KR20180026014A - Steel wire rod having excellent hydrogen embrittlement resistance for high strength spring, and method for manufacturing the same - Google Patents

Abstract

One aspect of the present invention relates to a steel material for a high-strength spring, having excellent hydrogen embrittlement resistance. The steel material for a high-strength spring comprises 0.45 to 0.60 wt% of C, 0.30 to 0.80 wt% of Mn, 1.40 to 1.80% of Si, less than or equal to 0.015 wt% of P, less than or equal to 0.015 wt% of S; 0.2 to 0.7 wt% of Cr, 0.05 to 0.25 wt% of Mo, 0.05 to 0.20 wt% of V, and the remainder consisting of Fe and other inevitable impurities. The steel material for a high-strength spring has the degree of grain refinement of prior austenite greater than or equal to 10, and comprises (V, Mo)C carbide, wherein the (V, Mo)C carbide has an average size in the range of 10 to 40 nm measured as a circle-equivalent diameter.

Description

TECHNICAL FIELD [0001] The present invention relates to a steel material for high-strength springs excellent in hydrogen embrittlement resistance and a method for manufacturing the steel material. [0002]

The present invention relates to a high strength spring steel material excellent in hydrogen embrittlement resistance and a method for manufacturing the same.

In recent years, the use of fossil fuels, especially petroleum fuels, has increased, and the seriousness of air pollution caused by the pollution sources generated by burning the petroleum fuels has risen worldwide. In order to avoid harm caused by petroleum fuels, There are many researches on saving technologies.

The automobile is a consumer demanding a large amount of the above-mentioned petroleum fuel, and the automobile manufacturer is continuously carrying out various attempts and studies to reduce the amount of petroleum fuel used. One of the most traditional ways to reduce the use of petroleum fuels is to improve the fuel economy of automobiles, which is the mainstream of the technology currently being developed and applied. As such fuel economy improvement methods, the combustion efficiency of the engine and the power transmission efficiency Another method is to reduce the amount of energy required to move the unit distance by reducing the weight of the automobile.

In order to reduce the weight of automobile body, it is possible to replace the parts of the automobile with lightweight materials with low specific gravity. However, there are not so many parts to replace the superiority of steel material. Therefore, steel materials are often used as automobile parts, and attempts to improve automobile fuel economy through weight reduction of steel parts are common.

If the weight of a steel material is simply reduced, a load that can be supported per unit weight is determined, which can cause a serious problem to the safety of an automobile. Therefore, weight reduction of parts is inevitably a problem to be solved after solving the problem of intensifying the parts. In addition, the hydrogen embrittlement sensitivity increases as the strength of the material increases. Especially, when the corrosion pits of the spring are generated by the calcium chloride or sodium chloride sprayed for the winter ice removal, the hydrogen embrittlement sensitivity is further increased.

In order to solve such a problem, Patent Document 1 discloses a method of increasing the hydrogen embrittlement resistance through microfine of old austenite grains.

Also, in Patent Document 2, it is described that the hydrogen embrittlement resistance can be improved by forming a fine austenite having 6 to 15% of retained austenite and having a fine austenite grain number of 10.0 or more.

However, there is a problem that satisfactory hydrogen embrittlement resistance can not be ensured only by the minute microstructure construction according to Patent Documents 1 and 2. [ Therefore, there is a demand for development of a steel material for a high-strength spring having excellent hydrogen embrittlement resistance and a manufacturing method thereof.

Korean Patent Laid-Open Publication No. 2015-0081366 Korean Patent Laid-Open Publication No. 2015-0002848

The present invention provides a steel material for high-strength springs having improved hydrogen embrittlement resistance by controlling the old austenite grain size and (V, Mo) C carbide, and a method for producing the same.

On the other hand, the object of the present invention is not limited to the above description. It will be understood by those of ordinary skill in the art that there is no difficulty in understanding the additional problems of the present invention.

