KR102113045B1 - River - Google Patents

Abstract

The steel according to one embodiment of the present invention is 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%, the balance consisting of Fe and impurities, and the ratio of Al content and N content is 2.6 Below, the ratio of Mn content and Ni content is 1.5 or more and 3.0 or less.

Description

River

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

In recent years, with the changes in energy circumstances, the movement to develop new energy resources has been active all over the world. Under these circumstances, as the development resources on the land were depleted, attention was paid to the subsea oil field, and development using a rig of oil drilling was carried out in a wide area centering around the continental shelf. In particular, in recent years, offshore structures represented by rigs for subsea oil drilling operated in the deep sea have been increasing, and in order to prevent damage to the excavation rig caused by a large hurricane, it is required to increase the strength of the excavation rig mooring chain. The breaking of the chain is directly related to serious accidents such as the fall of the league. In order to secure safety, which is an important task, both the high strength and high toughness of the chain are oriented. Specifically, a chain having a tensile strength of 1200 MPa or more and a Charpy impact value of -20 ° C or more of 75 J / cm 2 or more is required.

In such a chain, a hot-rolled steel bar having a diameter of φ50 mm or more is cut to a predetermined length, formed into an annular shape, and the butt end face is flash butt welded to produce a chain. After the flash butt welding, the stud may be pressed into the center of the chain ring. Thereafter, the chain is subjected to a quench tempering treatment to impart high strength and high toughness to the chain.

Examples of the invention of high-strength high-toughness chain steels include, for example, Patent Documents 1 to 6. However, any document aims to provide 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, it is known that a further increase in strength is required for the chain, but generally, when the strength of the steel is increased, the toughness of the steel decreases, and thereby the impact value of the steel decreases. When the steel having the components suggested in these documents is set to a strength of 1200 MPa or more, the target impact value cannot be obtained.

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

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

The gist of the present invention is as follows.

(1) The steel according to one embodiment of the present invention is, 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%, the balance consisting of Fe and impurities, defined by the following formula a The 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 content of each element in the steel in units of mass%.

(2)

The steel described in (1) may contain 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% in unit mass%.

According to the present invention, after quenching and tempering, it is possible to provide a steel having a tensile strength of 1200 MPa or more and a Charpy impact value of -JC2 or more at -20 ° C.

1 is a graph showing the relationship between the Y value of steel and the impact value at -20 ° C of steel after quenching and tempering.
2 is a graph showing the relationship between the Z value of steel and the impact value at -20 ° C of steel after quenching and tempering.

The present inventors continued various studies in order to realize a steel having high strength and excellent low-temperature toughness, and the following knowledge was obtained.

(A) Reducing the C content in order to reduce the amount of cementite that may be the starting point of destruction is effective to improve the low-temperature toughness of steel. However, in order to make the tensile strength of steel after quenching tempering to 1200 MPa 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, it is not possible to sufficiently improve the low-temperature toughness of steel by this means alone.

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

However, the ratio Y (Y = [Al] / [N]) of 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 non-metallic inclusions in steel increase, and rather lower the low-temperature toughness.

(D) In addition, in order to sufficiently improve the low-temperature toughness of the steel, it is necessary that the ratio Z (Z = [Mn] / [Ni]) of the Mn content and the Ni content is 1.5 or more and 3.0 or less. When the value of Z is less than 1.5, the amount of retained austenite increases, and when the value of Z exceeds 3.0, the content of Mn dissolved in steel increases. In either case, the low temperature toughness of the steel is lacking. 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 becomes less than 1.5, thereby impairing the low-temperature toughness.

(E) Further, 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) produced from V, Nb and Ti lower the low temperature toughness of the steel.

(F) In addition, it is necessary to contain Mo in order to sufficiently improve the low-temperature toughness of the steel. This is because Mo makes the cementite, which causes low-temperature toughness lower, fine and harmless.

