CN114250412B - High strength carburizing steel with improved durability - Google Patents

High strength carburizing steel with improved durability Download PDF

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CN114250412B
CN114250412B CN202110160012.0A CN202110160012A CN114250412B CN 114250412 B CN114250412 B CN 114250412B CN 202110160012 A CN202110160012 A CN 202110160012A CN 114250412 B CN114250412 B CN 114250412B
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carburizing steel
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CN114250412A (en
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姜敏宇
权纯佑
姜旻佑
金贤基
金桐辉
李仁范
李忠谚
洪承贤
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Hyundai Motor Co
Kia Corp
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Kia Motors Corp
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    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C8/06—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
    • C23C8/08—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases only one element being applied
    • C23C8/20—Carburising
    • C23C8/22—Carburising of ferrous surfaces
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Solid-Phase Diffusion Into Metallic Material Surfaces (AREA)
  • Heat Treatment Of Articles (AREA)

Abstract

The invention discloses a carburizing steel, which comprises the following components in percentage by weight based on the total weight of the carburizing steel: 0.1wt% to 0.3wt% C (carbon); 2.0wt% to 2.7wt% Cr (chromium); 0.4wt% to 0.7wt% Si (silicon); mo (molybdenum) at 0.3wt% or more and 0.7wt% or less; less than 0.6wt% Mn (manganese); and 0.6wt% to 1.5wt% of Ni (nickel).

Description

High strength carburizing steel with improved durability
Technical Field
The present disclosure relates to carburizing steel, and more particularly to high strength carburizing steel for vehicle parts with improved durability.
Background
Generally, gears of a transmission of a vehicle are members for transmitting engine power directly to a differential system and efficiently transmitting rotation or power between two or more shafts according to a running state of the vehicle, which are subjected to bending stress, contact stress, and the like at the same time. With the above gear, if the durability of the material is insufficient, fatigue fracture (tooth fracture) due to lack of bending fatigue strength and fatigue damage (pitting) due to lack of contact fatigue strength often occur. Therefore, the above gears require physical properties such as high hardness, strength, toughness, fatigue strength, and fatigue life.
The durability of carburizing steel for vehicle parts used as a countermeasure thereto is improved by 1) increasing the high hardness carbide fraction by increasing Cr, and 2) increasing the matrix structure strength and softening resistance by increasing Si.
However, when the content of Cr/Si is increased, the pitting start time point at which surface damage is caused by the carbide and hardness increasing effect is improved, but there is a tendency that crack growth and bending strength are remarkably reduced due to the matrix structure toughness reducing effect by Cr/Si.
The matters described in the background section are to aid in understanding the background of the disclosure and may include matters previously unknown to those skilled in the art to which the disclosure pertains.
Disclosure of Invention
The present disclosure aims to solve the above-described problems, and an object of the present disclosure is to provide a carburizing steel capable of simultaneously ensuring bending strength and pitting corrosion resistance.
A high strength carburized steel according to one aspect of the present disclosure comprising, based on the total weight of the carburized steel: 0.1wt% to 0.3wt% C (carbon); 2.0wt% to 2.7wt% Cr (chromium); 0.4wt% to 0.7wt% Si (silicon); mo (molybdenum) at 0.3wt% or more and 0.7wt% or less; less than 0.6wt% Mn (manganese); and 0.6wt% to 1.5wt% of Ni (nickel).
The content of Si (silicon) may be 0.6wt% or more and 0.7wt% or less.
The content of Ni (nickel) may be 0.6wt% or more and 1.0wt% or less.
The content of Mn (manganese) may be 0.1wt% or more and 0.5wt% or less.
The carburized steel may further comprise one or more of Ti (titanium), V (vanadium) and Nb (niobium).
The sum of the contents of Ti (titanium), V (vanadium) and Nb (niobium) may be 1.0wt% or less with respect to the total wt%.
The carburizing steel may further contain 1 to 30ppm of B (boron).
In the carburizing steel according to the present disclosure, the value of the following formula may be 0.2 or more,
([Ni]+0.3[Mn])/([Cr]+3[Si])，
wherein, [ Ni ], [ Mn ], [ Cr ], [ Si ] respectively represent Ni, mn, cr, si wt%.
