KR20090037631A - High strength carburizing steel with high fatigue resistance - Google Patents

Abstract

A high strength steel for high temperature vacuum carburizing is provided to shorten carburizing time by adding carbon material for preventing the growth of austenite crystal. A high strength steel for high temperature vacuum carburizing consists of carbon 0.17~0.27 weight%, chrome 0.85~1.55 weight%, silicon 0.15~0.90 weight%, nickel 0.25 weight%, manganese 0.45~0.90 weight%, sulfur 0.030 weight%, nitrogen 50~170 weight ppm, molybdenum 0.03 weight%, aluminium 0.010~0.040 weight%, niobium 0.015~0.12 weight%, vanadium 0.04~0.06 weight% and the rest of iron and impurity.

Description

High strength carburizing steel with high fatigue resistance

The present invention relates to high-strength steel for high temperature vacuum carburizing, and more particularly, to high-temperature vacuum carburizing high strength steel having excellent fatigue resistance.

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.

Most transmission parts, which are engine power transmission parts, are subjected to carburizing heat treatment on the surface in order to secure high strength, high durability, and sufficient toughness. Therefore, shortening the carburizing time reduces the cost of the final product. Conventionally, gas carburizing is mainly used, but when using high-temperature vacuum carburizing instead of gas carburizing, the carburizing time may be shortened to about 1/4 of the conventional gas carburizing.

However, gas carburizing steels that have been used up to now have not been used because the grain boundary grows when carburizing in high temperature vacuum, causing thermal deformation and breakage of the carburized products. Increasing the carburizing temperature to reduce the process cost had a problem of abnormal grain generation.

In addition, the size of a conventional component has a lot of shortcomings 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 component. Deterioration has a major problem that the fatigue life of the component is greatly reduced and the component life is shortened. Therefore, the development of high strength steel for carburizing, which is excellent in various fatigue performances that can withstand high outputs without increasing the size of components and excellent carburizing ability, is prioritized.

Although this improves the fatigue strength and temper softening resistance is high Cr and Si is very effective element I to which the carburizing method and the like are very high affinity with oxygen is SiO _2, Cr ₂ O ₃ at the time of carburizing Since there is a problem of lowering the fatigue strength of the component by generating a surface abnormality layer, etc., the addition of Cr or Si is restricted. It is also true that addition of tooth processing such as shot peening imposes a heavy burden on manufacturing costs.

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, carburization is possible even at a high temperature in a vacuum, and there is an urgent need for development of steel materials capable of securing various fatigue strength characteristics superior to conventional high-strength materials.

The present invention to solve the above problems, by adding Al, Ti, Nb, V to form a carbonitride to prevent the growth of austenite grain boundary to maintain a constant grain boundary even at high temperature, high temperature vacuum carburizing is possible, An object of the present invention is to provide a high-strength steel for high temperature vacuum carburization with excellent fatigue resistance with high contact fatigue strength.

In order to achieve the above object, the high-strength steel for high-temperature vacuum carburizing excellent fatigue properties according to the present invention is C: 0.17% to 0.27% by weight, Cr: 0.85% to 1.55% by weight, Si: 0.15% by weight to 0.90% by weight, Ni: 0.25% by weight or less, Mn: 0.45% by weight to 0.90% by weight, S: 0.030% by weight or less, N: 50% by weight to 170% by weight, Mo: 0.03% by weight or less, Al: 0.010% by weight ~ 0.040% by weight, Nb: 0.015% by weight-0.12% by weight and V: 0.04% by weight to 0.06% by weight, the balance consists of Fe and inevitable impurities.

Moreover, it is preferable to further contain 50 weight ppm-150 weight ppm Ti: as a high temperature crystal grain refinement element.

In addition, the inevitable impurities preferably include P: 0.030% by weight or less and O: 25% by weight or less.

According to the present invention as described above, the carburized high strength steel having excellent fatigue resistance of the present invention is excellent in tempering softening resistance and has an ideal grain generation temperature of 1000, while having various fatigue strengths higher than two times higher than conventional high strength steels. Since it is more than degreeC, the high temperature vacuum carburization of 1000 degreeC or more is possible.

In addition, the present invention forms a carbonitride that prevents the growth of austenite grain boundaries by adding Al, Ti, Nb, V to maintain a constant grain boundary even at high temperatures, thereby enabling high temperature carburizing and shortening the carburizing time. have.

Therefore, the present invention can be expected to increase the carburization temperature due to the abnormal grain generation temperature higher than the conventional steel, thereby improving productivity and cost reduction is expected, and is cheap for the quality but high strength Since it can fully respond to customer demand, it is possible to dramatically improve the quality and price competitiveness of the vehicle as well as profitability in steelmaking.