Based on the above knowledge, the present inventors have found a structural component having high strength and high low temperature toughness, particularly a steel capable of manufacturing a chain. Below, the concrete aspect of the steel which concerns on this embodiment is demonstrated. In addition, the steel according to the present embodiment is a steel having the effect that the tensile strength after quenching tempering is 1200 MPa or more and the Charpy impact value at -20 ° C is 75 J / cm 2 or more, but the strength and impact value before quenching tempering. Is not particularly limited. Hereinafter, unless otherwise specified, descriptions of mechanical properties such as strength and toughness relate to steels according to the present embodiment after quenching and tempering.

The reason for limiting the content of each alloying element in the steel according to the present embodiment will be described below. The unit "%" of the content of the alloying element means mass%.

C: 0.08 to 0.12%

C is an important element for determining the strength of steel. In order to obtain a tensile strength of 1200 MPa or more after quenching tempering, the lower limit of the C content is set to 0.08%. On the other hand, when the C content is excessive, the steel becomes excessively high in strength and the toughness of the steel decreases. In addition, when the C content is excessive, the amount of cementite serving as a starting point for destruction increases, and the toughness of the steel is significantly lowered. 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 a function as a deoxidizing agent as well as a function of securing the strength of the steel. When the Si content is less than 0.05%, a deoxidizing action is not sufficiently obtained, and non-metallic inclusions in the steel increase, deteriorating the toughness of the steel. On the other hand, when Si is contained exceeding 0.50%, Si causes the toughness of the steel to decrease. Therefore, Si content is made into 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 securing required quenching properties. In order to ensure sufficient quenchability to make the tensile strength of the steel after quenching tempering to 1200 MPa or more, the lower limit of the Mn content is set to 1.50%. 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 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 incorporated into the steel in the steel manufacturing process. When the P content exceeds 0.040%, the toughness of the steel is lowered beyond the allowable limit, so 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%. Since the steel according to the present embodiment does not require P, the lower limit of the P content is 0%, but considering the ability of the refining equipment, the lower limit of the P content may be 0.001%, 0.002%, or 0.003%. .

S: 0.020% or less

S, like P, is an impurity incorporated into the steel in the steel manufacturing process. When the S content exceeds 0.020%, S forms a large amount of MnS in the steel, thereby deteriorating 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 of the S content is 0%, but considering the ability of the refining equipment, the lower limit of the S content may be 0.001%, 0.002%, or 0.003%. .

Cr: 1.00 to 2.50%

Cr has an effect of increasing the quenchability of the steel. In order to secure sufficient quenchability, and thereby to make the tensile strength of the steel after quench tempering to 1200 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 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 quenching and corrosion resistance of steel materials. The lower limit of the Cu content is set to 0.01% in order to ensure sufficient quenchability and corrosion resistance to make the tensile strength of the steel after quench tempering to 1200 MPa or more. On the other hand, when the Cu content is excessive, the toughness of the steel decreases. Therefore, the upper limit 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 the steel, and is an essential element for high toughness of the steel according to the present embodiment after quenching and tempering. When the Ni content is less than 0.75%, it is difficult to sufficiently exhibit the effect. On the other hand, when the Ni content is excessive, the amount of retained austenite increases, so the low-temperature toughness is lowered. 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 to 0.50%