A carburizing steel according to another aspect of the present disclosure, comprising, based on the total weight of the carburizing steel: 0.1wt% to 0.3wt% C (carbon); 2.0wt% to 2.7wt% Cr (chromium); 0.4wt% to 0.7wt% Si (silicon); mo (molybdenum) at 0.3wt% or more and 0.7wt% or less; less than 0.6wt% Mn (manganese); 0.6wt% to 1.5wt% of Ni (nickel); 1.0wt% or less of the sum of Ti (titanium), V (vanadium) and Nb (niobium); and 1 to 30ppm of B (boron). The values of the following formulas satisfy 0.2 or more, ([ Ni ] +0.3[ Mn ])/([ Cr ] +3[ Si ]), wherein [ Ni ], [ Mn ], [ Cr ], [ Si ] respectively represent Ni, mn, cr, si wt%.
The content of Si (silicon) may be 0.6wt% or more and 0.7wt% or less.
The content of Ni (nickel) may be 0.6wt% or more and 1.0wt% or less.
The content of Mn (manganese) may be 0.1wt% or more and 0.5wt% or less.
The carburized hardness is 780Hv or more and the bending strength may be 3,000MPa or more.
The high-strength carburized steel according to the present disclosure can increase bending strength by 10% or more and increase carburized layer hardness by 20Hv or more, as compared to conventional steel, thereby achieving durability.
In general, conventional high-durability carburizing steel improves strength and durability by increasing the content of Cr-Si, in which case, due to the toughness reducing effect of Si and the formation of brittle (Cr, fe) 7 C 3 The base carbide, therefore, tends to embrittle the bulk material. The present disclosure optimizes the Ni/Mn content in a relatively low Cr/Si range based on the ratio of (ni+0.3mn)/(cr+3si) to ensure toughness of the matrix structure, thereby suppressing the propagation of fatigue cracks to ensure both strength and toughness.
In addition, carburization performance can be significantly improved by reducing the contents of Cr and Si, so that carburization heat treatment can be achieved only by gas carburization without applying a preheating treatment, thus saving costs such as heat treatment costs.
Furthermore, the present disclosure is not realized by Cr carbide of high hardness like conventional steel, but the contents of Ni and Mn can be controlled by low content of Cr, thereby ensuring toughness of a matrix structure and improving strength and hardness, particularly inhibiting propagation of fatigue cracks, thereby remarkably improving durability.
Accordingly, the fatigue life can be improved to reduce surface pitting caused by fatigue fracture, thereby suppressing an increase in transmission noise.
Drawings
FIG. 1 shows hardness according to Cr content, fe 3 C. High temperature (Cr, fe) 7 C 3 Carbide distribution.
FIG. 2 shows impact toughness and Fe according to Si content 3 C. High temperature (Cr, fe) 7 C 3 Carbide distribution.
Fig. 3 shows a bulk retained austenite distribution according to Ni content, and fig. 4A and 4B are tissue pictures showing the bulk retained austenite distribution.
FIG. 5 shows Fe according to the content of Mo 3 C and high temperature (Mo, ni, fe) composite carbide distribution.
Fig. 6A to 6F show pitting corrosion generation amount evaluation result images under severe conditions.
Fig. 7A to 7C show pitting corrosion generation amount evaluation result images under flat conditions (flat conditions).
Fig. 8A shows a microstructure according to comparative example 8, fig. 8B shows a microstructure according to example 1, and fig. 8C shows a microstructure according to example 8.
Detailed Description
For a full understanding of the present disclosure, the operational advantages of the present disclosure, and the objects attained by practicing the present disclosure, reference should be made to the drawings showing exemplary embodiments of the disclosure and to what is shown in the accompanying drawings.
In describing exemplary embodiments of the present disclosure, descriptions of known techniques or repeated descriptions that can unnecessarily obscure the subject matter of the present disclosure will be reduced or omitted.
An object of the present disclosure is to provide a carburized steel capable of simultaneously securing bending strength and pitting corrosion resistance exceeding conventional limits as a carburized steel for vehicle parts manufactured by carburizing heat treatment, quenching, tempering, and the like of steel materials.
To this end, the carburizing steel according to the exemplary embodiment of the present disclosure includes 0.1wt% or more and 0.3wt% or less of C (carbon), 2.0wt% or more and 2.7wt% or less of Cr (chromium), 0.4wt% or more of Si (silicon), 0.3wt% or more and 0.7wt% or less of Mo (molybdenum), less than 0.6wt% of Mn (manganese), 0.6wt% or more and 1.5wt% or less of Ni (nickel), 1.0wt% or less of Ti (titanium) +v (vanadium) +nb (niobium), and 1 to 30ppm of B (boron) with respect to 100wt% (wt%) of the total composition, and may include the balance of Fe (iron) and impurities; further, since the value ([ Ni ] +0.3[ Mn ])/([ Cr ] +3[ Si ]) satisfies 0.2 or more, no network carbide is generated in the carburized layer; and the carburized steel has a carburized hardness (Hv) of 780Hv or more and a bending strength of 3,000MPa or more.