EMBODIMENT OF THE INVENTION Hereinafter, the reason for addition of the alloy component of this invention and the limitation of a component range is demonstrated.

C: 0.17% to 0.27% by weight

C is one of the main elements determining strength and hardness in special steels, and it is necessary to contain C at least 0.17% by weight in order to secure strength. It is also necessary to increase the strength of the non-carburized layer. However, exceeding 0.27% by weight increases the possibility of deformation during quenching. Moreover, forging workability and machinability fall. Therefore, in consideration of these characteristics, the appropriate content range of C is set to 0.17% by weight to 0.27% by weight.

Cr : 0.85 wt% ~ 1.55 wt%

Cr is added more than 0.85% by weight to increase hardening and tempering resistance of steel, to improve fatigue strength, and to improve pitting resistance by increasing carbide wear resistance and tempering softening resistance with Si through promoting carbide. It is necessary. However, when added in excess of 1.55% by weight, toughness is lowered and workability and machinability are inferior. Therefore, the appropriate content range of Cr is set to 0.85% by weight to 1.55% by weight.

Si : 0.15 wt% ~ 0.90 wt%

Si is used as an effective deoxidizer (0.10% by weight or more) in steelmaking, and is an element that is dissolved in a matrix and increases fatigue strength through grain boundary strengthening, and it is preferable to add 0.15% by weight or more. However, when the content is excessive, it is added at 0.90% by weight or less because the toughness is lowered, the moldability is reduced, and forging and processing are difficult. In particular, Si combines with oxygen during carburization to form SiO ₂ grain boundary oxide layer on the carburized product, which can cause component breakage.However, since the grain boundary oxide layer is not formed during vacuum carburization, the steel wear resistance is improved along with Cr and the oil temperature rises. It is important to add the proper content because it serves to increase the temper softening resistance. Therefore, the content of Si is set to 0.15% by weight to 0.90% by weight.

Ni : 0.25 wt% or less

Ni is an element that increases the hardenability, improves toughness but increases the manufacturing cost of parts and decreases the manufacturability, so that the appropriate content range of Ni is limited to 0.25% by weight or less.

Mn : 0.45 wt% ~ 0.90 wt%

Mn improves yield strength by making pearlite fine and solidifying ferrite. It also improves the hardenability and strength of the steel, and increases the plasticity at high temperatures to improve castability. In particular, 0.45% by weight to 0.90% by weight is added to form MnS in combination with the harmful component S to prevent redness 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 it in 0.90% by weight or less because excessive toughness lowers toughness. Therefore, the stoichiometric ratio with other components is calculated and the appropriate content range of Mn is set to 0.45% by weight to 0.90% by weight.

S: 0.030 wt% or less

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 the steel.

N: 50 weight ppm  ~ 170 weight ppm

When N is added in an appropriate amount, N is combined with Al, Ti, and Nb to form nitrides to refine austenite grains and improve wear characteristics. Therefore, N must have a certain level or more, but excessive addition causes various brittleness and age hardening of steel. do. Tensile strength and yield strength increase due to quenching aging, strain aging by cold working, and clear brittleness at 200 to 300 ° C, and the impact value decreases to cause brittleness of steel. Therefore, the appropriate content range of N is set to 50 ppm by weight to 170 ppm by weight.

Mo : 0.03% by weight or less

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, increases manufacturing costs, and decreases the workability of parts. Therefore, the appropriate content range of Mo is limited to 0.03% by weight or less.

Al : 0.010 wt% ~ 0.040 wt%

Al is sikina refine the crystal grains to form the AlN in combination with that at the same time N which acts as a strong deoxidizer, 0.01 less than% by weight is not preferred since the smaller the deoxidation and grain refinement effect is in excess of 0.04% by weight as Al ₂ O ₃ Increasing nonmetallic inclusions may have a detrimental effect. In addition, when a large amount of AlN is present, the high temperature toughness is lowered, and in particular, AlN is precipitated at the austenite grain boundary during the forging process, causing grain boundary brittleness and lowering the high temperature creep strength. Therefore, the appropriate content range of Al is set to 0.010% to 0.040% by weight.

Nb : 0.015 wt% ~ 0.12 wt%

Nb is an element that combines with N or C to form NbN or NbC, respectively, to increase the grain coarsening temperature of steel at high temperature to prevent grain coarsening, and to refine the grain to improve ductility and toughness. It is necessary to add. However, since it is an expensive element, even a small amount of addition must obtain the maximum effect, so 0.12 weight% or more is not preferable. Accordingly, the stoichiometric ratio with other components is calculated to add 0.015% by weight to 0.12% by weight.