One aspect of the present invention is a steel sheet comprising, by weight, 0.45 to 0.60% of C, 0.30 to 0.80% of Mn, 1.40 to 1.80% of Si, 0.015% or less of P, (V, Mo) C carbide, wherein the content of Mo is 0.05 to 0.25%, V is 0.05 to 0.20%, the balance of Fe and other unavoidable impurities, the old austenite grain size is 10.0 or more, ) Carbide is intended to provide a steel material for high strength springs excellent in hydrogen embrittlement resistance having an average size of 10 to 40 nm as measured by a circle equivalent diameter.

In another aspect of the present invention, there is provided a steel sheet comprising, by weight, 0.45 to 0.60% of C, 0.30 to 0.80% of Mn, 1.40 to 1.80% of Si, 0.015% or less of P, ~ 0.7%, Mo: 0.05 ~ 0.25%, V: 0.05 ~ 0.20%, balance Fe and other unavoidable impurities; A quenching heat treatment step of heating the wire rod to a temperature range of 880 to 1000 ° C at an average heating rate of 50 ° C / sec or more, holding it for 40 seconds or less, and cooling the wire rod at an average cooling rate of 10 ° C / s or more; And the quenching heat treated wire material is heated to a temperature range of 360 to 480 캜 at an average heating rate of 30 캜 / second or more and maintained for 40 seconds or less, and then cooled to 60 캜 or lower at an average cooling rate of 10 캜 / And a heat treatment step, which are excellent in hydrogen embrittlement resistance.

In addition, the solution of the above-mentioned problems does not list all the features of the present invention. The various features of the present invention and the advantages and effects thereof can be understood in more detail with reference to the following specific embodiments.

According to the present invention, it is possible to provide a steel material having a tensile strength of 1920 MPa or more and excellent hydrogen embrittlement resistance, and a manufacturing method thereof.

1 is a photograph of a (V, Mo) C carbide of Inventive Example 1 taken by a transmission electron microscope.
2 is a graph showing the hydrogen release behavior according to the temperature obtained through the heat release test in Inventive Example 4 and Comparative Example 4. FIG.

Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention can be modified into various other forms, and the scope of the present invention is not limited to the embodiments described below. Further, the embodiments of the present invention are provided to more fully explain the present invention to those skilled in the art.

The present inventors have found that a method of coating paint, urethane or the like to prevent hydrogen embrittlement of coil springs is difficult to prevent hydrogen embrittlement due to generation of pits of a spring caused by gravel, sand, etc., There is a need to improve the hydrogen embrittlement resistance and it has been recognized that sufficient microfabrication of microstructure for ensuring hydrogen embrittlement can not ensure sufficient hydrogen embrittlement resistance and a study has been conducted to solve this problem.

As a result, the interface of (Mo, V) C carbide was used as the non-proliferative hydrogen trap part by the combined addition of Mo and V together with the fine microstructure implementation, so that the hydrogen emission peak appeared at 330 ~ 370 ° C Thereby improving the hydrogen embrittlement resistance. Thus, the present invention has been accomplished.

Hydrogen embrittlement  High Strength Spring Steel with Excellent Resistance

Hereinafter, a steel material for high-strength springs excellent in hydrogen embrittlement resistance according to one aspect of the present invention will be described in detail.

According to one aspect of the present invention, there is provided a steel material for high strength springs excellent in hydrogen embrittlement resistance according to one aspect of the present invention, which comprises 0.45 to 0.60% of C, 0.30 to 0.80% of Mn, 1.40 to 1.80% of Si, (V, Mo) of 0.015% or less, Cr of 0.2-0.7%, Mo of 0.05-0.25%, V of 0.05-0.20%, balance of Fe and other unavoidable impurities and having an old austenite grain size of 10.0 or more, C carbide, and the (V, Mo) C carbide has an average size of 10 to 40 nm as measured by a circle equivalent diameter.

