Multi-stage and multi-phase heat treatment method for improving toughness of high-strength low-alloy steel workpiece

Abstract

This application relates to the field of heat treatment technology for high-strength low-alloy steel workpieces, specifically providing a multi-stage multiphase heat treatment method for improving the toughness of high-strength low-alloy steel workpieces. This heat treatment method employs a multi-stage multiphase quenching process, resulting in a multiphase microstructure of lath bainite, sheet bainite, sheet martensite, and retained austenite in the steel workpiece. During the multi-stage multiphase quenching process, the phase formation sequence is lath bainite, sheet bainite, and sheet martensite, respectively. By pre-forming partial lath bainite, the nucleation and growth of sheet bainite are promoted, shortening the processing time and improving production efficiency. Through this technology, the toughness of high-strength low-alloy steel workpieces can be improved.

Description

Technical Field

[0001] This application relates to the field of heat treatment technology for high-strength low-alloy steel workpieces, and specifically provides a multi-stage multiphase heat treatment method for improving the toughness of high-strength low-alloy steel workpieces. Background Technology

[0002] Steel remains one of the most widely used materials. High-strength low-alloy steel, in particular, is widely used in transportation, construction, bridges, shipbuilding, offshore platforms, and engineering machinery, primarily due to its advantages such as low cost and excellent overall performance. As these sectors continue to expand towards extreme environmental conditions—such as faster transportation, taller buildings, and deeper oceans—the demands on its service performance are becoming increasingly stringent. Furthermore, under the dual pressures of environmental protection ("dual carbon") and overcapacity, the steel industry is facing industrial restructuring and upgrading, placing even more stringent requirements on the performance of high-strength low-alloy steel. Using high-strength low-alloy steel with superior performance and in smaller quantities can alleviate the pressure on resources, energy, and the environment.

[0003] Among the many performance requirements of high-strength low-alloy steel, strength and toughness are two particularly critical performance indicators. The properties of steel are closely related to its microstructure. Through the 973 Program project "Theoretical and Technological Foundation Research on Microstructure Control of High-Performance Steel," it was recognized that employing a "multiphase, metastable, and multiscale" approach... 3 The concept of microstructure control expands the matrix microstructure from a single phase (such as ferrite, bainite, or martensite) to a multiphase structure (bainite + martensite, ferrite + bainite, bainite + martensite, or bainite + martensite + austenite), effectively inhibiting crack nucleation and propagation, and improving the strength and toughness of steel. The microstructure and properties of high-strength low-alloy steel primarily depend on heat treatment. Therefore, developing novel heat treatment processes to obtain multiphase microstructures and improve the strength and toughness of steel materials is one of the important development directions for the steel industry.

[0004] In view of this, the present invention is hereby proposed. Summary of the Invention

[0005] The purpose of this application is to provide a multi-stage, multiphase heat treatment method for improving the toughness of high-strength low-alloy steel workpieces, so as to improve the toughness of high-strength low-alloy steel workpieces by improving the processing technology.

[0006] To achieve the above objectives, this application adopts the following technical solution.

[0007] A multi-stage multiphase heat treatment method for improving the toughness of high-strength low-alloy steel workpieces, the method comprising the following steps:

[0008] S1: Heating and heat preservation treatment

[0009] The high-strength low-alloy steel workpiece is heated to the first temperature T1 and held at that temperature for a period of time t1 to complete the austenitization and composition homogenization of the high-strength low-alloy steel workpiece.

[0010] S2: Multi-stage multiphase controlled quenching

[0011] Process S2-1: Speed ​​and temperature controlled cooling

[0012] The high-strength low-alloy steel workpiece that has completed S1 is cooled to the second temperature T2, so that the high-strength low-alloy steel workpiece forms partial lamellar bainite;

[0013] Process S2-2: Temperature and time controlled insulation

[0014] The high-strength low-alloy steel workpiece that has completed S2-1 is heated to the third temperature T3 and held at that temperature for a period of time t2, so that the high-strength low-alloy steel workpiece forms part of lath bainite.

[0015] Process S2-3: Rapid cooling treatment

[0016] The high-strength low-alloy steel workpiece that has completed S2-2 is cooled to room temperature, so that the high-strength low-alloy steel workpiece forms a partial lath martensite structure.

[0017] S3: Medium-low temperature tempering treatment

[0018] The high-strength low-alloy steel workpiece with S2-3 is heated to the fourth temperature T4 and held at that temperature for a period of time t3 for tempering treatment to obtain a multiphase microstructure of lath bainite, slab bainite, slab martensite and retained austenite.

[0019] Further, by mass percentage, the high-strength low-alloy steel workpiece comprises: 0.15%–0.32% C, 1.2%–3.0% Mn, 0.6%–1.8% Si, 0.4%–1.5% Cr, 0.2%–0.8% Mo, 0.2%–1.2% Ni; P≤0.0015%, S≤0.005%, with the remainder being Fe and unavoidable impurity elements. Simultaneously, the sum of the elemental contents of Mn, Cr, and Mo is not less than 2.6% and not more than 4.0%.

[0020] Furthermore, in S1, the first heating temperature T1 of the high-strength low-alloy steel workpiece is 30°C to 100°C higher than the austenite final transformation temperature of the high-strength low-alloy steel workpiece, and the holding time t1 is 120 min to 240 min.

[0021] Furthermore, in the process S2-1, the cooling rate of the high-strength low-alloy steel workpiece is the same as the cooling rate at which lamellar bainite is obtained;

[0022] Preferably, the cooling method is air jet cooling and / or spray cooling.

[0023] Furthermore, in the process S2-1, the second temperature T2 is higher than the martensitic transformation start temperature of the high-strength low-alloy steel workpiece and lower than the lamellar bainitic transformation start temperature, preferably 5°C to 30°C higher than the martensitic transformation start temperature.

[0024] Furthermore, in the process S2-2, the third temperature T3 is higher than the lath bainite formation temperature of the high-strength low-alloy steel workpiece but lower than the bainite initiation transformation temperature, and the holding time t2 is 120 min to 360 min.

[0025] Furthermore, in the process S2-3, the cooling method is air jet cooling and / or spray cooling.

[0026] Furthermore, in S3, the fourth temperature T4 is higher than the martensite completion transformation temperature and lower than the lamellar bainite initiation transformation temperature, and the holding time t3 is 240 min to 480 min.

