A heat treatment process for regulating fresh martensite in a martensite-based advanced high-strength steel

Abstract

The application discloses a heat treatment process for regulating fresh martensite in a martensite-based advanced high-strength steel, which comprises the following steps: (1) tempering martensite-based advanced high-strength steel plate at a temperature below the austenitizing start temperature Ac1 of the steel material at a predetermined heating speed; (2) maintaining the temperature for a period of time at the tempering temperature; and (3) cooling the tempered steel plate to room temperature at a certain speed. The application combines the tempering process with the conventional production process of the martensite-based advanced high-strength steel, can regulate the tempering degree of the fresh martensite in the structure by setting appropriate tempering parameters, reduce the hardness of the fresh martensite, and thus reduce the hardness difference between the phases in the structure. The application is basically applicable to all martensite-based advanced high-strength steels, has the advantages of low energy consumption, high production efficiency and low cost, and can be applied to industrial production.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of heat treatment of advanced high-strength steel, and relates to a heat treatment process for regulating fresh martensite in martensite-based advanced high-strength steel. BACKGROUND

[0002] With the global energy crisis and environmental problems becoming increasingly serious, higher requirements are put forward for automobiles in energy saving and emission reduction, etc., which makes the automobile industry develop towards energy saving, environmental protection and safety, etc. Considering the material cost and performance requirements, advanced high-strength steel becomes the preferred material for automobile weight reduction. Martensite-based advanced high-strength steel such as martensite steel, DP steel, Q&P steel, etc. is widely used in industrial production due to its low cost and high strength.

[0003] With the increase of strength level, the insufficient plasticity of high-strength martensite-based advanced high-strength steel has become a bottleneck problem restricting its industrialization process. The existing research results show that the fresh martensite in the martensite-based advanced high-strength steel is a key factor affecting its mechanical properties. Fresh martensite is a hard and brittle phase, and its hardness is much higher than that of other phases in the structure, which will affect the local stress / strain distribution and the uniformity of deformation, resulting in poor ability to coordinate deformation. In addition, during the deformation process, obvious stress concentration will occur near the fresh martensite, so that the fresh martensite interface becomes the most likely position for the initiation of holes and cracks.

[0004] Therefore, how to reduce the hardness difference between fresh martensite and other phases in the structure of the martensite-based advanced high-strength steel is the key to improving the formability of the martensite-based advanced high-strength steel. In the previous production and research, long-time tempering treatment is generally used to transform the hard and brittle fresh martensite into tempered martensite to reduce its hardness, so as to achieve the ideal combination of strength, ductility and toughness. Therefore, the tempering heat treatment process is applied to the production process of the martensite-based advanced high-strength steel, and by setting appropriate tempering parameters, the hardness difference between the phases in the structure is reduced while the high strength is ensured, so as to improve the plasticity of the martensite-based advanced high-strength steel, which has important significance for the popularization and application of the martensite-based advanced high-strength steel. SUMMARY

[0005] The purpose of the present application is to provide a heat treatment process for regulating fresh martensite in martensite-based advanced high-strength steel. By tempering heat treatment of the martensite-based advanced high-strength steel under specific process parameters, the hardness of fresh martensite in the structure is regulated, so as to effectively improve the plasticity while basically not affecting the original high strength.

[0006] In order to achieve the above purpose, the present application provides the following technical scheme:

[0007] A heat treatment process for regulating fresh martensite in martensite-based advanced high-strength steel, comprising the following steps:

[0008] (1) heating the martensite-based advanced high-strength steel sheet material to a predetermined temperature at a predetermined heating rate of ≥5℃ / min, the predetermined temperature being a tempering temperature below the austenitizing start temperature Ac1 of the steel material, to perform tempering;

[0009] The Ac1 is the start temperature of the transformation of pearlite to austenite when the martensite-based advanced high-strength steel is heated.

[0010] The chemical composition of the martensite-based advanced high-strength steel sheet material is as follows: C: 0.05%~0.77%; Si: 0~4.0%; Mn: 0~7.0%; Cr: 0~3.0%; Ni: 0~3.0%; V: 0~0.6%; Mo: 0~2.0%; Nb: 0~0.5%; the balance being Fe and inevitable impurities.

