KR100913172B1 - Ultra high strength carburizing steel with high fatigue resistance - Google Patents

Abstract

The present invention relates to carburizing ultra high strength steel having excellent contact fatigue strength, and the carburizing ultra high strength steel having excellent contact fatigue strength according to the present invention is C: 0.15 to 0.25 wt%, Cr: 1.70 to 2.30 wt%, and Si: 0.50 to 0.70% by weight, N: 100 to 200% by weight, Al: 0.010 to 0.040% by weight, Nb: 0.015 to 0.035% by weight, Ti: 50% by weight or less, Ni: 0.05% by weight or less, Fe and inevitable impurities as remainder It is made, including, rolling and heating at 1200 to 1300 ℃ characterized in that the carbonitride completely dissolved. The super high strength steel for carburizing having excellent contact fatigue characteristics according to the present invention has excellent contact fatigue characteristics and temper softening resistance by completely solidifying carbonitride, and since an ideal grain occurrence temperature is 1000 ° C. or higher, an increase in carburizing temperature can be expected. As a result, productivity and cost reduction are expected.

High strength, contact fatigue, ideal grain, carburizing

Description

Ultra high strength carburizing steel with high fatigue resistance

The present invention relates to high strength steel for carburizing, and more particularly, to carburizing ultra high strength steel having excellent contact fatigue strength.

Recently, the trend of high performance (high power, low fuel consumption, quietness) of automobile engines continues, and accordingly, research on improving durability of main components (particularly, transmission) has emerged as a core task of the industry. In addition, cost reduction, that is, research on low cost processes, is also a major area of concern in terms of profitability, and is essential for survival.

In addition, most transmission parts, which are engine power transmission parts, are mainly subjected to carburizing heat treatment to secure high strength, high durability, and sufficient toughness, thereby increasing the strength of securing high durability in the carburized finished state, as well as grain refinement and grain boundary oxide layer. The occurrence of abnormal grains for reduction and improvement of thermal deformation should be suppressed.

In the case of steels developed to date, the size of the conventional parts has a limitation in terms of strength in order to cope with the recent increase in automobile performance, and there is a restriction to increase the size of the parts. It caused the drop.

In addition, when the carburizing temperature is raised to reduce the process cost, there is a problem of occurrence of abnormal grains.As a result, the reduction in the temper softening resistance of the parts due to the oil temperature rise during rapid starting and sudden braking of the car significantly reduces the fatigue strength of the parts. It shows a big problem of shortening component life.

Therefore, the development of ultra-high strength steel with excellent carburizing ability, which is excellent in various fatigue performances that can withstand higher power than conventional parts without increasing the size of components, is prioritized.

Cr and Si are very effective elements to improve the fatigue strength and temper softening resistance, but they have a high affinity with oxygen, which causes the surface abnormality layer to reduce the fatigue strength when carburizing. Due to this, the addition of Cr or Si is limited, and a lot of manufacturing costs are added by adding a process called regrinding and shot peening after carburizing to prevent a drop in fatigue strength due to occurrence of surface abnormalities during carburization. It is also true.

On the other hand, with the introduction of the vacuum carburizing method which is carburized in vacuum recently, not only the reduction of the surface abnormality layer, but also the use of elements such as Cr and Si in terms of materials, and the productivity improvement effect due to carburization at high temperature, are expected. Due to the lack of suitable materials to save the effectiveness is falling.

Therefore, there is an urgent need for the development of steels that can be carburized even at high temperatures in the vacuum and that can provide excellent contact fatigue strength characteristics and temper softening resistance compared to existing high strength materials.

In order to solve the problems described above, the present invention employs 100% of carbonitride, thereby improving various fatigue strengths (rotational bending fatigue strength, torsional fatigue strength, contact fatigue strength, etc.) and tempering softening resistance, thereby providing excellent contact fatigue characteristics. It is an object to provide ultra high strength steel for carburizing.

