CN115094309A - Cr-Ni-Mo carburizing steel containing Nb, heat treatment method and part - Google Patents

Abstract

The application relates to a Cr-Ni-Mo carburizing steel containing Nb, a heat treatment method and parts, which comprises the following steps by mass percent: 0.15-0.19% of C, 0.25-0.40% of Si, 0.40-0.60% of Mn, less than or equal to 0.020% of P, 0.015-0.030% of S, 0.95-1.20% of Cr, 1.40-1.70% of Ni0.25-0.35% of Mo, 0.025-0.040% of Al, 0.020-0.060% of Nb0.010-0.020% of N, 0.10-0.25% of Cu, and the balance of Fe and inevitable impurities, wherein the content ratio of Al to N is 1.5-3.0. The method can solve the problems that the Cr-Ni-Mo series carburizing steel in the related technology can cause the crystal grain growth and the mixed crystal phenomenon under the low-pressure vacuum carburizing technology, and can not meet the technological requirements of the low-pressure vacuum carburizing.

Description

Cr-Ni-Mo carburizing steel containing Nb, heat treatment method and part

Technical Field

The application relates to the technical field of heat treatment of carburizing steel, in particular to Cr-Ni-Mo carburizing steel containing Nb, a heat treatment method and parts.

Background

The domestic heavy gearbox part uses Cr-Ni-Mo series carburizing steel, and the national standard GB/T5216 stipulates the content of alloy elements of the material: 0.17-0.23% of C, 0.17-0.37% of Si, 0.60-0.95% of Mn, less than or equal to 0.030% of P, less than or equal to 0.035% of S and 0.30-0.65% of Cr; 0.35 to 0.75 percent of Ni, 0.15 to 0.25 percent of Mo, less than or equal to 0.25 percent of Cu, and not coarser than grade 5 of austenite grain size. And the Cr-Ni-Mo series carburizing steel in the standard CGMA001-1 established by the gear professional association specifies that the mass percentage of the alloy elements of the material is as follows: 0.17-0.23% of C, 0.15-0.35% of Si, 0.60-0.95% of Mn, less than or equal to 0.030% of P, 0.017-0.032% of S and 0.35-0.65% of Cr; 0.35 to 0.75 percent of Ni, 0.15 to 0.25 percent of Mo, 0.020 to 0.045 percent of Al, less than or equal to 0.20 percent of Cu, and no coarser austenite grain size than grade 5.

The existing Cr-Ni-Mo series carburizing steel is adopted to produce the gear box shaft tooth type parts, and the isothermal annealing treatment is usually carried out after the part blank is forged in order to obtain the pearlite and ferrite balanced structure for the convenience of cutting processing. However, because the Cr-Ni-Mo series carburizing steel has high contents of Mn, Ni and Mo elements, the segregation problem of Mn, Ni, Mo and other alloy elements is often inevitable, so that bainite and even martensite and other abnormal structures appear in the Cr-Ni-Mo series carburizing steel forging stock, and compared with other series carburizing steel, serious banded structures (the grade of the banded structure can reach 4.0 grade at most) are more likely to appear, great troubles are brought to the subsequent cutting processing of the forging stock and the carburizing metallurgical quality of parts, and the production efficiency and the product quality are seriously influenced.

In recent years, low-pressure vacuum carburization has become popular among various main machinery factories and parts suppliers as an environmentally friendly green heat treatment technology. The low-pressure vacuum carburization can effectively avoid the tissue defects of non-martensite and the like on the surface, which are caused by the conventional carburization quenching, and the surface quality of the shaft and gear parts is obviously improved. Conventional carburization process temperatures are affected by furnace performance and the maximum service temperature is limited to within 950 ℃, whereas improvements in vacuum carburization furnace technology can increase the maximum service temperature to 1050 ℃. Because the diffusion coefficient of carbon is accelerated along with the increase of the temperature during the carburization, the carburization speed can be greatly improved, the time for obtaining the same carburization depth is greatly shortened, and the production efficiency is improved and the production cost is reduced. However, the temperature of the low-pressure vacuum carburization process is usually over 980 ℃, while the existing Cr-Ni-Mo series carburization steel adopts AlN refined grains, when the carburization temperature is increased to over 950 ℃, an AlN precipitated phase which plays a role of pinning a grain boundary can be continuously dissolved, and the dissolution is more sufficient when the temperature is higher and the time is longer, so that the grain boundary is not pinned and continuously migrates, the grain grows up, even a serious mixed crystal phenomenon can occur, and the process requirement of the low-pressure vacuum carburization can not be met.

Disclosure of Invention

The embodiment of the application provides Cr-Ni-Mo carburizing steel containing Nb, a heat treatment method and parts, which aim to solve the problems that Cr-Ni-Mo series carburizing steel in the related technology can cause the phenomena of crystal grain growth and mixed crystal generation under the low-pressure vacuum carburizing process, and can not meet the process requirement of low-pressure vacuum carburizing.

In a first aspect, there is provided a Cr-Ni-Mo carburized steel containing Nb, which comprises, in mass percent: 0.15-0.19% of C, 0.25-0.40% of Si, 0.40-0.60% of Mn, less than or equal to 0.020% of P, 0.015-0.030% of S, 0.95-1.20% of Cr, 1.40-1.70% of Ni, 0.25-0.35% of Mo, 0.025-0.040% of Al, 0.020-0.060% of Nb, 0.010-0.020% of N, 0.10-0.25% of Cu, and the balance of Fe and inevitable impurities, wherein the content ratio of Al to N is 1.5-3.0.

In some embodiments, the total Al + Nb content, in mass percent, is greater than or equal to 0.060%.

