Multistage, multiphase heat treatment method for increasing toughness of high-strength low-alloy steel workpiece

Abstract

A multistage, multiphase heat treatment method for increasing the toughness of a high-strength low-alloy steel workpiece. Using a multistage, multiphase quenching process causes the steel workpiece to ultimately obtain a multiphase structure of strip bainite, lath bainite, lath martensite, and residual austenite. The forming sequence of the phases in the multistage, multiphase quenching process is sequentially strip bainite, lath bainite, and lath martensite. Forming a portion of the strip bainite in advance promotes nucleation and growth of the lath bainite, shortens a process time, increases production efficiency, and increases the toughness of the high-strength low-alloy steel workpiece.

Description

Multi-stage multiphase heat treatment method to improve the toughness of high-strength low-alloy steel workpieces Technical Field

[0001] The present application relates to the technical field of heat treatment of high-strength low-alloy steel workpieces, and specifically provides a multi-stage multi-phase heat treatment method for improving the toughness of high-strength low-alloy steel workpieces. Background Art

[0002] Steel remains one of the most widely used materials. High-strength low-alloy steel is widely used in transportation, construction, bridges, ships, offshore platforms, and engineering machinery, primarily due to its low cost and excellent overall performance. As these areas continue to expand into more extreme natural environments, such as faster traffic, taller buildings, and deeper oceans, increasingly stringent requirements are being placed on their service performance. Furthermore, under the dual pressures of "dual carbon" environmental protection 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 improved performance and reduced usage can reduce pressure on resources, energy, and the environment.

[0003] Among the many performance requirements of high-strength low-alloy steel, strength and toughness are two of the most critical performance indicators. The performance of steel is closely related to its structure. Through the 973 project "Basic Research on Theory and Technology of Microstructure Control of High-Performance Steel", we realized that the use of "multiphase, metastable, multiscale" M 3 The concept of microstructure control, which expands the matrix structure from a single phase (such as ferrite, bainite, and martensite) to a multiphase (bainite + martensite dual phase, ferrite + bainite dual phase, bainite + martensite dual phase, bainite + martensite + austenite dual phase), can effectively prevent crack nucleation and propagation, and improve the strength and toughness of steel. The microstructure and performance of high-strength low-alloy steel mainly depend on heat treatment. Therefore, developing new heat treatment processes to obtain a multiphase structure to improve the strength and toughness of steel materials is one of the important development directions of the steel industry.

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

[0005] Summary of the Invention

[0006] The purpose of the present application is to provide a multi-stage multi-phase heat treatment method for improving the toughness of a high-strength low-alloy steel workpiece, so as to improve the toughness of the high-strength low-alloy steel workpiece by improving the treatment process.

[0007] In order to achieve the above objectives, this application adopts the following technical solutions.

[0008] A multi-stage multi-phase heat treatment method for improving the toughness of a high-strength low-alloy steel workpiece, the treatment method comprising the following steps:

[0009] S1: Heating and insulation treatment

[0010] Heating the high-strength low-alloy steel workpiece to a first temperature T1 and maintaining the temperature for a period of time t1 to complete austenitization and composition homogenization of the high-strength low-alloy steel workpiece;

[0011] S2: Multi-stage multi-phase controlled quenching

[0012] Process S2-1: Speed ​​and temperature controlled cooling

[0013] Cooling the high-strength low-alloy steel workpiece after S1 to a second temperature T2, so that the high-strength low-alloy steel workpiece forms partial lamellar bainite;

[0014] Process S2-2: Temperature and time control and heat preservation

[0015] heating the high-strength low-alloy steel workpiece after S2-1 to a third temperature T3 and maintaining the temperature for a period of time t2, so that the high-strength low-alloy steel workpiece forms partial lath bainite;

[0016] Step S2-3: Rapid cooling treatment

[0017] Cooling the completed S2-2 high-strength low-alloy steel workpiece to room temperature so that the high-strength low-alloy steel workpiece forms a partial lath martensite structure;

[0018] S3: Medium and low temperature tempering treatment

[0019] The high-strength low-alloy steel workpiece completed in S2-3 is heated to a fourth temperature T4 and kept at this temperature for a period of time t3 for tempering treatment to obtain a multiphase structure of lamellar bainite, lath bainite, lath martensite and retained austenite.

[0020] Furthermore, the high-strength low-alloy steel workpiece includes, by mass percentage, 0.15% to 0.32% C, 1.2% to 3.0% Mn, 0.6% to 1.8% Si, 0.4% to 1.5% Cr, 0.2% to 0.8% Mo, and 0.2% to 1.2% Ni; P≤0.0015%, S≤0.005%, and the rest are Fe and unavoidable impurity elements. At the same time, the sum of the element contents of Mn, Cr, and Mo is not less than 2.6% and not higher than 4.0%.

[0021] Furthermore, in the above S1, the first heating temperature T1 of the high-strength low-alloy steel workpiece is 30°C to 100°C higher than the austenite finish transformation temperature of the high-strength low-alloy steel workpiece, and the holding time t1 is 120min to 240min.

[0022] Furthermore, in the step S2-1, the cooling rate of the high-strength low-alloy steel workpiece is a cooling rate for obtaining lamellar bainite;

[0023] Preferably, the cooling method is air jet cooling and / or spray cooling.

[0024] Furthermore, in the step S2-1, the second temperature T2 is higher than the martensite start transformation temperature of the high-strength low-alloy steel workpiece and lower than the lamellar bainite start transformation temperature, preferably, 5°C to 30°C higher than the martensite start transformation temperature.

[0025] Furthermore, in the step S2-2, the third temperature T3 is higher than the lath bainite formation temperature of the high-strength low-alloy steel workpiece and lower than the bainite start transformation temperature, and the holding time t2 is 120 min to 360 min.

