to 1,200 MPa or more, it is not possible to obtain an intended impact value.
Patent Document 7 describes a steel material having superior toughness at the HAZ, even for high heat input welding. Patent Document 8 describes a case hardening steel with high toughness and a manufacturing method thereof and further discloses that after solidification the bloom or ingot is cooled to room temperature and soaked between 1,250-1,320 °C for 3-180 min. This soak generates finely precipitated MnS.

[Prior Art Document]

[Patent Document]

[Patent Document 1] Japanese Unexamined Patent Application, First Publication No. S58-22361
[Patent Document 2] Japanese Unexamined Patent Application, First Publication No. S58-96856
[Patent Document 3] Japanese Unexamined Patent Application, First Publication No. S59-159972
[Patent Document 4] Japanese Unexamined Patent Application, First Publication No. S59-159969
[Patent Document 5] Japanese Unexamined Patent Application, First Publication No. S62-202052
[Patent Document 6] Japanese Unexamined Patent Application, First Publication No. S63-203752
[Patent Document 7] EP 2977479 A1
[Patent Document 8] US 2013/048156 A1

[Disclosure of the Invention]

[Problems to be Solved by the Invention]

An object of the present invention is to provide a steel having a high strength and an excellent low-temperature toughness (particularly, fracture toughness at a low temperature) after quenching and tempering. Specifically, the object of the invention is to provide a steel in which the Charpy impact value at -20°C reaches 75 J/cm² or more, when quenching and tempering are carried out so that the tensile strength reaches 1,200 MPa or more.

[Means for Solving the Problem]

The present invention is as described below.

(1) According to an aspect of the present invention, there is provided a steel containing, by unit mass%, C: 0.08% to 0.12%, Si: 0.05% to 0.50%, Mn: 1.00% to 3.00%, P: 0.040% or less, S: 0.020% or less, Cr: 1.00% to 2.50%, Cu: 0.01% to 0.50%, Ni: 0.75% to 3.20%, Mo: 0.10% to 0.50%, Nb: 0.005% to 0.050%, Al: 0.010% to 0.100%, N: 0.0050% to 0.0150%, V: 0% to 0.300%, Ca: 0% to 0.0100%, Zr: 0% to 0.0100%, Mg: 0% to 0.0100%, and a remainder including Fe and impurities, in which a number density of Mn sulfides having an equivalent circle diameter of more than 5 µm is 0 pieces/mm² to 10 pieces/mm², and an average aspect ratio of the Mn sulfides having an equivalent circle diameter of 1.0 µm to 5.0 µm is 1.0 or more and 10.0 or less.
(2) The steel according to (1) may contain, by unit mass%, V: 0.010% to 0.300%.
(3) The steel according to (1) or (2) may contain, by unit mass%, one or more selected from the group consisting of Ca: 0.0005% to 0.0100%, Zr: 0.0005% to 0.0100%, and Mg: 0.0005% to 0.0100%.

[Effects of the Invention]

According to the present invention, it is possible to provide steel having a tensile strength of 1,200 MPa or more and a Charpy impact value at -20°C of 75 J/cm² or more after quenching and tempering.

[Embodiments of the Invention]

The present inventors have continued a variety of researches in order to realize steel having a high strength and an excellent low-temperature toughness, as a result, the present inventors obtained the following findings.

(a) In order to impart a tensile strength of 1,200 MPa or more to quenched and tempered steel, a C content in the steel needs to be set to 0.08% or more.
(b) When steel contains all of Ni, Mo, and Nb, the low-temperature toughness of the steel is improved. The present inventors found that, when steel contains all of Ni, Mo, and Nb, the impact value of the steel is improved. This is considered to be because, when steel contains all of Ni, Mo, and Nb, cementite in the steel which may generally act as an origin of fracture is refined to a level at which the cementite does not act as a fracture origin. In addition, when steel contains all of Ni, Mo, and Nb, the block size of a martensite becomes small, and thus it is assumed that the ductile-brittle transition temperature of the steel is decreased and brittle fracture does not easily occur even at a low temperature.
(c) The present inventors found that the low-temperature toughness of steel can be improved by decreasing the grain size and aspect ratios of Mn sulfides which may act as an origin of fracture.

On the basis of the above described findings, the present inventors found the chemical composition, inclusion state, and manufacturing method for steel that can be used to manufacture a structural component having a high strength and a high low-temperature toughness, particularly, chains. Hereinafter, a specific aspect of steel according to the present embodiment will be described. In addition, although the steel according to the present embodiment is steel having an effect in which the tensile strength reaches 1,200 MPa or more and the Charpy impact value at -20°C reaches 75 J/cm² or more after quenching and tempering, the strength and the impact value before quenching and tempering are not particularly limited. Hereinafter, unless particularly otherwise described, description of mechanical properties such as strength and toughness relates to the steel according to the present embodiment after quenching and tempering.
Hereinafter, the reasons for limiting the amounts of individual alloying elements of the steel according to the present embodiment will be described. The unit "%" of the amounts of the alloying elements indicates mass%.

C: 0.08% to 0.12%

C is an important element that determines the strength of the steel. In order to obtain a tensile strength of 1,200 MPa or more after quenching and tempering, the lower limit of the C content is set to 0.08%. On the other hand, when the C content is excessive, the strength of the steel is excessively increased, and thus the toughness of the steel is degraded. In addition, when the C content is excessive, the amount of cementite which acts as an origin of fracture is increased, and the toughness of the steel is significantly degraded. Therefore, the upper limit of the C content is set to 0.12%. The upper limit of the C content is preferably 0.11%. The lower limit of the C content is preferably 0.09%.

