JP6648659B2 - Machine structural parts - Google Patents

Description

  The present invention relates to a machine structural component, and more particularly to a machine structural component having excellent delayed fracture resistance.

  In order to reduce the weight and performance of automobiles and various industrial machines, or to reduce the construction costs of civil engineering and building structures, the strength of mechanical structural parts has been increased.

  For example, the bolt is manufactured by quenching and tempering after forming into a predetermined shape using low alloy steel such as SCM435 and SCM440 specified in JIS G 4053. Therefore, in order to easily increase the strength of the bolt, the tempering temperature may be lowered.

  However, in a mechanical structure component having a tensile strength exceeding 1200 MPa, delayed fracture, which is a kind of hydrogen embrittlement, becomes a problem. The above-mentioned "delay fracture" is a phenomenon in which a part placed under static stress suddenly and brittlely fractures after a certain period of time, and occurs even with a small amount of hydrogen. When the above parts are used outdoors, especially in an environment where salt such as seawater and snowmelt flies, the pH under the rust layer decreases, the amount of hydrogen penetration increases, and the risk of delayed fracture increases. .

Therefore, steel or steel parts in consideration of hydrogen embrittlement such as delayed fracture have been studied. For example, Patent Literature 1 discloses a steel having excellent hydrogen fatigue fracture resistance utilizing (Mo, V) ₂ C serving as a hydrogen trap site, and Patent Literature 2 discloses a hydrogen trap and an alloy carbide for high strength. Patent Document 3 discloses a steel excellent in delayed fracture resistance in which the microstructure is controlled in addition to the chemical composition of the steel material.

JP 2002-327235 A JP-A-2006-045670 JP 2007-031734 A

Akio Saito, Yunosuke Tokuhiro, Shiro Yoshizawa, Koji Yamakawa, Kazutoshi Nakao: Anticorrosion Technology, 26 (1977), pp. 503-508

  The steel disclosed in Patent Document 1 is excellent in hydrogen fatigue fracture resistance, and the steel disclosed in Patent Document 2 is excellent in delayed fracture resistance. Further, the bolt disclosed in Patent Document 3 also has excellent delayed fracture resistance. However, in each of the above-mentioned patent documents, it is not sufficient to particularly study means for suppressing intrusion of hydrogen (hereinafter, referred to as “hydrogen from the environment in use”) generated by corrosion that greatly affects the environment in use. Absent. Therefore, the technologies disclosed in Patent Documents 1 to 3 have room for improvement from the viewpoint of suppressing the amount of hydrogen entering from the environment in use.

  SUMMARY OF THE INVENTION It is an object of the present invention to provide a high-strength component for mechanical structure having excellent delayed fracture resistance by suppressing the amount of hydrogen entering from the environment in use by optimizing the chemical composition.

  In order to solve the above-mentioned problems, the present inventors have quenched and tempered low alloy steel materials having various chemical compositions to obtain a hydrogen content which causes delayed fracture of a machine structural component, among others, A detailed investigation was conducted on the amount of hydrogen intrusion from the environment. As a result, as shown in an example in Examples described later, a completely new finding has been obtained that the presence of dissolved Mo is particularly important for suppressing the amount of hydrogen entering from the environment during use.

  The present invention has been completed based on the above findings, and the gist of the present invention resides in the following components for machine structure.

(1) The chemical composition is expressed in mass%
C: 0.21-0.34%,
Mn: 0.10 to 1.0%,
Mo: 1.50 to 4.0%,
V: 0.10 to 0.35%,
Al: 0.01 to 0.10%,
N: 0.001 to 0.015%,
Nb: 0 to 0.04%,
Ti: 0 to 0.04%,
Cr: 0 to 1.0%,
B: 0 to 0.0060%,
Ca: 0 to 0.005%,
Mg: 0 to 0.005%,
Zr: 0 to 0.005%,
The balance is Fe and impurities,
Si, P and S as impurities are Si: 0.15% or less, P: 0.015% or less, and S: 0.015% or less;
Further, Fn1 represented by the following equation [1] exceeds 0,
The average size of MC type carbide in steel is 1 to 30 nm,
A tensile strength of 1200 MPa or more;
Parts for machine structure.
Fn1 = (Mo / 96) + (V / 51)-(C / 12) ... [1]
However, Mo, V and C in the formula [1] mean the contents (% by mass) of the respective elements in the steel.

