CN114686655B - A method for rapid spheroidizing annealing of GCr15 steel - Google Patents

Abstract

The invention discloses a rapid spheroidizing annealing method for GCr15 steel. According to the method, annealed GCr15 steel is directly placed in a Formastor high-frequency induction line device, the temperature rise and the temperature reduction are rapidly completed within 10 seconds, and austenitizing and spheroidizing processes are directly completed in a high-frequency coil, so that the time is greatly shortened; meanwhile, the invention effectively shortens the temperature fluctuation range, shortens the traditional process of normalizing and then spheroidizing annealing, adopts isothermal quenching to obtain a medium-temperature bainitic structure and then spheroidizing annealing, avoids a large amount of temperature raising and lowering processes, and greatly reduces energy consumption. The invention has simple flow, high efficiency and energy saving, and can obviously improve the spheroidizing annealing efficiency of GCr15 steel and improve the spheroidizing annealing effect by adjusting the heat preservation temperature, the heat preservation time and the cooling rate of spheroidizing annealing through high-frequency rapid heating and cooling.

Description

A method for rapid spheroidizing annealing of GCr15 steel

Technical field

The invention relates to the field of steel, and relates to an online rapid spheroidizing annealing method for GCr15 bearing steel wire rod.

Background technique

Bearing steel has become an important part of mechanical equipment due to its good strength, toughness, high fatigue resistance and wear resistance. With the continuous advancement of technical equipment such as smelting, rolling and heat treatment, the requirements for bearing steel are also constantly increasing, and the spheroidizing annealing method, which is one of the key links in controlling carbide quality, is particularly important.

GCr15 steel is a common and typical bearing steel. Its original structure is generally lamellar pearlite. It has high hardness, high brittleness, easy cracking, and is difficult to cut. Therefore, it often needs to be spheroidized. After spheroidizing treatment, it is organized into spherical pearlite, with slightly lower strength and hardness, but good plasticity, machinability and cold working formability, and low tendency of deformation and cracking during quenching, making the structure ready for subsequent heat treatment. However, the cooling speed of ordinary spheroidizing annealing is slow and the production cycle is long. Therefore, an important issue that must be considered is how to shorten the annealing time and reduce energy consumption in production practice to achieve the purpose of improving production efficiency. Therefore, how to make steel have faster production time, lower energy consumption and better performance has become an urgent technical problem to be solved in this field.

At present, for spheroidizing annealing technology in the steel field, more research has been done on spheroidizing annealing technology for GCr15 steel, H13 steel, gear steel, tool steel and other steel types. For example, patent No. CN102382962B discloses a rapid spheroidizing of GCr15 bearing steel pipe. Annealing process is relatively complex and takes a long time to heat up and cool down, which greatly increases the process time and energy consumption.

CN201710411463.0 patent "A method for online rapid spheroidizing annealing of GCr15 bearing steel after hot rolling" adopts an optimized spheroidizing treatment process to control the precipitation of proeutectoid carbides during the rolling process through deformation induction, shortening spheroidization The time required for annealing improves energy efficiency; CN201810318301.7 patent "A rapid spheroidizing annealing process method for GCr15 bearing steel" This method effectively controls the size of proeutectoid carbides and forms cementite during the eutectoid transformation process. The precipitation provides more nucleation sites, thereby effectively shortening the time required for spheroidizing annealing and improving energy efficiency; CN201110354494.X patent "GCr15 Bearing Steel Pipe Rapid Spheroidizing Annealing Process" This invention effectively shortens the network carbide precipitation interval The residence time can inhibit the precipitation of network carbides, obtain a good spheroidized structure in a short time, and greatly shorten the spheroidization time. The above three rapid spheroidizing annealing processes only improve the pearlite spheroidizing annealing process after rolling, but the pearlite spheroidizing annealing process itself takes a long time, and bearing steel generally needs to be normalized before spheroidizing annealing. Room temperature requires a lot of heating and cooling, which wastes a lot of energy and does not fundamentally solve the energy consumption problem of spheroidizing annealing.

Contents of the invention

The purpose of the present invention is to provide a method for rapid spheroidizing annealing of GCr15 steel in view of the above-mentioned problems existing in the spheroidizing annealing technology of existing GCr15 steel. This method directly places the annealed GCr15 steel into the Formastor high-frequency induction line device, quickly completes the temperature rise and fall in 10 seconds, and directly completes the austenitization and spheroidization process in the high-frequency coil, greatly shortening the time; at the same time, the invention is effective It shortens the temperature fluctuation range and reduces the traditional process of first normalizing to eliminate secondary carbides and then performing spheroidizing annealing. Instead, quenching is used to obtain a medium-temperature bainite structure and then spheroidized annealing, which avoids a large number of temperature rising and cooling processes and greatly reduces the temperature fluctuation range. Reducing energy consumption is also a unique feature of this invention. The invention has a simple process, high efficiency and energy saving. Through high-frequency rapid heating and cooling, it adjusts the holding temperature, holding time and cooling rate of the spheroidizing annealing, significantly improves the spheroidizing annealing efficiency of GCr15 steel and improves the spheroidizing annealing effect.

