WO2008075889A1

WO2008075889A1 - Ultra high strength carburizing steel with high fatigue resistance

Info

Publication number: WO2008075889A1
Application number: PCT/KR2007/006650
Authority: WO
Inventors: Sung Do Wang; Chul Woo Park; Dong Yun Kim
Original assignee: Seah Besteel Corporation
Priority date: 2006-12-19
Filing date: 2007-12-18
Publication date: 2008-06-26
Also published as: KR20080056945A

Abstract

Disclosed is ultra high strength carburizing steel with high fatigue resistance, comprising: 0.15 to 0.25% by weight of C; 1.70 to 2.30% by weight of Cr; 0.50 to 0.70% by weight of Si; 0.010 to 0.040% by weight of Al; and a balance of Fe and inevitable impurities. Since this ultra high strength carburizing steel with high fatigue resistance is excellent in fatigue-proof characteristics and resistance to temper softening, and has an abnormal grain growth temperature of 1000°C or higher, the ultra high strength carburizing steel can increase carburizing temperature. Therefore, productivity can be raised, and cost savings can be achieved.

Description

ULTRA HIGH STRENGTH CARBURIZING STEEL WITH HIGH

FATIGUE RESISTANCE

Technical Field

[1] The present invention relates to high strength carburizing steel, and more particularly to ultra high strength carburizing steel with high fatigue resistance. Background Art

[2] As the trend to high performance (high power, low fuel efficiency, and stillness) automotive engines has continued in recent years, research to improve the durability of main parts (in particular, transmission) is looming large as the most important project in the industry. Moreover, in view of ensuring profitability, great attention is being paid to research for cost savings, that is, research on low-cost processes, which is also requisite for survival in the industry.

[3] Additionally, in order to ensure high strength, high durability, and sufficient toughness, parts for transmitting engine power, including most transmission parts, are mainly subjected to a carburizing heat treatment. With regard to this, the occurrence of abnormal grain growth must be suppressed during the carburizing heat treatment so as to achieve high strength fine-grained carburized products with higher durability, reduce the formation of a grain boundary oxide layer, and improve thermal deformation characteristics.

[4] Steel materials developed so far have a limit in that the size of parts has to be reluctantly increased because typically sized parts do not have sufficient strength to cope with the recent trend to high performance vehicles, and such an increase in the size of parts results in high fuel-consuming vehicles.

[5] When carburizing temperature is increased for the sake of process cost saving, there is a problem in that abnormal grain growth occurs. Also, the lowering of resistance to temper softening, due to an increase in oil temperature during abrupt starting and braking, noticeably reduces the fatigue strengths of parts, which causes a serious problem of shortening the life of the parts.

[6] Therefore, it is necessary to first of all develop ultra high strength steel with various superior fatigue-proof characteristics and high carburizing ability, which can endure higher power than the existing engine power without increasing the size of parts.

[7] Some elements, such as Cr and Si, are very effective in enhancing fatigue strength and resistance to temper softening, but restrictions are placed on the addition of Cr or Si because this element has a high affinity to oxygen, and thus generates an abnormal surface layer when used with the current carburizing method, which has an adverse effect on the enhancement of fatigue strength. In order to prevent fatigue strength from being lowered by the generation of such an abnormal surface in the course of carburizing, the so-called re-polishing and short peening process is additionally performed after the carburizing, but in fact, this is a burden that increases the cost of production.

[8] Meanwhile, the recent introduction of a vacuum carburizing method in which carburizing is performed under a vacuum atmosphere has made it possible to reduce the generation of an abnormal surface layer, and made it easier to use the above-mentioned elements, such as Cr and Si. In addition, it is expected to improve productivity by performing carburizing at an elevated temperature. However, lack of suitable materials that can make the most of these advantages lowers the effectiveness of the vacuum carburizing method.

