CN108603259B

CN108603259B - Steel having high strength and excellent low-temperature toughness after quenching and tempering

Info

Publication number: CN108603259B
Application number: CN201680081304.2A
Authority: CN
Inventors: 寺本真也; 山崎真吾; 门田淳
Original assignee: Nippon Steel and Sumitomo Metal Corp
Current assignee: Nippon Steel Corp
Priority date: 2016-02-19
Filing date: 2016-02-19
Publication date: 2020-11-06
Anticipated expiration: 2036-02-19
Also published as: EP3418412A1; CN108603259A; US20200165711A1; KR20180099881A; KR102113045B1; EP3418412A4; WO2017141425A1; JPWO2017141425A1; EP3418412B1; JP6590001B2

Abstract

The steel according to one aspect of the present invention contains, in mass%, C: 0.08 to 0.12%, Si: 0.05 to 0.50%, Mn: 1.50-3.00%, P: 0.040% or less, S: 0.020% or less, V: 0.010% or less, Ti: 0.010% or less, Nb: 0.005% or less, Cr: 1.00-2.50%, Cu: 0.01 to 0.50%, Ni: 0.75 to 1.60%, Mo: 0.10 to 0.50%, Al: 0.025-0.050%, N: 0.0100-0.0200%, Ca: 0-0.0100%, Zr: 0-0.0100% and Mg: 0 to 0.0100%, the balance being Fe and impurities, the ratio of Al content to N content being 2.6 or less, and the ratio of Mn content to Ni content being 1.5 or more and 3.0 or less.

Description

Steel having high strength and excellent low-temperature toughness after quenching and tempering

Technical Field

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

Background

In recent years, with the change in energy state, the trend to develop new energy is being activated worldwide. Under such circumstances, submarine oil fields are attracting attention as development resources on land are depleted, and development of drilling platforms using oil excavation is performed in a wide area centered around the vicinity of continental rise. In particular, in recent years, the number of offshore structures represented by offshore oil drilling platforms operating in deep sea has increased, and in order to prevent damage to the drilling platforms due to large hurricanes, it has been required to increase the strength of chains for mooring the drilling platforms. The breakage of the chain can directly lead to serious accidents such as the collapse of the drilling platform. In order to ensure safety, which is an important issue, both high strength and high toughness of chains are being sought. Specifically, the tensile strength is 1200MPa or more and the Charpy impact value at-20 ℃ is required to be 75J/cm²The above chain.

Such a chain is manufactured by cutting a hot-rolled steel bar having a diameter of 50mm or more into a predetermined length, forming the cut bar into a circular ring shape, and then flash butt welding the butted end faces. The stud bolt is sometimes pressed into the center of the chain ring after flash butt welding. Thereafter, the chain is subjected to quenching and tempering treatment, thereby imparting high strength and high toughness to the chain.

Examples of the invention of steel for high-strength and high-toughness chains include patent documents 1 to 6. However, the object of any of the documents is to provide a chain having a tensile strength of 800 to 1000MPa, and no study has been made on the condition that the strength of steel is set to 1200MPa or more. In recent years, a chain is required to have further high strength, but it is known that: generally, when a steel material is strengthened, the toughness of the steel material is lowered, and thus, the impact value of the steel material is lowered. When the steel having the components shown in these documents has a strength of 1200MPa or more, the target impact value cannot be obtained.

Documents of the prior art

Patent document

Patent document 1: japanese patent application laid-open No. 58-22361

Patent document 2: japanese laid-open patent publication No. 58-96856

Patent document 3: japanese laid-open patent publication No. 59-159972

Patent document 4: japanese laid-open patent publication No. 59-159969

Patent document 5: japanese patent application laid-open No. 62-202052

Patent document 6: japanese patent application laid-open No. 63-203752

Disclosure of Invention

The present invention addresses the problem of providing a steel having high strength after quenching and tempering and excellent low-temperature toughness (particularly low-temperature fracture toughness at low temperatures). Specifically, the present invention addresses the problem of providing a steel sheet having a Charpy impact value of 75J/cm at-20 ℃ when quenched and tempered to a tensile strength of 1200MPa or more²The above steel.

The gist of the present invention is as follows.

