CN113789480B - A kind of cold forging gear steel and preparation method thereof - Google Patents

Abstract

The invention belongs to the technical field of steel, and particularly relates to cold-forged gear steel and a preparation method thereof. The gear steel comprises the following chemical components in percentage by mass: c:0.15% -0.25%, si: 0.05-0.40%, mn:0.30% -1.40%, P:0 to 0.03%, S: s:0 to 0.035%, cr: 0.80-1.50%, ti:0 to 0.05%, al:0 to 0.045%, cu:0 to 0.15%, N:0 to 0.0150 percent, and the balance of Fe and inevitable impurities; the metallographic structure of the gear steel comprises austenite, and the grain size of the austenite is 6-7. The gear steel meets the performance of the gear, has lower hardness in a hot rolling state, and solves the problem of large cold deformation resistance of the conventional gear steel; by reducing the contents of Si and Mn elements, the cold deformation resistance of the gear steel blank is reduced, and the problem of local coarse grains is solved.

Description

A kind of cold forging gear steel and preparation method thereof

technical field

The invention belongs to the technical field of iron and steel, in particular to a cold forging gear steel and a preparation method thereof.

Background technique

In recent years, with the rapid development of the machining industry, cold forging technology, as a less/no cutting process with less consumables, high efficiency and superior performance, has been widely used in industrial fields such as automobiles, machine tools, and instruments. The huge technical superiority and good economic benefits exhibited by cold forging technology have gradually become the development direction of precision production of small and medium forgings such as gears.

The conventional preparation process of gears is: gear steel raw material → blanking → hot forging → heat treatment after forging → gear processing (hobbing, shaping, milling, planing) → carburizing → quenching, tempering → surface cleaning → inspection → storage. At present, the production process of this gear product is very mature and is widely used in the gear manufacturing industry. However, the tooth profile processing after hot forging in this process cuts off the forging streamline of the blank, resulting in a decrease in the mechanical properties of the gear. In addition, the machining time of this process is long, the utilization rate of the tooth blank is low, and the production cost is high, which greatly affects the development of the gear industry.

The introduction of cold forging technology into the gear processing industry can fully solve the above problems. Its specific advantages are as follows: because the metal streamlines of cold precision forging gears are distributed along the tooth profile and the structure is dense, the strength of the gear, the wear resistance of the tooth surface, and the heat treatment performance are improved. and meshing noise are superior to cutting gears. Compared with cutting gears, the strength of cold precision forging gears can be increased by 20%, the bending fatigue life can be increased by 20%, the heat treatment deformation can be reduced by 30%, and the production cost can be reduced by more than 20%. Rapid production of high-quality gears by cold precision forging has become a global trend.

However, although the cold precision forging technology is good, it puts forward higher requirements for the gear steel raw materials. In order to reduce the wear of the blank on the mold, the blank is required to have high cold plasticity and small deformation resistance, so as to improve the use of the mold. life. The traditional solution is to use spheroidizing annealing to reduce the hardness of the raw material, which seriously reduces the production efficiency and greatly increases the production cost. Therefore, traditional gear steel raw materials have been difficult to meet the requirements of cold precision forging technology, and a new type of gear steel is urgently needed. On the basis of satisfying gear performance, it has low hardness and good cold plasticity, which is suitable for cold forging technology. .

Contents of the invention

In view of the above problems, the present invention provides a cold forged gear steel and a preparation method thereof to overcome the above problems or at least partly solve the above problems.

An embodiment of the present invention provides a cold-forged gear steel. The chemical composition of the gear steel is: C: 0.15%-0.25%, Si: 0.05%-0.40%, Mn: 0.30%-1.40%, P : 0～0.03%, S: 0～0.035%, Cr: 0.80%～1.50%, Ti: 0～0.05%, Al: 0～0.045%, Cu: 0～0.15%, N: 0～0.0150%, remainder The amount is Fe and unavoidable impurities; the metallographic structure of the gear steel includes austenite, and the grain size of the austenite is grade 6-7.

