KR101822293B1 - High strength special steel - Google Patents

Abstract

Disclosed is high strength special steel, comprising: 0.1-0.5 wt% of carbon (C); 0.1-2.3 wt% of silicon (Si); 0.3-1.5 wt% of manganese (Mn); 1.1-4.0 wt% of chromium (Cr); 0.3-1.5 wt% of molybdenum (Mo); 0.1-4.0 wt% of nickel (Ni); 0.01-0.50 wt% of vanadium (V); 0.001-0.010 wt% of boron (B); 0.01-0.65 wt% of tungsten (W); 0.01-1.0 wt% of zirconium (Zr); and the remaining including remaining iron (Fe) and other inevitable impurities. An object of the present invention is to provide the high strength special steel with improved strength and fatigue life.

Description

{HIGH STRENGTH SPECIAL STEEL}

The present invention relates to a high-strength special steel having improved strength and fatigue life through control of the shape, size and amount of formation of carbide and boride by controlling the components and the content.

In the case of stabilizer bars, drive shafts, or subframes applied to chassis suspensions of rally cars, which are applied to recent chassis modules, lightweight technology is developed to maximize fuel efficiency through manufacturing parts in hollow form or using polymer materials. Is underway.

In the case of conventional chassis steels applied to the above-mentioned components, high strength conditions are satisfied through addition of elements such as chromium (Cr), molybdenum (Mo) and vanadium (V), but relatively simple forms of carbides are formed in the structure, The amount of carbide is not large and the size is not fine, which means that the durability of parts is not supported.

JP 5200540 aims to manufacture high strength steels by adding elements such as boron (B) and zirconium (Cr), but the content of boron (B) is in the range of 0.0005 to 0.006% ) Content is not more than 0.0001%, which is problematic in that it is difficult to enhance the durability while satisfying the high strength.

It should be understood that the foregoing description of the background art is merely for the purpose of promoting an understanding of the background of the present invention and is not to be construed as an admission that the prior art is known to those skilled in the art.

KR 10-2015-0038426 JP 5200540

It is an object of the present invention to provide a high strength special steel having improved strength and fatigue life by controlling the shape, size and amount of formation of carbide and boride by controlling the components and the content.

In order to achieve the above object, the high strength special steel according to the present invention is characterized in that it comprises 0.1 to 0.5% of carbon (C), 0.1 to 2.3% of silicon (Si), 0.3 to 1.5% of manganese (Mn) (Cr): 1.1 to 4.0%, molybdenum (Mo): 0.3 to 1.5%, nickel (Ni): 0.1 to 4.0%, vanadium (V): 0.01 to 0.50%, boron (B) W: 0.01 to 0.65%, zirconium (Zr): 0.01 to 1.0%, the balance iron (Fe) and other unavoidable impurities.

(V, Fe, W, Cr) C and (Zr, Mo) C may be present in complex carbide form in the tissue.

(Fe, Cr, Mo) 7C3 may be present in complex carbide form within the tissue.

There may be (Fe, Cr, Mo, W) 23C6 in complex carbide form in the tissue.

There may be a (Boride) in the form of (Mo, Fe, W) 3B2 in the tissue.

The precipitate present in the tissue may have a mole fraction of 0.009 or more.

The size of the precipitate present in the tissue may be 10 nm or less.

A tensile strength of 1589 MPa or more, and a fatigue life of 590,000 cycles or more.

According to the high strength special steel of the present invention as described above, by controlling the content of elements to generate carbide and boride in the structure, strength and fatigue life can be expected to be improved.

FIG. 1 is a graph showing a change in the mole fraction according to temperature with respect to the phase of the presence of a base. FIG.
FIG. 2 is a graph showing a change in the mole fraction according to temperature with respect to the phase of the embodiment of the present invention. FIG.
3 is a graph showing changes in the molar fraction with time for the precipitate of the example of the present invention.
FIG. 4 is a graph showing a change in size with time of the precipitate of the embodiment of the present invention. FIG.

Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings.