It is also an object of the present invention to provide a carburizing ultra high strength steel having excellent contact fatigue characteristics capable of high temperature vacuum carburizing and shortening the carburizing time.

In order to achieve the above object, carburizing ultra high strength steel having excellent contact fatigue properties according to the present invention is C: 0.15 to 0.25% by weight, Cr: 1.70 to 2.30% by weight, Si: 0.50 to 0.70% by weight, and N: 100 to 200 ppm by weight, Al: 0.010 to 0.040% by weight, Nb: 0.015 to 0.035% by weight, Ti: 50% by weight or less, Ni: 0.05% by weight or less, remainder containing Fe and unavoidable impurities, 1200 to 1300 ° C It is characterized in that the complete heating of the carbonitride by rolling and heating.

Preferably, the ultra high strength steel further comprises Mn: 0.45 to 0.75% by weight, S: 0.030% by weight or less, and Mo: 0.25 to 0.50% by weight.

Also preferably, the super-strength steel is a solid solution of the solid carbide, so that the abnormal grain generation temperature is 1010 ~ 1150 ℃, the contact fatigue limit is 3.660 × 10 ⁶ ~ 6.822 × 10 ⁶ .

As described in detail above, according to the carburizing ultra-high strength steel excellent in contact fatigue characteristics of the present invention, by completely solidifying carbonitride, 30% or more of various fatigue strength compared to the existing high strength steel, excellent tempering softening resistance And since abnormal grain generation temperature is 1000 degreeC or more, high temperature carburization of 1000 degreeC or more is possible.

Therefore, the present invention can be expected to increase the carburizing temperature due to the abnormal grain generation temperature higher than the conventional steel, thereby reducing the carburizing time, it is expected to improve the productivity and cost reduction, inexpensive for quality At the same time, it is possible to sufficiently respond to customer demand for high strength steel, which can dramatically improve the quality and price competitiveness of the steel as well as profitability in steelmaking.

Hereinafter, the reason for setting the alloy component of the present invention having the above content range will be described.

C: C is one of the main elements for determining strength and hardness in special steels and needs to be contained in an amount of 0.15% by weight or more in order to secure strength. However, when it exceeds 0.25 weight%, toughness will fall. In addition, as the cold workability increases, tensile strength and yield point increase and elongation decreases. Therefore, in consideration of these characteristics, the C content range is set to 0.15 to 0.25 wt%.

Cr: Cr promotes carburizing because it increases steel hardening and tempering resistance, improves fatigue strength, and makes stable carbide. It is necessary to add more than 1.70% by weight to increase the wear resistance and temper softening resistance of the steel together with Si as a stable carbide forming element. However, when added in excess of 2.30% by weight, the toughness is degraded and at the same time, the cold forging is deteriorated. Therefore, the appropriate content range of Cr is set to 1.70 to 2.30% by weight.

Si: Si is used as an effective deoxidizer in steelmaking (0.10% by weight or more), and it is preferable to add 0.50% by weight or more as an element that is dissolved in a matrix to increase fatigue strength. However, if the content is excessive, the toughness is lowered, the moldability is lowered, and forging and processing are difficult, so it is added at 0.70% by weight or less. In particular, Si forms an intergranular oxide layer on the surface by combining with oxygen in the air during carburization, which may cause high temperature transformation layer due to depletion of alloying components on the surface, but also improves abrasion resistance of steel and temper softening by rising oil temperature. Since it plays a role of increasing the resistance, the addition of an appropriate content is important in the range that the fatigue strength due to the formation of the high temperature transformation layer is not lowered. Therefore, the content of Si is set to 0.50 to 0.70% by weight.

Al: Al is an element that not only acts as a powerful deoxidizer, but also binds with N to refine the grains. However, when Al is added less than 0.010% by weight, deoxidation or grain refining action is not preferable, and when it exceeds 0.040% by weight, the amount of non-metallic inclusions such as Al ₂ O _{3 is} rather detrimental. Can have Therefore, the appropriate content range of Al is set to 0.010 to 0.040% by weight.