First, the alloy composition of the present invention will be described in detail. Hereinafter, the unit of each element content is expressed by weight% unless otherwise specified.

C: 0.45 to 0.60%

C is an essential element added to secure the strength of the spring.

When the C content is less than 0.45%, the ingotability is not ensured and it is difficult to sufficiently secure the strength required for the spring steel. Therefore, the lower limit of the C content is preferably 0.45%, and the lower limit is more preferably 0.48%.

On the other hand, when the C content is more than 0.60%, plate-like martensite structure is formed for the purpose of heat resistance to corrosion, and cracking of the material occurs, and the resistance to hydrogen embrittlement is remarkably deteriorated. In addition, there is a problem that it is difficult to secure enough toughness with high strength and to suppress decarburization of material caused by high Si addition.

Mn: 0.30 to 0.80%

Mn, when present in the steel, is an element which is useful for improving the incombustibility of the steel and securing the strength. When the Mn content is less than 0.30%, it is difficult to obtain sufficient strength and entrapment property required for the high-strength spring material. If the Mn content exceeds 0.80%, the toughness may deteriorate. Therefore, the Mn content is preferably 0.30 to 0.80%.

Si: 1.40 to 1.80%

Si is dissolved in ferrite to enhance the strength of the base material and improve the deformation resistance. When the Si content is less than 1.4%, Si is dissolved in the ferrite and the effect of strengthening the base material strength and improving the deformation resistance is insufficient. On the other hand, when the Si content exceeds 1.8%, surface decarburization can be promoted during the heat treatment. Therefore, the Si content is preferably 1.4 to 1.8%.

P: not more than 0.015%

P is segregated in the grain boundaries as an impurity to lower the toughness. Therefore, it is important to control the upper limit thereof, and it is preferable to suppress it to 0.015% or less.

S: not more than 0.015%

S is segregated as a low-melting point element as an impurity to reduce the toughness and form an emulsion, which adversely affects the spring characteristics. Therefore, it is important to control the upper limit, and it is preferable to suppress it to 0.015% or less.

Cr: 0.2-0.7%

Cr is a useful element for ensuring oxidation resistance, softening of temper softening, prevention of surface decarburization and incombustibility. When the Cr content is less than 0.2%, it is difficult to ensure sufficient oxidation resistance, temper softening property, surface decarburization prevention and entrapping effect, and the like. On the other hand, when the Cr content exceeds 0.7%, the deformation resistance is lowered and the pitting corrosion can be promoted. Therefore, the Cr content is preferably 0.2 to 0.7%.

Mo: 0.05 to 0.25%

Mo serves to improve the precipitation hardening, refinement of old austenite grain and hydrogen embrittlement resistance by forming carbide. When the Mo content is less than 0.05%, sufficient carbides can not be formed. When the Mo content exceeds 0.25%, the production cost increases and the effect of retarding the phase transformation is large, resulting in a problem of lowering productivity. Therefore, the Mo content is preferably 0.05 to 0.25%.

V: 0.05 to 0.20%

V forms single or complex carbides to improve precipitation hardening, refinement of old austenite grain and hydrogen embrittlement resistance. When the V content is less than 0.05%, sufficient carbide can not be formed. When the V content is more than 0.20%, the production cost is increased and the amount of coarse carbide which is not dissolved in the heat treatment is increased, . Therefore, the V content is preferably 0.05 to 0.20%.

The remainder of the present invention is iron (Fe). However, in the ordinary manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably incorporated, so that it can not be excluded. These impurities are not specifically mentioned in this specification, as they are known to any person skilled in the art of manufacturing.

However, in order to further improve strength, toughness and corrosion resistance, it is preferable that 0.1 to 0.4% of Ni and 0.1 to 0.3% of Cu are contained in weight percent, And < / RTI >

Ni: 0.1 to 0.4%

Ni is an element added to improve the incombustibility and toughness. When the Ni content is less than 0.1%, the effect of improvement of the incombustibility and toughness is not sufficient. When the Ni content exceeds 0.4%, the amount of retained austenite is increased to decrease the fatigue life. Which can lead to an increase in unit price.