Si: 0.05% to 0.50%

Si has an action for ensuring the strength of the steel and also an action as a deoxidizing agent. When the Si content is less than 0.05%, the deoxidizing action cannot be sufficiently obtained, the number of non-metallic inclusions in the steel is increased, and the toughness of the steel is degraded. On the other hand, when the Si content is more than 0.50%, Si causes the degradation in the toughness of the steel. Therefore, the Si content is set to 0.05% to 0.50%. The upper limit of the Si content is preferably 0.40%, 0.30%, or 0.20%. The lower limit of the Si content is preferably 0.06%, 0.07%, or 0.08%.

Mn: 1.00% to 3.00%

Mn is an essential element for ensuring a desired hardenability. In order to ensure sufficient hardenability for setting a tensile strength of the steel after quenching and tempering to 1,200 MPa or more, the lower limit of the Mn content is set to 1.0%. On the other hand, when the Mn content is excessive, the toughness of the steel is degraded, and thus the upper limit of the Mn content is set to 3.00%. The upper limit of the Mn content is preferably 2.90%, 2.80%, or 2.70%. The lower limit of the Mn content is preferably 1.10%, 1.20%, or 1.30%.

P: 0.040% or less

P is an impurity that is incorporated into the steel during the manufacturing process of the steel. When the P content exceeds 0.040%, the toughness of the steel is degraded more than a permissible limit, and thus the P content is limited to 0.040% or less. The upper limit of the P content is preferably 0.030%, 0.025%, or 0.020%. The steel according to the present embodiment does not need P, and thus the lower limit of the P content is 0%; however, when the capability of a refining facility and the like are taken into account, the lower limit of the P content may be set to 0.001%, 0.002%, or 0.003%.

S: 0.020% or less

S is, similar to P, an impurity that is incorporated into the steel during the manufacturing process of the steel. When the S content exceeds 0.020%, S forms a large amount of Mn sulfides in the steel, and the toughness of the steel is degraded. Therefore, the S content is limited to 0.020% or less. When the S content is 0.020% or less, the number density of the Mn sulfides is sufficiently decreased, and the toughness of the steel is maintained at a high level. The upper limit of the S content is preferably 0.015%, 0.012%, or 0.010%. The steel according to the present embodiment does not need S, and thus the lower limit of the S content is 0%; however, when the capability of a refining facility and the like are taken into account, the lower limit of the S content may be set to 0.001 %, 0.002%, or 0.003%.

Cr: 1.00% to 2.50%

Cr has an action for enhancing the hardenability of the steel. In order to ensure sufficient hardenability for setting a tensile strength of the steel after quenching and tempering to 1,200 MPa or more, the lower limit of the Cr content is set to 1.00%. On the other hand, when the Cr content is excessive, the toughness of the steel is degraded. Therefore, the upper limit of the Cr content is set to 2.50%. The upper limit of the Cr content is preferably 2.40%, 2.30%, or 2.20%. The lower limit of the Cr content is preferably 1.30%, 1.40%, or 1.50%.

Cu: 0.01% to 0.50%

Cu is an effective element for improving the hardenability and corrosion resistance of the steel. In order to ensure sufficient hardenability and corrosion resistance for setting a tensile strength of the steel after quenching and tempering to 1,200 MPa or more, the lower limit of the Cu content is set to 0.01%. On the other hand, when the Cu content is excessive, the toughness of the steel is degraded. Therefore, the upper limit of the Cu content is set to 0.50%. The upper limit of the Cu content is preferably 0.40%, 0.30%, or 0.20%. The lower limit of the Cu content is preferably 0.02%, 0.03%, or 0.05%.

Ni: 0.75% to 3.20%

Ni is an extremely effective element for improving the toughness of the steel and an essential element for increasing the toughness of the steel according to the present embodiment in which a tensile strength after quenching and tempering is 1,200 MPa or more. When the Ni content is less than 0.75%, it is difficult to sufficiently exhibit the effects. On the other hand, when the Ni content exceeds 3.20%, the effect for improving toughness is saturated. Therefore, the Ni content is set to 0.75% to 3.20%. The upper limit of the Ni content is preferably 3.15%, 3.10%, or 3.05%. The lower limit of the Ni content is preferably 0.80%, 0.85%, or 0.90%.

Mo: 0.10% to 0.50%

The present inventors found that Mo has an effect for improving the low-temperature toughness of the steel, when Mo is contained in the steel together with Ni and Nb. This is considered to be because, when the steel contains Mo together with Ni and Nb, cementite in the steel which may generally act as an origin of fracture is refined to a level at which the cementite does not act as a fracture origin. In addition, when the steel contains Mo together with Ni and Nb, the block size of a martensite becomes small, and thus it is assumed that the ductile brittle transition temperature of the steel is decreased and brittle fracture does not easily occur even at a low temperature. When the Mo content is less than 0.10%, it is difficult to sufficiently exhibit the effects. On the other hand, when the Mo content exceeds 0.50%, the effect for improving toughness is saturated. Therefore, the Mo content is set to 0.10% to 0.50%. The upper limit of the Mo content is preferably 0.47%, 0.45%, or 0.42%. The lower limit of the Mo content is preferably 0.15%, 0.20%, or 0.25%.

Nb: 0.005% to 0.050%

Nb has an effect for improving the low-temperature toughness of the steel, when Nb is contained in the steel together with Ni and Mo. This is considered to be because, when the steel contains Nb together with Ni and Mo, cementite in the steel which may generally act as an origin of fracture is refined to a level at which the cementite does not act as a fracture origin. In addition, when the steel contains Nb together with Ni and Mo, the block size of a martensite becomes small, and thus it is assumed that the ductile brittle transition temperature of the steel is decreased and brittle fracture does not easily occur even at a low temperature. When the Nb content is less than 0.005%, it is difficult to sufficiently exhibit the effects. On the other hand, when the Nb content exceeds 0.050%, the effect for improving toughness is saturated. Therefore, the Nb content is set to 0.005% to 0.050%. The upper limit of the Nb content is preferably 0.045%, 0.040%, or 0.035%. The lower limit of the Nb content is preferably 0.007%, 0.010%, or 0.015%.