The high strength special steel according to the present invention comprises 0.1 to 0.5% of carbon (C), 0.1 to 2.3% of silicon (Si), 0.3 to 1.5% of manganese (Mn), 1.1 to 4.0% of chromium (Cr) , Molybdenum (Mo): 0.3 to 1.5%, nickel (Ni): 0.1 to 4.0%, vanadium (V): 0.01 to 0.50%, boron (B): 0.001 to 0.010%, tungsten (W) , Zirconium (Zr): 0.01 to 1.0%, iron (Fe), and other unavoidable impurities.

Hereinafter, the reason for restricting the constituent conditions of the steel in the high-strength special steel of the present invention will be described in detail.

Carbon (C): 0.1 to 0.5%

Carbon (C) serves to increase strength and hardness. (V, Fe, W, Cr) C, (Zr, Mo) C, (Fe, Cr, Mo) 7C3 and (Fe, Cr, Mo, W) 23C6 are formed by stabilizing the retained austenite . Further, the resistance to tempering is increased.

When the content of carbon (C) is less than 0.1%, the effect of increasing the strength is not large and the fatigue strength is lowered. On the other hand, when the content of carbon (C) is more than 0.5%, the undissolved large-sized carbides remain, resulting in poor fatigue characteristics and durability life. In addition, there is also a problem that the processability of patterning is lowered. Therefore, the content of carbon (C) is limited to the range of 0.1 to 0.5%.

Silicon (Si): 0.1 to 2.3%

Silicon (Si) serves to improve elongation. Further, the ferrite and martensite structure are cured to increase the heat resistance and hardenability. It is improved in the shape retentivity and heat resistance, but is sensitive to decarburization.

When the content of silicon (Si) is less than 0.1%, the effect of increasing the rate of the reduction of the sintering rate is insignificant. In addition, the heat resistance and the hardenability increase effect are not large. On the other hand, when the content of silicon (Si) is more than 2.3%, carbon (C) causes decarburization due to mutual penetration reaction in the structure. In addition, there is a problem that workability is lowered due to the increase of the overall hardness. Therefore, the content of silicon (Si) is limited within the range of 0.1 to 2.3%.

Manganese (Mn): 0.3 to 1.5%

Manganese (Mn) serves to improve hardenability and strength. It is dissolved in a matrix to increase bending fatigue strength and incombustibility and inhibit the formation of inclusions such as Al2O3 as a deoxidizing agent for producing oxides. On the other hand, MnS inclusions are formed when excessive amounts are present, resulting in high-temperature brittleness.

When the content of manganese (Mn) is less than 0.3%, the improvement of the ingot property becomes insignificant. On the other hand, when the content of manganese (Mn) exceeds 1.5%, there is a problem that the workability of the casting process is deteriorated, and the fatigue life is weakened due to the center segregation and precipitation of MnS inclusions. Therefore, the content of manganese (Mn) is limited within the range of 0.3 to 1.5%.

Chromium (Cr): 1.1 to 4.0%

Cr (Cr) dissolves in the austenite structure and forms CrC carbide during tempering, improves hardenability, improves strength by suppressing softening, and contributes to grain refinement.

When the content of chromium (Cr) is less than 1.1%, the effect of improving the strength and improving the hardenability is not significant. However, when the content of chromium (Cr) exceeds 4.0%, the production of various carbides is suppressed and the effect of the increase of the content is saturated, resulting in an increase of the cost. Therefore, the content of chromium (Cr) is limited within the range of 1.1 to 4.0%.

Molybdenum (Mo): 0.3 to 1.5%

Molybdenum (Mo) functions to improve microstructure and strength, and to improve heat resistance and fracture toughness. Further, the resistance to tempering is increased.

When the content of molybdenum (Mo) is less than 0.3%, the effect of improving the strength and fracture toughness is not significant. On the other hand, if the content of molybdenum (Mo) exceeds 1.5%, the effect of increasing the strength is saturated and the cost increases only. Therefore, the content of molybdenum (Mo) is limited within the range of 0.3 to 1.5%.

Nickel (Ni): 0.1 to 4.0%

Nickel (Ni) improves corrosion resistance and heat resistance, improves hardenability and prevents low-temperature embrittlement. It is an element that stabilizes austenite and extends the high-temperature region.