  (2) The component for a machine structure according to the above (1), which contains at least one selected from 0.005 to 0.04% of Nb and 0.005 to 0.04% of Ti in mass%. .

  (3) The component for a machine structure according to the above (1) or (2), containing Cr: 0.1 to 1.0% by mass%.

  (4) The component for a machine structure according to any one of the above (1) to (3), which contains B: 0.0003 to 0.0060% by mass%.

  (5) In mass%, it contains one or more kinds selected from Ca: 0.0002 to 0.005%, Mg: 0.0002 to 0.005% and Zr :: 0.0002 to 0.005%. The component for machine structure according to any one of (1) to (4).

  (6) The component for a machine structure according to any one of (1) to (5), wherein the entire surface is covered with a galvanized layer.

  Advantageous Effects of Invention According to the present invention, it is possible to provide a high-strength component for a mechanical structure having excellent delayed fracture resistance by suppressing intrusion of hydrogen from the environment in use. Therefore, the present invention has a remarkable industrial contribution.

It is the figure which arranged the test result of (Example 2) on the horizontal axis | shaft and the vertical axis | shaft, respectively taking the tensile strength (it showed in the range of 1200-1600 MPa) and the breaking strength ratio. It is the figure which arranged the test result of (Example 3) by taking the tensile strength and the breaking strength ratio on the horizontal axis and the vertical axis, respectively.

  Hereinafter, each requirement of the present invention will be described in detail. In addition, “%” of the content of each element means “% by mass”.

(A) Chemical composition:
C: 0.21 to 0.34%
C is an element that improves the strength of steel, and it is necessary to contain 0.21% or more in order to increase the strength of the component for mechanical structure. On the other hand, when the content of C exceeds 0.34%, the cold workability decreases. Therefore, the content of C is set to 0.21 to 0.34%. A preferred lower limit of the C content is 0.25%, and a preferred upper limit is 0.30%. In addition, the content of C needs to exceed 0 in the above-mentioned Fn1 [= (Mo / 96) + (V / 51)-(C / 12)].

Mn: 0.10-1.0%
Mn is an effective element for deoxidizing steel and is a useful element for improving the strength and hardenability of steel. In order to obtain the above effects, it is necessary to contain Mn at 0.10% or more. On the other hand, if the content of Mn exceeds 1.0%, segregation at the grain boundaries causes delayed fracture of a grain boundary cracking type, and the cold workability at the time of working into a part shape due to an increase in hardness increases. Or deteriorate. Therefore, the content of Mn is set to 0.10 to 1.0%. A preferred lower limit of the Mn content is 0.20%, and a preferred upper limit is 0.70%.

Mo: 1.50 to 4.0%
Mo is an important element in the present invention. Mo improves the strength and delayed fracture resistance by improving the strength and hardenability of steel, and increasing the strength by precipitation strengthening by forming fine carbides at the time of high-temperature tempering around 600 ° C. Make them compatible. Further, Mo improves delayed fracture resistance in a solid solution state through an effect of reducing the amount of intrusion of hydrogen from the environment in use. In order to obtain these effects, it is necessary to contain Mo at 1.50% or more. On the other hand, even if Mo is contained in excess of 4.0%, their effects are saturated and high-temperature workability such as hot rolling is impaired. Therefore, the content of Mo is set to 1.50 to 4.0%. A preferred lower limit of the Mo content is 2.2%, and a preferred upper limit is 2.9%. The content of Mo needs to be greater than 0 in the above Fn1 [= (Mo / 96) + (V / 51)-(C / 12)].

