Medium-temperature spheroidizing annealing method for medium-carbon alloy structural steel bar for cold forming

Abstract

The invention provides a medium carbon alloy structural steel bar sub-temperature spheroidizing annealing method for cold forming, relates to the technical field of metal material heat treatment, and designs a sub-temperature spheroidizing annealing process of a steel bar in a roller hearth type protective atmosphere continuous annealing furnace based on the Ac1 temperature of the steel bar. Comprising a front vacuum section, a preheating section, a heating section, a heat preservation section, a slow cooling section, a rapid cooling section and a rear vacuum section. Wherein the heating section comprises a first heating section and a second heating section; the temperature of the second heating section and the temperature of the heat preservation section are both 10-50 DEG C lower than the Ac1 temperature. The method has the characteristics of high heating speed, low heating temperature, short spheroidizing time and good spheroidizing effect.

Description

Technical Field

[0001] The invention relates to the technical field of heat treatment of metal materials, in particular to a sub-temperature spheroidizing annealing method for a medium-carbon alloy structural steel bar for cold forming. Background Art

[0002] In the parts manufacturing industry, the traditional hot forming process has many defects such as high energy consumption, high pollution, serious decarburization of parts blanks, and large processing allowances. It has gradually been replaced by cold forming technologies such as cold heading and cold extrusion. However, cold forming technology has high requirements on incoming materials. It is necessary to use materials with high plasticity, low hardness, and low sensitivity to work hardening. Therefore, it is usually necessary to spheroidize annealing treatment on the material.

[0003] Medium carbon alloy structural steel has a wide range of applications and a variety of grades. This series of alloy steel has high hardness, strength, wear resistance, sufficient toughness, high hardenability, hardenability and other excellent comprehensive mechanical properties. It is an important material for making heavy machinery crankshafts, connecting rods, bolts, gears, blades and other parts. Medium carbon alloy structural steel is generally produced by rolling. The rolled material has problems such as uneven structure, large residual stress, high hardness, and poor plasticity. It is a typical difficult-to-process material and cannot be directly produced by cold forming technology.

[0004] Taking 34CrNiMo6 structural steel for cold forming in a certain parts processing unit as an example, its main chemical composition (mass percentage) is: C: 0.3-0.38%, Si: ≤0.4%, Mn: 0.5-0.8%, P: ≤0.025%, S: ≤0.035%, Cr: 1.3-1.7%, Ni: 1.3-1.7%, Mo: 0.15-0.3%, Al: 0.015-0.08%, and the rest is matrix Fe and unavoidable impurities. The spheroidization rate is required to be ≥80% (tested according to SEP-15201998-09), the Brinell hardness value is ≤200HBW (performed according to GB / T231.1-2018), the full decarburization layer is 0 and the semi-decarburization layer is ≤0.2mm (performed according to GB / T224-2019), and the grain size is ≥6 (performed according to ISO 643-2012). 34CrNiMo6 is a hypoeutectoid steel. With the existing common annealing process, the spheroidization rate is less than about 10% ( Figure 1 As shown in the figure, most of the carbides are distributed in block and lamellar shapes, with a hardness between 210 and 230 HBW, which cannot meet the use requirements. In addition, in order to give full play to the characteristics of cold forming technology with small processing allowance, the cold forming structural steel requires the surface decarburization of raw materials to be 0 for the full decarburization layer and ≤0.2 mm for the semi-decarburization layer. Conventional heat treatment methods are also difficult to meet, and grinding decarburization treatment is often required, which increases the process cost and steel material loss. Therefore, in order to meet the quality requirements of 34CrNiMo6 cold forming structural steel, it is necessary to develop its spheroidizing annealing process. Summary of the invention

[0005] The object of the present invention is to provide a method for sub-temperature spheroidizing annealing of medium-carbon alloy structural steel bars for cold forming, which adopts a roller bottom protective atmosphere continuous annealing furnace for sub-temperature spheroidizing annealing, has fast heating speed, low heating temperature, short spheroidizing time and good spheroidizing effect.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] The present invention provides a sub-temperature spheroidizing annealing method for a medium carbon alloy structural steel bar for cold forming.