When the content of nickel (Ni) is less than 0.1%, the effect of improving the corrosion resistance and high temperature stability is not significant. On the other hand, when the content of nickel (Ni) exceeds 4.0%, there arises a problem that a hot brittleness is generated. Therefore, the content of nickel (Ni) is limited within the range of 0.1 to 4.0%.

Vanadium (V): 0.01 to 0.50%

Vanadium (V) forms fine precipitates and improves fracture toughness. The fine precipitates inhibit crystal grain boundary migration, dissolve vanadium (V) at the time of the austenization, and precipitate at the time of tempering to cause secondary curing. However, if it is used in an excessive amount, the post-machining hardness is lowered.

When the content of vanadium (V) is less than 0.01%, the effect of improving the strength and fracture toughness is not significant. On the other hand, when the content of vanadium (V) exceeds 0.50%, the workability is lowered and the productivity is lowered. Therefore, the content of vanadium (V) is limited to the range of 0.01 to 0.50%.

Boron (B): 0.001 to 0.010%

Boron (B) improves strength and elongation and prevents corrosion. Improves impact resistance and hardenability, and functions to prevent weldability degradation and low-temperature embrittlement. (Fe, Cr, Mn) 2B and (Mo, Fe, W) 3B2.

When the content of boron (B) is less than 0.001%, the strength is lowered and the formation of boride is lowered. On the other hand, when the content of boron (B) exceeds 0.010%, there is a problem that the impact resistance is lowered due to the decrease in toughness and ductility. Therefore, the content of boron (B) is limited within the range of 0.001 to 0.010%.

Tungsten (W): 0.01 to 0.65%

Tungsten (W) forms precipitation carbide and improves abrasion resistance and toughness at high temperature. It also inhibits tissue growth and reduces scale resistance.

When the content of tungsten (W) is less than 0.01%, the abrasion resistance is lowered. On the other hand, when the content of tungsten (W) exceeds 0.65%, the precipitated carbide is excessively formed and the toughness is lowered. Therefore, the content of tungsten (W) is limited within the range of 0.01 to 0.65%.

Zirconium (Zr): 0.01 to 1.0%

Zirconium (Zr) can improve the lifetime by forming precipitates and removing nitrogen (N), oxygen (O), sulfur (S) and the like to reduce the size of nonmetallic inclusions.

If the content of zirconium (Zr) is less than 0.01%, carbides are not formed and the inclusions are coarsened. If the content of zirconium (Zr) exceeds 1.0%, ZrO2 is formed, the effect becomes saturated, and only the cost increases. Therefore, the content of zirconium (Zr) is limited within the range of 0.01 to 1.0%.

Aluminum (Al), copper (Cu), oxygen (O), and the like may be included as inevitable impurities in addition to the above elements.

Aluminum (Al): not more than 0.003%

Aluminum (Al) serves to improve strength and impact toughness. It is possible to reduce the addition amount of vanadium for grain refining and nickel for securing toughness, which are high-priced elements. However, when the content of aluminum (Al) exceeds 0.003%, Al ₂ O ₃ which is a prismatic large inclusion is produced, which acts as a fatigue starting point, and the durability is weakened. Therefore, it is appropriate to limit the content of aluminum (Al) to 0.003% or less.

Copper (Cu): not more than 0.3%

Copper (Cu) can enhance the strength after tempering and improve the corrosion resistance of steel such as nickel (Ni). However, if the content of Cu exceeds 0.3%, the cost of alloy will increase. Therefore, it is reasonable to limit the content of copper (Cu) to 0.3% or less.

Oxygen (O): not more than 0.003%

Oxygen (O) is combined with silicon (Si) or aluminum (Al) to form hard oxide-based nonmetal inclusions, which leads to deterioration of fatigue life characteristics. Therefore, the content of oxygen (O) It is good. If the content of oxygen (O) exceeds 0.003%, Al ₂ O ₃ is formed due to the reaction with aluminum (Al), which acts as a fatigue starting point and weakens the durability. Therefore, it is appropriate to limit the content of oxygen (O) to 0.003% or less.