Here, [ Ni ], [ Mn ], [ Cr ], [ Si ] refer to Ni, mn, cr, si wt%, respectively.
That is, this relates to a novel Cr-Si-Ni carburizing steel component system which can improve pitting corrosion resistance and bending strength by increasing toughness as compared with conventional high-durability high Cr-Si carburizing steel.
In the prior art, there is a carburizing steel component system that has improved durability compared to conventional materials by ensuring basic toughness by using Mn and increasing the content of cr—si that is advantageous in controlling carbide.
However, since the prior art has a great possibility of excessively decreasing toughness by a maximum Si content of 2%, the present disclosure utilizes a method of increasing Ni having a better effect than Mn, rather than optimizing the Si content decreasing toughness of the material in a low range, thereby improving toughness of the matrix structure, thus realizing a method of utilizing an effect of controlling formation of cr—si carbide, thereby securing durability.
Hereinafter, each component and content will be described in more detail with reference to fig. 1 to 5.
FIG. 1 shows hardness according to Cr content, fe 3 C. High temperature (Cr, fe) 7 C 3 Composite carbide distribution. Cr is an alloying element that is typically added to ensure hardenability of the material, but the ranges are optimized in this disclosure to determine the type and formation temperature of precipitated carbides and hardenability. According to the present disclosure, there are three types of carbide as affected by Cr content: fe (Fe) 3 C/(Fe,Cr) 3 C/(Fe,Cr) 7 C 3 。
1)Fe 3 C: the carbide is the most basic carbide in the Fe-C component system formed at the temperature below the A1 temperature (730 ℃). In the case of martensite, to achieve high strength, fe is intentionally suppressed by optimizing the heat treatment, particularly tempering conditions 3 Precipitation of C thereby supersaturating C, but in many components supersaturated C combines with Fe due to heat generated during use to form Fe 3 C, thereby decreasing the strength of the matrix structure, so that the addition of Si delays Fe in many cases 3 And C precipitation.
2)(Fe,Cr) 3 C: when the content of Cr having higher C affinity than Fe is increased, a (Fe, cr) is formed 3 Carbide of C. (Fe, cr) 3 C is precipitated in the region of 800 ℃ or higher than A1 temperature, even if it is carburizedAlso ensures stability during handling and also reduces available carbon contained in the matrix structure, thereby inhibiting the additional formation of Fe during use 3 C。
3)(Fe,Cr) 7 C 3 : when Cr is further increased, a mixture of (Fe, cr) is formed 7 C 3 Rather than (Fe, cr) 3 C。(Fe,Cr) 7 C 3 Formed at high temperature but classified as unstable transitional carbides, and excessive use of C at the time of precipitation, thereby further decreasing strength.
If less than 2.0% Cr is added, the present disclosure may not sufficiently form stable (Fe, cr) at high temperatures during carburization 3 C-based carbide.
As a result, due to the formation of Fe during use 3 The total amount at C exceeds 1% and a rapid softening phenomenon occurs, so that the lower limit of Cr in the present disclosure is 2.0%.
In contrast, if Cr is added in excess of 2.7%, carbide formation due to Cr is very activated to form a carbide having instability and strong brittleness (Fe, cr) despite the high temperature of 900 ℃ or higher 7 C 3 To reduce the amount of available carbon and thereby reduce the surface hardness enhancing effect. Therefore, the upper limit of Cr in the present disclosure is 2.7%.
Next, FIG. 2 shows impact toughness, fe, according to Si content 3 C. High temperature (Fe, cr) 7 C 3 Carbide distribution.
Si is known to be the same as Fe 3 And an element having a cation solubility (cation solubility) of C cementite of 0. That is, fe may not be formed around Si element 3 C, due to this effect, most of the Si-containing martensite is very effectively suppressed from forming Fe during use 3 C, the hardness decreases. However, when Si is added at less than 0.4%, fe 3 The content of C cannot be effectively suppressed to less than 1%, thereby disadvantageously ensuring hardness, and thus the lower limit of Si in the present disclosure is 0.4%.