[0026] Furthermore, in the step S2-3, the cooling method is air jet cooling and / or spray cooling.

[0027] Furthermore, in said S3, the fourth temperature T4 is higher than the martensite transformation end temperature and lower than the lamellar bainite transformation start temperature, and the holding time t3 is 240 min to 480 min;

[0028] Preferably, the fourth temperature T4 is greater than 100°C and less than 360°C.

[0029] Furthermore, before S1, the high-strength low-alloy steel workpiece is in a forged state or a cast+annealed state, and the structure of the high-strength low-alloy steel workpiece is a bainite+martensite duplex structure or a ferrite+pearlite structure.

[0030] Furthermore, 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 wall thickness and the minimum wall thickness is 20 mm.

[0031] Compared with the prior art, the technical effects of this application are:

[0032] The multi-stage, multiphase heat treatment method of this application utilizes a multi-stage quenching process to preform a portion of lamellar bainite, promoting the nucleation and growth of lamellar bainite, shortening process time and improving production efficiency. By controlling the cooling rate and temperature during the multi-stage, multi-step quenching process, the microstructure of the high-strength low-alloy steel workpiece is composed of a multiphase structure of lamellar bainite, lamellar bainite, lamellar martensite, and retained austenite. This technique improves the toughness of the high-strength low-alloy steel workpiece. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The following further illustrates the various technical features of the present application and the relationships between them with reference to the accompanying drawings. The accompanying drawings are exemplary, and some technical features are not shown in actual proportion. In addition, some drawings may omit technical features that are commonly used in the technical field to which the present application belongs and are not essential for understanding and implementing the present application, or additional technical features that are not essential for understanding and implementing the present application may be shown. In other words, the combination of the various technical features shown in the accompanying drawings is not intended to limit the present application. In addition, throughout the present application, the same figure numbers refer to the same content. The specific description of the drawings is as follows:

[0034] FIG1 is a continuous cooling transformation curve of a high-strength low-alloy steel workpiece of Example 1; in the figure, Ac1 is the austenite austenite transformation temperature, Ac3 is the austenite end transformation temperature, Ms is the martensite start transformation temperature, Mf is the martensite end transformation temperature, A is austenite, UB is lath bainite, LB is lath bainite, and M is martensite;

[0035] FIG2 is a SEM micrograph of a high-strength low-alloy steel workpiece processed in Example 1;

[0036] FIG3 is a SEM micrograph of a high-strength low-alloy steel workpiece processed in Comparative Example 1-1;

[0037] FIG4 is a SEM micrograph of a high-strength low-alloy steel workpiece processed using Comparative Example 1-2;

[0038] FIG5 is a SEM micrograph of a high-strength low-alloy steel workpiece processed using Comparative Examples 1-3;

[0039] FIG6 is a continuous cooling transformation curve of a high-strength low-alloy steel workpiece of Example 2; in the figure, Ac1 is the austenite austenite transformation temperature, Ac3 is the austenite end transformation temperature, Ms is the martensite start transformation temperature, Mf is the martensite end transformation temperature, A is austenite, UB is lath bainite, LB is lath bainite, M is martensite, and F is ferrite;

[0040] FIG7 is a SEM micrograph of a high-strength low-alloy steel workpiece processed in Example 2;

[0041] FIG8 is a SEM micrograph of a high-strength low-alloy steel workpiece processed in Comparative Example 2-1;

[0042] FIG9 is a SEM micrograph of a high-strength low-alloy steel workpiece processed in Comparative Example 2-2;

[0043] FIG10 is a SEM micrograph of the high-strength low-alloy steel workpiece processed by Comparative Example 2-3. DETAILED DESCRIPTION

[0044] Glossary

[0045] High-strength low-alloy steel: refers to a type of steel material with a yield strength of 500-800 MPa and a total alloy element content of less than 5%.

[0046] Hardenability: refers to the material characteristics characterized by the depth of the hardened layer and the hardness distribution of the specimen under specified conditions, which mainly depends on the critical quenching cooling rate of the material; under specified conditions, it determines the characteristics of the steel's hardening depth and hardness distribution; it characterizes the ability of steel to obtain a hardened layer depth when quenched, indicates the steel's ability to accept quenching, and is related to the chemical composition / components of the steel.

[0047] Martensitic hardenability: The ability of a specimen to obtain martensitic structure under specified conditions is used to characterize the material characteristics.

[0048] Bainite hardenability: The ability of a specimen to obtain bainite structure under specified conditions is used to characterize the material characteristics.

[0049] Hardenability: refers to the ability of steel to harden during quenching, expressed as the maximum hardness that can be obtained by quenching into martensite. It mainly depends on the carbon content in martensite. The higher the carbon content, the higher the hardenability of the steel.

[0050] Austenite transformation temperature: The temperature at which the ferrite / pearlite / bainite / martensite structure begins to transform into austenite structure during the slow heating process of steel materials.

[0051] Austenite end transformation temperature: The temperature at which the ferrite / pearlite / bainite / martensite structures of steel materials are all transformed into austenite structures during the slow heating process.

[0052] Bainite transformation start temperature: The temperature at which the austenite structure begins to transform into bainite structure during the cooling process of steel materials, including granular bainite, lamellar bainite and lamellar bainite structures. Generally, the temperature for the formation of granular bainite structure is the highest, followed by lamellar bainite, and then lamellar bainite structure.

[0053] Lath bainite transformation start temperature: The temperature at which the austenite structure begins to transform into lath bainite structure during the cooling process of steel materials; this temperature is lower than the transformation start temperature of granular bainite and lath bainite.

[0054] Martensite transformation start temperature: The temperature at which the austenite structure of steel material completely transforms into martensite during the cooling process.