In a second aspect, there is provided a heat treatment method of the Nb-containing Cr-Ni-Mo carburized steel as described above, which includes the steps of:

a. forging: heating Cr-Ni-Mo carburizing steel containing Nb, and forging to form a part;

b. austenitizing: transferring the cooled part into a heating furnace, wherein the heating temperature is 940-960 ℃, and the heat preservation time is 30-60 min;

c. intercooling: carrying out air blowing cooling on the austenitized part;

d. three-stage isothermal normalizing: transferring the intercooled part into an isothermal furnace, preserving heat for 50-70 min at the temperature of 640-660 ℃, then heating to 715-735 ℃ within 50-70 min, and finally preserving heat for 50-70 min at the temperature of 715-735 ℃;

e. cooling along with the furnace: cooling the part subjected to isothermal normalizing to a certain temperature along with the furnace, and then discharging and air cooling;

f. and (3) sequentially carrying out rough machining, finish machining, low-pressure vacuum carburization, quenching and low-temperature tempering on the air-cooled part.

In some embodiments, in step f, the low pressure vacuum carburization process includes: and (3) placing the finished part into a vacuum carburizing furnace for vacuumizing, preheating and preserving heat at 650-700 ℃, → 4.5-6.6 kpa, and performing strong carburizing treatment at 1020-1050 ℃, controlling the environmental carbon potential to be 0.95-1.20% C → 2.5-5.0 kpa, and performing high-temperature diffusion at 1000-1030 ℃, controlling the environmental carbon potential to be 0.85-1.15% C → 1.3-3.0 kpa, and performing low-temperature diffusion at 980-1010 ℃, controlling the environmental carbon potential to be 0.75-0.95% C, and performing low-pressure carburizing vacuum treatment with the process layer depth of 0.5-1.5 mm.

In some embodiments, in step f, when the part is a shaft-type part or a thick-walled gear-type part, the quenching process is vacuum oil quenching.

In some embodiments, the vacuum oil quench comprises: after the parts are oiled, the stirring speed of a stirrer in a quenching oil groove is 350-400 r/min for 0 s-4-6 s; 5-7 s-15-17 s after the parts are oiled, and the stirring speed of a stirrer in the quenching oil tank is 950-1000 r/min; after the parts are oiled for 16-18 s until quenching is finished, the stirring speed of a stirrer in a quenching oil groove is 200-250 r/min; the oil temperature of the vacuum oil quenching is 110-155 ℃.

In some embodiments, in step f, when the part is a thin-wall gear sleeve type part or a thin-wall gear ring type part, the quenching treatment adopts vacuum high-pressure gas quenching.

In some embodiments, the vacuum high pressure gas quenching comprises: and in the vacuum gas quenching chamber, cooling for 100-180 s under the pressure of 17-20 bar, cooling for 350-500 s under the pressure of 9-12 bar, and discharging.

In some embodiments, in step f, the low temperature tempering treatment comprises: heating the quenched part to 160-200 ℃, and preserving heat for 2-3 h; and/or the presence of a gas in the gas,

in the step c, the cooling speed is 60-100 ℃/min, and the air cooling time is 180-300 s.

In a third aspect, there is provided a part manufactured by the heat treatment method as described in any one of the above.

The beneficial effect that technical scheme that this application provided brought includes:

the method is based on the existing Cr-Ni-Mo carburizing steel, microalloying elements in the steel are optimized and adjusted, 0.020-0.060% of Nb is added, and the content ratio of Al to N is set, so that when a large amount of NbC precipitated phases exist in the steel, more AlN precipitated phases exist, the NbC precipitated phases are stable at high temperature and can play a role in pinning grain boundaries, and the AlN precipitated phases have the characteristics of being fine and uniform.

The Nb-containing Cr-Ni-Mo carburizing steel provided by the application meets the requirements of low-pressure vacuum carburizing process temperature of 980-1050 ℃ and average grain size more than or equal to 7.0 grade.

Drawings

In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings needed to be used in the description of the embodiments are briefly introduced below, and it is obvious that the drawings in the following description are only some embodiments of the present application, and it is obvious for those skilled in the art to obtain other drawings based on these drawings without creative efforts.

FIG. 1 is a graph of a conventional isothermal normalizing process employed in the prior art;

FIG. 2 is a graph of a three-stage isothermal normalizing process provided herein;

FIG. 3 is a grain size photograph of Nb containing austenitic Cr-Ni-Mo carburized steel at 1000 ℃ for 9h as provided in example 1 of the present application;

FIG. 4 is a photograph of austenite grain size of a conventional Cr-Ni-Mo carburized steel currently in production at 1000 ℃ for 9 hours;

FIG. 5 is a metallographic photograph showing an abnormal structure of a forged blank of the ring gear produced at present after conventional isothermal normalizing;

FIG. 6 is a metallographic photograph showing a severe band-shaped structure of a forged blank of the ring gear which is produced at present after conventional isothermal normalizing;

FIG. 7 is a metallographic picture of a forged ring gear blank subjected to three-stage isothermal normalizing, provided in example 1 of the present application;

FIG. 8 is a photograph of a strip structure of an inner gear ring forging stock subjected to three-stage isothermal normalizing provided in embodiment 1 of the application;

FIG. 9 is a photograph of grain size of Nb containing austenitic Cr-Ni-Mo carburized steel at 1050 ℃ for 9h as provided in example 2 of the present application;

FIG. 10 is a photograph of austenite grain size of a conventional Cr-Ni-Mo carburized steel currently in production at 1050 ℃ for 9 hours;

FIG. 11 is a metallographic photograph showing an abnormal structure of a currently produced gear shaft forging stock after being subjected to conventional isothermal normalizing;

FIG. 12 is a metallographic picture showing the appearance of severe banding in a presently produced gear shaft forging after conventional isothermal normalizing;

FIG. 13 is a metallographic picture taken after three-stage isothermal normalizing of a gear shaft forging provided in example 2 of the present application;

FIG. 14 is a photograph of a strip structure of a gear shaft forging obtained by three-stage isothermal normalization according to example 2 of the present application.

