KR20180050305A - High strength spring, method of manufacturing the same, steel for high strength spring, and method of manufacturing the same - Google Patents

Abstract

The present invention relates to a ferritic stainless steel having a composition of 0.40 to 0.50% of C, 1.00 to 3.00% of Si, 0.30 to 1.20% of Mn, 0.05 to 0.50% of Ni, 0.35 to 1.50% of Cr, 0.03 to 0.50% : 0.05 to 0.50%, Al: 0.005 to 0.100%, V: 0.05 to 0.50%, Nb: 0.005 to 0.150% and N: 0.0100 to 0.0200%, P: not more than 0.015%, S: not more than 0.010% At least one of Nb compound containing at least one of Nb carbide, Nb nitride and Nb carbonitride and at least one of V carbide and V carbonitride precipitated around the Nb compound, ≪ RTI ID = 0.0 > V < / RTI >

Description

TECHNICAL FIELD [0001] The present invention relates to a high strength spring, a method of manufacturing the same, a steel for a high strength spring, and a manufacturing method thereof. BACKGROUND OF THE INVENTION [0002]

The present invention relates to a high strength spring, a method for manufacturing the same, and a steel for high strength springs and a manufacturing method thereof.

High strength springs are used in automobiles and the like. Since the high-strength spring has a high strength, it can be formed of a thin wire material, which contributes to weight reduction of the automobile and further improvement of fuel efficiency of the automobile. However, if the strength of the spring is increased, the fatigue strength, hydrogen embrittlement resistance and delayed fracture resistance in a corrosive environment deteriorate.

Thus, the spring steel disclosed in Patent Document 1 captures hydrogen that enters the steel from the external environment by the hydrogen trap site made of a precipitate containing V or the like, and suppresses diffusion of hydrogen in the steel.

Japanese Patent Application Laid-Open No. 2001-288539

In order to secure hydrogen embrittlement resistance, it is effective to increase the number of precipitates to be hydrogen trap sites. The precipitate contains V and the like.

However, even if the content of element such as V is simply increased, a large precipitate is formed and the number of precipitates is not increased.

In order to obtain a high strength, it is effective to increase the C content, but if the C content is too high, the corrosion durability decreases.

In order to obtain a high strength with a low C content, it is effective to perform the tempering treatment at a low temperature. However, when the N content is high, low temperature tempering brittleness occurs. As a result, toughness is lowered, and the delayed fracture resistance is lowered.

The main object of the present invention is to provide a high strength spring which is excellent in hydrogen embrittlement resistance, corrosion durability and delayed fracture resistance in consideration of the above problems.

In order to solve the above problems, according to one aspect of the present invention,

The steel sheet according to any one of claims 1 to 3, wherein the steel sheet has a composition of C: 0.40-0.50%, Si: 1.00-3.00%, Mn: 0.30-1.20%, Ni: 0.05-0.50%, Cr: 0.35-1.50%, Mo: 0.03-0.50% 0.005 to 0.100% of Al, 0.05 to 0.50% of V, 0.005 to 0.150% of Nb and 0.0100 to 0.0200% of N, 0.015% or less of P and 0.010% or less of S, Is made of Fe and unavoidable impurities,

An Nb compound containing at least one of Nb carbide, Nb nitride and Nb carbonitride,

And a V compound containing at least one of V carbide and V carbonitride precipitated around the Nb compound.

According to the present invention, it is possible to provide a high strength spring excellent in hydrogen embrittlement resistance, corrosion durability and delayed fracture resistance, and steel for high strength springs.

1A to 1E are SEM photographs of a part of a steel section after the tempering treatment according to the embodiment 1 is performed.
Figs. 2A to 2E are SEM photographs of different portions of steel sections after the tempering treatment according to Example 1. Fig.
3 is a view showing results of rotational bending fatigue tests of Example 1 and Comparative Example 1. Fig.
4 is a view showing the endurance test results of Example 3 and Comparative Example 2. Fig.

Hereinafter, embodiments for carrying out the present invention will be described.

The high-strength spring is used, for example, as a suspension spring of an automobile. Here, the high strength means that the tensile strength is 1800 MPa or higher. The shape of the test piece used for measuring the tensile strength conformed to the shape of the No. 4 test piece described in Japanese Industrial Standard (JIS Z2241).

