Annealing heat treatment method for high carbon chromium stainless steel

Abstract

The present invention belongs to the field of heat treatment technology of martensitic stainless steel in the metallurgical industry, and relates to an annealing heat treatment method for high-carbon chromium stainless steel. The annealing heat treatment method for high-carbon chromium stainless steel is to heat the high-carbon chromium stainless steel material to above the austenitizing temperature, keep it warm for a sufficient time, and then slowly cool it down to below the austenitizing temperature; heat it up to above the austenitizing temperature again, and then slowly cool it down to below the austenitizing temperature, and then quickly cool it down to room temperature using a rapid cooling method. The high-carbon chromium stainless steel material treated by the method has lower strength and hardness, higher elongation and shrinkage, and meets the needs of subsequent straightening, drawing, and cold heading processing. At the same time, it can increase the single-pass processing volume, reduce the number of production passes, improve production efficiency, save production costs, and has broad industrial application prospects.

Description

Technical Field

[0001] The invention belongs to the technical field of heat treatment of martensitic stainless steel in the metallurgical industry, and relates to a heat treatment method of a martensitic stainless steel material, in particular to an annealing heat treatment method of a high-carbon chromium stainless steel material. Background Art

[0002] High-carbon chromium stainless steel is a martensitic stainless steel. According to GB / T 1220-2007, "Stainless Steel Bar," and GB / T 3086-2019, "High-carbon Chromium Bearing Steel," high-carbon chromium stainless steel is a martensitic stainless steel with a carbon content greater than 0.60%. Heat treatment results in a higher hardness than conventional martensitic stainless steel. It is primarily used in precision shafts, bearings, guide rails, shears, and other applications requiring high corrosion and wear resistance.

[0003] The production of stainless steel raw materials requires hot processing and forming. The high-carbon chromium stainless steel material after hot processing has the characteristics of high hardness and low plasticity, and cannot be processed directly. Annealing heat treatment is required to reduce the strength and hardness of the material and improve the plasticity of the material.

[0004] The annealing heat treatment process directly determines the material's machinability. If the hardness and strength after annealing are high, but the elongation and shrinkage are low, the material's plasticity is poor, making it prone to cracking and breaking during processing. In severe cases, the material may break during transportation, making subsequent processing impossible. Due to its higher carbon content, high-carbon chromium stainless steel is more difficult to anneal than other martensitic stainless steels. The material's plasticity after annealing heat treatment according to the standard's recommended process is poor, forcing subsequent processing to be performed using multiple passes with small deformations, resulting in low production efficiency and high processing costs. Summary of the Invention

[0005] In view of this, the technical problem to be solved by the present invention is to provide an annealing heat treatment method for high-carbon chromium stainless steel materials, wherein the high-carbon chromium stainless steel materials heat-treated by the method have lower strength and hardness, higher elongation and shrinkage, thereby meeting the needs of subsequent straightening, drawing, cold heading and other processing, and at the same time can help increase the single-pass processing volume, reduce the number of production passes, improve production efficiency, and save production costs. Moreover, the annealing heat treatment process of the present invention is easy to control, has strong applicability with annealing equipment, is suitable for wide promotion, and has broad market application prospects. The annealing method of the high-carbon chromium stainless steel material described in the present invention is to heat the material to above the austenitizing temperature, keep it warm for a sufficient time, and then slowly cool it down to below the austenitizing temperature; heat it again to above the austenitizing temperature and keep it warm for a sufficient time, and then slowly cool it down to below the austenitizing temperature. After cooling to a sufficiently low temperature, it is cooled to room temperature by rapid cooling.

[0006] The technical solution adopted by the present invention to solve its technical problem is:

[0007] A method for annealing high carbon chromium stainless steel comprises the following steps:

[0008] (1) The high carbon chromium stainless steel material is heated to above the austenitizing temperature and then kept warm for a sufficient time; this step dissolves as many carbides as possible, and the undissolved carbides slowly spheroidize during the holding process to obtain spherical carbides and austenitized structure; the holding temperature should not be too high to avoid decarburization and coarse grains that lead to a decrease in material plasticity;

[0009] (2) Slowly cooling the structure obtained in the above step to below the austenitization temperature of the high-carbon chromium stainless steel material; the cooling process causes carbon elements to precipitate from the austenite matrix to form new carbides, and the newly formed carbides gradually grow and become spherical, thereby obtaining a spheroidized structure;