Mo has an effect of improving the low-temperature toughness of steel. Mo refines the cementite serving as a starting point for destruction and detoxifies it. In addition, Mo makes the block size of the martensite structure finer, lowers the ductile brittle transition temperature of the steel, thereby making brittle fracture difficult 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 toughness improving effect is saturated. Therefore, the content of Mo 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 deoxidation action, Al has a function of adjusting the crystallinity of the metal structure to precipitate the metal structure when precipitated as AlN. When the Al content is less than 0.025%, a sufficient atomization effect cannot be obtained, so the toughness of the steel is lowered. On the other hand, when Al is contained in the steel in excess of 0.050%, the precipitation amount of AlN is saturated, and the alumina-based non-metallic 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 has a function of combining with Al to precipitate effective AlN for adjusting the crystallinity of the metal structure. If the N content is less than 0.0100%, this action is not sufficiently exhibited. On the other hand, when N is contained in the steel in excess of 0.0200%, the solid solution N increases, and the toughness of the steel decreases. Therefore, the N content is set at 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, the content of V, Ti, and Nb is preferably smaller. This is because VN, NbC and Ti (C, N) produced from V, Nb and Ti lower the low-temperature toughness of steel. The present inventors have found that it is necessary to make 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 in order to prevent the low-temperature toughness of the steel from lowering. 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 content of V, Ti, and Nb is preferably small, the lower limit of the content of V, Ti, and Nb is 0%. However, when these elements are incorporated as impurities into the steel, it is sometimes undesirable to remove these elements completely from the steel, considering the cost-effectiveness. Therefore, the lower limit of the V content may be 0.003%, 0.002%, or 0.001% in consideration of the ability and economical efficiency of the refining equipment, and the lower limit of the Ti content may be 0.003%, 0.002%, or 0.001%, and the Nb content The lower limit value of may be 0.0010%, 0.0009%, or 0.0008%.

Ca: 0 to 0.0100%, Zr: 0 to 0.0100%, and Mg: 0 to 0.0100% or more.

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, all of Ca, Zr, and Mg form oxides, become crystallized 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 by 0.0005% or more, 0.0010% or more, or 0.0015% or more, and Zr may be contained in the steel by 0.0005% or more, 0.0010% or more, or 0.0015% or more, and Mg in the steel. You may contain 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%, excessive amounts 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 made 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%.

Balance: Fe and impurities

The remainder of the alloy component of the steel according to the present embodiment is made of Fe and impurities. The impurity is a component that is incorporated by various factors in the manufacturing process or raw materials such as ores or scraps when industrially manufacturing a steel material, and means that it is permitted in a range that does not adversely affect the steel according to the present embodiment. .

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

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

Y = (Al) / (N)… … Expression a

In Formula a, the symbol in parentheses indicates the content in terms of the unit mass% of the element related to the symbol.

AlN has an effect of improving the low-temperature toughness of steel by making crystal grains fine. However, when the ratio (Y value) of the Al content and the N content in the steel exceeds 2.6, the alumina-based nonmetallic inclusions in the steel increase, and the steel becomes brittle, so the low-temperature toughness is lowered. 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. Further, the lower limit of the Y value is not particularly limited, but considering the lower limit of the Al content and the upper limit of the N content, the Y value is not less than 1.25.

The present inventors obtained the above-mentioned knowledge by the experiment described below. The present inventors have all the features other than the Y value within the prescribed range of the steel according to the present embodiment, and quench tempering is performed on various steels having different Y values under the following conditions, and then Charpy at a temperature of -20 ° C. An impact test was performed.

Qing treatment: steel is heated to 900 ℃ and held for 30 minutes, then water cooled

· Tempering treatment: Steel is heated to 135 ℃ and held for 30 minutes, then air cooled

Thereby, the present inventors obtained a graph (FIG. 1) showing the relationship between the Y value and the impact value at -20 ° C. As shown in Fig. 1, steels having a Y value greater than 2.6 did not have sufficient low-temperature toughness after quenching and tempering.

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

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

Z = (Mn) / (Ni)… … Expression b

In formula b, the symbol in parentheses indicates the content in terms of unit mass% of the element related to the symbol.

As described above, Ni improves the low-temperature toughness of the 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. In addition, when the Z value is more than 3.0, the solid solution Mn content with respect to the Ni content becomes excessive, and the effect of improving the low temperature toughness by Ni is negated, so that the steel embrittles and the low temperature toughness decreases. Therefore, the Z value is set to be 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 obtained the above-mentioned knowledge by the experiment described below. The present inventors have all of the features other than the Z value within the prescribed range of the steel according to the present embodiment, and quench tempering is performed on various steels having different Z values under the following conditions, and then Charpy at a temperature of -20 ° C. An impact test was performed.

