CN116356206A - Large-specification high-static-torque-strength steel for output gear, manufacturing method thereof and carburization method - Google Patents

Abstract

The invention provides steel for a large-specification high-static-torque-strength output gear, a manufacturing method thereof and a carburizing method, which comprise the following components: c:0.20-0.25%, si:0.15-0.30%, mn:1.00-1.10%, cr:1.08-1.18%, ni:1.00-1.10%, al:0.030-0.050%, P: less than or equal to 0.015 percent, B:0.0007% -0.0030%, S:0.020-0.035%, T.O.: less than or equal to 15ppm, [ N ]:80-120ppm, and the balance of Fe and unavoidable impurity elements. The end hardenability is 42-47HRC, J15:41-45HRC, J25:38-43HRC, rotational bending fatigue strength after carburization is more than or equal to 920MPa, torsional fatigue strength is more than or equal to 620MPa, and torque is more than or equal to 309 N.m.

Description

Large-specification high-static-torque-strength steel for output gear, manufacturing method thereof and carburization method

Technical Field

The invention belongs to the field of gear steel, and relates to steel for a large-specification high-static-torque-strength output gear with a diameter of more than 90mm, a manufacturing method and a carburizing method thereof, which are suitable for manufacturing a high-static-torque-strength automobile variable-speed output gear.

Background

With the proposal and landing of a double-carbon target, the weight reduction and the high efficiency of an automobile become an important development direction of the automobile industry, and a differential gear is used as a power transmission device of the automobile and has higher requirements on the torque. The traditional gear steel material greatly increases the material cost by adding micro-alloying elements Nb, V and the like or alloying elements such as Ni and the like into the steel, and severely restricts the application of the high-torque automobile gear steel to popularization.

Hardenability is a key indicator for improving torsional strength of materials. The new energy gear steel material commonly used in the current market is 17CrNiMo6 gear steel, and the hardenability control range J9 is: 37-47 HRC, J15: 34-46 HRC, J25: 31-43 HRC. However, the material contains about 1.0wt% of Ni, and the cost is more than 40% of the total alloy cost, so that it is necessary to develop a gear material with low cost, large specification and high hardenability so as to achieve the requirement of the automobile industry for low-cost high-output torque high-performance gear steel.

The invention discloses ultra-pure high-temperature fine grain gear steel, a manufacturing method and application thereof, wherein the invention can ensure that the grain size is always above 6 grade after high-temperature treatment at 960 ℃ by controlling reasonable proportion of Al, B and N elements, but the hardenability is not strictly required, so that the requirements of low-cost high-output torque high-performance gear steel cannot be met.

The invention discloses a manufacturing method of steel for gears of electric automobile speed reducers, which is characterized in that the quenching degree of materials is effectively improved in a higher range by improving elements such as C, mn, cr and the like, and the quenching degree can reach J9:37-49HRC, J15:31-43hrc, j25:26-38HRC. Although the hardenability is further improved, the requirements of low-cost high-output-torque high-performance gear steel still cannot be met.

The invention discloses a high-hardenability Mn-Cr series carburizing steel, which is disclosed in Chinese patent with publication number of CN107604253A and publication date of 2018, 1 month and 19 days, and the hardenability of the material is effectively improved by improving elements such as C, mn, cr and the like in a higher range, and the hardenability can reach J9:40-46HRC, J15:34-40HRC, J25:30.5-33.5HRC; and the content control of Al and N ensures that austenite grains do not grow obviously in the carburizing process. Although the hardenability is further improved, the requirements of low-cost high-output-torque high-performance gear steel still cannot be met.

At present, the CrNiMo gear steel has good toughness, and meanwhile, certain technical accumulation is provided for the development of the CrNiMo gear steel, but the requirements of improving hardenability, having high output torque, and having performance equivalent to 20Cr2Ni4 and greatly reducing cost are still not met. In addition, passenger cars and commercial vehicles are developing in the directions of light weight and large instantaneous acceleration, so that higher demands are put forward on the static torque strength of gear steel, and the influence on the static torque strength is less studied at present, so that development of new technical means for improving hardenability, toughness, high output torque and low-cost high-performance gear steel is urgently needed, and the development of the high-performance gear steel and the automobile industry are continuous. In addition, for gear steel bars with a large specification of more than 90mm in diameter, the problem of banded structure generally exists, and once the gear steel bars are formed, the banded structure level of the gear steel is generally required to be not higher than 3.0 level, and is required to be not higher than 2.0 level when the gear steel bars are formed, so that the processing and the service performance of the gear steel are ensured.