Detailed Description

In order to make the objects, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application, and it is obvious that the described embodiments are some, but not all, embodiments of the present application. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present application.

The embodiment of the application provides a Cr-Ni-Mo carburizing steel containing Nb, which comprises the following components in percentage by mass: 0.15-0.19% of C, 0.25-0.40% of Si, 0.40-0.60% of Mn, less than or equal to 0.020% of P, 0.015-0.030% of S, 0.95-1.20% of Cr, 1.40-1.70% of Ni, 0.25-0.35% of Mo, 0.025-0.040% of Al, 0.020-0.060% of Nb, 0.010-0.020% of N, 0.10-0.25% of Cu, and the balance of Fe and inevitable impurities, wherein the content ratio of Al to N is 1.5-3.0.

The embodiment is based on the existing Cr-Ni-Mo carburizing steel, the microalloying elements in the steel are optimized and adjusted, 0.020-0.060% of Nb is added, and the content ratio of Al to N is set, so that a large amount of NbC precipitated phases exist in the steel, more AlN precipitated phases exist, the NbC precipitated phases are stable at high temperature and can play a role in pinning grain boundaries, and the AlN precipitated phases have the characteristics of being fine and uniform, when initial fine grains are obtained, the NbC precipitated phases can play a role in further hindering the grain boundary expansion during high-temperature carburizing, the NbC and AlN composite effect ensures the control effect of the grain size of low-pressure vacuum carburizing, and the serious mixed crystal phenomenon caused by the growth of the grains is avoided.

The Cr-Ni-Mo carburized steel containing Nb provided by the embodiment meets the requirements of 980-1050 ℃ low-pressure vacuum carburization process temperature and more than or equal to 7.0-grade average grain size.

The preparation process of the Nb-containing Cr-Ni-Mo carburizing steel in the embodiment comprises the following steps:

(1) smelting treatment: and mixing one or two of high-quality scrap steel and hot molten iron, and smelting by adopting an electric furnace or a converter to obtain molten steel.

(2) Refining treatment: and refining the molten steel by adopting a ladle furnace, and blowing Ar for stirring in the whole process to obtain refined molten steel.

(3) And (3) vacuum treatment: and placing the refined molten steel into a vacuum furnace for vacuum degassing, wherein the vacuum degree in the furnace is less than or equal to 66.7Pa, and obtaining the molten steel after vacuum degassing.

(4) Pouring treatment: and (3) carrying out protective casting on the molten steel subjected to vacuum degassing in the whole continuous casting process, controlling the superheat degree of the molten steel to be less than or equal to 30 ℃ in the casting process, and electromagnetically stirring by using a crystallizer to ensure that no secondary pollution is caused in the casting process so as to obtain a continuous casting billet.

(5) Heating a casting blank: and heating the continuous casting slab to 1250-1300 ℃, and keeping the temperature for 3-5 hours to completely dissolve the Nb-containing precipitated phase in the steel.

(6) Rolling: the initial rolling temperature is 1140-1180 ℃, the intermediate rolling temperature is 1060-1090 ℃, the final rolling temperature is controlled to be 1000-1040 ℃, and the temperature of the upper cooling bed is controlled to be 830-870 ℃.

In some preferred embodiments, the total content of Al and Nb, Al + Nb, is greater than or equal to 0.060 wt%, which has the advantages of ensuring the content of total precipitated phases in the steel, ensuring enough precipitated phases during low-pressure vacuum carburization, and effectively limiting the grain growth.

The embodiment of the application also provides a heat treatment method of the Cr-Ni-Mo carburizing steel containing Nb, which comprises the following steps:

a. forging: heating the round steel raw material to 1150-1250 ℃, forging and forming the round steel raw material into a part, controlling the finish forging temperature to 1000-1100 ℃, and placing the part in air for free cooling. The round steel raw material is the Nb-containing Cr-Ni-Mo carburizing steel provided in the above embodiment.

b. Austenitizing: and transferring the cooled part into a heating furnace, wherein the heating temperature is 940-960 ℃, and the heat preservation time is 30-60 min, so that the part is fully austenitized.

c. Intercooling: and (5) carrying out air blowing cooling on the austenitized part.

Specifically, the austenitized part is placed in a middle cooling area to be subjected to forced air cooling, the air temperature is 30-40 ℃, the air speed is 12-15 m/s, the average cooling speed is 60-100 ℃/min, the air cooling time of the part is controlled to be 180-300 s, and the phenomenon that super-cooled austenite in the part is converted into proeutectoid ferrite is avoided to the greatest extent.

d. Three-stage isothermal normalizing: transferring the intercooled part into an isothermal furnace for three-section isothermal normalizing treatment, which specifically comprises the following steps:

first-stage isothermal treatment: and (3) preserving the heat for 50-70 min at the temperature of 640-660 ℃, and mainly aiming at ensuring enough supercooling degree after austenitizing and cooling, and forming pearlite with smaller lamella spacing in the initial stage of isothermal transformation so as to ensure that the hardness of the part is in the higher range of 180-190 HB.

