KR100716344B1 - Heat treatment method of cr-mo alloy for transmission gear and shaft - Google Patents

Abstract

The present invention is to increase the chromium and silicon significantly compared to the existing chromium-molybdenum steel in order to improve the fatigue strength, torsional fatigue strength, contact fatigue properties (fitting resistance) and high strength, ultra-high strength chromium- for transmission gear and shaft The present invention relates to a heat treatment method of molybdenum alloy steel.

To this end, the present invention is based on iron (Fe), carbon (C) 0.17 to 0.21% by weight, silicon (Si) 0.50 to 0.70% by weight, manganese (Mn) 0.45 to 0.75% by weight, phosphorus (P) ) 0.020 wt% or less, sulfur (S) 0.030 wt% or less, chromium (Cr) 1.95 to 2.25 wt%, molybdenum (Mo) 0.33 to 0.43 wt%, niobium (Nb) 0.015 to 0.035 wt% Chromium-molybdenum alloy steel and chromium-molybdenum alloy steel made by adding 12 to 15 ppm or less of dissolved oxygen to the alloy for cleanliness are maintained at 2 to 3 at 920 to 940 ° C. to secure microstructure uniformity after forging and workability after forging. Performing austenizing for a time; Controlling cooling to a tempering temperature at a cooling rate of at least 150 ° C./min; Provided is a heat treatment method comprising tempering at 660 to 680 ° C. for 5 to 7 hours, followed by air cooling.

Transmission gears and shafts, ultra high strength chromium-molybdenum alloy steel, heat treatment method

Description

Heat treatment method of Cr-Mo alloy for transmission gear and shaft

1 is a process chart illustrating a heat treatment method of ultra-high strength chromium-molybdenum alloy steel for a transmission gear and a shaft according to the present invention;

2 is a graph showing the results of roughness test for alloy steels of Examples and Comparative Examples according to the present invention;

3 is a graph showing the fatigue limit test results for the alloy steel of the Examples and Comparative Examples according to the present invention,

4 is a process diagram illustrating a process of manufacturing a gear and a shaft for an existing vehicle transmission.

The present invention relates to a heat treatment method of ultra-high strength chromium-molybdenum alloy steel for transmission gears and shafts, and more specifically, to improve fatigue strength, torsional fatigue strength, contact fatigue characteristics (fitting resistance) and increase the strength of chromium and silicon. The present invention relates to a method of heat-treating ultra-high strength chromium-molybdenum alloy steel for transmission gears and shafts, which has significantly increased properties compared to chromium-molybdenum steel and greatly improved physical properties.

Typically, gears and shafts for automotive transmissions are shown in the accompanying Figure 4, material → hot or cold forging → cooling (air or cold) → ISO annealing or Normalizing → processing (Hobbing & Shaving) → It is manufactured through processes such as carburization heat treatment.

Recently, in order to solve consumer complaints about transmission gear noise, it is generally manufactured including post-heat treatment (honing).

In general, conventional transmission gears and shaft materials are used by adding alloy elements chromium, molybdenum and nickel in order to increase hardenability, fatigue strength, toughness and durability.

That is, chromium (Cr) alloy steel, chromium (Cr)-molybdenum (Mo) alloy steel, or chromium (Cr)-nickel (Ni)-molybdenum (Mo) alloy steel, etc. are mainly used by carburizing heat treatment.

In the case of nickel, the nickel is high cost and the toughness of the steel is too high when added to the steel, deterioration of roughness of the gear tooth surface during the machining of the gear, shortening the life of the machining tool, and eventually reducing productivity. Is in the trend.

Therefore, in recent years, Cr-Mo alloy steels having a higher Mo content than conventional Cr-Mo alloy steels have been developed and used to replace existing Cr-Ni-Mo steels.