Optionally, the properties of the gear steel are as follows: tensile strength ≥ 1080MPa, impact toughness ≥ 60J, hardenability J9 point hardness 30-40HRC.

Optionally, the metallographic structure further includes ferrite and pearlite, and the ratio of ferrite is >50%.

A preparation method of cold forged gear steel, the method comprising:

The smelted molten steel is sequentially poured and continuously cast to obtain a slab;

Heating the cast slab, and finally descaling to obtain the cast slab after descaling;

Rolling and cooling the cast slab after descaling in sequence to obtain the gear steel;

The rolling includes rough rolling and finishing rolling, the starting temperature of the rough rolling is 1050-1100°C, the temperature of the finishing rolling is 850-950°C, and the total deformation of the finishing rolling unit is 40% ~60%.

Optionally, the pouring temperature is 1500-1580° C.; the heating temperature is 1150-1200° C., and the heating time is 2.5-3.5 hours.

Optionally, the pouring and continuous casting of the smelted molten steel to obtain the slab also includes: smelting the molten steel containing the chemical composition, and performing vacuum degassing during the smelting process to obtain the smelted molten steel; The time for vacuum degassing is 4-7 minutes.

Optionally, the smelting includes vacuum refining outside the furnace, and the vacuum refining outside the furnace specifically includes: holding the pressure for 10-12 minutes in a vacuum pressure environment with a pressure less than 67Pa.

Optionally, the vacuum refining outside the furnace includes deoxidizing and aluminizing the molten steel, so that the refined molten steel has an aluminum content of 0.0450% to 0.070% in terms of mass fraction, and the deoxidizing agent for deoxidation is aluminum alloy .

Optionally, the cooling sequentially includes entering the cooling bed and covering the heat preservation cover, exiting the cooling bed, entering the slow cooling pit and exiting the slow cooling pit.

Optionally, the temperature entering the slow cooling pit is 550-650°C, and the temperature exiting the slow cooling pit is ≤300°C; the microstructure of the gear steel exiting the slow cooling pit is ferrite and pearlite.

One or more technical solutions in the embodiments of the present invention have at least the following technical effects or advantages:

The cold forging gear steel provided by the present invention has the following chemical components in terms of mass fraction: C: 0.15% to 0.25%, Si: 0.05% to 0.40%, Mn: 0.30% to 1.40%, P: 0 to 0.03%, S : 0~0.035%, Cr: 0.80%~1.50%, Ti: 0~0.05%, Al: 0~0.045%, Cu: 0~0.15%, N: 0~0.0150%, the balance is Fe and unavoidable impurities ; The metallographic structure of the gear steel includes austenite, and the grain size of the austenite is grade 6-7. On the basis of satisfying the performance of the gear, the gear steel has a lower hardness in the hot-rolled state, It can be directly used for cold forging, which solves the problem of large cold deformation resistance of conventional gear steel 20CrMnTi; by reducing the content of Si and Mn elements, the cold deformation resistance of gear steel billets is reduced; Increase the number of deformation bands inside austenite grains, provide a large number of nucleation sites for ferrite phase transformation, and promote ferrite phase transformation; the hardness of cold forged gear steel can reach below 220HBW, which is the same as that in the spheroidized annealed state. Conventional gear steel 20CrMnTi has the same hardness, has low cold deformation resistance, and can be used for cold forging; adopts appropriate heat treatment process, tensile strength ≥ 1080MPa, impact toughness ≥ 60J, hardenability J9 point hardness reaches 30-40HRC, In austenite, the grain size after carburizing at a temperature not greater than 980°C is not less than grade 6 and grade 7.0, which avoids the occurrence of local coarse grains.

The above description is only an overview of the technical solution of the present invention. In order to better understand the technical means of the present invention, it can be implemented according to the contents of the description, and in order to make the above and other purposes, features and advantages of the present invention more obvious and understandable , the specific embodiments of the present invention are enumerated below.

Description of drawings

In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following will briefly introduce the drawings that need to be used in the description of the embodiments. Obviously, the drawings in the following description are some embodiments of the present invention. For those skilled in the art, other drawings can also be obtained based on these drawings without creative effort.