Second-stage heating treatment: slowly heating to raise the temperature from 640-660 ℃ of the first-stage isothermal treatment to 715-735 ℃, wherein the heating time is 50-70 min, the heating rate is 55-95 ℃/h, and the main purpose is to provide driving energy for micro-alloying elements (Cr, Ni, Mo and the like) of the supercooled austenite which are easy to generate segregation in the isothermal process, so that the alloy elements in the part are in a dispersion distribution state, and the segregation phenomenon in the part cooling process is greatly improved.

And (3) third-stage isothermal treatment: and (3) preserving the heat for 50-70 min at the temperature of 715-735 ℃, ensuring that the microalloy elements in the part have enough time to continue diffusing, reducing the hardness of the part to 170-180 HB, and simultaneously ensuring that the level of a banded structure in the part is less than or equal to 1.5.

After the first stage of isothermal treatment, the supercooled austenite in the part structure is not completely transformed into pearlite, and microalloying elements (such as Cr, Ni, and Mo) are not sufficient in driving energy to realize diffusion distribution, so that bainite and even martensite are generated in the part if the temperature reduction treatment is performed, the uniformity of the structure in a forged piece is poor, the machinability of the part is deteriorated, and the subsequent carburizing heat treatment deformation is adversely affected, so that the temperature cannot be reduced after the first stage of isothermal treatment, and the temperature is increased in the second stage.

After the second stage of temperature rise treatment, the partially aggregated micro-alloying elements (Cr, Ni, Mo and the like) in the part are not completely diffused, the pearlite hardness with small interlayer spacing is not changed greatly, and the hardness of the part can be reduced only by continuing heat preservation. If the temperature reduction treatment is carried out at the stage, the risk of tissue segregation still exists in the parts, and the hardness of the forged piece is 180-190 HB, so that the cutting machining performance is poor, the cutting machining cost is high, the temperature cannot be reduced after the second-stage temperature rise treatment, and the isothermal treatment is carried out at the third stage.

The conventional isothermal normalizing process curve chart is shown in figure 1, the problem that bainite and other abnormal structures and banded structures are out of tolerance is caused by adopting the process, the three-section isothermal normalizing process curve chart provided by the application is shown in figure 2, and the problem that the bainite and other abnormal structures and banded structures in Cr-Ni-Mo carburizing steel are out of tolerance can be remarkably improved or even eliminated by adopting the three-section isothermal normalizing process, so that the subsequent machining performance of parts is improved, and the carburizing heat treatment deformation tendency is reduced.

e. Cooling along with the furnace: cooling the part subjected to isothermal normalizing to a certain temperature along with the furnace, wherein the temperature can be set according to actual requirements, such as 300 ℃, and then discharging and air cooling.

f. And (3) sequentially carrying out rough machining, finish machining, low-pressure vacuum carburization, quenching and low-temperature tempering on the air-cooled part.

In the step f, the low-pressure vacuum carburization process includes: setting carburizing time and diffusion time according to the technical requirements of parts, drawing up a corresponding process curve, putting the finished parts into a vacuum carburizing furnace for vacuumizing, preheating and preserving heat at 650-700 ℃, starting low-pressure carburizing treatment at 980-1050 ℃, and filling acetylene or propane in a pulse mode, wherein the control pressure is 1.3-6.6 kpa, and the method specifically comprises the following steps: 4.5 to 6.6kpa, 1020 to 1050 ℃, controlling the environmental carbon potential to be 0.95 to 1.20 percent C → 2.5 to 5.0kpa, performing high-temperature diffusion at 1000 to 1030 ℃, controlling the environmental carbon potential to be 0.85 to 1.15 percent C → 1.3 to 3.0kpa, performing low-temperature diffusion at 980 to 1010 ℃, controlling the environmental carbon potential to be 0.75 to 0.95 percent C, and requiring the process layer depth of low-pressure vacuum carburization to be 0.5 to 1.5 mm.

The low-pressure vacuum carburization can effectively avoid the tissue defects of non-martensite and the like on the surface, obviously improve the surface quality of the shaft-gear parts, and select three-stage vacuum oil quenching treatment for shaft-type parts and thick-wall gear-type parts with larger sizes according to the requirements of the sizes and the precision of the parts, so that the heat treatment deformation of the parts is effectively controlled; the thin-wall gear sleeve type part and the thin-wall gear ring type part are subjected to vacuum high-pressure gas quenching treatment, so that the investment of pressure quenching equipment can be reduced, carbon emission in the production process can be reduced, and green manufacturing is realized; meanwhile, the deformation of the carburizing heat treatment can be obviously reduced, the carburizing time can be reduced by increasing the carburizing temperature, and the energy consumption and the gas consumption are effectively reduced.

In the step f, the quenching treatment modes are different for different types of parts:

(1) when the part is a shaft type part or a thick-wall gear type part with larger size, vacuum oil quenching is adopted in the quenching treatment.

The method comprises the following steps of (1) selecting proper graded quenching oil by using a vacuum quenching furnace, and determining various characteristic time and temperature of steel during cooling according to an initial super-cooled austenite transformation curve of Nb-containing Cr-Ni-Mo carburizing steel, so as to formulate the following three-stage vacuum oil quenching process:

after the parts are oiled, a slower cooling mode is adopted for 0s to 4-6 s, and the stirring speed of a stirrer in a quenching oil groove is 350-400 r/min; after the parts are oiled, 5-7-15-17 s, adopting a rapid cooling mode, wherein the stirring speed of a stirrer in a quenching oil tank is 950-1000 r/min; after the oil is added to the parts for 16-18 s, a slow cooling mode is adopted until quenching is finished, and the stirring speed of a stirrer in a quenching oil groove is 200-250 r/min; the oil temperature of the vacuum oil quenching is 110-155 ℃.