[9] Accordingly, there is an urgent need to develop a steel material that can be subjected to carburizing at an elevated temperature and under a vacuum, and ensure higher fatigue resistance and resistance to temper softening than those of existing high strength materials. Disclosure of Invention Technical Problem

[10] Accordingly, the present invention has been made to solve at least the above- mentioned problems occurring in the prior art, and the present invention provides ultra high strength carburizing steel with high fatigue resistance by improving various fatigue strengths (rotary bending fatigue strength, torsion fatigue strength, etc.) and resistance to temper softening.

[11] Also, the present invention provides ultra high strength carburizing steel with high fatigue resistance, which can be subjected to carburizing at an elevated temperature. Technical Solution

[12] In accordance with an aspect of the present invention, there is provided ultra high strength carburizing steel with high fatigue resistance, comprising: 0.15 to 0.25% by weight of C; 1.70 to 2.30% by weight of Cr; 0.50 to 0.70% by weight of Si; 0.010 to 0.040% by weight of Al; and a balance of Fe and inevitable impurities.

[13] Preferably, the ultra high strength carburizing steel with high fatigue resistance further comprises: 0.015 to 0.035% by weight of Nb; and 100 to 200ppm by weight of N as elements for fining crystal grains. [14] It is preferred that the ultra high strength carburizing steel with high fatigue resistance further comprises 50ppm or less by weight of Ti. [15] It is also preferred that the ultra high strength carburizing steel with high fatigue resistance further comprises: 0.45 to 0.75% by weight of Mn; and 0.030% or less by weight of S as elements for improving machinability. [16] It is also preferred that the ultra high strength carburizing steel with high fatigue resistance further comprises 0.25 to 0.50% by weight of Mo as an element for improving quenchability. [17] It is also preferred that the ultra high strength carburizing steel with high fatigue resistance further comprises 0.30% or less by weight of M as an element for improving hardenability. [18] It is also preferred that the inevitable impurities comprise: 0.020% by weight of P; and 25ppm by weight of O.

Advantageous Effects

[19] Accordingly, the present invention can increase the carburizing temperature because the inventive steel has a higher abnormal grain growth temperature than that of the conventional steel, and thus can be expected to enhance productivity and reduce production costs. As a result, the present invention can satisfy customers demands for high strength steel that is more cost-effective while having the same quality as that of the convention steel. Rnally, the present invention can bring an unprecedented improvement in the quality and cost efficiency of vehicles. Brief Description of the Drawings

[20] The foregoing and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which:

[21] HGS. 1 to 3 are pictures illustrating various gears made of ultra high strength carburizing steel with high fatigue resistance, according to an exemplary embodiment of the present invention. Best Mode for Carrying Out the Invention

[22] Hereinafter, the reasons why the inventive alloy composition with the above- mentioned content ranges is established will be described.

[23]

[24] C: C is one of major elements determining the strength and hardness of special steel, and needs to be contained in a content of 0.15% or more by weight in order to ensure strength. However, if the content of C exceeds 0.25% by weight, toughness is lowered. Also, tensile strength and yield point increase and elongation decreases as the degree of cold working increases. Therefore, in consideration of these characteristics, the content range of C is set to 0.15 to 0.25% by weight.

[25]

[26] Cr: Cr strengthens resistance to quenching and tempering, improves fatigue strength, and favors carburizing by facilitating the formation of stable carbides. In order to increase the wear resistance and temper softening resistance of steel, together with Si, as stable carbide-forming elements, Cr needs to be added in an amount of 1.70% or more by weight. However, if the content of added Cr exceeds 2.30% by weight, toughness is lowered and at the same time cold forgeablity deteriorates. Therefore, the appropriate content range of Cr is set to 1.70 to 2.30% by weight.