[0008] Based on the Ac1 temperature of the steel bar, a sub-temperature spheroidizing annealing process of the steel bar in a roller bottom protective atmosphere continuous annealing furnace is designed, including: a front vacuum section, a preheating section, a heating section, a heat preservation section, a slow cooling section, a fast cooling section and a rear vacuum section;

[0009] Wherein, the heating section includes a first heating section and a second heating section;

[0010] The temperatures of the second heating section and the heat preservation section are both 10-50° C. lower than the Ac1 temperature.

[0011] Furthermore, based on the above technical solution, the Ac1 temperature of the steel bar is calculated using Andrew's formula.

[0012] Furthermore, based on the above technical solution, the sub-temperature spheroidizing annealing process is designed through the Oswald ripening mechanism.

[0013] Furthermore, based on the above technical solution, the vacuum degree of the pre-vacuum section is 30-70Pa, and the holding time is 15-30min.

[0014] Further, on the basis of the above technical solution, the preheating section includes a first preheating section and a second preheating section;

[0015] The temperature of the first preheating stage is 190-290°C and the holding time is 120-180 minutes;

[0016] The temperature of the second preheating stage is 420-480°C, and the holding time is 120-180 minutes.

[0017] Further, on the basis of the above technical solution, the temperature of the first heating section is 625-675°C, and the holding time is 70min-105min;

[0018] The temperature of the second heating section is 690-730°C, and the holding time is 70min-105min.

[0019] Further, on the basis of the above technical solution, the heat preservation section includes a first heat preservation section, a second heat preservation section and a third heat preservation section;

[0020] The temperature of the first insulation stage is 690-730°C, and the holding time is 100-150 minutes;

[0021] The temperature of the second insulation section is 690-730°C, and the holding time is 100min-150min;

[0022] The temperature of the third insulation section is 690-730°C, and the holding time is 100min-150min.

[0023] Further, on the basis of the above technical solution, the slow cooling section includes a first slow cooling section, a second slow cooling section and a third slow cooling section;

[0024] The temperature of the first slow cooling section is 670-710°C, and the holding time is 90min-135min;

[0025] The temperature of the second slow cooling section is 625-675°C, and the holding time is 90min-135min;

[0026] The temperature of the third slow cooling section is 565-635°C, and the holding time is 90min-135min.

[0027] Further, on the basis of the above technical solution, the rapid cooling section includes a first rapid cooling section and a second rapid cooling section;

[0028] The temperature of the first rapid cooling stage is 415-485°C, and the holding time is 60min-90min;

[0029] The temperature of the second fast cooling stage is 160-240°C, and the holding time is 60min-90min.

[0030] Further, on the basis of the above technical solution, the vacuum degree of the post-vacuum section is 30-70Pa, and the holding time is 15-30min;

[0031] And / or, the protective atmosphere in the roller bottom protective atmosphere continuous annealing furnace includes one of nitrogen, argon and helium;

[0032] And / or, the roller speed in the roller bottom protective atmosphere continuous annealing furnace is 2-3 m / h.

[0033] The invention provides a sub-temperature spheroidizing annealing method for a medium carbon alloy structural steel bar for cold forming, which has the following beneficial effects:

[0034] (1) The sub-temperature spheroidizing annealing in the present invention is carried out by rapidly heating to 10-50°C below the Ac1 temperature, and utilizing the Ostwald ripening mechanism, carbon diffuses from high-energy locations such as sharp corners and defects (small radius of curvature) of carbides to low-energy locations such as planes (large radius of curvature), and the original structure of the steel slowly completes the spheroidization transformation of irregular shapes, and the bainite and ferrite structures are not austenitized during the whole process. This technology has the characteristics of fast heating speed, low heating temperature, short spheroidizing time, and good spheroidizing effect.

[0035] (2) The present invention adopts a roller bottom protective atmosphere continuous annealing furnace for sub-temperature spheroidizing annealing, vacuum treatment is adopted when entering and exiting the furnace, and a protective atmosphere is introduced into the furnace, which has the characteristics of less surface oxidation and less decarburization.

