阅读说明：本技术 一种用于高强度紧固件的等温球化退火工艺 (Isothermal spheroidizing annealing process for high-strength fastener ) 是由 徐兵 黎水泉 吴细浩 于 2020-12-01 设计创作，主要内容包括：一种用于高强度紧固件的等温球化退火工艺,包括如下步骤：(1)轧制：对连铸后的钢坯进行开坯,将开坯后的钢坯进行轧制,钢坯轧制后以5-10℃/s的速率冷却至550～600℃,随后在辊道冷却线上一直保温到300-350℃集卷,得到待等温球化退火件；(2)等温球化退火：将上述待等温球化退火件放入电阻炉内,在700-720℃温度下,等温保温时间为4～5h,然后随炉冷却至500～550℃后,出炉空冷至室温,得到等温球化退火件；(3)组织检验。本发明所述的用于高强度紧固件的等温球化退火工艺,工艺步骤设置合理,在球化退火前先进行轧制,实现碳化物的在线直接球化,可大幅度减少球化退火时间,等温球化退火提高了球化质量,节能、环保、低成本,前景广泛。(An isothermal spheroidizing annealing process for high strength fasteners comprising the steps of: (1) rolling: cogging the continuously cast steel billet, rolling the cogging steel billet, cooling the billet to 550-600 ℃ at the speed of 5-10 ℃/s after rolling, and then carrying out heat preservation on a roller way cooling line until the temperature reaches 300 ℃ and 350 ℃ for coiling to obtain an isothermal spheroidizing annealing piece; (2) isothermal spheroidizing annealing: placing the piece to be subjected to isothermal spheroidizing annealing into a resistance furnace, keeping the isothermal temperature for 4-5 h at the temperature of 700-720 ℃, cooling to 500-550 ℃ along with the furnace, taking out of the furnace, and air-cooling to room temperature to obtain an isothermal spheroidizing annealing piece; (3) and (5) tissue inspection. The isothermal spheroidizing annealing process for the high-strength fastener has the advantages that the process steps are reasonably set, rolling is carried out before spheroidizing annealing, online direct spheroidizing of carbide is realized, the spheroidizing annealing time can be greatly reduced, the spheroidizing quality is improved by isothermal spheroidizing annealing, and the process is energy-saving, environment-friendly, low in cost and wide in prospect.)

1. An isothermal spheroidizing annealing process for high strength fasteners, comprising the steps of:

(1) rolling: cogging the continuously cast steel billet, rolling the cogging steel billet, cooling the billet to 550-600 ℃ at the speed of 5-10 ℃/s after rolling, and then carrying out heat preservation on a roller way cooling line until the temperature reaches 300 ℃ and 350 ℃ for coiling to obtain an isothermal spheroidizing annealing piece;

(2) isothermal spheroidizing annealing: placing the piece to be subjected to isothermal spheroidizing annealing into a resistance furnace, keeping the isothermal temperature for 4-5 h at the temperature of 700-720 ℃, cooling to 500-550 ℃ along with the furnace, taking out of the furnace, and air-cooling to room temperature to obtain an isothermal spheroidizing annealing piece;

(3) and (3) tissue examination: the specification and the components of the isothermal spheroidized annealing piece are inspected by a quality technical department, and the high-strength fastener can be put into production after the specification and the components are qualified.

2. The isothermal spheroidizing annealing process for the high-strength fastener according to claim 1, wherein the cogging in the step (1) is performed at a cogging heating temperature of 1100 ℃ and a heating time of 5-7 h.

3. The isothermal spheroidizing annealing process for the high-strength fastener according to claim 1, wherein in the rolling in the step (1), the initial rolling temperature in the rolling process is 1050-1080 ℃, the final rolling temperature is 875-895 ℃, and the spinning temperature is 800-815 ℃.

4. The isothermal spheroidizing annealing process for high strength fasteners according to claim 1, wherein before the piece to be isothermally spheroidized is put into a resistance furnace, the piece to be isothermally spheroidized is subjected to a carbide net inspection.