The technical solution of the present invention is:

A method for rapid spheroidizing annealing of GCr15 steel. The method includes the following steps: placing the wire rod of GCr15 steel into a high-frequency induction device and performing the following operations:

(1) Use 5 to 10 seconds to heat the wire to 880°C-980°C, and keep it at this temperature for 15 to 30 minutes until the sample is completely austenitized;

(2) Then quickly cool the wire to 400±10°C in 5 to 10 seconds and keep it warm for 425 to 445 seconds;

(3) Then quickly heat the wire to 678±10°C in 5 to 10 seconds and keep it warm for 90 to 120 seconds;

(4) Use 5 to 10 seconds to quickly lower the wire to 578±10°C, and then keep it warm for 50 to 75 seconds;

(5) Rapidly cool the re-insulated wire to room temperature in 5 to 10 seconds to complete spheroidization annealing.

The components of the GCr15 steel include C: 0.95-1.10%, Si: 0.15-0.35%, Mn: ≤0.5%, Cr: 1.30-1.60%, and the balance is Fe.

The high-frequency induction device is a Formastor high-frequency induction device, and the diameter of the GCr15 steel wire is 2.5 to 3.5 mm.

The starting temperature of the transformation from pearlite to austenite is specifically 628°C.

The starting temperature of the transformation from austenite to bainite is 400°C.

The substantive features of the present invention are:

In the current technology, the conventional spheroidizing annealing process is to perform normalizing first (needs to be cooled to room temperature), and then placed in an annealing furnace to perform a slow spheroidizing annealing process. The core innovation point of this invention is to merge the two processes, directly austenitizing the sample in a high-frequency induction furnace, without normalizing (that is, no need to cool to room temperature), directly undergoing medium-temperature bainite transformation (cooling to (about 400 degrees), and then quickly cool down to complete the insulation at two temperatures. The performance can also meet relevant requirements, but the energy consumption is greatly reduced. The rapid temperature rise and cooling of this process is ensured by the Formastor high-frequency induction device. Conventional annealing furnaces cannot achieve rapid temperature rise and cooling.

The mechanism is the conventional spheroidizing annealing process: the secondary carbides are broken by normalizing, and then through long-term annealing and heat preservation, the secondary carbides are partially integrated into the austenite, retaining a considerable amount of undissolved carbides, and then cooled during the subsequent cooling process. , carbides precipitate with these residual carbides as the core.

Rapid annealing process: directly obtain a certain amount of undissolved carbides through bainite transformation with short energy consumption (not lowered to room temperature, requiring less energy), and then crush a small amount of secondary carbides through the first rapid temperature isotherm, and the second Rapid insulation uses undissolved carbide as the core to precipitate carbide because it can achieve the same performance.

Another innovative point in the present invention is that the material is selected as wire. In the induction coil, the time of the heat treatment process is controlled by the moving speed of the wire. Secondly, the use of high-frequency induction coil and rapid cooling can combine the two processes of the conventional process, greatly improving the efficiency of the heat treatment process. Reduce equipment usage and time.

The beneficial effects of the present invention are:

The GCr15 steel rapid spheroidizing annealing process implemented by the above technical solution uses high-frequency induction rapid heating and two steam/water rapid cooling channels to spheroidize carbides online. The entire process time is only about 0.5h, which greatly shortens the spheroidization. The annealing cycle greatly reduces energy consumption, and the carbides are evenly distributed, avoiding carbide aggregation. The carbide particles are small, the process is simple, the principle is clear, and it is convenient for industrial production.

The invention overcomes the shortcomings of the current technology that the furnace equipment has to go through repeated high and low temperature cycles, which requires more electric energy and damages the equipment. It cleverly utilizes the precursor treatment (mentioned above) to save electric energy, shorten the heat treatment time, and achieve the same effect; not only It takes shorter time (as can be seen from the process comparison) and requires less electricity. One ton of products can save about 100kWh of electricity.

Description of the drawings

Figure 1 is a schematic diagram of the specific process described in the present invention.