[0055] Austenite: In terms of steel materials, it refers to a solid solution of carbon atoms in γ-Fe. It has a high ability to dissolve carbon atoms and is generally represented by the symbol "A" or "γ".

[0056] Ferrite: In terms of steel materials, it refers to a solid solution of carbon atoms in α-Fe. Its ability to dissolve carbon atoms is very low and is generally represented by the symbol "F" or "α".

[0057] Pearlite: In terms of steel materials, it is an ordered mixture of ferrite and cementite, which is the transformation product of austenite at high temperature and is generally represented by the symbol "P".

[0058] Bainite: In terms of steel materials, it is the austenite transformation product when the austenite is supercooled to a temperature range between below the pearlite transformation temperature and above the martensite transformation temperature, generally represented by the symbol "B".

[0059] Martensite: In terms of steel materials, it is a supersaturated solid solution of carbon atoms in α-Fe, which is the low-temperature transformation product of austenite and is generally represented by the symbol "M".

[0060] Retained austenite: In the present invention, it refers to the fact that due to the alloying regulation and process control of the steel material, a small amount of austenite in the high-strength alloy steel remains untransformed even after cooling to room temperature. This part of the untransformed austenite is called "retained austenite" and is generally represented by the symbol "RA".

[0061] The multi-stage multi-phase heat treatment method for improving the toughness of high-strength low-alloy steel workpieces provided by the present invention mainly includes three steps: S1 heating and heat preservation treatment, S2 multi-stage multi-phase controlled quenching treatment, and S3 medium-low temperature tempering treatment.

[0062] The high-strength low-alloy steel workpiece in the present invention refers to a steel workpiece having a C content of between 0.15% and 0.32% by mass, with Mn, Si, Cr, Mo, and Ni as alloying elements, the sum of which is no less than 2.6% and no 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% to 0.32% C, 1.2% to 3.0% Mn, 0.6% to 1.8% Si, 0.4% to 1.5% Cr, 0.2% to 0.8% Mo, and 0.2% to 1.2% Ni; P ≤ 0.0015%, S ≤ 0.005%, with the remainder being Fe and unavoidable impurity elements; and the sum of which is no less than 2.6% and no more than 4.0% for Mn, Cr, and Mo.

[0063] In the high-strength low-alloy steel workpiece of the present invention, adding 0.15% to 0.32% C is beneficial to improving the hardenability of the high-strength low-alloy steel workpiece, obtaining higher-strength bainite and martensite structures, and improving the stability of the retained austenite. However, too high a C element is not conducive to the formation of lath martensite and the toughness of the workpiece; by controlling the Si content, the formation of carbides is avoided during the S2 multi-stage controlled quenching treatment, which deteriorates the toughness of the high-strength low-alloy steel workpiece; by controlling the contents of Mn, Cr and Mo elements, the bainite hardenability of the high-strength low-alloy steel workpiece can be improved, so that it can avoid the formation of ferrite and pearlite under air jet or spray cooling conditions, which is beneficial to the control of the temperature of the high-strength low-alloy steel workpiece during the speed and temperature controlled cooling process of step S2-1 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 workpiece, but the price of Ni is relatively high and needs to be strictly controlled.

[0064] 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 treatment of S2, 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 wall thickness and the minimum wall thickness is 20 mm.

[0065] S1 heating and insulation treatment

[0066] In one embodiment, the heating and holding treatment includes: placing the high-strength low-alloy steel workpiece into a heating furnace that has been heated to a first temperature T1 and heating and holding the workpiece for a first time t1 to complete austenitization and composition homogenization of the high-strength low-alloy steel workpiece.

[0067] In some embodiments, the high strength low alloy steel workpiece to be subjected to S1 treatment is in a finished forging state or a finished casting + annealing state.

[0068] In some embodiments, to achieve the above-mentioned phase transformation purpose, the first temperature T1 is higher than Ac3 (austenite finish temperature) of the high-strength low-alloy steel workpiece material, preferably 30°C to 100°C higher than the austenite finish temperature of the high-strength low-alloy steel workpiece, and can be, but is 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.

[0069] In a preferred embodiment, before implementing the heating and holding treatment of S1, a continuous cooling transformation curve of the high-strength low-alloy steel workpiece material should be obtained to determine the key cooling rates and key temperatures such as the cooling rate range for obtaining lamellar bainite, the end transformation temperature of austenite, the start transformation temperature of bainite, the start transformation temperature of lamellar bainite and the start transformation temperature of martensite.

[0070] In a preferred embodiment, the specific process of the heating and insulation treatment is: placing the high-strength low-alloy steel workpiece that has completed forging or casting + annealing into a first heating furnace that has been heated to a first time T1 (for example, 880°C to 1000°C) and heating and insulation for a first time t1 (for example, 120min to 240min).

[0071] In a preferred embodiment, during the heating and holding process of S1, the structure of the high-strength low-alloy steel workpiece is transformed from the forged or cast + annealed bainite / martensite or ferrite + pearlite structure to a single-phase austenite structure, and the composition is homogenized.

[0072] The first temperature T1 may be, but is not limited to, 880°C, 900°C, 920°C, 940°C, 960°C, 980°C or 1000°C; the first time t1 may be, but is not limited to, 120 min, 130 min, 160 min, 200 min or 240 min.