[27]

[28] Si: Since Si is used as an effective deoxidzer (0.10% or more by weight) in steel making, and is solid-solutioned in the matrix of steel to thereby increase fatigue strength, it is preferably contained in a content of 0.5% or more by weight. However, if it is contained excessively, formability is reduced due to the lowering of toughness, and thus forging and working become difficult. On account of this, Si is contained in a content of 0.7% or less by weight. Particularly, since on one hand Si combines with atmospheric oxygen in the carburizing process to thereby form a grain boundary oxide layer at the surface of steel, which leads to the exhaustion of alloy components and thus the formation of a high-temperature transformation layer, and on the other hand Si, along with Cr, provides steel with improved wear resistance and resistance to temper softening according to a rise in oil temperature, it is important to appropriately add Si within a content range where fatigue strength is not lowered by the formation of the high-temperature transformation layer. Therefore, the content of Si is set to 0.50 to 0.70% by weight.

[29]

[30] Al: Al acts as not only a strong deoxidzer, but also an element for fining crystal grains by combining with N. However, if Al is added in an amount of less than 0.01% by weight, the effect of deoxidzation or crystal grain fining dsadvantageously decreases. Contrarily, if the content of Al exceeds 0.04% by weight, an adverse effect is caused by an increase in the amount of non-metallic inclusions, such as Al₂O₃. Therefore, the appropriate content of Al is set to 0.010 to 0.040% by weight. [31]

[32] Nb: Since Nb prevents crystal grain coarsening by increasing the grain coarsening temperature of steel, and improves ductility and toughness by fining crystal grains, it needs to be added in an amount of 0.015% or more by weight. However, it is not preferred that the content of Nb exceeds 0.035% by weight because Nb is a very expensive element, and thus it is required to obtain the maximum effect with the minimum amount. Therefore, the addition content of Nb is set to 0.015 to 0.035% by weight, based on the calculation of the stoichiometric ratio of Nb to other components.

[33]

[34] N: Since the addition of an appropriate amount of N fines crystal grains of austenite and improves the wear characteristic of steel by combining with Al to thereby form a nitride, the content of N must be equal to or more than a certain level. However, the addition of an excess amount of N lowers elongation, and causes age-hardening (blue embrittleness). Therefore, the appropriate content of N is set to 100 to 200ppm by weight.

[35]

[36] Mn: Mn improves the quenchability and strength of steel, and enhances the castability of steel at a high temperature by increasing its plasticity. Particularly, since Mn combines with S, which is a harmful component, to thereby form MnS, it prevents red brittleness, and improves the machinability of steel. In order to make the best of these effects, Mn is preferably added in an amount of 0.45% or more by weight. Contrarily, if Mn is added excessively, toughness is lowered, and thus it is preferred that Mn is added in an amount of 0.75% or less by weight. Therefore, the appropriate content of Mn is set to 0.45 to 0.75% by weight.

[37]

[38] Mo: On one hand, Mo is excellent in improving the quenchability of steel, and is very effective in increasing strength and toughness. On the other hand, Mo significantly increases hardness during a heat treatment, such as normalizing, causes high production cost, and lowers the workability of a part. Sufficient quenchability and resistance to temper softening cannot be ensured when Mo is added in an amount of less than 0.25% by weight, and the effect of improving quenchability is saturated when Mo is added in an amount of more than 0.50% by weight. Therefore, the content of Mo is limited to 0.25 to 0.50% by weight.

[39]

[40] Nt: M increases hardenability and improves toughness, but lowers productivity by increasing the production cost of a part. Therefore, the content of M is limited to 0.30% or less by weight.

[41]

[42] P: P is segregated at grain boundaries during solidification to thereby lower toughness and impact resistance, and promotes dual-phase heat treatment micro- structures (band microstructures). Therefore, the content of P is limited to 0.020% or less by weight.

[43]

[44] S: S improves the machinability of steel by combining with Mn to form MnS, but acts as the source of generation and path of defects during surface treatment by forming some coarse inclusions. Therefore, the content of S is limited to 0.030% by weight.