5. The isothermal spheroidizing annealing process for high strength fasteners according to claim 1, wherein the temperature control system of the electric resistance furnace in the step (3) is controlled by using a PID control algorithm.

6. The isothermal spheroidizing annealing process for the high-strength fastener according to claim 5, wherein a temperature sensor is arranged in the resistance furnace, and the temperature sensor inputs real-time temperature data in the resistance furnace to a PLC through an analog input module.

7. The isothermal spheroidizing annealing process for high strength fasteners according to claim 6, wherein the PID control algorithm comprises the following steps:

(1) establishing a variable universe fuzzy PID prediction controller in Simulink, establishing data connection with WINCC by utilizing an OPC read-write module in OPC Toolbox, acquiring real-time temperature data in the resistance furnace acquired by the WINCC from PLC, and setting characteristic parameters in open-loop dynamic response of a resistance furnace temperature control system by MATLAB according to a setting formula to obtain a real-time PID parameter value;

(2) the real-time PID parameter values are written into variables of WINCC through OPC communication, and then the real-time PID parameter values are written into a PID continuous controller of a PLC in real time through Profibus DP communication by the WINCC;

(3) and the PLC sends the output value of the PID controller to an analog output module to control an actuator of the resistance furnace to perform temperature control action.

Technical Field

The invention belongs to the technical field of automobile sun visor molds, and particularly relates to an isothermal spheroidizing annealing process for a high-strength fastener.

Background

A fastener (also referred to as a standard) is a generic term for a mechanical part of a type used when two or more parts (or members) are fastened and connected as a single body. The fastener features various kinds and specifications, different performance and use and very high standardization, serialization and universalization degree. The fastener is the most widely used mechanical basic member and has a large demand.

Among them, as for cold heading steels for manufacturing fasteners of 10.9 and 12.9 grades, since strict requirements are made on strength grade, fatigue property and delayed fracture resistance, Cr and Mo alloy steel series are generally used, and in the subsequent process, a spheroidizing annealing process is required to soften the materials for cold heading forming, and then thermal refining is performed to obtain fasteners with excellent performance.

At present, most of cold forging steel for fasteners is subjected to spheroidizing annealing process after raw materials are offline to improve the plastic deformation capacity of the cold forging steel. Generally, a steel mill provides hot rolled wire rods, and a standard part factory performs subsequent spheroidizing annealing treatment, but the cold heading performance of medium carbon alloy steel cannot completely meet the requirements, so that the production yield is low, and the phenomenon of surface cracks and turning-around of cold heading to generate a large amount of defective products is caused. Therefore, an isothermal spheroidizing annealing process suitable for high-strength fasteners is required to be developed, and the isothermal spheroidizing annealing process has high added value and wide application prospect.

Chinese patent application No. CN201921491019.5 discloses a fastener wire rod spheroidizing annealing device convenient to go up unloading, forms spheroidizing annealing furnace through spheroidizing annealing furnace base and annealing furnace left upper cover, annealing furnace right upper cover combination, and left upper cover and right upper cover make things convenient for the sideslip to open, make things convenient for the fastener wire rod to go up unloading, improve spheroidizing annealing efficiency, do not improve the isothermal spheroidizing annealing process of fastener.

Disclosure of Invention

The purpose of the invention is as follows: in order to overcome the defects, the invention aims to provide the isothermal spheroidizing annealing process for the high-strength fastener, which is simple, strong in operability and reasonable in process step arrangement, rolling is carried out before spheroidizing annealing, the on-line direct spheroidizing of carbide is realized, the spheroidizing annealing time can be greatly reduced, the spheroidizing quality is improved by the isothermal spheroidizing annealing, and the process has the advantages of energy conservation, environmental protection, low cost, high benefit and wide application prospect.