[0073] Multi-stage multi-phase controlled quenching treatment of S2

[0074] Quenching is a heat treatment method that includes continuous cooling quenching and austempering. Continuous cooling quenching of steel is a heat treatment method in which the steel is heated to a temperature above the critical temperature Ac3 (hypoeutectoid steel) or Ac1 (hypereutectoid steel) (i.e., S1 of the present invention), kept at this temperature for a period of time to fully or partially austenitize, and then rapidly cooled at a cooling rate greater than the critical cooling rate to below Ms (or isothermally cooled near Ms) to undergo martensite (or bainite) transformation. Isothermal quenching of steel is a heat treatment method in which the steel is heated to a temperature above the critical temperature Ac3 (hypoeutectoid steel) or Ac1 (hypereutectoid steel), kept at this temperature for a period of time to fully or partially austenitize, and then rapidly cooled at a cooling rate greater than the critical cooling rate to between the bainite start transformation temperature and the bainite end transformation temperature, and isothermally cooled for a period of time to undergo bainite transformation.

[0075] In the present invention, the multi-stage multi-phase controlled quenching treatment adopts a process of controlled speed and temperature cooling, controlled temperature and time holding, and rapid cooling. During the multi-stage multi-phase controlled quenching treatment, the high-strength low-alloy steel workpiece begins with controlled speed and temperature cooling, where the high-strength low-alloy steel workpiece is cooled at a certain cooling rate to a temperature between the start of bainite transformation and the start of martensite transformation, thereby forming a certain amount of lamellar bainite structure. This is followed by a controlled temperature and time holding treatment, where the high-strength low-alloy steel workpiece is heated to a temperature between the start of bainite transformation and the start of lath bainite transformation, and held at this temperature for a long time, thereby forming a certain amount of lath bainite. This is followed by a rapid cooling treatment, where the high-strength low-alloy steel workpiece is rapidly cooled to room temperature, thereby forming a certain amount of lath martensite structure, with a small amount of residual austenite structure.

[0076] In order to realize the above-mentioned tissue phase change process, S2 specifically includes the following steps:

[0077] Step S2-1: Controlled speed and temperature cooling: After step S1, the high-strength low-alloy steel workpiece is cooled to a 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;

[0078] Step S2-2: Temperature and time control and heat preservation: After S2-1, the high-strength low-alloy steel workpiece is heated to a third temperature T3 and kept at this temperature for a period of time t3, so that a portion of lath bainite is formed in the high-strength low-alloy steel workpiece;

[0079] 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.

[0080] Step 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 lath martensite structure is formed in the high-strength low-alloy steel workpiece.

[0081] In a preferred embodiment, the specific method of S2 is preferably as follows:

[0082] Step S2-1: The high-strength low-alloy steel workpiece after step S1 is removed from the first heating furnace and cooled by air jet or spray at a certain cooling rate V to a second temperature T2 which is higher than the martensite start transformation temperature of the high-strength low-alloy steel workpiece and lower than the lamellar bainite start transformation temperature, wherein the high-strength low-alloy steel workpiece first forms lamellar bainite structure instead of granular bainite or lath bainite structure at the cooling rate V;

[0083] The cooling rate V may be, but is not limited to, 0.1°C / s, 0.25°C / s, 0.5°C / s, 0.75°C / s, 1°C / s, 2°C / s, 3°C / s, 4°C / s, 5°C / s, 6°C / s, 7°C / s, 8°C / s, 9°C / s, 10°C / s, 11°C / s, 12°C / s, 13°C / s, 14°C / s or 15°C / s; the second temperature T2 may be, but is not limited to, 300°C, 310°C, 320°C, 330°C, 340°C, 350°C, 360°C or 380°C. Preferably, T2 is 5°C to 30°C higher than the martensitic start transformation temperature of the high-strength low-alloy steel workpiece material, for example, it may be, but is not limited to, 5°C, 10°C, 15°C, 20°C, 25°C or 30°C.

[0084] Step S2-2: The high-strength low-alloy steel workpiece that has completed step S2-1 is quickly placed into a second heating furnace that has been heated to a third temperature T3, and heated and kept warm for a second time t2, wherein the third heating temperature T3 is lower than the bainite start transformation temperature and higher than the lath bainite start formation temperature.

[0085] The third temperature T3 may be, but is not limited to, 330°C, 340°C, 350°C, 360°C, 370°C, 380°C, 390°C or 400°C; the second time t2 may be, but is not limited to, 120min, 150min, 180min, 210min, 240min, 270min, 300min, 330min or 360min.

[0086] Step S2-3: The high-strength low-alloy steel workpiece having completed step S2-2 is taken out from the second heating furnace, and cooled to room temperature by air jet or spray cooling.

[0087] S3 medium and low temperature tempering treatment

[0088] Tempering is a heat treatment method in which high-strength low-alloy steel workpieces are heated to a temperature below Ac1 (the starting temperature for the transformation of pearlite to austenite during heating) after hardening, held at that temperature for a certain period of time, and then cooled to room temperature. Tempering is divided into low-temperature tempering (tempering of high-strength low-alloy steel workpieces at 150℃ to 250℃), medium-temperature tempering (tempering of high-strength low-alloy steel workpieces between 360℃ and 500℃), and high-temperature tempering (tempering of high-strength low-alloy steel workpieces above 500℃ to 650℃).

[0089] The low-temperature tempering treatment in step S3 includes the following steps: placing the high-strength low-alloy steel workpiece after step S2 into a heating furnace heated to a fourth temperature T4, holding the temperature for a third time t3 (240 to 480 minutes), then removing the workpiece from the furnace and cooling it to room temperature. The resulting high-strength low-alloy steel workpiece has a multiphase structure of lath bainite, lath bainite, lath martensite, and retained austenite.

[0090] In a preferred embodiment, the fourth temperature T4 is greater than 100° C. and less than 360° C., and the third time t3 is 240 min to 480 min;

[0091] T4 may be, but is not limited to, 100°C, 150°C, 200°C, 250°C, 300°C or 360°C; t3 may be, but is not limited to, 240 min, 270 min, 300 min, 330 min, 360 min, 390 min, 420 min, 450 min or 480 min.