V: 0.04% to 0.06% by weight

V is a strong nitride forming element, and when added in an amount of 0.04% by weight or more, V is precipitated in the steel to increase the strength and effectively prevent coarsening of grain boundaries at high temperatures. However, since the manufacturing cost increases when V is added in excess of 0.06% by weight, the content of V is set to 0.04% by weight to 0.06% by weight.

Ti : 50 weight ppm  ~ 150 weight ppm

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. TiN precipitated by bonding with N deteriorates the quality of the tooth at the origin of fatigue cracking, and thus limits the Ti content range to 50 ppm by weight to 150 ppm by weight.

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

Table 1 shows the results of measuring the austenitic grain size and grain coarsening temperature (GCT) according to the difference in rolling heating temperature when SCM920H, which is a conventional low carbon steel, is based.

SCM920H Austenitic grain size (μm) Grain Coarsening Temperature (℃) Sample 1 1st rolling heating temperature (℃) 1184 13.2 1010 Second rolling heating temperature (℃) 1179 Sample 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 (Sample 2) of less than 1100 ° C, the austenite grain size is fine but insufficient for 100% solid solution of carbonitride, and the grain coarsening temperature has dropped to 970 ° C. .

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 lower 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.

(Example)

Hereinafter, the present invention will be described in more detail with reference to Examples.

Table 2 compares the chemical composition of the inventive steel according to the present embodiment and the conventional steel for comparison with the inventive steel of the present invention.

division C (% by weight) Si (% by weight) Mn (wt%) P (% by weight) S (% by weight) Ni (wt% Cr (% by weight) Mo (% by weight) V (% by weight) Al (% by weight) Ti (ppm) Nb (% by weight) N (ppm) O (ppm) Etc Invention steel A 0.22 0.24 0.86 0.010 0.013 0.15 1.21 0.02 0.00 0.025 100 0.005 92 9 - B 0.24 0.81 0.58 0.011 0.005 0.09 1.38 0.02 0.00 0.015 145 0.027 93 15 - C 0.24 0.80 0.62 0.012 0.005 0.07 1.05 0.03 0.05 0.027 55 0.087 83 15 - Zhonglai River D 0.22 0.27 0.88 0.010 0.002 0.05 1.23 0.01 0.00 0.026 37 0.005 120 7 Cu (0.09% by weight) E 0.18 0.10 0.70 0.009 0.005 0.25 1.30 0.59 0.00 0.021 16 0.019 112 13 Cu (0.13% by weight) F 0.15 to 0.30 0.25 to 1.10 0.30 to 1.20 - 0.005 ~ 0.100 - - - - - 0.02 to 0.06 0.020-0.120 - - Pb, Ca, Bi addition G 0.10 to 0.30 0.40 to 1.50 0.30 to 2.00 - Max. 0.030 0.03 to 0.35 0.50 to 3.00 0.03 to 1.50 0.02 0.15 - 0.02 to 0.30 0.02 to 0.150 - - B addition (10 ~ 50ppm)

In Table 2, the inventive steel is A, B, C and the conventional steel is denoted by D, E, F, G. Invented steel A was melted in an electric furnace according to the alloy design and process design described above, continuously cast into 160x160mm billet, reheated and rolled into a Φ40mm specimen, and the invented steels B and C were 390x510mm bloom. Continuous casting and reheating were rolled to a test piece of Φ 120 mm.

In addition, in order to improve the fatigue strength more than 30% compared to the existing high strength steel, and in order to secure the temper softening resistance, the inventive steels A, B, and C according to the present invention actively added Cr or Si, and After sufficiently deoxidizing with Al to prevent abnormal grain formation, Ti, Nb, and V were set in different amounts, and Ti, Nb, V-carbide, and Ti were set so that the rolling crack temperature was 1200 to 1300 ° C. , Nb, V-nitride were sufficiently formed.

Invented steels A and B are based on low carbon alloy steel (SCR420H), and contain a large amount of Ti as compared to conventional steels. As described above, abnormal grain formation temperature at high temperature due to TiN is expected to be improved. Invented steel C is low carbon. Based on the alloy steel (SCR420H), the conventional carburizing conventional steel D is SCR-based, E is SCM-based mass production transmission steel currently in use. Conventional steels F and G are invention steels of Japanese special instructors, and conventional steel F (Japanese Invention Patent No. 2003-77356) is a carburizing steel for improving surface hardness and strength due to precipitation of surface carbides by a high concentration vacuum carburizing method. Increasing Cr to promote carbide formation and forming Cr carbide of 5-15% level after carburizing to improve strength, and conventional steel G (Japan Invention Patent No. 2003-96908) was impacted by the addition of Ni and B. The purpose of the present invention was to improve toughness and to improve softening resistance by TiC dispersion and Si addition. The strength of TiC carbide was increased by increasing Ti and inducing TiC carbide dispersion after carburization.

