CN111286671B

CN111286671B - Ultra-pure high-temperature fine-grain gear steel, manufacturing method and application thereof

Info

Publication number: CN111286671B
Application number: CN202010128336.1A
Authority: CN
Inventors: 曹红福; 黄镇; 白云; 惠世杰; 李国忠; 许晓红; 吴小林; 狄梦龙; 卢明霞; 孙鸿平; 孟羽; 廖书全; 芦莎; 张丽娜; 高亮; 徐光琴; 赵福来
Original assignee: Jiangyin Xingcheng Special Steel Works Co Ltd
Current assignee: Jiangyin Xingcheng Special Steel Works Co Ltd
Priority date: 2020-02-28
Filing date: 2020-02-28
Publication date: 2021-06-29
Anticipated expiration: 2040-02-28
Also published as: CN111286671A

Abstract

The invention relates to an ultra-pure high-temperature fine-grain gear steel and a manufacturing method thereof, wherein the chemical components comprise: 0.15 to 0.21%, Si: less than or equal to 0.12 percent, Mn: 1.00-1.30%, Cr: 1.00-1.30%, S: 0.010-0.025%, P is less than or equal to 0.025%, Ni: 0.70-1.00%, Mo: 0.02-0.10%, B: 0.0020-0.0040%, Cu is less than or equal to 0.20%, Al is less than or equal to 0.05%, Ca is less than or equal to 0.0005%, Ti is less than or equal to 0.003%, N: 0.0080-0.016%, wherein N is (0.80-1.0) × (0.5% Al +0.7% B), the balance Fe and inevitable impurities; the characteristic value of the oxide in the steel reaches K1 not more than 10, Al, B and N added in proportion in the steel can be fully combined, and a large amount of fine particles are produced in a matrix, so that the grain size of the matrix is still above grade 6 after high-temperature carburization treatment at the temperature of 960 ℃.

Description

Ultra-pure high-temperature fine-grain gear steel, manufacturing method and application thereof

Technical Field

The invention belongs to the technical field of smelting of iron-based special steel, and particularly relates to gear steel and application thereof as a gear processing material of an electric automobile speed reducer.

Background

The speed reducer of the electric automobile is a precise mechanical component and has the function of reducing the rotating speed of a motor rotating at a high speed through the speed reducer and increasing torque, so that the high-speed running of the automobile is realized. The gear used by the speed reducer is subjected to great impact force due to the large moment transmitted by the high-speed motor, so that the gear material is required to have higher fatigue strength, and the influence of the purity on the fatigue life is obvious, so that the electric automobile has higher requirement on the purity of the gear steel, and the impact of the high motor rotating speed on the gear is met.

Due to the consideration of environmental protection, energy conservation, improvement of production efficiency, cost and other factors, the high-temperature low-pressure carburization technology of gears is being widely adopted. The high-temperature low-pressure carburizing technology is to perform carburizing treatment on the gear at the heating temperature of more than 960 ℃ in vacuum, and can shorten the carburizing time by more than 50 percent, thereby greatly improving the production efficiency and saving the mass production cost.

The high-temperature low-pressure carburization technology requires that the crystal grains of the gear material can not be coarsened at the heating temperature of more than 960 ℃, namely the gear material still keeps the state of fine crystal grains, so that higher requirements are put forward on the production technology and the component design of the gear steel. In order to obtain fine crystal grains, it is generally used to add Al and N elements to steel, and pin grain boundaries by AlN precipitated in the steel to prevent the grains from growing, but AlN particles in the steel start to be dissolved at a temperature of 930 ℃ or higher, and when the temperature reaches 960 ℃, substantially all of the AlN particles are dissolved in the steel to lose the effect of refining the grains, so that it is necessary to add elements such as Ti, Nb, and V, and to refine the grains by the composite action of the elements. Ti added into the steel has a large inhibiting effect on the coarsening of crystal grains, but large-grained TiN inclusions are easy to produce, and the fatigue performance of the steel is deteriorated. The principle of V grain refinement is to produce VC or V (C, N), but the precipitates have a low solid solution temperature and are solid-dissolved in the steel at high temperatures, thereby losing the effect of grain refinement.