Al: 0.010% to 0.100%

In addition to a deoxidizing action, Al has an action for adjusting the crystal grain size of a metallographic structure and miniaturizing the metallographic structure when Al is precipitated as AlN. When the Al content is less than 0.010%, it is not possible to obtain a sufficient miniaturizing effect, and thus the toughness of the steel is degraded. On the other hand, when the Al content in the steel exceeds 0.100%, the amount of AlN precipitated is saturated, the number of alumina based non-metallic inclusions in the steel is increased, and the toughness of the steel is degraded. Therefore, the Al content is set to 0.010% to 0.100%. The upper limit of the Al content is preferably 0.090%, 0.070%, or 0.050%. The lower limit of the Al content is preferably 0.012%, 0.015%, or 0.018%.

N: 0.0050% to 0.0150%

N has an action for precipitating AlN, which is effective for adjusting the crystal grain size of the metallographic structure, by bonding to Al. When the N content is less than 0.0050%, this action is not sufficiently exhibited. On the other hand, when the N content in the steel exceeds 0.0150%, the number of solute N is increased, and the toughness of the steel is degraded. Therefore, the N content is set to 0.0050% to 0.0150%. The upper limit of the N content is preferably 0.0140%, 0.0130%, or 0.0120%. The lower limit of the N content is preferably 0.0055%, 0.0060%, or 0.0065%.

V: 0% to 0.300%

The steel according to the present embodiment does not need V. Therefore, the lower limit of the V content is 0%. However, V has an action for adjusting the crystal grain size of the metallographic structure and miniaturizing the metallographic structure when V is precipitated as VN. Therefore, as an optional element, the steel may contain 0.010% or more, 0.020% or more, or 0.030% or more of V. On the other hand, when the V content in the steel exceeds 0.300%, coarse VN remains in the steel after heating for quenching, and this coarse VN degrades the toughness of the steel after quenching and tempering. Therefore, the V content is set to 0.300% or less. The upper limit of the V content is preferably 0.250% or less, 0.200%, or 0.150%.
One or more selected from the group consisting of Ca: 0% to 0.0100%, Zr: 0% to 0.0100% or less, and Mg: 0% to 0.0100%
The steel according to the present embodiment does not need Ca, Zr, and Mg. Therefore, the lower limit of each of the Ca content, the Zr content, and the Mg content is 0%. However, all of Ca, Zr, and Mg have an effect for forming an oxide, acting as a crystallization nucleus of MnS, and uniformly and finely dispersing MnS so as to improve the impact value of the steel. Therefore, as an optional element, the steel may contain 0.0005% or more, 0.0010% or more, or 0.0015% or more of Ca, may contain 0.0005% or more, 0.0010% or more, or 0.0015% or more of Zr, and may contain 0.0005% or more, 0.0010% or more, or 0.0015% or more of Mg. On the other hand, when each of the Ca content, the Zr content, and the Mg content exceeds 0.0100%, an excess amount of a hard inclusion such as an oxide and a sulfide is generated, and the toughness of the steel is degraded. Therefore, the upper limits of each of the Ca content, the Zr content, and the Mg content is set to 0.0100% or less. The upper limit of the Ca content is preferably 0.0090%, 0.0070%, or 0.0050%, the upper limit of the Zr content is preferably 0.0090%, 0.0070%, or 0.0050%, and the upper limit of the Mg content is preferably 0.0090%, 0.0070%, or 0.0050%.