( Example  And Comparative Example )

The following Tables 1 and 2 disclose examples and comparative examples based on specimens prepared by varying the composition components and contents. The specimens annealed at 950 ~ 1000 ℃ and annealed at about 200 ℃ were used.

wt% carbon
(C) silicon
(Si) manganese
(Mn) chrome
(Cr) molybdenum
(Mo) nickel
(Ni) vanadium
(V) Boron (B) tungsten
(W) Zirconium (Zr) Copper
(Cu) aluminum
(Al) Oxygen
(O) Example 1 0.12 0.11 0.31 1.12 0.32 0.13 0.03 0.003 0.04 0.02 0.065 0.0006 0.0018 Example 2 0.49 2.28 1.47 3.98 1.48 3.94 0.47 0.008 0.62 0.98 0.034 0.0010 0.0004 Example 3 0.35 1.22 0.66 1.44 0.74 1.98 0.24 0.005 0.37 0.54 0.055 0.0005 0.0001 Existence 0.15 0.15 1.0 1.50 0.90 - 0.25 - - - 0.062 0.0013 0.0016 Comparative Example 1 0.08 0.22 0.79 1.51 0.56 1.94 0.25 0.005 0.06 0.03 0.041 0.0006 0.0004 Comparative Example 2 0.51 0.19 0.38 2.17 0.38 0.33 0.33 0.009 0.63 0.95 0.043 0.0012 0.0021 Comparative Example 3 0.33 0.09 1.46 3.76 1.45 3.34 0.45 0.005 0.47 0.56 0.050 0.0026 0.0018 Comparative Example 4 0.16 2.32 0.83 1.54 0.67 2.54 0.17 0.003 0.04 0.02 0.034 0.0010 0.0011 Comparative Example 5 0.45 0.25 0.27 2.56 0.43 0.48 0.44 0.007 0.52 0.91 0.040 0.0009 0.0001 Comparative Example 6 0.37 0.58 1.53 3.96 1.45 3.81 0.41 0.004 0.37 0.54 0.062 0.0013 0.0016 Comparative Example 7 0.20 1.95 0.53 1.07 0.66 2.31 0.19 0.009 0.08 0.05 0.065 0.0018 0.0017 Comparative Example 8 0.47 0.86 0.43 4.14 1.45 0.84 0.16 0.005 0.48 0.48 0.041 0.0006 0.0012 Comparative Example 9 0.35 0.37 1.45 3.55 0.28 3.85 0.46 0.003 0.26 0.54 0.045 0.0014 0.0015 Comparative Example 10 0.14 1.85 1.26 1.15 1.53 2.64 0.21 0.008 0.05 0.09 0.056 0.0021 0.0022 Comparative Example 11 0.43 2.14 0.54 3.91 0.59 0.09 0.33 0.004 0.42 0.98 0.063 0.0019 0.0014 Comparative Example 12 0.34 1.64 1.47 1.54 0.44 4.12 0.49 0.006 0.55 0.54 0.041 0.0014 0.0002 Comparative Example 13 0.41 1.36 0.86 2.46 1.21 1.45 0.009 0.005 0.15 0.08 0.063 0.0017 0.0008 Comparative Example 14 0.48 0.27 0.78 3.98 0.67 1.93 0.52 0.002 0.62 0.88 0.062 0.0010 0.0009 Comparative Example 15 0.32 1.75 0.56 1.56 0.61 2.54 0.45 0.0009 0.09 0.16 0.064 0.0027 0.0023 Comparative Example 16 0.19 0.26 1.43 2.38 1.43 0.49 0.16 0.012 0.62 0.98 0.042 0.0007 0.0014 Comparative Example 17 0.21 1.94 0.93 1.28 0.66 2.31 0.18 0.008 0.008 0.44 0.067 0.0017 0.0015 Comparative Example 18 0.38 0.98 1.53 3.94 1.45 3.97 0.41 0.003 0.67 0.05 0.053 0.0011 0.0011 Comparative Example 19 0.35 2.25 1.45 3.54 0.52 3.11 0.45 0.002 0.59 0.008 0.054 0.0014 0.0013 Comparative Example 20 0.44 0.56 0.78 3.87 0.75 1.96 0.52 0.001 0.47 1.03 0.060 0.0010 0.0009