The patent CN103924030A discloses a production method of ultra-pure steel, which adopts a vacuum electric furnace or a vacuum induction furnace for smelting and vacuum casting, when the oxygen content is below 10ppm, 0.5-3 kg/t of Si-Ca-Al-Mg-RE composite deoxidizer is added, and after the addition, the mixture is kept stand for 1-10 min for vacuum casting, so that pure steel with the total oxygen content below 4ppm can be obtained. Meanwhile, the rare earth composite additive is adopted to modify the inclusions in the steel, so that the size of the inclusions is reduced, the number of the inclusions in the steel is reduced, and a new effective way is provided for the purification smelting of molten steel. The steel produced by the method is difficult to produce in batches due to the adoption of high-requirement equipment and technology, and is high in production cost and difficult to popularize.

Patent CN102560255A discloses a steel for high-temperature vacuum carburized gear, which comprises the following components by weight percent: 0.10 to 0.30%, Si: 0.15 to 0.25%, Mn: 0.60-0.90%, P is less than or equal to 0.025, S: 0.010-0.020%, Cr: 0.85-1.25%, Al: 0.033-0.055%, N: 0.0160-0.0300%, Ti: 0.001-0.009%, [ O ] less than or equal to 0.0020%, and the balance Fe and inevitable impurities. The microalloying mode is adopted to control the abnormal growth of austenite grains in the high-temperature vacuum carburization process of the gear steel, so that the grain size of the steel can be controlled to be 7.0-8.0 grade. The steel for the gear adopts higher content of N, Al and Ti to pin structure refined grains, the excessively high content of Ti is not beneficial to the surface quality and the purity of steel, and the precipitate of AlN is not high-temperature resistant and is dissolved in a matrix during high-temperature carburization.

Disclosure of Invention

In order to meet the use requirements of a core component gear of an electric automobile speed reducer, the invention develops the ultra-pure high-temperature fine-grain gear steel which can still keep finer grain size (more than 6 grades) after carburizing heat treatment at the temperature of more than 960 ℃.

The method is different from the common oxide characteristic value of the prior gear steel, which requires that K4 or K3 is less than or equal to 30, improves the purity of a steel matrix through the adjustment of molten steel smelting, and grades the oxide characteristic value in the steel matrix according to DIN50602K to reach K1 which is less than or equal to 10.

Different from the design idea that the common gear steel adopts one or more of Al and/or Ti, V and Nb to be matched with refined grains, the invention adopts the composite action of Al and B precipitates, so that the grain size of the matrix after high-temperature carburization is kept fine, and the effect of refining the grains is better than that of Al and/or Ti, V and Nb. B is a method for strengthening the strength of tooth root, which is different from the effect of improving the hardenability of steel in general, by producing BN in the steel to refine crystal grains and simultaneously improve the bending punching performance of the gear tooth steel.

The gear steel comprises the following chemical components in percentage by weight: 0.15 to 0.21%, Si: less than or equal to 0.12 percent, Mn: 1.00-1.30%, Cr: 1.00-1.30%, S: 0.010-0.025%, P is less than or equal to 0.025%, Ni: 0.70-1.00%, Mo: 0.02-0.10%, B: 0.0020-0.0040%, Cu is less than or equal to 0.20%, Al is less than or equal to 0.05%, Ca is less than or equal to 0.0005%, Ti is less than or equal to 0.003%, N: 0.0080-0.016%, wherein N is (0.80-1.0) × (0.5% Al +0.7% B), the balance Fe and inevitable impurities.

Preferably, the gear steel further comprises Nb: 0.01 to 0.06 percent.

The main functions and design basis corresponding to each element of the chemical components of the steel for the gear are as follows:

c is the most basic element in steel and is also the most economic strengthening element. In order to ensure that the center part of the carburized and quenched and tempered gear has good strength and toughness, the carbon content range of the steel is determined by considering the factors that some gears need to be welded and the like.

Si is a basic solid-solution strengthening element as a deoxidizing element to improve hardenability, but Si is oxidized in a carburized layer and affects the retained austenite amount during steel carburization, and therefore the Si content is determined to be 0.12% or less.