Remainder: Fe and impurities

The remainder of the chemical composition of the steel according to the present embodiment consists of Fe and impurities. The impurities refer to elements which are incorporated by a raw material such as an ore or a scrap, or a variety of causes in the manufacturing process during the industrial manufacturing of the steel, and the impurities are permitted to an extent in which the steel according to the present embodiment is not adversely influenced.
Next, the reason for limiting the inclusion state of the steel according to the present embodiment will be described.
Number density of Mn sulfides having an equivalent circle diameter of more than 5 µm is 0 pieces/mm² to 10 pieces/mm²
A Mn sulfide having an equivalent circle diameter of more than 5 µm (hereinafter, referred to as a "coarse Mn sulfide") significantly degrades the low-temperature toughness of the steel, and thus the number density of the coarse Mn sulfides is preferably set to substantially 0 pieces/mm². Therefore, the lower limit of the number density of the coarse Mn sulfides is 0 pieces/mm². However, when the number density is 10 pieces/mm² or less, the low-temperature toughness is not seriously impaired. Therefore, the upper limit of the number density of the coarse Mn sulfides is set to 10 pieces/mm². The upper limit of the number density of the coarse Mn sulfides is preferably 9 pieces/mm², 8 pieces/mm², or 7 pieces/mm².
Average aspect ratio of the Mn sulfides having an equivalent circle diameter of 1.0 µm to 5.0 µm is 1.0 or more and 10.0 or less
A Mn sulfide having an equivalent circle diameter of 1.0 µm to 5.0 µm (hereinafter, referred to as a "fine Mn sulfide") has a smaller adverse influence on the toughness of the steel than the coarse Mn sulfide. However, a fine Mn sulfide, in which the aspect ratio of the Mn sulfide that can be calculated by dividing the major axis of the Mn sulfide by the minor axis of the Mn sulfide is excessively large, may act as an origin of fracture and degrade the toughness of the steel, similar to the coarse Mn sulfide. The present inventors found that, when the average aspect ratio of the fine Mn sulfides is set to 10.0 or less, it is possible to make the fine Mn sulfides almost harmless. A preferred upper limit of the average aspect ratio of the fine Mn sulfides is 9.0, 7.5, or 6.0. When the major axis and the minor axis of the fine Mn sulfide are equal to each other, the aspect ratio of the fine Mn sulfide reaches 1.0, and thus the lower limit of the average aspect ratio of the fine Mn sulfides is set to 1.0.
The fine dispersion of Mn sulfides which may act as origins of fracture and a decrease in the aspect ratios thereof are extremely effective for improving the low-temperature toughness of the steel. In addition, since the state of the Mn sulfides does not change before and after quenching and tempering that are carried out under ordinary conditions, the state of the Mn sulfides is maintained even after quenching and tempering as long as the state of the Mn sulfides is controlled as described above before quenching and tempering, and the above described effects can be obtained.
In addition, in the steel according to the present embodiment, there is no need for limiting the number density of the fine Mn sulfides. Although there is concern that an extremely large amount of the fine Mn sulfides impairs the toughness of the steel, the number density of the fine Mn sulfides does not increase to an extent in which the toughness of the steel is impaired, as long as the S content is in the above described range. Furthermore, in the steel according to the present embodiment, a Mn sulfide having an equivalent circle diameter of less than 1.0 µm (hereinafter, referred to as an "ultrafine Mn sulfide") does not act as an origin of fracture, and thus the aspect ratio and number density of the ultrafine Mn sulfide are not particularly specified. Furthermore, in the steel according to the present embodiment, the Mn sulfides (the coarse Mn sulfides and the fine Mn sulfides) are almost uniformly dispersed, and thus a place where the state of the Mn sulfide is specified is not particularly limited.
A method for specifying the state of the Mn sulfide is as described below. First, a cross section of the steel is mirror-polished, and then optical microscopic photographs are captured at 10 or more random places on the cross section at a magnification of 1,000 times. The ten photographs obtained in the above described manner are processed using image analysis software, for example, Luzex (registered trademark) or the like, whereby the state of Mn sulfides in the steel, that is, the number density of the coarse Mn sulfides and the average aspect ratio of the fine Mn sulfides can be obtained. In the steel according to the present embodiment, Mn sulfides are elongated in a processing direction. For example, when the steel is hot rolled, the Mn sulfides are elongated in a hot rolling direction. Therefore, the cross section where the optical microscopic photographs are captured needs to be formed parallel to the processing direction (for example, the hot rolling direction). On the other hand, since the Mn sulfides are almost uniformly dispersed in the steel according to the present embodiment, and a place where the optical microscopic photographs are captured is not particularly specified.
Next, a method for manufacturing the steel according to the present embodiment will be described.
The method for manufacturing the steel according to the present embodiment includes a process of continuously casting molten steel having the chemical composition of the steel according to the present embodiment so as to obtain a slab and a process of soaking the slab twice or more. The conditions for continuously casting the molten steel are not particularly limited. In the process of soaking the slab, firstly, the slab is heated up to a temperature range of 1,300°C to 1,350°C, and then, the temperature of the slab is held in this temperature range for 300 seconds to 18,000 seconds, and furthermore, the slab is cooled to 900°C or lower. In addition, the soaking is carried out twice or more.

(Soaking)