Hereinafter, Table 3 summarizes the evaluation results for the inventive steel and the conventional steel.

Invention steel Conventional Steel A B C D E Oxygen content ppm 9 15 15 15 15 Nitrogen content ppm 92 93 83 120 112 Curability (HRC) J5 43 46 45 42 45 J11 32 35 34 31 39 Ideal grain coarsening temperature ℃ 1,070 1,020 1,060 940 1,000 Contact fatigue life x1000 1,206 3,239 2,407 578 1,924 Mechanical properties Yield strength Kgf / ㎠ 74 109 94 74 97 The tensile strength Kgf / ㎠ 103 136 114 102 114 Strain % 17 12 15 19 15 Section reduction rate % 30 34 32 43 45 Rolling crack temperature ℃ 1200-1300 1100-1180

As shown in Table 3, the inventive steels (A, B, C) had better mechanical properties than the conventional steels (D, E), showed excellent contact fatigue life results of carburized parts, and abnormal grain generation of conventional steels. While the temperature is 940 to 1000 ° C, the abnormal grain generation temperature of the inventive steel is greatly improved to 1020 to 1070 ° C.

Herein, the inventive steels B and C show a result of an increase in the contact fatigue life characteristics of 2 to 3 times or more compared with the conventional steel, and also in comparison with the inventive steel A, Nb-carbide and Nb- are increased by the increase of Nb. Nitride was sufficiently formed to improve fatigue strength and to secure tempering softening resistance.

Among them, the present invention steel C shows the most excellent abnormal grain coarsening temperature and contact fatigue life, and it is evaluated to be excellent in temper softening resistance in comparison with conventional steels D and E, and has the best fatigue life in comparison with other conventional steels. It shows the performance and is the most suitable for high strength steel for high temperature vacuum carburizing with excellent fatigue resistance.

This is because the inventive steel C according to the present invention actively added Cr or Si, and deoxidized with Al, which is a deoxidation and grain refining element, in order to prevent abnormal grain formation at a high temperature. , V was added to form Ti, Nb, V-carbide, and Ti, Nb, V-nitride sufficiently, and the rolling grain temperature was set to 1200 to 1300 ° C, so that various carbonitrides were dissolved 100%. Could increase. As a result, fatigue strength was improved by more than 30% compared to the existing high strength steel, and sufficient tempering softening resistance was obtained.

Therefore, the inventive steel has a very good temper softening resistance against the strength reduction of the carburized surface compared with the conventional steel, and the fitting of the surface layer due to the deterioration of the surface of the carburized parts during the rise of the oil temperature due to rapid start and sudden braking which may occur while driving a car Suppression of generation is greater than conventional steel.

Using the steel of the present invention as described above, as shown in Figures 1a to 1e, gears of automobile parts were manufactured. Compared to the conventional high-strength steel, the present invention is found to be suitable for the application of carburized tooth steel which requires the next generation high durability and fatigue resistance which is excellent in temper softening resistance while having at least two times higher contact fatigue life and various fatigue strengths. Can be.

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 1e is a view showing the gears made of high-strength steel for high temperature vacuum carburizing excellent fatigue resistance according to an embodiment of the present invention.

Claims (2)

High strength steel for high temperature vacuum carburizing with excellent fatigue resistance, C: 0.17 wt%-0.27 wt%, Cr: 0.85 wt%-1.55 wt%, Si: 0.15 wt%-0.90 wt%, Ni: 0.25 wt% or less, Mn: 0.45 wt%-0.90 wt%, S: 0.030 Less than or equal to%, N: 50 ppm to 170 ppm, Mo: 0.03 wt% or less, Al: 0.010 wt% to 0.040 wt%, Nb: 0.015 wt% to 0.12 wt%, and V: 0.04 wt% to 0.06 wt% A high-strength steel for high temperature vacuum carburizing, comprising, and having excellent fatigue resistance, comprising Fe and unavoidable impurities as the remainder. The method of claim 1, A high temperature grain refining element, the high strength steel for high temperature vacuum carburizing excellent in fatigue resistance, characterized in that it further comprises Ti: 50 ppm by weight to 150 ppm by weight.

Images