The high strength spring may be a coil spring. The coil spring is manufactured by hot spring forming, cold spring forming or the like. In hot spring forming, the wire is heat-molded into a coil shape, and then quenching and tempering are performed. In the cold spring forming, the wire is subjected to quenching treatment, tempering treatment, and the like, and then the wire is formed into a coil shape from the cold.

On the other hand, the high-strength spring is a coil spring in the present embodiment, but may be a leaf spring or the like. The shape of the high-strength spring is not particularly limited. Also, the use of the high strength spring is not limited to the automotive suspension system.

The high strength spring is made of steel for high strength springs. The steel for high strength springs is subjected to quenching treatment, tempering treatment and the like, and has a martensite structure obtained by quenching treatment. Prior to the quenching treatment, the pearlite structure dominates, while the quenching temperature dominates the austenite structure, and after the quenching treatment, the martensite structure becomes dominant.

The steel for high strength springs may be any steel as long as it is subjected to quenching treatment, tempering treatment or the like, and its shape is not particularly limited. For example, in the case of hot spring forming, the steel for high strength springs may have the shape of a spring (for example, a coil shape). On the other hand, in the case of cold spring forming, the steel for high strength springs may have a shape of a spring or a shape (for example, a rod shape) before being processed into a shape of a spring.

The steel for high strength springs comprises 0.40 to 0.50% of C, 1.00 to 3.00% of Si, 0.30 to 1.20% of Mn, 0.05 to 0.50% of Ni, 0.35 to 1.50% of Cr, 0.03 to 0.50% 0.05 to 0.50% of Cu, 0.005 to 0.100% of Al, 0.05 to 0.50% of V, 0.005 to 0.150% of Nb and 0.0100 to 0.0200 of N, 0.015% or less of P and 0.010% or less of S And the remainder is composed of Fe and unavoidable impurities. Hereinafter, each component will be described. In the description of each component, "%" means mass%.

C is an effective element for improving the strength of steel. The content of C is 0.40 to 0.50%. If the content of C is less than 0.40%, the required strength as a spring can not be obtained. On the other hand, when the content of C exceeds 0.50%, corrosion durability is lowered.

Si is an element effective for improving the strength of steel by solid solution in ferrite. The content of Si is 1.00 to 3.00%. If the Si content is less than 1.00%, the required strength as a spring can not be obtained. On the other hand, when the content of Si exceeds 3.00%, decarburization tends to occur on the surface when the spring is heated and molded in hot, thereby decreasing the durability of the spring.

Mn is an effective element for improving the hardenability of steel. The content of Mn is 0.30 to 1.20%. If the content of Mn is less than 0.30%, the effect of improving the hardenability can not be sufficiently obtained. On the other hand, if the content of Mn exceeds 1.20%, the toughness is deteriorated.

Ni is an element necessary to increase the corrosion durability of steel. The content of Ni is 0.05 to 0.50%. If the Ni content is less than 0.05%, the effect of increasing the corrosion durability of steel can not be sufficiently expected. Since Ni is expensive, the upper limit of the Ni content is 0.50%.

Cr is an effective element for improving the strength of steel. The Cr content is 0.35 to 1.50%. If the Cr content is less than 0.35%, the effect of improving the strength of steel can not be sufficiently expected. On the other hand, if the Cr content exceeds 1.50%, toughness tends to deteriorate.

Mo is an element which secures the hardenability of steel and improves the strength and toughness of steel. The Mo content is 0.03 to 0.50%. If the Mo content is less than 0.03%, the effect of adding Mo can not be sufficiently expected. On the other hand, when the Mo content exceeds 0.50%, the effect of adding Mo is saturated.

Cu is a component that increases corrosion durability. The content of Cu is 0.05 to 0.50%. If the Cu content is less than 0.05%, the effect of increasing the corrosion durability does not sufficiently appear. On the other hand, when the content of Cu exceeds 0.50%, cracking occurs in hot rolling.

