CN111893265A

CN111893265A - Isothermal spheroidizing annealing method for steel

Info

Publication number: CN111893265A
Application number: CN202010828640.7A
Authority: CN
Inventors: 王中忞; 王长文
Original assignee: Individual
Current assignee: Individual
Priority date: 2020-08-11
Filing date: 2020-08-11
Publication date: 2020-11-06

Abstract

An isothermal spheroidizing annealing method for steel. The cooling mode of isothermal (constant temperature) conversion method is adopted, and the heating speed is caused by the energy difference between the heating temperature field and the steel surface, so that the heating speed with large energy difference is high; on the contrary, the heating speed is slow, and in the austenitizing process of the steel, the heating speed is increased by increasing the heating temperature after charging, so that the rapid heating is realized; the characteristics of 'stability of supercooled austenite of steel' are applied to controlled cooling after austenitizing, the charging mode, the cooling method, the supercooling degree, the cooling speed, the cooling uniformity and the cooling efficiency after austenitizing provide conditions for controlling the transformation speed, the nucleation rate and the growth speed of the supercooled austenite; the method comprises the following steps: austenitizing the steel, maintaining the steel at an equal temperature, and controlling the cooling of the steel; the invention can save energy and resources, reduce cost, shorten heating and cooling time and isothermal time, shorten period, advance construction period, improve spheroidization quality, and obtain expected uniform spheroidization structure and performance up to standard.

Description

Isothermal spheroidizing annealing method for steel

Technical Field

The invention relates to a heat treatment method of steel, in particular to an isothermal spheroidizing annealing method of steel.

Background

In the prior art, the isothermal spheroidizing annealing of the steel has large energy and resource consumption, long period and high cost, and the quality technical indexes after spheroidizing annealing are difficult to reach the standard; the method is characterized in that:

the steel is rapidly heated by putting the steel into a furnace at a high temperature, raising the temperature of the furnace to be higher than the required heating temperature (100-200 ℃), then charging the steel piece and stopping heat supply; when the furnace temperature is reduced to the required heating temperature, heat supply is started, the required heating temperature is entered for heating, the heating temperature and time are controlled, and the next heat treatment procedure is carried out after thorough burning;

the heating temperature of eutectoid steel, hypereutectoid steel and alloy tool steel GCr15 steel is (730-750) DEG C or (780-810) DEG C;

the heating temperature of the cold extrusion formed low-carbon steel and low-carbon low-alloy steel 20CrMo steel is 750-760 ℃ or 800-840 ℃;

heating temperature of Steel-required heating temperature A₁+(20～30)℃；

The isothermal temperatures of eutectoid, hypereutectoid and alloy tool steels are: ar (Ar)₁-(20～30)℃；

The isothermal temperature of the low-carbon steel and the low-carbon low-alloy steel formed by cold extrusion is as follows:

Ar₁- (20-30) DEG C, wherein the isothermal temperature of the 20CrMo steel is 640 ℃;

austenitizing GCr15 steel, cooling to isothermal temperature at a cooling rate of (10-30) DEG C/h, keeping isothermal, cooling to 600 ℃ at a cooling rate of (10-30) DEG C/h, discharging and air cooling;

after austenitizing the steel, it is rapidly cooled to slightly below Ar₁At a temperature of 650 to 700 ℃ or Ar₁Isothermal holding is carried out at a temperature of about 20 ℃;

keeping the steel isothermal, discharging and air cooling;

keeping the steel isothermal, and then cooling the steel to 300 ℃ along with the furnace, discharging and air cooling;

the above problems can be solved by the technical scheme of the invention.

Disclosure of Invention

In order to solve the technical problems, the invention is realized by the following technical scheme;

the invention aims to provide an isothermal spheroidizing annealing method of steel, which overcomes the defects in the prior art, adopts a constant temperature transformation method isothermal spheroidizing annealing method, and has the advantages of good effect and short production period compared with other spheroidizing annealing methods; the method applies that the heating speed is caused by the energy difference between a heating temperature field and the steel surface, and the heating speed with large energy difference is high; on the contrary, the rule of low heating speed is that in the austenitizing process of the steel, the heating speed is increased by increasing the heating temperature, namely, a rapid heating method is adopted; controlled cooling after austenitizing is carried out by using the characteristic of 'super-cooled austenite stability of steel'.

The invention is realized as follows, which is characterized in that the method comprises the following steps:

1. in the austenitizing process of the isothermal spheroidizing annealing of the steel, the steel is put into a furnace at a low temperature, and is loaded into the furnace in a stacking mode which is favorable for heat circulation, uniform diathermy and thorough cooling of heating and cooling sections to the required temperature respectively, the heating speed is increased by increasing the heating temperature, the rapid heating is carried out, the temperature is increased along with the furnace, the temperature increasing speed is controlled, the temperature is increased to be higher than the required heating temperature, namely the rapid heating temperature, the rapid heating temperature is entered for rapid heating, and the rapid heating temperature and the heat preservation time are controlled; stopping heat supply after rapid heating and heat preservation, starting heat supply when the furnace is cooled to the required heating temperature, entering the required heating temperature for heating, and controlling the required heating temperature and heat preservation time; after austenitizing the steel, stopping heat supply, cooling along with the furnace, controlling the cooling speed, cooling to an isothermal temperature, starting heat supply, entering the isothermal temperature for isothermal maintenance, and controlling the isothermal temperature and isothermal time; after the steel is kept isothermally, stopping heat supply, cooling along with the furnace and controlling the cooling speed to cool to the nose tip temperature T of the C curve_POpening the furnace door (cover) at the temperature of minus 20-60 ℃; the temperature T of the nose tip part of the C curve is cooled along with the furnace_P- (180-220) DEG C, discharging from the furnace, and air-cooling to room temperature;

the method comprises the following steps:

austenitizing the steel;

(II) keeping the steel isothermal;

and (III) controlled cooling of the steel.

2. The austenitization of the steel is:

in the austenitizing process of isothermal spheroidizing annealing of steel, a charging mode, a heating method, a temperature rise speed, a heating speed, heating uniformity, heating efficiency, a heating temperature and heat preservation time are used for controlling the austenitizing degree, and a heating section and a cooling section are uniformly and respectively diathermically heated and fully cooled to required temperatures, so that the phase transformation of austenite is completed, austenite grains are not grown, the carbon concentration distribution of austenite is not uniform, a large number of undissolved cementite particles are reserved, the heating stress is reduced, and the cost is low;

secondly, the isothermal spheroidizing annealing of the steel is carried out, the steel is put into a furnace at a low temperature in the austenitizing process, and the steel is charged in a stacking mode which is favorable for heat circulation, uniform diathermy of heating and cooling cross sections and thorough cooling respectively;

in the austenitizing process of isothermal spheroidizing annealing of the steel, after the steel is charged into a furnace, the heating speed is increased by increasing the heating temperature, rapid heating is carried out, the temperature is increased along with the furnace and is controlled to be higher than the required heating temperature, namely the rapid heating temperature, the rapid heating temperature is entered for rapid heating, the heating time and the heating period are shortened, energy and resources are saved, crystal grains are refined, and the rapid heating temperature and the heat preservation time are controlled;

(1) the steel is rapidly heated by heating after charging, wherein the heating speed is (150-200) DEG C/h;

(2) the rapid heating temperature of the steel is higher than the required heating temperature;

(3) the quick heating zero heat preservation time of the steel is as follows: the surface temperature is 90-95% of the required heating temperature, and the core temperature is 80-85% of the required heating temperature; preparing for heating at the required heating temperature and controlling the austenitizing degree of the steel;

fourthly, after the isothermal spheroidizing annealing of the steel is rapidly heated and preserved, stopping heat supply, cooling along with the furnace, uniformly and thoroughly cooling the section to the required heating temperature, heating at the required heating temperature, and controlling the required heating temperature and the heat preservation time;

(1) heating temperature of steel-required heating temperature is: ac of₁+(20～35)℃

1) The eutectoid steel, hypereutectoid steel and alloy tool steel are Ac₁+(20～30)℃；

2) The low-carbon steel and the low-carbon low-alloy steel which are formed by cold extrusion are Ac₁+(25～35)℃；

(2) The heating temperature and zero heat preservation time of the steel is as follows: the surface temperature is the desired heating temperature and the core temperature is > 90% of the desired heating temperature.

3. The isothermal hold of the steel is:

after austenitizing, carrying out isothermal maintenance on the steel, and controlling the cooling speed, the isothermal temperature and the isothermal time for uniformly and thoroughly cooling the cross section to the isothermal temperature after austenitizing so as to provide conditions for fully transforming carbide spheroidized tissues;

secondly, after austenitizing the isothermal spheroidizing annealing of the steel, stopping heat supply, cooling along with the furnace at the cooling speed of 40-60 ℃ per hour, and uniformly and thoroughly cooling the section to isothermal temperature for isothermal maintenance;

and (III) the isothermal spheroidizing annealing isothermal temperature of the steel is as follows: ac of₁-(20～40)℃

(1) The eutectoid steel, hypereutectoid steel and alloy tool steel are Ac₁-(20～40)℃；

(2) The low-carbon steel and the low-carbon low-alloy steel which are formed by cold extrusion are Ac₁-(20～35)℃；

The isothermal time of the isothermal spheroidizing annealing of the steel is the time required for finishing the structure transformation, namely spheroidizing and the time for uniformly and thoroughly cooling the section to the isothermal temperature; the isothermal time is (4-6) h.