V: 0.10 to 0.35%
V is an important element in the present invention. By containing an appropriate amount of V in combination with Mo, the carbide of Mo is not Mo ₂ C but a fine MC-type carbide of (Mo, V) C. Therefore, V is effective in improving delayed fracture resistance by reducing the amount of hydrogen entering from the environment during use by securing solid solution Mo. In addition, fine (Mo, V) C increases strength by precipitation strengthening during high-temperature tempering at around 600 ° C., thereby achieving both improvement in strength and delayed fracture resistance. In order to obtain these effects, V must be contained at 0.10% or more. On the other hand, if the content of V is excessive, coarse carbonitrides are formed at a temperature of about 900 ° C. such as during quenching heating, so that the quenching heating temperature is increased to form a solid solution of the carbonitrides in the matrix. Necessity arises and the production cost increases. Therefore, the content of V is set to 0.10 to 0.35%. A preferred lower limit of the V content is 0.17%, and a preferred upper limit is 0.29%. Note that the content of V needs to be greater than 0 in the aforementioned Fn1 [= (Mo / 96) + (V / 51)-(C / 12)].

Al: 0.01 to 0.10%
Al is an element necessary for the deoxidation of steel, and has an effect of forming a nitride to reduce the austenite grain size during heat treatment. Therefore, Al is contained in an amount of 0.01% or more. On the other hand, even if Al is contained in excess of 0.10%, the above effect is saturated. Therefore, the content of Al is set to 0.01 to 0.10%. A preferred upper limit of the Al content is 0.05%. In addition, the Al content of the present invention indicates the content in total Al.

N: 0.001 to 0.015%
N is contained in an amount of 0.001% or more in order to form a nitride or a carbonitride which is effective in reducing the austenite grain size. On the other hand, if the content of N exceeds 0.015%, coarse nitrides are formed, and the ductility is reduced. Therefore, the content of N is set to 0.001 to 0.015%. A preferred lower limit of the N content is 0.003%, and a preferred upper limit is 0.008%.

Nb: 0 to 0.04%
Nb is a useful element that forms fine carbides, nitrides, and carbonitrides and suppresses coarsening of crystal grains. Therefore, Nb may be contained as needed. However, when the content of Nb is increased and exceeds 0.04%, a coarse carbonitride is formed, so that necessary strength cannot be obtained. Therefore, the upper limit of the Nb content when it is contained is set to 0.04%. In order to stably obtain the above effects, the lower limit of the Nb content is preferably 0.005%.

Ti: 0 to 0.04%
Ti is a useful element that forms fine carbides, nitrides, and carbonitrides and suppresses coarsening of crystal grains. For this reason, you may make it contain Ti as needed. However, if the content of Ti exceeds 0.04%, coarse carbonitrides are formed, and the required strength cannot be obtained. Therefore, the upper limit of the Ti content when it is contained is set to 0.04%. In order to stably obtain the above effects, the lower limit of the Ti content is preferably 0.005%.

  It is preferable that the total amount when the above-mentioned Ti and Nb are combined and contained is 0.05% or less.

Cr: 0 to 1.0%
Cr is an element effective for improving hardenability and increasing softening resistance during tempering. Therefore, Cr may be contained as necessary. However, even if the content of Cr exceeds 1.0%, the above effect is saturated. Therefore, the upper limit of the Cr content when it is contained is set to 1.0%. The upper limit of the Cr content is preferably 0.6%. In order to stably obtain the above effects, the lower limit of the Cr content is preferably 0.1%.

B: 0 to 0.0060%
B is a useful element that significantly improves the hardenability of steel. Therefore, B may be contained as necessary. However, when the content of B exceeds 0.0060%, the effect is saturated. Therefore, the upper limit of the B content when it is contained is set to 0.0060%. In order to stably obtain the above effects, the lower limit of the B content is preferably 0.0003%.

Ca: 0 to 0.005%
Ca is an element effective for controlling the form of sulfide. That is, Ca has a form control effect of preventing elongation of MnS in the rolling direction, and suppresses deterioration of workability and toughness. Further, Ca also has a deoxidizing effect. For this reason, Ca may be contained as necessary. However, when Ca is contained in excess of 0.005%, coarse inclusions are formed, and on the contrary, the toughness is reduced. Therefore, the upper limit of the Ca content when it is contained is set to 0.005%. In order to stably obtain the above effects, the lower limit of the Ca content is preferably 0.0002%.