[0036] (3) By controlling the holding temperature, holding time and cooling rate, the present invention obtains a 34CrNiMo6 rod with a spheroidization rate of ≥80%, a hardness of ≤200 HBW, a grain size of ≥6, a full decarburization layer of 0, and a semi-decarburization layer of ≤0.2 mm, thereby meeting the requirements for 34CrNiMo6 structural steel raw materials for cold forming. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0038] Figure 1 It is the organizational diagram of 34CrNiMo6 structural steel after ordinary spheroidizing annealing in the prior art;

[0039] Figure 2 The spheroidized surface organization diagram of 34CrNiMo6 medium carbon alloy structural steel provided by the present invention;

[0040] Figure 3 This is the spheroidized core structure diagram of the 34CrNiMo6 medium carbon alloy structural steel provided by the present invention. DETAILED DESCRIPTION

[0041] In order to make the purpose, technical scheme and advantages of the present invention clearer, the technical scheme in the embodiment of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. It should be understood by those skilled in the art that the embodiments are only to help understand the present invention and should not be regarded as specific limitations of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in the field without making creative work are within the scope of protection of the present invention. The process parameters of the following embodiments that do not specify specific conditions are usually based on conventional conditions.

[0042] The endpoints and any values ​​of the ranges disclosed in the present invention are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of each range, the endpoint values ​​of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be regarded as specifically disclosed in the present invention.

[0043] According to a first aspect of the present invention, there is provided a method for sub-temperature spheroidizing annealing of a medium carbon alloy structural steel bar for cold forming.

[0044] Based on the Ac1 temperature of the steel bar, a sub-temperature spheroidizing annealing process of the steel bar in a roller bottom protective atmosphere continuous annealing furnace is designed, including: a front vacuum section, a preheating section, a heating section, a heat preservation section, a slow cooling section, a fast cooling section and a rear vacuum section;

[0045] Wherein, the heating section includes a first heating section and a second heating section;

[0046] The temperatures of the second heating section and the heat preservation section are both 10-50° C. lower than the Ac1 temperature.

[0047] Specifically, the sub-temperature spheroidizing annealing in the present invention is heated to 10-50°C below the Ac1 temperature, and is limited to keeping 10-50°C lower than the Ac1 temperature during the second heating section and the insulation section. At such a temperature, based on the Ostwald ripening mechanism, carbon diffuses from the high-energy places such as the sharp corners and defects (small radius of curvature) of the carbide to the low-energy places at the plane (large radius of curvature), and the original organization of the steel slowly completes the irregular shape of the spheroidization transformation. During the whole process, the bainite and ferrite organizations are not austenitized. However, if the temperature of the second heating section or the insulation section is lower than the Ac1 temperature by more than 50°C, the atomic activity decreases, the atomic diffusion capacity weakens, and the cementite is difficult to spheroidize. In the end, most of the carbides are lamellar cementite organizations. If the temperature of the second heating section or the insulation section is higher than the Ac1 temperature, the organization will be transformed into austenite + pearlite + ferrite organization, and part of the carbides will dissolve in the austenite and cannot be spheroidized. In the subsequent cooling process, it will be transformed into lamellar cementite, which will eventually lead to a low spheroidization rate.

[0048] Specifically, the roller bottom protective atmosphere continuous annealing furnace is a commonly used annealing equipment, which has the characteristics of smooth roller operation, good production continuity, uniform and stable furnace temperature, fast heating, less surface decarburization layer, and good annealing quality. It is more and more widely used in industry. Therefore, this technical solution adopts a roller bottom protective atmosphere continuous annealing furnace to perform spheroidizing annealing on medium carbon alloy structural steel bars. The advantage of using a sub-temperature spheroidizing annealing process in a roller bottom protective atmosphere continuous annealing furnace is that after annealing, the steel has good surface quality, less decarburization, good flatness, high spheroidization rate, low hardness, and low tendency to work hardening. After annealing, it can be directly processed into parts by cold heading, cold drawing, and cold extrusion. Compared with the ordinary annealing + straightening + turning process, the part performance is better, the process is less, the material loss is less, and the energy consumption is lower; the box-type or desktop furnace sub-temperature spheroidizing annealing requires the annealed bars to be straightened and polished before cold processing, which weakens the advantages of sub-temperature spheroidizing annealing.