4. The controlled cooling of the steel is as follows:

the isothermal spheroidizing annealing of the steel utilizes the characteristic of supercooled austenite stability of the steel to carry out controlled cooling after austenitizing, and the furnace charging mode, the cooling method, the supercooling degree, the cooling speed, the cooling uniformity and the cooling efficiency after austenitizing provide conditions for controlling the transformation speed, the nucleation rate and the growth speed of the supercooled austenite;

secondly, in the austenitizing process of the isothermal spheroidizing annealing of the steel, the steel is rapidly heated to be fed into the furnace at a low temperature, and the steel is charged by adopting a stacking mode which is favorable for heat circulation, uniform diathermy of heating and cooling sections and complete cooling to the required temperature;

after the isothermal spheroidizing annealing of the steel is rapidly heated and insulated, stopping heat supply, cooling along with the furnace, uniformly and thoroughly cooling the section to the required heating temperature, starting heat supply, and heating at the required heating temperature;

fourthly, after austenitizing the isothermal spheroidizing annealing of the steel, stopping heat supply, and cooling along with the furnace at the cooling speed of 40-60 ℃ per hour;

after austenitizing the isothermal spheroidizing annealing of the steel, cooling along with the furnace, uniformly and thoroughly cooling the section to isothermal temperature, starting to supply heat, and entering the isothermal temperature for isothermal maintenance;

after isothermal maintenance of isothermal spheroidizing annealing of steel, stopping heat supply, and cooling along with the furnace at the cooling speed of 40-60℃/h;

and (seventhly) after isothermal spheroidizing annealing isothermal maintenance of the steel, cooling along with the furnace, uniformly and thoroughly cooling the section to the temperature of opening a furnace door (cover): C-Curve nose tip temperature T_P-(20～60)℃

(1) The eutectoid steel, hypereutectoid steel and alloy tool steel are T_P-(20～40)℃；

(2) The low-carbon steel and the low-carbon low-alloy steel formed by cold extrusion are T_P-(40～60)℃；

(eighth) opening a furnace door (cover) of the isothermal spheroidizing annealing furnace of the steel to cool along with the furnace, wherein the temperature from uniform through cooling of the cross section to air cooling of discharging from the furnace is as follows: C-Curve nose tip temperature T_P-(180～220)℃

(1) The eutectoid steel, hypereutectoid steel and alloy tool steel are T_P-(180～200)℃；

(2) The low-carbon steel and the low-carbon low-alloy steel formed by cold extrusion are T_P-(200～220)℃；

(nine) opening a furnace door (cover) of isothermal spheroidizing annealing of steel, cooling along with the furnace, uniformly and thoroughly cooling the cross section until the temperature of discharged air cooling is C curve nose tip temperature T_PAnd (4) discharging at the temperature of (180-220) DEG C, and air-cooling to room temperature.

5. The temperature is as follows:

isothermal spheroidizing annealing of steel the temperature is based on the furnace chamber indication to temperature.

Compared with the prior art, the invention has the beneficial effects that:

the heating and cooling time and the isothermal time are shortened, the period is shortened, the construction period is advanced, energy and resources are saved, the cost is reduced, the spheroidization quality is improved, and the expected uniform spheroidized tissue and performance reach the standard.

Drawings

FIG. 1: the invention discloses a flow chart of an isothermal spheroidizing annealing method of steel, wherein the abscissa represents time t; ordinate meterTemperature, deg.C; t is_SJThe required heating temperature is DEG C; t is_DWIs at an isothermal temperature, DEG C.

FIG. 2: the relationship diagram of the transformation speed and the supercooling degree of the austenite of the steel, namely a supercooling austenite isothermal transformation curve, wherein the abscissa represents the transformation speed, and the ordinate represents the transformation temperature at DEG C; Δ G is the phase transition driving force, D is the atomic diffusion coefficient; t is_PThe nose tip temperature is C curve.

FIG. 3: the isothermal spheroidizing annealing method for the steel has a schematic heating process, and the abscissa represents time t; the ordinate represents temperature, deg.C; t is_SJThe required heating temperature is set at DEG C, and s is the steel surface temperature at DEG C; c is the steel core temperature, DEG C; point D' is > 90% of the required heating temperature T_SJ，℃。

FIG. 4: the isothermal spheroidizing annealing method of the prior art steel has a schematic heating process, and the abscissa represents time t; the ordinate represents temperature, deg.C; t is_SJThe required heating temperature is set as s, the surface temperature of the steel is set as s, and the core temperature of the steel is set as c.

FIG. 5: austenitizing the 20CrMo steel by the isothermal spheroidizing annealing method, and cooling the austenitized steel to a C curve A at different cooling speeds₁Below line, isothermal temperature T in different temperature ranges_DWSchematic cooling process with isothermal hold, with time, lg τ, on abscissa; the ordinate represents temperature, deg.C; t is_PNose tip temperature, C, curve; t is_DWIsothermal temperature, deg.C; curve 1 is the cooling curve for the inventive steel: the cooling speed DE is (40-60) DEG C/h, and the isothermal temperature EF is T_DW(708-723) DEG C; curve 2 is the cooling curve for the prior art steel: the cooling speed is rapid cooling and the isothermal temperature T_DWIs 700 ℃; curve 3 is the cooling curve for the prior art steel: the cooling speed is rapid cooling and the isothermal temperature T_DWIs 650 ℃; curve 4 is the cooling curve for the prior art steel: the cooling speed is rapid cooling and the isothermal temperature T_DWWas 484 ℃.

FIG. 6: in the prior art, the heating temperature of isothermal spheroidizing annealing of GCr15 steel is 730 ℃, the metallographic phase map with normal isothermal temperature is shown, the metallographic phase structure is poor-heat spheroidization structure, and flaky and punctiform pearlite is mixed and distributed in a matrix.

FIG. 7: in the prior art, the heating temperature of isothermal spheroidizing annealing of GCr15 steel is 750 ℃, the metallographic phase map with normal isothermal temperature is shown, the metallographic phase structure is poor-heat spheroidization structure, and the matrix is mixed with a small amount of globular and flaky pearlite.

FIG. 8: in the prior art, the isothermal spheroidizing annealing heating temperature of 20CrMo steel is 800 ℃, the heat preservation time is 6h, the metallographic phase map with normal isothermal temperature is shown, and the metallographic structure is a spherical, rough flaky and punctate pearlite and white ferrite matrix.

FIG. 9: in the prior art, the isothermal spheroidizing annealing heating temperature of 20CrMo steel is 840 ℃, the heat preservation time is 6h, and the isothermal temperature is normal metallographic atlas; the metallographic structure is a coarse lamellar pearlite and white ferrite matrix, and the surface has serious oxidation and decarburization phenomena and a black structure.

FIG. 10: in the prior art, the isothermal spheroidizing annealing heating temperature of 20CrMo steel is normal, the isothermal temperature is 640 ℃, the metallographic phase map is maintained isothermally for 6 hours, and the metallographic structure is punctiform pearlite and a small amount of globular pearlite and fine lamellar pearlite.

Detailed Description

The present invention will be described in further detail with reference to the accompanying drawings and specific embodiments.

The present invention will now be described in detail with reference to a few preferred embodiments thereof as illustrated in the accompanying drawings, wherein the several specific details are set forth in order to provide a thorough understanding of the present invention, but it will be apparent to one skilled in the art that the present invention may be practiced without some or all of these specific details; in some instances, well known process steps have not been described in detail in order to not unnecessarily obscure the present invention.

Zero heat preservation time:

the concept of the heat preservation time is as follows: when the steel is subjected to heat treatment, the steel stays in a heating field with a process specified temperature for a certain time; the heat preservation time comprises the following steps:

(1) heating the steel surface to the process temperature for the required time;

(2) at the moment of the thorough-roasting time when the surface of the steel is heated to the process temperature, the temperature difference between the surface and the core is less than 10 percent of the surface temperature, namely the surface temperature of which the core temperature is more than 90 percent, which represents that the whole steel reaches the process temperature without prolonging the thorough-roasting time;

(3) the heating temperature of the isothermal spheroidizing annealing of the steel for finishing the structure transformation time exceeds Ac₁After finishing the process, the structure transformation of the steel, namely the austenitizing of pearlite, is finished without heat preservation;

from (2) and (3): when the isothermal spheroidizing annealing of the steel is heated, the through-burning time and the austenite homogenization time in the heat preservation time are removed, and only the time from surface heating to process temperature, namely the zero heat preservation time, in which the surface temperature is the required heating temperature, and the temperature difference between the surface and the core is less than 10 percent of the surface temperature, namely the time from the core temperature to the required heating temperature of more than 90 percent, is reserved, so that the heating and heat preservation time and period are shortened, the construction period is advanced, and on the premise of obtaining the expected structure and performance, energy and resources are saved, and the cost is reduced; the steel has short retention time at high temperature, austenite grains are not grown in time, and the grains are refined.

The isothermal spheroidizing annealing of the steel comprises the following steps: by adopting the cooling mode of the isothermal (constant temperature) transformation method, the organization transformation of the steel, namely the spheroidization of carbide is fully completed in the isothermal (constant temperature transformation) process, the spheroidized organization and the performance are uniform and consistent to reach the standard, the cutting performance is improved, the cold deformation forming is smoothly carried out, the organization preparation is made for the final quenching, the quenching performance is improved, the quenching defect is reduced, and the production period is shorter than that of other spheroidizing annealing methods.