Mg: 0 to 0.005%
Mg is an element effective for controlling the form of the sulfide, like Ca. Therefore, Mg may be contained as necessary. However, even if Mg is contained in excess of 0.005%, the effect is saturated. Therefore, the upper limit of the Mg content when it is contained is set to 0.005%. In order to stably obtain the above effects, the lower limit of the Mg content is preferably 0.0002%.

Zr: 0 to 0.005%
Zr, like Ca and Mg, is an effective element for controlling the form of sulfide. Therefore, Zr may be contained as necessary. However, even if Zr is contained in excess of 0.005%, the effect is saturated. Therefore, the upper limit of the Zr content when it is contained is set to 0.005%. In order to stably obtain the above effects, the lower limit of the Zr content is preferably 0.0002%.

  When Ca, Mg and Zr are combined and contained, the total amount is preferably 0.006% or less.

  The mechanical structural component of the present invention is composed of the above-described elements and the balance of Fe and impurities. Si, P, and S as impurities are as follows: Si: 0.15% or less; P: 0.015% or less; S: 0.015% or less, and Fn1 represented by the above formula [1] has a chemical composition exceeding 0.

  Here, “impurities” are components that are mixed in due to various factors in the ore, scrap and other raw materials and the production process when the steel material is industrially produced, and are allowable as long as they do not adversely affect the present invention. Means what is done.

Si: 0.15% or less Si is an element mixed as an impurity in the steel manufacturing process. When the Si content exceeds 0.15%, the cold workability when working into a part shape is deteriorated due to an increase in hardness. Therefore, the content of Si is set to 0.15% or less. The content of Si is preferably 0.10% or less, more preferably 0.07% or less, and is preferably as low as possible.

P: 0.015% or less P is contained as an impurity, and embrittles the crystal grain boundaries of the mechanical structure component after quenching and tempering, thereby reducing delayed fracture resistance. Therefore, the content of P is set to 0.015% or less. It is preferable that the content of P is as low as possible.

S: 0.015% or less S is an impurity, usually present in steel as Mn sulfide, and promotes hydrogen penetration by dissolving in neutral and acidic aqueous solutions, thereby deteriorating delayed fracture resistance. . Therefore, the content of S is set to 0.015% or less. The S content is preferably as low as possible.

Fn1 exceeds 0 In the mechanical structure component of the present invention, Fn1 represented by the following formula [1] exceeds 0.
Fn1 = (Mo / 96) + (V / 51)-(C / 12) ... [1]
However, Mo, V and C in the formula [1] mean the contents (% by mass) of the respective elements in the steel.

  Fn1 is an index by which the mechanical structural component of the present invention can secure solid solution Mo, reduce the amount of hydrogen entering from the environment in use, and obtain good delayed fracture resistance. As described above, by adding Mo and V in combination, a fine MC-type carbide of (Mo, V) C is formed by high-temperature tempering at around 600 ° C. This MC type carbide is effective in ensuring strength when high-temperature tempering is performed, which is effective in improving delayed fracture resistance. On the other hand, even if the MC type carbide is used, solid solution Mo is consumed which is effective in suppressing the amount of hydrogen entering from the environment in use. Will be invited. However, when Fn1 exceeds 0, solid solution Mo is ensured even when the MC-type carbide is formed, so that the amount of hydrogen penetration from the environment in use is reduced and good delayed fracture resistance is obtained. Can be A preferred lower limit of Fn1 is 0.01. Note that the upper limit of Fn1 may be a value close to 0.031 when the contents of Mo, V, and C are 4.0%, 0.35%, and 0.21%, respectively. 0.020.

(B) MC type carbide:
In the mechanical structural component of the present invention having the chemical composition described in the above section (A), the average size of MC type carbide in steel is 1 to 30 nm. Since the MC-type carbide has a disc shape, the average size of the MC-type carbide is measured by measuring the major axis of the carbide observed in a TEM image at a magnification of about 400,000 times, and averaging the number of carbides to be observed. Indicates the calculated value. Note that MC-type carbide having a major axis of 1 nm or more was used as an observation target.

  If the average size of the MC type carbide is less than 1 nm, the amount of precipitation strengthening is insufficient, so that the delayed fracture resistance deteriorates. On the other hand, if the average size of the MC-type carbide exceeds 30 nm, the amount of precipitation strengthening is rather reduced and C in the matrix is consumed, so that the strength required for a machine structural component cannot be obtained.