Cu: 0.1 to 0.3%

Cu is a useful element for preventing decarburization and improving corrosion resistance. The decarburized layer remarkably lowers the fatigue life after the spring working. When the Cu content is less than 0.1%, the above-mentioned effect is insufficient, and when the Cu content is more than 0.3%, rolling defect due to high temperature brittleness may occur.

(V, Mo) C carbide, and the average size of the (V, Mo) C carbide measured by the circle equivalent diameter is 10 to 40 nm.

Since the grain size is large when the old austenite grain size is less than 10.0, it is difficult to uniformly distribute the hydrogen diffused into the grain boundary, so that it reaches the critical hydrogen concentration early to cause fracture, There is a problem.

The (V, Mo) C carbide refers to VC, MoC, and complex carbides thereof. By pinning austenite grains during the annealing process to prevent grain growth, the size of the old austenite grains of the steel for spring is miniaturized Role and a non-proliferative hydrogen trap.

Carbide in the steel is known to act as a trap for hydrogen introduced during the manufacturing process or hydrogen introduced by corrosion during use of the vehicle springs. The (V, Mo) C carbide in the carbide has a high activation energy at the interface, And a non-proliferative hydrogen trap.

When the average size of the (V, Mo) C carbide measured by the circle equivalent diameter is more than 40 nm, the hydrogen embrittlement becomes disadvantageous because it serves as a starting point that induces hydrogen embrittlement. On the other hand, the lower limit of the average size is not particularly limited, but it is difficult to control the average size to less than 10 nm.

At this time, the (V, Mo) C carbide may be 8 to 40 / 탆 ² .

(V, Mo) C carbide is less than 8 / 탆 ² , it is difficult to sufficiently perform the role of non-proliferative hydrogen trap. When (V, Mo) C carbide exceeds 40 / 탆 ² , (V, Mo) C carbide increases the coarsening tendency.

In addition, the microstructure of the steel of the present invention may contain 90 to 95% of tempered martensite and 5 to 10% of retained austenite in an area fraction.

In addition, the steel material of the present invention may exhibit a hydrogen emission peak at a temperature of 330 to 370 DEG C in a heat release test.

Hydrogen in steel is largely classified into diffusible hydrogen and non-diffusible hydrogen. Diffusible hydrogen is hydrogen which diffuses by mechanical driving force or chemical driving force according to external stress to cause hydrogen embrittlement, and non-proliferative hydrogen means hydrogen which is not diffused by driving force. These diffusible and non-diffusible hydrogens can be categorized through a heat release test that observes the amount of hydrogen released from the material while heating the material. The hydrogen released until the temperature rise temperature reaches 300 ° C can be defined as diffusible hydrogen. In addition, when a temperature above the activation energy is received at the hydrogen trapping portion, the hydrogen trapping portion of the material can be deduced through the peak of the hydrogen emission amount at a specific temperature, and the hydrogen emission peak at the temperature of 330 to 370 ° C (V, Mo) C carbide of the non-proliferative hydrogen trap.

On the other hand, the tensile strength of the steel material of the present invention may be 1920 MPa or more.

When the tensile strength is less than 1920 MPa, the vehicle weight reduction effect due to high strength may be insufficient, and it may be difficult to satisfy the design shear stress of the coil spring of 1200 MPa or more.

Hydrogen embrittlement  Manufacturing method of high strength spring steel excellent in resistance

Hereinafter, a method of manufacturing a steel material for a high strength spring having excellent hydrogen embrittlement resistance, which is another aspect of the present invention, will be described in detail.

