WO2017141425A1

WO2017141425A1 - Steel

Info

Publication number: WO2017141425A1
Application number: PCT/JP2016/054853
Authority: WO
Inventors: 真也寺本; 真吾山▲崎▼; 門田　淳
Original assignee: 新日鐵住金株式会社
Priority date: 2016-02-19
Filing date: 2016-02-19
Publication date: 2017-08-24
Also published as: US20200165711A1; JPWO2017141425A1; EP3418412A4; KR20180099881A; CN108603259A; EP3418412A1; KR102113045B1; EP3418412B1; JP6590001B2; CN108603259B

Abstract

Steel according to one aspect of the present invention contains, in unit mass %, C: 0.08-0.12%, Si: 0.05-0.50%, Mn: 1.50-3.00%, P: no more than 0.040%, S: no more than 0.020%, V: no more than 0.010%, Ti: no more than 0.010%, Nb: no more than 0.005%, 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: 0-0.0100% and Mg: 0-0.0100%, with the remainder being made up of Fe and impurities. The ratio of Al content to N content does not exceed 2.6, and the ratio of the Mn content to Ni content falls within the range of 1.5-3.0.

Description

steel

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

In recent years, with the changing energy situation, movements to develop new energy resources have become active throughout the world. Under these circumstances, as the onshore development resources are depleted, attention has been focused on subsea oil fields, and development using oil drilling rigs has been conducted in a wide range of areas, mainly around the continental shelf. Has come to be done. Particularly in recent years, the number of offshore structures represented by subsea oil drilling rigs operating in the deep sea has increased. To prevent damage to drilling rigs caused by large hurricanes, the strength of drilling rig mooring chains has increased. It has been demanded. Chain breaks are directly linked to serious accidents such as rig collapses. In order to ensure safety, which is an important issue, both higher strength and higher toughness of chains have been directed. Specifically, a chain having a tensile strength of 1200 MPa or more and a Charpy impact value at −20 ° C. of 75 J / cm ² or more is required.

Such a chain is manufactured by cutting a hot-rolled steel bar having a diameter of 50 mm or more into a predetermined length, forming it into an annular shape, and then flash-butt welding the butted end faces. In some cases, a stud is pressed into the center of the chain ring after flash butt welding. Thereafter, the chain is subjected to quenching and tempering treatment to impart high strength and toughness to the chain.

Examples of inventions for high strength and high toughness chain steel include, for example, Patent Documents 1 to 6 and the like. However, none of these documents is aimed at providing a chain having a tensile strength of 800 to 1000 MPa, and the case where the strength of the steel is set to 1200 MPa or more has not been studied. In recent years, chains have been required to have higher strength, but it is generally known that when steel materials are strengthened, the toughness of the steel materials decreases, thereby reducing the impact value of the steel materials. When the steel having the components presented in these documents is made to have a strength of 1200 MPa or more, the intended impact value cannot be obtained.

Japanese Laid-Open Patent Publication No. 58-22361 Japanese Unexamined Patent Publication No. 58-96856 Japanese Unexamined Patent Publication No. 59-159972 Japanese Unexamined Patent Publication No. 59-159969 Japanese Unexamined Patent Publication No. Sho 62-202052 Japanese Unexamined Patent Publication No. Sho 63-203752

An object of the present invention is to provide a steel having high strength and excellent low temperature toughness (particularly fracture toughness at low temperature) after quenching and tempering. Specifically, an object of the present invention is to provide a steel having a Charpy impact value at −20 ° C. of 75 J / cm ² or more when quenched and tempered so that the tensile strength is 1200 MPa or more. It is.

The gist of the present invention is as follows.

(1) The steel according to one embodiment of the present invention is unit mass%, C: 0.08 to 0.12%, Si: 0.05 to 0.50%, Mn: 1.50 to 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 to 2.50 %, Cu: 0.01 to 0.50%, Ni: 0.75 to 1.60%, Mo: 0.10 to 0.50%, Al: 0.025 to 0.050%, N: 0.00. 0-100 to 0.0200%, Ca: 0 to 0.0100%, Zr: 0 to 0.0100%, and Mg: 0 to 0.0100%, with the balance being Fe and impurities, The defined Y value is 2.6 or less, and 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)
The symbols (Al), (N), (Mn), and (Ni) in the formula are the contents of the elements according to the symbols in the steel in unit mass%.
(2)
The steel described in (1) above is composed of Ca: 0.0005 to 0.0100%, Zr: 0.0005 to 0.0100%, and Mg: 0.0005 to 0.0100% in unit mass%. You may contain 1 or more types selected from a group.