Nb: Nb is an element that raises the grain coarsening temperature of steel at high temperature to prevent grain coarsening, and refines the grain to improve ductility and toughness. Therefore, Nb needs to be added at least 0.015% by weight. However, since it is an expensive element, maximum effect should be acquired even with a small amount addition, and it is not preferable to exceed 0.035 weight%. Therefore, the stoichiometric ratio with other components is calculated and the addition content is set to 0.015 to 0.035% by weight.

N: When N is added in an appropriate amount, N is combined with Al to form nitrides to refine austenite grains and improve abrasion characteristics. Therefore, N must have a certain level or more, but excessive addition may cause a decrease in elongation and age hardening (clear brittleness). cause. Therefore, the proper content range is set to 100 ~ 200ppm.

Mn: Mn improves the hardenability and strength of steel, and increases the plasticity at high temperatures to improve castability. In particular, MnS is formed by combining with S, which is a harmful component, to prevent red brittleness and improve cutting processability. In order to exert such an effect, it is preferable to add at least 0.45% by weight or more, but it is preferable to add 0.75% by weight or less because excessive toughness lowers toughness. Therefore, the content of Mn is set to 0.45 to 0.75% by weight.

Mo: Mo is an element that plays an excellent role in improving the hardenability of steel and has a great effect on improving the strength and toughness, but significantly increases the hardness during heat treatment such as normalizing, and increases the manufacturing cost and decreases the workability of parts. Such Mo is not preferable when added to less than 0.25% by weight, it is not preferable because sufficient hardenability and softening resistance cannot be secured, and when it exceeds 0.50% by weight, the hardenability-improving effect is saturated. Therefore, it is limited to 0.25 to 0.50% by weight.

Ni: Ni is an element that increases the hardenability, improves toughness but decreases the manufacturability by increasing the manufacturing cost of the part, and is limited to 0.30% by weight or less.

P: P is an element that segregates at grain boundaries during solidification, lowers toughness of steel, reduces impact resistance, and is limited to 0.020% by weight or less.

S: S is limited to 0.030% by weight or less as a cause of defects and paths during surface treatment by forming elements or some large inclusions which form MnS by bonding with Mn to improve the machinability of steel.

Ti: Ti is a strong nitride-forming element that finely precipitates TiC in steel by bonding with C to disperse and strengthen the base and delays the formation and propagation of cracks due to fatigue fracture or pitting. Although the toughness is improved, TiN precipitated by N-bonding is the starting point of fatigue cracking, and the quality of the gear is reduced, so it is limited to 50 ppm or less.

O: O forms non-metallic inclusions by the bonding of oxidative elements in the steel, thereby inhibiting the mechanical and fatigue properties of the steel. Therefore, the content is limited to 25 ppm or less.

Hereinafter, the influence of the crystal grains according to the rolling heating temperature of the present invention will be described in detail.

Table 1 shows the results of measurement of austenite grain size and grain coarsening temperature (GCT) according to the rolling heating temperature difference based on the conventional low carbon steel SCM920H.

SCM920H Austenitic grain size (μm) Grain Coarsening Temperature (℃) No.1 1st rolling heating temperature (℃) 1184 13.2 1010 Second rolling heating temperature (℃) 1179 No.2 1st rolling heating temperature (℃) 1189 9.3 970 Second rolling heating temperature (℃) 1043

According to Table 1, it can be seen that when rolling at the secondary rolling heating temperature (No. 2) of less than 1100 ° C, the austenite grain size is minute but insufficient for 100% solid solution of carbonitride, and the grain coarsening temperature has dropped to 970 ° C. have.

Rolling heating temperature (rolling crack temperature) is a factor that determines the degree of solid solution of carbonitride which makes carburizing particle fine. Too low rolling heating temperature is less effective in carburizing particle size due to formation of unemployed carbonitride. Drop or decrease the grain coarsening temperature. On the contrary, too high a rolling heating temperature causes decarburization, a reduction in carburizing particle size reduction effect and energy waste due to coarse austenite grain formation.