Figure 2 is a schematic diagram of the application equipment of the process described in the present invention (in the figure: 1. Quartz tube, 2. Air pipe, 3. Cooling water pipe, 4. Sample, 5. Thermocouple, 6. High-frequency induction coil).

Figure 3 is the metallographic structure of the samples before processing in all examples.

Figure 4 is the metallographic structure of the sample after the process in Example 4.

Detailed ways:

The content of the present invention will be further described in detail below through specific examples and in conjunction with the accompanying drawings. An online rapid spheroidizing annealing process for GCr15 carbon tool steel is shown in Figure 1. Figure 2 shows a schematic diagram of the application equipment (high frequency induction device) of the process described in the present invention. The sample 4 is placed in the quartz tube 1. The quartz tube 1 surrounds the air pipe 2, the cooling water pipe 3 and the high-frequency induction coil 6 in sequence, and the temperature of the sample is collected through the thermocouple 5. When the sample is heated, the high-frequency induction coil 6 is used for heating to achieve the purpose of rapid heating of the sample, and the protective gas nitrogen is purged through the trachea 2 to achieve the purpose of rapid cooling of the sample. Figure 3 is the metallographic structure of the sample before the process described in the present invention. The original microstructure of GCr15 before rapid spheroidization treatment was lamellar pearlite.

In the embodiment of the present invention, the spheroidizing annealing high-frequency induction device is a Formaster-II fully automatic phase change dilatometer manufactured by Fuji Electric Co., Ltd., hereinafter referred to as the Formastor high-frequency induction device. This device can realize rapid heating of materials and rapid cooling of materials.

The equipment used to observe the metallographic microstructure in the embodiment of the present invention is a Japanese Olympus metallographic microscope.

The national standard used to measure the spheroidization grade in the embodiment of the present invention is the rating method in GB/T 18254-2016 high carbon chromium bearing steel.

The content of the present invention will be further described in detail below with reference to the accompanying drawings. Through the GCr15 rapid spheroidizing annealing process shown in Figure 4, the microstructure after heat treatment is fine and uniform spherical carbides dispersed on the ferrite matrix.

The rapid spheroidizing annealing process of GCr15 bearing steel is shown in Figure 1. It specifically includes the following steps: rapid heating, insulation, rapid cooling, insulation above Ac1, insulation below Ac ₁ , and rapid cooling to room temperature:

1. Rapid heating process and heat preservation process. The most basic problem of spheroidizing annealing is how to solve the formation of granular carbide core. The granular carbide in the structure is grown by the remaining carbide particles during heating austenitization. The more remaining carbide particles, the more complete spherical carbide particles will be obtained. The easier it is to spheroidize the structure, so specific requirements must be put forward for heating austenitization during spheroidization. In addition to retaining as many remaining carbide particles as possible during austenitization, it is also necessary to obtain austenite with as large a non-uniform carbon concentration as possible. The non-uniformity of the austenite composition is conducive to the formation of pearlite transformation. During the nucleation and growth process, undissolved carbonized material points can become non-uniform nucleation centers for pearlite transformation, which can make the abnormal decomposition rate of supercooled austenite 6 to 7 times faster than that of uniform austenite. Therefore, for the austenitization stage, it is necessary to quickly heat up to the appropriate temperature and maintain the temperature for a short time. The present invention takes 10 seconds to rapidly raise the temperature to 980°C and maintain the temperature for 20 minutes.

2. Rapidly cool down to the bainite transformation zone. Before hypereutectoid steel spheroidization, the network carbides need to be eliminated. Generally, a quenching or normalizing process is used to eliminate the chain-like network carbides distributed along the grain boundaries. However, the present invention only reduces the temperature to the bainite region at medium temperature to achieve the effect of uniform distribution of carbides. In the present invention, the temperature is maintained at the bainite starting transformation temperature (400°C) for 435 seconds.

3. Rapid cooling and insulation above Ac ₁ . The process of rapid cooling increases the heterogeneity of austenite composition while retaining more undissolved carbonized material points. Proper insulation can promote the significant increase in crystal defects and structural heterogeneity, and the remaining carbide material points become more dispersed and fine. . The present invention keeps the temperature above Ac1 (678°C) for 102s.

4. Quickly cool down and keep warm below Ac ₁ . In this temperature range, the carbide structure can precipitate at locations with high linear defect density, forming dispersed granular carbides in some areas, and finally forming a spheroidized structure. The carbide particles formed are small in size and round and uniform in shape. , the distribution is diffuse. The present invention keeps the temperature below Ac ₁ (578°C) for 60 seconds.