According to the present invention, a steel having a tensile strength of 1200 MPa or more and a Charpy impact value at −20 ° C. of 75 J / cm ² or more after quenching and tempering can be provided.

3 is a graph showing the relationship between the Y value of steel and the impact value at −20 ° C. of the steel after quenching and tempering. 4 is a graph showing the relationship between the Z value of steel and the impact value at −20 ° C. of the steel after quenching and tempering.

The inventor has made various studies in order to realize a steel having high strength and excellent low-temperature toughness, and has obtained the following knowledge.

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

(B) By incorporating Ni into the steel, the low temperature toughness of the steel is improved. However, this means alone cannot sufficiently improve the low temperature toughness of steel.

(C) The low temperature toughness of the steel is further improved by appropriately containing Al and N in addition to Ni. This is because the fine AlN formed from Al and N refines the crystal grains and promotes the effect of improving the low temperature toughness by Ni. In order to obtain this effect, it is necessary that the Al content is 0.025% or more and the N content is 0.0100% or more.

However, the ratio Y (Y = [Al] / [N]) between the Al content and the N content needs to be 2.6 or less. When the value of Y is more than 2.6, alumina-based nonmetallic inclusions in the steel increase, and instead the low temperature toughness is reduced.

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

(E) In order to sufficiently improve the low temperature toughness of the steel, it is necessary to limit the contents of V, Nb and Ti. VN, NbC and Ti (C, N) generated from V, Nb and Ti reduce the low temperature toughness of the steel.
(F) Furthermore, in order to sufficiently improve the low temperature toughness of steel, it is necessary to contain Mo. This is because Mo refines cementite that causes low temperature toughness to deteriorate and renders it harmless.

Based on the above findings, the present inventors have found a steel capable of producing a structural component, particularly a chain, having high strength and high low temperature toughness. Below, the specific aspect of steel which concerns on this embodiment is demonstrated. The steel according to the present embodiment is a steel having an effect that the tensile strength after quenching and tempering is 1200 MPa or more and the Charpy impact value at −20 ° C. is 75 J / cm ² or more. The impact value is not particularly limited. Hereinafter, unless otherwise specified, the description explaining the 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 content of each alloy element of the steel according to the present embodiment will be described. The unit “%” of the content of the alloy element means mass%.

C: 0.08 to 0.12%
C is an important element that determines the strength of steel. In order to obtain a tensile strength of 1200 MPa or more after quenching and tempering, the lower limit of the C content is 0.08%. On the other hand, when the C content is excessive, the steel becomes excessively strong and the toughness of the steel decreases. On the other hand, if the C content is excessive, the amount of cementite that is the starting point of fracture increases, and the toughness of the steel is significantly reduced. Therefore, the upper limit of the C content is 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 as a deoxidizer as well as an action for securing the strength of steel. When the Si content is less than 0.05%, a sufficient deoxidation effect cannot be obtained, and the nonmetallic inclusions in the steel increase to lower the toughness of the steel. On the other hand, when Si is contained exceeding 0.50%, Si causes toughness reduction of 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 to 3.00%
Mn is an essential component for ensuring the required hardenability. In order to ensure sufficient hardenability to make the tensile strength of the steel after quenching and tempering 1200 MPa or more, the lower limit value of the Mn content is 1.50%. On the other hand, when the Mn content is excessive, the toughness of the steel decreases, so the upper limit value of the Mn content is 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: 0.040% or less P is an impurity mixed in steel in the steel manufacturing process. However, if the P content exceeds 0.040%, the toughness of the steel is reduced to an allowable limit or more. The content is limited to 0.040% or less. The upper limit of the P content is preferably 0.030%, 0.025%, or 0.020%. Since the steel according to the present embodiment does not require P, the lower limit value of the P content is 0%, but considering the capacity of the refining equipment and the like, the lower limit value of the P content is 0.001%, 0 It may be 0.002% or 0.003%.

S: 0.020% or less S, like P, is an impurity mixed in the steel in the steel manufacturing process. If the S content exceeds 0.020%, S contains a large amount of MnS in the steel. Forming and lowering 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%. Since the steel according to the present embodiment does not require S, the lower limit value of the S content is 0%, but considering the capacity of the refining equipment and the like, the lower limit value of the S content is 0.001%, 0 It may be 0.002% or 0.003%.