(1) The steel according to one aspect of the present invention contains, in mass%: c: 0.08 to 0.12%, Si: 0.05 to 0.50%, Mn: 1.50-3.00%, P: 0.040% or less, S: 0.020% or less, V: 0.010% or less, Ti: 0.010% or less, Nb: 0.005% or less, Cr: 1.00-2.50%, Cu: 0.01 to 0.50%, Ni: 0.75 to 1.60%, Mo: 0.10 to 0.50%, Al: 0.025-0.050%, N: 0.0100-0.0200%, Ca: 0-0.0100%, Zr: 0-0.0100% and Mg: 0 to 0.0100%, and the balance of Fe and impurities, wherein the Y value defined by the following formula a is 2.6 or less, the Z value defined by the following formula b is 1.5 or more and 3.0 or less,

Y＝(Al)/(N)…(a)

Z＝(Mn)/(Ni)…(b)

in the formula, symbols (Al), (N), (Mn) and (Ni) are contents in mass% of elements of each symbol in the steel.

(2)

The steel according to the above (1), which may contain one or more elements selected from the following elements, Ca: 0.0005 to 0.0100%, Zr: 0.0005-0.0100% and Mg: 0.0005 to 0.0100%.

According to the present invention, it is possible to provide a steel sheet having a tensile strength of 1200MPa or more after quenching and tempering and a Charpy impact value of 75J/cm at-20 DEG C²The above steel.

Drawings

FIG. 1 is a graph showing the relationship between the Y value of steel and the impact value at-20 ℃ of steel after quenching and tempering.

FIG. 2 is a graph showing the relationship between the Z value of steel and the impact value at-20 ℃ of the steel after quenching and tempering.

Detailed Description

The present inventors have continued various studies to realize a steel having high strength and excellent low-temperature toughness, and as a result, have obtained the following findings.

(A) Reducing the C content in order to reduce the amount of carburized steel that can act as a fracture starting point is effective in improving the low-temperature toughness of the steel. However, in order to make the tensile strength of the steel after quenching and tempering 1200MPa or more, the C content cannot be made less than 0.08%.

(B) By containing Ni in the steel, the low-temperature toughness of the steel is improved. However, the low-temperature toughness of steel cannot be sufficiently improved by this means alone.

(C) By appropriately containing Al and N in addition to Ni, the low-temperature toughness of the steel is further improved. This is because fine AlN made of Al and N makes crystal grains fine, thereby promoting the effect of improving low-temperature toughness by Ni. In order to obtain this effect, it is necessary to set the Al content to 0.025% or more and the N content to 0.0100% or more.

However, the ratio Y (Y ═ Al/[ N ]) of the Al content to the N content needs to be 2.6 or less. When the value of Y exceeds 2.6, the amount of alumina-based non-metallic inclusions in the steel increases, and the low-temperature toughness is rather lowered.

(D) In order to sufficiently improve the low-temperature toughness of the steel, it is necessary to set the ratio Z (Z ═ Mn ]/[ Ni ]) of the Mn content to the Ni content to 1.5 or more and 3.0 or less. When the value of Z is less than 1.5, the retained austenite amount increases, and when the value of Z exceeds 3.0, the content of Mn solid-dissolved in the steel increases. In either case, the low-temperature toughness of the steel is insufficient. That is, as described above, Ni has an effect of improving the low-temperature toughness of the steel, but when the Ni content is excessive, Z is less than 1.5 and the low-temperature toughness is impaired.

(E) In addition, in order to sufficiently improve the low-temperature toughness of steel, the content of V, Nb and Ti needs to be limited. VN, NbC and Ti (C, N) produced from V, Nb and Ti lowered the low temperature toughness of the steel.

(F) In addition, Mo needs to be contained in order to sufficiently improve the low-temperature toughness of the steel. This is because Mo makes cementite, which causes a decrease in low-temperature toughness, finer and less harmful.

Based on the above findings, the present inventors have found that a structural member, particularly a chain, having high strength and high low-temperature toughness can be manufactured. Hereinafter, a specific embodiment of the steel according to the present embodiment will be described. The steel according to the present embodiment has a tensile strength of 1200MPa or more after quenching and tempering and a Charpy impact value of 75J/cm at-20 ℃²The steel having the above effects is not particularly limited in strength and impact value before quenching and tempering. Unless otherwise specified, the description below of mechanical properties such as strength and toughness relates to the present invention after quenching and temperingThe steel of the embodiment.