Compared with the common quenching process, the three-section vacuum oil quenching process better utilizes the super-cooled austenite transformation curve of the carburizing steel and the cooling characteristic of the graded quenching oil, the heat treatment deformation of the quenched part can be effectively controlled, and the product quality is improved.

(2) When the part is a thin-wall gear sleeve type part or a thin-wall gear ring type part, vacuum high-pressure gas quenching is adopted for quenching treatment.

The vacuum high-pressure gas quenching comprises the following steps: and (3) performing gas quenching heat treatment by adopting high-pressure nitrogen in a vacuum gas quenching chamber, cooling for 100-180 s under the pressure of 17-20 bar, cooling for 350-500 s under the pressure of 9-12 bar, and discharging.

The process of low-pressure vacuum carburization and then vacuum high-pressure gas quenching is adopted, so that the procedures of slow cooling, reheating, subsequent pressure quenching, sizing quenching and the like after the thin-wall part is carburized can be omitted.

In the step f, the low temperature tempering treatment includes: and heating the quenched part to 160-200 ℃, and preserving heat for 2-3 hours.

Example 1:

the Cr-Ni-Mo carburizing steel containing Nb is applied to an inner gear ring of a heavy-duty gearbox, and comprises the following chemical components in percentage by mass: 0.16% of C, 0.32% of Si, 0.51% of Mn, 0.014% of P, 0.025% of S, 1.03% of Cr, 1.48% of Ni, 0.28% of Mo, 0.031% of Al, 0.045% of Nb, 0.012% of N, 0.14% of Cu, and the balance of Fe and inevitable impurities. The content ratio of Al to N is 2.58, and the content ratio of Al + Nb is 0.076%.

The preparation process of the Nb-containing Cr-Ni-Mo carburizing steel comprises the following steps:

(1) smelting treatment: and mixing one or two of high-quality scrap steel and hot molten iron, and smelting by adopting an electric furnace or a converter to obtain molten steel.

(2) Refining treatment: and refining the molten steel by adopting a ladle furnace, and blowing Ar for stirring in the whole process to obtain refined molten steel.

(3) Vacuum treatment: and placing the refined molten steel into a vacuum furnace for vacuum degassing, wherein the vacuum degree is 33.4Pa, and obtaining the molten steel after vacuum degassing.

(4) Pouring treatment: and (3) carrying out protective pouring on the molten steel subjected to vacuum degassing in the whole continuous casting process, wherein the superheat degree of the molten steel is 21 ℃, and electromagnetically stirring by using a crystallizer to obtain a continuous casting billet.

(5) Heating a casting blank: heating the continuous casting billet to 1280 ℃, and preserving heat for 4 hours to completely melt the Nb-containing precipitated phase in the steel.

(6) Rolling: the initial rolling temperature is 1150 ℃, the intermediate rolling temperature is 1080 ℃, the final rolling temperature is 1020 ℃ and the temperature of the upper cooling bed is controlled to be 850 ℃.

The heat treatment method for producing the heavy gearbox ring gear by adopting the Nb-containing Cr-Ni-Mo carburizing steel comprises the following steps:

a. forging: heating Cr-Ni-Mo carburizing steel raw material containing Nb to 1230 ℃, rolling, expanding, forging and forming into an inner gear ring forging blank, wherein the finish forging temperature is 1050 ℃, and placing in air for cooling.

b. Austenitizing: and heating the forging stock of the inner gear ring to 950 ℃, and keeping the temperature for 30min to ensure that the forging stock of the inner gear ring is fully austenitized.

c. Intercooling: and placing the austenitized forged inner gear ring blank in a middle cooling area for forced air cooling, wherein the air temperature is 35 ℃, the air speed is 13m/s, the average cooling speed is 90 ℃/min, and the air cooling time of the forged inner gear ring blank is 200 s.

d. Three-stage isothermal normalizing: transferring the internally cooled annular gear forging stock to an isothermal furnace for three-stage isothermal normalizing treatment:

first-stage isothermal treatment: the temperature is set to 650 ℃, and the holding time is 1 h.

Second-stage heating treatment: slowly heating from 650 ℃ to 715 ℃, controlling the heating rate to be 65 ℃/h and the heating time to be 1 h.

And (3) third-stage isothermal treatment: the temperature is set to 715 ℃, and the holding time is 1 h.

e. Cooling along with the furnace: and cooling the inner gear ring forging blank subjected to isothermal normalizing to 300 ℃ along with the furnace, and then discharging the inner gear ring forging blank out of the furnace for air cooling.

f. The method comprises the steps of inner gear ring forging blank rough machining → fine machining → low-pressure vacuum carburization treatment → vacuum high-pressure gas quenching treatment → low-temperature tempering treatment.

Wherein the low-pressure vacuum carburization in step f includes: placing the finish-machined inner gear ring forging blank into a vacuum carburizing furnace for vacuumizing, starting heating, preheating at 650 ℃, keeping the temperature → starting a low-pressure carburizing process, and filling acetylene in a pulse mode, wherein the method comprises the following steps: performing strong cementation treatment at the temperature of 1030 ℃ and 5.2kpa, controlling the environmental carbon potential to be 1.05 percent C → 3.9kpa, performing high-temperature diffusion at the temperature of 1010 ℃, controlling the environmental carbon potential to be 0.95 percent C → 2.1kpa, performing low-temperature diffusion at the temperature of 990 ℃, controlling the environmental carbon potential to be 0.85 percent C, and requiring the depth of a process layer of low-pressure vacuum cementation treatment to be 0.6-1.2 mm.