[0049] As an optional implementation manner of the present invention, the Ac1 temperature of the steel bar is calculated using Andrew's formula.

[0050] Specifically, the Ac1 temperature of 34CrNiMo6 calculated according to the Andrew formula of the present invention is ≥735°C, such as 735°C, 737°C, 740°C, 743°C, 745°C, 747°C, 750°C, etc.

[0051] As an optional embodiment of the present invention, the sub-temperature spheroidizing annealing process is designed by Ostwald ripening mechanism.

[0052] Specifically, the Ostwald ripening mechanism refers to the irregular spheroidization transformation of the original structure of the steel.

[0053] In the prior art, most steels can meet the requirements of conventional turning through ordinary annealing processes. Therefore, the conventional technology for existing parts is ordinary annealing + straightening + turning, and sub-temperature spheroidizing annealing with high requirements for furnace temperature uniformity will not be selected. However, the present invention is aimed at 34CrNiMo6 structural steel for cold forming, and it is difficult to meet the requirements by conventional spheroidizing annealing process. Therefore, sub-temperature spheroidizing annealing process is selected: sub-temperature spheroidizing annealing is to heat the steel to a certain temperature below the Ac1 temperature, and through long-term heat preservation, the bainite and pearlite in the original structure are first melted at the high surface energy of the edges and corners inside it, and transformed into short rod-shaped cementite with lower energy. Subsequently, C atoms are separated from bainite and pearlite and diffuse rapidly through the grain boundaries and precipitate at the grain boundaries. According to the Ostwald ripening mechanism of the growth of granular cementite, carbon diffuses from the high-energy locations such as the sharp corners and defects (small radius of curvature) of the carbide to the low-energy locations of the plane (large radius of curvature), and the original structure of the steel slowly completes the irregular spheroidization transformation. During the whole process, the bainite and ferrite structures are not austenitized. Sub-temperature spheroidizing annealing has the characteristics of low annealing temperature, less new decarburization layer, and good spheroidizing effect. It is suitable for spheroidizing annealing of hypoeutectoid steel. Medium carbon alloy structural steel belongs to hypoeutectoid steel. Therefore, this technical solution adopts sub-temperature spheroidizing annealing for medium carbon alloy structural steel.

[0054] As an optional embodiment of the present invention, the purpose of the fore-vacuum section is to prevent oxygen in the air from entering the furnace body along with the steel when the continuous furnace is fed with steel, resulting in a high oxygen content in the furnace, high oxidation loss and excessive decarburization. The vacuum degree of the fore-vacuum section is 30-70Pa (such as 40Pa, 50Pa, 60Pa, etc.), and the holding time is 15-30min.

[0055] As an optional embodiment of the present invention, the preheating section includes a first preheating section and a second preheating section;

[0056] The temperature of the first preheating stage is 190-290°C and the holding time is 120-180 minutes;

[0057] The temperature of the second preheating stage is 420-480°C, and the holding time is 120-180 minutes.

[0058] As an optional embodiment of the present invention, the purpose of the first heating section is to prepare for the temperature increase of the second heating section, and its temperature is 625-675°C and the holding time is 70min-105min;

[0059] The temperature of the second heating section is 690-730°C (such as 700°C, 710°C, 720°C, etc.), and the holding time is 70min-105min (such as 80min, 90min, 100min, etc.).

[0060] As an optional embodiment of the present invention, the heat preservation section includes a first heat preservation section, a second heat preservation section and a third heat preservation section;

[0061] The temperature of the first insulation stage is 690-730°C (such as 700°C, 710°C, 720°C, etc.), and the holding time is 100min-150min (such as 110min, 120min, 130min, 140min, etc.);

[0062] The temperature of the second insulation section is 690-730°C (such as 700°C, 710°C, 720°C, etc.), and the holding time is 100min-150min (such as 110min, 120min, 130min, 140min, etc.);

[0063] The temperature of the third insulation section is 690-730°C (such as 700°C, 710°C, 720°C, etc.), and the holding time is 100min-150min (such as 110min, 120min, 130min, 140min, etc.).