Fig. 1 is the hardenability curve contrast figure of the embodiment of the present invention and comparative example 1 steel;

Fig. 2 is the cold state stress-strain curve comparative figure of embodiment of the present invention and comparative example steel 1;

Fig. 3 is the morphology of prior austenite grains after carburizing at 980°C for 1 hour in the gear steel of Example 1 of the present invention;

Fig. 4 is the morphology of prior austenite grains after carburizing at 980°C for 1 hour in comparative example 1 steel of the present invention;

Fig. 5 is the morphology of prior austenite grains after carburizing at 980°C for 1 hour in comparative example 2 steel of the present invention;

Fig. 6 is the morphology of prior austenite grains after carburizing at 980°C for 6 hours in the gear steel of Example 1 of the present invention;

Fig. 7 is the morphology of prior austenite grains after carburizing at 980°C for 6 hours in comparative example 1 steel of the present invention;

Fig. 8 is the morphology of prior austenite grains after carburizing at 980°C for 6 hours in steel of Comparative Example 2 of the present invention.

detailed description

The present invention will be described in detail below in conjunction with specific embodiments and examples, and the advantages and various effects of the present invention will be presented more clearly. Those skilled in the art should understand that these specific implementations and examples are used to illustrate the present invention, not to limit the present invention.

Throughout the specification, unless otherwise specified, terms used herein should be understood as commonly used in the art. Therefore, unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. In case of conflict, this specification shall take precedence.

Unless otherwise specified, various raw materials, reagents, instruments and equipment used in the present invention can be purchased from the market or prepared by existing methods.

The technical solution of the embodiment of the present application is to solve the above-mentioned technical problems, and the general idea is as follows:

According to a typical embodiment of the present invention, a cold forged gear steel, the chemical composition of the gear steel is: C: 0.15%-0.25%, Si: 0.05%-0.40%, Mn: 0.30% ～1.40%, P: 0～0.03%, S: 0～0.035%, Cr: 0.80%～1.50%, Ti: 0～0.05%, Al: 0～0.045%, Cu: 0～0.15%, N: 0 ~0.0150%, the balance is Fe and unavoidable impurities; the metallographic structure of the gear steel includes austenite, and the grain size of the austenite is grade 6-7.

By reducing the content of Si and Mn elements, the cold deformation resistance of the gear steel billet is reduced; through the method of low-temperature large-deformation finish rolling, the number of deformation bands inside the austenite grains is greatly increased, and a large number of ferrite phase transformations are provided. The nucleation position promotes the ferrite phase transformation, and the method of slow cooling after rolling is adopted to increase the residence time of the ferrite phase region, which can increase the proportion of ferrite in the rolled material structure and reduce the cold deformation resistance.

In austenite, the grain size after carburizing at a temperature not higher than 980°C is 6-7, which avoids the occurrence of local coarse grains.

As an optional implementation manner, the metallographic structure further includes ferrite and pearlite, and the ferrite ratio is greater than 50%.

Significantly increase the number of deformation bands inside the austenite grains, provide a large number of nucleation sites for ferrite transformation, and promote ferrite transformation.

As an optional embodiment, the properties of the gear steel are as follows: tensile strength ≥ 1080 MPa, impact toughness ≥ 60 J, hardenability J9 point hardness of 30-40 HRC.

The hardness of the gear steel can reach below 220HBW, which is determined by its metallographic structure, which is equivalent to the hardness of the spheroidized annealed conventional gear steel 20CrMnTi, has low cold deformation resistance, and can be used for cold forging.

As an optional implementation, the finished product specifications of the cold forged gear steel range from Φ8mm to Φ250mm.

A preparation method of cold forged gear steel, the method comprising:

The smelted molten steel is sequentially poured and continuously cast to obtain a slab;

Heating the cast slab, and finally descaling to obtain the cast slab after descaling;

Rolling and cooling the cast slab after descaling in sequence to obtain the gear steel;

The rolling includes rough rolling and finishing rolling, the starting temperature of the rough rolling is 1050-1100°C, the temperature of the finishing rolling is 850-950°C, and the total deformation of the finishing rolling unit is 40% ~60%.