[0027] Preferably, the fourth temperature T4 is greater than 100°C and less than 360°C.

[0028] Furthermore, the high-strength low-alloy steel workpiece before S1 is in a forged state or a cast + annealed state, and the microstructure of the high-strength low-alloy steel workpiece is a bainite + martensite multiphase structure or a ferrite + pearlite structure.

[0029] Furthermore, the maximum wall thickness of the high-strength low-alloy steel workpiece is 60mm, the minimum wall thickness is 10mm, and the maximum difference between the maximum and minimum wall thickness is 20mm.

[0030] Compared with the prior art, the technical effects of this application are as follows:

[0031] This application's multi-stage multiphase heat treatment method utilizes a multi-stage quenching process. By pre-forming a partial lamellar bainite structure, it promotes the nucleation and growth of lath bainite, shortening process time and improving production efficiency. Through the control of cooling rate and temperature in the multi-stage, multi-step quenching process, the microstructure of the high-strength low-alloy steel workpiece consists of a multiphase structure of lamellar bainite, lath bainite, lath martensite, and retained austenite. These techniques improve the toughness of the high-strength low-alloy steel workpiece. Attached Figure Description

[0032] The various technical features of this application and their relationships will be further explained below with reference to the accompanying drawings. The drawings are exemplary; some technical features are not shown to scale, and some drawings may omit technical features commonly used in the art to which this application pertains that are not essential for understanding and implementing this application, or additionally show technical features that are not essential for understanding and implementing this application. In other words, the combination of various technical features shown in the drawings is not intended to limit this application. Furthermore, throughout this application, the same reference numerals refer to the same things. Specific descriptions of the drawings are as follows:

[0033] Figure 1 This is the continuous cooling transformation curve of the high-strength low-alloy steel workpiece in Example 1; in the figure, Ac1 is the austenitic austenitic transformation temperature, Ac3 is the austenitic end transformation temperature, Ms is the martensitic start transformation temperature, Mf is the martensitic end transformation temperature, A is austenite, UB is lath bainite, LB is lath bainite, and M is martensite.

[0034] Figure 2 These are SEM images of high-strength low-alloy steel workpieces processed using Example 1;

[0035] Figure 3 These are SEM images of a high-strength low-alloy steel workpiece processed using Comparative Example 1-1.

[0036] Figure 4 These are SEM micrographs of high-strength low-alloy steel workpieces processed using Comparative Examples 1-2;

[0037] Figure 5 These are SEM micrographs of high-strength low-alloy steel workpieces processed using Comparative Examples 1-3;

[0038] Figure 6 This is the continuous cooling transformation curve of the high-strength low-alloy steel workpiece in Example 2; in the figure, Ac1 is the austenitic austenitic transformation temperature, Ac3 is the austenitic end transformation temperature, Ms is the martensitic start transformation temperature, Mf is the martensitic end transformation temperature, A is austenite, UB is lath bainite, LB is lath bainite, M is martensite, and F is ferrite.

[0039] Figure 7 These are SEM images of the high-strength low-alloy steel workpiece processed using Example 2;

[0040] Figure 8 These are SEM images of a high-strength low-alloy steel workpiece processed using Comparative Example 2-1;

[0041] Figure 9 These are SEM images of a high-strength low-alloy steel workpiece processed using Comparative Example 2-2;

[0042] Figure 10 These are SEM images of high-strength low-alloy steel workpieces processed using Comparative Example 2-3. Detailed Implementation

[0043] Glossary

[0044] High-strength low-alloy steel: refers to a class of steel materials with a yield strength of 500-800 MPa and a total alloy element content of less than 5%.

[0045] Hardenability: refers to the material characteristics characterized by the depth of the hardened layer and the distribution of hardness under specified conditions. It mainly depends on the critical quenching rate of the material. Under specified conditions, it determines the hardened depth and hardness distribution of steel. It characterizes the ability of steel to obtain the depth of the hardened layer during quenching, and represents the steel's ability to accept quenching. It is related to the chemical composition / components of the steel.

[0046] Martensitic hardenability: The ability of a sample to obtain a martensitic structure under specified conditions to characterize the material properties.

[0047] Bainitic hardenability: The ability of a material to obtain a bainitic microstructure using a sample under specified conditions to characterize the material properties.

[0048] Hardenability: refers to the ability of steel to harden during quenching. It is expressed as the highest hardness that can be obtained by quenching into martensite. It mainly depends on the carbon content in the martensite. The higher the carbon content, the higher the hardenability of the steel.

[0049] Austenite transformation temperature: The temperature at which the ferrite / pearlite / bainite / martensite structure of steel begins to transform into austenite structure during the slow heating process.

[0050] Austenite transformation end temperature: The temperature at which ferrite / pearlite / bainite / martensite structures in steel materials are completely transformed into austenite structures during the slow heating process.

[0051] Bainite transformation temperature: The temperature at which the austenitic structure of steel begins to transform into bainite during the cooling process. This includes granular bainite, lath bainite, and lamellar bainite structures. Generally, the highest temperature is the formation temperature of granular bainite, followed by lath bainite, and then lamellar bainite.

[0052] The temperature at which lamellar bainite begins to transform is the temperature at which austenite begins to transform into lamellar bainite during the cooling process of steel materials; this temperature is lower than the temperature at which granular bainite and lath bainite begin to transform.

[0053] The martensitic transformation temperature: the temperature at which the austenitic structure of steel begins to transform completely into martensite during the cooling process.

[0054] Austenite: In the context of steel materials, it refers to a solid solution in which carbon atoms are dissolved in γ-Fe. It has a high ability to dissolve carbon atoms and is generally represented by the symbol "A" or "γ".

[0055] Ferrite: In the context of steel materials, it refers to a solid solution in which carbon atoms are dissolved in α-Fe. Its ability to dissolve carbon atoms is very low, and it is generally represented by the symbol "F" or "α".

[0056] Pearlite: In the case of steel materials, it is an ordered mixture of ferrite and cementite, and is a product of the transformation of austenite at high temperature. It is generally represented by the symbol "P".

[0057] Bainite: In the case of steel materials, it is the austenite transformation product when austenite is supercooled to a temperature range between the pearlite transformation temperature and the martensite transformation temperature. It is generally represented by the symbol "B".