Disclosure of Invention

The invention aims to provide a large-size high static torque strength steel for output gears and a manufacturing method thereof, wherein the end hardenability of the obtained steel for gears is J9:42-47HRC, J15:41-45HRC, J25:38-43HRC, and the band structure grade of the gear steel is controlled below 2.0 grade, no noble alloy is added, and the cost is low.

The invention also provides a carburization method of the steel for the large-specification high-static-torque-strength output gear, which is controlled by a carburization process, wherein the rotational bending fatigue strength of the steel for the gear is more than or equal to 920MPa, the torsional fatigue strength is more than or equal to 620MPa, and the torque is more than or equal to 309 N.m after carburization treatment of the steel for the large-specification high-static-torque-strength output gear.

The specific technical scheme of the invention is as follows:

the steel for the large-specification high-static-torque-strength output gear comprises the following components in percentage by mass:

c:0.20-0.25%, si:0.15-0.30%, mn:1.00-1.10%, cr:1.08-1.18%, ni:1.00-1.10%, al:0.030-0.050%, P: less than or equal to 0.015 percent, B:0.0007% -0.0030%, S:0.020-0.035%, T.O.: less than or equal to 15ppm, [ N ]:80-120ppm, and the balance of Fe and unavoidable impurity elements.

The components of the steel for the large-specification high-static-torque-strength output gear also meet the following conditions: alf=Al-1.52 x [ N ] is more than or equal to 0.01% and less than or equal to 0.030%, al/N is more than or equal to 2.5 and less than or equal to 5, wherein Al/N is more than or equal to 2.5 and less than or equal to 5 ensures that the second phase of austenitic AlN can be separated out by a sufficient amount to ensure fine and uniform grain size, and excessive Al easily causes uneven AlN distribution and mixed crystal; the free Alf can greatly improve the hardenability, but excessively high but reduces the hardenability, wherein the coefficient of [ N ] is subjected to fitting revision again, the critical value of the free Alf for improving the hardenability is 0.01% -0.030%, the too low effect is not great, and the too high effect reduces the hardenability.

The end hardenability of the steel for the large-specification high-static-torque-strength output gear is J9:42-47HRC, J15:41-45HRC, J25:38-43HRC.

The diameter of the large-specification high-static-torque-strength output gear is more than 90 mm.

The invention provides a manufacturing method of steel for a large-specification high-static-torque-strength output gear, which comprises the following process flows:

smelting, LF refining, RH vacuum treatment, continuous casting, heating, rolling and slow cooling;

the smelting comprises the following steps: smelting by adopting an electric arc furnace/converter;

the LF refining is carried out, the steel is fully deoxidized in the refining process, the oxygen content is kept below 12ppm, and Al is regulated by adding an aluminum wire, so that the steel meets the control of free aluminum.

The heating: the higher the heating temperature and the longer the time, the more uniform the composition, but the more serious the oxidation. The optimal heating temperature for rolling the casting blank of the large gear steel bar is preferably 1200-1250 ℃. The soaking temperature of the billet in the heating furnace is controlled to be 1200-1250 ℃, and the total time of preheating, heating and soaking is controlled to be 5.0-10.0 h.

The rolling comprises the following steps: the initial rolling temperature is 1130-1180 ℃ and the final rolling temperature is 780-820 ℃.

The slow cooling: after rolling, the steel is quickly moved into a slow cooling pit by a cooling bed, the pit entering temperature is 600-650 ℃, and the slow cooling time is more than or equal to 24 hours.

Grinding and peeling are carried out after the slow cooling is carried out, so that no decarburization and zero defect on the surface are ensured.

The invention provides a carburization method of steel for a large-specification high-static-torque-strength output gear, which comprises the following steps of:

1) Normalizing the gear material, and preserving heat;

2) Then carburizing treatment is carried out;

3) Reducing the temperature after carburization and preserving heat;

4) Then carrying out oil cooling quenching;

5) And (5) tempering at low temperature.