As shown in fig. 1, the present invention is realized by the following method:

the heating speed is caused by the energy difference between a heating temperature field and the surface of the steel, and the heating speed with large energy difference is high; conversely, the rule of slow heating rate, in the austenitizing process of the steel, the heating rate is increased by increasing the heating temperature, i.e. the rapid heating temperature is higher than the required heating temperature, as shown in table 1; thereby shortening the heating time and period, advancing the construction period, saving energy and resources and reducing the cost; this is due to:

(1) for steel with the same diameter and the same grade, the heating speed of high temperature of the heating temperature field is greater than the heating speed of low temperature;

TABLE 1 time to temperature of 45 steel surfaces and cores heated in box furnaces of different diameters

From table 1, the surface s and the core c of 45 steel with the same diameter are heated to the same temperature by a box furnace at different temperatures, and the following are obtained: the heating time of 700 ℃ is more than 840 ℃ and the heating time of 920 ℃ is more than

Examples are: looking up a table 1, the diameter phi 80mm of the 45 steel is heated at the three temperatures, and the corresponding temperature time of the surface s and the core c is as follows:

heating at 700 deg.C for s-62min, c-64min > 840 deg.C for s-50min, c-52min > 920 deg.C for s-38min, and c-40min

Thus: heating speed of 920 ℃ and more than 840 ℃ and heating speed of 700 DEG C

(2) The internal part of the material is the change from high energy to low energy, and the heating of the steel in the heating temperature field is the process from the high energy of the heating temperature field to the low energy of the steel;

the steel is heated in a heating temperature field, wherein high energy of the heating temperature field is firstly transferred to the surface of the low-energy steel, and the high energy is formed on the surface of the steel and then transferred to the core part of the low energy from the high energy on the surface of the steel; the temperature of the heating temperature field is increased, namely the energy of the heating temperature field is increased, and the energy difference between the heating temperature field and the steel surface is increased, so that the heating speed is increased;

just like a hydroelectric power station, a barrage is built, the potential energy of water is improved, namely the energy of the water is increased, then the gate is opened to generate electricity, the potential energy of the water is converted into kinetic energy, and the flow speed is accelerated;

(3) as long as the rapid heating temperature and the heat preservation time are controlled, the surface s temperature of the steel is the required heating temperature, and the core c temperature is more than 90 percent of the required heating temperature, the rapid heating can not cause overheating; see table 2;

TABLE 2 temperature variation for heating 45 steel with 200mm section in a box furnace

Note: delta T is the difference between the surface s temperature and the core c temperature

The thorough burning time of the steel is influenced by the heating coefficient and is determined by the size of the steel part;

from Table 2, the temperature change of the section of 45 steel heated in a box furnace within 200mm of phi for the rated time is obtained: surface s temperature > 1/3R temperature > 2/3R temperature > core c temperature

Examples are: cold work die steel Cr4W2MoV steel Ac₁760 ℃ and the heating temperature T required by isothermal spheroidizing annealing_SJTable 2 is looked up for (780-790) DEG C, when the surface s temperature of the steel (without considering the influence of the heating coefficient k) is 784 ℃, the core c temperature is 737 ℃, the temperature difference Delta T between the surface s temperature and the core c temperature is 47 ℃, and the core c temperature is 94 percent of the required heating temperature T_SJI.e. > 90% of the required heating temperature T_SJAt the moment, the zero heating and heat preservation time is 85min, and as long as the zero heating and heat preservation time is controlled to be 85min, the cold-work die steel Cr4W2MoV with the diameter of 200mm can not be overheated;

the super-cooled austenite stability characteristics of the steel are as follows:

(1) the relation between the supercooled austenite and the supercooling degree of the steel, namely the supercooled austenite isothermal transformation curve is similar to the shape of the C curve, and the directions are opposite, so that the supercooled austenite isothermal transformation curve has the characteristic of the C curve; as shown in FIG. 2;

(2) the transformation speed of the supercooled austenite is related to the nucleation rate and the growth speed, and the nucleation rate and the growth speed depend on the supercooling degree;

1) when the supercooling degree is small, Δ G is driven due to phase change_VSmaller, slower transition speeds;

2) as the supercooling degree increases, the phase change driving force Δ G_VIncreasing, the atomic diffusion coefficient D decreases;

3) the transition speed being driven by the phase change driving force deltag before the temperature falls to a certain value_VControlling, increasing with increasing supercooling degree; then, the transformation speed is controlled by the atomic diffusion coefficient D and is reduced along with the increase of the supercooling degree;

4) phase (C)Variable driving force Δ G_VAnd the atomic diffusion coefficient D, resulting in the transition temperature at the nose tip temperature T_PNearby, the transition speed reaches a maximum value;

the performance of the steel finally depends on the structure after the austenite cooling transformation, and the characteristics of 'supercooled austenite stability of the steel' are applied, so that technical support is provided for the charging mode of isothermal spheroidizing annealing, the cooling method after austenitizing, and the control of the supercooling degree, the cooling speed, the cooling uniformity and the cooling efficiency;

1. in the austenitizing process of the isothermal spheroidizing annealing of the steel, the steel is put into a furnace at a low temperature, and after the steel is loaded into the furnace in a stacking mode which is favorable for heat circulation, uniform diathermy of heating and cooling sections and thorough cooling to the required temperature, the heating speed is increased by increasing the heating temperature, and rapid heating is realized; starting to heat up along with the furnace from the point O and controlling the heating speed until the point A is higher than the required heating temperature T_SJThe rapid heating temperature AB is entered for rapid heating, and the rapid heating temperature AB and the heat preservation time A are controlled₁B₁(ii) a After rapid heating and heat preservation, stopping heat supply from the point B, and cooling to the point C along with the furnace to obtain the required heating temperature T_SJStarting to supply heat, entering the required heating temperature CD for heating, and controlling the required heating temperature CD and the heat preservation time C₁D₁(ii) a After austenitizing the steel, stopping heat supply from a point D, cooling along with the furnace and controlling the cooling speed, wherein the temperature is isothermal when the temperature is cooled to a point E_DWStarting heat supply, entering isothermal temperature EF for isothermal maintenance, and controlling the isothermal temperature EF and the isothermal time E₁F₁(ii) a After the steel is kept isothermally, stopping heat supply from a point F, cooling along with the furnace and controlling the cooling speed until a point G is the nose tip temperature T of the C curve_POpening the furnace door (cover) at the temperature of minus 20-60 ℃; the nose tip temperature T of a C curve is obtained when the furnace is cooled to a point H_P- (180-220) DEG C, discharging from the furnace, and air-cooling to room temperature J point;

the method comprises the following steps:

austenitizing the steel;

(II) keeping the steel isothermal;

and (III) controlled cooling of the steel.

2. The austenitization of the steel is:

in the austenitizing process of isothermal spheroidizing annealing of steel, a charging mode, a heating method, a temperature rise speed, a heating speed, heating uniformity, heating efficiency, a heating temperature and heat preservation time are used for controlling the austenitizing degree, and heating and cooling sections are uniformly and respectively diathermized to required temperatures, so that austenite phase transformation is completed, austenite grains are not grown, the carbon concentration distribution of austenite is non-uniform, a large number of undissolved cementite particles are reserved, a spontaneous core of carbide spheroidization is formed, heating stress is reduced, and conditions are provided with low cost, so that the expected uniform spheroidized structure and performance reach the standard are obtained;

secondly, the isothermal spheroidizing annealing of the steel is carried out, the steel is put into a furnace at a low temperature in the austenitizing process, and the steel is charged in a stacking mode which is favorable for heat circulation, uniform diathermy of heating and cooling sections and complete cooling to the required temperature respectively; the cross section is uniform in heat transmission and cold transmission, so that the heating uniformity and the cooling uniformity are improved, the temperature is uniform, the heating and cooling time and the isothermal time are shortened, and the period is shortened;

(1) the steel section is uniformly diathermically heated to the required temperature;

1) the heating uniformity is improved, the heating stress is reduced, the energy difference of a temperature field of the steel is reduced, and the temperature is uniform;

2) shortening the time for uniformly penetrating heat of the steel section to reach the required temperature;

shortening the heating period, advancing the construction period, saving energy and resources and reducing the cost;

secondly, a large amount of undissolved carbide particles are reserved in austenite, and the solubility distribution of austenite carbon is non-uniform, so that a spontaneous core for carbide spheroidization is formed, and the spheroidization is facilitated;

thirdly, the high-temperature retention time is short, and austenite grains are not grown up in time, so that fine austenite grains are obtained;

(2) uniformly and thoroughly cooling the steel section to the required temperature;

1) the cooling uniformity is improved, the cooling stress is reduced, the energy difference of a temperature field of the steel is reduced, and the temperature is uniform;

2) the time for uniformly and thoroughly cooling the steel section to the required temperature is shortened;

shortening the cooling period, advancing the construction period, saving energy and resources and reducing the cost;

secondly, the time of uniformly and thoroughly cooling the section of the austenitized steel to isothermal temperature is shortened, and the temperature of Ar is increased₁Cooling rate near temperature, avoiding in Ar₁The cooling speed near the temperature is too low, and coarse cementite appears on the grain boundary of the prior precipitation ferrite or wide strip ferrite appears around the prior precipitation ferrite, so as to prevent the generation of non-uniform spheroidized structure and performance;

thirdly, the time for uniformly and thoroughly cooling the cross section to the tapping temperature after the isothermal maintenance of the steel and the time for uniformly and thoroughly cooling the cross section to the room temperature after tapping are shortened, and Ar is shortened₁The time of heat preservation below the temperature is avoided from being in Ar₁Keeping the temperature below the temperature for a long time, and generating coarse cementite on the grain boundary of the prior precipitation ferrite or generating wide strip ferrite around the prior precipitation ferrite so as to prevent the generation of non-uniform spheroidized structure and performance;

in the austenitizing process of the isothermal spheroidizing annealing of the steel, after the steel is charged into a furnace, the heating speed is increased by increasing the heating temperature, the rapid heating is carried out, and the temperature rising speed is controlled; as shown in FIG. 3;

(1) the heating speed is increased by increasing the heating temperature, and the rapid heating has the advantages that:

1) the heating time and period are shortened, the construction period is advanced, energy and resources are saved, and the cost is reduced;

2) fine austenite grains are obtained; this is due to:

firstly, the steel has certain heating temperature, high heating speed and large superheat degree, austenite is formed at higher temperature, the number of formed cores is large, and the initial austenite crystal grains are small;

secondly, the heating speed is improved, and the heating time is shortened, namely the high-temperature retention time is short, and austenite grains are not grown in time;

(2) after the steel is charged into the furnace, heating the steel to a point A along with the furnace from a point O, wherein the heating speed OA is (150-200) DEG C/h;

(3) the steel is heated up to the point A along with the furnace from the point O to be the rapid heating temperature AB, enters the rapid heating temperature AB for rapid heating, and controls the rapid heating temperature AB and the heat preservation time A₁B₁Preparing for controlling the austenitizing degree; as shown in FIG. 3;

(4) the rapid heating temperature AB of the steel is higher than the required heating temperature T_SJThe temperature is 60-160 ℃ higher;

namely: AB ═ T_SJ+(60～160)℃＝Ac₁+(20～35)℃+(60～160)℃

1) The eutectoid steel, hypereutectoid steel and alloy tool steel are as follows:

Ac₁+(20～30)℃+(60～160)℃

examples are: GCr15 Steel Ac₁At a temperature of 745 c,

then: AB ═ Ac₁+(20～30)+(60～160)＝745+(20～30)+(60～160)