[0058] Martensite: In the case of steel materials, it is a supersaturated solid solution of carbon atoms in α-Fe. It is a product of the low-temperature transformation of austenite and is generally represented by the symbol "M".

[0059] Retained austenite: In this invention, it refers to the small amount of austenite that remains unchanged even after cooling to room temperature in high-strength alloy steel through alloying regulation and process control. This untransformed austenite is called "retained austenite" and is generally represented by the symbol "RA".

[0060] The multi-stage multiphase heat treatment method for improving the toughness of high-strength low-alloy steel workpieces provided by this invention mainly includes three steps: S1 heating and holding treatment, S2 multi-stage multiphase controlled quenching treatment, and S3 medium-low temperature tempering treatment.

[0061] The high-strength low-alloy steel workpiece in this invention refers to a steel workpiece with a carbon element mass percentage between 0.15% and 0.32%, using Mn, Si, Cr, Mo, and Ni as alloying elements, and the sum of the contents of Mn, Cr, and Mo is not less than 2.6% and not more than 4.0%, with the remainder being Fe and other unavoidable impurity elements. In a preferred embodiment, the composition of the high-strength low-alloy steel workpiece, by mass percentage, includes: 0.15%–0.32% C, 1.2%–3.0% Mn, 0.6%–1.8% Si, 0.4%–1.5% Cr, 0.2%–0.8% Mo, 0.2%–1.2% Ni; P ≤ 0.0015%, S ≤ 0.005%, with the remainder being Fe and unavoidable impurity elements, while the sum of the contents of Mn, Cr, and Mo is not less than 2.6% and not more than 4.0%.

[0062] In the high-strength low-alloy steel workpieces of this invention, adding 0.15% to 0.32% C is beneficial to improving the hardenability of the high-strength low-alloy steel workpieces, obtaining high-strength bainitic and martensitic structures, and improving the stability of retained austenite. However, excessive C content is detrimental to the formation of lath martensite and the toughness of the workpiece. By controlling the Si content, carbide formation during the S2 multi-stage controlled quenching process is avoided, which would worsen the toughness of the high-strength low-alloy steel workpieces. By controlling the Mn, Cr, and Mo content, the bainitic hardenability of the high-strength low-alloy steel workpieces can be improved, preventing the formation of ferrite and pearlite under air-jet or spray cooling conditions, which is beneficial to the temperature control of the high-strength low-alloy steel workpieces during the S2-1 speed-controlled and temperature-controlled cooling process in the S2 multi-stage controlled quenching process. Adding a certain amount of Ni can improve the impact toughness of the high-strength low-alloy steel workpieces, but Ni is expensive and needs to be strictly controlled.

[0063] In a preferred embodiment, in order to improve the temperature uniformity of the high-strength low-alloy steel workpiece during the multi-stage controlled quenching process of S2, the maximum wall thickness of the high-strength low-alloy steel workpiece is 60mm, the minimum wall thickness is 10mm, and the maximum difference between the maximum and minimum wall thickness is 20mm.

[0064] S1 heating and heat preservation treatment

[0065] In one embodiment, the heating and holding process includes: placing the high-strength low-alloy steel workpiece into a heating furnace that has been heated to a first temperature T1 and holding it at that temperature for a first time t1 to complete the austenitization and composition homogenization of the high-strength low-alloy steel workpiece.

[0066] In some implementations, the low-alloy steel workpiece to be subjected to S1 treatment is in a forged state or in a cast + annealed state.

[0067] In some embodiments, to achieve the aforementioned phase transformation, the first temperature T1 is higher than the Ac3 (austenite completion transformation temperature) of the high-strength low-alloy steel workpiece material, preferably 30°C to 100°C higher, for example, but not limited to, 30°C, 40°C, 50°C, 60°C, 70°C, 80°C, 90°C, or 100°C. In some embodiments, the first time t1 is 120 to 240 minutes.

[0068] In a preferred embodiment, before performing the heating and heat preservation treatment of S1, a continuous cooling transformation curve of the high-strength low-alloy steel workpiece material should be obtained to determine key cooling rates and key temperatures such as the cooling rate range for obtaining lamellar bainite, the austenite completion transformation temperature, the bainite initiation transformation temperature, the lamellar bainite initiation transformation temperature, and the martensite initiation transformation temperature.

[0069] In a preferred embodiment, the heating and holding process is as follows: the high-strength low-alloy steel workpiece that has been forged or cast and annealed is placed in a first heating furnace that has been heated to a first time T1 (e.g., 880°C to 1000°C) and held for a first time t1 (e.g., 120 min to 240 min).

[0070] In a preferred embodiment, during the heating and holding process of S1, the microstructure of the high-strength low-alloy steel workpiece is transformed from a forged or cast + annealed bainite / martensite or ferrite + pearlite microstructure into a single-phase austenite microstructure, and the composition is homogenized.

[0071] The first temperature T1 may be, but is not limited to, 880℃, 900℃, 920℃, 940℃, 960℃, 980℃ or 1000℃; the first time t1 may be, but is not limited to, 120min, 130min, 160min, 200min or 240min.

[0072] S2 Multi-stage Multiphase Controlled Quenching Treatment

[0073] Quenching is a heat treatment method, including continuous cooling quenching and isothermal quenching. Continuous cooling quenching of steel involves heating the steel to a temperature above the critical temperature Ac3 (hypoeutectoid steel) or Ac1 (hypereutectoid steel) (i.e., S1 in this invention), holding it at that temperature for a period of time to achieve complete or partial austenitization, and then rapidly cooling it at a rate greater than the critical cooling rate to below Ms (or isothermally near Ms) to undergo martensitic (or bainitic) transformation. Isothermal quenching of steel involves heating the steel to a temperature above the critical temperature Ac3 (hypoeutectoid steel) or Ac1 (hypereutectoid steel), holding it at that temperature for a period of time to achieve complete or partial austenitization, and then rapidly cooling it at a rate greater than the critical cooling rate to between the bainitic initiation temperature and the bainitic termination temperature, and isothermally cooling it for a period of time to undergo bainitic transformation.