Qing treatment: steel is heated to 900 ℃ and held for 30 minutes, then water cooled

· Tempering treatment: Steel is heated to 135 ℃ and held for 30 minutes, then air cooled

Thereby, the present inventors obtained a graph (FIG. 2) showing the relationship between the Z value and the impact value at -20 ° C. As shown in Fig. 2, steels with a Z value of 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, particle size, and dispersion state of AlN in the steel vary depending on the conditions of heat treatment (for example, quenching and tempering) performed on the steel. In addition, when the content of Al and N is controlled as described above, regardless of the state of AlN before quenching and tempering, AlN effectively functions during quenching and tempering under conditions selected to set the tensile strength of steel to 1200 MPa or more. , Improve the toughness of the steel. That is, the subject of the steel according to the present embodiment is to perform a heat treatment on the steel so that the tensile strength is 1200 MPa, so that the Charpy impact value at -20 ° C of the steel is 75 J / cm 2 or more. It is not necessary for solving the lecture subject according to the embodiment. Therefore, in the steel according to the present embodiment, the state of AlN is not particularly defined. In addition, the present inventors, as a result of the experiment, when the steel is heated to 850 to 900 ° C, AlN is preferably precipitated regardless of the state of the steel before heating, and when the steel in this state is cooled, the structure is preferably refined by AlN. Estimate.

Even if the steel according to the present embodiment is quenched and tempered so that the tensile strength is 1200 MPa or more, the Charpy impact value at -20 ° C can be maintained 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, when the steel according to the present embodiment is subjected to a quenching treatment that is performed by heating at 900 ° C for 30 minutes and then water-cooling, and also by performing a tempering treatment that is heated at 135 ° C and maintained for 30 minutes, the tensile strength is 1200 MPa or more. In addition, steel having a Charpy impact value of at least -20 ° C of 75 J / cm 2 or more is obtained. The steel according to the present embodiment after heat treatment under this quenching tempering condition has an average particle diameter of cementite of 0.05 µm or less, an average size of martensite blocks of 5.5 µm or less, and a content of residual austenite of 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 tempering condition. Normally, when the tensile strength of the steel is 1200 MPa or more, low-temperature toughness (especially 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, so even when heat treated under this quenching tempering condition, martensite Block and cementite are sufficiently refined, and have high low-temperature toughness. Moreover, 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 this quenching tempering condition. Excess amounts of Al and Ni may impair low-temperature toughness, but the steel according to the present embodiment has a low Al-N content and an N-ratio and a Ni-Mn content, so the low-temperature toughness is impaired. Does not work. In addition, 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, so even when heat treated under this quenching tempering condition, inclusion precipitates It is suppressed and has high low temperature toughness.

Note that the quenching tempering treatment according to the above-described conditions is only an example of the use of steel according to the present embodiment. The steel according to the present embodiment can be subjected to heat treatment in any condition, depending on the purpose. In addition, the characteristics of the steel according to the present embodiment after heat treatment based on an example of the quenching tempering condition described above is not limited to the technical scope of the steel according to the present embodiment. The subject of the steel according to the present embodiment is to set the Charpy impact value at -20 ° C to 75 J / cm 2 or more after heat treatment so that the tensile strength is 1200 MPa or more. In order to solve this problem, it is necessary to control the chemical component, the ratio of the Al content and the N content, and the ratio of the Ni content and the Mn content, as described above. However, other configurations, such as control of martensite, cementite, and retained austenite 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, particularly excellent effects can be exhibited when used as a material such as a chain for mooring submarine oil rigs.