Normalizing in step 1), normalizing temperature: 910-930 ℃ and keeping the temperature for 1+/-0.5 h;

the carburizing treatment in step 2): carburizing at 910-930 ℃ for 6-8h;

the temperature is reduced to 800-880 ℃ in the step 3), and the temperature is kept for 30-40min;

in the step 4), quenching oil is cooled to room temperature;

in the step 5), low-temperature tempering is carried out at 160-180 ℃ for not less than 2 hours.

After nitriding treatment, the grain size of the gear is 8.5 grade or more, the rotational bending fatigue strength is more than or equal to 920MPa, the torsional fatigue strength is more than or equal to 620MPa, and the torque is more than or equal to 309 N.m; the surface hardness is more than or equal to 715HV.

The design idea of the invention is as follows: the quenching oil temperature, i.e., the temperature range, is expressed herein without the need to express time

C: c is the most effective and economic strengthening element in steel, is the most effective element affecting hardenability, and in order to ensure that the gear steel has enough strength and enough hardenability, the C content needs to be improved, and the improvement of the C content is helpful for improving the hardness of a core matrix, so that the carbon content in martensite laths in the matrix is increased, and the improvement of the carbon content of the core is helpful for improving the content of residual austenite of the core, the residual austenite can increase the coordinated deformability of the matrix, so that the coordinated deformation is performed under the action of high torque, cracks are not easy to initiate, the improvement of the hardness of martensite is far greater than the reduction of the hardness caused by the improvement of the residual austenite, the improvement of the hardness of the core can increase the static torsion strength of the alloy, so that the carbon content is not lower than 0.20, but the excessively high carbon content has great damage to toughness, and the carbon content is determined to be 0.20-0.25%.

Si: si is a deoxidizer, meanwhile, the strength and the hardness of steel are improved through solid solution strengthening, the hardenability of gear steel can be improved, the content of Si cannot be lower than 0.15%, excessive silicon increases the activity of C, and the decarburization and graphitization tendency of the steel in the rolling and heat treatment processes are promoted, so that a carburized layer is easy to oxidize, and the content of Si is controlled to be 0.15% -0.30%.

Mn: mn can enlarge an austenite phase region, stabilize an austenite structure and improve the hardenability of steel, but Mn is soluble in ferrite, so that the hardness and strength of ferrite and austenite in the steel are improved, and meanwhile, mn can improve the stability of the austenite structure and remarkably improve the hardenability of the steel. However, excessive Mn reduces the plasticity of the steel, and the toughness of the steel deteriorates upon hot rolling. The Mn content is controlled to be 1.00-1.10%.

Cr: is an element that enhances hardenability of steel and contributes to strength enhancement. Under the condition of lower C content, a proper amount of Cr is added, so that the steel can be ensured to reach the required hardenability and strength. Therefore, the content of the modified starch is controlled to be 1.08-1.18%.

Ni: ni can effectively improve the core toughness of steel, reduce ductile-brittle transition temperature, improve low-temperature impact performance, has the effect of improving the fatigue strength of steel materials, has another effect of improving the stacking fault energy, improving dislocation crossing potential barrier, improving anti-torsion performance, has higher Ni cost, and has the excessively high Ni content which can reduce machinability after hot working. Therefore, the Ni content is controlled within 1.00-1.10%.

B: the segregation to the grain boundary improves the grain boundary strength and the high-temperature plasticity and hardenability; below 0.0007%, the above effect is insignificant, and above 0.0030%, the increase in effect is insignificant and ferrite may be generated, affecting the performance thereof. In addition, the toughness of the carburizing steel carburized layer can be improved, and the crack expansion resistance of the material can be improved. For high torque output gears, wear resistance in teeth and bending resistance of tooth roots are improved, and tooth surface wear and gear breakage are avoided. Therefore, the B content should be controlled to be 0.0007% -0.0030%.

Al: al is an effective deoxidizer, and forms AlN refined grains, and when the Al content is less than 0.030%, the effect is insignificant, and when the Al content is more than 0.040%, coarse inclusions are easily formed, and the performance of the steel is deteriorated. Therefore, the adding time of Al is required to be adjusted in the steelmaking process, and the content of Al is ensured to be controlled to be 0.030-0.050%.