The other effect of Si is to increase the reactivity (activity) of C when added, thus suppressing Fe 3 C, but promotes another form of carbide forming reaction. Has already been provided withIt was demonstrated that in the component system according to the present disclosure, in order to suppress Fe 3 Si is added in the formation of C, but when the Si content exceeds 0.7%, unstable high temperature (Fe, cr) is formed due to the recombination with Cr 7 C 3 Thereby reducing the amount of available carbon, and thus is quite disadvantageous in ensuring hardness, and even the addition of Ni tends to fail to sufficiently ensure toughness.
Therefore, the upper limit of Si in the present disclosure is 0.7% by weight or more and 0.6% by weight or less is more preferable.
Next, fig. 3 shows a bulk retained austenite distribution according to Ni content, and fig. 4A and 4B are tissue pictures showing the bulk retained austenite distribution.
In the present disclosure, ni is an important element for achieving the expected reduction in propagation speed of fatigue cracks due to the reinforcement of toughness of materials. In particular, cr/Si has a greater effect of reducing the toughness of the matrix structure than Ni increases the toughness of the material, and therefore, in order to achieve the effect of enhancing toughness by Ni, it is necessary to optimize the Ni/Mn content based on the Cr/Si content while considering the synergistic effect of Ni and Mn that plays the same role although the relative effect and content are small.
The toughness reducing effect is required to consider both the influence of Cr and Si, and it is confirmed that the present disclosure can ensure the effect of sufficiently enhancing toughness only when 0.2 or more is satisfied, based on the evaluation result based on (ni+0.3mn)/(cr+3si).
When the content of Ni exceeds 1.5%, bulk austenite starts to exist at room temperature due to strong austenite stabilizing action of Ni, thereby decreasing the fraction of martensite formed at the time of heat treatment, thus improving toughness, but causing adverse effects of strength decrease. Therefore, the upper limit of Ni in the present disclosure is 1.5%.
In addition, ni is a toughness-enhancing improving element and a hardenability-improving element, and when the content of Ni is less than 0.6%, toughness is reduced, resulting in an increase in propagation speed of fatigue cracks and a reduction in fatigue life.
Therefore, the Ni content is required to be 0.6wt% or more and 1.5wt% or less, more preferably 0.6wt% or more and 1.0wt% or less.
Next, fig. 5 shows Fe according to the content of Mo 3 Distribution of C and high temperature (Mo, ni, fe) composite carbides. The (Mo, ni, fe) composite carbide can be (Mo, ni, fe) 2 C or (Mo, ni, fe) 6 And C carbide.
Mo inhibits Fe at tempering by high affinity with C 3 C forms and causes micronization to enhance the main elements of uniformity. In addition, mo is also used to delay the movement of potential to enhance the strength and toughness of the material. However, when less than 0.3% of Mo is added, fe is hardly suppressed 3 C and causes no effect of micronization, so that the hardness enhancing effect is not achieved, and thus the lower limit of Mo in the present disclosure is 0.3%.
On the other hand, when Mo exceeds 0.7%, mo reacts with Ni and forms (Mo, ni, fe) -based composite carbide at high temperature, reducing the amount of available carbon, and thus a phenomenon that surface hardness is lowered again occurs.
Therefore, the upper limit of Mo in the present disclosure is 0.7%.
Mn is known to be an austenite stabilizing element that can enhance toughness similar to Ni, although it acts at about 1/3 level. In addition, mn has effects similar to those of Ni in terms of hardenability and toughness of a reinforcing material, but due to an austenite stabilizing effect caused by Ni that has been sufficiently contained, when the content of Mn exceeds 0.6%, a phenomenon occurs in which the martensite fraction is reduced while bulk austenite is stabilized at room temperature, resulting in a reduction in hardness.
Therefore, the upper limit of Mn in the present disclosure is 0.6% by weight or more, and more preferably 0.1% by weight or more and 0.5% by weight or less.
Nb/Ti/V is an alloy element having a large strength improvement effect by forming MC-based carbides at high temperature to miniaturize grain boundaries and enhance precipitation. However, since Nb/Ti/V is formed at a higher temperature than Cr carbide, there is a tendency to reduce the amount of Cr carbide required for improving durability desired in the present disclosure when Nb/Ti/V is excessively added. Therefore, the sum of Nb/Ti/V is limited to 1% or less.