The purpose of the invention is realized by the following technical scheme:

an isothermal spheroidizing annealing process for high strength fasteners, comprising the steps of:

(1) rolling: cogging the continuously cast steel billet, rolling the cogging steel billet, cooling the billet to 550-600 ℃ at the speed of 5-10 ℃/s after rolling, and then carrying out heat preservation on a roller way cooling line until the temperature reaches 300 ℃ and 350 ℃ for coiling to obtain an isothermal spheroidizing annealing piece;

(2) isothermal spheroidizing annealing: placing the piece to be subjected to isothermal spheroidizing annealing into a resistance furnace, keeping the isothermal temperature for 4-5 h at the temperature of 700-720 ℃, cooling to 500-550 ℃ along with the furnace, taking out of the furnace, and air-cooling to room temperature to obtain an isothermal spheroidizing annealing piece;

(3) and (3) tissue examination: the specification and the components of the isothermal spheroidized annealing piece are inspected by a quality technical department, and the high-strength fastener can be put into production after the specification and the components are qualified.

The isothermal spheroidizing annealing process for the high-strength fastener is characterized in that a steel billet for producing the high-strength fastener is cold heading steel, the cold heading steel has the advantages of high steel utilization rate, good mechanical property, suitability for automatic production and the like, and is still the main forming mode of the high-strength fastener at present, so that the cold heading steel is required to have small enough deformation resistance and high enough deformation capacity, and except for cold-work strengthened non-quenched and tempered steel, low-carbon boron steel and other steel types, the cold heading steel for the high-strength fastener produced according to the traditional metallurgical production process flow needs to be subjected to spheroidizing annealing treatment before cold forming so as to soften the material. The aim of the spheroidizing annealing is to obtain a spheroidized structure in which dispersed carbides are uniformly distributed on a ferrite matrix, and the structure has optimal plasticity and lowest hardness.

The invention firstly rolls before spheroidizing annealing, firstly cogging the continuously cast steel billet, and aims to effectively control the frame segregation and the center segregation of the continuously cast steel billet by changing the heating temperature and the heating time at the cogging position, then control the austenite grain size and the growth of the steel billet by controlling the cogging temperature, the finishing temperature and the spinning temperature in the rolling process, and finally, the room-temperature structure of the wire rod obtained in the cooling process after rolling is that bainite and a small amount of pearlite carbon are uniformly distributed on a matrix, thereby realizing the online direct spheroidizing of carbide, greatly reducing the spheroidizing annealing time and being beneficial to the subsequent isothermal spheroidizing annealing.

In the isothermal spheroidizing annealing process, heat preservation is carried out for 4-5 hours at the temperature of 700-720 ℃, so that the cementite pieces in the lamellar structure are dissolved through and dissolved and broken by themselves and grow into spheres, then the temperature is preserved at the sub-temperature, the furnace is cooled to 500-550 ℃, and then the cementite pieces are discharged from the furnace for air cooling, in the furnace cooling process, the dissolved cementite particles are automatically spheroidized, the temperature is cooled to about 500 ℃, carbides are completely spheroidized, if the cementite is continuously kept in the furnace for cooling, large-particle carbides swallow small particles to be spheroidized and grow, so that the small-particle carbides are dissolved and disappear, the purpose of shortening the spheroidizing annealing time is achieved, and the spheroidizing quality is greatly improved.

Further, in the isothermal spheroidizing annealing process for the high-strength fastener, the cogging in the step (1) is carried out at the cogging heating temperature of 1100-1130 ℃ for 5-7 hours.

The cogging parameters are reasonably set, the heating temperature is increased to 1100-1130 ℃ during cogging, and the heating time is prolonged to 5-7 hours, so that dendritic segregation elements (C, Mn and the like) of the continuously cast billet are uniformly diffused, the austenite homogenization is achieved, and the subsequent spheroidizing annealing is facilitated.

Further, in the isothermal spheroidizing annealing process for the high-strength fastener, in the rolling in the step (1), the initial rolling temperature in the rolling process is 1050-1080 ℃, the final rolling temperature is 875-895 ℃, and the spinning temperature is 800-815 ℃.

The method has reasonable rolling parameter setting, the initial rolling temperature in the rolling process is 1050-1080 ℃, the finish rolling temperature is 875-895 ℃, and the spinning temperature is 800-815 ℃, so that the size and the growth of austenite grains of the cogging steel billet are controlled, and abnormal grains are avoided.