P and S: sulfur is easy to form MnS inclusion with manganese in steel, so that the steel is hot and brittle, but a small amount of S is added, the cutting performance of gear steel can be obviously improved while the product performance is not influenced, and MnS has the effect of grain refinement; p is an element with strong segregation tendency, increases the cold brittleness of steel, reduces plasticity, and is harmful to uniformity of product structure and performance. Control P: less than or equal to 0.015 percent, S:0.020-0.035%.

T.O: T.O forms oxide inclusion in steel, and T.O is controlled to be less than or equal to 15ppm.

[ N ]: can form compound with Nb, B, al, etc. to refine crystal grains, and reasonable Al/[ N ] has obvious effect on grain refinement, while too high [ N ] can form continuous casting defects such as bubbles, etc. Al/[ N ] is less than or equal to 2.5 and less than or equal to 5, thus ensuring that the crystal grains are fine and uniform and ensuring that a certain proportion of free Alf exists. Therefore, the [ N ] content should be controlled to 80-120ppm.

Alf=al-1.52× [ N ] is the free aluminum content in steel, and can function to improve the hardenability of the material when the free aluminum content is higher than 0.010%, but when the free aluminum content exceeds 0.030%, the hardenability improvement effect on steel is not obvious, and therefore, should be controlled to 0.01% -0.030%.

The hardenability of steel mainly depends on the stability of supercooled austenite, and the smaller the critical cooling speed of steel is, the larger the hardenability is. Factors influencing supercooled austenite stability mainly include chemical composition of steel, austenite uniformity, austenite grain size, austenitization state, and the like. It has been found that alloying elements such as Cr, ni, mo, etc. can increase the hardenability of the material, decrease the content of residual elements in embrittled grain boundaries such as P, sn, etc., and repeated quenching to refine the crystal grains also contributes to improving the hardenability of the material. However, since resources are scarce and expensive, it is desirable to avoid the use of alloying elements such as Mo; the need for strict screening of scrap steel is required in pursuit of extreme reductions such as P, sn; further, although repeated quenching is an effective method for refining crystalline grains, it is necessary to increase the heat treatment time and add equipment. Al is generally added to serve as a deoxidizer, and AlN has a grain boundary pinning effect, and can also be added to prevent coarsening of grains. Previous studies have also shown that free Al in austenite can retard the transformation of austenite to ferrite for reasons believed to be related to the distribution of Al near the ferrite-austenite transformation interface. However, few reports have been made about improvement of hardenability by Al. Free aluminum can play a role in improving hardenability to a certain extent, but is required to be matched in steel making and rolling for application.

The invention relates to a large-specification output gear, the control difficulty of the large-specification output gear with the size diameter of more than 90mm is the problem of large level of a banded structure, and the banded structure is lightened by a means of reducing the segregation degree of alloy elements through a rolling process. The invention adopts reasonable rolling technological measures, and the rolling technological measures mainly comprise the heating process, the final rolling temperature and the cooling process measures after rolling of a casting blank. The higher the heating temperature and the longer the time, the more uniform the composition, but the more serious the oxidation. The optimal heating temperature for rolling the casting blank of the large gear steel bar is preferably 1200-1250 ℃. In addition, the large-specification gear steel has higher requirements on hardenability, so the invention adds the alloy element B to improve the hardenability.

Compared with the prior art, the invention mainly improves the carbon content, reduces the Ni content with high cost, does not add noble metals Nb and Mo, can improve the core hardness, improves the core hardness by 1-2 HRC, can obviously improve the torque, obtains the torque and torsional fatigue close to those of the comparative steel on the basis, greatly reduces the cost, ensures certain Ni content and does not reduce the toughness; in addition, the free aluminum can play a role in improving the hardenability to a certain extent, so that the overall hardenability is improved; alloy cost is reduced, hardenability is improved, core hardness is improved, and static torsion strength is improved. In addition, compared with the secondary quenching process of other patents, the invention reduces the heat treatment cost by adopting the direct quenching process after carburization.

The end hardenability of the gear steel of the large-specification high-static-torque-strength output gear steel produced by adopting the components and the reasonable manufacturing method is 42-47HRC, J15:41-45HRC, J25:38-43HRC, the rotational bending fatigue strength of the gear steel after carburization treatment is more than or equal to 920MPa, the torsional fatigue strength is more than or equal to 620MPa, and the torque is more than or equal to 309 N.m. The invention provides a method for controlling the banded structure of the large-specification gear steel bar material, which is suitable for more than 90mm, by adopting a means of combining component control (reducing segregation alloy elements) and rolling process control, so that the banded structure is controlled within 2 levels.