[0064] Specifically, the use of segmented insulation not only improves the annealing effect, but also makes the surface quality of the steel after annealing good, with less decarburization, good flatness, high spheroidization rate, low hardness and low tendency to work hardening.

[0065] As an optional embodiment of the present invention, the slow cooling section includes a first slow cooling section, a second slow cooling section and a third slow cooling section;

[0066] The temperature of the first slow cooling section is 670-710°C, and the holding time is 90min-135min;

[0067] The temperature of the second slow cooling section is 625-675°C, and the holding time is 90min-135min;

[0068] The temperature of the third slow cooling section is 565-635°C, and the holding time is 90min-135min.

[0069] As an optional embodiment of the present invention, the rapid cooling section includes a first rapid cooling section and a second rapid cooling section;

[0070] The temperature of the first rapid cooling stage is 415-485°C, and the holding time is 60min-90min;

[0071] The temperature of the second fast cooling stage is 160-240°C, and the holding time is 60min-90min.

[0072] As an optional embodiment of the present invention, the purpose of the post-vacuum section is to prevent oxygen in the air from entering the furnace body when the continuous furnace is tapping steel, causing a high oxygen content in the furnace, resulting in high oxidation loss and excessive decarburization. The vacuum degree of the post-vacuum section is 30-70Pa (such as 40Pa, 50Pa, 60Pa, etc.), and the holding time is 15-30min.

[0073] Specifically, the present invention limits the vacuum degree of the front vacuum section and the rear vacuum section to 30-70Pa, which can achieve the purpose of less surface burning and less additional decarburization after annealing. If it is lower than 30Pa, it has high requirements on equipment performance and high equipment cost, which is overcapacity in rod annealing; if it is higher than 70Pa, it will increase the risk of surface burning and decarburization of the rod.

[0074] As an optional embodiment of the present invention, the protective atmosphere in the roller bottom protective atmosphere continuous annealing furnace includes one of nitrogen, argon and helium.

[0075] As an optional implementation manner of the present invention, the roller speed in the roller bottom protective atmosphere continuous annealing furnace is 2-3 m / h.

[0076] The present invention will be further described in detail below with reference to specific embodiments and comparative examples.

[0077] Example 1

[0078] The raw materials of this embodiment can be obtained from public commercial channels. The present invention is described in detail below through specific examples, but the present invention is not limited thereto. The hot-rolled bar grade used in the following examples is 34CrNiMo6, and the standard DIN EN 10083-3:2006 is implemented. The mass percentage of each element component is shown in Table 1.

[0079] Table 1 Mass percentage of each element in the hot-rolled medium-carbon alloy structural steel bar with the grade of 34CrNiMo6 (the balance is Fe and unavoidable impurities)

[0080] element C Si Mn P S Cr Ni Mo Al Mass percentage 0.34 0.25 0.65 0.018 0.015 1.61 1.35 0.22 0.03

[0081] The Ac1 temperature calculated by Andrew's formula according to the composition of each element in the grade 34CrNiMo6 is 740℃.

[0082] The hot-rolled structural steel bars are placed on the transmission roller table for distribution. After distribution, they are driven into the roller bottom protective atmosphere continuous annealing furnace according to the set roller speed of 2-3m / h for spheroidizing annealing. The roller bottom protective atmosphere continuous annealing furnace consists of seven major parts: front vacuum chamber, preheating zone, heating zone, insulation zone, slow cooling zone, fast cooling zone, and rear vacuum chamber. Each zone is divided into 1-3 small sections. The temperature measuring points in the heat treatment area are located at the tail of the small section. The main temperature control of each section is shown in Table 2. The atmosphere in the furnace is protected by nitrogen, the roller speed is 2-3m / h, and the bars are vacuumed, preheated, heated, kept warm, slowly cooled, quickly cooled, vacuumed, and discharged from the furnace in turn to obtain spheroidizing annealed bars.