Therefore, the establishment of a sufficient heating section capable of solidifying 100% of various carbonitrides is important for securing steel quality and economy. Therefore, in the present invention, the rolling heating temperature is set to 1200 to 1300 ° C. to ensure sufficient solid solution of various carbonitrides.

Through the following examples will be described the present invention in more detail.

Table 2 shows the chemical composition of the inventive steel according to the present invention and the conventional steel for comparison with the inventive steel of the present invention, the invention steel is represented by A, B, C and D and the conventional steel by E, F and G. . Invented steel A was melted in an electric furnace according to alloy design and process design, continuously cast into a blow of 370x480mm, rolled to 160mm billet, reheated, and then rolled into a test material of Φ60mm. Invented steels B, C and D Was melted in a vacuum induction furnace (VIM), forged to 160 mm billets, reheated and rolled into a Φ40 mm specimen.

In addition, ultra-high strength steels A, B, C and D according to the present invention actively added Cr or Si to improve the fatigue strength more than 30% and secure temper softening resistance, compared to the existing high strength steel, high temperature In order to prevent abnormal grain formation in, sufficient deoxidation was performed with Al, and Nb and Ti in appropriate amounts were added to sufficiently form niobium, titanium-carbide, niobium, and titanium-nitride such that the rolling cracking temperature was 1200 to 1300 ° C. It was.

division C (% by weight) S (% by weight) Mn (wt%) P (% by weight) S (% by weight) Ni (% by weight) Cr (% by weight) Mo (% by weight) Al (% by weight) Nb (% by weight) Ti (ppm) O (ppm) N (ppm) Invention steel A 0.21 0.60 0.60 0.012 0.006 0.05 2.05 0.38 0.032 0.029 31 14 155 B 0.21 0.52 0.60 0.006 0.011 0.02 1.82 0.40 0.018 0.021 25 21 151 C 0.17 0.58 0.54 0.007 0.006 0.02 2.22 0.41 0.013 0.020 19 25 135 D 0.18 0.58 0.54 0.006 0.004 0.02 1.71 0.29 0.015 0.019 21 14 110 Conventional Steel E 0.19 0.12 0.68 0.010 0.013 0.11 1.31 0.58 0.025 0.023 23 15 122 F 0.18 0.07 0.52 0.017 0.019 1.57 0.58 0.61 0.028 0.023 - 13 110 G 0.22 0.28 0.88 0.012 0.008 0.20 1.23 0.07 0.027 0.026 37 10 123

Invented steels A, B, C, and D are based on low carbon alloy steel (SCM920H), and contain much more Cr and Si than conventional steels, and thus, as described above, the tempering softening resistance is expected to be improved. By adding Nb, a synergistic effect of the abnormal grain generation temperature can be expected. Further, by further adding a predetermined amount of Ti, titanium-carbonitride was formed to refine the crystal grains, further contributing to the increase in the abnormal grain generation temperature. The conventional carburizing conventional steel, E is SCM, F is SNCM, and G is SCR based mass production transmission steel. The evaluation results for the inventive steel and the conventional steel are shown in Table 3 below.