Through the method of low-temperature and large-deformation finish rolling, the number of deformation bands inside the austenite grains is greatly increased, and a large number of nucleation sites are provided for the ferrite transformation to promote the ferrite transformation. The method of slow cooling after rolling is adopted. , increasing the residence time of the ferrite phase region can increase the proportion of ferrite in the rolled product structure and reduce the cold deformation resistance.

After refining, the slab can be obtained by continuous casting directly, and the molten steel can also be vacuum degassed (one or more of VD, RH or VOD is selected.

As an optional embodiment, the pouring temperature is 1500-1580° C.; the heating temperature is 1150-1200° C., and the heating time is 2.5-3.5 hours.

As an optional implementation, the pouring and continuous casting of the smelted molten steel to obtain the slab also includes: smelting the molten steel containing the chemical composition, and performing vacuum degassing during the smelting process to obtain the smelted After molten steel; the time for the vacuum degassing is 4-7 minutes.

As an optional implementation manner, the smelting includes vacuum refining outside the furnace, and the vacuum refining outside the furnace specifically includes: maintaining the pressure for 10-12 minutes in a vacuum pressure environment with a pressure less than 67 Pa.

As an optional implementation, the vacuum refining outside the furnace includes deoxidizing and aluminizing the molten steel, so that the refined molten steel has an aluminum content of 0.0450% to 0.070% in terms of mass fraction, and the deoxidized The deoxidizer is aluminum alloy.

On the basis of ensuring the low oxygen content in the steel, the cold deformation ability is improved.

As an optional implementation manner, the molten steel is smelted in an electric furnace or a converter, and the aluminum content of the refined primary component J1 is controlled to be 0.0450%-0.070%.

The carbon content (mass fraction) at the end of tapping is controlled at 0.08% to 0.12%; then it enters the LF furnace for out-of-furnace refining, feeds aluminum wire at 4 to 5 m/t after the ladle, and controls the target aluminum content of the primary composition J1 of the refining station to reach 0.050 %～0.070%, the amount of lime and fluorite should be added according to the slag condition and sulfur content during tapping to ensure good slag fluidity. In order to obtain good cold forging performance,

Aluminum wire can be effectively added to molten steel to obtain a high and stable aluminum yield. On the basis of controlling the aluminum content in the steelmaking process and ensuring low oxygen content in the steel, aluminum nitrogen fixation is used to improve the cold deformation capacity; The quality of molten steel is improved, which can greatly reduce the cost of deoxidation and alloying, and the economic benefits are obvious. The reason for controlling the aluminum content is: on the basis of ensuring the low oxygen content in the steel, aluminum is used to fix nitrogen to improve the cold deformation capacity.

As an optional implementation manner, the cooling rate on the cooling bed is 0-2° C./s. Preferably, the cooling rate range is 0-0.5°C/s.

As an optional implementation manner, the cooling sequentially includes entering the cooling bed and covering the heat preservation cover, exiting the cooling bed, entering the slow cooling pit and exiting the slow cooling pit.

As an optional implementation, the temperature of the entering slow cooling pit is 550-650°C, the temperature of the exiting slow cooling pit is ≤300°C; the gear steel structure of the exiting slow cooling pit is ferrite and Pearlite.

The reason for entering the cooling bed and covering the heat preservation cover is to reduce the air convection near the cooling bed and slow down the cooling speed of the rolled material as much as possible; it adopts the method of slow cooling after rolling to increase the residence time of the ferrite phase zone, which can improve the ferrite in the rolled material structure. Ratio, reduce cold deformation resistance.

The temperature entering the slow cooling pit is 550-650° C., the temperature exiting the slow cooling pit is ≤ 300° C., and the structure exiting the slow cooling pit is ferrite and pearlite. Control the temperature of entering and exiting the slow cooling pit to avoid abnormal tissue

As an optional embodiment, the casting slab is a rectangular billet with a size range of (120-350) mm×(120-350) mm or a round billet with a diameter of Φ100 mm-Φ350 mm.