The soaking is carried out in order to finely disperse Mn sulfides included in the slab. During the continuous casting, coarse Mn sulfides are crystallized in the slab. When the slab is heated up to a temperature range of 1,300°C to 1,350°C and then held in this temperature range for 300 seconds to 18,000 seconds, the coarse Mn sulfides are solutionized, and the Mn sulfides are precipitated when the slab is cooled to 900°C or lower. The Mn sulfides are refined by solutionizing and precipitation.
When the holding temperature of the slab is lower than 1,300°C and when the holding time of the slab at the temperature is shorter than 300 seconds, the Mn sulfides are not sufficiently solutionized. In addition, when the soaking is carried out only once, the Mn sulfides are not sufficiently refined. In order to set the dispersion state of the Mn sulfides in the steel in the above described range, the soaking under the above described conditions needs to be carried out twice or more. When the cooling stop temperature of the slab is set to higher than 900°C and the subsequent soaking is initiated, the Mn sulfides are not precipitated during cooling, and thus the refinement of the Mn sulfides becomes insufficient.
Meanwhile, when the heating temperature of the slab is higher than 1,350°C, the ductility of the slab is degraded, and a problem of cracking is caused. In addition, when the heating time of the slab is longer than 18,000 seconds, it is not preferable in consideration of the economic efficiency.
On the slab in which the Mn sulfides are sufficiently refined by the above described treatment, it is possible to carry out an optional processing and an optional heat treatment afterwards. For example, blooming and hot rolling are performed on this slab so as to produce a steel bar, and a chain processing is performed on this steel bar, whereby a chain can be obtained. In addition, during or after the chain processing, it is possible to quench and temper the chain. Since the Mn sulfides included in the slab obtained using the above described method are sufficiently refined, and it is assumed that the fine Mn sulfides included in the slab are not outside the specification range described above due to blooming, hot rolling, the chain processing, and quenching and tempering that are carried out under ordinary conditions.
Even when the steel according to the present embodiment is quenched and tempered so that the tensile strength reaches 1,200 MPa or more, the Charpy impact value at -20°C can be maintained at 75 J/cm2 or more. Therefore, the steel according to the present embodiment is particularly preferably used as steel for quenching and tempering.
For example, when a quenching treatment in which steel is heated to 900°C, held for 30 minutes, and then cooled with water is performed on the steel according to the present embodiment, and furthermore, a tempering treatment in which the steel is heated to 135°C and held for 30 minutes is performed on the steel, steel having a tensile strength of 1,200 MPa or more and a Charpy impact value at -20°C of 75 J/cm² or more is obtained. In the steel according to the present embodiment on which the heat treatment under the above described quenching and tempering conditions is performed, the number density of Mn sulfides having an equivalent circle diameter of more than 5 µm is 0 pieces/mm² to 10 pieces/mm², the average aspect ratio of the Mn sulfides having an equivalent circle diameter of 1.0 µm to 5.0 µm is 1.0 or more and 10.0 or less, the average grain size of cementite is 0.05 µm or less, and the average size of martensite blocks is 5.5 µm or less. The steel according to the present embodiment contains 0.08% or more of C and thus has a tensile strength of 1,200 MPa or more, when the heat treatment under the above described quenching and tempering conditions is performed. Generally, the low-temperature toughness (particularly, low-temperature toughness) is impaired when the tensile strength of the steel is 1,200 MPa or more. However, the steel according to the present embodiment contains 0.75% to 3.20% of Ni, 0.10% to 0.50% of Mo, and 0.005% to 0.050% of Nb, and thus, when the heat treatment under the above described quenching and tempering conditions is performed, martensite blocks and cementite are sufficiently refined, and the steel has a high low-temperature toughness. In addition, in the steel according to the present embodiment on which the heat treatment under the above described quenching and tempering conditions is performed, similar to the steel according to the present embodiment before quenching and tempering, the number density of Mn sulfides having an equivalent circle diameter of more than 5 µm is 0 pieces/mm² to 10 pieces/mm², and the average aspect ratio of the Mn sulfides having an equivalent circle diameter of 1.0 µm to 5.0 µm is 1.0 or more and 10.0 or less, and thus the steel has a high low-temperature toughness.
Meanwhile, quenching and tempering under the above described conditions are simply an example of the use of the steel according to the present embodiment. According to the purposes, a heat treatment under optional conditions can be performed on the steel according to the present embodiment. In addition, the properties of the steel according to the present embodiment on which the heat treatment is performed on the basis of an example of the above described quenching and tempering conditions do not limit the technical scope of the steel according to the present embodiment. The object of the steel according to the present embodiment is to obtain a Charpy impact value at -20°C of 75 J/cm² or more after a heat treatment is carried out so that the tensile strength reaches 1,200 MPa. As described above, in order to achieve this object, it is necessary to control the chemical composition and the Mn sulfide state before the heat treatment. However, other constitutions, for example, the states of martensite and cementite before the heat treatment and the like do not need to be controlled in order to achieve the object of the steel according to the present embodiment.
In addition, quenching and tempering under ordinary conditions do not have any influences on the Mn sulfide state. Therefore, when the Mn sulfide state in quenched and tempered steel is in the specification range described above, it is assumed that the Mn sulfide state before quenching and tempering in the steel is also in the specification range described above.
The steel according to the present embodiment is capable of exhibiting particularly excellent effects, when the steel according to the present embodiment is used as a material for chains for mooring offshore oil-drilling rigs which needs to have a high tensile strength and a high low-temperature toughness.

[Examples]

Hereinafter, the present invention will be described in detail using examples. Meanwhile, these examples are intended to describe the technical meaning and effects of the present invention and do not limit the scope of the present invention.

[Example 1]

Steel A having a chemical composition shown in Table 1 was continuously cast so as to obtain a slab, then, a soaking was performed on the slab once or more, and furthermore, a blooming was performed on the slab, thereby obtaining a 162 mm x 162 mm rolled material. The conditions for the soaking and the number of times of the soaking are shown in Table 2. After that, hot rolling was performed on the rolled material, thereby producing a round steel bar having a diameter of 86 mm. Next, a quenching treatment in which the round steel bar was cut, heated to 900°C, held for 30 minutes, and furthermore, cooled with water was carried out, and then a tempering treatment in which the round steel bar was heated to 135°C and held for 30 minutes was carried out, thereby obtaining round steel bars Nos. A1 to A5. These quenching conditions and tempering conditions are the same as heat treatment conditions that are recommended for the production of chains using the present invention steel.
Three JIS No. 14A tensile test pieces and four JIS No. 4 V-notch Charpy impact test pieces were produced from a 1/4D portion (a region at a depth of approximately 1/4 of a diameter D of the round steel bar from the surface of the round steel bar) of a C cross section of each of the quenched and tempered round steel bars Nos. A1 to A5. A tensile test was carried out at normal temperature and a rate of 20 mm/min according to JIS Z 2241. A Charpy impact test was carried out at -20°C according to JIS Z 2242.
Furthermore, a 10 mm x 10 mm sample was cut out from the 1/4D portion of the C cross section of each of the quenched and tempered round steel bars Nos. A1 to A5, and the metallographic structure of the steel and the state of inclusions were observed on a cross section parallel to a rolling direction. In order to observe Mn sulfides present in the steel, the cross section was mirror-polished, then, 10 metallographic photographs were captured using an optical microscope at a magnification of 1,000 times, and the equivalent circle diameters and aspect ratios of Mn sulfides included in the photographs were obtained by means of an image analysis (Luzex (registered trademark)). In addition, in order to observe cementite present in the steel, the cross section was corroded with a nital etching solution, five metallographic photographs were captured using a scanning electron microscope at a magnification of 5,000 times, and the average grain size of the cementite included in the photographs was obtained by means of an image analysis (Luzex (registered trademark)). Furthermore, a crystal orientation analysis was carried out on the sample using an electron backscatter diffraction pattern, and the area-weighted average equivalent circle diameter of crystal grains surrounded by high angle grain boundaries which had an orientation difference angle of 15 degrees, which was obtained from the above described analysis, was considered as the average grain size of martensite blocks.