[0074] In this invention, the multi-stage multiphase controlled quenching process employs controlled-speed and controlled-temperature cooling, controlled-temperature and controlled-time holding, and rapid cooling. During the multi-stage multiphase controlled quenching process, the high-strength low-alloy steel workpiece undergoes controlled-speed and controlled-temperature cooling at a certain rate to a temperature between the bainite initiation temperature and the martensite initiation temperature, forming a certain amount of lamellar bainite. Next, controlled-temperature and controlled-time holding is performed, heating the workpiece to a temperature between the bainite initiation temperature and the lath bainite initiation temperature and holding it for a relatively long time to form a certain amount of lath bainite. Finally, rapid cooling is performed, rapidly cooling the workpiece to room temperature, forming a certain amount of lath martensite, with a small amount of austenite remaining.

[0075] To achieve the above-mentioned phase transition process, S2 specifically includes the following steps:

[0076] Process S2-1: Speed ​​and temperature controlled cooling: After S1, the high-strength low-alloy steel workpiece is cooled to the second temperature T2 at a certain cooling rate, so that a portion of lamellar bainite is formed in the high-strength low-alloy steel workpiece.

[0077] Process S2-2: Temperature and time control and heat preservation: After S2-1, the high-strength low-alloy steel workpiece is heated to the third temperature T3 and held for a period of time t3, so that a portion of lath bainite is formed in the high-strength low-alloy steel workpiece.

[0078] In some embodiments, the third temperature T3 to which the high-strength low-alloy steel workpiece is heated in S2-2 is higher than the second temperature T2 to which the high-strength low-alloy steel workpiece is cooled in S2-1.

[0079] Process S2-3: Rapid cooling treatment: After S2-2, the high-strength low-alloy steel workpiece is cooled to room temperature, so that a portion of the high-strength low-alloy steel workpiece forms a lath martensite structure.

[0080] In a preferred embodiment, the specific method of S2 is preferably as follows:

[0081] Process S2-1: The high-strength low-alloy steel workpiece that has completed step S1 is taken out from the first heating furnace and cooled by air spray or mist spray at a certain cooling rate V to a second temperature T2 that is higher than the martensite transformation temperature of the high-strength low-alloy steel workpiece material but lower than the lamellar bainite transformation temperature. The high-strength low-alloy steel workpiece forms lamellar bainite structure first at the cooling rate V, rather than granular bainite or lath bainite structure.

[0082] The cooling rate V can be, but is not limited to, 0.1℃ / s, 0.25℃ / s, 0.5℃ / s, 0.75℃ / s, 1℃ / s, 2℃ / s, 3℃ / s, 4℃ / s, 5℃ / s, 6℃ / s, 7℃ / s, 8℃ / s, 9℃ / s, 10℃ / s, 11℃ / s, 12℃ / s, 13℃ / s, 14℃ / s, or 15℃ / s; the second temperature T2 can be, but is not limited to, 300℃, 310℃, 320℃, 330℃, 340℃, 350℃, 360℃, or 380℃. Preferably, T2 is 5℃ to 30℃ higher than the martensitic transformation start temperature of the high-strength low-alloy steel workpiece material, for example, it can be, but is not limited to, 5℃, 10℃, 15℃, 20℃, 25℃, or 30℃.

[0083] Process S2-2: The high-strength low-alloy steel workpiece that has completed process S2-1 is quickly placed into the second heating furnace that has been heated to the third temperature T3, and heated and held at that temperature for a second time t2. The third heating temperature T3 is lower than the bainite transformation temperature but higher than the lath bainite formation temperature.

[0084] The third temperature T3 can be, but is not limited to, 330℃, 340℃, 350℃, 360℃, 370℃, 380℃, 390℃ or 400℃; the second time t2 can be, but is not limited to, 120min, 150min, 180min, 210min, 240min, 270min, 300min, 330min or 360min.

[0085] Step S2-3: Remove the high-strength low-alloy steel workpiece that has completed step S2-2 from the second heating furnace and cool it to room temperature using air jet or spray cooling method.

[0086] S3 Low-temperature tempering treatment

[0087] Tempering is a heat treatment method in which high-strength low-alloy steel workpieces are hardened, heated to a temperature below Ac1 (the starting temperature of the transformation from pearlite to austenite during heating), held at that temperature for a certain time, and then cooled to room temperature. Tempering is divided into low-temperature tempering (tempering of high-strength low-alloy steel workpieces at 150℃~250℃), medium-temperature tempering (tempering of high-strength low-alloy steel workpieces between 360℃~500℃), and high-temperature tempering (tempering of high-strength low-alloy steel workpieces above 500℃~650℃).

[0088] The low-temperature tempering treatment of S3 includes the following steps: the high-strength low-alloy steel workpiece after S2 is placed in a heating furnace heated to the fourth temperature T4 and held at that temperature for a third time t3 (240 min to 480 min), then removed from the furnace and cooled to room temperature. The resulting high-strength low-alloy steel workpiece has a multiphase microstructure consisting of lath bainite, slab bainite, slab martensite, and retained austenite.

[0089] In a preferred embodiment, the fourth temperature T4 is greater than 100°C but less than 360°C, and the third time t3 is 240 min to 480 min.

[0090] T4 can be, but is not limited to, 100℃, 150℃, 200℃, 250℃, 300℃ or 360℃; t3 can be, but is not limited to, 240min, 270min, 300min, 330min, 360min, 390min, 420min, 450min or 480min.

[0091] In a preferred embodiment, the cooling method of S3 can be either air jet cooling or natural air cooling.

[0092] The specific embodiments of this application will now be described in detail with reference to the accompanying drawings.

[0093] Example 1

[0094] The composition and content of the high-strength low-alloy steel workpiece in Example 1 are shown in Table 1 below:

[0095] Table 1. Chemical composition and content (mass percentage) of high-strength low-alloy steel workpieces. content 0.28 1.78 1.85 1.23 0.61 0.22

[0096] Several high-strength low-alloy steel workpieces were prepared using conventional steelmaking, forging, and cutting methods according to the chemical composition and content in Table 1. The initial microstructure was bainitic / martensite dual phase. The dimensions of the high-strength low-alloy steel workpieces were 500mm*300mm*20mm (length*width*thickness).