The reasons for limiting the contents of the respective alloying elements in the steel of the present embodiment will be described below. The unit "%" of the content of the alloying element means mass%.

C：0.08～0.12％

C is an important element determining the strength of steel. The lower limit of the C content is set to 0.08% in order to obtain a tensile strength of 1200MPa or more after quenching and tempering. On the other hand, if the C content is excessive, the strength of the steel becomes excessively high, and the toughness of the steel decreases. When the C content is excessive, the amount of cementite that becomes a fracture starting point increases, and the toughness of the steel significantly decreases. 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：0.05～0.50％

Si has an effect of securing the strength of steel and also has an effect as a deoxidizer. When the Si content is less than 0.05%, the deoxidation effect cannot be sufficiently obtained, and the non-metallic inclusions in the steel increase, so that the toughness of the steel decreases. On the other hand, if Si is contained in an amount exceeding 0.50%, Si causes a decrease in 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：1.50～3.00％

Mn is a component necessary for ensuring necessary hardenability. The lower limit of the Mn content is set to 1.50% in order to ensure sufficient hardenability by setting the tensile strength of the steel after quenching and tempering to 1200MPa or more. On the other hand, when the Mn content is excessive, the toughness of the steel decreases, so 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 1.70%, 1.90% or 2.00%.

P: less than 0.040%

P is an impurity mixed into steel in a steel production process, and when the content of P exceeds 0.040%, the toughness of the steel is reduced to the allowable limit or more, so the content of P is limited to 0.040% or less. The upper limit of the P content is preferably 0.030%, 0.025% or 0.020%. The lower limit of the P content is 0% because P is not required in the steel of the present embodiment, but may be 0.001%, 0.002% or 0.003% in consideration of the capacity of refining facilities and the like.

S: 0.020% or less

Like P, S is an impurity mixed into steel in a steel production process, and when the S content exceeds 0.020%, S forms a large amount of MnS in the steel, and deteriorates the toughness of the steel. Therefore, the S content is limited to 0.020% or less. The upper limit of the S content is preferably 0.015%, 0.012% or 0.010%. The lower limit of the S content is 0% because the steel of the present embodiment does not require S, but may be 0.001%, 0.002% or 0.003% in consideration of the capacity of refining facilities and the like.

Cr：1.00～2.50％

Cr has an effect of increasing the hardenability of steel. In order to ensure sufficient hardenability and thereby to make the tensile strength of the steel after quenching and tempering 1200MPa or more, the lower limit of the Cr content is set to 1.00%. On the other hand, if the Cr content is excessive, the toughness of the steel decreases. 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：0.01～0.50％

Cu is an element effective for improving the hardenability and corrosion resistance of steel. The lower limit of the Cu content is set to 0.01% in order to ensure sufficient hardenability and corrosion resistance by setting the tensile strength of the steel after quenching and tempering to 1200MPa or more. On the other hand, if the Cu content is excessive, the toughness of the steel decreases. 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：0.75～1.60％

Ni is an element extremely effective for improving the toughness of the steel, and is an element necessary for increasing the toughness of the steel of the present embodiment after quenching and tempering. When the Ni content is less than 0.75%, it is difficult to sufficiently exert the effect thereof. On the other hand, if the Ni content is excessive, the retained austenite amount increases, and therefore, the low-temperature toughness decreases. Therefore, the upper limit of the Ni content is set to 1.60%. The upper limit of the Ni content is preferably 1.50%, 1.35% or 1.20%. The lower limit of the Ni content is preferably 0.80%, 0.85% or 0.90%.

Mo：0.10～0.50％

Mo has the effect of improving the low temperature toughness of the steel. Mo makes cementite which becomes a fracture starting point fine and harmless. Further, Mo makes the block size of the martensite structure fine, lowers the ductile-brittle transition temperature of the steel, and thereby makes brittle fracture difficult to occur even at low temperatures. When the Mo content is less than 0.10%, it is difficult to sufficiently exert the effect thereof. On the other hand, when the Mo content exceeds 0.50%, the toughness-improving effect 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%.