Al is an element necessary for adjusting the deoxidizing agent and the austenite crystal grain size of steel. The content of Al is 0.005 to 0.100%. If the Al content is less than 0.005%, the crystal grains can not be made finer. On the other hand, if the Al content exceeds 0.100%, the castability tends to decrease.

V is an effective element for improving the strength of steel and suppressing embrittlement of hydrogen. The content of V is 0.05 to 0.50%. If the content of V is less than 0.05%, the effect of adding V can not be sufficiently expected. On the other hand, if the content of V exceeds 0.50%, carbides which are not dissolved in austenite are increased to deteriorate the spring characteristics.

Nb is an element which improves the strength and toughness of steel by the refinement of crystal grains and the precipitation of fine carbides. Further, Nb is an element that contributes to the fine dispersion of the V compound (hereinafter referred to as "V compound") containing at least one of V carbide and V carbonitride, thereby increasing hydrogen embrittlement resistance. The content of Nb is 0.005 to 0.150%. If the content of Nb is less than 0.005%, the effect of adding Nb can not be sufficiently expected. On the other hand, when the content of Nb exceeds 0.150%, carbides which are not dissolved in austenite are increased and the spring characteristics are deteriorated.

N is combined with Al and Nb to form AlN and NbN, and is an element that is effective in making finer the austenite crystal grain size. The toughness is improved by such miniaturization. The content of N is 0.0100 to 0.0200%. When the content of N is 0.0100% or more, the effect of improving the toughness is sufficiently exhibited. On the other hand, if N is added excessively, bubbles are formed on the surface of the steel lump during solidification, deterioration of the casting composition of the steel is caused, and the upper limit of the N content is 0.0200%.

P precipitates at the austenite grain boundaries and embrittles the grain boundary, which causes the impact value to decrease. To suppress this problem, the P content is limited to 0.015% or less.

S exists as an intervening material of MnS in steel, which causes deterioration in fatigue life and corrosion durability. An inclusion means that steel is already present in a dissolved state. In order to reduce inclusions, the S content is limited to 0.010% or less, preferably 0.005% or less.

Steel for high strength springs is prepared by dissolving a V compound in iron at a quenching temperature to finely disperse the V compound as a hydrogen trap site and then precipitating the V compound around the Nb compound to be finely dispersed in steel. Thus, the steel for high-strength springs includes an Nb compound and a V compound deposited around the Nb compound. The V compound may be completely precipitated adjacent to the Nb compound, so that it does not completely surround the range of the Nb compound, but may completely surround it. In steel for high strength springs, the Nb compound may be present inside the V compound.

The Nb compound is a precipitate precipitated in iron during the solidification of molten steel. The Nb compound includes at least one of Nb nitride, Nb carbide and Nb carbonitride. The Nb compound is finely dispersed in steel before quenching so that it does not solidify into iron at the quenching temperature and becomes the starting point at which the V compound precipitates in the quenching or tempering treatment from the quenching temperature. As the starting point at which the V compound is precipitated, Nb nitride which is more finely dispersed than Nb carbide or Nb carbonitride is preferably used.

V compound is present as a large precipitate in steel before quenching treatment, so it is solidified into iron at quenching temperature and then precipitated from the Nb compound as a starting point. Since the Nb compound is finely dispersed, the V compound to be precipitated from the Nb compound can be finely dispersed. The number of the V compounds can be increased according to the miniaturization of the V compound, and thus a steel for high strength springs excellent in hydrogen embrittlement can be obtained.

V compound is dissolved in the iron at the quenching temperature, the quenching temperature becomes 950 DEG C or more and 1000 DEG C or less. The quenching temperature is higher than the solidus temperature at which the V compound is dissolved in iron, so that when the V content is 0.50% or less as described above, the V compound is completely solubilized in the calculation of the solubility. Since the quenching temperature is high, an appropriate amount of Nb, Al, N or the like is added in order to suppress the crystal grain from becoming large. Thereby, deterioration of toughness can be suppressed, and deterioration of resistance to delayed fracture can be suppressed. Therefore, it is possible to obtain a steel for high strength springs excellent in delayed fracture resistance.