On the other hand, the molybdenum is very effective in improving the hardenability and strength, and also has the advantage of increasing the grain coarsening temperature, but as the element that caused the recent increase in the raw material price of alloy steel, it is a disadvantage that many uses are limited because of too high price There is this.

Accordingly, the present inventors adjust the chemical components such that fatigue strength, torsional fatigue strength, contact fatigue properties (fitting resistance), etc. are significantly improved based on the elements of the existing Cr-Mo alloy steel, and workability is improved. To improve the physical properties by improving chromium and silicon significantly compared to existing chromium-molybdenum steels to improve fatigue strength, torsional fatigue strength, contact fatigue characteristics (fitting resistance) and high strength by improving heat treatment conditions before processing after forging. It is possible to solve problems such as reduced productivity due to the difficulty of workability of the gears applying nickel-chromium-molybdenum steel, shortening the life of machining tools, and improvement of physical properties, and it is equivalent to high Mo-containing Cr-Mo steel. It is possible to achieve breakthrough physical properties at a price, and also to meet the high-strength, high durability, and compact design, which are recently developed by automobiles. It is an object of the present invention to provide a heat treatment method of ultra-high strength chromium-molybdenum alloy steel for a transmission gear and a shaft.

The present invention for achieving the above object is based on iron (Fe), carbon (C) 0.17 to 0.21% by weight, silicon (Si) 0.50 to 0.70% by weight, manganese (Mn) 0.45 to 0.75 weight %, 0.020 wt% or less of phosphorus (P), 0.030 wt% or less of sulfur (S), 1.95 to 2.25 wt% of chromium (Cr), 0.33 to 0.43 wt% of molybdenum (Mo), and 0.015 to 0.035 wt% of niobium (Nb) Provided is a high-strength chromium-molybdenum alloy steel for a transmission gear and a shaft characterized by adding 12 to 15 ppm or less of dissolved oxygen for cleanliness of an alloy to an alloy-based component.

In addition, the present invention is subjected to austenizing for 2 to 3 hours at 920 ~ 940 ℃ in the heat treatment before processing to secure the microstructure uniformity and the workability after forging to secure the chromium-molybdenum alloy steel of the composition as described above Making a step; Controlling cooling to a tempering temperature at a cooling rate of at least 150 ° C./min; It provides a heat treatment method of high-strength chromium-molybdenum alloy steel for a transmission gear and a shaft, which comprises the step of tempering at 660 to 680 ° C for 5 to 7 hours and then air-cooling.

Hereinafter, the present invention will be described in more detail.

The chromium-molybdenum alloy steel of the present invention, unlike the conventional chromium-molybdenum alloy steel (SCM920HVSi) and nickel-chromium-molybdenum alloy steel (SNCM518H) described in Table 1, has iron (Fe) as a main component, and carbon (C) 0.17 -0.21 wt%, silicon (Si) 0.50-0.70 wt%, manganese (Mn) 0.45-0.75 wt%, phosphorus (P) 0.020 wt% or less, sulfur (S) 0.030 wt% or less, chromium (Cr) 1.95-2.25 12-15 ppm or less of dissolved oxygen is added to an alloy component including weight%, molybdenum (Mo) 0.33 to 0.43 weight%, and niobium (Nb) 0.015 to 0.035 weight% for the cleanliness of the alloy (to reduce impurities). It is done.

In particular, the chromium-molybdenum alloy steel of the present invention is not added nickel to improve the workability, and the chemical composition is greatly increased so that fatigue strength, torsional fatigue strength, contact fatigue properties (fitting resistance), etc. are significantly improved compared to the existing alloy steel. Adjusted.

In addition, since the affinity with oxygen is very strong, the content of chromium and silicon, which has limited the content of the alloy steel for gears until now, has been greatly increased than that of the conventional chromium-molybdenum alloy steel. Mo) content was reduced to improve hardness and fatigue strength.