Al：0.025～0.050％

Al has an effect of adjusting the grain size of the metal structure and refining the metal structure into fine grains when AlN is precipitated, in addition to the deoxidation effect. When the Al content is less than 0.025%, a sufficient grain-refining effect cannot be obtained, and therefore, the toughness of the steel is lowered. On the other hand, when Al is contained in the steel in an amount exceeding 0.050%, the precipitation amount of AlN is saturated, and alumina-based nonmetallic inclusions in the steel increase, thereby decreasing the toughness of the steel. Therefore, the Al content is set to 0.025 to 0.050%. The upper limit of the Al content is preferably 0.045%, 0.042% or 0.040%. The lower limit of the Al content is preferably 0.027%, 0.029%, or 0.030%.

N：0.0100～0.0200％

N has an effect of bonding to Al to precipitate AlN effective for adjusting the grain size of the metal structure. When the N content is less than 0.0100%, the effect cannot be sufficiently exhibited. On the other hand, if the content of N in the steel exceeds 0.0200%, the amount of dissolved N increases, and the toughness of the steel decreases. Therefore, the N content is set to 0.0100 to 0.0200%. The upper limit of the N content is preferably 0.0180%, 0.0170% or 0.0160%. The lower limit of the N content is preferably 0.0110%, 0.0120%, or 0.0130%.

V: 0.010% or less

Ti: 0.010% or less

Nb: less than 0.005%

In the steel according to the present embodiment, the contents of V, Ti and Nb are preferably small. This is because VN, NbC and Ti (C, N) generated from V, Nb and Ti degrade the low-temperature toughness of the steel. The inventor has the following findings: in order to prevent the low temperature toughness of the steel from being lowered, it is necessary to set the V content to 0.010% or less, the Ti content to 0.010% or less, and the Nb content to 0.005% or less. The upper limit of the V content is preferably 0.009%, 0.007% or 0.005%. The upper limit of the Ti content is preferably 0.009%, 0.007% or 0.005%. The upper limit of the Nb content is preferably 0.004%, 0.003% or 0.002%.

In the steel according to the present embodiment, since the contents of V, Ti and Nb are preferably small, the lower limit of the contents of V, Ti and Nb is 0%. However, when these elements are mixed into steel as impurities, it is not preferable to completely remove these elements from the steel in consideration of the cost-effect ratio. Therefore, in consideration of the capacity and economy of the refining facility, the lower limit of the V content may be set to 0.003%, 0.002% or 0.001%, the lower limit of the Ti content may be set to 0.003%, 0.002% or 0.001%, and the lower limit of the Nb content may be set to 0.0010%, 0.0009% or 0.0008%.

Is selected from Ca: 0-0.0100%, Zr: 0-0.0100% and Mg: 0-0.0100% or less

The steel according to the present embodiment does not require Ca, Zr, and Mg. Therefore, the lower limit of the contents of Ca, Zr and Mg is 0%. However, Ca, Zr, and Mg all have the effect of forming oxides to become nuclei of MnS and improving the impact value of steel by uniformly and finely dispersing MnS. Therefore, as optional elements, Ca may be contained in the steel by 0.0005% or more, 0.0010% or more, or 0.0015% or more, Zr may be contained in the steel by 0.0005% or more, 0.0010% or more, or 0.0015% or more, and Mg may be contained in the steel by 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, if the content of each of Ca, Zr, and Mg exceeds 0.0100%, hard inclusions such as oxides and sulfides are produced in excess amounts, and the toughness of the steel is lowered. Therefore, the upper limit values of Ca, Zr, and Mg are set to 0.0100% or less, respectively. 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%.

And the balance: fe and impurities

The balance of the alloy components of the steel according to the present embodiment is made up of Fe and impurities. The impurities mean raw materials such as ores and scraps, or components mixed by various factors of the manufacturing process in the industrial production of steel materials, and mean components that are acceptable within a range that does not adversely affect the steel strip of the present embodiment.

Ratio of Al content to N content (Y value): 2.6 or less

In the steel according to the present embodiment, the ratio of the Al content to the N content (Y value) is defined by the following formula a.

Y ═ Al)/(N) … … formula a

In the formula a, the parenthesized symbol indicates the content of the element of the symbol in mass%.

AlN has the effect of refining crystal grains and improving the low-temperature toughness of steel. However, when the ratio of the Al content to the N content (Y value) in the steel exceeds 2.6, the aluminum oxide-based non-metallic inclusions in the steel increase and the steel becomes brittle, so that the low-temperature toughness is rather lowered. Therefore, the Y value is set to 2.6 or less. The upper limit of the value of Y is preferably 2.55, 2.50 or 2.45. The lower limit of the Y value is not particularly limited, but when the lower limit of the Al content and the upper limit of the N content are considered, the Y value is not less than 1.25.