Example

The present invention will be described in detail below by way of examples. In addition, these examples are intended to illustrate 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, the steel of the chemical composition shown in Table 1 was solvent and hot forged to obtain a round bar steel having a diameter of 86 mm. The round bar steel was cut, heated to 900 ° C, held for 30 minutes, and then quenched by water cooling, and then heated to 135 ° C to hold for 30 minutes, followed by a tempering treatment. These quench 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 JIS 14A tensile test specimens and four JIS 4 V notch Charpy impact test specimens were produced from the 1 / 4D section of the C section of the round bar steel (a region of about 1/4 of the diameter D of the round bar from the surface of the round bar steel). The tensile test was performed in accordance with JIS Z 2241 at a rate of 20 mm / min at room temperature. The Charpy impact test was performed in accordance with JIS Z 2242 at -20 ° C.

In addition, a square sample of 10 mm on one side is cut out from the 1 / 4D section on the C cross section of the round bar steel after quenching tempering, and the cross section of the sample is corroded using a nitrile etchant, and the sample is magnified 5000 times using a scanning electron microscope. Five cross-sectional tissue photographs were taken, and the average particle diameter of cementite contained in these pictures was determined by image analysis using Luzex (registered trademark), and this was referred to as the average particle diameter of cementite of the round bar steel. Further, crystal orientation analysis was performed using a backscattered electron beam diffraction pattern, and the area weighted average circle equivalent diameter of the crystal grains surrounded by a high angle grain boundary of 15 degrees or more of the azimuth angle obtained by this analysis was measured, and marten of round bar steel It was called the average particle diameter of the site block. In addition, the amount of retained austenite in the round bar steel was measured by X-ray diffraction.

The results of the above-described experiments are shown in Table 1-1, Table 1-2, and Table 2. Table 1-1 and Table 1-2 show the chemical components of the Example Steel and Comparative Example Steel, and Table 2 shows the tensile strength, impact value, and cementite of the Example Steel and Comparative Example Steel after quenching and tempering under the above-described conditions. It shows the average particle diameter of, the average size of a martensite block, and the amount of retained austenite. In Table 1-2, values outside the prescribed range of the present invention are underlined.

[Table 1-1]

[Table 1-2]

[Table 2]

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

In contrast, Comparative Example No. Since 24 had insufficient C content, the required tensile strength was not obtained after quenching and tempering. Comparative Example No. Since the content of C was excessive in 25, it became excessively high in strength after quenching tempering, and 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 excess Si, Mn, and Cr lowered the toughness of the steel, after quenching tempering, the low-temperature toughness of Comparative Examples 26, 27, and 34 was insufficient.

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

Comparative Example No. 35 had insufficient Ni content, and because the effect of improving the low temperature toughness by Ni was small, the low temperature toughness was insufficient. On the other hand, Comparative Example No. 36 had a large amount of Ni, and the amount of retained austenite increased after quenching tempering, and therefore, low temperature toughness was insufficient after quenching tempering.

Comparative Example No. Since 37 had insufficient Mo content, after quenching tempering, cementite serving as a starting point for fracture became coarse, and martensite structure (block size) became coarse, so low-temperature toughness was low.

Comparative Example No. Since the Al content of 38 was insufficient, a sufficient particulate effect was not obtained, and the martensite structure (block size) became coarse after quenching tempering, and low-temperature toughness was insufficient.

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

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

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

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

Claims (2)

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%
It contains, the balance is made of Fe and impurities,
Y value defined by the following formula a is 2.6 or less,
Z value defined by the following formula b is 1.5 or more, 3.0 or less
River characterized in that.
Y = (Al) / (N)… (a)
Z = (Mn) / (Ni)… (b)
The symbols (Al), (N), (Mn), and (Ni) in the formula are the content of each element in the steel in units of mass%. The method of claim 1, wherein the unit mass%,
Ca: 0.0005 to 0.0100%,
Zr: 0.0005 to 0.0100%, and
Mg: 0.0005 to 0.0100%
Steel comprising at least one selected from the group consisting of.

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