Chromium, the content of which is increased according to the present invention, is a very effective element for improving the hardenability of steel, and it is very useful for improving the bending fatigue strength by increasing the core hardness, making stable fine carbide to promote carburization and making carburization suitable, In case of tempering, the softening resistance is greatly improved, and the contact fatigue property (fitting resistance) can be significantly increased.

The silicon contained according to the present invention is dissolved in the base to help improve the fatigue strength, and greatly improve the softening resistance during tempering, thereby significantly increasing the contact fatigue property (fitting resistance).

As such, since chromium and silicon greatly improve the softening resistance, it is possible to minimize the occurrence of tooth fitting, which is a form of gear surface damage generated after a long time of use, thereby providing a great advantage of increasing the vehicle warranty life.

Although chromium and silicon are very effective elements for improving the physical properties of automobile gears, their use was limited because they were very good at making grain boundary oxide layers during carburization.

Nevertheless, the reason for increasing the content of chromium and silicon is that the vacuum carburizing method, which is an anoxic atmosphere-free carburizing process, has been developed and used, and the addition of a post-heat treatment process in order to satisfy consumer requirements for automobile gear noise has recently been popularized. This is because the restrictions on its use have been removed.

Referring to the main members of the present invention and the reason for limitation of the content in more detail as follows.

1) Carbon (C): 0.17 to 0.21 wt%

Addition of at least 0.17% or more is necessary to obtain the desired core hardness from HV 440 to 472. If the core content is 0.21% by weight or more, the core hardness is too high to sufficiently introduce the compressive residual stress on the surface after quenching. It is limited to 0.17 to 0.21% by weight.

2) Silicon (Si): 0.50 to 0.70 wt%

In the chromium-molybdenum alloy steel of the present invention, silicon is preferably limited to 0.50 to 0.70 wt%, and if it is less than 0.50 wt%, the above softening resistance does not increase, and if it exceeds 0.70 wt%, the known solid solution strengthening effect is too large. Forging and machining are difficult, with extremely low formability.

3) Chromium (Cr): 1.95 to 2.25 wt%

It is preferable to limit the content of chromium (Cr) to 1.95 to 2.25% by weight. At this time, when the content is less than 1.95% by weight, there is no significant difference compared with the hardenability of the high strength chromium-molybdenum alloy steel, so that the fatigue strength improvement is not large. In addition, the softening resistance does not significantly increase during tempering, so that the contact fatigue characteristics are not improved. When the content is 2.25% by weight or more, the processability due to the precipitation of a large amount of fine carbide causes a sharp drop in processing and forging formation.

4) Molybdenum (Mo): 0.33 to 0.43 wt%

In general, it is preferable to add 0.33 to 0.43% by weight, which is smaller than high strength chromium-molybdenum alloy steel, and the reason why the molybdenum content is usually kept smaller than that of high strength chromium-molybdenum alloy steel is to prevent the formation of a large amount of harmful carbides due to the high chromium content. To reduce brittleness.

5) Sulfur (S): 0.030 wt% or less

In order to improve workability despite improving physical properties (high strength), the upper limit content is generally raised within a range that is not more harmful than that of chromium-molybdenum alloy steel, and it is better to manage it to 0.03% by weight or less.

6) Phosphorus (P): 0.020 wt% or less

In general, it is preferable to control the content to 0.02% by weight or less within a range that is not harmful, such as chromium-molybdenum alloy steel.

7) Niobium (Nb): 0.015 to 0.035 wt%

In order to prevent grain coarsening and maximize grain refinement during high temperature carburizing, niobium (Nb) is preferably limited to 0.015 to 0.035% by weight, and niobium (Nb) forms fine niobium and carbide in alloy steel. If the content is less than 0.015% by weight as an element that prevents grain coarsening, the effect is not great. If the content exceeds 0.035% by weight, carbides are excessively precipitated at the grain boundaries, causing brittleness.