5. Quickly cool to room temperature. The key to the rapid spheroidizing annealing process of GCr15 bearing steel provided by the invention is to select the appropriate heat treatment process, heating temperature, and holding time. This process can greatly shorten the spheroidizing annealing cycle, save energy consumption, and obtain dispersion on the ferrite matrix. The structure is distributed with fine, small, uniform and round carbide particles, and the hardness is 28HRC. The invention can ensure that the workpiece to be processed has good shaping processability and cutting performance, thereby improving the quality and usability of the product.

Example 1

(1) The selected GCr15 steel is a wire rod with a diameter of 3mm. Its composition contains C: 0.95-1.10%, Si: 0.15-0.35%, Mn: ≤0.5%, Cr: 1.30~1.60%, and the balance is Fe.

(2) Rapidly raise the temperature (10s) to 680°C, and keep it at this temperature for 20 minutes until the sample is completely austenitized;

(3) Rapid cooling (5s) to 400℃ and heat preservation for 435s;

(4) Rapidly heat up (5s) to 678°C and keep warm for 102s;

(5) Rapidly cool down (5 seconds) to 578°C and keep warm for 60 seconds;

(6) Cool to room temperature in 10 seconds to complete spheroidization annealing.

Through the above comprehensive control, some of the carbides obtained after spheroidization annealing are in the form of spherical particles, and some are flaky pearlite. The distribution is uneven, and there are a large number of reticular cementite areas, which is rated as level 5.

Example 2

(1) The selected GCr15 steel is a wire rod with a diameter of 3mm. Its composition contains C: 0.95-1.10%, Si: 0.15-0.35%, Mn: ≤0.5%, Cr: 1.30~1.60%, and the balance is Fe.

(2) Rapidly raise the temperature (10 seconds) to 980°C, and keep it at this temperature for 20 minutes until the sample is completely austenitized;

(3) Rapidly cool down (5s) to room temperature;

(4) After the sample has cooled to room temperature, quickly raise the temperature (5 seconds) to 678°C and keep it warm for 102 seconds;

(5) Rapidly cool down (5 seconds) to 578°C and keep warm for 60 seconds;

(6) Cool to room temperature in 10 seconds to complete spheroidization annealing.

Through the above comprehensive control, the carbide obtained after spheroidization annealing is uniformly distributed in spherical particles, without the appearance of large cementite areas, and is rated as level 3. However, energy consumption and time are virtually increased.

Example 3

(1) The selected GCr15 steel is a wire rod with a diameter of 3mm. Its composition contains C: 0.95-1.10%, Si: 0.15-0.35%, Mn: ≤0.5%, Cr: 1.30~1.60%, and the balance is Fe.

(2) Rapidly raise the temperature (10 seconds) to 980°C, and keep it at this temperature for 20 minutes until the sample is completely austenitized;

(3) Rapid cooling (5s) to 400℃ and heat preservation for 435s;

(4) Rapidly heat up (5 seconds) to 678°C and keep warm for 300 seconds;

(5) Rapidly cool down (5 seconds) to 578°C and keep warm for 60 seconds;

(6) Cool to room temperature in 10 seconds to complete spheroidization annealing.

Through the above comprehensive control, the carbide obtained after spheroidization annealing is uniformly distributed in spherical particles, without the appearance of large cementite areas, and is rated as level 4. The second hold time is too long.

Example 4

(1) The selected GCr15 steel is a wire rod with a diameter of 3mm. Its composition contains C: 0.95-1.10%, Si: 0.15-0.35%, Mn: ≤0.5%, Cr: 1.30~1.60%, and the balance is Fe.

(2) Rapidly raise the temperature (10 seconds) to 980°C, and keep it at this temperature for 20 minutes until the sample is completely austenitized;

(3) Rapid cooling (5s) to 400℃ and heat preservation for 435s;

(4) Rapidly heat up (5s) to 678°C and keep warm for 102s;

(5) Rapidly cool down (5 seconds) to 578°C and keep warm for 60 seconds;

(6) Cool to room temperature in 10 seconds to complete spheroidization annealing.

Through the above comprehensive control, the photo of the metallographic structure of the sample obtained in this example is shown in Figure 4. It can be seen from the photo that the carbide obtained after spheroidizing annealing is uniformly distributed in spherical granular shape, and there is no large piece of carburization. The appearance of the body area is rated as Level 2. And the process time used is the shortest.