The vacuum high-pressure gas quenching treatment in the step f comprises the following steps: and transferring the carburized inner gear ring forging blank into a vacuum gas quenching chamber, performing gas quenching heat treatment by adopting high-pressure nitrogen, cooling for 160s under the pressure of 20bar, cooling for 450s under the pressure of 12bar, and discharging.

In step f, the low-temperature tempering treatment comprises the following steps: and heating the quenched ring gear forging stock to 180 ℃, and preserving heat for 2 hours.

Example 1 microalloying elements in steel were optimally adjusted based on the conventional Cr-Ni-Mo carburized steel, and 0.045% Nb was added, where N is 2.58 and Al + Nb is 0.076%. Under the heat treatment condition of 1000 ℃ for 9h, the austenite grain size of the Nb-containing Cr-Ni-Mo carburizing steel is detected to be 8.0 grade, as shown in figure 3; the austenite grain size of the conventional Cr-Ni-Mo carburized steel adopted in the prior production is mixed crystal 2 grade (85 percent) and +7.5 grade (15 percent), as shown in FIG. 4.

After the conventional Cr-Ni-Mo carburizing steel adopted in the prior production is adopted in the conventional isothermal normalizing process during the production of the inner gear ring forging stock, due to the fact that alloying elements such as Mn, Cr, Ni and Mo are prone to segregation in the cooling process, abnormal structures such as bainite can appear in the structure of the inner gear ring forging stock in the prior production, as shown in FIG. 5, and along with a serious banded structure, the grade is 3.0, as shown in FIG. 6. The inner gear ring forging stock of the embodiment 1 adopts a three-section isothermal normalizing process: the austenitizing process adopts 950 ℃ multiplied by 30min to fully austenitize the forging stock; the middle cooling area is cooled by blowing air with strong force, the air temperature is 35 ℃, the air speed is 13m/s, the average cooling speed is 90 ℃/min, the air cooling time of the inner gear ring forging stock is 200s, and the occurrence of proeutectoid ferrite in the forging stock is avoided as much as possible; three-stage isothermal normalizing: the first stage of isothermal treatment is carried out, the temperature is set to be 650 ℃, the heat preservation time is 1h, so that the adequate supercooling degree of the forging stock of the inner gear ring is ensured after austenitizing and cooling, pearlite with smaller lamellar spacing is formed at the initial stage of isothermal transformation, and the hardness of the forging stock is ensured to be 185-190 HB; the second stage of heating treatment, namely slowly heating from 650 ℃ to 715 ℃, controlling the heating rate to be 65 ℃/h and the heating time to be 1h, providing driving energy for micro-alloying elements (Cr, Ni, Mo and the like) of supercooled austenite which are easy to generate segregation in the isothermal process, enabling the alloy elements in the forging stock to be in a dispersion distribution state, and greatly improving the segregation phenomenon in the forging stock cooling process; and (3) carrying out isothermal treatment for the third stage, wherein the temperature is set to be 715 ℃, the heat preservation time is 1h, the microalloy elements in the forging stock have enough time to continue diffusing, the hardness of the forging stock is reduced to 175-180 HB, and meanwhile, the metallographic structure in the forging stock is ensured to be uniform ferrite and pearlite, as shown in figure 7, and the level of the banded structure is 0.5, as shown in figure 8.

In the prior production, an inner gear ring forging stock adopts an atmosphere carburizing and pressure quenching process, and through detection, the depth of a non-martensitic layer at the tooth surface of the inner gear ring is 15-20 mu m, and the depth of intergranular oxidation is 8-12 mu m; the depth of a non-horse layer at the tooth root is 13-19 mu m, and the depth of intergranular oxidation is 7-10 mu m; however, after the forged ring gear blank of example 1 is produced by the process of low-pressure vacuum carburization and vacuum high-pressure gas quenching, the phenomena of non-martensite and intergranular oxidation are not detected at the tooth surface and tooth root of the ring gear. According to the detection results, the process of low-pressure vacuum carburization and vacuum high-pressure gas quenching obviously improves the surface quality of the inner gear ring, and can effectively improve the fatigue performance of parts. Meanwhile, the process of vacuum high-pressure gas quenching after low-pressure vacuum carburization is adopted, so that the processes of slow cooling, reheating, subsequent pressure quenching, sizing quenching and the like after carburization of the annular gear part can be omitted, and the production cost of the part is obviously reduced.

Example 2:

the Cr-Ni-Mo carburizing steel containing Nb is applied to a gear shaft of a heavy-duty gearbox and comprises the following chemical components in percentage by mass: 0.18% of C, 0.37% of Si, 0.58% of Mn, 0.012% of P, 0.029% of S, 1.15% of Cr, 1.65% of Ni, 0.32% of Mo, 0.038% of Al, 0.051% of Nb, 0.011% of N, 0.21% of Cu, and the balance of Fe and inevitable impurities. The content ratio of Al to N was 3.45, and Al + Nb was 0.089%.

The preparation process of the Nb-containing Cr-Ni-Mo carburizing steel comprises the following steps:

(1) smelting treatment: one or two of high-quality scrap steel and hot molten iron are mixed and smelted by an electric furnace or a converter to obtain molten steel.

(2) Refining treatment: and refining the molten steel by adopting a ladle furnace, and blowing Ar for stirring in the whole process to obtain refined molten steel.

(3) And (3) vacuum treatment: and placing the refined molten steel into a vacuum furnace for vacuum degassing, wherein the vacuum degree is 55.6Pa, and obtaining the molten steel subjected to vacuum degassing.

(4) Pouring treatment: and (3) carrying out protective pouring on the molten steel subjected to vacuum degassing in the whole continuous casting process, wherein the superheat degree of the molten steel is 28 ℃, and electromagnetically stirring by using a crystallizer to obtain a continuous casting billet.

