CN114086085B

CN114086085B - Forming process for controlling residual ferrite of 38Si7 elastic strip

Info

Publication number: CN114086085B
Application number: CN202111350772.4A
Authority: CN
Inventors: 蒋波; 彭天恩; 胡芳菲; 何国宁; 胡青凤; 刘雅政
Original assignee: China United Test & Certification Co ltd; University of Science and Technology Beijing USTB
Current assignee: China United Test & Certification Co ltd; University of Science and Technology Beijing USTB
Priority date: 2021-11-15
Filing date: 2021-11-15
Publication date: 2022-06-07
Anticipated expiration: 2041-11-15
Also published as: CN114086085A

Abstract

The invention belongs to the field of metal materials, and relates to a forming process for controlling residual ferrite of a 38Si7 elastic strip. The method comprises the steps of heating a 38Si7 bar with the diameter of phi 13mm to a certain temperature through medium frequency induction, preserving heat for a certain time, performing quenching treatment at the certain temperature by using waste heat after stamping forming, cooling to room temperature by using a commercially available inorganic water-soluble quenching agent, tempering at the certain temperature, preserving heat for a certain time, taking out, and air-cooling to the room temperature. The microstructure of the elastic strip obtained by the process is fine tempered troostite (the length of the strip is 10-20 mu m) and residual ferrite (less than 1 grade), and the elastic strip has proper Rockwell hardness, and the hardness is controlled to be 44-46 HRC.

Description

Forming process for controlling residual ferrite of 38Si7 elastic strip

Technical Field

The invention belongs to the field of metal materials, relates to spring steel 38Si7 for a high-speed railway fastener, and particularly relates to a forming process meeting the performance requirement of the high-speed railway fastener on a 38Si7 elastic strip.

Background

In recent years, China develops rapidly in the aspect of railway systems, and the requirement of railway elastic strips is increased increasingly due to the fact that a large number of steel rails are laid. The sleeper elastic strip is used as an important component of a railway system and plays a main role in fixing the position of a steel rail and preventing the steel rail from sliding. As the railway system gradually advances to high speed and light weight, it becomes important to improve the speed and ensure the safety, so that the performance improvement of the sleeper elastic strip is very important. The spring strips work mainly under elastic conditions, and therefore need to have high elastic limit, high yield ratio, sufficient plasticity and toughness and high fatigue strength to ensure the stability of train operation. At present, commercial spring steel produced in China mainly comprises 60Si2Mn, 60Si2Cr and the like, while German SKL12 type spring steel mainly adopts 38Si7 material, and German standard 38Si7 spring strips enter the research field from the aspects of saving material and energy, opening international markets by connecting international and the like.

38Si7 belongs to medium-low carbon steel, the original structure is ferrite plus pearlite, the induction heating speed is higher in the elastic strip forming process, the ferrite is difficult to austenitize, and the residual ferrite after forming is increased. If conventional oil cooling is adopted in the subsequent cooling process, on one hand, proeutectoid ferrite is often generated in the cooling process due to the slow cooling speed, on the other hand, a large amount of oil smoke generated by direct contact of oil and high-temperature elastic strips can pollute the environment, and the deformation and cracking tendency of the elastic strips can be increased by adopting water cooling. When the ferrite in the elastic strip is distributed in a net shape, the strength and toughness of a grain boundary can be reduced, so that the service life of the elastic strip is influenced, and therefore, the reduction of the ferrite proportion as much as possible while the environmental protection is ensured during forming is an important direction for the research and development of the elastic strip at present. The heat treatment process for the 38Si7 spring steel bar material is given in German standard BS-EN-10089-2002, but the process given by the standard is mainly suitable for the bar material with the diameter phi of 9mm, and has no guiding significance for the heat treatment process for the 38Si7 spring steel bar material with the diameter phi of 13 mm. However, in the patent application No. 2020109603160, "a heat treatment process of spring steel 38Si7 with controlled surface decarburization and mechanical properties", a heat treatment process of a 38Si7 spring steel bar with a diameter of Φ 13mm "is given, but the key target is a 38Si7 spring steel bar, the heating mode is common heating and the quenching and cooling mode is water cooling, and the process is suitable for meeting the requirements of water quenching heat treatment performance of a 38Si7 elastic bar raw material bar after common heating, and is not suitable for a process of further induction heating and forming the bar into an elastic bar. In addition, the patent 2020109603160 is not concerned with the regulation of the organization of the spring strips.

Disclosure of Invention

The invention aims to provide a forming process for controlling residual ferrite of 38Si7 elastic strips, wherein 38Si7 elastic strips with the diameter phi 13mm processed by the forming process are fine tempered troostite (the strip length is 10-20 mu m) and residual ferrite (less than 1 level), and have proper Rockwell hardness, and the hardness is controlled between 44-46 HRC.

The 38Si7 steel comprises the following chemical components in percentage by mass according to Germany standard BS-EN-10089-2002: 0.35 to 0.42 percent of C, 1.50 to 1.80 percent of Si, 0.50 to 0.80 percent of Mn, less than or equal to 0.35 percent of Cr, less than or equal to 0.10 percent of Ni, less than or equal to 0.025 percent of P, less than or equal to 0.025 percent of S, and the balance of Fe and inevitable impurities.