[0083] Table 2: Sub-temperature spheroidizing annealing process of carbon alloy structural steel in the embodiment of bar grade 34CrNiMo6

[0084]

[0085] According to the designed sub-temperature spheroidizing annealing process, 8 batches of 34CrNiMo6 medium carbon alloy structural steel were produced, with specifications of 20-70mm and lengths of 4-7m. The spheroidizing annealing process parameters of each batch are shown in Table 3:

[0086] Table 3 Process parameters of spheroidizing annealing for 8 batches

[0087] The unit of time is min, the unit of temperature is ℃, and the unit of vacuum degree is Pa.

[0088] The spheroidization rate of the 34CrNiMo6 medium carbon alloy structural steel produced in the above 8 batches was tested according to SEP-15201998-09, the decarburization layer was tested according to GB / T224-2019, the hardness test was carried out according to GBT231.1-2018, and the grain size test was carried out according to ISO 643-2012. The spheroidized structure and properties after annealing are shown in Table 4:

[0089] Table 4 Spheroidized structure and properties of structural steel bar grade 34CrNiMo6 after sub-temperature spheroidizing annealing

[0090]

[0091] According to the designed sub-temperature spheroidizing annealing process, the surface microstructure of 34CrNiMo6 medium carbon alloy structural steel produced in batch 4 is as follows Figure 2 As shown, the core metallographic structure is as follows Figure 3 As shown, Figure 2-3As shown in the figure, most of the carbides in the pearlite have been transformed into spherical carbides, a small amount of which are short rod-shaped carbides, and there is no obvious blocky or lamellar pearlite structure. The carbide spheroidization rate is 83% to 90%, the hardness value is 188HBW to 195HBW, the grain size is 7-8, the full decarburization layer depth is 0, and the semi-decarburization layer depth is 0.12-0.17mm, which meets the requirements of the agreement.

[0092] Comparative Example 1

[0093] The difference between this comparative example and the first batch in Example 1 is that the common spheroidizing annealing process in the prior art is used to anneal the bar grade 34CrNiMo6, and the annealing furnace is a trolley annealing furnace, which specifically includes the following steps:

[0094] 1. Heat the steel to 860℃ (±20℃) at ≤80℃ / h and keep it warm for 8-14h.

[0095] 2. Cool the steel to 720℃ (±20℃) at ≤30℃ / h and keep it warm for 2-4h.

[0096] 3. Cool the steel to ≤550℃ at ≤30℃ / h and then take it out of the furnace.

[0097] Comparative Example 2

[0098] The main difference between this comparative example and the first batch in Example 1 is that the temperature of the second heating stage is 750°C, and the temperature of the insulation stage is also 750°C, which is higher than the Ac1 temperature. The remaining operating steps and technical parameters are the same as those of the first batch in Example 1.

[0099] Comparative Example 3

[0100] The main difference between this comparative example and the first batch in Example 1 is that the temperature of the second heating stage is 660°C, and the temperature of the insulation stage is also 660°C, which is more than 50°C lower than the Ac1 temperature. The remaining operating steps and technical parameters are the same as those of the first batch in Example 1.

[0101] Comparative Example 4

[0102] The main difference between this comparative example and the first batch in Example 1 is that the vacuum degrees of the front vacuum section and the rear vacuum section are both 90Pa, and the remaining operating steps and technical parameters are the same as those of the first batch in Example 1.

[0103] Table 5 Spheroidized structure and properties of structural steel bar grade 34CrNiMo6 after annealing in the comparative example

[0104]

[0105]

[0106] As shown in Table 5, compared with the first batch in Example 1, since Comparative Example 1 adopts the ordinary spheroidizing annealing process for annealing, the surface and core hardness of the prepared rods are relatively high, difficult to process, and the spheroidization rate is low, the depth of the decarburized layer is increased, and the crystal grade particle size is low.

[0107] As shown in Table 5, compared with the first batch in Example 1, since the temperature of the second heating stage in Comparative Example 2 is 750°C and the temperature of the insulation stage is also 750°C, which is higher than the Ac1 temperature, the structure in the rod is transformed into austenite + pearlite + ferrite structure, resulting in a low spheroidization rate.

[0108] As shown in Table 5, compared with the first batch in Example 1, since the temperature of the second heating stage in Comparative Example 3 is 660°C and the temperature of the insulation stage is also 660°C, which is more than 50°C lower than the Ac1 temperature, it is difficult to spheroidize the cementite in the organization, resulting in a low spheroidization rate.