Tensile Strength (MPa) Hardness (HV) Fatigue strength (MPa) Fatigue life Example 1 1589 562 1198 590,000 times Example 2 1595 556 1201 600,000 times Example 3 1592 572 1192 61 thousand times Existence 979 344 689 280,000 times Comparative Example 1 1366 455 804 250,000 times Comparative Example 2 1385 462 968 280,000 times Comparative Example 3 1486 495 1104 370,000 times Comparative Example 4 1375 459 1025 260,000 times Comparative Example 5 1563 521 1156 34 million times Comparative Example 6 1438 479 1003 250,000 times Comparative Example 7 1181 394 884 220,000 times Comparative Example 8 1492 496 1102 310,000 times Comparative Example 9 1316 441 966 290,000 times Comparative Example 10 1183 396 837 250,000 times Comparative Example 11 1333 448 1038 420,000 times Comparative Example 12 1418 461 1125 250,000 times Comparative Example 13 1192 396 837 250,000 times Comparative Example 14 1468 491 1105 330,000 times Comparative Example 15 1324 443 967 280,000 times Comparative Example 16 1476 489 1104 370,000 times Comparative Example 17 1393 464 837 250,000 times Comparative Example 18 1418 459 1175 250,000 times Comparative Example 19 1359 454 951 310,000 times Comparative Example 20 1456 471 1106 370,000 times

Table 1 shows the composition and contents of the examples and comparative examples. Table 2 shows tensile strength, hardness, fatigue strength, and fatigue life of Examples and Comparative Examples.

Tensile strength and yield strength were measured according to KS B 0802 or ISO 6892, hardness was measured according to KS B 0811 or ISO 1143 and fatigue life was measured according to KS B ISO 1143.

In the case of Comparative Example 1 and Comparative Example 2, the content of the other components was controlled within the range of the high-strength special steel according to the present invention to the same extent as that of the embodiment, and only the content of carbon (C) Or less than the predetermined range.

As shown in Table 2, it is confirmed that tensile strength, hardness, fatigue strength, and fatigue life are inferior to those of the examples even when they fall short of the range, even when they exceed the range.

In the case of Comparative Example 3 and Comparative Example 4, the content of other components was controlled within the range of the high-strength special steel according to the present invention in the same range as that of the embodiment, and only the content of silicon (Si) Or less than the predetermined range.

As shown in Table 2, the fatigue strength in the case of being under the range is comparable to those in Examples, but is lower than those of the Examples in terms of tensile strength, hardness and fatigue life, and the tensile strength, hardness, And the fatigue life is lower than those of the examples.

In the case of Comparative Example 5 and Comparative Example 6, the content of the other components was controlled within the range of the high-strength special steel according to the present invention in the same range as that of the example, and only the content of manganese (Mn) Or less than the predetermined range.

As shown in Table 2, the tensile strength and the fatigue strength in the case of being under the range are comparable to those in Examples but lower than those in the Examples in terms of hardness and fatigue life, and the tensile strength, hardness, And the fatigue life is lower than those of the examples.

In the case of Comparative Example 7 and Comparative Example 8, the content of the other components was controlled within the range of the high-strength special steel according to the present invention in the same range as that of the embodiment, and only the content of chromium (Cr) Or less than the predetermined range.

As shown in Table 2, the tensile strength, hardness, fatigue strength, and fatigue life are both inferior to those in the case of being under the range, and when exceeding the range, the fatigue strength is equivalent to the tensile strength, It can be confirmed that the lifetime is lower than those of the embodiments.

In the case of Comparative Example 9 and Comparative Example 10, the content of the other components was controlled within the range of the high-strength special steel according to the present invention in the same range as that of the embodiment, and only the content of molybdenum (Mo) Or less than the predetermined range.

As shown in Table 2, it is confirmed that tensile strength, hardness, fatigue strength, and fatigue life are inferior to those of the examples even when they fall short of the range, even when they exceed the range.