The invention relates to a forming process for controlling residual ferrite of a 38Si7 elastic strip, which comprises the following main process flows: 38Si7 bar processing preparation → medium frequency induction heating → punch forming → inorganic water-soluble quenching agent waste heat quenching → heating → medium temperature tempering, wherein:

(1) the induction heating and waste heat quenching process comprises the following steps: carrying out medium-frequency induction heating at the austenitizing heating temperature of 950-970 ℃ for 30-40 s, and carrying out punch forming after heating; then quenching the steel plate to room temperature by using an inorganic water-soluble quenching agent at 800-820 ℃, as shown in figure 1.

Further, in the technical scheme, the induction heating temperature is 950-970 ℃. The austenitizing heating temperature is determined mainly by the critical point of phase transformation of the steel. The heating temperature is too low to be completely austenitized, so that the residual ferrite after heat treatment is increased; while too high coarsens austenite grains, which makes the structure coarse after heat treatment, and is not favorable for obtaining the target structure and performance.

Further, in the technical scheme, the heating time is 30-40 s. The heating time is too short, austenitization is insufficient, and the austenite grains are coarsened due to too long heating time, so that the target structure and performance are not easy to obtain, and the production efficiency is reduced.

Further, in the scheme, the waste heat quenching temperature is 800-820 ℃. The waste heat quenching temperature is mainly set according to the heating temperature, and the temperature range measured after the molding is finished is 800-820 ℃ due to the temperature reduction in the molding process.

(2) And (3) tempering process: tempering and heating at 430-440 ℃, keeping the temperature for 90-120 min, and air-cooling to room temperature after tempering, as shown in figure 1.

Further, in the above technical scheme, the heating temperature is 430-440 ℃. In order to ensure that the elastic strip has good elasticity, medium-temperature tempering is adopted. The tempering temperature is too low, the hardness is too high, and the hardness after tempering is reduced due to too high tempering temperature, so that the target performance cannot be met.

Furthermore, in the technical scheme, the heat preservation time is 90-120 min, the tempering time is too short, the hardness of the steel is too high, the internal stress is large, and if the hardness is too long, the hardness is reduced and the cost is increased.

The reason for adopting the molding process is as follows: the induction heating, water-based quenching liquid quenching and medium-temperature tempering processes of the elastic strips effectively control the residual ferrite content of the elastic strips and ensure that tempered troostite tissues in the microscopic structures of the elastic strips are fine and have proper hardness. Compared with the patent 2020109603160, the invention is a process suitable for further induction heating and forming the 38Si7 spring steel bar into the elastic strip, and then performing waste heat quenching and medium temperature tempering, and can effectively control the structure and hardness of the elastic strip. Therefore, there are great differences in the process flow and the setting of parameters, which are specifically as follows:

(1) a heating process: the forming process adopts waste heat quenching treatment after medium-frequency induction heating forming, the induction heating speed is high, the heating time is 30-40 s, the obtained microstructure is fine, and the production efficiency can be improved. The heating temperature is set to be favorable for punch forming, and can reduce the content of residual ferrite after heat treatment and obtain a fine tempered troostite structure, so that the heating temperature is set to be 950-970 ℃. The heat treatment process in patent 2020109603160 is common heating, the influence of the forming process is not needed to be considered, the heating temperature is 900-920 ℃, the common heating speed is slow, and the heat preservation time is 30-40 min.

(2) A waste heat quenching process: the quenching process is carried out after the induction heating punch forming, the temperature is reduced in the forming process, the measured temperature range after the forming is 800-820 ℃, and the quenching process is directly carried out by utilizing waste heat after the forming. In addition, the quenching cooling medium is water-based quenching liquid which is a commercially available inorganic water-soluble quenching agent, the cooling speed of the quenching cooling medium is lower than that of water cooling, the internal stress generated in the forming and cooling process of the elastic strip is reduced, and the deformation and the cracking generated in the quenching process are prevented. In patent 2020109603160, the main object is a bar, and the quenching process is performed after the bar is heated, forming is not needed, the shape is simple, and the internal stress is small, so the quenching mode of the heat treatment process is water cooling.

(3) And (3) tempering: because the elastic strip is required to have higher elasticity, moderate temperature tempering is adopted to obtain a tempered troostite tissue. The tempering temperature of the quenched elastic strip is 430-440 ℃, and the tempering at the temperature can ensure that the fine martensite structure obtained after quenching is converted into a fine tempered troostite structure. In the heating stage, the residual ferrite after quenching is less due to the temperature control, so that a fine troostite structure and a very small amount of residual ferrite are obtained after tempering, and the elastic strip is ensured to have relatively proper hardness; the tempering time is 90-120 min, and the internal stress generated during the quenching and forming of the elastic strip can be fully eliminated by longer tempering time, so that the structure is uniform, and the elastic strip is ensured to have proper hardness so as to improve the ductility and the toughness of the elastic strip. In patent 2020109603160, the internal stress generated during forming is not considered, so the tempering time is short.