A cold forged gear steel of the present application and its preparation method will be described in detail below in combination with examples, comparative examples and experimental data.

Example 1

In this example, the constituent elements and mass percentages used in the gear steel are: C 0.17%, Si 0.08%, Mn 0.60%, P 0.010%, S 0.025%, Cr 1.30%, Ti 0.04%, Al 0.033%, Cu 0.13%, N0.0065%, the balance is Fe and unavoidable impurities. The metallographic structure of the gear steel includes austenite, and the grain size of the austenite is grade 6-7.

The production specification is Φ80mm new cold forged gear steel. The specific control parameters are:

(1) Electric furnace is used for smelting, and the carbon content (mass fraction) at the end of tapping is controlled at 0.10%; then it enters the LF furnace for out-of-furnace refining, and feeds aluminum wire 4.5m/t after the seat cladding, and controls the target aluminum content of J1 to reach 0.060%, Add lime and fluorite according to slag condition and sulfur content during tapping to ensure good slag fluidity; then carry out VD vacuum degassing on the molten steel, the pumping time is 5 minutes, and the vacuum holding time of less than 67Pa reaches 12 minutes.

(2) The molten steel is protected and poured throughout the whole process. The pouring temperature is 1550°C and the superheat is 25°C. Two-stage electromagnetic stirring and a reasonable cooling system are used to solidify to obtain a cast slab.

(3) The slab enters the heating furnace at room temperature, the heating temperature is 1180 ° C, and the heating time is 3 hours; it exits the heating furnace for high-pressure water descaling; after descaling, it enters the hot continuous rolling unit, the starting rolling temperature is 1080 ° C, and the finishing rolling temperature is 900 ℃, the total deformation of the finishing rolling unit is controlled at 40%; after finishing rolling, the cooling rate is 0.3℃/s; It is 520°C, and the temperature out of the slow cooling pit is 260°C.

Example 2

In this embodiment, the constituent elements and mass percentages of the gear steel are: C: 0.15%, Si: 0.05%, Mn: 0.30%, Cr: 0.80%, Ti: 0-0.05%, and the balance is Fe and not Avoid impurities. The metallographic structure of the gear steel includes austenite, and the grain size of the austenite is grade 6-7.

A kind of cold forging gear steel and preparation method thereof, described method comprises continuous casting, rolling successively, and specific process parameter is as follows:

pouring the molten steel at a temperature of 1500°C to obtain a slab;

Heating the billet at a temperature of 1150°C for 2.5 hours, and then descaling;

The rolling start temperature in the rolling is 1050°C, and the finish rolling temperature in the rolling is 850°C.

Example 3

In this embodiment, the constituent elements and mass percentages of the gear steel are: C: 0.25%, Si: 0.40%, Mn: 1.40%, P: 0.03%, S: 0.01%, Cr: 1.50%, Ti: 0.05 %, Al: 0.045%, Cu: 0.15%, N: 0.0150%, and the balance is Fe and unavoidable impurities. The metallographic structure of the gear steel includes austenite, and the grain size of the austenite is grade 6-7.

A kind of cold forging gear steel and preparation method thereof, described method comprises continuous casting, rolling successively, and specific process parameter is as follows:

pouring the molten steel at a temperature of 1580°C to obtain a slab;

Heating the billet at a temperature of 1200°C for 3.5 hours, and then descaling;

The rolling start temperature in the rolling is 1080°C, and the finish rolling temperature in the rolling is 900°C.

Example 4

In this embodiment, the constituent elements and mass percentages of the gear steel are: C: 0.21%, Si: 0.35%, Mn: 0.36%, P: 0.01%, S: 0.008%, Cr: 1.10%, Ti: 0.03 %, Al: 0-0.045%, Cu: 0-0.15%, N: 0-0.0150%. The metallographic structure of the gear steel includes austenite, and the grain size of the austenite is grade 6-7.