＝(745+20～745+30)+(60～160)＝(765～775)+(60～160)

＝765+60～775+160＝(825～935)℃

The rapid heating temperature AB of GCr15 steel is (825-935) DEG C;

2) the cold extrusion molding low-carbon steel and low-carbon low-alloy steel comprises the following steps:

Ac₁+(25～35)℃+(60～160)℃

examples are: 20CrMo Steel Ac₁At a temperature of 743 c,

then: AB ═ Ac₁+(25～35)+(60～160)＝743+(25～35)+(60～160)

＝(743+25～743+35)+(60～160)＝(768～778)+(60～160)

＝768+60～778+160＝(828～938)℃

The rapid heating temperature AB of the 20CrMo steel is (828-938) DEG C;

(5) the quick heating zero heat preservation time of the steel is as follows: the surface temperature is 90-95% of the required heating temperature, and the core temperature is 80-85% of the required heating temperature; this is:

improving the heating uniformity and the temperature field temperature of the uniform steel to prevent the surface temperature from exceeding the required heating temperature T_SJTurning to the required heating temperature CD for heating, and slowing the rest (5-10)% of the required heating temperature T_SJIn preparation for controlling the austenitizing degree of the steel;

(6) in the prior art, steel is rapidly heated at a high temperature, the temperature of the furnace is increased to be higher than the required heating temperature (100-200) DEG C in advance, then a steel piece is charged and heat supply is stopped, when the temperature of the furnace is reduced to the required heating temperature, heat supply is started, heating is carried out at the required heating temperature, the required heating temperature and heat preservation time are controlled, and the next heat treatment process is carried out after thorough burning; as shown in FIG. 4;

1) heat loss is caused, the higher the furnace temperature is, the more serious the heat loss is, and the energy conservation is limited; as shown in fig. 3 and 4, the heating time of the prior art is longer than that of the present invention;

2) the personal safety of operators is damaged, and the serious personal safety accident of electric shock casualty occurs because the high-temperature air is conductive; the following should be: stopping heat supply (power off), and then charging the steel piece;

3) the operator is charged at high temperature, so that the labor intensity is increased, and the personal safety of the operator is not good;

after the isothermal spheroidizing annealing of the steel AB is rapidly heated and the temperature is kept, the heat supply is stopped from the point B, the steel AB is cooled along with the furnace, and the section is uniformly and thoroughly cooled until the point C is the required heating temperature T_SJStarting to supply heat, and entering the required heating temperature CD for heating, as shown in FIG. 3; the heating method is that the rapid heating temperature AB is higher than the required heating temperature T_SJShifting to the required heating temperature CD to the required heating temperature T at 60-160 DEG C_SJHeating is carried out because AB is rapid heating, the heating speed is higher than CD heating speed, large temperature deviation of the temperature field of the steel is caused, the surface temperature is higher than the core temperature, and the rest (5-10)% of the required heating temperature T is slowed down_SJEffect of heating rate:

(1) at the moment that the steel is heated to a certain temperature, the surface temperature is higher than the core temperature, so that the heating uniformity is improved, and the heating stress is reduced;

(2) to uniform the temperature of the steel temperature field, reduce the temperature difference between the surface and the core, and prevent the steel surface temperature from exceeding the required heating temperature T_SJThe uniform and consistent austenitizing degree of the steel section is controlled to obtain the expected uniform and consistent spheroidized structure and the performance reaches the standard;

the internal part of the substance is the change from high energy to low energy state, and the heating of the steel in a heating temperature field is the process of transferring the high energy of the steel surface to the center of the low energy;

the steel is heated to a certain degree in a heating temperature field, the heat transfer speed from the surface to the core part is continuously attenuated and tends to be gentle, and the steel is in a sub-equilibrium state because the energy difference between the surface and the core part of the steel is reduced; at this point, it continues to be rapid (above the desired heating temperature T)_SJ) Heating, the surface energy of the steel is increased more than the energy transferred to the core, and cumulatively, the surface temperature of the steel exceeds the required heating temperature T_SJI.e., the surface heating temperature of the steel is high, resulting in:

1) the heating temperature of the steel surface is higher;

firstly, generating a poor spheroidizing tissue;

secondly, austenite grains are large, the carbon content in austenite is distributed more uniformly, undissolved carbide is less, and spherical, coarse sheet and punctate pearlite structures are formed after isothermal spheroidizing annealing;

the austenite grains are large, the carbon content in austenite is distributed uniformly, a large amount of undissolved carbide particles are not reserved, and a spontaneous core for carbide spheroidization cannot be formed, so that the spheroidization is not facilitated;

high strength and hardness, and reduced plasticity;

the isothermal spheroidizing annealing of the steel has high heating temperature, poor spheroidizing tissues are generated, tissue preparation cannot be carried out for final quenching, uniform quenching tissues cannot be obtained, the performance of the quenched steel is reduced, and quenching defects are increased; the hardness of eutectoid steel, hypereutectoid steel and alloy tool steel is higher, and the cutting processing difficulty is increased; the low-carbon steel and the low-carbon low-alloy steel formed by cold extrusion have high strength and hardness, reduce plasticity, have poor cold deformation capability and cannot be subjected to cold deformation smoothly;

2) the heating temperature of the steel surface is higher;

firstly, the surface of the steel has oxidation and decarburization phenomena and a black structure;

secondly, poor spheroidization tissue is generated;

thirdly, the structure is completely austenitized, and carbide is completely dissolved in austenite, so that the carbide core without non-spontaneous nucleation is supplied for carbon atoms to diffuse and grow in the subsequent isothermal and slow cooling process, and coarse lamellar pearlite and ferrite are generated by the austenite directly in the slow decomposition process;

high strength and hardness, reduced plasticity and large cold deformation resistance;

the isothermal spheroidizing annealing of the steel has higher heating temperature, produces a spheroidized poor structure, can not prepare the structure for final quenching, can not obtain a uniform quenching structure, reduces the performance of the quenched steel and increases the quenching defect; the eutectoid steel, hypereutectoid steel and alloy tool steel have higher hardness, and the cutting processing difficulty is increased; the low-carbon steel and the low-carbon low-alloy steel formed by cold extrusion have higher strength and hardness, reduce plasticity and have large cold deformation resistance, thus causing cold deformation forming failure;

(V) after the isothermal spheroidizing annealing rapid heating AB of the steel is subjected to heat preservation, the steel enters the required heating temperature CD to be heated, and the required heating temperature CD and the heat preservation time C are controlled₁D₁；

Isothermal spheroidizing annealing heating temperature of steel-required heating temperature T_SJThe heat preservation time is used for controlling the austenitizing degree, so that the austenite phase transformation is completed, austenite grains do not grow, a large number of undissolved carbide particles are remained in austenite, the non-uniformity of the carbon solubility of austenite is caused, a spontaneous core for carbide spheroidization is formed, the spheroidization is facilitated, the cross section is uniformly diathermized to the required heating temperature, the heating stress is reduced, the cost is low, and the expected uniform spheroidized tissue and the performance reach the standard;

(1) heating temperature CD of steel-required heating temperature T_SJComprises the following steps: ac of₁+(20～35)℃

The heating temperature of the steel, namely the required heating temperature is low, after isothermal spheroidizing annealing, a part of rolled or forged structure is reserved in the structure, and a part of original lamellar pearlite is reserved; the heating temperature is high, austenite grains grow, the carbon content in austenite is uniformly distributed, undissolved carbide is less, and a carbide spheroidization spontaneous core cannot be formed; the heating temperature of the steel, the required heating temperature, is therefore: ac of₁+(20～35)℃；

1) Eutectoid steel, hypereutectoid steel and alloy tool steel T_SJIs Ac₁+ (20-30) DEG C; namely:

CD＝T_SJ＝Ac₁+(20～30)℃

examples are:

(ii) GCr15 Steel Ac₁Is 745 ℃ then

CD＝T_SJ＝Ac₁+(20～30)＝745+(20～30)＝745+20～745+30

＝(765～775)℃

Heating temperature CD-required heating temperature T of GCr15 steel_SJIs (765-775) DEG C;

heating temperature of GCr15 Steel in prior art-required heating temperature T_SJIs (730-750) DEG C; the low heating temperature causes:

i) when the heating temperature is 730 ℃, an under-heat spheroidization defective structure is generated, and because the heating temperature is too low, part of rolled or forged structures are reserved in the spheroidization structure, most of carbides in austenite are not dissolved, and part of original flaky pearlite is reserved; partially dissolved carbide in austenite is reduced in size or broken in a sheet shape due to low temperature, and a plurality of undissolved broken carbide cores cannot grow to form point pearlite; as shown in FIG. 6; the hardness is higher;

ii) when the heating temperature is 750 ℃, poor heat spheroidization structures are generated, a small part of rolling or forging structures are reserved in the spheroidization structures due to the lower heating temperature, a small part of carbides in austenite are not dissolved, and a small part of original lamellar pearlite is preserved; partially dissolved carbide in austenite, because of low temperature, a plurality of undissolved broken carbide cores can not grow to form point pearlite; as shown in FIG. 7; the hardness is higher;

③ heating temperature of GCr15 Steel in prior art-required heating temperature T_SJ780-810 ℃ C; the higher heating temperature causes:

i) poor spheroidized tissue is generated;

ii) austenite grains are large, the carbon content in austenite is distributed more uniformly, undissolved carbide is less, and spherical and flaky pearlite structures are formed after spheroidizing annealing;

iii) the austenite has large crystal grains, the carbon content in the austenite is distributed more uniformly, a large amount of undissolved carbide particles are not reserved, and a spontaneous core for carbide spheroidization cannot be formed, so that the spheroidization is not facilitated;

iv) poor spheroidization and high hardness;

heating temperature of steel-required heating temperature T in prior art_SJIs A₁+(20～30)℃；

Examples are: GCr15 Steel A_1sAt 750 ℃ A_1fAt 795 ℃ A₁The temperature range is (750-795) DEG C;

i) when A is₁T is at the middle limit (770-775) DEG C_SJIs A₁+ (20-30) DEG C; namely:

T_SJ＝A₁+(20～30)＝(770～775)+(20～30)

＝770+20～775+30＝(790～805)℃

heating temperature-required heating temperature T of GCr15 steel in prior art_SJAt 790-805 ℃ C; the higher heating temperature causes: austenite grains are large, the carbon content in austenite is distributed more uniformly, undissolved carbide is less, and spherical, coarse sheet and punctate pearlite structures are formed after isothermal spheroidizing annealing; the carbon content in austenite is distributed more uniformly, a large amount of undissolved carbide particles are not reserved, and a spontaneous core for carbide spheroidization cannot be formed, so that the spheroidization is not facilitated; poor spheroidized tissues are generated, and the hardness is higher;

ii) when A is₁At an upper limit of 775 ℃, T_SJIs A₁+ (20-30) DEG C; namely:

T_SJ＝A₁+(20～30)＝775+(20～30)

＝795+20～795+30＝(815～825)℃

heating temperature-required heating temperature T of GCr15 steel in prior art_SJIs (815-825) DEG C; the higher heating temperature causes: austenite grains are large, the carbon content in austenite is uniformly distributed, and a coarse lamellar pearlite structure is formed after isothermal spheroidizing annealing; the carbon content in austenite is uniformly distributed, undissolved carbide particles are not reserved, and spontaneous cores of carbide spheroidization cannot be formed, so that the method is not favorable forSpheroidizing; poor spheroidized tissues are generated, and the hardness is higher;

in the prior art, the heating temperature of GCr15 steel is too low, too high or too high, after isothermal spheroidizing annealing, the hardness is too high or too high, and the difficulty of cutting processing is increased; poor spheroidized structure is generated, the structure preparation can not be carried out for the final quenching, uniform quenching structure can not be obtained, the performance of the quenched steel is reduced, and the quenching defect is increased;

2) the low-carbon steel and the low-carbon low-alloy steel which are formed by cold extrusion are Ac₁+ (25-35) DEG C; namely, it is

CD＝T_SJ＝Ac₁+(25～35)℃

Examples are:

20CrMo Steel Ac₁At 743 deg.C, then

CD＝T_SJ＝Ac₁+(25～35)＝743+(25～35)＝743+25～743+35

＝(768～778)℃

Heating temperature CD-required heating temperature T of 20CrMo steel_SJIs (768-778) DEG C;

heating temperature-required heating temperature T of 20CrMo steel in prior art_SJIs (750-760) DEG C; the lower heating temperature causes:

i) generating poor-heat spheroidizing tissues, and reserving part of rolling or forging tissues in the spheroidizing tissues;

ii) most of the carbides in the austenite are not dissolved, and part of the original lamellar pearlite is remained;

iii) partially dissolved carbide in austenite, because of low temperature, only size is reduced or flaky broken, and a plurality of undissolved broken carbide cores can not grow to form point pearlite;

(iv) poor spheroidization, high strength and hardness, reduced plasticity and poorer cold deformation capability;

③ heating temperature of 20CrMo steel in the prior art-required heating temperature T_SJIs (800-840) DEG C; the high heating temperature causes:

i) when the heating temperature is 800 ℃, the heating temperature is higher than Ac (close to Ac)₃(818 ℃ C.) to produce a poorly spheroidized structure of austeniteThe grain size is large, the carbon content in austenite is distributed more uniformly, undissolved carbide is less, a large amount of undissolved carbide particles are not reserved, a spontaneous core for carbide spheroidization cannot be formed, and spheroidization is not facilitated; forming spherical, coarse sheet and punctate pearlite structures after isothermal spheroidizing annealing; as shown in FIG. 8; the strength and the hardness are higher, the plasticity is reduced, and the cold deformation capability is poorer;

ii) when the heating temperature is 840 ℃, the heating temperature is too high (Ac > C)₃(818 ℃ C.), the surface has serious oxidation and decarburization phenomena and black tissues; heating at the temperature to completely austenitize the structure, completely dissolving carbide in austenite, so that the carbide core without non-spontaneous nucleation in the subsequent isothermal and slow cooling process is supplied with carbon atoms for diffusion and growth, and coarse lamellar pearlite and ferrite are generated from the austenite directly in the slow decomposition process; as shown in FIG. 9; the strength and the hardness are higher, the plasticity is reduced, and the cold deformation resistance is large;

in the prior art, the isothermal spheroidizing annealing heating temperature of 20CrMo steel is low, high and overhigh, and after spheroidizing annealing, a spheroidized poor tissue is generated, so that tissue preparation can not be carried out for final quenching, a uniform quenching tissue can not be obtained, the performance of the quenched steel is reduced, and the quenching defect is increased; the heating temperature is low or high, the strength and the hardness are high, the plasticity is reduced, the cold deformation capability is poor, and the cold deformation forming cannot be smoothly carried out; the heating temperature is too high, the strength and the hardness are higher, the plasticity is reduced, the cold deformation resistance is large, and the cold deformation forming failure is caused;

(2) required heating temperature holding time C of steel₁D₁The heat preservation time is zero, namely the heat preservation is carried out until the temperature of the D point on the surface of the steel is the required heating temperature T_SJThe temperature of the D' point of the core part is more than 90 percent of the required heating temperature T_SJ(ii) a As shown in fig. 3.

3. The isothermal hold of the steel is:

the method comprises the following steps of (I) austenitizing the isothermal spheroidizing annealing of steel, then keeping the steel isothermal, controlling the cooling speed, the cooling uniformity, the cooling efficiency, the supercooling degree, the isothermal temperature and the isothermal time from austenitizing to isothermal temperature, and providing conditions for fully transforming carbide spheroidized tissues so as to obtain the expected uniform spheroidized tissues and reach the performance standard;

secondly, after austenitizing the isothermal spheroidizing annealing of the steel, stopping heat supply from a point D, cooling along with the furnace, uniformly and thoroughly cooling the section to a point E, wherein the cooling speed DE is (40-60) DEG C/h;

(III) the section of the isothermal spheroidizing annealing of the steel is uniformly and thoroughly cooled to the point E which is the isothermal temperature T_DWStarting to supply heat, and entering an isothermal temperature EF for isothermal maintenance;

isothermal temperature EF of the steel being T_DWThe transformation from super-cooled austenite to pearlite is shown in curve C₁Line to nose tip temperature T_PIn the temperature range of isothermal maintenance, i.e. A₁＞T_DW＞T_P(ii) a And the condition that supercooled austenite is directly transformed into spherical pearlite is that the supercooling degree is smaller at a curve A of C₁Higher temperature isothermal hold below line, due to A₁＞Ac₁＞Ar₁I.e. slightly below Ac₁Temperature, form a large number of cementite crystal nuclei dispersed uniformly in austenite grains, avoid in Ar₁The following temperature is kept isothermally for a long time to prevent coarse cementite from appearing on the grain boundary of the prior precipitation ferrite or wide strip ferrite from appearing around the prior precipitation cementite; the isothermal temperature EF of the steel is therefore:

A₁＞Ac₁＞T_DW＞T_Pand A is₁＞Ac₁＞T_DW＞Ar₁

I.e. EF ═ T_DW＝Ac₁-(20～40)℃

This is because the isothermal temperature of the steel is low, the supercooling degree is large, and the transition temperature is at the nose tip temperature T of the supercooled austenite isothermal transition curve_PAbove, phase transition driving force Δ G_VLarge, the conversion speed is fast; otherwise, the transition speed is slow; isothermal transformation from supercooled austenite to globular pearlite is at curve C₁Line to nose tip temperature T_PIn a temperature range of between, i.e. at A₁＞T_DW＞T_PWithin a temperature range and with a small supercooling degree in curve C₁The transformation is carried out under the condition of keeping the temperature below the line at a higher temperature and keeping the temperature isothermally; the isothermal temperature of the steel is thus Ac₁-(20～40)℃；

(1) The isothermal temperatures EF of eutectoid, hypereutectoid and alloy tool steels are:

EF＝T_DW＝Ac₁-(20～40)℃

examples are:

1) GCr15 Steel A₁At 750 ℃ Ac₁At 745 ℃ Ar₁At 700 ℃ C, T_PThe temperature of the reaction kettle is 600 ℃,

EF＝T_DW＝Ac₁-(20～40)＝745-(20～40)＝745-20～745-40

＝(705～725)℃

the isothermal temperature EF of the GCr15 steel is (705-725) DEG C;

2) isothermal temperature T of GCr15 Steel of the prior art_DWComprises the following steps: ar (Ar)₁-(20～30)

T_DW＝Ar₁-(20～30)＝700-(20～30)＝700-20～700-30

＝(670～680)℃

In the prior art, the isothermal temperature of GCr15 steel is 670-680 ℃;

this is due to this isothermal temperature T_DWAlthough the temperature is higher than the nose tip temperature T of the C curve_PBut less than Ar₁Temperature, isothermal temperature T_DWThe lower is caused by:

the supercooling degree is high, carbon and alloy elements are difficult to diffuse in austenite, a large amount of uniformly dispersed cementite crystal nuclei cannot be formed in austenite crystal grains, and the spheroidization process is not facilitated;

secondly, increasing supercooling degree during isothermal transformation; as the nucleation rate of precipitated carbide is increased along with the increase of supercooling degree during the austenite decomposition, most of carbon atoms are point-like carbides except for the carbide which is not dissolved during the heating and grows up, which is not beneficial to the spheroidization process;

the supercooling degree is large, so that a spontaneous core for carbide spheroidization cannot be formed in austenite grains, and the spheroidization process is not facilitated;

fourthly, in Ar₁Isothermal maintenance at a temperature of Ar₁The long-term heat preservation at the following temperature causes the wide periphery of the first-precipitation cementiteThe nodulizing structure and the performance of the strip ferrite are not uniform;

producing poor spheroidization tissue with high hardness;

in the prior art, the GCr15 steel has low isothermal temperature and high hardness after isothermal spheroidizing annealing, so that the difficulty of cutting processing is increased; poor spheroidized structure is generated, the structure preparation can not be carried out for the final quenching, uniform quenching structure can not be obtained, the performance of the quenched steel is reduced, and the quenching defect is increased;