According to another aspect of the present invention, there is provided a method of manufacturing a high strength spring steel material excellent in hydrogen embrittlement resistance, comprising the steps of: preparing a wire material having the above-described alloy composition; A quenching heat treatment step of heating the wire rod to a temperature range of 880 to 1000 占 폚 at an average heating rate of 50 占 폚 / s or more, maintaining the same for 40 seconds or less, and then cooling the wire rod at an average cooling rate of 10 占 폚 / sec or more; And the quenching heat treated wire rod is heated to a temperature range of 360 to 480 캜 at an average heating rate of 30 캜 / s or more and maintained for 40 seconds or less, and then cooled to 60 캜 or lower at an average cooling rate of 10 캜 / And a heat treatment step.

Wire rod  Preparation phase

A wire rod having the above-described alloy composition is prepared.

At this time, preparing the wire rod may include heating the steel strip having the alloy composition described above to 1000 to 1150 캜; Subjecting the heated billet to final hot rolling in a temperature range of 810 to 910 캜 to obtain a wire rod; And cooling the wire rod at a cooling rate of 0.15 to 1.5 占 폚 / s.

During the wire fabrication process, VC is first generated and then MoC is generated. The nuclei of MoC are generated at the interface of the VC that is formed first, and coarse complex carbide can be formed. To prevent this, the hot rolling temperature should be lowered and the cooling rate lowered.

Coarse carbides can be formed when the final hot rolling temperature exceeds 910 DEG C or the cooling rate exceeds 1.5 DEG C / s.

On the other hand, if the final hot rolling temperature is lower than 810 DEG C, surface ferrite decarburization layer and low temperature structure may be generated, and equipment load problems may occur. If the cooling rate is less than 0.15 DEG C / s, the production speed may be slowed down.

Quenching heat treatment  step

The wire is heated to a temperature range of 880 to 1000 ° C at an average heating rate of 50 ° C / s or more, maintained for 40 seconds or less, cooled at an average cooling rate of 10 ° C / s or more and quenched (quenched).

Heating to a temperature range of 880 to 1000 ° C is for austenitization. If the heating temperature is lower than 880 캜, unused pearlite may remain during heating, and if it is higher than 1000 캜, decarburization and grain growth can be promoted.

When the average heating rate is less than 50 캜 / s, there is a problem that the fine carbide produced in the wire rod is reused and can not serve as a hydrogen trapping portion.

If the holding time is more than 40 seconds, the old austenite grain size may be less than 10.0.

When the average cooling rate is less than 10 占 폚 / s, there is a problem that an undesirable structure such as bainite is obtained in addition to a homogeneous martensite structure.

Heat treatment  step

The quenched and annealed wire rod is heated to a temperature range of 360 to 480 ° C at an average heating rate of 30 ° C / s or more and maintained for 40 seconds or less, and then subjected to a sag heat treatment for cooling to 60 ° C or less at an average cooling rate of 10 ° C / (Tempering).

If the heating temperature for the baking heat treatment is less than 360 ° C, the toughness of the martensite is not sufficient and the toughness of the spring deteriorates. If the heating temperature exceeds 480 ° C, it is difficult to secure the strength.

When the average heating rate is less than 30 ° C / s, there is a problem that the structure of tempered martensite to be obtained through rapid heating can not be obtained. Here, the tempered martensite structure obtained through rapid heating means a structure in which a carbide precipitated in a martensite structure such as epsilon carbide and cementite is concentrated on the old austenite grain boundaries and precipitated in the crystal grains without precipitation.

If the holding time is more than 40 seconds, the epsilon carbide of the tempered martensite may be transformed into cementite in the rapid heating, and the strength and toughness may be lowered.

If the average cooling rate is less than 10 캜 / s, it is difficult to control the structure during cooling, which may lead to a structural deviation of the carbides in the tempered martensite structure obtained through rapid heating.

At this time, the heating in the quenching heat treatment and the baking heat treatment step can be performed by a high frequency heat treatment. This is because it is easy to secure a high heating rate.