It is known that B (boron) is an element that can effectively suppress ferrite formation during cooling even in a small amount to improve hardenability. However, since B preferentially locates in the grain boundary, the grain boundary is weakened when B is excessively added, and thus the B content in the present disclosure is limited to 1 to 30ppm.
Hereinafter, test examples and experimental results by examples and comparative examples within the scope of the components according to the present disclosure will be described.
Test examples are shown in table 1 below.
TABLE 1
It can be seen that when the value of (ni+0.3mn)/(cr+3si) is 0.2 or more, each of examples 1 to 10 satisfies target values of carburized hardness and bending strength, and when the carburized hardness and bending strength are comprehensively determined, examples 1 and 8 may be referred to as the best examples.
Example 2 is an example in which the lower limit value of Ni is applied to example 1, example 3 is an example in which Cr is added in example 1, example 4 is an example in which Si is added in example 1, and example 5 corresponds to the case in which the value of Si/Cr is the largest in example 1.
Further, example 6 is an example of adding Mo in example 1, and example 7 is an example of reducing Mn in example 1.
Next, example 9 is an example in which the upper limit value of Ni is applied based on example 8, and example 10 is an example in which the upper limit value of most components is applied.
On the other hand, comparative example 1 has Mo smaller than that of example 1, and thus shows a result of small carburized hardness, and comparative example 2 has Mo larger than that of example 1, and thus shows a result of small carburized hardness.
It can be seen that comparative example 3 has Mn greater than that of example 1, and thus shows a result of small carburized hardness and bending strength, that comparative example 4 has Ni less than that of example 1 and a value of (ni+0.3mn)/(cr+3si) less than 0.2, and thus shows a result of small bending strength.
Next, it can be seen that Ni of comparative example 5 is greater than Ni of example 1, thus showing a result of small carburized hardness, cr of comparative example 6 is greater than Cr of example 1 and the value of (ni+0.3mn)/(cr+3si) is less than 0.2, thus showing a result of small bending strength, and Si of comparative example 7 is greater than Si of example 1 and the value of (ni+0.3mn)/(cr+3si) is less than 0.2, thus showing a result of small bending strength.
Further, comparative example 8 has Ni smaller than that of example 1 and a value of (ni+0.3mn)/(cr+3si) smaller than 0.2, and thus shows a result of small bending strength, and comparative example 9 has Cr smaller than that of example 1, and thus shows a result of small carburized hardness and bending strength, and comparative example 10 adopts an upper limit value of most of the components and Ni larger than that of example 1, and shows a result of small carburized hardness.
Fig. 6A to 6F show pitting corrosion generation amount evaluation result images under severe conditions, in which fig. 6A shows the result of example 1, fig. 6B shows the result of example 2, fig. 6C shows the result of comparative example 4, fig. 6D shows the result of comparative example 5, fig. 6E shows the result of comparative example 6, and fig. 6F shows the result of comparative example 8.
Test methods the sun gear, pinion, and carrier may be tested using a planetary gear assembly evaluation apparatus and 400Nm of torque applied to the carrier to drive the planet gear formed of carburized steel of the present disclosure at a speed of 4,000rpm based on the sun gear for 24 hours and then disassemble the planet gear and confirm the pitting-generating state of the gear surface.
From the results, it can be seen that the amounts of pitting corrosion generated in comparative examples 4, 5, 6 and 8 were visually confirmed to be larger than those in examples 1 and 2.
Further, fig. 7A to 7C show pitting generation amount evaluation result images under flat conditions, in which fig. 7A shows the result of example 1, fig. 7B shows the result of example 2, and fig. 7C shows the result of comparative example 8.
Here, the flat condition refers to a level lower than the severe condition in fig. 6A to 6F, and by the same test method as the severe condition, a torque of 300Nm can be applied to the carrier, the planetary gear formed of the carburizing steel of the present disclosure is driven at a speed of 3,000rpm based on the sun gear for 24 hours and then disassembled, and the pitting generation state of the gear surface is confirmed.
From the results, it can be seen that the amount of pitting corrosion generated in comparative example 8 was visually confirmed to be larger than that in examples 1 and 2.
As described above, the exemplary embodiments of the present disclosure may increase the bending strength by 10% or more and increase the carburized layer hardness by 20Hv or more, as compared to conventional steels, thereby achieving durability.
Fig. 8A shows a microstructure according to comparative example 8, fig. 8B shows a microstructure according to example 1, and fig. 8C shows a microstructure according to example 8.