  The above-mentioned MC-type carbide in the machine structural component of the present invention refers to an MC-type composite carbide of Mo and V. The mechanical structural component of the present invention includes Nb and / or Ti as an optional element in its chemical composition. When contained, it also includes carbides containing Nb and / or Ti of the MC type.

(C) Regarding tensile strength of machine structural parts:
The mechanical structure component of the present invention has a tensile strength of 1200 MPa or more. If the tensile strength is 1200 MPa or more, it can be suitably used as a machine structural part in a field where high strength is being promoted in recent years, for example, in applications such as automobiles, various industrial machines, and civil engineering / building structures. Can be.

(D) About plating for rust prevention of machine structural parts:
The entire surface of the machine structural component of the present invention can be rust-proofed by zinc plating. When a component having a tensile strength of 1200 MPa or more is subjected to rust prevention treatment by zinc plating, when the component is damaged and the iron base is exposed, the iron base becomes a cathode and becomes a hydrogen generation site. Therefore, the delayed fracture resistance of conventional steel parts for high-strength mechanical structures is reduced by galvanization. However, the mechanical structural component of the present invention having a tensile strength of 1200 MPa or more is excellent in delayed fracture resistance, and the delayed fracture resistance is deteriorated even when the rust-proofing treatment is performed by zinc plating to expose the iron base. Nothing. The thickness of the galvanized layer may be about 2 to 50 μm.

  The component for a mechanical structure of the present invention can be manufactured, for example, by the following method, but is not limited to this method.

  After melting the steel having the chemical composition described in the section (A), it is cast into an ingot or a slab. The cast ingot or slab is finished into a steel material having a required rough shape such as a steel plate or a round bar by hot working such as hot rolling, hot extrusion, and hot forging. Thereafter, the steel material is subjected to hot forging, cutting, or the like, and formed into a predetermined machine structural component shape. For parts requiring a cold working process such as cold forging, the steel material after hot working may be subjected to annealing or spheroidizing annealing in order to improve cold workability. In the case of a bolt or the like that requires dimensional accuracy, drawing may be performed before cold forging.

After forming into a predetermined part shape, in order to impart strength, the steel is heated to a temperature of _three or more Ac and then quenched by water cooling or oil cooling. If the heating temperature for quenching (hereinafter referred to as “quenching heating temperature”) is too low, the solid solution of Mo, V, Nb and Ti in the matrix of the carbonitride becomes insufficient and the MC-type carbide Has an average size of more than 30 nm, it is not possible to obtain the strength required for a machine structural component. On the other hand, from the viewpoint of operation, the furnace body and the accessory parts of the heat treatment furnace are significantly damaged, and the production cost is increased. Therefore, the quenching heating temperature is preferably set to 870 to 950 ° C.

  In order to improve the delayed fracture resistance, it is necessary to perform tempering after performing the above quenching treatment. The tempering temperature is preferably 500 to 650 ° C. If the tempering temperature is lower than 500 ° C., the average size of the MC-type carbide is less than 1 nm, so that sufficient delayed fracture resistance may not be obtained. On the other hand, when tempering is performed at 650 ° C. or higher, the MC type carbide may grow Ostwald and its average size exceeds 30 nm, and the strength may be reduced.

  The galvanizing process for covering the entire surface for rust prevention may be performed by a general method such as electroplating or hot-dip galvanizing. For example, the plating may be performed by a barrel plating method using an ammonium chloride bath and a method of performing a trivalent chromate treatment, and the thickness of the plating layer may be about 2 to 50 μm.

  Hereinafter, the present invention will be described more specifically with reference to Examples, but the present invention is not limited to these Examples.

(Example 1)
Steels A to E having the chemical compositions shown in Table 1 were melted, and the ingot obtained by casting in a mold was heated to 1250 ° C., and then formed into a round bar having a diameter of 70 mm by hot forging.

  Steels A and B in Table 1 are steels whose chemical compositions are within the range specified by the present invention, while steels C to E are steels whose chemical compositions deviate from the conditions specified by the present invention. .