Hereinafter, the present invention will be described more specifically by way of examples. It should be noted, however, that the following examples are intended to illustrate the invention in more detail and not to limit the scope of the invention. The scope of the present invention is determined by the matters set forth in the claims and the matters reasonably inferred therefrom.

( Example )

The steel strip having the composition shown in the following Table 1 was heated to 1030 占 폚, finally hot rolled at 880 占 폚 and cooled at a cooling rate of 0.8 占 폚 / s to prepare a wire rod having a diameter of 15 mm.

The wire rod was heated to the temperature shown in Table 2 at an average temperature raising rate of 60 占 폚 / s and maintained for the holding time shown in Table 2, followed by water cooling and quenching heat treatment. The quenched and heat-treated wire rod was heated to 415 ° C at an average heating rate of 60 ° C / s and maintained for 15 seconds, followed by water cooling and heat treatment to produce a steel material.

The tensile strength, the grain size, the average size of the (V, Mo) C carbide, the heat release test hydrogen peak temperature and the hydrogen embrittlement breaking time of the steel material were measured and reported in Table 2 below.

Tensile strength measurements were made using the ASTM E-8M subsize standard.

Microstructure was measured by grinding and corrosion to determine the austenite grain size, Respectively. The average size of (V, Mo) C carbide was measured by TEM (Transmission Electron Microscope) replica specimen, and the circle equivalent diameter was measured. The carbide distribution was evaluated by scanning five areas of STEM (scanning transmission electron microscope) image.

In the thermal desorption analysis, the steel was heated at a heating rate of 100 ° C / hr with a quadruple mass spectrometry equipment to observe the amount of hydrogen released.

The hydrogen brittle fracture time was measured by processing the steel material into a 1.5 mm x 8.0 mm x 65 mm plate-like specimen. The specimens were clamped in a 4 point bending jig to give a shear stress of 1400 MPa. The specimens were immersed in distilled water (0.5 mol / l H ₂ SO ₄ + 0.05 mol / l KSCN) A potential of 700 mV was applied to measure the break time of the specimen. In this case, the time required to break the specimen was defined as the brittle brittle fracture time, the specimen with 600 seconds or longer was passed and the specimen with the fracture time less than 600 seconds defined the risk of hydrogen embrittlement. The definition of the time, which is 600 seconds, is the injection time of hydrogen in excess of the hydrogen content of the critical hydrogen concentration which can cause hydrogen embrittlement in the commercial spring steel.

division Alloy composition (% by weight) C Mn Si P S Cr Mo V Inventory 1 0.53 0.42 1.64 0.010 0.005 0.31 0.07 0.08 Inventory 2 0.52 0.41 1.64 0.010 0.004 0.30 0.21 0.07 Inventory 3 0.54 0.43 1.63 0.010 0.005 0.30 0.07 0.15 Honorable 4 0.52 0.42 1.62 0.010 0.005 0.31 0.20 0.15 Inventory 5 0.49 0.60 1.45 0.010 0.005 0.60 0.10 0.10 Inventory 6 0.59 0.72 1.50 0.010 0.004 0.65 0.10 0.10 Comparative Example 1 0.44 0.40 1.65 0.010 0.004 0.30 0.07 0.07 Comparative Example 2 0.52 0.41 1.30 0.010 0.004 0.30 0.21 0.07 Comparative Example 3 0.52 0.42 1.60 0.010 0.005 0.30 0.27 0.22 Comparative Example 4 0.52 0.42 1.60 0.010 0.005 0.30 0.01 0.15 Comparative Example 5 0.53 0.41 1.60 0.010 0.005 0.30 0.12 - Comparative Example 6 0.51 0.91 1.58 0.010 0.005 0.87 0.07 0.10 Comparative Example 7 0.54 0.70 1.55 0.010 0.005 0.71 - - Comparative Example 8 0.52 0.40 1.61 0.010 0.005 0.30 0.10 0.07