Comparative example 1

The conventional hot-rolled gear steel 20CrMnTi is used as a comparative example, and its composition elements and mass percentages are: C0.20%, Si 0.29%, Mn 0.96%, P 0.015%, S 0.013%, Cr 1.21%, Ti 0.07%, N0 .0075%, the balance is Fe and unavoidable impurities.

Comparative example 2

The conventional annealed gear steel 20CrMnTi is used as a comparative example, and its composition elements and mass percentages are: C 0.20%, Si 0.25%, Mn 0.70%, P 0.015%, S 0.013%, Cr 0.91%, N 0.0065%, the balance It is Fe and unavoidable impurities. Spheroidizing annealing process: holding at 780°C for 1.5 hours, cooling to 680°C with the furnace and holding for 3 hours, cooling to 500°C in the furnace, and slow cooling after taking out the furnace.

Related experiments:

The properties of the steels prepared in Example 1 and Comparative Examples 1-2 were tested, and the test results are as follows.

The comparison of hardness, hardenability, cold deformation resistance, grain size after carburizing, and mechanical properties after heat treatment is carried out to the above-mentioned embodiments and comparative ratio gear steel billets, and the results are as follows:

(1) Hardness: Example 1: 148HBW: Comparative Example 1: 184HBW: Comparative Example 2: 143HBW. The hardness of the embodiment and the comparative example all meet the requirements of the gear steel billet; the hardness of the comparative example is better than that of the embodiment, and the low hardness of the embodiment can be directly used for cold forging.

(2) Hardenability: The hardenability test is carried out according to GB/T 5216-2014 "Structural Steel with Guaranteed Hardenability" and GB/T 225-2006 "End Quenching Test Method for Hardenability of Steel (Jominy Test)", The results are shown in Fig. 1, wherein the hardenability J9 point hardness of Example 1 reaches 34HRC, and the hardenability J9 point hardness of Comparative Example 1 is 31HRC. The hardenability of the embodiment is good, which is better than that of the comparative example.

(3) Cold deformation resistance: According to YB/T5293-2014 "Metallic Material Upsetting Test Method", the cold upsetting test is carried out, and the gear steel blanks of the examples and comparative examples are compressed to 1/3 position, and the cold deformation of the samples is observed The stress-strain curve is obtained, and the results are shown in Figure 2.

(4) Grain size after carburizing: Carburize according to GB/T 28694-2012 "Technical Requirements for Deep Carburizing". The particle morphology is shown in Fig. 3-8.

(5) Mechanical properties of heat treatment: After carburizing, oil cooling quenching at 850°C + tempering at 190°C for 2 hours, the tensile strength of Example 1 reached 1096MPa, and the impact energy reached 63J; the tensile strength of Comparative Example 1 reached 1086MPa, and the impact energy reached 66J; The tensile strength of Comparative Example 2 reaches 1090MPa, and the impact energy reaches 65J. The tensile strength and impact energy of the examples and the comparative examples all meet the requirements of the gear steel billets, and the heat treatment mechanical properties of the comparative examples are better than those of the examples.

It can be seen that the hardness and cold deformation resistance of the gear steel of the present invention are significantly lower than those of the conventional gear steel 20CrMnTi in the hot-rolled state, which are close to the annealed state, and can be used for cold forging; at the same time, its hardenability meets the national standard and is better than the conventional gear steel 20CrMnTi The embodiment of the present invention can maintain a finer austenite grain size after heat preservation for 1h and 6h, and the grain size level is less than 7.0, and there is no local coarse grain phenomenon; the mechanical properties after heat treatment are similar to those of conventional gear steel 20CrMnTi.

Detailed description of accompanying drawings 1-8:

As shown in FIG. 1 , the hardness at J9 point of hardenability in Example 1 reaches 34HRC, and the hardness at J9 point of hardenability in Comparative Example 1 is 31HRC. The hardenability of Example 1 is good, which is better than that of Comparative Example 1. The hardenability of Comparative Example 1 is low, and local coarse grains tend to appear after carburizing and quenching after cold forging.