(5) Heating a casting blank: heating the continuous casting billet to 1300 ℃ and preserving the temperature for 5h to completely melt the Nb-containing precipitated phase in the steel.

(6) Rolling: the initial rolling temperature is 1180 ℃, the intermediate rolling temperature is 1090 ℃, the final rolling temperature is 1040 ℃, and the temperature of the upper cooling bed is controlled to be 870 ℃.

The heat treatment method for producing the heavy gearbox gear shaft by adopting the Nb-containing Cr-Ni-Mo carburizing steel comprises the following steps:

a. forging: heating Cr-Ni-Mo carburizing steel raw materials containing Nb to 1250 ℃, performing die forging to form a gear shaft forging stock, setting the finish forging temperature to 1100 ℃, and cooling in air.

b. Austenitizing: and heating the gear shaft forging stock to 960 ℃, and keeping the temperature for 60min to ensure that the gear shaft forging stock is fully austenitized.

c. Intercooling: and placing the austenitized gear shaft forging stock in a middle cooling area for forced air cooling, wherein the air temperature is 40 ℃, the air speed is 12m/s, the average cooling speed is 60 ℃/min, and the air cooling time of the gear shaft forging stock is 300 s.

d. Three-stage isothermal normalizing: transferring the intercooled gear shaft forging stock to an isothermal furnace for three-section isothermal normalizing treatment:

first-stage isothermal treatment: the temperature is set to 660 ℃ and the holding time is 1 h.

And (3) second-stage heating treatment: slowly heating from 660 ℃ to 735 ℃, controlling the heating rate to be 75 ℃/h and the heating time to be 1 h.

A third stage of isothermal treatment: the temperature is set to 735 ℃ and the holding time is 1 h.

e. Cooling along with the furnace: and cooling the gear shaft forging stock subjected to the isothermal normalizing treatment to 300 ℃ along with a furnace, and discharging the gear shaft forging stock out of the furnace.

f. Rough machining of a forged gear shaft blank → fine machining → low-pressure vacuum carburization treatment → vacuum oil quenching treatment → low-temperature tempering treatment.

Wherein the low-pressure vacuum carburization in step f includes: placing the gear shaft forging stock after finish machining into a vacuum carburizing furnace for vacuumizing, starting heating, preheating at 690 ℃, keeping the temperature → starting performing a low-pressure carburizing process, and filling propane in a pulse mode, wherein the method specifically comprises the following steps: performing strong cementation treatment at 1050 ℃ under 6.4kpa, controlling the environmental carbon potential to be 1.15% C → 4.7kpa, performing high-temperature diffusion at 1030 ℃, controlling the environmental carbon potential to be 1.05% C → 2.5kpa, performing low-temperature diffusion at 1010 ℃, controlling the environmental carbon potential to be 0.95% C, and requiring the depth of a process layer of low-pressure vacuum cementation treatment to be 1.1-1.5 mm.

The vacuum oil quenching treatment of the gear shaft forging stock in the step f comprises the following steps: transferring the carburized gear shaft forging stock into a vacuum quenching furnace, selecting proper graded quenching oil, determining various characteristic time and temperature of steel during cooling according to an initial super-cooled austenite transformation curve of the Nb-containing Cr-Ni-Mo carburizing steel in the embodiment, and thus formulating a three-stage vacuum oil quenching process for the gear shaft forging stock:

firstly, after a carburized gear shaft forging blank is oiled for 0-5 s, a slow cooling mode is adopted, and the stirring speed of a stirrer in a quenching oil groove is set to 380 r/min; secondly, 6-15 s after the oil is added to the parts, a rapid cooling mode is adopted, and the stirring speed of a stirrer in a quenching oil tank is set to be 1000 r/min; thirdly, after the oil is filled into the parts for 16 seconds, the parts are quenched, a slow cooling mode is adopted, and the stirring speed of a stirrer in a quenching oil groove is set to be 200 r/min; the temperature of the quenching oil was set to 140 ℃.

Example 2 microalloying elements in steel were optimally adjusted based on the conventional Cr-Ni-Mo carburized steel, and 0.051% of Nb was added, with the setting of Al: N being 3.45 and Al + Nb being 0.089%. Under the heat treatment condition of 1050 ℃ for 9h, the austenite grain size of the Nb-containing Cr-Ni-Mo carburizing steel is detected to be 7.0 grade, as shown in figure 9; while the conventional Cr-Ni-Mo carburized steel used in the present production had an austenite grain size of grade 1.5, as shown in FIG. 10.

After the conventional Cr-Ni-Mo carburizing steel adopted in the prior production is adopted in the conventional isothermal normalizing process when producing the gear shaft forging stock, the abnormal structures such as martensite and the like appear in the structure of the gear shaft forging stock in the prior production due to the fact that the alloy elements such as Mn, Cr, Ni, Mo and the like are easy to be subjected to segregation in the cooling process, and the abnormal structures are shown in figure 11 and accompanied with serious banded structures, and the grade of the abnormal structures is 4.0, and is shown in figure 12. After the three-stage isothermal normalizing process was performed on the gear shaft forged blank of example 2, the structure in the forged blank was uniform ferrite + pearlite, as shown in fig. 13; and the banding is significantly improved, with the worst field of view banding ranked only 1.5, as shown in fig. 14.