(2) the isothermal temperature of the low-carbon steel and the low-carbon low-alloy steel formed by cold extrusion is as follows:

EF＝T_DW＝Ac₁-(20～35)℃

examples are:

1)20CrMo Steel A₁Ac at 755 deg.C₁At 743 ℃ Ar₁At 504 ℃ and T_PAt 700 ℃ is then

EF＝T_DW＝Ac₁-(20～35)＝743-(20～35)＝743-20～743-35

＝(708～723)℃

Isothermal temperature T of 20CrMo steel_DWIs (708-723) DEG C;

2) the isothermal temperature of the low-carbon low-alloy steel formed by cold extrusion in the prior art is as follows:

T_DW＝Ar₁- (20-30) DEG C, wherein the isothermal temperature T of the 20CrMo steel_DWAt 640 ℃;

T_DW＝Ar₁-(20～30)＝504-(20～30)＝504-20～504-30

＝(474～484)℃

isothermal temperature T of low-carbon low-alloy steel 20CrMo steel formed by cold extrusion in prior art_DWAt an isothermal temperature T of (474-484) ° C_DWLess than Ar₁Temperature, and less than C curve nose tip temperature T_P(700 ℃), which does not belong to the constant temperature transformation of the isothermal cooling mode pearlite after austenitizing;

due to the isothermal temperature T of the 20CrMo steel in the prior art_DWIs (474-484) DEG C, does not belong to isothermal (constant temperature transformation) spheroidizing annealing after austenitizing, and therefore, the isothermal temperature T of 20CrMo steel is adopted in the prior art_DWAdjusting the temperature to 640 ℃; this isothermal temperature T_DWStill less than the nose tip temperature T of the C curve_P(700 ℃), which does not belong to the isothermal cooling mode after austenitizing to generate the constant temperature transformation of pearlite, but belongs to the continuous cooling mode after austenitizing to obtain mixed tissues with uneven thickness or different types along with the transformation of continuous change of supercooling degree in a temperature range; as shown in FIG. 10; similar to ordinary spheroidizing annealing, the steel is heated to slightly above Ac₁Slowly cooling to 500-600 ℃ after austenitizing, discharging and air cooling;

this is due to the isothermal transformation from supercooled austenite to globular pearlite at curve C A₁Line to nose tip temperature T_PIn a temperature range of between, i.e. at A₁＞T_DW＞T_PAnd at a lower supercooling degree in curve C₁The transformation is carried out under the condition of keeping the temperature below the line at a higher temperature and keeping the temperature isothermally;

the isothermal temperature of 20CrMo steel in the prior art is Ar₁Carrying out isothermal (constant temperature transformation) spheroidizing annealing at the temperature of minus 20-30 ℃ or 640 ℃, wherein the temperature does not belong to pearlite, and obtaining mixed tissues with uneven thickness or different types; no tissue preparation for final quenching; the strength and the hardness are higher, the plasticity is reduced, and the cold deformation resistance number deformation hardening index is increased, so that cold deformation forming failure is caused;

isothermal spheroidizing annealing isothermal time E of steel₁F₁The time required for finishing the structure transformation, namely spheroidization, and the time for uniformly and thoroughly cooling the section to the isothermal temperature; isothermal time E₁F₁Is (4-6) h; this is due to: (ii) a

(1) Isothermal time E₁F₁Short, complete the structure transformation-insufficient spheroidization and high hardness;

(2) isothermal time E₁F₁Long time, waste of energy and resources, and in Ar₁Keeping the temperature for a long time, wherein abnormal structures, namely eutectoid steel, hypereutectoid steel and alloy tool steel, appear wide strip-shaped ferrite around the first precipitated cementite, and coarse cementite appears on the grain boundary of the first precipitated ferrite in the cold extrusion formed low-carbon steel and low-carbon low-alloy steel;

(3) isothermal time E₁F₁Is (4-6) h; after austenitizing the steel, cooling the steel along with the furnace, wherein the cooling speed DE is (40-60) DEG C/h;

1) when the cooling speed DE is 60 ℃/h at the upper limit and the isothermal time E₁F₁Is 6 h;

2) when the cooling speed DE is 50 ℃/h at the middle limit and the isothermal time E₁F₁Is 5 h;

3) when the cooling speed DE is lower limit 40 ℃/h, isothermal time E₁F₁Is 4 h;

4. the controlled cooling of the steel is as follows:

firstly, the isothermal spheroidizing annealing of the steel utilizes the characteristic of 'the stability of undercooled austenite of the steel' to carry out controlled cooling after austenitizing,

the supercooled austenite stability characteristics of the steel are:

(1) the supercooling austenite isothermal transformation curve is similar to the shape of the C curve, and the direction is opposite, so that the supercooling austenite isothermal transformation curve has the characteristic of the C curve; as shown in FIG. 2;

the transformation temperature of the super-cooled austenite of the steel is at the nose tip temperature T of the super-cooled austenite isothermal transformation curve_PNear, phase transition driving force Δ G_VThe transformation speed reaches a maximum value as a result of the combined action of two factors of the atomic diffusion coefficient D; transition temperature at the nose tip temperature T of super-cooled austenite isothermal transition curve_PAbove, the transition speed is driven by the phase change driving force Δ G_VControlling, namely increasing along with the increase of the supercooling degree in a proportional relation; transition temperature at the nose tip temperature T of super-cooled austenite isothermal transition curve_PThe transformation speed is controlled by the atomic diffusion coefficient D, and is reduced along with the increase of the supercooling degree, and the transformation speed is in inverse proportion relation;

the supercooled austenite stability characteristic of the steel provides technical support for a charging mode of isothermal spheroidizing annealing, a cooling method after austenitizing, and control of supercooling degree, cooling speed, cooling uniformity and cooling efficiency, so as to obtain expected uniform spheroidized tissues and provide conditions for reaching the performance standard;

secondly, in the austenitizing process of the isothermal spheroidizing annealing of the steel, the steel is charged in a stacking mode which is favorable for heat circulation, uniform diathermy of heating and cooling sections and thorough cooling to the required temperature respectively;

(III) after the isothermal spheroidizing annealing of the steel is quickly heated and preserved, stopping heat supply from the point B, cooling along with the furnace, uniformly and thoroughly cooling the section to the point C which is the required heating temperature T_SJHeating, and heating at the required heating temperature CD;

after austenitizing the isothermal spheroidizing annealing of the steel, stopping heat supply at a point D, cooling along with the furnace, uniformly and thoroughly cooling the section to an isothermal temperature T at a point E_DWThe general rule of the influence of the cooling speed on the structure and the performance of the steel after the isothermal spheroidizing annealing is as follows: the cooling speed is high, and the austenite decomposition temperature is low, so that the pearlite transformation product is fine, the stress is large, and the hardness is high; if the cooling rate is too slow, coarse cementite can appear on the proeutectoid ferrite grain boundary or ferrite strips can appear around the proeutectoid cementite to form net ferrite or cementite; in order to achieve the expected standard of tissue and performance, the cooling speed DE is (40-60) DEG C/h;

this is due to the fact that the transition temperature is at the nose tip temperature T of the supercooled austenite isothermal transition curve before the temperature of the steel is reduced to the isothermal temperature_PThe cooling speed is high, the supercooling degree is large, and the phase change driving force delta G is realized_VLarge, the conversion speed is fast; otherwise, the transition speed is slow; therefore, the cooling speed is (40-60) DEG C/h; as shown in FIG. 2;

(1) in the prior art, after austenitizing GCr15 steel, cooling the steel at a cooling speed of 10-30 ℃ per hour to a temperature of discharging for air cooling; this is due to:

1) austenitizing GCr15 steel, and cooling to isothermal temperature T at cooling speed of 10-30 ℃/h_DWTransition temperature at the super-cooled austenite isothermal transition curve nose tip temperature T_PThe above-mentioned materials are cooled too slowly, and the supercooling degree is too small, and the transition speed is influenced by phase-change driving force delta G_VControl, increasing with increasing supercooling degree, direct proportional relation, i.e. transition speed, decreasing with decreasing supercooling degree, when supercooling degree is too small, due to phase change driving force deltag_VToo small because ofThis transition speed is too slow;

2) the GCr15 steel is cooled at Ar at the over-slow cooling speed of (10-30) DEG C/h₁Cooling at the temperature nearby, the aggregation and growth of carbides cause carbide segregation, wide strip-shaped ferrite appears around the cementite which is precipitated firstly, and the spheroidization structure and the performance are not uniform;

3) the GCr15 steel is cooled at Ar at the over-slow cooling speed of (10-30) DEG C/h₁Cooling below temperature, increasing at Ar₁Cooling below the temperature for a time, the carbide aggregates and grows to cause carbide segregation, wide strip-shaped ferrite appears around the cementite which is precipitated first, and the spheroidization structure and the performance are not uniform;

in summary, after austenitizing the GCr15 steel in the prior art, the steel is cooled to the isothermal temperature T at a cooling speed of (10-30) DEG C/h_DWIsothermal holding is carried out, after isothermal spheroidizing annealing, spheroidized tissues and performances are not uniform, the difficulty of cutting processing is increased, and tissue preparation cannot be carried out for final quenching;

(2) the prior art steel is cooled to slightly less than Ar after austenitizing₁At a temperature of 650 to 700 ℃ or Ar₁Isothermal temperature T of about 20 ℃ below_DWCarrying out isothermal maintenance; this is due to:

after austenitizing the steel, cooling to C curve A at different cooling rates₁Isothermal temperature T of different temperature ranges below the line_DWIsothermal holding is carried out, different isothermal transformation occurs, the transformation characteristics are different, and the formed structure and the performance are also different;

taking 20CrMo steel as an example, A₁Ac at 755 deg.C₁At 743 ℃ Ar₁At 504 ℃ and T_PAt 700 ℃ and M_SIs 380 ℃;