Cr: 1.00-2.50%
Cr has the effect of increasing the hardenability of the steel. In order to ensure sufficient hardenability and thereby make the tensile strength of the steel after quenching and tempering be 1200 MPa or more, the lower limit value of the Cr content is 1.00%. On the other hand, when the Cr content is excessive, the toughness of the steel decreases. Therefore, the upper limit of the Cr content is 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 material. In order to ensure sufficient hardenability and corrosion resistance to bring the tensile strength of the steel after quenching and tempering to 1200 MPa or more, the lower limit value of the Cu content is 0.01%. On the other hand, when the Cu content is excessive, the toughness of the steel decreases. Therefore, the upper limit value of the Cu content is 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 1.60%
Ni is an extremely effective element for improving the toughness of steel, and is an essential element for increasing the toughness of the steel according to the present embodiment after quenching and tempering. If the Ni content is less than 0.75%, it is difficult to sufficiently exert its effect. On the other hand, when the Ni content is excessive, the amount of retained austenite increases, so the low temperature toughness is reduced. Therefore, the upper limit of the Ni content is 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 to 0.50%
Mo has the effect of improving the low temperature toughness of the steel. Mo refines cementite, which is the starting point of destruction, and renders it harmless. Moreover, Mo refines the block size of the martensite structure and lowers the ductile brittle transition temperature of steel, thereby making it difficult for brittle fracture to occur even at low temperatures. When the Mo content is less than 0.10%, it is difficult to sufficiently exhibit the effect. On the other hand, when the Mo content exceeds 0.50%, the effect of 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%.

Al: 0.025 to 0.050%
In addition to the deoxidizing action, Al has the action of adjusting the crystal grain size of the metal structure and making the metal structure finer when precipitated as AlN. When the Al content is less than 0.025%, a sufficient grain refining effect cannot be obtained, so that the toughness of steel is lowered. On the other hand, if Al exceeds 0.050%, the precipitation amount of AlN is saturated, and alumina-based nonmetallic inclusions in the steel increase to lower 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 to 0.0200%
N binds to Al and has the effect of precipitating AlN effective for adjusting the crystal grain size of the metal structure. If the N content is less than 0.0100%, this effect is not sufficiently exhibited. On the other hand, when N is contained in steel exceeding 0.0200%, solid solution N will increase and the toughness of steel will fall. 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: 0.005% or less In the steel according to the present embodiment, it is preferable that the contents of V, Ti, and Nb are small. This is because VN, NbC and Ti (C, N) generated from V, Nb and Ti lower the low temperature toughness of the steel. In order to prevent the low temperature toughness of the steel from being lowered, the inventors 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. I found out that it was necessary. 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 value of the Nb content is preferably 0.004%, 0.003%, or 0.002%.

In the steel according to this embodiment, it is preferable that the contents of V, Ti, and Nb are small, so the lower limit of the contents of V, Ti, and Nb is 0%. However, when these elements are mixed into the steel as impurities, it may not be preferable to completely remove these elements from the steel in view of cost effectiveness. Therefore, the lower limit value of the V content may be set to 0.003%, 0.002%, or 0.001% in consideration of the capacity and economics of the refining equipment, and the lower limit value of the Ti content is set to 0.00. 003%, 0.002%, or 0.001%, and the lower limit of the Nb content may be 0.0010%, 0.0009%, or 0.0008%.

One or more selected from the group consisting of Ca: 0 to 0.0100%, Zr: 0 to 0.0100%, and Mg: 0 to 0.0100% or less. The steel according to this embodiment includes Ca, Zr, And Mg is not required. Therefore, the lower limit of the contents of Ca, Zr, and Mg is 0%. However, Ca, Zr, and Mg all form oxides and become crystallization nuclei of MnS, and have an effect of improving the impact value of steel by uniformly and finely dispersing MnS. Therefore, as an optional element, Ca may be contained in the steel in an amount of 0.0005% or more, 0.0010% or more, or 0.0015% or more, and Zr may be contained in the steel in an amount of 0.0005% or more, 0.0010%. In addition, 0.0015% or more may be contained, and Mg may be contained in steel in an amount of 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, when the content of each of Ca, Zr, and Mg exceeds 0.0100%, an excessive amount of hard inclusions such as oxides and sulfides are generated, and the toughness of the steel is lowered. Therefore, the upper limit of each of Ca, Zr, and Mg is 0.0100% or less. The upper limit value of Ca content is preferably 0.0090%, 0.0070%, or 0.0050%, and the upper limit value of Zr content is preferably 0.0090%, 0.0070%, or 0. The upper limit of the Mg content is preferably 0.0090%, 0.0070%, or 0.0050%.