  From the round bar having a diameter of 70 mm obtained as described above, a disk having a diameter of 70 mm and a length of 20 mm was cut out and subjected to heat treatment of quenching and tempering. The quenching heating temperature was 900 ° C and the tempering temperature was 500 to 600 ° C.

  After quenching and tempering, the disc having a diameter of 70 mm and a length of 20 mm is machined and polished with emery paper to finish the disc with a diameter of 70 mm and a thickness of 0.5 mm. Ni plating of 0.5 μm was applied.

  Next, the hydrogen permeation coefficient of each steel type was determined by a method using a double cell type cathode charge hydrogen permeation test apparatus based on Non-Patent Document 1. Table 1 also shows the hydrogen permeability coefficients determined as described above.

  From Table 1, the steel A in which the above-mentioned Fn1 [= (Mo / 96) + (V / 51)-(C / 12)] exceeds 0 (becomes a positive value) and solid solution Mo is secured, and It is clear that the hydrogen permeability coefficient of steel B is smaller than the hydrogen permeability coefficient of steels C to E where Fn1 is negative, that is, the amount of invading hydrogen is small.

(Example 2)
Steels 1 to 33 having the chemical compositions shown in Table 2 were melted and cast into a mold, and the obtained ingot was heated to 1250 ° C., and then formed into a round bar having a diameter of 20 mm by hot forging.

  Steels 1 to 26 and steel 33 in Table 2 are steels whose chemical compositions are within the range specified in the present invention, while steels 27 to 32 are steels whose chemical compositions are out of the conditions specified in the present invention. It is.

  The above-mentioned round bar having a diameter of 20 mm was subjected to normalizing treatment at 900 to 960 ° C., and thereafter, was cut into a round bar having a diameter of 14 mm.

  Next, the round bar finished to 14 mm in diameter was quenched and tempered under the conditions shown in Table 3. The holding time for quenching and heating was set to 60 minutes, and the holding time for tempering was set to 90 minutes.

  For each steel, the following various investigations were performed using the quenched and tempered round bars.

<1> Tensile properties:
In accordance with JIS Z 2241, from the center of each of the quenched and tempered round bars having a diameter of 14 mm, a round bar tensile test piece of No. 14A having a parallel portion diameter of 6 mm was sampled in the longitudinal direction. Was subjected to a tensile test in the air to determine the tensile strength.

<2> Average size of MC type carbide:
For the test numbers for which a tensile strength of 1200 MPa or more was obtained in the investigation of <1> above, the position of D / 4 of the round bar having a diameter of 14 mm after the quenching-tempering treatment ("D" represents the diameter of the round bar). ) Was prepared, and the average size of MC type carbide was determined by measuring as described above using a photograph taken at a magnification of 400,000 times using a TEM with a total visual field of 100,000 nm ² using TEM. Was. TEM-EDS was used to confirm MC type carbide.

<3> Delayed fracture resistance:
Regarding the test number for which a tensile strength of 1200 MPa or more was obtained in the investigation of <1> above, the parallel part diameter was 7 mm in the longitudinal direction from the center of the round bar having a diameter of 14 mm after quenching-tempering, and the center was A test piece having an annular notch with a depth of 1.4 mm was sampled. The notch was a V-shaped 60 °, and the radius of curvature at the bottom of the notch was 0.175 mm. Using this test piece, at room temperature, it was immersed in an aqueous solution of 0.005% HCl + 3% NaCl at a concentration of mass% for 24 hours, and then a tensile test was performed at a low speed of 0.006 mm / min in the immersion state. The breaking stress (T1) in the aqueous solution was determined. Similarly, a tensile test was performed in the air at room temperature at the above-mentioned speed of 0.006 mm / min, and the breaking stress (T2) in the air was determined. From the rupture stress (T1) in the aqueous solution and the rupture stress (T2) in the atmosphere, T1 / T2 as a fracture strength ratio was calculated to evaluate the delayed fracture resistance. The closer the fracture strength ratio is to 1, the better the delayed fracture resistance is.

  Table 3 also shows the results of each of the above surveys. Further, in FIG. 1, the horizontal axis and the vertical axis respectively show the tensile strength (shown in the range of 1200 to 1600 MPa) and the fracture strength ratio, and compare the delayed fracture resistance of each steel. .