1) after austenitizing, the steel is cooled to a C curve A at a cooling speed of 40-60℃/h and a small supercooling degree₁An isothermal temperature T at a higher temperature (708-723) DEG C below the line_DWIsothermal holding is carried out, and is (A)₁～T_P) High temperature transformation in a temperature range, wherein the transformation characteristic is diffusion type phase transformation with small supercooling degree to form a spherical pearlite structure; FIG. 5 shows cooling curve 1;

2) prior art steel austeniteCooling to curve A at a low supercooling degree at a fast cooling rate after formation₁T at 700 ℃ below the line_PIsothermal temperature T of_DWIsothermal holding is carried out, and is (A)₁～T_P) High temperature transformation in a temperature range, wherein the transformation characteristic is diffusion type phase transformation with low supercooling degree to form a pearlite type structure; equivalent to isothermal normalizing, as shown in fig. 5, cooling curve 2, forms lamellar pearlite structure, has higher strength and hardness, reduces plasticity, and causes cold deformation forming failure;

3) in the prior art, after austenitizing, the steel is cooled to a C curve A at a high cooling speed and a high supercooling degree₁Under line (T)_P＞T_DW＞M_S) Isothermal temperature T of_DWIsothermal holding is carried out, and is_P～M_S) The medium temperature transformation in the temperature range has the transformation characteristic that the semi-diffusion type phase transformation with larger supercooling degree forms a bainite structure;

curve A when cooling to C₁Below the line to 650 ℃ [ less than 700 ℃ (T)_P) Isothermal temperature T of_DWIsothermal holding is carried out, and is_P～M_S) The medium temperature transformation within the temperature range is characterized in that the semi-diffusion type phase transformation with large supercooling degree forms a bainite structure, which is equal to bainite transformation isothermal quenching, and as shown in a cooling curve 3 in figure 5, an upper bainite structure is formed, the strength and hardness are too high, the brittleness is large, and cold deformation forming cannot be carried out;

curve A when cooling to C₁Slightly below the line Ar₁Temperature or Ar₁An isothermal temperature T of about 484 ℃ below 20 DEG C_DWIsothermal holding is carried out, and is_P～M_S) The medium temperature transformation within the temperature range is characterized in that the semi-diffusion type phase transformation with large supercooling degree forms a bainite structure, which is equal to bainite transformation isothermal quenching, as shown in a cooling curve 4 of figure 5, the bainite structure is formed, the strength and hardness are too high, the brittleness is large, and cold deformation forming cannot be carried out;

in summary, the prior art austenitization followed by rapid cooling to slightly less than Ar₁The temperature is 650-700 ℃ or Ar₁Isothermal temperature T of about 20 ℃ below_DWIsothermal holding is carried out, 20CrMo steel is equivalently subjected to isothermal normalizing to form a lamellar pearlite structure, the strength and hardness are higher, the plasticity is reduced, the cold deformation resistance is large, cold deformation forming fails and is equivalently subjected to bainite transformation isothermal quenching, an upper bainite structure and a bainite structure are formed, the strength and hardness are too high, the brittleness is large, and cold deformation forming cannot be carried out;

after austenitizing the isothermal spheroidizing annealing of the steel, the section is uniformly and thoroughly cooled to the point E which is the isothermal temperature T_DWStarting to supply heat, and entering an isothermal temperature EF for isothermal maintenance;

after isothermal maintenance of isothermal spheroidizing annealing of steel, stopping heat supply from a point F, and cooling to a point G along with furnace cooling, wherein the cooling speed FG is (40-60) DEG C/h;

the reason is that the cooling temperature of the steel is constant, the cooling speed is high, the supercooling degree is large, and the transformation temperature is at the nose tip temperature T of the supercooling austenite isothermal transformation curve_PAbove, phase transition driving force Δ G_VLarge, the conversion speed is fast; otherwise, the transition speed is slow; therefore, the cooling speed is (40-60) DEG C/h;

1) after the GCr15 steel is kept at the same temperature, the steel is cooled to the temperature of air cooling after tapping at the cooling speed of (10-30) DEG C/h, and the transition temperature is at the nose tip temperature T of the super-cooled austenite isothermal transition curve_PThe above-mentioned materials are cooled too slowly, and the supercooling degree is too small, and the transition speed is influenced by phase-change driving force delta G_VControl, increasing with increasing supercooling degree, direct proportional relation, i.e. transition speed, decreasing with decreasing supercooling degree, when supercooling degree is too small, due to phase change driving force deltag_VToo small, and therefore too slow, transition speed;

2) the GCr15 steel is cooled at Ar at the over-slow cooling speed of (10-30) DEG C/h₁Cooling below temperature, increasing at Ar₁Cooling below the temperature for a time, the carbide aggregates and grows to cause carbide segregation, wide strip-shaped ferrite appears around the cementite which is precipitated first, and the spheroidization structure and the performance are not uniform;

(2) in the prior art, isothermal spheroidizing annealing of steel is kept isothermally and then discharged from a furnace for air cooling; this is due to:

1) the steel is discharged from the furnace for air cooling after being isothermally kept, and the transformation temperature is at the nose tip temperature T of the supercooling austenite isothermal transformation curve_PThe air cooling is carried out, namely the rapid cooling is carried out, the supercooling degree is large, and the transformation speed is driven by the phase change driving force delta G_VControl to increase with increasing supercooling degree, and when supercooling degree is larger, driving force Δ G due to phase change_VLarger, and therefore faster transition speeds;

2) curve A of steel at C₁Line to nose tip temperature T_PThe cooling speed in the temperature range is high, the austenite decomposition temperature is low, pearlite transformation products are fine, carbides are fine and dispersed, the spheroidization is poor, the stress is large, and the hardness is high;

after isothermal spheroidizing annealing and isothermal holding of the steel, cooling along with the furnace from a point F, uniformly and thoroughly cooling the section to a point G, and opening a furnace door (cover) at the temperature of: C-Curve nose tip temperature T_P-(20～60)℃

(1) The eutectoid steel, hypereutectoid steel and alloy tool steel are as follows:

C-Curve nose tip temperature T_P-(20～40)℃；

Examples are: GCr15 Steel T_PThe temperature of the reaction kettle is 600 ℃,

G＝T_P-(20～40)＝600-(20～40)＝600-20～600-40

＝(560～580)℃

the temperature of opening a furnace door (cover) of GCr15 steel when the steel is cooled to a point G along with the furnace is 560-580 ℃;

(2) the low-carbon steel and the low-carbon low-alloy steel formed by cold extrusion are as follows:

C-Curve nose tip temperature T_P-(40～60)℃；

Examples are: 20CrMo Steel T_PIs 700 ℃ at the temperature of the furnace,

G＝T_P-(40～60)＝700-(40～60)＝700-40～700-60

＝(640～660)℃

the temperature for opening a furnace door (cover) when the 20CrMo steel is cooled to a point G along with the furnace is 640-660 ℃;

(3) this is due to:

1) transformation temperature of super-cooled austenite of steel in super-cooled austenite isothermalNose tip temperature T of transformation curve_PNearby, the transition speed reaches a maximum value; transition temperature at the nose tip temperature T of super-cooled austenite isothermal transition curve_PThe transition speed is controlled by the atomic diffusion coefficient D and decreases with increasing supercooling degree;

2) in the cooling process of the steel, the cooling speed is attenuated continuously, the steel is cooled to a certain degree and tends to be gentle, and the steel is in a sub-equilibrium state, which is caused by the fact that the energy difference between the temperature field of the steel and the surrounding environment is reduced;

after the steel is kept isothermally, the temperature T of the nose tip part of the C curve is obtained when the temperature is cooled to the G point along with the furnace_PTemperature of (20-60) DEG C, opening a furnace door (cover), cooling along with the furnace, increasing the energy difference between a steel temperature field and the ambient environment, and further increasing the nose tip temperature T from the G point to the C curve_PTemperature T of the tip of nose of curve C from (20-60) DEG C to point H_PCooling speed at (-180-220) DEG C with the furnace;

3) avoiding the transition temperature of the supercooled austenite of the steel from being at the nose tip temperature T of the supercooled austenite isothermal transition curve_PThen, the cooling is carried out too slowly, the supercooling degree is too small, the transformation speed is controlled by the atomic diffusion coefficient D and is reduced along with the increase of the supercooling degree, and the inverse proportion relation is that the transformation speed is increased along with the reduction of the supercooling degree, so that the atomic diffusion coefficient D is prevented from being too large and the atomic diffusion speed is prevented from being too high when the supercooling degree is too small;

4) avoid too slow cooling speed of steel in Ar₁Cooling below temperature, increasing at Ar₁The cooling time below the temperature prevents carbide from aggregating and growing to cause carbide segregation and uneven spheroidization structure and performance;

eutectoid steel, hypereutectoid steel and alloy tool steel firstly precipitate cementite and generate wide strip-shaped ferrite around the cementite;

coarse cementite appears on the grain boundary of the pre-precipitation ferrite of the low-carbon steel and the low-carbon low-alloy steel formed by cold extrusion;

5) the cooling time and the cooling period are shortened, and the construction period is advanced;

(eighth), the isothermal spheroidizing annealing of the steel is cooled along with the furnace from the G point, the temperature of the uniform through cooling of the section to the H point and the air cooling of the section out of the furnace is as follows: C-Curve nose tip temperature T_P-(180～220)℃

C-Curve nose tip temperature T_P-(180～200)℃；

Examples are: the TP of GCr15 steel is 600 ℃,

H＝T_P-(180～200)＝600-(180～200)＝600-180～600-200

＝(400～420)℃

the temperature of the GCr15 steel from furnace cooling to the air cooling H point of discharging is (400-420) DEG C;

C-Curve nose tip temperature T_P-(200～220)℃；

Examples are: 20CrMo Steel T_PIs 700 ℃ at the temperature of the furnace,

H＝T_P-(200～220)＝700-(200～220)＝700-200～700-220

＝(480～500)℃

the temperature from the furnace cooling of the 20CrMo steel to the air cooling H point of the discharged steel is (480-500) DEG C;

(3) this is due to:

1) the transformation temperature of the super-cooled austenite of the steel is at the nose tip temperature T of the super-cooled austenite isothermal transformation curve_PThe transformation speed is controlled by the atomic diffusion coefficient D, and is reduced along with the increase of the supercooling degree, and the transformation speed is in inverse proportion relation;