The gear shaft forging stock of embodiment 2 adopts "low pressure vacuum carburization + vacuum oil quenching" technology, and through the detection, compare with "atmosphere carburization + direct oil quenching" technology of present production, the radial average runout after the gear shaft carburization quenching is reduced to 0.06mm from 0.12mm, and there is 30% gear shaft radial runout less than or equal to 0.05mm, need not to carry on subsequent alignment treatment, make the gear shaft heat treatment after the quenching warp can be effectively controlled, the alignment fracture risk of the greatly reduced gear shaft, the product quality of the gear shaft has been promoted conscientiously.

In the description of the present application, it should be noted that the terms "upper", "lower", and the like indicate orientations or positional relationships based on the orientations or positional relationships shown in the drawings, which are merely for convenience of describing the present application and simplifying the description, and do not indicate or imply that the referred device or element must have a specific orientation, be constructed in a specific orientation, and be operated, and thus, should not be construed as limiting the present application. Unless expressly stated or limited otherwise, the terms "mounted," "connected," and "connected" are intended to be inclusive and mean, for example, that they may be fixedly connected, detachably connected, or integrally connected; can be mechanically or electrically connected; they may be connected directly or indirectly through intervening media, or they may be interconnected between two elements. The specific meaning of the above terms in the present application can be understood by those of ordinary skill in the art as appropriate.

It is noted that, in the present application, relational terms such as "first" and "second", and the like, are used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. Also, the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising an … …" does not exclude the presence of other identical elements in a process, method, article, or apparatus that comprises the element.

The above description is merely exemplary of the present application and is presented to enable those skilled in the art to understand and practice the present application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the application. Thus, the present application is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims (10)

1. An Nb-containing Cr-Ni-Mo carburizing steel, characterized by comprising, in mass percent: 0.15-0.19% of C, 0.25-0.40% of Si, 0.40-0.60% of Mn, less than or equal to 0.020% of P, 0.015-0.030% of S, 0.95-1.20% of Cr, 1.40-1.70% of Ni, 0.25-0.35% of Mo, 0.025-0.040% of Al, 0.020-0.060% of Nb, 0.010-0.020% of N, 0.10-0.25% of Cu, and the balance of Fe and inevitable impurities, wherein the content ratio of Al to N is 1.5-3.0.

2. The Nb-containing Cr-Ni-Mo carburizing steel according to claim 1, characterized in that:

the total content of Al and Nb, calculated by mass percent, is more than or equal to 0.060 percent.

3. A heat treatment method of Cr-Ni-Mo carburized steel containing Nb according to claim 1 or 2, characterized by comprising the steps of:

a. forging: heating Cr-Ni-Mo carburizing steel containing Nb, and forging to form a part;

b. austenitizing: transferring the cooled part into a heating furnace, wherein the heating temperature is 940-960 ℃, and the heat preservation time is 30-60 min;

c. intercooling: blowing air for cooling the austenitized part;

d. three-stage isothermal normalizing: transferring the intercooled part into an isothermal furnace, preserving heat for 50-70 min at the temperature of 640-660 ℃, then heating to 715-735 ℃ within 50-70 min, and finally preserving heat for 50-70 min at the temperature of 715-735 ℃;

e. cooling along with the furnace: cooling the part subjected to isothermal normalizing to a certain temperature along with the furnace, and then discharging and air cooling;

f. and (3) sequentially carrying out rough machining, finish machining, low-pressure vacuum carburization, quenching and low-temperature tempering on the air-cooled parts.

4. The heat treatment method according to claim 3, wherein:

in step f, the low-pressure vacuum carburization treatment includes: and (3) placing the finished part into a vacuum carburizing furnace for vacuumizing, preheating and preserving heat at 650-700 ℃, → 4.5-6.6 kpa, and performing strong carburizing treatment at 1020-1050 ℃, controlling the environmental carbon potential to be 0.95-1.20% C → 2.5-5.0 kpa, and performing high-temperature diffusion at 1000-1030 ℃, controlling the environmental carbon potential to be 0.85-1.15% C → 1.3-3.0 kpa, and performing low-temperature diffusion at 980-1010 ℃, controlling the environmental carbon potential to be 0.75-0.95% C, and performing low-pressure carburizing vacuum treatment with the process layer depth of 0.5-1.5 mm.

5. The heat treatment method according to claim 3, wherein:

in the step f, when the part is a shaft type part or a thick-wall gear type part, vacuum oil quenching is adopted for quenching treatment.

6. The heat treatment method according to claim 5, wherein:

the vacuum oil quenching comprises the following steps: after the parts are oiled, the stirring speed of a stirrer in a quenching oil groove is 350-400 r/min for 0 s-4-6 s; 5-7 s-15-17 s after the parts are oiled, and the stirring speed of a stirrer in the quenching oil tank is 950-1000 r/min; after the parts are oiled for 16-18 s until quenching is finished, the stirring speed of a stirrer in the quenching oil tank is 200-250 r/min; the oil temperature of the vacuum oil quenching is 110-155 ℃.

7. The heat treatment method according to claim 3, wherein:

and f, when the part is a thin-wall gear sleeve type part or a thin-wall gear ring type part, performing vacuum high-pressure gas quenching on the quenching treatment.

8. The heat treatment method according to claim 7, wherein:

the vacuum high-pressure gas quenching comprises the following steps: and in the vacuum gas quenching chamber, cooling for 100-180 s under the pressure of 17-20 bar, cooling for 350-500 s under the pressure of 9-12 bar, and discharging.

9. The heat treatment method according to claim 3, wherein:

in step f, the low-temperature tempering treatment comprises: heating the quenched part to 160-200 ℃, and preserving heat for 2-3 h; and/or the presence of a gas in the gas,

in the step c, the cooling speed is 60-100 ℃/min, and the air cooling time is 180-300 s.

10. A part manufactured by the heat treatment method according to any one of claims 3 to 9.

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