[0010] (3) heating the structure obtained in the above steps to a temperature above the austenitizing temperature of the material and keeping the temperature for a sufficient time; the heating temperature is equal to or slightly lower than the first heating temperature, and the holding time is equal to or slightly less than the first holding time; so that the small particles of carbide precipitated during the first cooling process are dissolved during the next heating and holding process, and the undissolved carbides continue to slowly spheroidize during the holding process;

[0011] (4) The structure obtained in the above steps is slowly cooled to below the austenitizing temperature and then rapidly cooled to room temperature; the slow cooling process allows the carbide particles to continue to grow slowly and evenly, obtaining a spheroidized structure with larger particles and more uniformity; the rapid cooling process uses a rapid cooling method to reduce the temperature to room temperature, avoiding the material staying in the low-temperature brittle zone for a long time to produce a brittle phase, while improving the heat treatment production efficiency.

[0012] Preferably, in the annealing heat treatment method, the high-carbon chromium stainless steel material is placed in a well-sealed heat treatment apparatus, preferably with a protective atmosphere, for heat treatment to prevent decarburization of the material during the heat treatment process. The protective atmosphere is preferably an inert atmosphere, preferably at least one of conventional inert atmospheres such as nitrogen, argon, and helium.

[0013] Preferably, the high carbon chromium stainless steel material in step (1) and step (3) is heated to above the austenite temperature, and the preferred heating temperature is not less than 880°C, more preferably 880-920°C.

[0014] Preferably, the high carbon chromium stainless steel material in step (1) and step (3) is kept warm for a sufficient time, with the optimal holding time being not less than 6 hours, to ensure uniform temperature in the heat treatment equipment.

[0015] Preferably, in step (2), the high carbon chromium stainless steel material is cooled to below the austenitizing temperature, and the preferred slow cooling temperature should not be higher than 700°C.

[0016] Preferably, in step (2) and step (4), the high carbon chromium stainless steel material is slowly cooled to below the austenite temperature of the material, and the preferred slow cooling rate should not exceed 20°C / h. The lower the cooling rate, the slower the cooling rate, the slower the carbide growth, the fewer newly formed particles, and the better the spheroidization effect of the original carbide. At the same time, the cooling rate of step (4) is lower than the cooling rate of step (2), which is more effective. However, in practice, cost pressure needs to be considered and the selection can be made based on actual conditions.

[0017] Preferably, in step (4), the high carbon chromium stainless steel material is slowly cooled again to below the austenitizing temperature, and the preferred slow cooling temperature range is 550-600°C. In fact, the cooling temperature may not be limited to the temperature range described in the present invention. The lower the terminal temperature of the slow cooling, the longer the transformation time, the better the structural transformation effect, and the better the annealing effect. However, too low a temperature will cause further energy loss and further increase economic costs. Therefore, the present invention comprehensively considers that the cooling temperature of step (2) and step (4) is the temperature range with the best overall balance. However, it can be further selected based on the actual working conditions such as the specific engineering volume.

[0018] Preferably, the rapid cooling process in step (4) is carried out by rapidly cooling the temperature to room temperature, and the preferred rapid cooling rate should be no less than 30°C / h.

[0019] Preferably, the main chemical elements of the processed high carbon chromium stainless steel material include: C: 0.60-1.20%, Si≤1.00%, Mn≤1.00%, Cr: 12.00-19.00%, Mo≤1.5% by mass percentage ， The remainder is Fe and unavoidable impurities. More preferably, the main chemical elements of the material include, by mass percentage: C: 0.60-1.20%, Si≤1.00%, Mn≤1.00%, Cr: 12.00-19.00%, Mo≤1.5%, P≤0.04%, S≤0.03%, Ni≤0.6%, Cu≤0.6%, N≤0.1%, and the remainder is Fe and unavoidable impurities.

[0020] Preferably, the room temperature in the present invention is 10°C-40°C, more preferably 20°C-30°C.

[0021] The high-carbon chromium stainless steel material after the annealing heat treatment process described above has low hardness and strength, with a hardness of ≤200HB and a strength of ≤670MPa. It also has high elongation and shrinkage, with an elongation of ≥25% and a reduction of area of ​​≥40%, and exhibits excellent overall processing properties.