Further, before the piece to be subjected to isothermal spheroidizing annealing is placed in a resistance furnace, the piece to be subjected to isothermal spheroidizing annealing is subjected to carbide net inspection.

Before isothermal spheroidizing annealing begins, carbide net inspection is firstly carried out, so that the piece to be subjected to isothermal spheroidizing annealing does not have a bulky and closed net carbide structure, and subsequent isothermal spheroidizing annealing is facilitated. The invention can reduce or even eliminate subjectivity in the inspection process by using the Image contrast enhancement function of Image-Pro Plus 6.0 through an SEM microstructure topography.

Further, in the isothermal spheroidizing annealing process for the high-strength fastener, the temperature control system of the resistance furnace in the step (3) is controlled by adopting a PID control algorithm.

The temperature is the most important control factor of the isothermal spheroidizing annealing process, and the PID control algorithm is used for controlling the temperature of the resistance furnace, so that the method has the advantages of simple structure, easiness in implementation and high reliability.

Further, according to the isothermal spheroidizing annealing process for the high-strength fastener, the temperature sensor is arranged in the resistance furnace, and the temperature sensor inputs real-time temperature data in the resistance furnace to the PLC through the analog input module.

The temperature in the resistance furnace is closely related to isothermal spheroidizing annealing effect and safety production, the temperature sensor is arranged in the resistance furnace and used for detecting temperature information in the resistance furnace, the temperature information over-analog input module inputs real-time temperature data in the resistance furnace to the PLC, the PLC is uploaded to an upper computer and is displayed and stored in real time, an operator can know the running condition in the resistance furnace in real time, rapid adjustment is made on the temperature, and the efficiency and the quality are improved.

Further, the PID control algorithm for the isothermal spheroidizing annealing process of the high strength fastener specifically includes the following steps:

(1) establishing a variable universe fuzzy PID prediction controller in Simulink, establishing data connection with WINCC by utilizing an OPC read-write module in OPC Toolbox, acquiring real-time temperature data in the resistance furnace acquired by the WINCC from PLC, and setting characteristic parameters in open-loop dynamic response of a resistance furnace temperature control system by MATLAB according to a setting formula to obtain a real-time PID parameter value;

(2) the real-time PID parameter values are written into variables of WINCC through OPC communication, and then the real-time PID parameter values are written into a PID continuous controller of a PLC in real time through Profibus DP communication by the WINCC;

(3) and the PLC sends the output value of the PID controller to an analog output module to control an actuator of the resistance furnace to perform temperature control action.

In order to solve a series of problems of large inertia, time lag, nonlinearity and time variation in the temperature control of the resistance furnace, such as large overshoot, long adjustment time, system oscillation, divergence and the like brought to a system, the invention adopts a variable universe fuzzy PID predictive controller which constructs a correlation function between a temperature error value and an adjustment parameter of the PID continuous controller, so that the control parameter of the PID continuous controller is correspondingly changed along with the change of the error value, the action degrees of proportional, integral and differential links are adjusted in real time, and after the time constant of a controlled object is reduced through the closed-loop control of a PID control principle, the link with lag is subjected to predictive control. Therefore, the requirements of the system on the control precision are met, the overshoot phenomenon of the system is avoided, the problems of poor self-adaptive capacity and poor temperature control precision of a fuzzy PID parameter self-tuning algorithm are solved, and the fuzzy PID parameter self-tuning algorithm has good tracking performance and strong anti-interference capacity. Wherein the setting formula is a setting formula proposed by Ziegler and Nichols.