Remainder: Fe and impurities The remainder of the alloy component of the steel according to the present embodiment is composed of Fe and impurities. Impurities are components that are mixed due to various factors of raw materials such as ore or scrap, or manufacturing process when industrially manufacturing steel materials, and a range that does not adversely affect the steel according to the present embodiment. Means what is allowed.

Ratio of Al content to N content (Y value): 2.6 or less In the steel according to the present embodiment, the ratio of Al content to N content (Y value) is defined by the following formula a. The
Y = (Al) / (N) …… Formula a
In the formula a, the symbol with parentheses indicates the content in unit mass% of the element according to the symbol.

AlN has the effect of refining crystal grains and improving the low temperature toughness of steel. However, when the ratio of Al content to N content in steel (Y value) exceeds 2.6, alumina-based nonmetallic inclusions in steel increase and the steel becomes brittle. Decreases. Therefore, the Y value is set to 2.6 or less. The upper limit of the Y value is preferably 2.55, 2.50, or 2.45. The lower limit value of the Y value is not particularly limited, but the Y value does not become less than 1.25 in consideration of the lower limit value of the Al content and the upper limit value of the N content described above.

The present inventors have obtained the above knowledge through experiments described below. The inventors have all the features other than the Y value within the specified range of the steel according to the present embodiment, and various steels having different Y values are quenched and tempered under the following conditions, and then at a temperature of −20 ° C. A Charpy impact test was conducted.
Quenching treatment: heating the steel to 900 ° C. and holding for 30 minutes, then water cooling / tempering treatment: heating the steel to 135 ° C. and holding for 30 minutes, then air cooling. A graph (FIG. 1) showing the relationship with the impact value at 20 ° C. was obtained. As shown in FIG. 1, the steel having a Y value of greater than 2.6 did not have sufficient low temperature toughness after quenching and tempering.

Ratio of Mn content to Ni content (Z value): 1.5 or more, 3.0 or less In the steel according to this embodiment, the ratio of Mn content to Ni content (Z value) is as follows: Defined by equation b.
Z = (Mn) / (Ni) ...... Formula b
In the formula b, the symbol in parentheses indicates the content in unit mass% of the element related to the symbol.

As described above, Ni improves the low temperature toughness of steel. However, when the Ni content is excessive and the ratio (Z value) of the Mn content and the Ni content in the steel is less than 1.5, the amount of retained austenite increases and the low temperature toughness of the steel is impaired. . Further, when the Z value is more than 3.0, the solid solution Mn content with respect to the Ni content becomes excessive, the effect of improving the low temperature toughness by Ni is negated, the steel becomes brittle, and the low temperature toughness is lowered. Therefore, the Z value is 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 knowledge through experiments described below. The inventors have all the features other than the Z value within the specified range of the steel according to the present embodiment, and various steels having different Z values are quenched and tempered under the following conditions, and then at a temperature of −20 ° C. A Charpy impact test was conducted.
Quenching treatment: heating the steel to 900 ° C. and holding it for 30 minutes, then water cooling / tempering treatment: heating the steel to 135 ° C. and holding it for 30 minutes, then air cooling. A graph (FIG. 2) showing the relationship with the impact value at 20 ° C. was obtained. As shown in FIG. 2, steels with Z values less than 1.5 or greater than 3.0 did not have sufficient low temperature toughness after quenching and tempering.

In addition, the number density of AlN in steel, a particle size, a dispersed state, etc. change according to the conditions of the heat processing (for example, quenching tempering etc.) performed with respect to steel. Further, when the contents of Al and N are controlled as described above, regardless of the state of AlN before quenching and tempering, during the quenching and tempering under conditions selected to make the tensile strength of steel 1200 MPa or more. AlN functions effectively and improves the toughness of the steel. In other words, the problem with the steel according to the present embodiment is that the Charpy impact value at −20 ° C. of the steel is 75 J / cm ² or more after the steel is heat-treated so that the tensile strength becomes 1200 MPa. Control of the state of AlN is not required for solving the problem of the steel according to the present embodiment. Therefore, in the steel according to the present embodiment, the state of AlN is not particularly defined. As a result of the experiment, the present inventors have shown that when the steel is heated to 850 to 900 ° C., AlN is preferably precipitated regardless of the state of the steel before heating. It is estimated that.