FIG. 8A shows 2.4Cr-0.7Si-0.08Ni steel in comparative example 8, and it can be confirmed that the retained austenite was in the form of a block; FIG. 8B shows the 2.0Cr-0.6Si-0.7Ni steel of example 1, and it can be seen that Fe 3 The size and fraction of C decreases by the increase in Ni.
In addition, FIG. 8C shows the 2.7Cr-0.7Si-1.0Ni steel in example 8, and it can be confirmed that Fe due to the increase of Cr+Ni 3 C is suppressed in grain boundaries and (Cr, fe) 7 C 3 Formed into a few particles.
As described above, the present disclosure has been described with reference to the exemplary drawings, but the present disclosure is not limited to the described exemplary embodiments, and it will be apparent to those skilled in the art that various modifications and changes may be made to the present disclosure without departing from the spirit and scope of the present disclosure. Accordingly, these modified examples or altered examples fall within the claims of the present disclosure, and the scope of the present disclosure should be construed based on the appended claims.

Claims (10)

1. A carburizing steel comprising, based on the total weight of the carburizing steel:
0.1wt% to 0.3wt% C, carbon;
2.0wt% or more and 2.7wt% or less of Cr, i.e., chromium;
si, i.e., silicon, in an amount of 0.4wt% to 0.7 wt%;
molybdenum, which is Mo in an amount of 0.3wt% to 0.7 wt%;
less than 0.6wt% Mn, i.e., mn; and
ni, namely nickel, in an amount of 0.6wt% to 1.5wt%,
wherein the value of the following formula is 0.2 or more,
([Ni]+0.3[Mn])/([Cr]+3[Si])，
wherein, [ Ni ], [ Mn ], [ Cr ], [ Si ] respectively represent Ni, mn, cr, si wt%.
2. The carburizing steel of claim 1, further comprising one or more of Ti, V, and Nb.
3. The carburizing steel according to claim 2, wherein the sum of the Ti, i.e., titanium, the V, i.e., vanadium, and the Nb, i.e., niobium is 1.0wt% or less with respect to the total wt% of the carburizing steel.
4. The carburizing steel of claim 1, further comprising 1 to 30ppm B, boron.
5. A carburizing steel comprising, based on the total weight of the carburizing steel:
0.1wt% to 0.3wt% C, carbon;
2.0wt% to 2.7wt% Cr, i.e., cr, 0.4wt% to 0.7wt% Si, i.e., si;
molybdenum, which is Mo in an amount of 0.3wt% to 0.7 wt%;
less than 0.6wt% Mn, i.e., mn; ni, namely nickel, in an amount of 0.6wt% to 1.5 wt%;
less than 1.0wt% of Ti, i.e., titanium, V, i.e., vanadium, and Nb, i.e., niobium; and
1 to 30ppm of B, namely boron,
wherein the value of the following formula satisfies 0.2 or more,
([Ni]+0.3[Mn])/([Cr]+3[Si])，
wherein, [ Ni ], [ Mn ], [ Cr ], [ Si ] respectively represent Ni, mn, cr, si wt%.
6. The carburizing steel according to claim 1 or 5, wherein the content of Si, i.e., silicon, is 0.6wt% or more and 0.7wt% or less.
7. The carburizing steel according to claim 1 or 5, wherein the content of Ni, i.e., nickel, is 0.6wt% or more and 1.0wt% or less.
8. A carburizing steel according to claim 1 or 5, wherein the Mn, i.e. manganese, content is 0.1wt% or more and 0.5wt% or less.
9. The carburized steel according to claim 1 or 5, wherein the carburized steel has a carburized hardness of 780Hv or more.
10. The carburizing steel according to claim 1 or 5, wherein the carburizing steel has a bending strength of 3,000mpa or more.
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Publication number Priority date Publication date Assignee Title
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JP2001329338A (en) * 2000-05-17 2001-11-27 Sanyo Special Steel Co Ltd Case hardening steel excellent in bending strength
CN102317490A (en) * 2009-03-30 2012-01-11 新日本制铁株式会社 Carburized steel part
CN104024444A (en) * 2011-11-01 2014-09-03 新日铁住金株式会社 Method for producing steel part
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CN106065455A (en) * 2015-04-20 2016-11-02 现代自动车株式会社 There is carburizing alloy steel and the manufacture method thereof of the durability of improvement

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