Drawings

FIG. 1 is a metallographic photograph of a gear steel structure after carburization in example 1;

FIG. 2 is a metallographic photograph of the gear steel structure after carburization in example 2;

FIG. 3 is a metallographic photograph of the gear steel structure after carburization in example 3;

FIG. 4 is a metallographic view of the structure of the gear steel after carburization of comparative example 1;

FIG. 5 is a metallographic photograph of a gear steel structure after carburization of comparative example 2.

Detailed Description

Example 1 to example 5

The steel for the large-specification high-static-torque-strength output gear comprises the following components in percentage by mass:

c:0.20-0.25%, si:0.15-0.30%, mn:1.00-1.10%, cr:1.08-1.18%, ni:1.00-1.10%, al:0.030-0.050%, P: less than or equal to 0.015 percent, B:0.0007% -0.0030%, S:0.020-0.035%, T.O.: less than or equal to 15ppm, [ N ]:80-120ppm, alf=Al-1.52 x [ N ] 0.030%, al/N2.5 x 5 x 0.30, fe and unavoidable impurity elements, and 5 furnace steel is produced, and the steel composition of each example is shown in Table 1, the balance being Fe and unavoidable impurities, which are not shown in Table 1.

Comparative example 1-comparative example 2

The steel for the gear comprises the following components in percentage by mass: as shown in table 1, the balance not shown in table 1 is Fe and unavoidable impurities. Comparative examples 1 and 2 are 2-furnace 18CrNiMo7-6 steels produced as a comparative steel according to the requirements in EN 10084.

TABLE 1 chemical compositions (unit: N in ppm and others in wt%) of examples and comparative examples of the present invention

Examples 1-5 and comparative examples 1-2 were produced using an electric arc furnace smelting-refining-vacuum-continuous casting-heating-rolling-slow cooling- (finishing) process flow. The continuous casting blank is heated and kept at 1200-1250 ℃ for more than or equal to 5 hours, the total time of preheating, heating and soaking is controlled to be 5.0-10.0 hours, round steel rolling is carried out, and the starting rolling temperature is: 1130-1180 ℃, the finishing temperature is 780-820 ℃, and after rolling, the materials are cooled to 600-650 ℃ by a cooling bed, put into a pit and are slowly cooled, and the slow cooling time is 48 hours.

Comparative examples 3, 4 and 5 used the above-described components of examples 3, 4 and 5, respectively, but did not use the above-described rolling processes of examples 1 to 5 and comparative examples 1 to 2, namely, comparative example 3 used the component of example 3, comparative example 4 used the component of example 4, and comparative example 5 used the component of example 5, except that the rolling processes of comparative examples 3, 4 and 5 were different from those of examples and comparative examples, and the rolling processes of comparative examples 3, 4 and 5 were: heating and preserving heat of the continuous casting blank at 1130-1200 ℃ for more than or equal to 5 hours, controlling the total time of preheating, heating and soaking for 5.0-10.0 hours, rolling round steel, and starting rolling temperature: and (3) cooling the rolled steel to 600-650 ℃ by a cooling bed at 1030-1080 ℃ and a final rolling temperature of 680-780 ℃ and slowly cooling the rolled steel in a pit for 48 hours. The production process parameters and corresponding band tissue grades for examples 1-5 and comparative examples 1-5 are shown in Table 2, from which it can be seen that examples 1-5 each control band tissue grade to grade 2 and below.

TABLE 2 Steel Rolling production Process parameters for examples and comparative examples of the present invention

Table 3 shows the end hardenability values of the examples and comparative examples of the present invention, and it can be seen from Table 3 that the gear steel hardenability control J9, J15 and J25 values of examples 1 to 5 of the present invention are all within the range required for the steel for the output gear of the high torque automobile transmission, and the hardenability is equivalent to that of the comparative example, but the cost is lower.