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

(1) the isothermal spheroidizing annealing process for the high-strength fastener has the advantages that the process steps are reasonably set, rolling is carried out before spheroidizing annealing, frame segregation and center segregation of a continuously cast steel billet are effectively controlled by changing the heating temperature and the heating time at a cogging position, the austenite grain size and growth of the steel billet are controlled by controlling the cogging temperature, the finish rolling temperature and the spinning temperature, after cooling after rolling, the room-temperature structure is bainite and a small amount of pearlite carbon are uniformly distributed on a matrix of an annealing piece to be subjected to isothermal spheroidizing, the online direct spheroidizing of carbide is realized, the spheroidizing annealing time can be greatly reduced, and the subsequent isothermal spheroidizing annealing is facilitated;

(2) according to the isothermal spheroidizing annealing process for the high-strength fastener, the purpose of shortening the spheroidizing annealing time is achieved, and the spheroidizing quality can be greatly improved;

(3) the isothermal spheroidizing annealing process for the high-strength fastener adopts a PID control algorithm to control the temperature of the resistance furnace, is easy to realize and high in reliability, and solves a series of problems of large overshoot, long adjusting time, system oscillation and divergence caused by large inertia, time lag, nonlinearity and time-varying property in the temperature control of the resistance furnace.

Detailed Description

In the following, the technical solutions in the embodiments of the present invention are clearly and completely described in the embodiments with reference to specific experimental data, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, but not all embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

The following examples 1, 2, 3, 4, and 5 provide an isothermal spheroidizing annealing process for high strength fasteners, wherein the temperature control system of the resistance furnace in step (3) is controlled using a PID control algorithm.

Furthermore, a temperature sensor is arranged in the resistance furnace, and the temperature sensor inputs real-time temperature data in the resistance furnace to the PLC through an analog input module.

Further, the PID control algorithm specifically includes the following steps:

(1) establishing a variable universe fuzzy PID prediction controller in Simulink, establishing data connection with WINCC by utilizing an OPC read-write module in OPC Toolbox, acquiring real-time temperature data in the resistance furnace acquired by the WINCC from PLC, and setting characteristic parameters in open-loop dynamic response of a resistance furnace temperature control system by MATLAB according to a setting formula to obtain a real-time PID parameter value;

(2) the real-time PID parameter values are written into variables of WINCC through OPC communication, and then the real-time PID parameter values are written into a PID continuous controller of a PLC in real time through Profibus DP communication by the WINCC;

(3) and the PLC sends the output value of the PID controller to an analog output module to control an actuator of the resistance furnace to perform temperature control action.

The invention adopts a variable-discourse-domain fuzzy PID predictive controller, which constructs a correlation function between a temperature error value and an adjusting parameter of a PID continuous controller, so that the control parameter of the PID continuous controller changes correspondingly along with the change of the error value, the action degrees of proportional, integral and differential links are adjusted in real time, and after the time constant of a controlled object is reduced through closed-loop control of a PID control principle, the link with hysteresis is subjected to predictive control. Therefore, the requirements of the system on the control precision are met, the overshoot phenomenon of the system is avoided, the problems of poor self-adaptive capacity and poor temperature control precision of a fuzzy PID parameter self-tuning algorithm are solved, and the fuzzy PID parameter self-tuning algorithm has good tracking performance and strong anti-interference capacity. Wherein the setting formula is a setting formula proposed by Ziegler and Nichols.

Example 1

(1) Rolling: cogging the continuously cast steel billet, wherein the cogging heating temperature is 1100 ℃, and the heating time is 5 hours; rolling the cogging steel billet, wherein the initial rolling temperature in the rolling process is 1080 ℃, the finish rolling temperature is 880 ℃, and the spinning temperature is 800 ℃; cooling the rolled steel billet to 550 ℃ at the speed of 6 ℃/s, and then keeping the temperature on a roller way cooling line until the temperature reaches 320 ℃ for coiling, thereby obtaining an isothermal spheroidizing annealing piece to be annealed;

(2) isothermal spheroidizing annealing: before the piece to be isothermally spheroidized is put into a resistance furnace, carrying out carbide net inspection on the piece to be isothermally spheroidized to ensure that the piece to be isothermally spheroidized does not have a net carbide structure which is thick and closed, putting the piece to be isothermally spheroidized into the resistance furnace, keeping the isothermal temperature for 5 hours at the temperature of 700 ℃, then cooling the piece to 500 ℃ along with the furnace, taking out the piece from the furnace, and carrying out air cooling to the room temperature to obtain the piece to be isothermally spheroidized;

(3) and (3) tissue examination: the specification and the components of the isothermal spheroidized annealing piece are inspected by a quality technical department, and the high-strength fastener can be put into production after the specification and the components are qualified.