[0011] (2) performing heat preservation at the tempering temperature in step (1) for a heat preservation time ≤180 min, to obtain the steel sheet after tempering is completed;

[0012] (3) cooling the steel sheet after tempering to room temperature to obtain the martensite-based advanced high-strength steel after tempering.

[0013] Further, the martensite content in the martensite-based advanced high-strength steel sheet material in step (1) is not less than 20%, and the tensile strength is not less than 780 MPa.

[0014] Further, the martensite-based advanced high-strength steel sheet material in step (1) is a steel sheet with a thickness of 0.5 mm~5 mm.

[0015] Further, the heating mode in step (1) includes but is not limited to induction heating, salt bath heating, heating furnace heating, etc.

[0016] Further, the tempering temperature in step (1) is 150℃~Ac1.

[0017] Further, the cooling speed in step (3) is ≥5℃ / min, and the cooling mode includes but is not limited to air cooling, water cooling, oil cooling, etc.

[0018] Further, the tensile strength of the martensite-based advanced high-strength steel after heat treatment decreases by not more than 150 MPa compared with the tensile strength before heat treatment.

[0019] Compared with the prior art, the beneficial effects of the present application are:

[0020] (1) The present application combines the tempering heat treatment process with the traditional martensite-based advanced high-strength steel production process, can set the appropriate tempering parameters, control the tempering degree of fresh martensite in the organization, reduce the hardness of fresh martensite, thereby reducing the hardness difference between the phases in the organization. The microstructure and hardness distribution before and after tempering are shown in Figures 1 and 2. The hardness of fresh martensite in the steel organization decreases after tempering heat treatment, which reduces the hardness difference between the phases in the organization. It can effectively improve the plasticity of the martensite-based advanced high-strength steel without affecting the original high strength of the martensite-based advanced high-strength steel; Figure 1 and Figures Figure 2 The hardness of fresh martensite in the steel organization decreases after tempering heat treatment, which reduces the hardness difference between the phases in the organization. It can effectively improve the plasticity of the martensite-based advanced high-strength steel without affecting the original high strength of the martensite-based advanced high-strength steel;

[0021] (2) The heat treatment process for regulating fresh martensite in the martensite-based advanced high-strength steel proposed by the present application has a wide application range and is basically applicable to all martensite-based advanced high-strength steels, and has the advantages of low energy consumption, high production efficiency and low cost, and can be applied to industrial production. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 The microstructure of the steel before and after tempering in Example 1 is shown in Figure 1, wherein (a) is before tempering, and (b) is after tempering;

[0023] Figure 2 The hardness distribution of the microstructure of the steel before and after tempering in Example 1 is shown in Figure 2, wherein (a) is before tempering, and (b) is after tempering;

[0024] Figure 3 The mechanical property curve of the steel before and after tempering in Example 1 is shown in Figure 3;

[0025] Figure 4 The mechanical property curve of the steel before and after tempering in Example 3 is shown in Figure 4. DETAILED DESCRIPTION

[0026] The technical solutions in the embodiments of the present application will be described below in conjunction with the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application. The present application will be further described in detail below in conjunction with specific implementation cases. However, the protection scope of the present application is not limited by the specific embodiments.

[0027] In the specific implementation process, the present application proposes a heat treatment process for regulating fresh martensite in the martensite-based advanced high-strength steel, and the specific steps are as follows:

[0028] (1) Put the 0.5 mm-5 mm martensite-based advanced high-strength steel (tensile strength ≥ 780 MPa, martensite content not less than 20%) plate into the heating furnace, and heat to the tempering temperature of 150℃-A c1 (steel austenitizing starting temperature) at a heating rate of ≥5℃ / min;

[0029] (2) The steel is kept at the tempering temperature for a period of time, and the holding time is not higher than 180 min.

[0030] (3) The steel plate after tempering is cooled to room temperature, and the cooling method includes but is not limited to air cooling, water cooling, oil cooling, etc.

[0031] In view of the above technical scheme, the embodiments of the present application are as follows:

[0032] Example 1

[0033] A heat treatment process for regulating fresh martensite in martensite-based advanced high-strength steel, comprising the following steps:

[0034] The martensite-based advanced high-strength steel selected in this example is a Q&P steel with a thickness of 0.9 mm, a martensite content of 36%, a tensile strength of 1208 MPa, and an elongation of 20.7%. The chemical composition is as follows in terms of weight percentage: C: 0.18%, Si: 1.51%, Mn: 2.48%, Nb: 0.41%, and the rest is Fe and unavoidable impurities.