Even when the steel according to the present embodiment is quenched and tempered so that the tensile strength becomes 1200 MPa or more, the Charpy impact value at −20 ° C. can be kept at 75 J / cm ² or more. Therefore, the steel according to the present embodiment is particularly preferably used as a quenching steel.

For example, if the steel according to the present embodiment is subjected to a quenching process in which the steel is heated to 900 ° C. and held for 30 minutes and then cooled with water, and further tempered to be heated to 135 ° C. and held for 30 minutes, the tensile strength is 1200 MPa. Thus, a steel having a Charpy impact value at −20 ° C. of 75 J / cm ² or more is obtained. The steel according to the present embodiment after being heat-treated under this quenching and tempering condition has an average particle size of cementite of 0.05 μm or less, an average size of martensite blocks of 5.5 μm or less, and a residual austenite content. Is 5% or less. Since the steel according to the present embodiment contains 0.08% or more of C, it has a tensile strength of 1200 MPa or more when heat-treated under this quenching and tempering condition. Normally, when the tensile strength of steel is 1200 MPa or more, low temperature toughness (particularly low temperature toughness) is impaired. However, the steel according to the present embodiment contains 0.025 to 0.050% Al, 0.0100 to 0.0200% N, and 0.10 to 0.50% Mo. Therefore, even when heat-treated under the quenching and tempering conditions, the martensite block and cementite are sufficiently refined and have high low temperature toughness. Further, since the steel according to the present embodiment contains 0.75 to 1.60% Ni, it has high low temperature toughness even when heat-treated under the quenching and tempering conditions. Excessive amounts of Al and Ni may impair low-temperature toughness, but the steel according to this embodiment has a controlled ratio of Al content to N content and a ratio of Ni content to Mn content. Therefore, low temperature toughness is not impaired. Further, the steel according to the present embodiment is limited to V content of 0.010% or less, Ti content of 0.010% or less, and Nb content of 0.005% or less. Even when heat-treated under the conditions, precipitation of inclusions is suppressed and high low temperature toughness is achieved.

Note that the quenching and tempering treatment according to the above-described conditions is merely an example of the use of steel according to the present embodiment. The steel according to the present embodiment can be heat-treated under any conditions depending on the purpose. Moreover, the characteristics of the steel according to the present embodiment after the heat treatment based on the example of the quenching and tempering condition described above is not limited to the technical scope of the steel according to the present embodiment. The problem with the steel according to this embodiment is that the Charpy impact value at −20 ° C. is 75 J / cm ² or more after heat treatment is performed so that the tensile strength is 1200 MPa or more. In order to solve this problem, as described above, it is necessary to control the chemical component, the ratio between the Al content and the N content, and the ratio between the Ni content and the Mn content. However, other configurations, for example, control of martensite, cementite, retained austenite, etc. before heat treatment are not required for solving the problems 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, when used as a material for a submarine oil drilling rig mooring chain or the like, a particularly excellent effect can be exhibited.

The present invention will be described in detail below by examples. These examples are for explaining the technical significance and effects of the present invention, and do not limit the scope of the present invention.

Using a 180 kg vacuum melting furnace, steels having chemical components shown in Table 1 were melted and hot forged to obtain a round bar steel having a diameter of 86 mm. The round steel bar was cut, heated to 900 ° C. and held for 30 minutes, and then quenched by water cooling, and then tempered by heating to 135 ° C. and held for 30 minutes. These quenching conditions and tempering conditions are the same as the heat treatment conditions recommended when creating a chain using the steel of the present invention. Three JIS14A tensile specimens and JIS4 V notch Charpy impact from 1 / 4D part of the C section of this round steel bar (region from the surface of the round steel bar to a depth of about 1/4 of the diameter D of the round steel bar). Four test pieces were prepared. The tensile test was carried out at room temperature and at a speed of 20 mm / min in accordance with JIS Z 2241. The Charpy impact test was performed at −20 ° C. according to JIS Z 2242.

Further, a 10 mm square sample was cut out from a 1 / 4D portion of the C cross section of the round steel bar after quenching and tempering, the sample cross section was corroded using a nital corrosive liquid, and the sample was magnified 5000 times using a scanning electron microscope. Five cross-sectional structure photographs were taken, and the average particle diameter of cementite contained in these photographs was determined by image analysis using Luzex (registered trademark), and this was used as the average particle diameter of cementite of the round bar steel. In addition, the crystal orientation analysis was performed using the backscattered electron diffraction pattern, and the area weighted average equivalent circle diameter of the crystal grain surrounded by the large angle grain boundary having an orientation difference angle of 15 degrees or more obtained by this analysis It was set as the average particle diameter of the martensite block of the steel bar. Further, the amount of retained austenite of the round bar steel was measured by an X-ray diffraction method.