[0092] In a preferred embodiment, the cooling method of S3 can be one of air jet and natural air cooling.

[0093] Below, the specific implementation methods of the present application are described in detail with reference to the accompanying drawings.

[0094] Example 1

[0095] The components and contents of the high-strength low-alloy steel workpiece of Example 1 are shown in Table 1 below:

[0096] Table 1 Chemical composition and content of high-strength low-alloy steel workpieces (mass percentage)

[0097] Several high-strength low-alloy steel workpieces were prepared using conventional steelmaking, forging, and cutting methods according to the chemical compositions and contents listed in Table 1. The initial microstructure of the workpieces was a bainite / martensite duplex structure. The dimensions of the high-strength low-alloy steel workpieces were 500 mm * 300 mm * 20 mm (length * width * thickness).

[0098] A cylinder with a diameter of 4 mm and a length of 10 mm was randomly taken from a high-strength low-alloy steel workpiece, and the continuous cooling transformation curve of Example 1 was obtained according to the national standard "YB / T 5128-2018 Steel Continuous Cooling Transformation Curve Measurement Dilatometer Method", as shown in Figure 1. It can be seen that the cooling rate range for obtaining lamellar bainite is 0.25°C / s to 8°C / s, and during the heating process, the austenite start transformation temperature is 760°C, the austenite end transformation temperature is 860°C, the bainite start transformation temperature is 420°C, the lamellar bainite start transformation temperature is 340°C, the martensite start transformation temperature is 295°C, the martensite end transformation temperature is 140°C, and the lath bainite start transformation temperature is 340°C.

[0099] The multi-stage multi-phase heat treatment method of the present invention is used to treat the workpiece, and the specific steps are as follows:

[0100] S1: placing the high-strength low-alloy steel workpiece into a first heating furnace heated to 920° C. (higher than the austenite end transformation temperature of the high-strength low-alloy steel workpiece in Example 1), heating and holding the workpiece for 180 minutes;

[0101] S2-1: The high-strength low-alloy steel workpiece that has completed the S1 treatment is quickly removed from the first heating furnace and cooled to 310°C (above the martensite start transformation temperature and below the lamellar bainite start transformation temperature) using an industrial fan. The cooling rate is approximately 2°C / s.

[0102] S2-2: The high-strength low-alloy steel workpiece that has completed the S2-1 treatment is quickly placed into the second heating furnace that has been heated to 360°C, and heated and kept warm for 180 minutes;

[0103] S2-3: The high-strength low-alloy steel workpiece that has completed the S2-2 treatment is taken out of the second heating furnace and the high-strength low-alloy steel workpiece is cooled to room temperature using the industrial fan of S2-1.

[0104] 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 keep warm for 240 minutes, then take out the high-strength low-alloy steel workpiece and use the industrial fan in S2-1 to cool it to room temperature.

[0105] 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 / T 229-1994, as shown in Table 2.

[0106] Table 2 Conventional mechanical properties and impact toughness of high-strength low-alloy steel workpieces

[0107] Compared with the following comparative examples 1-1, 1-2 and 1-3, the impact toughness of the high-strength low-alloy steel workpiece treated in Example 1 is improved while the strength remains almost unchanged.

[0108] The microstructure of the high-strength low-alloy steel workpiece treated in Example 1 is shown in Figure 2, and its final microstructure is lamellar bainite, lath bainite, lath martensite and austenite; and according to XRD detection, the high-strength low-alloy steel workpiece treated in Example 1 also contains 6% to 8% retained austenite.

[0109] Comparative Example 1-1

[0110] The same high-strength low-alloy steel workpiece as in Example 1 was selected and processed as follows:

[0111] N1: Place the high-strength low-alloy steel workpiece into a first heating furnace heated to 920° C. (higher than the austenite end transformation temperature of the high-strength low-alloy steel workpiece in Example 1) and heat and hold for 180 minutes;

[0112] N2-1: The high-strength low-alloy steel workpiece that has completed the N1 treatment is quickly removed from the first heating furnace and cooled to 310°C (above the martensite start transformation temperature and below the lamellar bainite start transformation temperature) using industrial quenching oil. The cooling rate at this time is about 18°C / s.

[0113] N2-2: The high-strength low-alloy steel workpiece that has completed the N2-1 treatment is quickly placed into the second heating furnace that has been heated to 360°C, and heated and kept warm for 180 minutes;

[0114] N2-3: The high-strength low-alloy steel workpiece that has completed the N2-2 treatment is taken out of the second heating furnace and cooled to room temperature using an industrial fan.

[0115] N3: Place the high-strength low-alloy steel workpiece that has completed the N2-3 treatment into the third heating furnace that has been heated to 280°C, heat and keep warm for 240 minutes, then take it out and use an industrial fan to cool the high-strength low-alloy steel workpiece to room temperature.

[0116] 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, and the cooling rate is about 2°C / s; while in Comparative Example 1-1, N2-1 uses industrial quenching oil to cool the workpiece, and the cooling rate is about 18°C / s; the other processes are the same.

[0117] 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 / T 229-1994, as shown in Table 3.

[0118] Table 3 Conventional mechanical properties and impact toughness of high-strength low-alloy steel workpieces treated with Comparative Example 1-1

[0119] The microstructure of the high-strength low-alloy steel workpiece treated with Comparative Example 1-1 is shown in Figure 3. Its final microstructure consists of lath bainite and lath martensite. XRD analysis also reveals that the high-strength low-alloy steel workpiece treated with Comparative Example 1-1 also contains 6% to 8% retained austenite.