Mn plays a role in solid solution strengthening on steel, and the steel content range is determined by utilizing the characteristics that Mn strongly improves the hardenability of the steel and has low cost.

Cr plays a role in solid solution strengthening on steel, improves the hardenability of the steel by utilizing Cr and has the characteristic of low cost to determine the content range of the steel.

The function of Ni in steel is to improve hardenability, and the Ni is matched with Cr and Mo to ensure proper carbon concentration of a carburized layer, so that the toughness of the carburized layer is improved. The other function of Ni is to improve the impact toughness of the steel and finally improve the fatigue strength of the material, but the high price of Ni increases the cost of the steel, and the controlled content of Ni in the steel is determined on the premise of ensuring the performance of the steel.

Mo is a carbide forming element, can improve the hardenability of steel, refine crystal grains and improve toughness, has high Mo cost, and is determined to be 0.02-0.10% as a supplementary element for improving the hardenability in consideration of the fact that the impact toughness of the steel can be ensured due to the high Ni content in the invention.

B does not play a role in improving hardenability in the steel of the present invention, but precipitates fine grains at grain boundaries as BN to improve the impact toughness of the steel.

The addition of S can improve the cutting performance of steel, but can cause the steel to generate hot brittleness and reduce the ductility and toughness of the steel, so the content range of S is determined to be 0.010-0.025 percent by combining the factors.

Besides serving as deoxidation in steel, Al and N form dispersed fine aluminum nitride particles to mainly refine grains, and proper Al content is matched with N in the steel for use.

Ti exists in the steel in the form of titanium carbonitride inclusions, affects the fatigue life of the gears, and is strictly controlled at a low level.

The high Ca content increases the amount and size of the spot oxides in the steel, and deteriorates the properties of the steel, and should be strictly controlled at a low level.

N is combined with elements such as Al, B, Ti, Nb, V and the like and precipitates to form corresponding precipitates for refining crystal grains, the content of the precipitates is determined according to the content of the elements such as Al, B, Ti, Nb, V and the like in steel, and Ti, Nb and V exist as residual elements in the application, wherein the content of Ti is strictly controlled because hard particles are formed to influence the fatigue resistance of gears. In the application, only the relative content control of Al, B and N which are positive effects needs to be considered, and the content of N is controlled according to (0.80-1.0) × (0.5% Al +0.7% B) according to the combination characteristics.

The manufacturing method of the ultra-pure high-temperature fine-grain gear steel mainly comprises the steps of primarily smelting molten steel by an electric furnace or a converter, refining outside the furnace and carrying out vacuum degassing treatment, pouring the molten steel into a continuous casting billet, heating and then rolling to obtain a finished product;

the primary smelting: charging the raw materials and molten iron into an electric furnace or a converter, electrifying and assisting with oxygen blowing for fluxing, controlling the content of C at the initial smelting end point to be more than 0.05 percent, controlling the temperature of the molten steel to be more than 1650 ℃, stopping slag and tapping, and carrying out strong deoxidation and partial alloying in the tapping process;

refining: hoisting a primary molten steel ladle to an LF refining furnace by a crane, switching on a bottom argon blowing device, transmitting electricity for heating, adding lime after the LF furnace transmits electricity for 10 minutes, strengthening the forced deoxidation of molten steel by an Al wire feeding method, adding SiC powder and aluminum powder to the surface of refined slag for diffusion deoxidation, controlling the quaternary alkalinity B of the slag to be 1.15-1.35, controlling MgO in the slag to be less than or equal to 5.0% for ensuring the fluidity of the slag, simultaneously ensuring that [ S ] in the steel is more than 0.015% and not more than 0.025% at most, adding more than 0.005% according to the upper limit of a target value, refining for more than 30 minutes, and completely adjusting all alloy elements to the target value before tapping;

vacuum degassing: hanging the refined steel ladle into a VD tank, connecting an Ar gas pipe, covering a vacuum cover, vacuumizing, and vacuum degassing; after breaking the hollow, feeding nitrogen lines according to the calculated ratio of the contents of Al and B in the molten steel, and carrying out soft argon blowing after VD degassing and tapping;

continuous casting: hoisting the ladle to a continuous casting ladle revolving platform, controlling the superheat degree of the tundish ladle at 15-30 ℃, and carrying out protective casting in the whole process to obtain a continuous casting billet;

rolling: reheating the continuous casting blank to 1150-plus-1280 ℃, rolling the continuous casting blank, descaling the continuous casting blank by high-pressure water, rolling the continuous casting blank at the initial rolling temperature of 1050-plus-1180 ℃ and the final rolling temperature of not less than 850 ℃, rolling the square blank into round steel, and collecting and slowly cooling the round steel in time after rolling.