The present inventors have obtained the above findings through experiments described below. The inventors carried out quenching and tempering under the following conditions and then carried out Charpy impact test at a temperature of-20 ℃ for various steels having different Y values, all of which characteristics other than the Y value were within the predetermined range of the steel of the present embodiment.

Quenching treatment: heating the steel to 900 ℃ and keeping for 30 minutes, and then carrying out water cooling

Tempering treatment: heating the steel to 135 deg.C and maintaining for 30 min, and air cooling

Thus, the inventors obtained a graph showing the relationship between the Y value and the impact value at-20 ℃ (FIG. 1). As shown in fig. 1, the steel having the Y value exceeding 2.6 does not have sufficient low-temperature toughness after the quenching and tempering.

Ratio of Mn content to Ni content (Z value): 1.5 or more and 3.0 or less

In the steel according to the present embodiment, the ratio (Z value) of the Mn content to the Ni content is defined by the following formula b.

Z ═ Mn)/(Ni) … … formula b

In the formula b, the parenthesized symbol indicates the content of the element of the symbol in mass%.

As described above, Ni improves the low-temperature toughness of steel. However, if the Ni content is excessive and the ratio of the Mn content to the Ni content (Z value) in the steel is less than 1.5, the retained austenite amount increases, and the low-temperature toughness of the steel is impaired. When the Z value exceeds 3.0, the content of solid-solution Mn becomes excessive relative to the content of Ni, and the effect of improving low-temperature toughness by Ni is offset, whereby steel is embrittled and low-temperature toughness is lowered. Therefore, the Z value is set to 1.5 or more and 3.0 or less. The upper limit of the Z value is preferably 2.9, 2.8 or 2.7. The lower limit of the Z value is preferably 1.6, 1.7 or 1.8.

The present inventors have obtained the above findings through experiments described below. The inventors carried out quenching and tempering under the following conditions and then carried out a Charpy impact test at a temperature of-20 ℃ for various steels having different Z values, all of which had characteristics other than the Z value within the predetermined range of the steel of the present embodiment.

Thus, the inventors obtained a graph showing the relationship between the Z value and the impact value at-20 ℃ (FIG. 2). As shown in fig. 2, the steels having the Z value less than 1.5 or exceeding 3.0 do not have sufficient low-temperature toughness after the quenching and tempering.

Further, the AlN in the steel has a number density, a particle diameter and a fractionThe dispersed state and the like vary depending on the conditions of heat treatment (for example, quenching and tempering) performed on the steel. In addition, when the contents of Al and N are controlled as described above, AlN effectively functions when the steel is quenched and tempered under conditions selected to make the tensile strength of the steel 1200MPa or more regardless of the state of AlN before the quenching and tempering, and the toughness of the steel is improved. That is, the steel according to the present embodiment has a problem in that after the steel is heat-treated so that the tensile strength becomes 1200MPa, the Charpy impact value at-20 ℃ of the steel is 75J/cm²As described above, the state control of AlN is not essential to solve the problem of the steel according to the present embodiment. Therefore, the state of AlN is not particularly specified in the steel according to the present embodiment. Further, as a result of experiments performed by the present inventors, it is estimated that: when the steel is heated to 850 to 900 ℃, AlN is favorably precipitated regardless of the state of the steel before heating, and when the steel in this state is cooled, the structure is favorably refined by AlN.

The steel according to the present embodiment can maintain the Charpy impact value at-20 ℃ of 75J/cm even when quenched and tempered to a tensile strength of 1200MPa or more²The above. Therefore, the steel according to the present embodiment is particularly preferably used as a steel for quenching.