  From Table 3 and FIG. 1, all of the test numbers 1 to 26 of the examples of the present invention satisfying the conditions of the chemical composition and the average size of MC type carbide specified in the present invention can obtain a tensile strength exceeding 1200 MPa, and It is clear that the sample has excellent delayed fracture resistance as compared with Test Nos. 28 to 33 of Comparative Examples having the same tensile strength level.

  In the case of the test numbers 27 to 33 of the comparative examples, the tensile strength of 1200 MPa or more was not obtained or the inferior in the delayed fracture resistance as compared with the test numbers 1 to 26 of the present invention example having the same tensile strength level. ing. Hereinafter, comparative examples after the test number 27 will be individually described.

  In Test No. 27, since the C content of the steel 27 used was as small as 0.15%, which was outside the conditions specified in the present invention, the tensile strength was less than 1200 MPa.

  In Test No. 28, the Fn1 of the steel 28 used was -0.0069, which was out of the conditions specified in the present invention, and thus the steel 28 was inferior in delayed fracture resistance.

  In Test No. 29, the Mo content of Steel 29 used was as low as 1.13%, and Fn1 was -0.0101, which was out of the conditions specified in the present invention. I have.

  In Test No. 30, the contents of P and S in the impurities of the steel 30 used were as high as 0.046% and 0.036%, respectively, and deviated from the conditions specified in the present invention. ing.

  Test No. 31 is inferior in the delayed fracture resistance because the V content of the steel 31 used is as small as 0.05%, which is out of the conditions specified in the present invention.

  In Test No. 32, since the Mn content of the steel 32 used was as large as 1.44%, which was out of the conditions specified in the present invention, the delayed fracture resistance was inferior.

  In Test No. 33, although the chemical composition of the steel 33 used was within the range specified in the present invention, no MC-type carbide having a major axis of 1 nm or more, which was an observation target, was found. For this reason, the average size of the MC type carbide is less than 1 nm, which is out of the conditions specified in the present invention, and is inferior in delayed fracture resistance.

(Example 3)
Steels 34 to 45 having the chemical compositions shown in Table 4 were melted, and the ingot obtained by casting in a mold was heated to 1250 ° C., and then formed into a round bar having a diameter of 20 mm by hot forging.

  Steels 34 to 41 and steel 45 in Table 4 are steels whose chemical compositions are within the range specified in the present invention, while steels 42 to 44 are steels whose chemical compositions are out of the conditions specified in the present invention. It is.

  The above-mentioned round bar having a diameter of 20 mm was subjected to normalizing treatment at 900 to 960 ° C., and thereafter, was cut into a round bar having a diameter of 14 mm.

  Next, the round bar finished to a diameter of 14 mm was quenched and tempered under the conditions shown in Table 5. The holding time for quenching and heating was set to 60 minutes, and the holding time for tempering was set to 90 minutes.

  For each steel, "tensile properties" and "average size of MC type carbide" were investigated by the same method as in the above (Example 2) using the above-mentioned quenched and tempered round bar. An investigation of “delay resistance to fracture” was conducted by the method shown in 4>.

<4> Delayed fracture resistance:
From the center of the round bar having a diameter of 14 mm after the quenching and tempering treatment, a test piece having a parallel portion diameter of 7 mm was sampled in the longitudinal direction. The test piece was provided with a zinc plating having a thickness of 10 μm. The zinc plating was performed by a barrel plating method using an ammonium chloride bath, and a trivalent chromate treatment was performed. Thereafter, an annular notch having a depth of 1.4 mm was provided at the center of the test piece. The notch was a V-shaped 60 °, and the radius of curvature at the bottom of the notch was 0.175 mm. This test piece was immersed in a 5% NaCl aqueous solution at a concentration of 5% by mass at room temperature for 24 hours, and then a tensile test was performed at a low speed of 0.006 mm / min in the immersed state. At (3) was determined. Similarly, a tensile test was performed in the air at room temperature at the above-mentioned speed of 0.006 mm / min, and the breaking stress (T2) in the air was determined. From the above-described fracture stress (T3) in the aqueous solution and the fracture stress (T2) in the atmosphere, T3 / T2, which is a fracture strength ratio, was calculated, and the delayed fracture resistance was evaluated. The closer the fracture strength ratio is to 1, the better the delayed fracture resistance. In addition, in order to simulate the phenomenon in which the iron base exposed when the plating is damaged becomes a cathode and hydrogen enters from there, the notch bottom was examined in the exposed state without plating as described above. Was.