[0035] (1) The Q&P steel plate is processed into a dog bone-shaped tensile sample with a gauge length of 6 mm;

[0036] (2) Heat to 500℃ in a salt bath furnace at a heating rate of about 100℃ / s and keep for 10 s to complete tempering;

[0037] (3) After tempering, water quenching to room temperature, Figure 1 is the microstructure of the steel before and after tempering, wherein (a) is before tempering, and (b) is after tempering; Figure 2 is the hardness distribution of the microstructure of the steel before and after tempering, wherein (a) is before tempering, and (b) is after tempering;

[0038] (4) After completing the tempering treatment, the tensile properties of the martensite-based advanced high-strength steel are tested according to the national standard method, and the obtained properties are as follows: tensile strength 1152 MPa, total elongation 34.2%. Figure 3 is the mechanical property curve of the martensite-based advanced high-strength steel before and after tempering.

[0039] Example 2

[0040] A heat treatment process for regulating fresh martensite in a martensite-based advanced high-strength steel, comprising the following steps:

[0041] The martensite-based advanced high-strength steel selected in this example is a Q&P steel with a thickness of 1.1 mm, a martensite content of 36%, a tensile strength of 1208 MPa, and an elongation of 20.7%. The chemical composition in terms of weight percentage is as follows: C: 0.18%, Si: 1.51%, Mn: 2.48%, Nb: 0.41%, and the remaining elements are Fe and unavoidable impurities.

[0042] (1) Process the Q&P steel sheet into a dog bone-shaped tensile sample with a gauge length of 6 mm;

[0043] (2) Heat to 400°C at a heating rate of 5°C / min in a muffle furnace and hold for 160 min to complete tempering;

[0044] (3) After tempering is completed, air cool to room temperature;

[0045] (4) After completing the tempering treatment, the tensile properties of the martensite-based advanced high-strength steel are tested according to the national standard method, and the obtained properties are as follows: tensile strength 1106 MPa, total elongation 36.2%.

[0046] Example 3

[0047] A heat treatment process for regulating fresh martensite in a martensite-based advanced high-strength steel, comprising the following steps:

[0048] The martensite-based advanced high-strength steel selected in this example is a Q&P steel with a thickness of 1.7 mm, a martensite content of 42%, a tensile strength of 1423 MPa, and an elongation of 12.5%. The chemical composition in terms of weight percentage is as follows: C: 0.18%, Si: 1.51%, Mn: 2.48%, Mo: 0.25%, V: 0.38%, and the remaining elements are Fe and unavoidable impurities.

[0049] (1) Process the Q&P steel sheet into a dog bone-shaped tensile sample with a gauge length of 10 mm;

[0050] (2) Heat to 300°C at a heating rate of about 100°C / s in a salt bath furnace and hold for 100 s to complete tempering;

[0051] (3) After tempering is completed, water quench to room temperature;

[0052] (4) After completing the tempering treatment, the tensile properties of the martensite-based advanced high-strength steel are tested according to the national standard method, and the obtained properties are as follows: tensile strength 1356 MPa, total elongation 16.8%. Figure 4 The mechanical property curves of the martensite-based advanced high-strength steel before and after tempering.

[0053] Example 4

[0054] A heat treatment process for regulating fresh martensite in a martensite-based advanced high strength steel, comprising the following steps:

[0055] The martensite-based advanced high strength steel selected in this example is a Q&P steel with a thickness of 1.7 mm, a martensite content of 42%, a tensile strength of 1423 MPa, and an elongation of 12.5%. The chemical composition in terms of weight percentage is as follows: C: 0.20%, Si: 1.51%, Mn: 2.48%, Mo: 0.25%, V: 0.38%, and the rest is Fe and inevitable impurities.

[0056] (1) Process the Q&P steel sheet into a dog-bone tensile sample with a gauge length of 10 mm;

[0057] (2) Heat to 300°C at a heating rate of 50°C / min in a muffle furnace and hold for 10 min to complete tempering;

[0058] (3) After tempering is completed, quench to room temperature in water;

[0059] (4) After completing the tempering treatment, the tensile properties of the martensite-based advanced high strength steel are tested according to the national standard method. The obtained properties are as follows: tensile strength 1298 MPa, total elongation 17.2%.