Rapid heating to the complete austenite region is the key to ensuring complete bainite or martensite transformation in the later stage. If there is no complete austenitization, it will easily lead to the formation of a reticular cementite region (Example 1), reticular carburizing The body cannot be eliminated in the later spheroidizing annealing and will be brought to the later products. Reticular cementite is the main cause of material cracking in the later use of materials. This patent cleverly uses pearlite to form precursor carbides (available in both Example 3 and Example 4), which provides prerequisites for the later formation of spherical pearlite. The samples of Examples 3 and 4 were austenitized without normalizing (that is, no need to be cooled to room temperature), and directly underwent medium-temperature bainite transformation (cooled to about 400 degrees), thus avoiding damage to the equipment caused by repeated heating of the samples (implementation Example 2), and reduce energy consumption. Although proper holding in the bainite zone increases the time (Example 4), it avoids the long waiting time and energy loss when the sample reaches room temperature and tends to completely equilibrate (Example 2). In Example 3, the complete transformation of bainite is completed and the fracture of carbides is completed. If the transformation time is further increased, it will only increase the energy consumption.

By using rapid induction heating and short-time heating/insulation, the present invention quickly cools down to the bainite zone of the medium-temperature transformation zone, rapidly breaks and dissolves the carbides in the original annealed lamellar spherical pearlite, and precipitates fine spherical carbides. The nucleation core is kept warm for a short period of time for the next two temperature sections to complete the spheroidization process. This short-time rapid induction heating treatment shortens the overall spheroidizing annealing time of GCr15 steel, increases the spheroidizing rate, greatly reduces energy consumption, and the carbide size and distribution are smaller and more uniform than those obtained by conventional spheroidizing annealing heat treatment.

The GCr15 spheroidizing annealing heat treatment process of the present invention and the comparative spheroidizing annealing heat treatment process parameters are shown in Table 1.

Table 1 Comparison of process parameters between the spheroidizing annealing heat treatment process of the present invention and the existing conventional spheroidizing annealing heat treatment process

From the above examples, we can see that in order to control the temperature rise and fall speed, GCr15 steel can only be made into rods; and the diameter of the rod cannot be too large; if the diameter is too large (more than 3.5mm), rapid short-time induction heating can only achieve The effect of surface treatment cannot achieve the effect of heat treatment on the core tissue of the sample, resulting in uneven distribution of sample tissue. This invention only targets wires with a diameter of 3mm. Induction heating can easily reach the core tissue in a short time, saving time. It also reduces the craftsmanship, which is also the ingenuity of this invention.

The Ac1 line in Figure 1 is also called the eutectoid line. It means that when an iron-carbon alloy with a carbon content of 0.77%-2.11% is cooled to this line, a eutectoid transformation occurs at a constant temperature of 727 degrees, that is, A _0.77% → F _{0.0218 %} +Fe ₃ C. Ac3 is the final temperature at which ferrite transforms into austenite during heating. Due to the difference in carbon content and changes in alloying elements, AC1 and AC3 will change. The heat treatment temperature is generally between AC1 and AC3, so the heat treatment temperature changes accordingly, and the process needs to be re-researched.

The above are only preferred embodiments of the present invention, and the protection scope of the present invention is not limited thereto. Therefore, any equivalent substitutions or changes based on the scope of the present invention are within the protection scope of the present invention.

Matters not covered in the present invention are known technologies.

Claims (4)

1. A method for rapid spheroidizing annealing of GCr15 steel, characterized in that the method includes the following steps: place the wire rod of GCr15 steel into a high-frequency induction device, and perform the following operations: (1) Use 5 to 10 seconds to heat the rod to 880°C-980°C, and keep it at this temperature for 15 to 30 minutes until the sample is completely austenitized; (2) Then quickly cool the wire to 400±10°C in 5 to 10 seconds and keep it warm for 425 to 445 seconds; (3) Then quickly heat the wire to 678±10°C in 5 to 10 seconds and keep it warm for 90 to 120 seconds; (4) Use 5 to 10 seconds to quickly lower the wire to 578±10°C, and then keep it warm for 50 to 75 seconds; (5) Rapidly cool the re-insulated wire to room temperature in 5 to 10 seconds to complete spheroidization annealing. 2. The rapid spheroidizing annealing method of GCr15 steel as claimed in claim 1, characterized in that the composition of the GCr15 steel contains C: 0.95-1.10%, Si: 0.15-0.35%, Mn: ≤0.5% by weight. , Cr: 1.30~1.60%, the balance is Fe. 3. The method for rapid spheroidizing annealing of GCr15 steel as claimed in claim 1, characterized in that the high-frequency induction device is a Formastor high-frequency induction device. 4. The rapid spheroidizing annealing method of GCr15 steel as claimed in claim 1, characterized in that the diameter of the GCr15 steel wire rod is 2.5-3.5 mm.