division Quenching heat treatment Tensile Strength (MPa) Grain number Average carbide size
(nm) Carbide count
(Number / 탆 ² ) Hydrogen peak temperature
(° C) Hydrogen embrittlement
Break time
(second) Heating temperature
(° C) Retention time
(second) Inventory 1 935 10 1979 10.4 26 12 340 669 Inventory 2 935 10 1946 10.6 22 18 360 874 Inventory 3 955 10 2008 10.9 29 29 340 778 Honorable 4 955 10 2046 11.0 39 39 360 865 Inventory 5 935 10 2011 10.7 32 22 340 688 Inventory 6 945 10 2033 10.8 36 21 340 644 Comparative Example 1 955 10 1889 10.3 26 10 340 - Comparative Example 2 935 10 1910 10.5 31 16 360 - Comparative Example 3 935 10 2135 10.9 54 44 360 218 Comparative Example 4 925 10 1933 10.5 30 21 240 342 Comparative Example 5 925 10 1926 10.6 32 3 140 488 Comparative Example 6 925 10 1998 10.7 41 13 340 470 Comparative Example 7 925 10 1885 8.6 - 0 140 455 Comparative Example 8 935 50 1935 9.7 30 12 340 492

Inventive Example 1 to Inventive Example 6 satisfy the alloy composition and manufacturing conditions proposed in the present invention, so that the old austenite grain size is 10.0 or more and the average size of the (V, Mo) C carbide is 40 nm or less and the tensile strength is 1920 MPa or more and a hydrogen peak appears at 330 to 370 DEG C, the hydrogen embrittlement breaking time is 600 seconds or more, which indicates that hydrogen embrittlement resistance is excellent. Despite its high strength, it disperses hydrogen through fine grain size and traps hydrogen in the fine carbide, which is excellent in hydrogen embrittlement resistance.

FIG. 1 is a photograph of a (V, Mo) C carbide of Inventive Example 1 taken by a transmission electron microscope, confirming the existence of VC, MoC, and (V, Mo) C fine carbide.

FIG. 2 is a graph showing the hydrogen release behavior according to the temperature obtained by the heat release test of Examples 4 and Comparative Example 4. In Example 4, the hydrogen peak temperature is shown at 360 ° C., Can be confirmed to be as low as 240 ° C.

In Comparative Example 1, the C content was 0.44 wt%, which was below the alloy component of the present invention, and in Comparative Example 2, the Si content was 1.30 wt%, which was below the alloy component, which was below the intended tensile strength in the present invention.

In Comparative Example 3, the hydrogen brittle fracture time was failed as the Mo and V contents were excessively added to produce coarse carbides.

In Comparative Example 4, an effective non-proliferative hydrogen trapping portion was not formed as the amount of addition of Mo was insufficient, and the hydrogen embrittlement breaking time was failed. The non-proliferative hydrogen trapping portion is not effectively produced is found through the fact that the peak of the hydrogen release test occurs at 240 캜, which is lower than 300 캜.

FIG. 2 is a graph showing the hydrogen release behavior according to the temperature obtained by the heat emission test of Examples 4 and Comparative Example 4. In Example 4, the hydrogen peak temperature is shown at 360 ° C., Lt; 0 > C.

When the diffusible hydrogen trapping portion capable of inducing hydrogen embrittlement is defined as a trap portion of hydrogen released at 300 ° C or less, Comparative Example 4 predicts that the hydrogen embrittlement breaking time is failed due to the presence of the diffusible hydrogen trapping portion in the non-proliferative hydrogen trap portion . Therefore, even if the size and the number of the carbides are satisfied, it can be seen that the hydrogen embrittlement is caused when the Mo content is out of the range suggested by the present invention.

In Comparative Example 5, the amount of addition of V was insufficient and the hydrogen embrittlement breaking time was failed. It is believed that the addition of Mo alone resulted in the formation of fine carbides in the solid state due to the absence of the carbide nuclei, and the hydrogen brittle fracture time would have failed.