TABLE 3 terminal hardenability values (HRC) for examples and comparative examples of the present invention

Examples	J9	J15	J25
				Requirements for	42～47	41～45	38～43
Example 1	43.2	42.3	40.3
				Example 2	43.4	42.8	40.5
Example 3	43.5	42.7	41.3
				Example 4	43.6	42.9	41.5
Example 5	43.9	43.0	41.6
				Comparative example 1	43.6	42.5	40.1
Comparative example 2	43.0	42.9	40.6

Each example and comparative example gear steel was subjected to a carburizing process (examples 1, 2, 3, 4, 5 were sampled on bars of 120mm diameter gauge, comparative examples 1, 2 were sampled on bars of 50mm diameter gauge): the gear material is subjected to normalizing treatment at 910-930 ℃ for 1+/-0.5 h, then subjected to carburizing treatment at 910-930 ℃ for 6-8h, the temperature after carburizing is reduced to 840-880 ℃ for heat preservation, oil quenching is carried out after heat preservation for 30-40min, the quenching oil temperature is 120-200 ℃, and low-temperature tempering is carried out at 160-180 ℃ after quenching, wherein the tempering time is not less than 2h. Specific parameters of carburization treatment after carburization treatment are shown in table 4, and the spin-bending fatigue strength, torsional fatigue strength and static torsion strength of each example and comparative example are also shown in table 5.

TABLE 4 carburizing process of examples and comparative examples of the present invention

TABLE 5 grain size after carburization, fatigue strength and static torsion strength of examples and comparative examples of the present invention

The formula of the invention improves the core hardness of the gear steel by improving the carbon content and the microalloying of B, further improves the static torsion strength of the gear steel, and reasonable Al/[ N ] has obvious effect on grain refinement.

Claims (9)

1. The steel for the large-specification high-static-torque-strength output gear is characterized by comprising the following components in percentage by mass:

C：0.20-0.25％，Si：0.15-0.30％，Mn：1.00-1.10％，Cr：1.08-1.18％，Ni：1.00-1.10％，Al：0.030-0.050％，P：≤0.015％，B：0.0007％-0.0030％，S：

0.020-0.035%, T.O.: less than or equal to 15ppm, [ N ]:80-120ppm, and the balance of Fe and unavoidable impurity elements.

2. The steel for large-format high static torque strength output gears according to claim 1, wherein the composition of the steel for large-format high static torque strength output gears satisfies: alf=Al-1.52 x [ N ] is less than or equal to 0.01% and less than or equal to 0.030%.

3. The steel for large-format high static torque strength output gears according to claim 1 or 2, wherein the composition of the steel for large-format high static torque strength output gears satisfies: al/N is more than or equal to 2.5 and less than or equal to 5.

4. The steel for large-format high static torque strength output gears according to claim 1, wherein the steel for large-format high static torque strength output gears has a terminal hardenability of J9:42-47HRC, J15:41-45HRC, J25:38-43HRC.

5. A method for manufacturing a steel for a large-format high static torque strength output gear according to any one of claims 1 to 4, characterized in that the method comprises heating, specifically: the soaking temperature of the billet in the heating furnace is controlled to be 1200-1250 ℃, and the total time of preheating, heating and soaking is controlled to be 5.0-10.0 h.

6. The method for manufacturing a steel for a large-format high static torque strength output gear according to claim 5, wherein the manufacturing method comprises the steps of: the initial rolling temperature is 1130-1180 ℃ and the final rolling temperature is 780-820 ℃.

7. The method for producing a steel for a large-format high static torque strength output gear according to claim 5 or 6, wherein the slow cooling: cooling to 600-650 ℃ by a cooling bed, and slowly cooling for more than or equal to 24 hours.

8. A carburization method of the steel for large-sized high static torque strength output gear according to any one of claims 1 to 4, characterized by comprising the steps of:

1) Normalizing the gear material, and preserving heat;

2) Then carburizing treatment is carried out;

3) Reducing the temperature after carburization and preserving heat;

4) Then carrying out oil cooling quenching;

5) And (5) tempering at low temperature.

9. The carburizing process of claim 8, wherein after nitriding treatment, the gear grain size is 8.5 grade or more, the rotational bending fatigue strength is equal to or more than 920MPa, the torsional fatigue strength is equal to or more than 620MPa, and the torque is equal to or more than

309n·m; the surface hardness is more than or equal to 715HV.

Images