[0097] A cylinder with a diameter of 4 mm and a length of 10 mm was randomly selected from a high-strength low-alloy steel workpiece. The continuous cooling transformation curve of Example 1 was obtained according to the national standard YB / T 5128-2018 "Determination of Continuous Cooling Transformation Curve of Steel - Expansion Method". Figure 1 As shown, the cooling rate range for obtaining lamellar bainite is 0.25℃ / s to 8℃ / s. During the heating process, the austenite initiation transformation temperature is 760℃, the austenite termination transformation temperature is 860℃, the bainite initiation transformation temperature is 420℃, the lamellar bainite initiation transformation temperature is 340℃, the martensite initiation transformation temperature is 295℃, the martensite termination transformation temperature is 140℃, and the lath bainite initiation transformation temperature is 340℃.

[0098] The multi-stage multiphase heat treatment method of the present invention is used to process the workpiece, and the specific steps are as follows:

[0099] S1: Place the high-strength low-alloy steel workpiece into the first heating furnace that has been heated to 920°C (higher than the austenite end transformation temperature of the high-strength low-alloy steel workpiece in Example 1), heat and hold for 180 minutes.

[0100] S2-1: The high-strength low-alloy steel workpiece that has completed the S1 treatment is quickly taken out from the first heating furnace and cooled to 310℃ (above the martensite transformation temperature and below the lamellar bainite transformation temperature) using an industrial fan. At this time, the cooling rate is about 2℃ / s.

[0101] S2-2: Quickly place the high-strength low-alloy steel workpiece that has been treated in S2-1 into the second heating furnace that has been heated to 360°C, and heat and hold it for 180 minutes.

[0102] S2-3: The high-strength low-alloy steel workpiece that has completed the S2-2 process is removed from the second heating furnace and cooled to room temperature using the industrial fan from S2-1.

[0103] S3: Place the high-strength low-alloy steel workpiece that has completed the S2-3 treatment into the third heating furnace that has been heated to 280°C, heat and hold for 240 minutes, and then take it out and use the industrial fan of S2-1 to cool the high-strength low-alloy steel workpiece to room temperature.

[0104] The conventional mechanical properties of the high-strength low-alloy steel workpiece of Example 1 were tested according to the national standard GB / T 228-2002, and the impact toughness of the high-strength low-alloy steel workpiece of Example 1 was tested according to the national standard GB / T229-1994, as shown in Table 2.

[0105] Table 2 Conventional mechanical properties and impact toughness of high-strength low-alloy steel workpieces

[0106] Compared with Comparative Examples 1-1, 1-2 and 1-3 below, the impact toughness of the high-strength low-alloy steel workpiece treated with Example 1 is improved while the strength remains almost unchanged.

[0107] The microstructure of the high-strength low-alloy steel workpiece treated in Example 1 is as follows: Figure 2 As shown, its final microstructure consists of lath bainite, slab bainite, slab martensite, and austenite; and XRD analysis revealed that the high-strength low-alloy steel workpiece treated in Example 1 also contained 6% to 8% residual austenite.

[0108] Comparative Example 1-1

[0109] The same high-strength low-alloy steel workpiece as in Example 1 was selected and subjected to the following treatment:

[0110] N1: Place the high-strength low-alloy steel workpiece into the first heating furnace that has been heated to 920°C (higher than the austenite end transformation temperature of the high-strength low-alloy steel workpiece in Example 1), heat and hold for 180 min;

[0111] N2-1: The high-strength low-alloy steel workpiece that has completed N1 treatment is quickly taken out from the first heating furnace and cooled to 310℃ (higher than the martensite transformation temperature and lower than the lamellar bainite transformation temperature) using industrial quenching oil. At this time, the cooling rate is about 18℃ / s.

[0112] N2-2: Quickly place the high-strength low-alloy steel workpiece that has completed N2-1 treatment into the second heating furnace that has been heated to 360°C, and heat and hold for 180 minutes;

[0113] N2-3: The high-strength low-alloy steel workpiece that has completed the N2-2 treatment is removed from the second heating furnace and cooled to room temperature using an industrial fan.

[0114] N3: Place the high-strength low-alloy steel workpiece that has undergone N2-3 treatment into a third heating furnace that has been heated to 280°C, heat and hold for 240 minutes, then remove it and use an industrial fan to cool the high-strength low-alloy steel workpiece to room temperature.

[0115] It should be noted that, compared with Example 1, the difference between the two is that: in Example 1, S2-1 uses an industrial fan to spray air to cool the workpiece, with a cooling rate of about 2°C / s; while in Comparative Example 1-1, N2-1 uses industrial quenching oil to cool the workpiece, with a cooling rate of about 18°C / s; the other processes are the same.

[0116] The conventional mechanical properties of the high-strength low-alloy steel workpiece of Comparative Example 1-1 were tested according to the national standard GB / T 228-2002, and the impact toughness of the high-strength low-alloy steel workpiece of Comparative Example 1-1 was tested according to the national standard GB / T229-1994, as shown in Table 3.

[0117] Table 3. Conventional mechanical properties and impact toughness of high-strength low-alloy steel workpieces treated according to Comparative Example 1-1

[0118] The microstructure of the high-strength low-alloy steel workpiece treated in Comparative Example 1-1 is as follows: Figure 3 As shown, its final microstructure consists of lath bainite and lath martensite. Furthermore, XRD analysis revealed that the high-strength low-alloy steel workpiece treated in Comparative Example 1-1 also contained 6%–8% retained austenite.

[0119] Comparative Examples 1-2

[0120] The same high-strength low-alloy steel workpiece as in Example 1 was selected and subjected to conventional quenching and tempering treatments. The specific treatment steps are as follows:

[0121] O1: Place the high-strength low-alloy steel workpiece into the first heating furnace which has been heated to 920°C (higher than the austenite end transformation temperature of the high-strength low-alloy steel workpiece in Example 1), heat and hold for 180 minutes;

[0122] O2: The high-strength low-alloy steel workpiece that has completed the O1 treatment is quickly removed from the first heating furnace and cooled to room temperature using an industrial fan. At this time, the cooling rate is about 2℃ / s.

[0123] O3: Place the high-strength low-alloy steel workpiece that has undergone O2 treatment into a second heating furnace that has been heated to 280°C, heat and hold for 240 minutes, then remove it and use an industrial fan to cool the high-strength low-alloy steel workpiece to room temperature.