As shown in Figure 2, when the strain is above 0.55, the true stresses of the comparative examples are respectively greater than those of the examples. If the true stress is too large, it is easy to cause deformation of the steel during subsequent processing.

As shown in Figure 3-8, it is the morphology of prior austenite grains after carburizing at 980°C for 1h and 6h in the examples and comparative examples; finer austenite grains can be maintained in the examples of the present invention after carburizing at 980°C for 1h and 6h. The grain size of the tenite is not less than grade 6, and less than grade 7.0 at the same time. There is no local mixed crystal phenomenon, which solves the problem of local coarse grain; while comparative example 1 and comparative example 2 appear after heat preservation carburizing for 1h and 6h. Mixed crystal phenomenon, local coarse crystal problem occurs.

Finally, it should also be noted that the term "comprises", "comprises" or any other variation thereof is intended to cover a non-exclusive inclusion such that a process, method, article or apparatus comprising a set of elements includes not only those elements, but also Other elements not expressly listed, or inherent to the process, method, article, or apparatus are also included.

While preferred embodiments of the invention have been described, additional changes and modifications to these embodiments can be made by those skilled in the art once the basic inventive concept is appreciated. Therefore, it is intended that the appended claims be construed to cover the preferred embodiment as well as all changes and modifications which fall within the scope of the invention.

Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalent technologies, the present invention also intends to include these modifications and variations.

Claims (5)

1. The cold-forging gear steel is characterized by comprising the following chemical components in percentage by mass: c:0.15% -0.25%, si: 0.05-0.40%, mn:0.30% -1.40%, P:0 to 0.03%, S:0 to 0.035%, cr: 0.80-1.50%, ti:0 to 0.05%, al:0 to 0.045%, cu:0 to 0.15%, N:0 to 0.0150 percent, and the balance of Fe and inevitable impurities; the metallographic structure of the gear steel comprises austenite, and the grain size of the austenite is 6-7 grades;

the properties of the gear steel are as follows: the tensile strength is more than or equal to 1080MPa, the impact toughness is more than or equal to 60J, and the hardness of a hardenability J9 point is 30-40 HRC;

the metallographic structure further comprises ferrite and pearlite, the ferrite ratio being > 50%;

the preparation method of the cold forging gear steel comprises the following steps:

pouring and continuously casting the smelted molten steel in sequence to obtain a casting blank;

heating the casting blank, and then descaling to obtain a descaled casting blank;

sequentially rolling and cooling the descaled casting blank to obtain the gear steel;

the rolling comprises rough rolling and finish rolling, wherein the initial rolling temperature of the rough rolling is 1050-1100 ℃, the finish rolling temperature is 850-950 ℃, and the total deformation of a finishing mill set is 40-60%;

the casting temperature is 1500-1580 ℃; the heating temperature is 1150-1200 ℃, and the heating time is 2.5-3.5 h;

the cooling comprises a cooling bed inlet cover, a cooling bed outlet cover, a slow cooling pit inlet and a slow cooling pit outlet in sequence.

2. The cold forging gear steel of claim 1, wherein the smelted molten steel is poured and continuously cast to obtain a casting blank, and the method further comprises the following steps: smelting the molten steel containing the chemical components, and performing vacuum degassing in the smelting process to obtain smelted molten steel; the vacuum degassing time is 4-7 min.

3. The cold-forged gear steel as claimed in claim 1, wherein said smelting comprises an external vacuum refining, said external vacuum refining comprising in particular: and maintaining the pressure in a vacuum pressure environment with the pressure less than 67Pa for 10-12 min.

4. The cold forging gear steel of claim 3, wherein the out-of-furnace vacuum refining comprises deoxidizing and adjusting aluminum content of the molten steel, so that the molten steel is refined to a station, the aluminum content is 0.0450-0.070% in mass fraction, and the deoxidizer for deoxidation is an aluminum alloy.

5. The cold-forging gear steel according to claim 1, wherein the temperature of the entering slow cooling pit is 550-650 ℃, and the temperature of the exiting slow cooling pit is less than or equal to 300 ℃; the structure of the gear steel out of the slow cooling pit is ferrite and pearlite.

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