[0022] Compared with the prior art, the beneficial effects of the present invention are as follows: the present invention reasonably designs the annealing heat treatment process, heats the material to above the austenitizing temperature, keeps it warm for a sufficient time, and then slowly cools it down to below the austenitizing temperature; heats it again to above the austenitizing temperature and keeps it warm for a sufficient time, then slowly cools it down to below the austenitizing temperature, and after it is cooled to a sufficiently low temperature, it is cooled down to room temperature by rapid cooling. The high-carbon chromium stainless steel material heat-treated by the method has lower strength and hardness, higher elongation and shrinkage, and thus meets the needs of subsequent straightening, drawing, cold heading and other processing, while increasing the single-pass processing volume, reducing production passes, improving production efficiency, and saving production costs. Moreover, the annealing heat treatment process of the present invention is easy to control, has strong compatibility with annealing equipment, is suitable for wide promotion, and has broad market and industrial application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is a schematic diagram of an annealing heat treatment method of the present invention;

[0024] Figure 2 This is a schematic diagram of the heat treatment process of Example 1 of the present invention;

[0025] Figure 3 Schematic diagram of the heat treatment process of Example 2 and Example 3.

[0026] Figure 4 This is the metallographic structure diagram of Example 1 after heat treatment of the present invention.

[0027] Figure 5 This is the metallographic structure diagram of Example 2 after heat treatment of the present invention.

[0028] Figure 6 This is the metallographic structure diagram of Example 3 after heat treatment of the present invention.

[0029] Figure 7 This is the metallographic structure diagram after heat treatment of Example 1 annealing under the standard recommended heat treatment system.

[0030] Figure 8 This is the metallographic structure diagram after heat treatment of example 2 of annealing of the standard recommended heat treatment system. DETAILED DESCRIPTION

[0031] In order to further understand the present invention, the technical solution of the present invention is further described below in conjunction with the embodiments. However, it should be understood that these descriptions are only for further illustrating the technical solution of the present invention and further illustrating the features and advantages of the present invention, and do not constitute a present limitation to the claims of the invention.

[0032] Example

[0033] High carbon chromium stainless steel material: High carbon chromium stainless steel wire rods with diameters of Φ5.5 mm and Φ9.5 mm were respectively selected for heat treatment (the process scheme described in the present invention is applicable to high carbon chromium stainless steel materials of different shapes, and the present invention mainly uses wire rods as an example). The main chemical compositions of the materials are shown in Table 1.

[0034] Table 1 Basic information and chemical composition of materials in Example

[0035]

[0036] Annealing heat treatment process:

[0037] (1) The selected high carbon chromium stainless steel material is placed in an atmosphere protection hood annealing furnace for heat treatment.

[0038] (2) The high-carbon chromium stainless steel material is heated to a temperature above the austenitizing temperature and then kept at this temperature for a sufficient period of time. In Example 1, the material is heated to 890°C at a heating rate of 150°C / h and then kept at this temperature for 10 hours; in Examples 2 and 3, the material is heated to 900°C at a heating rate of 150°C / h and then kept at this temperature for 12 hours. The heating rates in Steps 2 and 4 can be set and adjusted based on the heating capacity and production efficiency of the actual heat treatment equipment, and have little effect on the heat treatment performance of the present invention.

[0039] (3) The material is slowly cooled to below the austenitizing temperature, wherein Example 1 is cooled to 700°C at a rate of 20°C / h; Examples 2 and 3 are cooled to 700°C at a rate of 10°C / h.

[0040] (4) Heat the material to a temperature above the austenitizing temperature and keep it at that temperature for a sufficient time. The heating temperature is equal to or slightly lower than the first heating temperature, and the holding time is equal to or slightly less than the first holding time. In Example 1, the material is heated to 880°C at a heating rate of 100°C / h and then kept at that temperature for 10 hours; in Examples 2 and 3, the material is heated to 890°C at a heating rate of 100°C / h and then kept at that temperature for 12 hours.

[0041] (5) Slowly cool the material to below the austenitizing temperature. In Example 1, the temperature is lowered to 550°C at a rate of 20°C / h; in Examples 2 and 3, the temperature is lowered to 550°C at a rate of 10°C / h.

[0042] (6) After the material is cooled to the sufficiently low target temperature of step (5), it is rapidly cooled to room temperature using a rapid cooling method. In Examples 1, 2, and 3, the temperature is respectively cooled to 550°C, then cooled to 250°C using a hood cooling method at a rate of 50°C / h, and then removed from the furnace and air-cooled. A high-carbon chromium stainless steel material is obtained after heat treatment. The mechanical properties of the heat-treated materials of each Example are shown in Table 2.