Nb compound and a V compound precipitated around the Nb compound, a complex precipitate is formed. The average particle diameter of the composite precipitate may be 0.01 탆 or more and 1 탆 or less. The number of complex precipitates per unit area may be 100 / mm ² or more and 100000 / mm ² or less. The average particle diameter and the number per unit area are measured using, for example, an SEM (Scanning Electron Microscope). The average particle diameter is determined as the average value of the measured values by measuring the area equivalent diameter of each of the 100 complex precipitates. The number per unit area is obtained by measuring the number of complex precipitates present in the area having a total area of 15 mm ² and dividing the number by the total area.

The steel for high strength springs is limited to a content of C of 0.5% or less in order to suppress deterioration of corrosion durability. In order to secure the strength of steel in a range where the content of C is 0.5% or less, . Thus, steel for high strength springs excellent in corrosion durability and strength can be obtained. On the other hand, the lower limit of the tempering temperature is set to 250 deg. C, more preferably 300 deg. C, so as to sufficiently obtain the toughness improving effect by the tempering treatment.

Steel for high-strength springs contains N 0.0100 to 0.0200% in order to finely disperse the nitride sufficiently. In order to suppress the low-temperature tempering brittleness due to N, the high-strength spring steel is made harmless by precipitating NbN, AlN or the like instead of N by appropriately adding Nb and Al. Thereby, deterioration of toughness can be suppressed, and deterioration of resistance to delayed fracture can be suppressed. Therefore, steel for high strength springs excellent in delayed fracture resistance can be obtained.

y Example z

Hereinafter, specific examples, comparative examples and the like will be described.

[Example 1]

In Example 1, quenching treatment and tempering treatment were applied to steel having the following compositions, and rotary bending fatigue test pieces and hydrogen embrittlement test pieces were produced by machining.

0.45% of Mo, 0.35% of Cr, 0.35% of Mo, 0.035% of Al, 0.023% of Al, 0.25% of Al, Steel containing 0.020% of Nb, 0.030% of N, 0.0130% of P, 0.010% or less of P and 0.003% or less of S and the balance being Fe and inevitable impurities.

The quenching temperature was 950 占 폚 and the holding time was 30 minutes. Cooling from quenching temperature was made by oil cooling (oil cooling).

The tempering temperature was 360 占 폚, and the holding time was 1 hour. Cooling from the tempering temperature was carried out by air cooling.

The Vickers hardness of the steel after the tempering treatment was 590 Hv.

The obtained steel was observed with an electron microscope. Figs. 1A to 1E are SEM photographs of a portion of a steel section after tempering treatment according to Embodiment 1, and Figs. 2A to 2E are SEM pictures of another portion of a steel section after tempering treatment according to Embodiment 1. Fig. Figs. 1A and 2A are reflection electron images, Figs. 1B and 2B are characteristic X-ray maps of Nb, Figs. 1C and 2C are characteristic X-ray maps of N, Figs. 1D and 2D are characteristic X- 1E and 2E are characteristic X-ray maps of C, respectively. On the other hand, in Figs. 1A and 2A, the white portion of the reflected electron image represents the Nb compound, and the black portion around the white color represents the V compound. In the characteristic X-ray map of each element in FIG. 1B to FIG. 1E and FIG. 2B to FIG. 2E, the brightness of the color represents the amount of the element, and the amount of the element is larger as the color becomes brighter. The reflected electron image in Figs. 1A and 2A is an image of reflected electrons protruding from the vicinity of the end face of the steel, and therefore, the reflected electron image shows almost the same size as seen on the surface to be inspected. On the other hand, the characteristic X-ray map of FIG. 1B to FIG. 1E and FIG. 2B to FIG. 2E is an image of a characteristic X-ray generated from the end face of steel to the inside of steel. A threshold value is set for the intensity of the characteristic X-ray to be detected. Thus, the image of the characteristic X-ray map is different from the size shown on the inspected surface.

1A, the steel of Example 1 has a portion (black portion) having a higher concentration of V than the surrounding portion, and the Nb concentration is higher than the outside of the black portion on the inner side of the black portion It was observed that there was a high part (white part). From the characteristic X-ray map of FIG. 1B to FIG. 1E, the black portion and the white portion of FIG. 1A each have portions with high N and C concentrations, and portions with high N concentrations and portions with high C concentrations At least duplication was observed. Therefore, it can be said that at least V carbonitride is precipitated in the steel of Example 1 so as to cover at least the periphery of the Nb carbonitride after the tempering treatment.