  Table 5 also shows the results of each of the above surveys. Further, in FIG. 2, the horizontal axis and the vertical axis respectively show the tensile strength and the fracture strength ratio, and show the delayed fracture resistance of each steel in comparison.

  From Table 5 and FIG. 2, the test numbers 34 to 41 of the examples of the present invention satisfying the conditions of the chemical composition and the average size of the MC type carbide specified in the present invention can all obtain a tensile strength exceeding 1200 MPa, and It is clear that the samples have excellent delayed fracture resistance as compared with Test Nos. 42 to 45 of Comparative Examples having the same tensile strength level.

  In the case of the test numbers 42 to 45 of the comparative examples, the delayed fracture resistance is inferior to those of the test numbers 34 to 41 of the examples of the present invention having the same tensile strength level. Hereinafter, comparative examples after the test number 42 will be individually described.

  In Test No. 42, the Mo content of Steel 42 used was as low as 1.14%, and Fn1 was also −0.0097, which was out of the conditions specified in the present invention. I have.

  In Test No. 43, the Mn content of the steel 43 used was as high as 1.44%, and Fn1 was also −0.0014, which was out of the conditions specified in the present invention, and thus was inferior in delayed fracture resistance.

  In Test No. 44, Fn1 of the steel 44 used was -0.0032, which is out of the conditions specified in the present invention, and thus the steel 44 is inferior in delayed fracture resistance.

  In Test No. 45, although the chemical composition of the steel 45 used was within the range specified in the present invention, no MC type carbide having a major axis of 1 nm or more, which was the observation target, was found. For this reason, the average size of the MC type carbide is less than 1 nm, which is out of the conditions specified in the present invention, and is inferior in delayed fracture resistance.

Advantageous Effects of Invention According to the present invention, it is possible to provide a high-strength component for a mechanical structure having excellent delayed fracture resistance by suppressing intrusion of hydrogen from the environment in use. Therefore, the present invention has a remarkable industrial contribution.

Claims (6)

Chemical composition in mass%
C: 0.21-0.34%,
Mn: 0.10 to 1.0%,
Mo: 1.50 to 4.0%,
V: 0.10 to 0.35%,
Al: 0.01 to 0.10%,
N: 0.001 to 0.015%,
Nb: 0 to 0.04%,
Ti: 0 to 0.04%,
Cr: 0 to 1.0%,
B: 0 to 0.0060%,
Ca: 0 to 0.005%,
Mg: 0 to 0.005%,
Zr: 0 to 0.005%,
The balance is Fe and impurities,
Si, P and S as impurities are Si: 0.15% or less, P: 0.015% or less, and S: 0.015% or less;
Further, Fn1 represented by the following equation [1] exceeds 0,
The average size of MC type carbide in steel is 1 to 30 nm,
A tensile strength of 1200 MPa or more;
Parts for machine structure.
Fn1 = (Mo / 96) + (V / 51)-(C / 12) ... [1]
However, Mo, V and C in the formula [1] mean the contents (% by mass) of the respective elements in the steel.   The component for a mechanical structure according to claim 1, comprising at least one member selected from the group consisting of 0.005 to 0.04% by mass of Nb and 0.005 to 0.04% by mass of Ti.   The component for machine structural use according to claim 1 or 2, which contains 0.1 to 1.0% of Cr by mass%.   The component for a machine structure according to any one of claims 1 to 3, which contains 0.0003 to 0.0060% of B by mass%.   The composition contains at least one selected from the group consisting of Ca: 0.0002 to 0.005%, Mg: 0.0002 to 0.005%, and Zr :: 0.0002 to 0.005% by mass%. The component for a machine structure according to any one of 1 to 4. The machine structural part according to any one of claims 1 to 5, wherein the entire surface is covered with a galvanized layer.

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