Example 2

(1) Rolling: cogging the continuously cast steel billet, wherein the cogging heating temperature is 1100 ℃, and the heating time is 5.5 hours; rolling the cogging steel billet, wherein the initial rolling temperature in the rolling process is 1050 ℃, the finish rolling temperature is 890 ℃, and the spinning temperature is 810 ℃; cooling the rolled steel billet to 560 ℃ at the speed of 8 ℃/s, and then keeping the temperature on a roller way cooling line until the temperature reaches 320 ℃ for coiling, thereby obtaining an isothermal spheroidizing annealing piece to be annealed;

(2) isothermal spheroidizing annealing: before the piece to be isothermally spheroidized is put into a resistance furnace, carrying out carbide net inspection on the piece to be isothermally spheroidized to ensure that the piece to be isothermally spheroidized does not have a net carbide structure which is formed by coarseness and closed, putting the piece to be isothermally spheroidized into the resistance furnace, keeping the isothermal temperature for 4.5h at the temperature of 710 ℃, then cooling the piece to 500 ℃ along with the furnace, taking out the piece from the furnace, and carrying out air cooling to the room temperature to obtain the piece to be isothermally spheroidized;

(3) and (3) tissue examination: the specification and the components of the isothermal spheroidized annealing piece are inspected by a quality technical department, and the high-strength fastener can be put into production after the specification and the components are qualified.

Example 3

(1) Rolling: cogging the continuously cast steel billet at the cogging heating temperature of 1130 ℃ for 5-7 h; rolling the cogging steel billet, wherein the initial rolling temperature in the rolling process is 1050 ℃, the finish rolling temperature is 895 ℃, and the spinning temperature is 800 ℃; cooling the rolled steel billet to 600 ℃ at the speed of 7 ℃/s, and then keeping the temperature on a roller way cooling line until the temperature reaches 350 ℃ for coiling to obtain an isothermal spheroidizing annealing piece to be obtained;

(2) isothermal spheroidizing annealing: before the piece to be isothermally spheroidized is put into a resistance furnace, carrying out carbide net inspection on the piece to be isothermally spheroidized to ensure that the piece to be isothermally spheroidized does not have a net carbide structure which is formed by coarseness and is closed, putting the piece to be isothermally spheroidized into the resistance furnace, keeping the temperature at 720 ℃ for 4h, cooling to 530 ℃ along with the furnace, taking out the piece from the furnace, and carrying out air cooling to room temperature to obtain the piece to be isothermally spheroidized;

(3) and (3) tissue examination: the specification and the components of the isothermal spheroidized annealing piece are inspected by a quality technical department, and the high-strength fastener can be put into production after the specification and the components are qualified.

Example 4

(1) Rolling: cogging the continuously cast steel billet, wherein the cogging heating temperature is 1120 ℃, and the heating time is 6.5 hours; rolling the cogging steel billet, wherein the initial rolling temperature in the rolling process is 1050 ℃, the final rolling temperature is 875 ℃, and the spinning temperature is 815 ℃; cooling the rolled steel billet to 590 ℃ at the speed of 10 ℃/s, and then keeping the temperature on a roller way cooling line until the temperature reaches 330 ℃ for coiling to obtain an isothermal spheroidizing annealing piece to be annealed;

(2) isothermal spheroidizing annealing: before the piece to be isothermally spheroidized is put into a resistance furnace, carrying out carbide net inspection on the piece to be isothermally spheroidized to ensure that the piece to be isothermally spheroidized does not have a net carbide structure which is formed by coarseness and is closed, putting the piece to be isothermally spheroidized into the resistance furnace, keeping the temperature at 720 ℃ for 5h, cooling to 500 ℃ along with the furnace, taking out the piece from the furnace, and carrying out air cooling to room temperature to obtain the piece to be isothermally spheroidized;

(3) and (3) tissue examination: the specification and the components of the isothermal spheroidized annealing piece are inspected by a quality technical department, and the high-strength fastener can be put into production after the specification and the components are qualified.