Compared with the prior art, the invention is characterized in that:

the invention adopts the compatibility of Al and B as core elements for grain refinement, is easy to realize and has low cost. By controlling the reasonable proportion of Al, B and N elements and considering the unstable content of N element in molten steel, the content of N is necessary to be accurately controlled, the combination of experience and test can control the content of N in the range of (0.80-1.0) × (0.5% Al +0.7% B) to ensure that [ N ] is fully combined with [ Al ] and [ B ] in steel, B, N is precipitated at grain boundary when molten steel is solidified, namely continuous casting, Al and N are precipitated at the grain boundary in the heating process, AlN is precipitated at the grain boundary because the particles are very small (the size is in the range of 0.04-0.20 mm), the grain boundary is pinned before the steel is heated to the complete solid solution temperature to prevent the crystal grains from growing, a large amount of fine particles can be produced because Al, B and N added in proportion in the steel can be fully combined, the grain size can be ensured to be always above 6 grade after high-temperature treatment above 960 ℃, the gear steel can be completely suitable for low-pressure high-temperature carburization treatment in the later stage, the gear machining cost is saved, the requirements for energy conservation, environmental protection and efficiency improvement are met, and the gear steel has a good application prospect.

In the production process of the gear steel, the content of the steel tapping terminal point [ C ] of the electric furnace (primary smelting) is controlled to control the O content at a lower level at the primary smelting terminal point, and a deoxidizer is added in the steel tapping process, so that the fluidity of molten steel during steel tapping can be utilized to strengthen deoxidation, and the deoxidation burden during LF refining is reduced. The quaternary alkalinity is controlled to be 1.15-1.35 during LF refining, MgO in slag is controlled to be less than or equal to 50%, the purpose is to ensure good fluidity of the slag and improve the adsorption of the slag on impurities, particularly, the S content is controlled to be more than 0.015% during the refining process, and the N content in molten steel is stabilized, because the fluidity of the slag is improved, the active sites of the slag are increased, N is easy to float upwards as a light element and is adsorbed by the slag, so that the loss of the N element is caused, the experience of molten steel smelting is that the loss of the N can be inhibited by controlling the S element, the activity of the S is higher than that of the N, and the S occupies the active sites of the slag, so that the loss of the N can be reduced to a certain extent.

The soft argon blowing time after VD high vacuum treatment is more than 30 minutes, which is different from the traditional large slag amount and high alkalinity slag adopted by LF and the limitation on VD vacuum treatment time, the application can enable fine inclusions in steel to fully float and slag to be adsorbed, and eliminate the existence of large-particle inclusions, the characteristic value of oxides can reach K1 which is less than or equal to 10 according to DIN50602K method rating, and the process cost is reduced because the slag amount and alkalinity are moderate and the fluidity is good.

Drawings

FIG. 1 is a crystal phase diagram of a gear steel of the present invention, in which AlN and BN precipitated particles are clearly seen.

Detailed Description

The present invention is described in further detail below with reference to examples, which are intended to be illustrative and not to be construed as limiting the invention.

The specific production process of the gear steel comprises the following steps: 100t electric furnace-external refining-VD vacuum degassing-300X 340mm continuous casting billet-casting billet reheating-rolling-steel slow cooling-finishing and coping. The details are as follows

Electric furnace smelting (Primary smelting)

1) The pretreated molten iron in the raw materials fed into the furnace accounts for 30-60%, and the balance is selected high-quality scrap steel;

2) the key point control of the electric furnace smelting and tapping process is as follows: controlling the end point tapping carbon of the electric furnace, wherein the end point is not less than 0.05 percent and the end point is 0.015 percent; partially alloying in steel ladle, quantitatively adding synthetic slag and deoxidant for predeoxidizing, and retaining steel to prevent oxidizing slag.