Here, in the present invention, the upper limit of the content of Nb is higher than that of chromium-molybdenum alloy steel in general, which is to prevent over-precipitation at the grain boundary of chromium carbide due to the increase in chromium (Cr) content.

8) Manganese (Mn): 0.45 to 0.75 wt%

To ensure hardenability of the steel, an amount of at least 0.45% by weight should be added. However, since Mn is liable to cause particle oxidation, it should be limited to 0.75% by weight or less.

The chromium-molybdenum alloy steel of the present invention having such a composition and content ratio is much harder than the existing high strength chromium-molybdenum alloy steel, and thus requires special heat treatment before processing to ensure uniformity after micro forging and to secure workability after forging. Suitable heat treatment through heat treatment simulation test is as described in FIG.

As shown in Figure 1, the heat treatment of the present invention is composed of an austenizing section, a control cooling section, a tempering section and an air cooling section. Looking at the detailed holding temperature, the holding time and the cooling rate, The chromium-molybdenum alloy steel of the present invention is subjected to a heat treatment before processing to ensure microstructure uniformity after forging and workability after forging, performing austenizing at 920 to 940 ° C for 2 to 3 hours, Control cooling to cool to the tempering temperature at a cooling rate of 150 ° C./min or more (air, steam injection, etc.), and tempering at 660 to 680 ° C. for 5 to 7 hours, followed by air cooling.

In the heat treatment conditions of the present invention, the reason for regulating the cooling rate in the controlled cooling section is because of polygonal ferrite. Polygonal ferrite is a structure that locally precipitates in a stable phase at a cooling rate of 100 ° C./min or less, and it is difficult to secure microstructure homogeneity by generating segregation between components in a later process.

Accordingly, control at a cooling rate of 100 ° C./min or more can prevent precipitation of ferrite (polygonal ferrite) (experimental results). However, in consideration of mass production, a cooling rate of 150 ° C./min or more is suitable.

In addition, the tempering temperature is regulated to 660 to 680 ° C to exclude a polygonal ferrite precipitation section near 700 ° C.

In addition, the alloy steel material of the present invention is subjected to austenizing for 2 to 3 hours at 920 ~ 940 ℃ in consideration of the fact that the interior is hard to work well.

Hereinafter, the embodiment of the present invention will be described in more detail with a comparative example, but the present invention is not limited by the following examples.

Example 1

As weight percent, carbon (C) 0.18, silicon (Si) 0.40, manganese (Mn) 0.69, phosphorus (P) 0.009, (S) 0.020, nickel (Ni) 0.05, chromium (Cr) 1.79, molybdenum (Mo) 0.25 , A chromium-molybdenum alloy steel material containing 12 ppm of dissolved oxygen for the cleanliness of an alloy component containing niobium (Nb) 0.025 for 2 hours was carburized at 900 ° C. for 2 hours, and then quenched at an oil temperature of 150 ° C., followed by 170 ° C. Tempering was carried out for 2 hours to prepare a specimen (see Table 2).

Example 2

As weight percent, carbon (C) 0.21, silicon (Si) 0.59, manganese (Mn) 0.62, phosphorus (P) 0.010, (S) 0.019, nickel (Ni) 0.05, chromium (Cr) 2.07, molybdenum (Mo) 0.37 , A chromium-molybdenum alloy steel material containing 14 ppm of dissolved oxygen for the cleanliness of an alloy component containing niobium (Nb) 0.029 for 2 hours, carburized at 900 ° C. for 2 hours, and quenched at an oil temperature of 150 ° C., followed by 170 ° C. Tempering was carried out for 2 hours to prepare a specimen (see Table 2).

Example 3

As weight percent, carbon (C) 0.19, silicon (Si) 0.35, manganese (Mn) 0.62, phosphorus (P) 0.010, (S) 0.023, nickel (Ni) 0.06, chromium (Cr) 1.42, molybdenum (Mo) 0.38 , A chromium-molybdenum alloy steel material containing 13 ppm of dissolved oxygen for the cleanliness of an alloy component containing niobium (Nb) 0.026 for 2 hours, carburized at 900 ° C. for 2 hours, and then quenched at an oil temperature of 150 ° C., followed by 170 ° C. Tempering was carried out for 2 hours to prepare a specimen (see Table 2).