Example 5

(1) Rolling: cogging the continuously cast steel billet, wherein the cogging heating temperature is 1120 ℃, and the heating time is 5-7 h; rolling the cogging steel billet, wherein the initial rolling temperature in the rolling process is 1050 ℃, the finish rolling temperature is 890 ℃, and the spinning temperature is 815 ℃; cooling the rolled steel billet to 580 ℃ at the speed of 5 ℃/s, and then keeping the temperature on a roller way cooling line until the temperature reaches 310 ℃ for coiling to obtain an isothermal spheroidizing annealing piece to be obtained;

(2) isothermal spheroidizing annealing: before the piece to be isothermally spheroidized is put into a resistance furnace, carrying out carbide net inspection on the piece to be isothermally spheroidized to ensure that the piece to be isothermally spheroidized does not have a net carbide structure which is formed by coarseness and is closed, putting the piece to be isothermally spheroidized into the resistance furnace, keeping the temperature at 720 ℃ for 4h, cooling the piece to be isothermally spheroidized to 500 ℃ along with the furnace, taking the piece out of the furnace, and carrying out air cooling to room temperature to obtain the piece to be isothermally spheroidized;

(3) and (3) tissue examination: the specification and the components of the isothermal spheroidized annealing piece are inspected by a quality technical department, and the high-strength fastener can be put into production after the specification and the components are qualified.

Effect verification:

the isothermal spheroidized articles obtained in examples 1, 2, 3, 4 and 5 were subjected to a performance test according to the following criteria.

(1) Vickers hardness test: according to the regulation of GB4340-1989, the test piece is pressed into the surface of a test piece under the action of a load 1009f by measuring on an FM-700 digital display microhardness meter, and after the load is removed for 5s, the length of the two diagonal lines of the reserved indentation is measured, and the microhardness value is obtained. 5 points are selected on the surface of each sample, the first point and the point with larger fluctuation are removed, and then the average value is taken as the Vickers hardness value of the sample.

(2) Tensile test: in the isothermal spheroidized annealed articles obtained in the above examples 1, 2, 3, 4 and 5, samples were taken from the head and the J according to GB/2975-82, respectively, and subjected to the corresponding heat treatment processes, and then processed into standard tensile specimens according to GB/T228-2002. The tensile test is carried out on an electronic universal testing machine controlled by an SANS microcomputer, and the test process is kept stable, stress state and loading speed are constant. In addition, the surface of the sample is smooth so as to avoid stress concentration caused by the notch, and simultaneously the sample is clamped, so that the axial direction of the sample is consistent with the direction of the load, and the sample cannot be inclined or eccentric. The experiment was carried out in an atmosphere and room temperature environment with a chuck movement rate of 2 mm/min. The yield strength, tensile strength, elongation and reduction of area of the test piece were measured by an extensometer, and the yield ratio was calculated.

TABLE 1 sample Performance test results

Performance index	Example 1	Example 2	Example 3	Example 4	Example 5
						Hardness (HV)	190.230	192.681	194.232	191.562	193.204
Yield strength (Mpa)	450.03	451.24	453.21	457.36	456.26
						Tensile strength (Mpa)	623.15	633.20	634.08	635.01	634.27
Elongation (%)	29.261	29.452	29.875	29.014	29.722
						Reduction of area (%)	72.321	73.304	73.687	74.015	73.987
Yield ratio	0.722	0.713	0.715	0.720	0.719

The invention has many applications, and the above description is only a preferred embodiment of the invention. It should be noted that the above examples are only for illustrating the present invention, and are not intended to limit the scope of the present invention. It will be apparent to those skilled in the art that various modifications can be made without departing from the principles of the invention and these modifications are to be considered within the scope of the invention.