For example, when the steel according to the present embodiment is subjected to a quenching treatment in which the steel is heated to 900 ℃ and kept at 30 minutes and then water-cooled, and further subjected to a tempering treatment in which the steel is heated to 135 ℃ and kept at 30 minutes, the Charpy impact value at-20 ℃ and a tensile strength of 1200MPa or more can be obtained²The above steel. In the steel according to the present embodiment after heat treatment under the quenching and tempering conditions, the average grain size of cementite is 0.05 μm or less, the average size of martensite lumps is 5.5 μm or less, and the content of retained austenite is 5% or less. Since the steel according to the present embodiment contains 0.08% or more of C, the steel has a tensile strength of 1200MPa or more when heat-treated under the quenching and tempering conditions. Generally, when the tensile strength of steel is 1200MPa or more, the low-temperature toughness (particularly, the low-temperature toughness) is impaired. However, the steel according to this embodiment contains 0.025 to 0.050% of Al and 0.0100 to 0.0200% of AlN and 0.10 to 0.50% of Mo, so that the martensite blocks and cementite are sufficiently refined and have high low-temperature toughness even when heat treatment is performed under the quenching and tempering conditions. Further, since the steel according to the present embodiment contains 0.75 to 1.60% of Ni, it has high low-temperature toughness even when heat treatment is performed under the quenching and tempering conditions. While there is a possibility that the low-temperature toughness is impaired by excessive amounts of Al and Ni, the steel of the present embodiment controls the ratio of the Al content to the N content and the ratio of the Ni content to the Mn content, and therefore does not impair the low-temperature toughness. In addition, since the steel according to the present embodiment has a V content of 0.010% or less, a Ti content of 0.010% or less, and an Nb content of 0.005% or less, precipitation of inclusions can be suppressed even when heat treatment is performed under the quenching and tempering conditions, and high low-temperature toughness can be obtained.

The quenching and tempering treatment under the above conditions is merely an example of the application of the steel according to the present embodiment. The steel according to the present embodiment can be heat-treated under any conditions according to the purpose. The characteristics of the steel according to the present embodiment after the heat treatment performed under the above-described quenching and tempering conditions are not intended to limit the technical scope of the steel according to the present embodiment. The steel according to the present embodiment has a Charpy impact value of 75J/cm at-20 ℃ after heat treatment to a tensile strength of 1200MPa or more²The above. In order to solve this problem, it is necessary to control the chemical composition, the ratio of Al content to N content, and the ratio of Ni content to Mn content, as described above. However, control of other structures such as martensite, cementite, retained austenite, and the like before heat treatment is not essential to solve the problem of the steel according to the present embodiment.

Since the steel according to the present embodiment has high tensile strength and excellent low-temperature toughness after quenching and tempering, it can exhibit particularly excellent effects when used as a material for chains for mooring, etc. of offshore oil drilling platforms.

Examples

The present invention will be described in detail below with reference to examples. The examples are intended to illustrate the technical meaning and effect of the present invention, and do not limit the scope of the present invention.

The steel having the chemical composition shown in Table 1 was melted and hot forged using a 180kg vacuum melting furnace to obtain a round bar steel having a diameter of 86 mm. The round bar steel was cut, quenched by water cooling after heating to 900 ℃ and holding for 30 minutes, and then tempered by heating to 135 ℃ and holding for 30 minutes. The quenching and tempering conditions are the same as the heat treatment conditions recommended when using the steel of the invention for making chains. Three tensile test pieces of JIS14A and four Charpy impact test pieces of JIS 4V notch were prepared from 1/4D portion (the region from the surface of the round bar steel to a depth of about 1/4 of the diameter D of the round bar steel) of the C-section of the round bar steel. The tensile test was carried out at a rate of 20 mm/min at room temperature in accordance with JIS Z2241. The Charpy impact test was carried out at-20 ℃ in accordance with JIS Z2242.

Then, a 10mm square sample was cut out from 1/4D portion of the C section of the round bar steel after the quenching and tempering, the section of the sample was corroded with a nital solution, 5 photographs of the structure of the section of the sample were taken at a magnification of 5000 times with a scanning electron microscope, and the average grain size of cementite included in these photographs was determined by image analysis using Luzex (registered trademark), and this was used as the average grain size of cementite of the round bar steel. Further, crystal orientation analysis was performed using a backscattered electron diffraction pattern, and the area-weighted average circle equivalent diameter of crystal grains surrounded by high-angle grain boundaries having a misorientation angle of 15 degrees or more, which were obtained by the analysis, was taken as the average grain diameter of martensite blocks in round bar steel. Then, the retained austenite amount of the round bar steel was measured by an X-ray diffraction method.

The results of the above experiments are shown in tables 1-1, 1-2 and 2. Tables 1 to 1 and tables 1 to 2 show chemical compositions of example steels and comparative example steels, and table 2 shows tensile strengths, impact values, average grain sizes of cementite, average sizes of martensite lumps, and retained austenite amounts of example steels and comparative example steels after quench-tempering under the above-described conditions. In tables 1-2, values outside the range specified in the present invention are underlined.