Comparative Example 1

Chromium-molybdenum alloy steel (SCM920HVSi) according to Comparative Example 1 is the representative high strength transmission gear steel of the applicant of the present application, by weight% carbon (C) 0.19, silicon (Si) 0.11, manganese (Mn) 0.70, phosphorus (P) 0.012, (S) Chromium-molybdenum alloy steel added with dissolved dissolved oxygen of 15 ppm for alloy cleanness to alloying components containing 0.018, nickel (Ni) 0.06, chromium (Cr) 1.31, molybdenum (Mo) 0.62, niobium (Nb) 0.019 The material was carburized at 900 ° C. for 2 hours, quenched at an oil temperature of 150 ° C., and then tempered at 170 ° C. for 2 hours to prepare a specimen (see Table 2).

Comparative Example 2

Nickel-chromium-molybdenum alloy steel (SNCM518H) according to Comparative Example 2 is a representative high-strength transmission gear steel made by Mitsubishi Steel, Japan, in terms of weight% of carbon (C) of 0.18, silicon (Si) of 0.09, manganese (Mn) of 0.55, and phosphorus (P). ) Chromium added with 14 ppm of dissolved oxygen to the alloys containing 0.006, (S) 0.015, nickel (Ni) 1.57, chromium (Cr) 0.55, molybdenum (Mo) 0.61 and niobium (Nb) 0.022 The molybdenum alloy steel material was carburized at 900 ° C. for 2 hours, quenched at an oil temperature of 150 ° C., and then tempered at 170 ° C. for 2 hours to prepare a specimen (see Table 2).

Test Example 1

In order to find out the hardenability to the alloy steel of Example 1-3 and Comparative Example 1, the hardness was measured by using the jominy method, as shown in the accompanying graph of FIG. Compared to the conventional chromium-molybdenum alloy steel of Comparative Example, the alloy steel according to the example was found to have a high hardness in proportion to the measured distance, thereby increasing the hardenability significantly.

Test Example 2

Hardness measurement, fatigue test, impact test, contact fatigue test and torsion fatigue test were carried out as described in the footnote of Table 3 for Example 1-3 and Comparative Example 1-2, and the results are shown in Table 3 and FIG. It is as shown in the graph of 3.

As shown in Table 3 above, it can be seen that the alloy steel according to Example 2 of the present invention is superior in surface hardness and core hardness, and also excellent in fatigue limit and contact fatigue limit, as compared to the conventional alloy steels of Comparative Examples 1 and 2. there was.

In addition, the alloy steels according to Examples 1 and 3 exhibited superior physical properties compared to the alloy steels of Comparative Examples 1 and 2, but were inferior in the contact fatigue limit, which is an important characteristic of the gear.

As described above, according to the heat treatment method of the ultra-high strength chromium-molybdenum alloy steel for the transmission gear and shaft according to the present invention, compared to the existing high-strength nickel-chromium-molybdenum alloy steel and chromium-molybdenum steel has excellent fatigue properties (durability) It can provide the advantages that can be applied to compact high power transmission gears and shafts.

Claims (2)

delete In the chromium-molybdenum alloy steel having the composition and content of claim 1 in the heat treatment before processing to ensure microstructure uniformity after forging and workability after forging, Performing austenizing at 920-940 ° C. for 2-3 hours; Controlling cooling to a tempering temperature at a cooling rate of at least 150 ° C./min; A method of heat treatment of high strength chromium-molybdenum alloy steel for a transmission gear and a shaft, comprising tempering at 660 to 680 ° C. for 5 to 7 hours and then performing air cooling.

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