[0124] The conventional mechanical properties of the high-strength low-alloy steel workpieces in Comparative Examples 1-2 were tested according to the national standard GB / T 228-2002, and the impact toughness of the high-strength low-alloy steel workpieces in Comparative Examples 1-2 was tested according to the national standard GB / T229-1994, as shown in Table 4.

[0125] Table 4. Conventional mechanical properties and impact toughness of high-strength low-alloy steel workpieces treated according to Comparative Examples 1-2

[0126] The microstructure of the high-strength low-alloy steel workpiece treated using Comparative Examples 1-2 is as follows: Figure 4 As shown, its final microstructure consists of lamellar bainite and lath martensite. Furthermore, XRD analysis revealed that the high-strength low-alloy steel workpieces treated in Comparative Examples 1-2 also contained 2%–4% retained austenite.

[0127] Comparative Examples 1-3

[0128] The same high-strength low-alloy steel workpiece as in Example 1 was selected and subjected to conventional isothermal quenching and tempering treatment. The specific treatment steps are as follows:

[0129] P1: Place the high-strength low-alloy steel workpiece into the first heating furnace which has been heated to 920°C (higher than the austenite end transformation temperature of the high-strength low-alloy steel workpiece in Example 1), heat and hold for 180 min;

[0130] P2-1: The high-strength low-alloy steel workpiece that has completed P1 treatment is quickly removed from the first heating furnace and cooled to 360°C (above the temperature at which lamellar bainite begins to transform) using an industrial fan. At this time, the cooling rate is about 2°C / s.

[0131] P2-2: Place the high-strength low-alloy steel workpiece that has completed the P2-1 treatment into the second heating furnace that has been heated to 360°C and hold it at that temperature for 180 minutes.

[0132] P2-3: The high-strength low-alloy steel workpiece that has completed the P2-2 treatment is removed from the second heating furnace and cooled to room temperature using the industrial fan from P2-1.

[0133] P3: Place the high-strength low-alloy steel workpiece that has completed P2 treatment into the third heating furnace that has been heated to 280°C, heat and hold for 240 minutes, then take it out and use an industrial fan to cool the high-strength low-alloy steel workpiece to room temperature.

[0134] The conventional mechanical properties of the high-strength low-alloy steel workpieces of Comparative Examples 1-3 were tested according to the national standard GB / T 228-2002, and the impact toughness of the high-strength low-alloy steel workpieces of Comparative Examples 1-3 was tested according to the national standard GB / T229-1994, as shown in Table 5.

[0135] It should be noted that, compared with Example 1, the difference between the two is that: in Example S2-1, an industrial fan is used to spray air to cool the workpiece to 310°C; while in Comparative Example 1-3, an industrial fan is used to cool the workpiece to 360°C in P2-1, which is the same as the heat preservation temperature in Comparative Example 1-3 and Example 1 in S2-2; the other processes are the same.

[0136] Table 5. Conventional mechanical properties and impact toughness of high-strength low-alloy steel workpieces treated with Comparative Examples 1-3.

[0137] The microstructure of high-strength low-alloy steel workpieces treated using Comparative Examples 1-3 is as follows: Figure 5 As shown, its final microstructure consists of lath bainite and lath martensite. Furthermore, XRD analysis revealed that the high-strength low-alloy steel workpieces treated in Comparative Examples 1-3 also contained 8%–10% retained austenite.

[0138] Example 2

[0139] The composition and content of the high-strength low-alloy steel workpiece in Example 2 are shown in Table 6 below:

[0140] Table 6. Chemical composition and content (mass percentage) of high-strength low-alloy steel workpieces in Example 2. content 0.25 1.8 1.68 1.28 0.41 0.26

[0141] According to the chemical composition and content in Table 6, several high-strength low-alloy steel workpieces were prepared using conventional steelmaking, casting, annealing and cutting methods. The dimensions of the high-strength low-alloy steel workpieces were 500mm*300mm*20mm (length*width*thickness).

[0142] It should be noted that the initial state of the high-strength low-alloy steel workpieces in Examples 2, 2-1, 2-2, and 2-3 was the cast + annealed state, and their initial microstructure was ferrite and pearlite; which is different from Examples 1, 1-1, 1-2, and 1-3.

[0143] A cylinder with a diameter of 4 mm and a length of 10 mm was randomly selected from a high-strength low-alloy steel workpiece. The continuous cooling transformation curve of Example 2 was obtained according to the national standard YB / T 5128-2018 "Determination of Continuous Cooling Transformation Curve of Steel - Expansion Method". Figure 6 As shown, the cooling rate range for obtaining lamellar bainite is 1℃ / s to 15℃ / s. During the heating process, the austenite initiation transformation temperature is 760℃, the austenite termination transformation temperature is 842℃, the bainite initiation transformation temperature is 400℃, the lamellar bainite initiation transformation temperature is 350℃, the martensite initiation transformation temperature is 327℃, the martensite termination transformation temperature is 192℃, and the lath bainite initiation transformation temperature is 350℃.

[0144] The workpiece is processed using the multi-stage multiphase heat treatment method of the present invention, and the specific steps are as follows:

[0145] S1: Place the high-strength low-alloy steel workpiece into the first heating furnace that has been heated to 920°C (higher than the austenite end transformation temperature of the high-strength low-alloy steel workpiece in Example 1), heat and hold for 180 minutes.

[0146] S2-1: The high-strength low-alloy steel workpiece that has completed the S1 treatment is quickly taken out of the first heating furnace and cooled to 335℃ (higher than the martensite transformation temperature and lower than the lamellar bainite transformation temperature) using an industrial fan. At this time, the cooling rate is about 2℃ / s.

[0147] S2-2: Quickly place the high-strength low-alloy steel workpiece that has been treated in S2-1 into the second heating furnace that has been heated to 360°C, and heat and hold it for 180 minutes.

[0148] S2-3: The high-strength low-alloy steel workpiece that has completed the S2-2 process is removed from the second heating furnace and cooled to room temperature using the industrial fan from S2-1.

[0149] S3: Place the high-strength low-alloy steel workpiece that has completed the S2-3 treatment into the third heating furnace that has been heated to 280°C, heat and hold for 240 minutes, and then take it out and use the industrial fan of S2-1 to cool the high-strength low-alloy steel workpiece to room temperature.

