Classifications

• • 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
• • 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
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • 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
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • 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
  • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium 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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium 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

Definitions

• the present invention relates to high strength carburizing steel, and more particularly to ultra high strength carburizing steel with high fatigue resistance.
• 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.
• the present invention provides ultra high strength carburizing steel with high fatigue resistance, which can be subjected to carburizing at an elevated temperature.
• 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.
• 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.
• the ultra high strength carburizing steel with high fatigue resistance further comprises 50ppm or less by weight of Ti.
• 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.
• 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.
• 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.
• the inevitable impurities comprise: 0.020% by weight of P; and 25ppm by weight of O.
• 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.
• 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.
• 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
• 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.
• Cr strengthens resistance to quenching and tempering, improves fatigue strength, and favors carburizing by facilitating the formation of stable carbides.
• Cr needs to be added in an amount of 1.70% or more by weight.
• the appropriate content range of Cr is set to 1.70 to 2.30% by weight.
• 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.
• 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
• Si, along with Cr provides steel with improved wear resistance and resistance to temper softening according to a rise in oil temperature
• the content of Si is set to 0.50 to 0.70% by weight.
• Al acts as not only a strong deoxidzer, but also an element for fining crystal grains by combining with N.
• Al acts as not only a strong deoxidzer, but also an element for fining crystal grains by combining with N.
• Al is added in an amount of less than 0.01% by weight, the effect of deoxidzation or crystal grain fining dsadvantageously decreases.
• 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 Al 2 O 3 . Therefore, the appropriate content of Al is set to 0.010 to 0.040% by weight.
• 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.
• 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.
• 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.
• Mo is excellent in improving the quenchability of steel, and is very effective in increasing strength and toughness.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• the heating temperature for rolling 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.
• GCT Gar Coarsening Temperature
• the heating temperature for rolling is set to 1200 to 1300 0 C so that various carbonitrides are sufficiently solid- soluble.
• 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 ⁇ 60mm 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 ⁇ 40mm through reheating.
• VIP vacuum induction melting
• 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
• 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.
• 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.
• 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 115O 0 C whereas the GCT of the comparative steel ranges from 950 to 1000 0 C.
• 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.
• 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
• 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 1000 0 C or higher, and thus can be subjected to carburizing at a temperature of 1000 0 C or higher.
• 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.
• 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.

Applications Claiming Priority (2)

Publications (1)

Family

ID=39536464

Family Applications (1)

Country Status (2)

Cited By (1)

Families Citing this family (1)

Citations (7)

• 2006
  • 2006-12-19 KR KR1020060130169A patent/KR20080056945A/ko not_active Application Discontinuation
• 2007
  • 2007-12-18 WO PCT/KR2007/006650 patent/WO2008075889A1/en active Application Filing

Patent Citations (7)

Also Published As

Similar Documents

Legal Events