2A, in the other portions of the steel of Example 1, a portion having a higher concentration of V than the ambient (black portion) exists, and the portion of Nb (White portion) was observed in the vicinity of the surface of the sample. From the characteristic X-ray map of FIG. 2B to FIG. 2E, the black portion and the white portion of FIG. 2A each have portions with high N and C concentrations, and portions with high N concentrations and portions with high C concentrations At least duplication was observed. Therefore, in the steel of Example 1, after the tempering treatment, at least the V-carbonitride is deposited so as to surround at least the Nb carbonitride.

Thus, in the steel of Example 1, it was confirmed that the V compound precipitated around the Nb compound after the tempering treatment.

The shape of the test piece conformed to the shape of No. 1 test piece described in Japanese Industrial Standard (JIS Z2274). This test piece has a constricted portion called a parallel portion at the center of the round rod.

The rotational bending fatigue test specimen had a diameter of both ends of 15 mm, a diameter of a parallel portion of 8 mm, and a length of a parallel portion of 20 mm.

The hydrogen embrittlement test specimen had a diameter of both ends of 10 mm, a diameter of a parallel portion of 4 mm, and a length of a parallel portion of 15 mm.

[Comparative Example 1]

In Comparative Example 1, quenching treatment and tempering treatment were applied to steels having the following composition, and rotational bending fatigue test pieces and hydrogen embrittlement test pieces were produced by machining.

0.50% of Si, 1.50% of Mn, 0.45% of Mn, 0.26% of Ni, 0.80% of Cr, 0.09% of Mo, 0.12% of Cu, 0.023% of Al, % Of N, 0.020% of N, 0.0120% of N, 0.010% of P, 0.009% of S, and the balance of Fe and unavoidable impurities.

The quenching temperature was 900 캜 and the holding time was 30 minutes. Cooling from the quenching temperature was made by oil cooling.

The tempering temperature was 420 캜, and the holding time was 1 hour. Cooling from the tempering temperature was carried out by air cooling.

The Vickers hardness of the steel after the tempering treatment was 570 Hv.

The shape of the test piece was the same as that of the test piece of Example 1. [

[Rotational bending fatigue test]

In the rotational bending fatigue test, the test piece subjected to a constant bending moment was rotated at 3000 rpm, and a load of sinusoidal stress was applied to the test piece to investigate the number of times of repeated stress until the test piece was broken.

Fig. 3 shows results of rotational bending fatigue tests of Example 1 and Comparative Example 1. Fig. 3, the solid line shows the result of the rotational bending fatigue test of Example 1, and the broken line shows the rotational bending fatigue test result of Comparative Example 1. [

As clearly shown in FIG. 3, it was confirmed that the steel of Example 1 had an excellent bending fatigue strength as compared with the steel of Comparative Example 1.

[Hydrogen embrittlement test]

In the hydrogen embrittlement test, a parallel portion of a test piece was immersed in an electrolytic solution, hydrogen generated by the electric field of the electrolytic solution was charged by a test piece for 48 hours, a parallel portion was immersed in the electrolytic solution, The maximum stresses were investigated. As the electrolytic solution, an aqueous solution at 50 占 폚 containing 5% of ammonium thiocyanate was used. A lever-type static load tester was used as a tester for applying a load to the test piece. The test time for confirming the maximum stress not fractured (hereinafter referred to as "notch stress") was 96 hours. This hydrogen embrittlement test also serves as a corrosion endurance test and an internal delayed fracture test, and an aqueous solution containing 5% ammonium thiocyanate doubles as an electrolytic solution and a corrosive solution.

The test specimen of Example 1 had a fine breaking stress of 325 MPa, while a test piece of Comparative Example 1 had a poor breaking stress of 240 MPa. Therefore, it was confirmed that the steel of Example 1 was superior to the steel of Comparative Example 1 in hydrogen embrittlement resistance, corrosion durability, and delayed fracture resistance.