Invention steel Conventional Steel A B C D E F G Oxygen content ppm 14.3 20.9 24.6 13.5 15.0 13.0 9.6 Nitrogen content ppm 155.4 151.2 134.7 110.3 121.9 110.0 123.5 Curability (HRC) J5 47.2 46.7 45.6 45.3 44.0 43.2 44.9 J11 45.2 46.7 43.4 38.3 36.9 36.0 35.1 J13 44.2 44.8 42.4 34.3 35.4 33.6 33.2 J20 41.3 44.5 39.0 29.0 32.0 29.2 30.0 Ideal grain coarsening temperature ℃ 1150 1020 1010 1010 1000 950 1000 Contact fatigue life x1000 4154 3660 6666 6822 2200 - 1059 Mechanical properties Yield strength Kgf / ㎠ 95 98 98 92 90 81 86 The tensile strength Kgf / ㎠ 134 137 135 121 110 110 107 Strain % 12 17 19 22 14 17 17 Section reduction rate % 30 51 59 54 40 58 41 QT Rotary bending Kgf / ㎠ 51.4 51.1 48.8 46.3 45.6 45.0 40.9 torsion Kgf / ㎠ 45.0 46.0 42.7 33.0 35.0 35.0 33.4 Carburizing torsion Kgf / ㎠ 67.5 65.0 58.0 47.0 42.5 45.0 39.0 Carburized Surface Hardness 180 ℃ HRC 60.9 61.2 61.0 60.5 61.2 60.7 61.0 300 ℃ HRC 57.4 58.0 57.0 55.5 55.3 54.2 55.0 Decline % 5.7 5.2 6.6 8.3 9.7 10.7 9.8 Rolling crack temperature ℃ 1200-1300 1100-1180

 The steel of the present invention is superior in mechanical properties to conventional steels, and exhibits fatigue performance such as bending fatigue and torsional fatigue and excellent contact fatigue life of carburized parts. In particular, the present invention steel A exhibits the most excellent abnormal grain generation temperature, it is evaluated to be excellent in temper softening resistance in comparison with the conventional steel E, and shows the best fatigue performance in comparison with other conventional steels, thus providing excellent carburizing performance. It is the most suitable steel for ultra high strength steel.

As shown in Table 3, the contact fatigue life characteristics of the inventive steel were increased by 2 to 6 times or more compared to the conventional steel, and the mechanical bending properties, as well as the rotational bending and torsional fatigue characteristics, were excellent. In addition, the invention steel increased the temperature of occurrence of abnormal grains by adding Al, which is a deoxidation and grain refining element, and Nb and Ti, each having an appropriate amount of N and abnormal grain growth inhibitory effect. That is, the abnormal grain generation temperature of the conventional steel is significantly improved to 1010 ~ 1150 ℃, while the abnormal grain generation temperature of the invention steel is 950 ~ 1000 ℃.

In addition, the present invention steel has excellent temper softening resistance against the strength reduction of the carburized surface compared with the conventional steel, and thus suppresses the occurrence of fittings due to deterioration of the surface of the carburized parts when the oil temperature rises due to rapid start and sudden braking that can occur during driving. It's bigger than that.

From these results, it can be seen that the present invention steel is suitable for the application of carburized tooth steel, which requires high durability and next-generation fatigue properties. Gears which are automobile parts manufactured using the present invention steel are shown in Figs. 1A to 1C.

In the above, the present invention has been described by specific preferred embodiments, but the present invention is not limited to the above-described embodiments, and those skilled in the art to which the present invention pertains should be described in the following claims. Various modifications can be made without departing from the spirit of the invention.

1a to 1c is a view showing the gears made of super high strength steel for carburization excellent contact fatigue characteristics according to an embodiment of the present invention.

Claims (3)

C: 0.15 to 0.25 weight%, Cr: 1.70 to 2.30 weight%, Si: 0.50 to 0.70 weight%, N: 100 to 200 weight ppm, Al: 0.010 to 0.040 weight%, Nb: 0.015 to 0.035 weight%, Ti: 50 weight ppm or less (0 is not included), Ni: 0.05% or less (0 is not included), Mn: 0.45 to 0.75% by weight, S: 0.030% or less (0 is not included) Mo: 0.25 to 0.50% by weight, balance As containing Fe and unavoidable impurities, Vacuum carburizing with excellent contact fatigue strength, characterized in that the ideal grain generation temperature is 1010 ~ 1150 ℃ and the contact fatigue limit is 3.660 × 10 ⁶ ~ 6.822 × 10 ⁶ by rolling and heating at 1200 ~ 1300 ℃ to completely solidify the carbonitride. Ultra high strength steel. delete delete

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