The results of the above described experiments are shown in Table 1 and Table 2. Table 1 shows the chemical compositions of the steel A (that is, the chemical compositions of the steels No. Al to No. A5). Table 2 shows soaking conditions and the number of times of soaking during the manufacturing of the steels No. A1 to No. A5, and the average aspect ratios of Mn sulfides having an equivalent circle diameter of 1.0 µm to 5.0 µm, the number densities of Mn sulfides having an equivalent circle diameter of more than 5.0 µm, the tensile strengths, the impact values, the average grain sizes of the cementite, and the average sizes of the martensite blocks in the steels No. A1 to No. A5 which were quenched and tempered under the above described conditions. In Table 2, values outside the specification ranges of the present invention are underlined. Meanwhile, quenching and tempering under the above described conditions do not have any influences on the state of the Mn sulfides, and thus the states of the Mn sulfides in the quenched and tempered steels No. A1 to No. A5 disclosed in Table 2 are the same as those of the steels No. A1 to No. A5 before quenching and tempering.

[Table 1]

Steel type	Chemical composition (mass%)
Steel type	C	Si	Mn	P	S	Cr	Cu	Ni	Mo	Nb	Al	N
A	0.10	0.21	1.81	0.006	0.007	1.72	0.09	1.52	0.33	0.015	0.023	0.0085

[Table 2]

No.	Soaking conditions		Number of times of soaking	Average aspect ratio of Mn sulfides having equivalent circle diameter of 1.0 µm to 5.0 µm	Number density of Mn sulfides having equivalent circle diameter of more than 5.0 µm (pieces/mm²)	Tensile strength (MPa)	Impact value (J/cm²)	Average grain size of cementite (µm)	Average size of martensite blocks (µm)	Classification
No.	Heating temperature (°C)	Holding time (sec.)	Number of times of soaking			Tensile strength (MPa)	Impact value (J/cm²)	Average grain size of cementite (µm)	Average size of martensite blocks (µm)	Classification
A1	1300	7200	3	1.3	0.0	1308	171	0.03	3.7	Invention Example
A2	1350	7200	2	4.5	1.1	1321	154	0.04	3.8	Invention Example
A3	1290	7200	2	10.8	10.2	1324	68	0.03	3.8	Comparative Example
A4	1350	290	2	10.5	10.8	1311	54	0.03	3.8
A5	1300	7200	1	10.3	11.1	1284	65	0.03	4.1

As shown in Table 1 and Table 2, in the steels No. A1 and No. A2 which were the present invention example, the chemical compositions and the manufacturing conditions were appropriate, and thus the form of the Mn sulfides was in the specification range of the present invention. Therefore, the steels No. A1 and A2 had a tensile strength of 1,200 MPa or more and a Charpy impact value at -20°C of 75 J/cm² or more after quenching and tempering. In contrast, in the steels No. A3 to A5 which were comparative examples, the manufacturing conditions were not appropriate, and thus the Mn sulfides coarsened or the aspect ratios of the Mn sulfides increased, and the low-temperature toughness after quenching and tempering was insufficient.

[Example 2]

Each of steel B to AH having a chemical composition shown in Table 3 was continuously cast so as to obtain a slab, then, a soaking in which the holding temperature was 1,300°C and the holding time was 7,200 seconds was performed on the slab twice, and furthermore, blooming was performed on the slab, thereby obtaining a 162 mm x 162 mm rolled material. After that, hot rolling was performed on the rolled material, thereby producing a round steel bar having a diameter of 86 mm. Next, a quenching treatment in which the round steel bar was cut, heated to 900°C, held for 30 minutes, and furthermore, cooled with water was carried out, and then a tempering treatment in which the round steel bar was heated to 135°C and held for 30 minutes was carried out, thereby obtaining round steel bars Nos. B to AH. These quenching conditions and tempering conditions are the same as heat treatment conditions that are recommended for the production of chains using the present invention steel.
Three JIS No. 14A tensile test pieces and four JIS No. 4 V-notch Charpy impact test pieces were produced from a 1/4D portion of a C cross section of each of the quenched and tempered round steel bars Nos. B to AH. A tensile test was carried out at normal temperature and a rate of 20 mm/min according to JIS Z 2241. A Charpy impact test was carried out at -20°C according to JIS Z 2242.
Furthermore, a 10 mm x 10 mm sample was cut out from the 1/4D portion of the C cross section of each of the quenched and tempered round steel bars Nos. B to AH, and the metallographic structure of the steel and the state of inclusions were observed on a cross section parallel to a rolling direction. In order to observe Mn sulfides present in the steel, the cross section was mirror-polished, then, 10 metallographic photographs were captured using an optical microscope at a magnification of 1,000 times, and the equivalent circle diameters and aspect ratios of Mn sulfides included in the photographs were obtained by means of an image analysis (Luzex (registered trademark)). In addition, in order to observe cementite present in the steel, the cross section was corroded with a nital etching solution, five metallographic photographs were captured using a scanning electron microscope at a magnification of 5,000 times, and the average grain size of the cementite included in the photographs was obtained by means of an image analysis (Luzex (registered trademark)). Furthermore, a crystal orientation analysis was carried out on the sample using a backscattered electron beam diffraction pattern, and the area-weighted average equivalent circle diameter of crystal grains surrounded by high angle grain boundaries which had an orientation difference angle of 15 degrees, which was obtained from the above described analysis, was considered as the average grain size of martensite blocks.