[0150] It should be noted that the heat treatment process in Example 2 is the same as that in Example 1, but the initial states in Example 1 and Example 2 are different.

[0151] The conventional mechanical properties of the high-strength low-alloy steel workpiece of Example 2 were tested according to the national standard GB / T 228-2002, and the impact toughness of the high-strength low-alloy steel workpiece of Example 2 was tested according to the national standard GB / T229-1994, as shown in Table 7.

[0152] Table 7. Conventional mechanical properties and impact toughness of high-strength low-alloy steel workpieces treated in Example 1.

[0153] Compared with Comparative Examples 2-1, 2-2, and 2-3 below, the impact toughness of the high-strength low-alloy steel workpiece treated with Example 2 is improved while the strength remains almost unchanged.

[0154] The microstructure of the high-strength low-alloy steel workpiece treated in Example 2 is as follows: Figure 7 As shown, its final microstructure consists of lath bainite, slab bainite, slab martensite, and austenite; and XRD analysis revealed that the high-strength low-alloy steel workpiece treated in Example 2 also contained 4% to 6% residual austenite.

[0155] Comparative Example 2-1

[0156] The same high-strength low-alloy steel workpiece as in Example 2 was selected and subjected to the following treatment:

[0157] X1: Place the high-strength low-alloy steel workpiece into the first heating furnace that has been heated to 920°C (higher than the austenite end transformation temperature of the high-strength low-alloy steel workpiece in Example 1), heat and hold for 180 min;

[0158] X2-1: The high-strength low-alloy steel workpiece that has completed X1 treatment is quickly taken out from the first heating furnace and cooled to 335℃ (higher than the martensite transformation temperature and lower than the lamellar bainite transformation temperature) using industrial quenching oil. At this time, the cooling rate is about 18℃ / s.

[0159] X2-2: Quickly place the high-strength low-alloy steel workpiece that has been treated in X2-1 into the second heating furnace that has been heated to 360°C, and heat and hold it for 180 minutes.

[0160] X2-3: The high-strength low-alloy steel workpiece that has completed X2-2 treatment is removed from the second heating furnace and cooled to room temperature using an industrial fan.

[0161] X3: Place the high-strength low-alloy steel workpiece that has completed X2-3 treatment into the third heating furnace that has been heated to 280°C, heat and hold for 240 minutes, then take it out and use an industrial fan to cool the high-strength low-alloy steel workpiece to room temperature.

[0162] It should be noted that the difference between Comparative Example 2 and Example 2 is as follows: In Example 2, S2-1 uses an industrial fan to cool the workpiece with airflow at a cooling rate of approximately 2°C / s; while in Comparative Example 2-1, X2-1 uses industrial quenching oil to cool the workpiece at a cooling rate of approximately 18°C / s. Other processes are the same. Furthermore, compared to Comparative Example 1-1, Comparative Example 2-1 uses a different initial state of high-strength low-alloy steel workpiece, but undergoes the same heat treatment.

[0163] The conventional mechanical properties of the high-strength low-alloy steel workpiece of Comparative Example 2-1 were tested according to the national standard GB / T 228-2002, and the impact toughness of the high-strength low-alloy steel workpiece of Comparative Example 2-1 was tested according to the national standard GB / T229-1994, as shown in Table 8.

[0164] Table 8. Conventional mechanical properties and impact toughness of high-strength low-alloy steel workpieces treated according to Comparative Example 2-1.

[0165] The microstructure of the high-strength low-alloy steel workpiece treated in Comparative Example 2-1 is as follows: Figure 8 As shown, its final microstructure consists of lath bainite and lath martensite. Furthermore, XRD analysis revealed that the high-strength low-alloy steel workpiece treated in Comparative Example 2-1 also contained 6%–8% retained austenite.

[0166] Comparative Example 2-2

[0167] The same high-strength low-alloy steel workpiece as in Example 1 was selected and subjected to conventional quenching and tempering treatments. The specific treatment steps are as follows:

[0168] Y1: Place the high-strength low-alloy steel workpiece into the first heating furnace which has been heated to 920°C (higher than the austenite end transformation temperature of the high-strength low-alloy steel workpiece in Example 1), heat and hold for 180 min;

[0169] Y2: The high-strength low-alloy steel workpiece that has completed the Y1 treatment is quickly removed from the first heating furnace and cooled to room temperature using an industrial fan. At this time, the cooling rate is about 2℃ / s.

[0170] Y3: Place the high-strength low-alloy steel workpiece that has completed Y2 treatment into a second heating furnace that has been heated to 280°C, heat and hold for 240 minutes, then remove it and use an industrial fan to cool the high-strength low-alloy steel workpiece to room temperature.

[0171] The conventional mechanical properties of the high-strength low-alloy steel workpiece of Comparative Example 2-2 were tested according to the national standard GB / T 228-2002, and the impact toughness of the high-strength low-alloy steel workpiece of Comparative Example 2-2 was tested according to the national standard GB / T229-1994, as shown in Table 9.

[0172] Table 9. Conventional mechanical properties and impact toughness of high-strength low-alloy steel workpieces treated according to Comparative Example 2-2

[0173] The microstructure of the high-strength low-alloy steel workpiece treated in Comparative Example 2-2 is as follows: Figure 9 As shown, its final microstructure consists of lamellar bainite and lath martensite. Furthermore, XRD analysis revealed that the high-strength low-alloy steel workpiece treated in Comparative Example 2-2 also contained 2%–4% retained austenite.

[0174] Comparative Examples 2-3

[0175] The same high-strength low-alloy steel workpiece as in Example 1 was selected and subjected to conventional isothermal quenching and tempering treatment. The specific treatment steps are as follows:

[0176] Z1: Place the high-strength low-alloy steel workpiece into the first heating furnace that has been heated to 920°C (higher than the austenite end transformation temperature of the high-strength low-alloy steel workpiece in Example 1), heat and hold for 180 min;

[0177] Z2-1: The high-strength low-alloy steel workpiece that has completed Z1 treatment is quickly removed from the first heating furnace and cooled to 360°C (above the temperature at which lamellar bainite begins to transform) using an industrial fan. At this time, the cooling rate is about 2°C / s.

[0178] Z2-2: Place the high-strength low-alloy steel workpiece that has been treated in Z2-1 into a second heating furnace that has been heated to 360°C and hold it at that temperature for 180 minutes.