After the hydrogen embrittlement test, the amount of diffusible hydrogen contained in the test piece was measured. The amount of diffusible hydrogen is determined by continuously measuring the amount of hydrogen released from the test piece by heating the test piece and raising the temperature of the test piece at a constant rate by gas chromatography.

Hydrogen released at temperatures below 300 ° C is diffusible hydrogen and hydrogen released at temperatures above 300 ° C is non-diffusible hydrogen. The release of diffusible hydrogen is almost complete before the temperature of the specimen reaches 220 ° C. When the temperature of the specimen exceeds 400 ° C, non-proliferative hydrogen begins to be released. The hydrogen captured at the hydrogen trap site is not released at temperatures below 300 ° C.

The amount of diffusible hydrogen in the test piece of Example 1 was 0.36 mass ppm, while the amount of diffusible hydrogen in the test piece of Comparative Example 1 was 1.87 mass ppm. Therefore, it was confirmed that the steel of Example 1 had more hydrogen trapping sites than the steel of Comparative Example 1, and the hydrogen embrittlement resistance was excellent.

[Example 2]

In Example 2, quenching treatment and tempering treatment were applied to steel having the same composition as that of the steel of Example 1, and a tensile strength test piece was produced by machining and subjected to a tensile test.

The quenching temperature was 950 占 폚 and the holding time was 30 minutes. Cooling from the quenching temperature was made by oil cooling.

The tempering temperature was 380 캜 or 350 캜, and the holding time was 1 hour. Cooling from the tempering temperature was carried out by air cooling.

The shape of the tensile test specimen conformed to the shape of the No. 4 test piece described in Japanese Industrial Standard (JIS Z2241).

In the tensile test, tensile strength, 0.2% proof stress, elongation at break, and section shrinkage were measured.

The tempering temperature, the tensile test result, and the Vickers hardness are shown in Fig.

As can be clearly seen from Table 1, the steel of Example 2 has a high strength.

[Example 3]

In Example 3, steel having the same composition as the steel of Examples 1 and 2 was heat-molded into a coil shape. Thereafter, quenching treatment, tempering treatment, shot-peening and setting were performed on the obtained molded article to prepare a coil spring. Thereafter, a durability test of the obtained coil spring was carried out. The quenching temperature was 990 占 폚 and the holding time was 20 minutes. Cooling from the quenching temperature was made by oil cooling. The tempering temperature was 360 占 폚, and the holding time was 1 hour. Cooling from the tempering temperature was carried out by air cooling. The Vickers hardness of the coil springs after the tempering treatment was 580 Hv.

[Comparative Example 2]

In Comparative Example 2, steel having the same composition as that of the steel of Comparative Example 1 was formed into a coil shape in the same manner as in Example 3 to obtain a molded article having the same shape as in Example 3. Thereafter, quenching treatment, tempering treatment, shot peening and setting were performed on the obtained molded article to prepare a coil spring having the same shape as in Example 3. Thereafter, a durability test of the obtained coil spring was carried out. The quenching temperature was 940 占 폚 and the holding time was 20 minutes. Cooling from the quenching temperature was made by oil cooling. The tempering temperature was 420 캜, and the holding time was 1 hour. Cooling from the tempering temperature was carried out by air cooling. The Vickers hardness of the coil springs after the tempering treatment was 560 Hv.

[Durability test]

In the durability test, a repetitive stress load was applied to the coil spring at various stress amplitudes at an average stress of 735 MPa, and the number of repetitions of the stress was measured until the coil spring was broken. In Example 3, the stress amplitude was 735 MPa ± 620 MPa (maximum stress: 1355 MPa, minimum stress: 115 MPa) and 735 MPa ± 550 MPa (maximum stress: 1285 MPa, minimum stress: 185 MPa). In Comparative Example 2, the stress amplitude was 735 MPa ± 525 MPa (maximum stress: 1260 MPa, minimum stress: 210 MPa) and 735 MPa ± 500 MPa (maximum stress: 1235 MPa, minimum stress: 235 MPa).

Fig. 4 shows the endurance test results of Example 3 and Comparative Example 2. Fig. On the other hand, in FIG. 4, the solid line shows the endurance test result of Example 3, and the broken line shows the endurance test result of Comparative Example 2. As is clear from FIG. 4, it was confirmed that the coil spring of Example 3 was superior in durability to the coil spring of Comparative Example 2.