[ tables 1-1]

[ tables 1-2]

[ Table 2]

In examples of steels Nos. 1 to 23 having chemical compositions within the specified range of the present invention, after quenching and tempering under the above-mentioned conditions, the tensile strength was 1200MPa or more, and the Charpy impact value at-20 ℃ was 75J/cm²The above. After quenching and tempering under the above conditions, steels Nos. 1 to 23 were refined in cementite and martensite blocks and reduced in retained austenite amount.

In contrast, comparative example No.24 has an insufficient C content, and therefore does not provide the required tensile strength after quenching and tempering. Since comparative example No.25 had an excessive C content, the strength was excessively high after quenching and tempering, and the low-temperature toughness was insufficient.

Comparative example No.26 had an excessive Si content, comparative example 27 had an excessive Mn content, and comparative example 34 had an excessive Cr content. Since these excessive Si, Mn and Cr degrade the toughness of the steel, the low temperature toughness of comparative examples 26, 27 and 34 was insufficient after the quenching and tempering.

Since the content of at least one of V, Ti and Nb in comparative examples 28 to 33 is excessive, the toughness of the steel is lowered by precipitation strengthening by VN, NbC, or Ti (C, N), and the low-temperature toughness of comparative examples 28 to 33 after quenching and tempering is insufficient.

Comparative example No.35 had an insufficient Ni content, and the effect of improving the low-temperature toughness by Ni was small, so that the low-temperature toughness was insufficient. On the other hand, comparative example No.36 has a large Ni content, and the retained austenite amount increases after the quenching and tempering, so that the low-temperature toughness is insufficient after the quenching and tempering.

Since comparative example No.37 had an insufficient Mo content, after quenching and tempering, cementite that becomes the starting point of fracture became coarse, and the martensite structure (block size) became coarse, and therefore, the low-temperature toughness was low.

Comparative example No.38 had an insufficient Al content, and thus had insufficient grain refining effect, and the martensite structure (block size) became coarse after quenching and tempering, and the low-temperature toughness was insufficient.

Comparative example No.39 had an excessive N content, and therefore had insufficient low-temperature toughness because the content of dissolved N increased after quenching and tempering.

Since comparative examples No.40 to 42 contained excessive amounts of Ca, Zr and Mg, these elements lowered the toughness of the steel, and the low-temperature toughness after quenching and tempering was insufficient.

The contents of the respective alloying elements in comparative examples Nos. 43 to 45, 48 and 49 were within the predetermined ranges, but since the Y value or the Z value exceeded the predetermined ranges, the steel was rather embrittled and the low-temperature toughness after quenching and tempering was insufficient.

The contents of the respective alloying elements in comparative examples 46 and 47 were within the predetermined ranges, but since the Y value was less than the predetermined range, the retained austenite amount increased after the quenching and tempering, and the low-temperature toughness was insufficient.

Claims

1. A steel having high strength and excellent low-temperature toughness after quenching and tempering, which has a Charpy impact value of 75J/cm at-20 ℃ when quenched and tempered to have a tensile strength of 1211MPa or more and 1383MPa or less²The above steel is characterized in that it is,

contains, in mass%:

C：0.08～0.12％、

Si：0.05～0.50％、

Mn：1.50～3.00％、

p: less than 0.040%,

S: less than 0.020%,

V: less than 0.010%,

Ti: less than 0.010%,

Nb: less than 0.005 percent,

Cr：1.00～2.50％、

Cu：0.01～0.50％、

Ni：0.75～1.60％、

Mo：0.10～0.50％、

Al：0.025～0.050％、

N：0.0100～0.0200％、

Ca：0～0.0100％、

Zr: 0 to 0.0100% and

Mg：0～0.0100％，

the balance of Fe and impurities,

the value of Y defined by the following formula a is 2.6 or less,

a Z value defined by the following formula b is 1.5 or more and 3.0 or less,

Y＝(Al)/(N)…(a)

Z＝(Mn)/(Ni)…(b)

2. The steel having high strength and excellent low-temperature toughness after quench tempering according to claim 1,

contains one or more elements selected from the following elements in mass%,

Ca：0.0005～0.0100％、

zr: 0.0005 to 0.0100% and

Mg：0.0005～0.0100％。