The results of the above described experiments are shown in Table 3 and Table 4. Table 3 shows the chemical compositions of the steels Nos. B to AH. Table 4 shows the aspect ratios of Mn sulfides having an equivalent circle diameter of 1.0 µm to 5.0 µm, the number densities of Mn sulfides having an equivalent circle diameter of more than 5.0 µm, the tensile strengths, the impact values, the average grain sizes of the cementite, and the average sizes of the martensite blocks in the steels Nos. B to AH which were quenched and tempered under the above described conditions. In Table 3 and Table 4, values outside the specification ranges of the present invention are underlined. Meanwhile, quenching and tempering under the above described conditions do not have any influences on the state of the Mn sulfides, and thus the states of the Mn sulfides in the quenched and tempered steels Nos. B to AH disclosed in Table 4 are the same as those of the steels Nos. B to AH before quenching and tempering.

[Table 3]

Steel type	Chemical composition (mass%)														Classification
Steel type	C	Si	Mn	P	S	Cr	Cu	Ni	Mo	Nb	Al	N	V	Others	Classification
B	0.10	0.07	2.07	0.009	0.007	1.82	0.07	0.75	0.36	0.019	0.023	0.0059	-	-
C	0.08	009	1.78	0.008	0.003	1.81	0.10	2.52	0.35	0.013	0.024	0.0093	0.038	-
D	0.12	0.10	1.33	0.007	0.007	1.78	0.08	3.18	0.35	0.015	0.017	0.0113	-	-
E	0.10	0.11	1.66	0.008	0.005	1.56	0.09	2.45	0.33	0.022	0.018	0.0075	0.031	-
F	0.10	0.05	1.37	0.005	0.004	1.57	0.10	2.90	0.37	0.020	0.016	0.0119	-	Ca:0.003%
G	0.10	0.09	1.79	0.008	0.004	1.66	0.08	2.50	0.40	0.021	0.020	0.0109	0.042	Mg:0.005%
H	0.10	0.50	1.92	0.008	0.006	1.56	0.10	2.62	0.33	0.021	0.024	0.0080	-	Zr:0.009%
I	0.10	0.10	1.01	0.007	0.005	1.57	0.10	2.42	0.34	0.019	0.017	0.0054	-	-
J	0.10	0.09	2.98	0.006	0.004	1.03	0.09	2.74	0.35	0.015	0.021	0.0113	-	-
K	0.10	0.09	1.51	0.005	0.005	2.47	0.11	2.77	0.35	0.016	0.021	0.0087	-	-	Invention Example
L	0.09	0.07	1.35	0.007	0.005	1.58	0.07	2.46	0.11	0.015	0.017	0.0118	0.053	-	Invention Example
M	0.09	0.10	1.40	0.007	0.005	1.54	0.10	2.62	0.50	0.022	0.018	0.0077	-	-
N	0.10	0.08	1.34	0.008	0.006	1.55	0.08	2.64	0.31	0.005	0.021	0.0112	-	-
O	0.09	0.09	1.41	0.008	0.008	1.61	0.09	2.74	0.32	0.048	0.019	0.0091	-	-
P	0.09	0.10	1.62	0.009	0.007	1.63	0.08	2.81	0.35	0.015	0.011	0.0073	-	-
Q	0.10	0.10	1.58	0.006	0.007	1.54	0.10	2.68	0.33	0.016	0.098	0.0077	-	-
R	0.10	0.09	1.63	0.039	0.006	1.55	0.11	2.52	0.32	0.021	0.022	0.0064	0.028	Ca:0.005%
S	0.09	0.09	1.58	0.007	0.019	1.62	0.09	2.37	0.32	0.019	0.020	0.0084	-	-
T	0.10	0.10	1.52	0.008	0.007	1.54	0.02	2.41	0.37	0.018	0.019	0.0093	-	-
U	0.10	0.08	1.42	0.008	0.008	1.42	0.48	2.57	0.35	0.022	0.022	0.0078	-	-
V	0.10	0.10	1.41	0.006	0.005	1.71	0.11	2.82	-	-	0.022	0.0062	-	-	Comparative Example
w	0.09	0.10	1.61	0.005	0.004	1.82	0.08	2.98	0.32	-	0.024	0.0096	-	-
X	0.09	0.07	1.67	0.007	0.004	1.47	0.08	2.73	-	0.022	0.021	0.0117	-	-
Y	0.09	0.08	1.51	0.006	0.006	1.64	0.09	0.73	0.45	0.019	0.021	0.0080	-	-
Z	0,10	0.08	1.37	0.006	0.005	1.72	0.08	2.47	0.08	0.014	0.019	0.0081	0.032	Ca:0.005%
AA	0.10	0.09	1.43	0.008	0.006	1.64	0.09	2.72	0.34	0.003	0.022	0.0077	-	Mg:0.007%
AB	0.07	0.10	1.91	0.006	0.005	1.73	0.10	2.65	0.30	0.023	0.025	0.0065	0.045	-
AC	0.13	0.09	1.46	0.007	0.004	1.81	0.07	2.52	0.30	0.017	0.024	0.0055	-	-
AD	0.09	0.51	1.74	0.006	0.007	1.70	0.09	2.55	0.43	0.019	0.022	0.0083	-	-
AE	0.09	0.09	3.20	0.008	0.004	1.68	0.08	2.81	0.40	0.015	0.023	0.0057	-	-
AF	0.09	0.09	1.32	0.009	0.006	0.97	0.09	3.08	0.33	0.022	0.018	0.0063	-	-
AG	0.10	0.10	1.37	0.008	0.021	1.63	0.09	2.74	0.33	0.019	0.020	0.0094	0.024	Zr:0.009%
AH	0.09	0.09	2.08	0.007	0.004	1.84	0.08	2.90	0.38	0.023	0.023	0.0154	-	-