In Comparative Example 6, Mn and Cr were excessively added, and it was considered that hydrogen embrittlement breaking time was failed due to grain boundary softening, and that the average size of carbide was increased due to formation of (V, Cr) C due to the addition of Cr do.

Comparative Example 7 was a steel material to which Mo and V were not added, and the hydrogen embrittlement breaking time failed even though the strength was low. This indicates that there is a risk of hydrogen embrittlement even in general steels, which are known to have a high grain size and low hydrogen embrittlement sensitivity if no fine carbides are present.

In Comparative Example 8, it can be seen that hydrogen embrittlement resistance is caused by grain growth when the osteonizing heat treatment holding time is maintained for a long time.

It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined in the appended claims. It will be possible.

Claims (10)

The steel sheet according to any one of claims 1 to 3, wherein the steel sheet contains 0.45 to 0.60% of C, 0.30 to 0.80% of Si, 1.40 to 1.80% of Si, 0.015% or less of P, , V: 0.05 to 0.20%, balance Fe and other unavoidable impurities,
(V, Mo) C carbide and having an average size of 10 to 40 nm, measured in terms of the circle equivalent diameter, of the high strength spring having excellent hydrogen embrittlement resistance Steel for use.
The method according to claim 1,
The steel material according to claim 1, further comprising at least one of Ni: 0.1 to 0.4% and Cu: 0.1 to 0.3% by weight, and further exhibiting excellent hydrogen embrittlement resistance.
The method according to claim 1,
Wherein the steel material exhibits a hydrogen emission peak at a temperature of 330 to 370 占 폚 in a heat release test.
The method according to claim 1,
Wherein the steel has a tensile strength of 1920 MPa or more and is excellent in hydrogen embrittlement resistance.
The method according to claim 1,
Wherein the microstructure of the steel comprises 90 to 95% of tempered martensite and 5 to 10% of retained austenite in an area fraction, and is excellent in hydrogen embrittlement resistance.
The method according to claim 1,
The (V, Mo) C carbide is 8 to 40 pieces / 탆 ² , and is excellent in hydrogen embrittlement resistance.
The steel sheet according to any one of claims 1 to 3, wherein the steel sheet contains 0.45 to 0.60% of C, 0.30 to 0.80% of Si, 1.40 to 1.80% of Si, 0.015% or less of P, , V: 0.05 to 0.20%, balance Fe and other unavoidable impurities;
A quenching heat treatment step of heating the wire rod to a temperature range of 880 to 1000 ° C at an average heating rate of 50 ° C / sec or more, holding it for 40 seconds or less, and cooling the wire rod at an average cooling rate of 10 ° C / s or more; And
The quenched and annealed wire rod is heated to a temperature range of 360 to 480 캜 at an average heating rate of 30 캜 / sec or more and maintained for 40 seconds or less, and then subjected to a punching heat treatment for cooling to 60 캜 or lower at an average cooling rate of 10 캜 / Wherein the method comprises the steps of: preparing a steel material for a high-strength spring having excellent hydrogen embrittlement resistance.
8. The method of claim 7,
Wherein the step of preparing the wire rod comprises the steps of: 0.45 to 0.60% of C; 0.30 to 0.80% of Mn; 1.40 to 1.80% of Si; 0.015% or less of P; To 0.25%, V: 0.05 to 0.20%, the balance Fe and other unavoidable impurities to 1000 to 1150 캜;
Subjecting the heated billet to final hot rolling in a temperature range of 810 to 910 캜 to obtain a wire rod; And
And cooling the wire rod at a cooling rate of 0.15 to 1.5 占 폚 / s.
8. The method of claim 7,
Wherein the heating in the quenching heat treatment and the heating heat treatment step is performed by a high frequency heat treatment.
8. The method of claim 7,
Wherein the wire rod further comprises at least one of Ni: 0.1 to 0.4% and Cu: 0.1 to 0.3% in weight percent, and wherein the wire material further has excellent hydrogen embrittlement resistance.

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