In the case of Comparative Example 11 and Comparative Example 12, the content of the other components was controlled within the range of the high-strength special steel according to the present invention in the same range as that of the embodiment, and only the content of nickel (Ni) Or less than the predetermined range.

As shown in Table 2, the tensile strength, hardness, fatigue strength, and fatigue life are both inferior to those in the case of being under the range, and when exceeding the range, the fatigue strength is equivalent to the tensile strength, It can be confirmed that the lifetime is lower than those of the embodiments.

In the case of Comparative Example 13 and Comparative Example 14, the content of the other components was controlled within the range of the high-strength special steel according to the present invention in the same range as that of the example, and only the content of vanadium (V) Or less than the predetermined range.

As shown in Table 2, the tensile strength, hardness, fatigue strength, and fatigue life are both inferior to those in the case of being under the range, and when exceeding the range, the fatigue strength is equivalent to the tensile strength, It can be confirmed that the lifetime is lower than those of the embodiments.

In the case of Comparative Example 15 and Comparative Example 16, the content of the other component was controlled within the range of the high-strength special steel according to the present invention to the same extent as that of the Example, and only the content of boron (B) Or less than the predetermined range.

As shown in Table 2, the tensile strength, hardness, fatigue strength, and fatigue life are both inferior to those in the case of being under the range, and when exceeding the range, the fatigue strength is equivalent to the tensile strength, It can be confirmed that the lifetime is lower than those of the embodiments.

In the case of Comparative Example 17 and Comparative Example 18, the content of the other components was controlled within the range of the high-strength special steel according to the present invention in the same range as that of the example, and only the content of tungsten (W) Or less than the predetermined range.

As shown in Table 2, the tensile strength, hardness, fatigue strength, and fatigue life are both inferior to those in the case of being under the range, and when exceeding the range, the fatigue strength is equivalent to the tensile strength, It can be confirmed that the lifetime is lower than those of the embodiments.

In the case of Comparative Example 19 and Comparative Example 20, the content of the other components was controlled within the range of the high-strength special steel according to the present invention to the same extent as that of the example, and only the content of zirconium (Zr) Or less than the predetermined range.

As shown in Table 2, the tensile strength, hardness, fatigue strength, and fatigue life are both inferior to those in the case of being under the range, and when exceeding the range, the fatigue strength is equivalent to the tensile strength, It can be confirmed that the lifetime is lower than those of the embodiments.

Hereinafter, a high strength special steel according to the present invention will be described with reference to FIGS. 1 to 4. FIG.

Fig. 1 is a graph showing the thermodynamic-based calculation results for the alloy components 0.15C-0.15Si-1.0Mn-1.5Cr-0.9Mo-0.25V (the numbers before the symbol are wt%) as a group. You can see the change.

Fig. 2 is a graph showing the results of the thermodynamic basis for the alloy component 0.3C-0.2Si-0.7Mn-1.5Cr-2.0Ni-0.5Mo-0.15V-0.005B-0.3W-0.1Zr as an example of the high- As a graph showing the calculation results, it is possible to know a change in the mole fraction with respect to temperature.

Comparing FIG. 1 and FIG. 2, the examples show that the amount of carbon (C) and nickel (Ni) as the austenite stabilizing element are larger than those of the base, and that the temperatures of A1 and A3 are lowered and thus the austenite region is expanded. .

(V, Fe, W, Cr) C type carbide is precipitated in the form of complex carbide in the case of the embodiment in contrast to the presence of VC carbide in the tissue. This is because vanadium (V), tungsten (W), and chromium (Cr), which are primarily formed by carbide formation, are added. Unlike the presence of carbides, carbides of form (V, Fe, W, Cr) And is formed in a shape having a small size and high distribution of carbide. On the other hand, due to the addition of zirconium (Zr), (Zr, Mo) C together with molybdenum (Mo) is generated from the ferrite region and exists in the form of complex carbide. Precipitation refers to the formation of another solid phase in the solid phase.

As the complex carbide of small size is uniformly distributed in the tissue, the strength and fatigue life can be improved. The results are shown in Table 2.