Although the embodiment of the high-strength spring has been described above, the present invention is not limited to the embodiments and the like, and various modifications and improvements can be added within the scope of the present invention described in the claims.

The present application is based on patents Nos. 2016-205535 filed with the Japanese Patent Office on October 19, 2016, Patents 2017-061981 filed with the Japanese Patent Office on March 27, 2017, and on May 11, 2017 Which is based on Japanese Patent Application No. 2017-095054 filed with the Japanese Patent Office, and the entire contents of Japanese Patent Application No. 2016-205535, Japanese Patent Application No. 2017-061981, Japanese Patent Application No. 2017-095054 are incorporated herein by reference.

Claims (4)

The steel sheet according to any one of claims 1 to 3, wherein the steel sheet has a composition of C: 0.40-0.50%, Si: 1.00-3.00%, Mn: 0.30-1.20%, Ni: 0.05-0.50%, Cr: 0.35-1.50%, Mo: 0.03-0.50% 0.005 to 0.100% of Al, 0.05 to 0.50% of V, 0.005 to 0.150% of Nb and 0.0100 to 0.0200% of N, 0.015% or less of P and 0.010% or less of S, Is made of Fe and unavoidable impurities,
An Nb compound containing at least one of Nb carbide, Nb nitride and Nb carbonitride,
And a V compound containing at least one of V carbide and V carbonitride deposited around the Nb compound. The steel sheet according to any one of claims 1 to 3, wherein the steel sheet has a composition of C: 0.40-0.50%, Si: 1.00-3.00%, Mn: 0.30-1.20%, Ni: 0.05-0.50%, Cr: 0.35-1.50%, Mo: 0.03-0.50% 0.005 to 0.100% of Al, 0.05 to 0.50% of V, 0.005 to 0.150% of Nb and 0.0100 to 0.0200% of N, 0.015% or less of P and 0.010% or less of S, A tempering treatment in which the quenching temperature is 950 DEG C or more and 1000 DEG C or less and the tempering treatment in which the tempering temperature is 250 DEG C or more and less than 390 DEG C is applied to steel made of Fe and unavoidable impurities,
The V compound containing at least one of the V carbide and the V carbonitride is solid-solved in Fe at the quenching temperature, and then the V compound containing at least one of Nb carbide, Nb nitride and Nb carbonitride is solid- V compound is precipitated. The steel sheet according to any one of claims 1 to 3, wherein the steel sheet has a composition of C: 0.40-0.50%, Si: 1.00-3.00%, Mn: 0.30-1.20%, Ni: 0.05-0.50%, Cr: 0.35-1.50%, Mo: 0.03-0.50% 0.005 to 0.100% of Al, 0.05 to 0.50% of V, 0.005 to 0.150% of Nb and 0.0100 to 0.0200% of N, 0.015% or less of P and 0.010% or less of S, Is made of Fe and unavoidable impurities,
An Nb compound containing at least one of Nb carbide, Nb nitride and Nb carbonitride,
And a V compound containing at least one of V carbide and V carbonitride precipitated around the Nb compound. The steel sheet according to any one of claims 1 to 3, wherein the steel sheet has a composition of C: 0.40-0.50%, Si: 1.00-3.00%, Mn: 0.30-1.20%, Ni: 0.05-0.50%, Cr: 0.35-1.50%, Mo: 0.03-0.50% 0.005 to 0.100% of Al, 0.05 to 0.50% of V, 0.005 to 0.150% of Nb and 0.0100 to 0.0200% of N, 0.015% or less of P and 0.010% or less of S, A tempering treatment in which the quenching temperature is 950 DEG C or more and 1000 DEG C or less and the tempering treatment in which the tempering temperature is 250 DEG C or more and less than 390 DEG C is applied to steel made of Fe and unavoidable impurities,
The V compound containing at least one of the V carbide and the V carbonitride is solid-solved in Fe at the quenching temperature, and then the V compound containing at least one of Nb carbide, Nb nitride and Nb carbonitride is solid- V compound is precipitated.

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