[0043] Table 2 Mechanical properties and drawing properties after annealing heat treatment of the embodiment

[0044]

[0045]

[0046] Note: Single-pass drawing reduction rate (single-pass drawing reduction rate) = (1-cross-sectional area after drawing / cross-sectional area before drawing)%

[0047] Comparative Example

[0048] The material was heat treated using the heat treatment process recommended by the GB / T 1220-2007 "Stainless Steel Rod" standard as a comparative example. The standard recommends "800-920°C slow cooling" as the heat treatment process. Historical data using this recommended heat treatment process were selected for comparison. Heat treatment was also performed in a nitrogen-protected bell-type annealing furnace. High-carbon chromium stainless steel wire rods were heated to 870°C, held at this temperature for 8 hours, then cooled at a rate of 30°C / h to 550°C. Cooling was then performed using a bell-type annealing furnace. After cooling to 250°C, the wire rods were removed from the furnace and air-cooled.

[0049] The basic information and chemical composition of the materials treated by conventional process are shown in Table 3, and the mechanical properties and drawing properties after treatment are shown in Table 4.

[0050] Table 3 Basic information and chemical composition of materials processed by conventional process

[0051]

[0052] Table 4 Mechanical properties and drawing properties after conventional annealing heat treatment

[0053]

[0054] From the embodiment and the accompanying drawings ( Figure 4 、 Figure 5 、 Figure 6 ) and comparative examples and accompanying drawings ( Figure 7 、 Figure 8 Comparison of the metallographic microstructure of the annealed high-carbon chromium stainless steel reveals that the microstructure of the annealed high-carbon chromium stainless steel consists of a ferrite matrix (light gray) with discretely distributed granular carbides (pure white particles and black dot-like particles). Through the annealing process provided by the present invention, the high-carbon chromium stainless steel obtained after heat treatment in each example exhibits a microstructure characterized by larger, predominantly spherical carbide particles (pure white particles), relatively fewer small carbides (black dot-like particles), and a more uniform carbide distribution, resulting in lower strength and better plasticity.

[0055] As can be seen from Table 2 of the examples and the comparison with Table 4 of the comparative examples, the high carbon chromium stainless steel materials obtained after heat treatment in each example through the annealing process provided by the present invention have significantly reduced strength and hardness, wherein the strength is not greater than 670 MPa and the hardness is not greater than 200 HB; the elongation and shrinkage after fracture are significantly improved, with the elongation after fracture being not less than 25% and the shrinkage after fracture being not less than 40%, and having good comprehensive processing performance.

[0056] The above-described embodiments are only preferred solutions of the present invention and are not intended to limit the present invention in any form. Other variations and modifications are possible without exceeding the technical solutions described in the claims.

Claims

1. A method for annealing high carbon chromium stainless steel, characterized in that: The following steps are involved: (1) Heating the high carbon chromium stainless steel material to above the austenitizing temperature and keeping it warm; (2) slowly cooling the structure obtained in the above step to below the austenitizing temperature of the high carbon chromium stainless steel material; (3) heating the tissue obtained in the above step to a temperature above the austenitizing temperature of the material and keeping it warm; wherein the heating temperature is not higher than the temperature in step (1) and the keeping warm time is not longer than the keeping warm time in step (1); (4) slowly cooling the structure obtained in the above step to below the austenitizing temperature, and then rapidly cooling it to room temperature; Wherein: the heating temperature in said step (1) and step (3) is not less than 880°C, and the holding time is not less than 6 hours; The temperature of the slow cooling in step (2) is not higher than 700°C; the temperature range of the slow cooling in step (4) is 550-600°C; The slow cooling rate of step (2) and step (4) does not exceed 20°C / h, and the fast cooling rate of step (4) is not less than 30°C / h.

2. The annealing heat treatment method for high carbon chromium stainless steel according to claim 1, characterized in that: The heat treatment process is carried out under the protection of an inert atmosphere.

3. The annealing heat treatment method for high carbon chromium stainless steel according to claim 1, characterized in that: The main chemical elements of the processed high carbon chromium stainless steel material include, by mass percentage, C: 0.60-1.20%, Si≤1.00%, Mn≤1.00%, Cr: 12.00-19.00%, Mo≤1.5%, and the rest are Fe and unavoidable impurities.

4. The annealing heat treatment method for high carbon chromium stainless steel according to claim 1, characterized in that: The hardness of the high carbon chromium stainless steel material after heat treatment is ≤200HBW, the tensile strength is ≤670MPa, the elongation after fracture is ≥25%, and the section shrinkage is ≥40%.

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