[0120] Comparative Example 1-2

[0121] The same high-strength low-alloy steel workpiece as in Example 1 was selected and subjected to conventional quenching and tempering treatment. The specific treatment steps are as follows:

[0122] O1: Place the high-strength low-alloy steel workpiece into a first heating furnace heated to 920° C. (higher than the austenite end transformation temperature of the high-strength low-alloy steel workpiece in Example 1) and heat and hold for 180 minutes;

[0123] O2: The high-strength low-alloy steel workpiece that has completed the O1 treatment is quickly taken out of the first heating furnace and cooled to room temperature using an industrial fan. The cooling rate is about 2°C / s.

[0124] O3: Place the high-strength low-alloy steel workpiece that has completed the O2 treatment into the second heating furnace that has been heated to 280°C, heat and keep warm for 240 minutes, then take it out and use an industrial fan to cool the high-strength low-alloy steel workpiece to room temperature.

[0125] The conventional mechanical properties of the high-strength low-alloy steel workpieces of 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 of Comparative Examples 1-2 was tested according to the national standard GB / T 229-1994, as shown in Table 4.

[0126] Table 4 Conventional mechanical properties and impact toughness of high-strength low-alloy steel workpieces treated with Comparative Examples 1-2

[0127] The microstructure of the high-strength low-alloy steel workpiece treated using Comparative Examples 1-2 is shown in Figure 4 . Its final microstructure consists of lamellar bainite and lath martensite. XRD analysis also reveals that the high-strength low-alloy steel workpiece treated using Comparative Examples 1-2 also contains 2% to 4% retained austenite.

[0128] Comparative Examples 1-3

[0129] The same high-strength low-alloy steel workpiece as in Example 1 was selected and subjected to conventional austempering and tempering treatment. The specific treatment steps are as follows:

[0130] P1: Place the high-strength low-alloy steel workpiece into a first heating furnace heated to 920° C. (higher than the austenite end transformation temperature of the high-strength low-alloy steel workpiece in Example 1) and heat and hold for 180 minutes;

[0131] P2-1: The high-strength low-alloy steel workpiece that has completed the P1 treatment is quickly removed from the first heating furnace and cooled to 360°C (above the lamellar bainite transformation temperature) using an industrial fan. The cooling rate is approximately 2°C / s.

[0132] 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 keep it warm for 180 minutes.

[0133] P2-3: Take the high-strength low-alloy steel workpiece that has completed the P2-2 treatment out of the second heating furnace and use the industrial fan of P2-1 to cool the high-strength low-alloy steel workpiece to room temperature.

[0134] P3: Place the high-strength low-alloy steel workpiece that has completed the P2 treatment into the third heating furnace that has been heated to 280°C, heat and keep warm for 240 minutes, then take it out and use an industrial fan to cool the high-strength low-alloy steel workpiece to room temperature.

[0135] 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 / T 229-1994, as shown in Table 5.

[0136] It should be noted that, compared with Example 1, the difference between the two is that the industrial fan used in S2-1 in Example 1 cools the workpiece by spraying air to 310°C; while the industrial fan used in P2-1 in Comparative Example 1-3 cools the workpiece to 360°C, which is the same as the insulation temperature in P2-2 in Comparative Example 1-3 and S2-2 in Example 1; the other processes are the same.

[0137] Table 5 Conventional mechanical properties and impact toughness of high-strength low-alloy steel workpieces treated with Comparative Examples 1-3

[0138] The microstructures of the high-strength low-alloy steel workpieces treated using Comparative Examples 1-3 are shown in Figure 5 . Their final microstructures consist of lath bainite and lath martensite. XRD analysis also revealed that the high-strength low-alloy steel workpieces treated using Comparative Examples 1-3 also contained 8% to 10% retained austenite.

[0139] Example 2

[0140] The components and contents of the high-strength low-alloy steel workpiece of Example 2 are shown in Table 6 below:

[0141] Table 6 Chemical composition and content (mass percentage) of high-strength low-alloy steel workpiece of Example 2

[0142] Several high-strength low-alloy steel workpieces were prepared using conventional steelmaking, casting, annealing, and cutting methods according to the chemical compositions and contents in Table 6. The dimensions of the high-strength low-alloy steel workpieces were 500 mm * 300 mm * 20 mm (length * width * thickness).

[0143] It should be noted that the initial state of the high-strength low-alloy steel workpieces in Example 2, Comparative Example 2-1, Comparative Example 2-2, and Comparative Example 2-3 is a casting + annealing state, and its initial structure is ferrite and pearlite; which is different from Example 1, Comparative Example 1-1, Comparative Example 1-2, and Comparative Example 1-3.

[0144] A cylinder with a diameter of 4 mm and a length of 10 mm was randomly taken from a high-strength low-alloy steel workpiece, and the continuous cooling transformation curve of Example 2 was obtained according to the national standard "YB / T 5128-2018 Steel Continuous Cooling Transformation Curve Measurement Dilatometric Method", as shown in Figure 6. It can be seen that the cooling rate range for obtaining lamellar bainite is 1°C / s to 15°C / s, and during the heating process, the austenite start transformation temperature is 760°C, the austenite end transformation temperature is 842°C, the bainite start transformation temperature is 400°C, the lamellar bainite start transformation temperature is 350°C, the martensite start transformation temperature is 327°C, the martensite end transformation temperature is 192°C, and the lath bainite start transformation temperature is 350°C.

[0145] The multi-stage multi-phase heat treatment method of the present invention is used to treat the workpiece, and the specific steps are as follows:

[0146] S1: placing the high-strength low-alloy steel workpiece into a first heating furnace heated to 920° C. (higher than the austenite end transformation temperature of the high-strength low-alloy steel workpiece in Example 1), heating and holding the workpiece for 180 minutes;

[0147] S2-1: The high-strength low-alloy steel workpiece that has completed the S1 treatment is quickly removed from the first heating furnace and cooled to 335°C (above the martensite start transformation temperature and below the lamellar bainite start transformation temperature) using an industrial fan. The cooling rate is approximately 2°C / s.