[0060] Example 5

[0061] A heat treatment process for regulating fresh martensite in a martensite-based advanced high strength steel, comprising the following steps:

[0062] The martensite-based advanced high strength steel selected in this example is a DP steel with a thickness of 2.2 mm, a martensite content of 76%, a tensile strength of 1872 MPa, and an elongation of 4.7%. The chemical composition in terms of weight percentage is as follows: C: 0.68%, Si: 0.51%, Mn: 0.98%, Cr: 0.5%, and the rest is Fe and inevitable impurities.

[0063] (1) Process the DP steel sheet into a dog-bone tensile sample with a gauge length of 10 mm;

[0064] (2) Heat to 300°C at a heating rate of 20°C / min in a muffle furnace and hold for 30 min to complete tempering;

[0065] (3) After tempering is completed, quench to room temperature in water;

[0066] (4) After the tempering treatment, the tensile properties of the martensite-based advanced high-strength steel are tested according to the national standard method, and the obtained properties are as follows: tensile strength 1735 MPa, total elongation 9.6%.

[0067] Example 6

[0068] A heat treatment process for regulating fresh martensite in a martensite-based advanced high-strength steel, comprising the following steps:

[0069] The martensite-based advanced high-strength steel selected in this example is a DP steel with a thickness of 2.9 mm, the martensite content is 89%, the tensile strength is 1462 MPa, and the elongation is 3.9%. The chemical composition in terms of weight percentage is as follows: C: 0.35%, Si: 0.52%, Mn: 1.05%, Cr: 0.52%, and the rest is Fe and unavoidable impurities.

[0070] (1) The DP steel plate material is processed into a dog bone-shaped tensile sample with a gauge length of 10 mm;

[0071] (2) In a salt bath furnace, heat to 350℃ at a heating rate of about 100℃ / s and keep for 100s to complete tempering;

[0072] (3) After tempering, water quenching to room temperature;

[0073] (4) After the tempering treatment, the tensile properties of the martensite-based advanced high-strength steel are tested according to the national standard method, and the obtained properties are as follows: tensile strength 1386 MPa, total elongation 10.3%.

Claims

1. A heat treatment process for regulating fresh martensite in a martensite-based advanced high strength steel, characterized in that, The method comprises the following steps: (1) heating the martensite-based advanced high-strength steel sheet at a predetermined heating rate of greater than or equal to 5 ℃ / min to a predetermined temperature, which is a tempering temperature below the austenitizing start temperature Ac1 of the steel material, and performing tempering at the tempering temperature of 150 ℃ to Ac1; The Ac1 is the start temperature of the transformation of pearlite to austenite when the martensite-based advanced high-strength steel is heated; The martensite-based advanced high-strength steel sheet has a chemical composition by weight percentage of: C: 0.05% to 0.77%; Si: 0 to 4.0%; Mn: 0 to 7.0%; Cr: 0 to 3.0%; Ni: 0 to 3.0%; Mo: 0 to 2.0%; Nb: 0 to 0.5%; and the balance of Fe and inevitable impurities; (2) performing heat preservation at the tempering temperature in step (1) for a heat preservation time of less than or equal to 180 min, to obtain a tempered steel sheet; V：0~0.6％； (3) cooling the tempered steel sheet to room temperature at a cooling rate of greater than or equal to 5 ℃ / min, to obtain a martensite-based advanced high-strength steel after tempering, wherein the cooling mode includes but is not limited to air cooling, water cooling, and oil cooling. The martensite content in the martensite-based advanced high-strength steel sheet in step (1) is not less than 20%, and the tensile strength is not less than 780 MPa. The martensite-based advanced high-strength steel sheet in step (1) is a steel sheet with a thickness of 0.5 mm to 5 mm.

2. The heat treatment process for regulating fresh martensite in a martensite-based advanced high strength steel according to claim 1, characterized in that, The tensile strength of the martensite-based advanced high-strength steel after heat treatment is decreased by not more than 150 MPa compared with the tensile strength before heat treatment.

3. The heat treatment process for regulating fresh martensite in a martensite-based advanced high strength steel according to claim 1, characterized in that, ​ 4. The heat treatment process for regulating fresh martensite in a martensite-based advanced high strength steel according to claim 1, characterized in that, ​

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