[0109] As shown in Table 5, compared with the first batch in Example 1, since the vacuum degrees of the front vacuum section and the rear vacuum section in Comparative Example 4 are both 90Pa, the surface of the rod is burned, thereby significantly increasing the depth of the decarburized layer.

[0110] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for sub-temperature spheroidizing annealing of medium carbon alloy structural steel bars for cold forming, characterized in that: Based on the Ac1 temperature of the steel bar, a sub-temperature spheroidizing annealing process of the steel bar in a roller bottom protective atmosphere continuous annealing furnace is designed, including: a front vacuum section, a preheating section, a heating section, a heat preservation section, a slow cooling section, a fast cooling section and a rear vacuum section; Wherein, the heating section includes a first heating section and a second heating section; The temperatures of the second heating section and the heat preservation section are both 10-50° C. lower than the Ac1 temperature.

2. The method for sub-temperature spheroidizing annealing of medium carbon alloy structural steel bars for cold forming according to claim 1, characterized in that: The Ac1 temperature of the steel bar is calculated using Andrew's formula.

3. The method for sub-temperature spheroidizing annealing of medium carbon alloy structural steel bars for cold forming according to claim 1, characterized in that: The sub-temperature spheroidizing annealing process is designed through the Oswald ripening mechanism.

4. The method for sub-temperature spheroidizing annealing of medium carbon alloy structural steel bars for cold forming according to claim 1, characterized in that: The vacuum degree of the pre-vacuum section is 30-70 Pa, and the holding time is 15-30 min.

5. The method for sub-temperature spheroidizing annealing of medium carbon alloy structural steel bars for cold forming according to claim 1, characterized in that: The preheating section includes a first preheating section and a second preheating section; The temperature of the first preheating stage is 190-290°C and the holding time is 120-180 minutes; The temperature of the second preheating stage is 420-480°C, and the holding time is 120-180 minutes.

6. The method for sub-temperature spheroidizing annealing of medium carbon alloy structural steel bars for cold forming according to claim 1, characterized in that: The temperature of the first heating section is 625-675°C, and the holding time is 70min-105min; The temperature of the second heating section is 690-730°C, and the holding time is 70min-105min.

7. The method for sub-temperature spheroidizing annealing of medium carbon alloy structural steel bars for cold forming according to claim 1, characterized in that: The heat preservation section includes a first heat preservation section, a second heat preservation section and a third heat preservation section; The temperature of the first insulation stage is 690-730°C, and the holding time is 100-150 minutes; The temperature of the second insulation section is 690-730°C, and the holding time is 100min-150min; The temperature of the third insulation section is 690-730°C, and the holding time is 100min-150min.

8. The method for sub-temperature spheroidizing annealing of medium carbon alloy structural steel bars for cold forming according to claim 1, characterized in that: The slow cooling section includes a first slow cooling section, a second slow cooling section and a third slow cooling section; The temperature of the first slow cooling section is 670-710°C, and the holding time is 60min-90min; The temperature of the second slow cooling section is 625-675°C, and the holding time is 60min-90min; The temperature of the third slow cooling section is 565-635°C, and the holding time is 60min-90min.

9. The method for sub-temperature spheroidizing annealing of medium carbon alloy structural steel bars for cold forming according to claim 1, characterized in that: The rapid cooling section includes a first rapid cooling section and a second rapid cooling section; The temperature of the first rapid cooling stage is 415-485°C, and the holding time is 60min-90min; The temperature of the second fast cooling stage is 160-240°C, and the holding time is 60min-90min.

10. The method for sub-temperature spheroidizing annealing of medium carbon alloy structural steel bars for cold forming according to claim 1, characterized in that: The vacuum degree of the post-vacuum stage is 30-70 Pa, and the holding time is 15-30 min; And / or, the protective atmosphere in the roller bottom protective atmosphere continuous annealing furnace includes one of nitrogen, argon and helium; And / or, the roller speed in the roller bottom protective atmosphere continuous annealing furnace is 2-3 m / h.

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