[0148] S2-2: The high-strength low-alloy steel workpiece that has completed the S2-1 treatment is quickly placed into the second heating furnace that has been heated to 360°C, and heated and kept warm for 180 minutes;

[0149] S2-3: The high-strength low-alloy steel workpiece that has completed the S2-2 treatment is taken out of the second heating furnace and the high-strength low-alloy steel workpiece is cooled to room temperature using the industrial fan of S2-1.

[0150] 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 keep warm for 240 minutes, then take out the high-strength low-alloy steel workpiece and use the industrial fan in S2-1 to cool it to room temperature.

[0151] It should be noted that the heat treatment process of Example 2 is the same as that of Example 1, but the initial states of Example 1 and Example 2 are different.

[0152] 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 / T 229-1994, as shown in Table 7.

[0153] Table 7 Conventional mechanical properties and impact toughness of high-strength low-alloy steel workpieces treated in Example 1

[0154] Compared with the following comparative examples 2-1, 2-2 and 2-3, the impact toughness of the high-strength low-alloy steel workpiece treated in Example 2 is improved while the strength remains almost unchanged.

[0155] The microstructure of the high-strength low-alloy steel workpiece treated in Example 2 is shown in Figure 7. Its final microstructure is lamellar bainite, lath bainite, lath martensite and austenite. XRD detection shows that the high-strength low-alloy steel workpiece treated in Example 2 also contains 4% to 6% retained austenite.

[0156] Comparative Example 2-1

[0157] The same high-strength low-alloy steel workpiece as in Example 2 was selected and processed as follows:

[0158] X1: Place the high-strength low-alloy steel workpiece into a first heating furnace heated to 920° C. (higher than the austenite end transformation temperature of the high-strength low-alloy steel workpiece in Example 1) and heat and hold for 180 minutes;

[0159] X2-1: The high-strength low-alloy steel workpiece that has completed the X1 treatment is quickly removed from the first heating furnace and cooled to 335°C (above the martensite start temperature and below the lamellar bainite start temperature) using industrial quenching oil. The cooling rate is approximately 18°C / s.

[0160] X2-2: The high-strength low-alloy steel workpiece that has completed the X2-1 treatment is quickly placed into the second heating furnace that has been heated to 360°C and heated and kept warm for 180 minutes;

[0161] X2-3: Take the high-strength low-alloy steel workpiece that has completed the X2-2 treatment out of the second heating furnace and use an industrial fan to cool the high-strength low-alloy steel workpiece to room temperature.

[0162] 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 keep warm for 240 minutes, then take out and use an industrial fan to cool the high-strength low-alloy steel workpiece to room temperature.

[0163] It should be noted that the difference between Example 2 and Comparative Example 2 is that S2-1 in Example 2 uses an industrial fan to cool the workpiece with air jets at a cooling rate of approximately 2°C / s, while X2-1 in Comparative Example 2-1 uses industrial quenching oil to cool the workpiece at a cooling rate of approximately 18°C / s. The other processes are the same. Furthermore, the initial state of the high-strength low-alloy steel workpiece in Comparative Example 2-1 differs from that in Comparative Example 1-1, but the heat treatments they undergo are the same.

[0164] 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 / T 229-1994, as shown in Table 8.

[0165] Table 8 Conventional mechanical properties and impact toughness of high-strength low-alloy steel workpieces treated with Comparative Example 2-1

[0166] The microstructure of the high-strength low-alloy steel workpiece treated with Comparative Example 2-1 is shown in Figure 8 . Its final microstructure consists of lath bainite and lath martensite. XRD analysis also reveals that the high-strength low-alloy steel workpiece treated with Comparative Example 2-1 contains 6% to 8% retained austenite.

[0167] Comparative Example 2-2

[0168] The same high-strength low-alloy steel workpiece as in Example 1 was selected and subjected to conventional quenching and tempering treatment. The specific treatment steps are as follows:

[0169] Y1: Place the high-strength low-alloy steel workpiece into a first heating furnace heated to 920° C. (higher than the austenite end transformation temperature of the high-strength low-alloy steel workpiece in Example 1) and heat and hold for 180 minutes;

[0170] 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. The cooling rate is approximately 2°C / s.

[0171] Y3: Place the high-strength low-alloy steel workpiece that has completed Y2 treatment into the second heating furnace that has been heated to 280°C, heat and keep warm for 240 minutes, then take out and use an industrial fan to cool the high-strength low-alloy steel workpiece to room temperature.

[0172] 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 / T 229-1994, as shown in Table 9.

[0173] Table 9 Conventional mechanical properties and impact toughness of high-strength low-alloy steel workpieces treated with Comparative Example 2-2

[0174] The microstructure of the high-strength low-alloy steel workpiece treated in Comparative Example 2-2 is shown in Figure 9 . Its final microstructure consists of lamellar bainite and lath martensite. XRD analysis also reveals that the high-strength low-alloy steel workpiece treated in Comparative Example 2-2 contains 2% to 4% retained austenite.

[0175] Comparative Examples 2-3

[0176] The same high-strength low-alloy steel workpiece as in Example 1 was selected and subjected to conventional austempering and tempering treatment. The specific treatment steps are as follows:

[0177] Z1: Place the high-strength low-alloy steel workpiece into a first heating furnace heated to 920° C. (higher than the austenite end transformation temperature of the high-strength low-alloy steel workpiece in Example 1) and heat and hold for 180 minutes;

[0178] Z2-1: The high-strength low-alloy steel workpiece that has completed the Z1 treatment is quickly removed from the first heating furnace and cooled to 360°C (above the lamellar bainite transformation temperature) using an industrial fan. The cooling rate is approximately 2°C / s.