[0179] Z2-3: The high-strength low-alloy steel workpiece that has completed the Z2-2 treatment is removed from the second heating furnace and cooled to room temperature using the industrial fan from Z2-1.

[0180] Z3: Place the high-strength low-alloy steel workpiece that has completed Z2 treatment into the third heating furnace that has been heated to 280°C, heat and hold for 240 minutes, then take it out and use an industrial fan to cool the high-strength low-alloy steel workpiece to room temperature.

[0181] The conventional mechanical properties of the high-strength low-alloy steel workpieces of Comparative Example 2-3 were tested according to the national standard GB / T 228-2002, and the impact toughness of the high-strength low-alloy steel workpieces of Comparative Example 2-3 was tested according to the national standard GB / T229-1994, as shown in Table 10.

[0182] It should be noted that, compared with Example 2, the difference between the two is that: in Example 2, the industrial fan used in S2-1 is used to spray air to cool the workpiece to 335°C; while in Comparative Example 2-3, the industrial fan used in P2-1 is used to cool the workpiece to 360°C, which is the same as the heat preservation temperature in Comparative Example 1-3 and Example 1; the other processes are the same.

[0183] Table 10 shows the conventional mechanical properties and impact toughness of high-strength low-alloy steel workpieces treated according to Comparative Examples 2-3.

[0184] The microstructure of the high-strength low-alloy steel workpiece treated using Comparative Examples 2-3 is as follows: Figure 10 As shown, its final microstructure consists of lath bainite and lath martensite. Furthermore, XRD analysis revealed that the high-strength low-alloy steel workpieces treated in Comparative Examples 2-3 also contained 4%–6% retained austenite.

[0185] It is understood that those skilled in the art can combine the features mentioned in one or more embodiments throughout this application with features from other embodiments in any appropriate manner to implement this application.

[0186] Note that the above are merely preferred embodiments and the technical principles employed in this application. Those skilled in the art will understand that this application is not limited to the specific embodiments described herein, and various obvious changes, readjustments, and substitutions can be made without departing from the scope of protection of this application. Therefore, although this application has been described in detail through the above embodiments, this application is not limited to the above embodiments. Many other equivalent embodiments may be included without departing from the technical concept of this application, all of which fall within the scope of protection of this application.

Claims

1. A multi-stage, multiphase heat treatment method for improving the toughness of high-strength low-alloy steel workpieces, characterized in that, The processing method includes the following steps: S1: Heating and heat preservation treatment The high-strength low-alloy steel workpiece is heated to the first temperature T1 and held at that temperature for a period of time t1 to complete the austenitization and composition homogenization of the high-strength low-alloy steel workpiece. The first heating temperature T1 of the high-strength low-alloy steel workpiece is 30℃~100℃ higher than the austenite final transformation temperature of the high-strength low-alloy steel workpiece, and the holding time t1 is 120min~240min. S2: Multi-stage multiphase controlled quenching Process S2-1: Speed ​​and temperature controlled cooling The high-strength low-alloy steel workpiece that has completed S1 is cooled to the second temperature T2, so that the high-strength low-alloy steel workpiece forms partial lamellar bainite; The second temperature T2 is higher than the martensite initiation temperature of the high-strength low-alloy steel workpiece but lower than the lamellar bainite initiation temperature; the cooling rate of the high-strength low-alloy steel workpiece is the cooling rate at which lamellar bainite is obtained. Process S2-2: Temperature and time control for heat preservation The high-strength low-alloy steel workpiece that has completed S2-1 is heated to the third temperature T3 and held at that temperature for a period of time t2, so that the high-strength low-alloy steel workpiece forms part of lath bainite. The third temperature T3 is higher than the lath bainite formation temperature of the high-strength low-alloy steel workpiece but lower than the bainite initiation transformation temperature, and the holding time t2 is 120 min ~ 360 min. Process S2-3: Rapid cooling treatment The high-strength low-alloy steel workpiece that has completed S2-2 is cooled to room temperature, so that the high-strength low-alloy steel workpiece forms a partial lath martensite structure. S3: Medium-low temperature tempering treatment The high-strength low-alloy steel workpiece with S2-3 is heated to the fourth temperature T4 and held at that temperature for a period of time t3 for tempering treatment to obtain a multiphase microstructure of lath bainite, slab bainite, slab martensite and retained austenite. The fourth temperature T4 is higher than the martensite completion transformation temperature but lower than the lamellar bainite initiation transformation temperature, and the holding time t3 is 240 min ~ 480 min.

2. The heat treatment method according to claim 1, characterized by, The high-strength low-alloy steel workpiece, by mass percentage, comprises: 0.15%~0.32% C, 1.2%~3.0% Mn, 0.6%~1.8% Si, 0.4%~1.5% Cr, 0.2%~0.8% Mo, and 0.2%~1.2% Ni; P≤0.0015%, S≤0.005%, with the remainder being Fe and unavoidable impurity elements. Furthermore, the sum of the elemental contents of Mn, Cr, and Mo is not less than 2.6% and not more than 4.0%.

3. The heat treatment method according to claim 1, characterized by, In the process S2-1, the cooling method is air jet cooling and / or spray cooling.

4. The heat treatment method according to claim 1, characterized by, In the process S2-1, the second temperature T2 is 5°C to 30°C higher than the martensitic transformation temperature of the high-strength low-alloy steel workpiece and lower than the lamellar bainitic transformation temperature.

5. The heat treatment method according to claim 1, characterized in that, In the process S2-3, the cooling method is air jet cooling and / or spray cooling.

6. The heat treatment method according to claim 1, characterized by, In S3, the fourth temperature T4 is greater than 100°C and less than 360°C.

7. The heat treatment method according to any one of claims 1 to 6, characterized in that, Before S1, the high-strength low-alloy steel workpiece is in a forged state or a cast + annealed state, and the microstructure of the high-strength low-alloy steel workpiece is a bainite + martensite dual phase structure or a ferrite + pearlite structure.

8. The heat treatment method according to claim 7, characterized in that, The maximum wall thickness of the high-strength low-alloy steel workpiece is 60 mm, the minimum wall thickness is 10 mm, and the maximum difference between the maximum and minimum wall thickness is 20 mm.

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