Refining in LF furnace

1) Adding 200kg of lime after the LF furnace is powered on for 10 minutes, strengthening the forced deoxidation of the molten steel by using an Al wire feeding method, adding SiC powder and aluminum powder on the surface of the refined slag for diffusion deoxidation, and keeping good slag fluidity (quaternary alkalinity is controlled to be 1.15-1.35, and MgO is less than or equal to 5.0%);

2) refining and taking a 1 st chemical component sample, adjusting Cr, Ni, Mn and Mo to an internal control lower limit, analyzing a 2 nd sample, finely adjusting all components to enter a target value, [ S ] adding according to a standard upper limit +0.005%, and tapping for at least 5 minutes;

3) the LF refining time is controlled to be more than 40 minutes.

VD vacuum degassing and soft argon blowing

1) The holding time under high vacuum degree (133Pa) is more than 10 minutes;

2) sampling and analyzing after the blank is broken, feeding a nitrogen line according to the calculated ratio of the contents of Al and B, and adjusting the contents of other elements, wherein elements which do not make requirements in the steel cannot be added;

3) the soft argon blowing time after VD is discharged is more than or equal to 30 minutes.

Continuous Casting

1) Protecting the casting in the whole continuous casting process, and setting the electromagnetic stirring parameters of a crystallizer and a secondary cooling section;

2) controlling the superheat degree target of the continuous casting molten steel to be 15-30 ℃, allowing the superheat degree of the first furnace to be 10 ℃ higher than that of the continuous casting furnace, and controlling the pulling speed to be 0.5-0.85 m/min;

3) continuous casting blank is taken out of line at a high temperature of more than 550 ℃, and is put into a slow cooling pit for heat preservation for more than 24 hours.

Rolling of

1) Heating the continuous casting billet in a heating furnace with low oxidizing atmosphere, controlling the temperature of a preheating section at 800 ℃ plus 600 ℃, controlling the temperature of a heating section and a soaking section at 1280 ℃ plus 1150 ℃, and preserving heat for 4-6 hours;

2) the initial rolling temperature is 1050 plus 1180 ℃, the final rolling temperature is more than or equal to 900 ℃, and the square steel is rolled into the round steel.

3) The steel cooling process comprises the following steps: the steel is put into a pit and slowly cooled for more than 24 hours.

The chemical compositions (wt%) of the gear steels of the three examples according to the above preparation method are shown in Table 1.

TABLE 1

Examples	C	Si	Mn	P	S	Cr	Ni	Mo	B
										1	0.18	0.10	1.15	0.015	0.015	1.20	0.02	0.05	0.0026
2	0.20	0.08	1.13	0.020	0.013	1.18	0.02	0.03	0.0023
										3	0.18	0.08	1.16	0.012	0.018	1.22	0.02	0.05	0.0027

TABLE 1 (continuation)

Examples	Al	Cu	Nb	Ca	Ti	N	O
								1	0.033	0.04	-	0.0001	0.0020	0.014	0.0008
2	0.032	0.03	-	0.0001	0.0022	0.0013	0.0006
								3	0.035	0.04	0.021	0.0001	0.0015	0.0015	0.0009

(II) the results of the high-temperature grain size examination of the steels of the examples are shown in Table 2

TABLE 2

Quenching temperature	Rolled stock specification	Water quenching at 960 deg.C for 4h	Water quenching at 980 deg.C for 4h	Water quenching at 1000 deg.C for 4h
					Examples	mm	Grain size, grade	Grain size, grade	Grain size, grade
1	φ55	7.0	6.5	6-1
					2	φ70	7.0	6.0	6-1
3	φ110	8.0	7.5	7.5

The three embodiments relate to gear round steel with different specifications, and the grain size is still above grade 6 after the gear round steel is subjected to high-temperature treatment at 960 ℃, 980 ℃ and 1000 ℃. As is apparent from fig. 1, the gear steel matrix had uniformly dispersed precipitated particles, and mass spectrometry confirmed that these particles were AlN and BN particles.