[0179] Z2-2: Place the high-strength low-alloy steel workpiece that has completed the Z2-1 treatment into the second heating furnace that has been heated to 360°C and keep it warm for 180 minutes.

[0180] Z2-3: Take the high-strength low-alloy steel workpiece that has completed the Z2-2 treatment out of the second heating furnace and use the industrial fan of Z2-1 to cool the high-strength low-alloy steel workpiece to room temperature.

[0181] Z3: Place the high-strength low-alloy steel workpiece that has completed the Z2 treatment into the third heating furnace that has been heated to 280°C, heat and keep warm for 240 minutes, then take it out and use an industrial fan to cool the high-strength low-alloy steel workpiece to room temperature.

[0182] The conventional mechanical properties of the high-strength low-alloy steel workpieces of Comparative Examples 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 Examples 2-3 was tested according to the national standard GB / T 229-1994, as shown in Table 10.

[0183] It should be noted that, compared with Example 2, the difference between the two is that the industrial fan used in S2-1 in Example 2 sprays air to cool the workpiece to 335°C; while the industrial fan used in P2-1 in Comparative Example 2-3 cools the workpiece to 360°C, which is the same as the insulation temperature in P2-2 in Comparative Example 1-3 and S2-2 in Example 1; the other processes are the same.

[0184] Table 10 Conventional mechanical properties and impact toughness of high-strength low-alloy steel workpieces treated with Comparative Examples 2-3

[0185] The microstructure of the high-strength low-alloy steel workpiece treated using Comparative Examples 2-3 is shown in Figure 10 . Its final microstructure consists of lath bainite and lath martensite. XRD analysis also reveals that the high-strength low-alloy steel workpiece treated using Comparative Examples 2-3 also contains 4% to 6% retained austenite.

[0186] It can be understood that those skilled in the art can combine the features mentioned in one or more embodiments mentioned throughout the present application with the features of other embodiments in any appropriate manner to implement the present application.

[0187] Note that the above are only preferred embodiments of the present application and the technical principles employed. Those skilled in the art will understand that the present application is not limited to the specific embodiments described herein, and that various obvious changes, readjustments, and substitutions can be made by those skilled in the art without departing from the scope of protection of the present application. Therefore, although the present application has been described in more detail through the above embodiments, the present application is not limited to the above embodiments, and may include many other equivalent embodiments without departing from the technical concept of the present application, all of which fall within the scope of protection of the present application.

Claims

1. A multi-stage and multi-phase 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 Heat the high-strength low-alloy steel workpiece to the first temperature T1 and keep it warm for a period of time t1 to complete the austenitization and composition homogenization of the high-strength low-alloy steel workpiece; S2: Multi-stage multi-phase controlled quenching Process S2-1: Cooling with controlled speed and temperature Cool the high-strength low-alloy steel workpiece completed in S1 to the second temperature T2, so that partial lath bainite is formed in the high-strength low-alloy steel workpiece; Process S2-2: Heat preservation with controlled temperature and time Heat the high-strength low-alloy steel workpiece completed in S2-1 to the third temperature T3 and keep it warm for a period of time t2, so that partial plate bainite is formed in the high-strength low-alloy steel workpiece; Process S2-3: Rapid cooling treatment Cool the high-strength low-alloy steel workpiece completed in S2-2 to room temperature, so that partial lath martensite structure is formed in the high-strength low-alloy steel workpiece; S3: Medium and low temperature tempering treatment Heat the high-strength low-alloy steel workpiece completed in S2-3 to the fourth temperature T4 and keep it warm for a period of time t3 for tempering treatment to obtain a multi-phase structure of lath bainite, plate bainite, lath martensite and retained austenite.

2. The heat treatment method according to claim 1, characterized in that, By mass percentage, the high-strength low-alloy steel workpiece 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%, and the rest are Fe and inevitable impurity elements. At the same time, the sum of the element contents of Mn, Cr and Mo is not less than 2.6% and not higher than 4.0%.

3. The heat treatment method according to claim 1, wherein In the S1, the first heating temperature T1 of the high-strength low-alloy steel workpiece is 30°C - 100°C higher than the austenite end transformation temperature of the high-strength low-alloy steel workpiece, and the heat preservation time t1 is 120 min - 240 min.

4. The heat treatment method according to claim 1, characterized in that, In the process S2-1, the cooling rate of the high-strength low-alloy steel workpiece is the cooling rate for obtaining lath bainite; Preferably, the cooling method is air spraying cooling and / or spray cooling.

5. The heat treatment method according to claim 1, characterized in that, In the process S2-1, the second temperature T2 is higher than the martensite start transformation temperature of the high-strength low-alloy steel workpiece and lower than the lath bainite start transformation temperature; Preferably, it is 5°C - 30°C higher than the martensite start transformation temperature.

6. The heat treatment method according to claim 1, wherein In the process S2-2, the third temperature T3 is higher than the plate bainite formation temperature of the high-strength low-alloy steel workpiece and lower than the bainite start transformation temperature, and the heat preservation time t2 is 120 min - 360 min.

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

8. The heat treatment method according to claim 1, characterized in that In the S3, the fourth temperature T4 is higher than the martensite end transformation temperature and lower than the lath bainite start transformation temperature, and the heat preservation time t3 is 240 min - 480 min; Preferably, the fourth temperature T4 is greater than 100°C and less than 360°C.

9. The heat treatment method according to any one of claims 1 to 8, characterized in that, The high-strength low-alloy steel workpiece before S1 is in a forging state or a casting + annealing state, and the structure of the high-strength low-alloy steel workpiece is a bainite + martensite duplex structure or a ferrite + pearlite structure.

10. The heat treatment method according to claim 9, wherein 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 wall thickness and the minimum wall thickness is 20 mm.

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