[45]

[46] Ti: Ti is a strong nitride-forming element that delays the occurrence and propagation of cracks due to fatigue fracture or pitting by combining with C to precipitate fine TiC in steel and thus dispersion-strengthen the matrix of steel. On account of this, Ti increases strength and toughness during a heat treatment. However, TiN precipitated by combining with N acts as the starting point of fatigue cracks, which deteriorates the quality of a gear. Therefore, the content of Ti is limited to 50ppm or less by weight.

[47]

[48] O: Since O forms non-metallic inclusions by combining with oxidizing elements in steel, it deteriorates the mechanical properties and fatigue characteristics of steel. Therefore, the content of O is limited to 25ppm or less by weight.

[49]

[50] Reference will now be made in detail to the effect of the heating temperature for rolling of the present invention on crystal grains.

[51] Table 1 shows results of measuring austenite grain size and grain coarsening temperature (GCT) according to a difference between first and second heating temperatures for rolling, based on SCM920H that is a conventional low-carbon steel material.

[52] As seen from Table 1, when the steel material is rolled using the second heating temperature for rolling of 1100⁰C or less (sample No. 2), the austenite grain size is finer, but the grain coarsening temperature (GCT) is lowered as compared to sample No. 1 because the heating temperature for rolling is insufficient to enable carbonitrides to be solid-soluble up to 100%. [53] Table 1

[Table 1]

[54] The heating temperature for rolling (rolling crack initiation temperature) is a factor determining the solid solubility of carbonitrides that fine carburized grain size, and too low heating temperature for rolling reduces the effect of fining carburized grain size or lowers the GCT (Grain Coarsening Temperature) due to the formation of solid- insoluble carbonitrides. On the contrary, too high heating temperature for rolling gives rise to decarbonization, reduces the effect of fining carburized grain size due to the formation of coarse austenite grains, and causes a waste of energy.

[55] That is, setting an appropriate heating zone in which various carbonitrides are solid- soluble up to 100% is important to ensuring the quality and economy of steel. Accordingly, in the present invention, the heating temperature for rolling is set to 1200 to 1300⁰C so that various carbonitrides are sufficiently solid- soluble. Mode for the Invention

[56] Reference will now be made in detail to exemplary embodiments of the present invention. [57] Table 2 shows the chemical compositions of the inventive steels, together with those of the conventional steels as comparative steel. The inventive steels are designated by A, B, C and D, and the comparative steels are designated by E, F and G. The inventive steel A was melted in an electric furnace according to alloy and process designs, was continuously cast into bloom of 370mm , was rolled into billet of 160mm, and then was rolled into a testing material of Φ60mm through reheating. Each of the inventive steels B, C and D was melted in a vacuum induction melting (VIM) furnace, was forged into billet of 160mm, and then was rolled into a testing material of Φ40mm through reheating.

[58] Also, in order to improve fatigue resistance by 30% or more as compared to the existing high strength and ensure resistance to temper softening, Cr or Si was actively added to the inventive ultra high strength steels A, B, C and D. Moreover, in order to prevent the occurrence of abnormal grain growth, the inventive steels were sufficiently deoxidized with Al, and then niobium-/titanium-carbides and niobium- /titanium-nitrides were sufficiently formed by adding Nb and Ti thereto in an appropriate amount so as to output a heating temperature for rolling of 1200 to 1300⁰C.

[59] Table 2 [Table 2] [Table ]

[60] [61] Since the inventive steels A, B, C and D are based on low-carbon alloy steel(SCM920H), and contain large amounts of Cr and Si as compared to the comparative steels E, F and G, they are expected to have improved resistance to temper softening, as described above. Also, the inventive steels are expected to increase the GCT where abnormal grains occur because a certain amount of Nb is added thereto. Moreover, the addition of a given amount of Ti to the inventive steel and thus the formation of titanium carbonitrides contribute to fine crystal grains, which further increases the GCT. The comparative steels E, F and G are based on SCM, SNCM and SCR, and correspond to steel materials for mass production of gears in use. Evaluation results of the inventive and comparative steels are summarized in Table 3. [62] Table 3

[Table 3] [Table ]

[63] [64] The inventive steels is excellent in mechanical properties over the comparative steels, and exhibit higher fatigue performance (e.g., rotary bending fatigue strength or torsion fatigue strength) and a longer contact fatigue life in a carburized part than those of the comparative steels. In particular, since the inventive steel A exhibits the most superior GCT, is evaluated as excellent in resistance to temper softening as compared to the comparative steel E, and has the highest fatigue resistance as compared to the other comparative steels, it is the best steel material suitable to ultra high strength car- burizing steel with high fatigue resistance.