Unlike the case where (Cr, Fe) 7C3 type carbide is produced in the tissue and the carbide disappears at a temperature of 500 ° C or lower, the (C, Fe, Mo) 7C3 type carbide And is formed into a composite carbide form. The temperature range that is generated compared to the base is also formed at a high temperature so that it is formed in a stable state. In addition, it can be expected that the fatigue life as well as the bar strength, which is uniformly distributed in the tissue, can be improved. have.

Unlike the case where the (Mo, Fe) 6C type carbide is formed in the low temperature region in the structure, the content of molybdenum (Mo) is small in the embodiment and the carbide of the form (Mo, Fe) 6C is formed in the low temperature region (Fe, Cr, Mo, W) 23C6 type carbide is precipitated and formed into complex carbide form.

In the case of carbides such as (Mo, Fe) 6C formed in a low temperature region, the strength and fatigue life are lowered rather than being unstable. In the embodiment, molybdenum (Mo) forms boride from the austenite region, (Fe, Cr, Mo, W) 23C6, (Cr, Fe, Mo) 7C3, (Zr, Mo) C to form stable complex carbides. As a result, the formation of (Mo, Fe) 6C type carbide is suppressed due to the lack of molybdenum (Mo) in the temperature range, and the fatigue life as well as the strength can be expected to be improved.

On the other hand, in the case of the embodiment, boron (B) is added so that boride such as (Fe, Cr, Mn) 2B and (Mo, Fe, W) 3B2 can be precipitated in the tissue. From the thermodynamic point of view, (Fe, Cr, Mn) 2B can be generated and disappear. (Mo, Fe, W) 3B2 may remain in the structure at 500 ° C or less to improve the strength and fatigue life.

FIG. 3 is a graph showing a change in the mole fraction of a precipitate containing carbide and boride with annealing time. As shown in FIG. 3, the annealing time is 10 hours, It can be confirmed that a much larger amount of precipitate is formed compared to the presence of only 0.002. This can be expected to have an effect of improving the strength and fatigue life as described above. Means the mole fraction of the precipitate in the whole structure, and may be expressed as 0.9%.

FIG. 4 is a graph showing a change in size of a precipitate containing carbide and boride with annealing time. As shown in FIG. 4, a precipitate having a size of 40 nm or more is formed on the basis of 10 hours of annealing time. In the other examples, precipitates having a size of 10 nm or less are formed as indicated by d. This can likewise entail improvements in strength and fatigue life.

As described above, the high-strength special steel according to the present invention can improve the strength and fatigue life by controlling the content of elements to generate carbide and boride.

The tensile strength can be increased by about 62%, and thus, when it is applied to a vehicle body, it can be lightened by about 37%, so fuel efficiency can be improved. Fatigue strength can increase by about 73% and fatigue life can increase by about 111%.

While the present invention has been particularly shown and described with reference to specific embodiments thereof, 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 by the following claims It will be apparent to those of ordinary skill in the art.

Claims (8)

(C): 0.1 to 0.5%, silicon (Si): 0.1 to 2.3%, manganese (Mn): 0.3 to 1.5%, chromium (Cr): 1.1 to 4.0%, molybdenum (B): 0.001 to 0.010%, tungsten (W): 0.01 to 0.65%, zirconium (Zr): 0.01 to 0.5% ~ 1.0%, balance iron (Fe) and other unavoidable impurities,
A tensile strength of 1589 MPa or more, and a fatigue life of 590,000 cycles or more. The method according to claim 1,
Characterized in that (V, Fe, W, Cr) C and (Zr, Mo) C are present in complex carbide form in the structure. The method according to claim 1,
Characterized in that (Fe, Cr, Mo) 7C3 is present in complex carbide form in the structure. The method according to claim 1,
Characterized by the presence of (Fe, Cr, Mo, W) 23C6 in the complex carbide form in the structure. The method according to claim 1,
A high strength special steel characterized by the presence of (Mo, Fe, W) 3B2 boride in the structure. The method according to claim 1,
Characterized in that the precipitate present in the tissue has a mole fraction of 0.009 or more. The method of claim 6,
Characterized in that the size of the precipitate present in the tissue is 10 nm or less. delete

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