[Table 4]

Steel type	Average aspect ratio of Mn sulfides having equivalent circle diameter of 1.0 µm to 5.0 µm	Number density of Mn sulfides having equivalent circle diameter of more than 5 µm (pieces/mm²)	Tensile strength (MPa)	Impact value (J/cm²)	Average grain size of cementite (µm)	Average size of martensite blocks (µm)	Classification
B	4.1	2.1	1278	156	0.03	3.2
C	3.9	0.0	1222	150	0.04	3.9
D	4.5	1.3	1370	170	0.04	3.8
E	4.2	0.7	1315	181	0.04	4.4
F	3.7	0.0	1301	167	0.04	4.8
G	4.3	0.4	1307	153	0.04	3.6
H	4.1	1.1	1290	127	0.04	4.4
I	4.2	1.6	1304	131	0.04	3.4
J	3.8	0.6	1318	187	0.03	3.7
K	4.1	0.8	1285	170	0.04	4.9	Invention Example
L	4.3	1.3	1285	164	0.04	4.1	Invention Example
M	4.5	1.7	1296	128	0.03	3.1
N	3.9	1.1	1311	127	0.03	3.8
O	4.3	0.3	1308	134	0.04	3.7
P	4.4	0.7	1297	130	0.03	3.6
Q	4.1	1.2	1287	136	0.03	3.8
R	3.8	1.8	1303	129	0.04	3.9
S	4.2	1.2	1298	132	0.03	3.6
T	4.0	2.0	1311	138	0.04	3.7
U	4.3	1.4	1318	128	0.04	3.7
V	4.2	2.2	1279	49	0.07	10.2	Comparative Example
W	4.0	0.3	1296	70	0.06	8.7
X	3.7	0.0	1294	57	0.06	9.1
Y	4.4	2.1	1267	41	0.06	9.5
Z	4.1	0.8	1302	63	0.07	9.8
AA	4.3	1.2	1297	54	0.06	9.9
AB	4.2	0.7	1193	162	0.04	3.6
AC	4.1	0.6	1415	68	0.04	3.3
AD	4.5	1.4	1282	58	0.04	5.1
AE	4.3	2.1	1294	46	0.04	4.7
AF	4.3	0.2	1266	71	0.04	5.4
AG	4.1	5.6	1309	52	0.03	4.3
AH	4.0	0.0	1282	67	0.03	3.8

As shown in Table 3 and Table 4, in all of the steels Nos. B to U which were the present invention example, the chemical compositions and the states of the Mn sulfides were in the specification range of the present invention. Therefore, the steels Nos. B to U had a tensile strength of 1,200 MPa or more and a Charpy impact value at -20°C of 75 J/cm² or more after quenching and tempering.
In contrast, in the steels Nos. V, W, X, Y, Z, and AA which were comparative examples, the amount of one or more of Mo, Nb, and Ni was insufficient or one or more of Mo, Nb, and Ni was not included, and thus, after quenching and tempering, cementite which acted as an origin of fracture became coarse, furthermore, the average size of martensite blocks became coarse, and the low-temperature toughness was insufficient.
In the steel No. AB which was a comparative example, the C content was insufficient, and thus a necessary tensile strength could not be obtained after quenching and tempering. Meanwhile, in the steel No. AC which was a comparative example, the C content was excessive, and thus the strength became excessively high, and the low-temperature toughness after quenching and tempering was insufficient.
In the steel No. AD which was a comparative example, the Si content was excessive, and in the steel No. AE, the Mn content was excessive. The excess Si or Mn degraded the toughness of the steel, and thus the low-temperature toughness of the steels No. AD and No. AE after quenching and tempering was insufficient.
In the steel No. AF which was a comparative example, the Cr content was insufficient, and thus sufficient hardenability could not be obtained, and the low-temperature toughness after quenching and tempering was insufficient.
In the steel No. AG which was a comparative example, the S content was excessive, and thus an excess amount of Mn sulfides were formed, and the low-temperature toughness after quenching and tempering was insufficient. In the steel No. AH which was a comparative example, the N content was excessive, and thus the amount of solute N became excessive, and the low-temperature toughness after quenching and tempering was insufficient.

Claims

A steel comprising, by unit mass%,
C: 0.08% to 0.12%;
Si: 0.05% to 0.50%;
Mn: 1.00% to 3.00%;
P: 0.040% or less;
S: 0.020% or less;
Cr: 1.00% to 2.50%;
Cu: 0.01% to 0.50%;
Ni: 0.75% to 3.20%;
Mo: 0.10% to 0.50%;
Nb: 0.005% to 0.050%;
Al: 0.010% to 0.100%;
N: 0.0050% to 0.0150%;
V: 0% to 0.300%;
Ca: 0% to 0.0100%;
Zr: 0% to 0.0100%;
Mg: 0% to 0.0100%; and
a remainder including Fe and impurities,
wherein a number density of Mn sulfides having an equivalent circle diameter of more than 5 µm is 0 pieces/mm² to 10 pieces/mm², and
an average aspect ratio of the Mn sulfides having an equivalent circle diameter of 1.0 µm to 5.0 µm is 1.0 or more and 10.0 or less.
The steel according to claim 1 comprising, by unit mass%,
V: 0.010% to 0.300%.
The steel according to claim 1 or 2 comprising, by unit mass%, one or more selected from the group consisting of
Ca: 0.0005% to 0.0100%;
Zr: 0.0005% to 0.0100%; and
Mg: 0.0005% to 0.0100%.