Drawings

FIG. 1 is a schematic view of a molding process;

FIG. 2 is a transverse microstructure of the shaped elastic band, wherein (a) is a structure diagram of example 1, (b) is a structure diagram of example 2, (c) is a structure diagram of example 3, (d) is a structure diagram of comparative example 1, and (e) is a structure diagram of comparative example 2.

Detailed Description

The 38Si7 steel has the following chemical composition in percentage by mass: 0.39% C, 1.73% Si, 0.68% Mn, 0.009% P, 0.006% S, 0.21Cr, 0.02Ni, and the balance Fe.

The molding process parameters used in the examples are shown in table 1. A transverse core microstructure photograph of the molded spring band is shown in FIG. 2, and Rockwell hardness is shown in Table 2. It can be seen that the spring strips of examples 1, 2 and 3 are substantially composed of fine tempered troostite after forming, the length of the tempered troostite strip is 10-20 μm, the content of residual ferrite is very small, and the grade is less than 1. Example 1 the Rockwell hardness after molding was 44.2 HRC; example 2 the Rockwell hardness after molding was 45.9 HRC; example 3 the Rockwell hardness after molding was 45.3 HRC. Therefore, after the forming process is adopted, the residual ferrite in the elastic strip is less than 1 grade, the length of the tempered troostite strip is 10-20 mu m, and the Rockwell hardness is 44-46 HRC.

The molding process parameters used in comparative example 1 are shown in table 1. The photographs of the transverse core structures of the molded spring strips are shown in FIG. 2, and the Rockwell hardness is shown in Table 2. It can be seen that the spring strip of comparative example 1 is composed of tempered troostite and ferrite after forming, the length of the tempered troostite strip is 10-20 μm, the content of residual ferrite is obviously more than that in the example, and the grade reaches 2. Comparative example 1 has a Rockwell hardness of 42.4HRC after heat treatment. Therefore, after the forming process is adopted, the residual ferrite in the elastic strip is obviously increased, and the Rockwell hardness is lower.

The molding process parameters used in comparative example 2 are shown in table 1. The photographs of the transverse core structures of the molded spring strips are shown in FIG. 2, and the Rockwell hardness is shown in Table 2. It can be seen that the spring strip of comparative example 2 after forming is composed of tempered troostite and ferrite, the content of residual ferrite is very small, the grade is less than 1 grade, and the length of the tempered troostite strip is 30-40 μm. Comparative example 2 has a rockwell hardness of 44.0HRC after heat treatment. It can be seen that the tempered troostite strip inside the elastic strip is thicker after the forming process.

TABLE 1 Heat treatment Process parameters used in the examples and comparative examples

Numbering	Quenching heating temperature/. degree.C	Quenching heating time/s	Quenching temperature/. degree.C	Tempering temperature/. degree.C	Tempering time/min
						Example 1	966	30	810	430	90
Example 2	954	30	802	430	90
						Example 3	968	30	816	430	90
Comparative example 1	946	30	804	430	60
						Comparative example 2	1042	30	880	430	90

TABLE 2 ferrite contents and hardness after heat treatment of examples and comparative examples

Numbering	Length of tempered troostite strip, μm	Residual ferrite grade	Rockwell hardness/HRC
				Example 1	10～20	< level 1	44.2
Example 2	10～20	< level 1	45.9
				Example 3	10～20	< level 1	45.3
Comparative example 1	10～20	Stage 2	42.4
				Comparative example 2	30～40	< level 1	44.0

Claims

1. A forming process for controlling residual ferrite of a 38Si7 elastic strip is characterized by comprising the following steps: medium-frequency induction heating a 38Si7 bar with the diameter of phi 13mm to a certain temperature, keeping the temperature for a certain time, performing quenching treatment at the certain temperature by using waste heat after stamping forming, cooling to room temperature by using a commercially available inorganic water-soluble quenching agent, performing medium-temperature tempering at the certain temperature, keeping the temperature for a certain time, taking out, and air-cooling to the room temperature;

the method mainly comprises the following steps:

(1) an induction heating process: heating a 38Si7 bar with the diameter of phi 13mm by medium frequency induction until the bar is fully austenitized, wherein the austenitizing heating temperature is 950-970 ℃, the heating time is 30-40 s, and then performing punch forming;

(2) a waste heat quenching process: directly quenching the formed 38Si7 elastic strip by using waste heat, wherein the quenching temperature is 800-820 ℃, and the quenching liquid is a commercially available inorganic water-soluble quenching agent;

(3) and (3) tempering: after the treatment in the step (2), performing medium-temperature tempering treatment, wherein the heating temperature is 430-440 ℃, the heat preservation time is 90-120 min, and air cooling to room temperature after tempering;

after the processing method is adopted for processing, the residual ferrite content of the 38Si7 elastic strip with the diameter of phi 13mm is less than 1 grade, the tempered troostite structure is fine, the strip length is 10-20 mu m, the Rockwell hardness is proper, and the hardness is controlled to be 44-46 HRC.