Although preferred embodiments of the present invention have been described in detail hereinabove, it should be clearly understood that modifications and variations of the present invention are possible to those skilled in the art. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. The utility model provides an ultra-pure high temperature fine grain gear steel which characterized in that: the gear steel comprises the following chemical components in percentage by weight: 0.15 to 0.21%, Si: less than or equal to 0.12 percent, Mn: 1.00-1.30%, Cr: 1.00-1.30%, S: 0.010-0.025%, P is less than or equal to 0.025%, Ni: 0.70-1.00%, Mo: 0.02-0.10%, B: 0.0020-0.0040%, Cu is less than or equal to 0.20%, Al is less than or equal to 0.05%, Ca is less than or equal to 0.0005%, Ti is less than or equal to 0.003%, N: 0.0080-0.016%, and N = (0.80-1.0) × (0.5% Al +0.7% B), wherein the symbol of the element represents the content of the element, and the balance is Fe and inevitable impurities;

the characteristic value of oxides in a steel matrix is graded according to a DIN50602K method to reach K1 not more than 10, AlN and BN particles are precipitated in the steel matrix, the size of the AlN and BN particles is 0.02-0.20 mu m, and the grain size of the gear steel is still above grade 6 after high-temperature carburization treatment at the temperature of above 960 ℃.

2. The ultra-pure high temperature fine grain gear steel of claim 1, wherein: the gear steel further comprises Nb: 0.01 to 0.06 percent.

3. A method of producing the ultra-pure high temperature fine grain gear steel of claim 1 or 2, characterized by: primarily smelting molten steel by an electric furnace or a converter, refining outside the furnace and vacuum degassing treatment, casting the molten steel into a continuous casting billet, heating and rolling to obtain a finished product;

and (3) refining: hoisting a primary molten steel ladle to an LF refining furnace by a crane, switching on a bottom argon blowing device, transmitting electricity for heating, adding a slag former and a diffusion deoxidizer in the process, controlling the quaternary alkalinity B of the slag to be = 1.15-1.35, controlling MgO in the slag to be less than or equal to 5.0% for ensuring the fluidity of the slag, simultaneously ensuring that [ S ] in the steel is more than 0.015% and not more than 0.025% at most, adding more than 0.005% according to the upper limit of a target value, refining for more than 30 minutes, and completely adjusting all alloy elements to the target value before tapping;

and (3) pouring: hoisting the ladle to a continuous casting ladle revolving platform, controlling the superheat degree of the tundish ladle at 15-30 ℃, and carrying out protective casting in the whole process to obtain a continuous casting billet;

and (3) rolling: reheating the continuous casting blank to 1150-plus-1280 ℃, rolling the continuous casting blank, descaling the continuous casting blank by high-pressure water, rolling the continuous casting blank at the initial rolling temperature of 1050-plus-1180 ℃ and the final rolling temperature of not less than 850 ℃, rolling the square blank into round steel, and collecting and slowly cooling the round steel in time after rolling.

4. The method of producing an ultrapure high temperature fine grain gear steel according to claim 3 wherein: and (4) stopping slag and discharging from the furnace by adopting an EBT sliding plate mechanism at the initial smelting end point.

5. The method of producing an ultrapure high temperature fine grain gear steel according to claim 3 wherein: the duration of the vacuum degassing is not less than 10 minutes.

6. The method of producing an ultrapure high temperature fine grain gear steel according to claim 3 wherein: the casting adopts the technical measures of constant casting speed, automatic control of the liquid level of the crystallizer, electromagnetic stirring of the crystallizer and electromagnetic stirring of the tail end.

7. The method of producing an ultrapure high temperature fine grain gear steel according to claim 3 wherein: during refining, the LF is electrified for 10 minutes, then lime is added, forced deoxidation of molten steel is enhanced by an Al wire feeding method, and SiC powder and aluminum powder are added to the surface of refined slag for diffusion deoxidation.

8. Use of the ultra-pure high temperature fine grain gear steel according to claim 1 or 2, characterized in that: the gear steel is used as a gear processing material of an electric automobile speed reducer.