2) in the cooling process of the steel, the cooling speed is attenuated continuously, the steel is cooled to a certain degree and tends to be gentle, and the steel is in a sub-equilibrium state because the energy difference between the temperature field of the steel and the surrounding environment is reduced;

the temperature of the steel is the nose tip temperature T of the C curve when the steel is cooled to the H point along with the furnace_P- (180-220) DEG C, tapping and air cooling, and increasing the energy difference between the temperature field of the steel and the ambient environment, thereby increasing the temperature T of the nose tip part of the curve C from the H point to the C point_PCooling at a temperature ranging from (180-220) DEG C to J point at room temperature;

3) avoiding the transition temperature of the supercooled austenite of the steel from being at the nose tip temperature T of the supercooled austenite isothermal transition curve_PThereafter, the cooling is too slow, the supercooling degree is too small, and the transition speed is affectedThe sub-diffusion coefficient D is controlled to be reduced along with the increase of the supercooling degree, and the inverse proportion relation, namely the transformation speed is increased along with the reduction of the supercooling degree, so that the phenomenon that when the supercooling degree is too small, the atomic diffusion coefficient D is too large and the atomic diffusion speed is too high is prevented;

coarse cementite appears on the grain boundary of the pre-precipitation ferrite of the low-carbon steel and the low-carbon low-alloy steel formed by cold extrusion; the spheroidized structure and the performance are not uniform;

(3) in the prior art, after the isothermal maintenance of steel, the steel is discharged from a furnace and cooled by air when the temperature of the steel is reduced to 300 ℃; this is due to:

1) super-cooled austenite transformation temperature of steel from nose tip temperature T_PTo 300 ℃, is the nose tip temperature T of the supercooled austenite isothermal transformation curve_PThe transformation speed is controlled by the atomic diffusion coefficient D and is reduced along with the increase of the supercooling degree, and the transformation speed is in inverse proportion relation, namely the transformation speed is increased along with the reduction of the supercooling degree, when the supercooling degree is over-low, the atomic diffusion coefficient D is over-high, and the atomic diffusion speed is over-high;

2) in the cooling process of the steel, the cooling speed is attenuated continuously, the steel is cooled to a certain degree and tends to be gentle, and the cooling speed is increased to Ar₁The cooling time below the temperature causes carbide aggregation and growth to cause carbide segregation, and spheroidized tissues and properties are not uniform; the cooling time and period are prolonged, and the construction period is delayed;

(nine) the isothermal spheroidizing annealing section of the steel is uniformly and thoroughly cooled to H point, the temperature of the H point discharged from the furnace is the nose tip temperature T of the C curve_PAnd (4) discharging at the temperature of (180-220) DEG C, and air-cooling to room temperature J point.

5. Isothermal spheroidizing annealing method of steel the temperature is based on the indication of the temperature of a hearth.

Claims

1. An isothermal spheroidizing annealing method of steel is characterized in that: in the austenitizing process of steel, the steel is put into a furnace at a low temperature, after the steel is loaded into the furnace in a stacking mode which is favorable for heat circulation, uniform heat penetration and cooling of the heating and cooling sections to the required temperature respectively, the heating speed is increased by increasing the heating temperature, the steel is quickly heated, the temperature is increased along with the furnace and the temperature increasing speed is controlled, the steel is increased to be higher than the required heating temperature, namely the quick heating temperature, the steel is quickly heated at the quick heating temperature, and the quick heating temperature and the heat preservation time are controlled; stopping heat supply after rapid heating and heat preservation, starting heat supply when the furnace is cooled to the required heating temperature, entering the required heating temperature for heating, and controlling the required heating temperature and heat preservation time; after austenitizing the steel, stopping heat supply, cooling along with the furnace, controlling the cooling speed, cooling to an isothermal temperature, starting heat supply, entering the isothermal temperature for isothermal maintenance, and controlling the isothermal temperature and isothermal time; after the steel is kept isothermally, stopping heat supply, cooling along with the furnace and controlling the cooling speed to cool to the nose tip temperature T of the C curve_P- (20-60) DEG C, opening a furnace door (cover), and cooling to the nose tip temperature T of the C curve along with the furnace_P- (180-220) DEG C, discharging from the furnace, and air-cooling to room temperature;

the method comprises the following steps:

austenitizing the steel;

(II) keeping the steel isothermal;

and (III) controlled cooling of the steel.

2. Austenitizing the steel according to claim 1, characterized in that the austenitizing of the steel is:

determining the furnace charging mode, the heating method, the heating speed, the heating uniformity, the heating efficiency, the heating temperature and the heat preservation time of the isothermal spheroidizing annealing of the steel in the austenitizing process as the control of the austenitizing degree, uniformly and respectively carrying out diathermy and cold permeation on a heating section and a cooling section to the required temperature so as to finish the phase transformation of the austenite, ensuring that austenite crystal grains are not grown, the carbon concentration distribution of the austenite is not uniform, reserving a large amount of undissolved cementite particles, reducing the heating stress and providing conditions with low cost;

secondly, determining that the steel enters the furnace at a low temperature in the austenitizing process of the isothermal spheroidizing annealing of the steel, and charging the steel by adopting a stacking mode which is favorable for heat circulation, uniform diathermy of heating and cooling sections and complete cooling to the required temperature;

determining the isothermal spheroidizing annealing of the steel in the austenitizing process, increasing the heating speed by increasing the heating temperature after the steel is charged into a furnace, carrying out rapid heating, raising the temperature along with the furnace and controlling the temperature rise speed to be higher than the required heating temperature, namely the rapid heating temperature, entering the rapid heating temperature for rapid heating, and controlling the rapid heating temperature and the heat preservation time;

(1) determining that the steel is rapidly heated, namely, heating the steel along with a furnace after charging, wherein the heating speed is (150-200) DEG C/h;

(2) determining that the rapid heating temperature of the steel is higher than the required heating temperature;

(3) determining the quick heating zero heat preservation time of steel as follows: the surface temperature is 90-95% of the required heating temperature, and the core temperature is 80-85% of the required heating temperature;

fourthly, after the isothermal spheroidizing annealing of the steel is determined to be rapidly heated and kept warm, stopping heat supply, cooling along with the furnace, uniformly and thoroughly cooling the section to the required heating temperature, heating at the required heating temperature, and controlling the required heating temperature and the heat preservation time;

(1) determining the heating temperature of the steel, wherein the required heating temperature is as follows: ac of₁+(20～35)℃

(2) Determining the required heating temperature and heating zero heat preservation time of the steel as follows: the surface temperature is the desired heating temperature and the core temperature is > 90% of the desired heating temperature.

3. Isothermal holding of steel according to claim 1, characterized in that the isothermal holding of steel is:

determining that the austenitizing of the steel by isothermal spheroidizing annealing enters isothermal maintenance, and controlling the cooling speed, the isothermal temperature and the isothermal time of the austenitized section which is uniformly and thoroughly cooled to the isothermal temperature;

secondly, after austenitizing of isothermal spheroidizing annealing of the steel is determined, stopping heat supply, cooling along with the furnace, wherein the cooling speed is 40-60 ℃ per hour, and the section is uniformly and thoroughly cooled to isothermal temperature for isothermal maintenance;

and (III) determining the isothermal spheroidizing annealing isothermal temperature of the steel as follows: ac of₁-(20～40)℃

Fourthly, determining the isothermal time of the isothermal spheroidizing annealing of the steel as the time required for finishing the structure transformation, namely spheroidizing and the time for uniformly and thoroughly cooling the section to the isothermal temperature; the isothermal time is (4-6) h.

4. Controlled cooling of steel according to claim 1, characterised in that the controlled cooling of steel is:

determining a charging mode of isothermal spheroidizing annealing of steel, a cooling method, a supercooling degree, a cooling speed, cooling uniformity and cooling efficiency after austenitizing, and providing conditions for controlling the transformation speed, nucleation rate and growth speed of supercooled austenite;

secondly, determining that the steel is rapidly heated into steel to enter a furnace at a low temperature in the austenitizing process of the isothermal spheroidizing annealing of the steel, and charging the steel by adopting a stacking mode which is favorable for heat circulation, uniform diathermy of heating and cooling sections and complete cooling to the required temperature respectively;

thirdly, after the isothermal spheroidizing annealing of the steel is determined to be rapidly heated and kept warm, stopping heat supply, cooling along with the furnace, uniformly and thoroughly cooling the section to the required heating temperature, starting heat supply, and heating at the required heating temperature;

fourthly, after austenitizing of isothermal spheroidizing annealing of the steel is determined, stopping heat supply, and cooling along with the furnace at the cooling speed of 40-60 ℃ per hour;

after austenitizing of isothermal spheroidizing annealing of the steel is determined, stopping heat supply, cooling along with the furnace, uniformly and thoroughly cooling the section to isothermal temperature, starting heat supply, and entering the isothermal temperature for isothermal maintenance;

after isothermal maintenance of isothermal spheroidizing annealing of the steel is determined, stopping heat supply, and cooling along with the furnace at the cooling speed of (40-60) DEG C/h;

and (seventhly), after the isothermal maintenance of the isothermal spheroidizing annealing of the steel is determined, cooling along with the furnace, uniformly and thoroughly cooling the section to the temperature of opening a furnace door (cover) as follows: C-Curve nose tip temperature T_P-(20～60)℃

(eighth) determining that the isothermal spheroidizing annealing of the steel opens a furnace door (cover) to be cooled along with the furnace, and the temperature from uniform through cooling of the section to air cooling of discharging is as follows: C-Curve nose tip temperature T_P-(180～220)℃

(ninthly) determining that the isothermal spheroidizing annealing of the steel opens a furnace door (cover) to be cooled along with the furnace, the temperature from uniform through cooling of the cross section to air cooling of the discharged furnace is the nose tip temperature T of the C curve_PAnd (4) discharging at the temperature of (180-220) DEG C, and air-cooling to room temperature.

5. The temperature of claim 1, wherein the temperature is:

and determining the isothermal spheroidizing annealing temperature of the steel according to the indication temperature of the hearth.