[65] As seen from Table 3, the contact fatigue life characteristic of the inventive steel is 2 to 6 times as long as the comparative steel, and is excellent in mechanical properties, rotary bending fatigue strength, and torsion fatigue strength over the comparative steel. Also, since Al and N as deoxidizing and crystal grain fining elements, and Nb and Ti as elements for suppressing abnormal grain growth are added to the inventive steel, the inventive steel exhibits an increased GCT. That is, the GCT of the inventive steel ranges from 1010 to 115O⁰C whereas the GCT of the comparative steel ranges from 950 to 1000⁰C.

[66] In addition, the inventive steel is also excellent in resistance to temper softening according to a decrease in carburized surface hardness, so that when compared to the comparative steel, it can have greater capability to suppress pitting from being caused by the surface deterioration of a carburized part when oil temperature increases according to abrupt starting and braking.

[67] From the above-dscussed results, it can be noted that the inventive steel is very suitable for applying to a next generation carburizing steel for gears, which is required to have high durability and high fatigue resistance. From among automobile parts fabricated using the inventive steel, several gears are illustrated in HGS. 1 to 3. Industrial Applicability

[68] As described above, the inventive ultra high strength carburizing steel with high fatigue resistance has various fatigue strengths that are higher than those of existing high strength steel materials by 30% or greater, is excellent in resistance to temper softening, exhibits an abnormal grain growth temperature of 1000⁰C or higher, and thus can be subjected to carburizing at a temperature of 1000⁰C or higher.

[69] Accordingly, the present invention can increase the carburizing temperature because the inventive steel has a higher abnormal grain growth temperature than that of the conventional steel, and thus can be expected to enhance productivity and reduce production costs. As a result, the present invention can satisfy customers demands for high strength steel that is more cost-effective while having the same quality as that of the convention steel. Rnally, the present invention can bring an unprecedented improvement in the quality and cost efficiency of vehicles.

[70] While this invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the present invention is not limited to the disclosed embodiment and the drawings, but, on the contrary, it is intended to cover various modifications and variations within the spirit and scope of the appended claims.

Claims

[1] Ultra high strength carburizing steel with high fatigue resistance, comprising:

0.15 to 0.25% by weight of C; 1.70 to 2.30% by weight of Cr; 0.50 to 0.70% by weight of Si; 0.010 to 0.040% by weight of Al; and a balance of Fe and inevitable impurities. [2] The ultra high strength carburizing steel as claimed in claim 1, further comprising: 0.015 to 0.035% by weight of Nb; and 100 to 200ppm by weight of

N as elements for fining crystal grains. [3] The ultra high strength carburizing steel as claimed in claim 2, further comprising 50ppm or less by weight of Ti. [4] The ultra high strength carburizing steel as claimed in claim 1 or 2, further comprising: 0.45 to 0.75% by weight of Mn; and 0.030% or less by weight of S as elements for improving machinability. [5] The ultra high strength carburizing steel as claimed in claim 1 or 2, further comprising 0.25 to 0.50% by weight of Mo as an element for improving quenchability. [6] The ultra high strength carburizing steel as claimed in claim 1 or 2, further comprising 0.30% or less by weight of N as an element for improving harde- nability. [7] The ultra high strength carburizing steel as claimed in claim 1 or 2, wherein the inevitable impurities comprise: 0.020% by weight of P; and 25ppm by weight of

O.