The results of the above experiment are shown in Table 1-1, Table 1-2, and Table 2. Table 1-1 and Table 1-2 show the chemical composition of the example steel and the comparative example steel, and Table 2 shows the tensile strength of the example steel and the comparative example steel after quenching and tempering under the above-mentioned conditions. The impact value, the average particle size of cementite, the average size of the martensite block, and the amount of retained austenite are shown. In Table 1-2, values outside the specified range of the present invention are underlined.

Steel No. whose chemical composition is within the specified range of the present invention. In Examples 1 to 23, after quenching and tempering under the above conditions, the tensile strength was 1200 MPa or more, and the Charpy impact value at −20 ° C. was 75 J / cm ² or more. Steel No. In Nos. 1 to 23, after quenching and tempering under the above-described conditions, cementite and martensite blocks were refined, and the amount of retained austenite was reduced.

In contrast, Comparative Example No. In No. 24, since the C content was insufficient, the necessary tensile strength could not be obtained after quenching and tempering. Comparative Example No. No. 25 was excessive in strength after quenching and tempering because the C content was excessive, and low temperature toughness was insufficient.

Comparative Example No. 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 lowered the toughness of the steel, the low temperature toughness of Comparative Examples 26, 27, and 34 was insufficient after quenching and tempering.

Comparative Example No. In Nos. 28 to 33, since the content of one or more of V, Ti, and Nb was excessive, the toughness of the steel decreased due to precipitation strengthening by VN, NbC, or Ti (C, N), and after quenching and tempering Comparative Example No. The low temperature toughness of 28 to 33 was insufficient.

Comparative Example No. No. 35 lacks the Ni content, and the low temperature toughness improvement effect by Ni is small, so the low temperature toughness is insufficient. On the other hand, Comparative Example No. No. 36 had a high Ni content, and the amount of retained austenite increased after quenching and tempering, so that low temperature toughness was insufficient after quenching and tempering.

Comparative Example No. In No. 37, the Mo content was insufficient, so that after quenching and tempering, cementite that became the starting point of fracture became coarse, and the martensite structure (block size) became coarse, so the low-temperature toughness was low.

Comparative Example No. No. 38 had insufficient Al content, so that a sufficient fine grain effect could not be obtained, the martensite structure (block size) became coarse after quenching and tempering, and low temperature toughness was insufficient.

Comparative Example No. In No. 39, since the N content was excessive, after quenching and tempering, the low temperature toughness was insufficient due to the increase in the solid solution N content.

Comparative Example No. In Nos. 40 to 42, since the Ca, Zr or Mg content was excessive, these elements lowered the toughness of the steel and the low temperature toughness was insufficient after quenching and tempering.

Comparative Example No. In Nos. 43 to 45, 48, and 49, the content of each alloy element was within the specified range, but because the Y value or Z value exceeded the specified range, the steel was embrittled and low temperature toughness was insufficient after quenching and tempering. .

Comparative Example No. In Nos. 46 and 47, the content of each alloy element was within the specified range, but since the Y value was below the specified range, the amount of retained austenite increased after quenching and tempering, and the low temperature toughness was insufficient.

Claims

In unit mass%
C: 0.08 to 0.12%,
Si: 0.05 to 0.50%,
Mn: 1.50 to 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 to 2.50%,
Cu: 0.01 to 0.50%,
Ni: 0.75 to 1.60%,
Mo: 0.10 to 0.50%,
Al: 0.025 to 0.050%,
N: 0.0100 to 0.0200%,
Ca: 0 to 0.0100%,
Zr: 0 to 0.0100% and Mg: 0 to 0.0100%
And the balance consists of Fe and impurities,
Y value defined by the following formula a is 2.6 or less,
A steel having a Z value defined by the following formula b of 1.5 or more and 3.0 or less.
Y = (Al) / (N) (a)
Z = (Mn) / (Ni) (b)
The symbols (Al), (N), (Mn), and (Ni) in the formula are the contents of the elements according to the symbols in the steel in unit mass%.
In unit mass%
Ca: 0.0005 to 0.0100%,
Zr: 0.0005 to 0.0100% and Mg: 0.0005 to 0.0100%
The steel according to claim 1, comprising at least one selected from the group consisting of: