A kind of toughening heat treatment process suitable for 20CrMnTi steel building pipe

Abstract

The application relates to a strengthening and toughening heat treatment process suitable for 20CrMnTi steel building pipes, and the heat treatment process comprises two steps of heating and controlled cooling; in the heating step, the 20CrMnTi steel building pipes are heated to 860-900 DEG C at a rate of 15-30 DEG C / s, and after the heating is completed, the 20CrMnTi steel building pipes are directly subjected to the controlled cooling step without heat preservation; in the controlled cooling step, the 20CrMnTi steel building pipes after induction heating are cooled to 360-400 DEG C at a rate of 30-50 DEG C / s through water spraying or water penetration, and then are naturally cooled to room temperature in air. The 20CrMnTi steel building pipes prepared by the method have good strength and plasticity matching, the yield strength is in the range of 500-550 MPa, the tensile strength is in the range of 1100-1180 MPa, the elongation after fracture can reach more than 12%, and the yield strength ratio is lower than 0.5; on the basis of meeting the requirements of high strength and toughness and low yield strength ratio of building structure steel, the use of the 20CrMnTi steel is expanded.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of metallurgy, and particularly relates to a heat treatment process capable of effectively improving the comprehensive mechanical properties of 20CrMnTi steel building pipe. BACKGROUND

[0002] In the field of building structures, various steel pipe products have a wide range of applications. In recent years, with the development trend of large-scale, high-rise and lightweight of building structures, higher and higher requirements for the comprehensive mechanical properties such as strength and plasticity of building steel pipes are put forward; at the same time, in order to meet the requirements of earthquake resistance and safety performance, the building steel pipes generally require a lower yield ratio (usually below 0.6); in addition, the selection of materials and processing technology of building steel pipes also need to take into account factors such as cost and efficiency.

[0003] 20CrMnTi steel is a commonly used low-carbon low-alloy steel. According to GB / T 3077-2015 Alloy Structure Steel, the chemical composition of 20CrMnTi steel is: C 0.17-0.23%, Si 0.17-0.37%, Mn 0.80-1.10%, Cr 1.00-1.30%, Ti 0.04-0.10, and the balance is Fe and unavoidable impurities. 20CrMnTi steel has good hardenability, and is usually used as the preferred heat treatment process of carburizing + quenching + low-temperature tempering for various gears, shafts and other parts. From the perspective of strength, material cost and welding performance, 20CrMnTi steel can theoretically be used as a candidate material for preparing building steel pipes. However, when the conventional heat treatment process is used, if the tensile strength exceeds 1000MPa, quenching + low-temperature tempering heat treatment must be used. At this time, due to the microstructure of high-strength and low-plasticity tempered martensite, the elongation after fracture is difficult to exceed 10%, and the yield ratio is also difficult to reach below 0.6. Therefore, it is necessary to develop a heat treatment process suitable for industrial production, so as to realize the good matching and synchronous improvement of high strength, high plasticity and high toughness.

[0004] To this end, the application provides a heat treatment process capable of effectively improving the comprehensive mechanical properties of 20CrMnTi steel building pipe, which aims to meet the requirements of high strength and toughness and low yield ratio of building structure steel, and further expand the use of 20CrMnTi steel. SUMMARY

[0005] The technical problem to be solved by the application is to provide a heat treatment process capable of effectively improving the comprehensive mechanical properties and suitable for 20CrMnTi steel building pipe.

[0006] To solve the above technical problems, the technical scheme adopted by the present application is: a kind of toughening heat treatment process suitable for 20CrMnTi steel building pipe, the heat treatment process includes heating and control cooling two steps;The heating step, 20CrMnTi steel building pipe is heated to 860-900 ℃ at the rate of 15-30 ℃ / s, after heating, no need to keep warm, directly into the cooling step.

[0007] The heating step of the present application, the heating mode is induction heating.

[0008] The control cooling step of the present application, after the induction heating of 20CrMnTi steel building pipe, it is cooled to 360-400 ℃ at the rate of 30-50 ℃ / s, then it is naturally cooled to room temperature in air, that is, building pipe finished product is obtained.

[0009] The control cooling step of the present application, the cooling mode is water spray cooling or water cooling.

[0010] The 20CrMnTi steel building pipe obtained by the heat treatment process of the present application has good strength and plasticity matching, yield strength is 500-550 MPa, tensile strength is 1100-1180 MPa.

[0011] The 20CrMnTi steel building pipe obtained by the heat treatment process of the present application has good strength and plasticity matching, yield strength is 500-550 MPa, tensile strength is 1100-1180 MPa.

[0012] The 20CrMnTi steel building pipe obtained by the heat treatment process of the present application has good strength and plasticity matching, yield strength is 500-550 MPa, tensile strength is 1100-1180 MPa.

[0013] The heat treatment process suitable for the 20CrMnTi steel building pipe includes two steps of induction heating and controlled cooling; the design idea is: 1) the temperature control in the heating step is in a reasonable range which can ensure that the pipe organization is completely austenitized and can avoid the rapid thickening of the surface iron oxide scale, so as to ensure the strength and surface quality of the finished product, and a reasonable heating rate range is adopted, so that it can match the conditions of commonly used industrial induction heating equipment; after the heating step is completed, there is no conventional heat preservation or soaking stage, so that the austenite grain growth and the thickening of the surface iron oxide scale can be effectively inhibited, and the production rhythm can be maximized and the power consumption cost can be reduced; 2) the controlled cooling successively experiences water spraying or water penetrating cooling and natural cooling in air, the termination temperature of the water spraying or water penetrating cooling stage is controlled in the best range between the Ms point and the Mf point of the 20CrMnTi steel, and the cooling rate is controlled in the range of 30-50℃ / s, so as to obtain the martensite and austenite organization, and the carbon distribution can be realized in the subsequent natural cooling stage in air, so as to obtain and retain the residual austenite organization, and the synchronous improvement of the strength and the plasticity is achieved. If the termination temperature of the water spraying or water penetrating cooling stage is lower than 360℃, it is difficult to obtain enough residual austenite organization, resulting in insufficient plasticity; if the termination temperature of the water spraying or water penetrating cooling stage is higher than 400℃, it is difficult to obtain enough martensite organization, resulting in insufficient strength; if the cooling rate of the water spraying or water penetrating cooling stage is less than 30℃ / s, it is difficult to obtain enough martensite organization, and then it is difficult to obtain high enough strength; if the cooling rate exceeds 50℃ / s, not only the risk of cracking and deformation is increased, but also it is difficult to control the termination temperature of the water spraying or water penetrating cooling stage in a reasonable range. In addition, if the wall thickness specification of the building pipe is less than 3.0mm, the pipe is prone to deformation and bending in the water spraying or water penetrating cooling stage; if the wall thickness specification exceeds 5.5mm, due to the large difference between the cooling rates inside and outside the pipe, it is difficult to ensure the uniformity of the microstructure and mechanical properties.

[0014] The 20CrMnTi steel building pipe obtained by the above technical scheme has good strength and plasticity matching, the yield strength is in the range of 500-550MPa, the tensile strength is in the range of 1100-1180MPa, the elongation after fracture can reach more than 12%, and the yield strength ratio is not higher than 0.5, which meets the requirements of building structure steel for high strength and toughness and low yield strength ratio. The application expands the use of 20CrMnTi steel, and has the advantages of simple operation and stable performance. BRIEF DESCRIPTION OF DRAWINGS

[0015] Figure 1 It is a time-temperature diagram of the heat treatment process of the application. DETAILED DESCRIPTION

[0016] The present invention will now be described in further detail with reference to specific embodiments.

[0017] The process steps of this heat treatment method are described below:

[0018] (1) Induction heating step: The induction heating step involves heating the 20CrMnTi steel building pipe to 860-900℃ at a rate of 15-30℃ / s. The heating rate of 15-30℃ / s can match the conditions of commonly used industrial induction heating equipment, especially the power conditions, thus enabling industrial mass production. The final heating temperature is controlled within the range of 860-900℃ because the measured value of the Ac3 temperature (complete austenitization temperature) of 20CrMnTi steel is approximately 810℃. Controlling the final heating temperature and holding temperature within the range of 50-90℃ of the Ac3 temperature ensures that the pipe achieves complete austenitization during the heating process, thereby obtaining the martensite and austenite structure required for high strength and high plasticity during the subsequent controlled cooling process. If the heating temperature is below 860℃, the pipe may not achieve complete austenitization, resulting in fluctuations and instability in mechanical properties. If the heating temperature is above 900℃, the iron oxide scale on the surface of the pipe will thicken rapidly, thus affecting the surface quality of the finished product.

[0019] (2) Controlled cooling steps: The controlled cooling steps are to first cool the 20CrMnTi steel building pipe after induction heating to 360-400℃ by spraying or passing water through it at a rate of 30-50℃ / s, and then let it cool naturally to room temperature in the air. During the water spraying or piercing cooling process, the cooling rate of the pipe can reach over 30℃ / s, which meets the cooling rate requirements for the martensitic transformation of 20CrMnTi steel. The termination temperature of the water spraying or piercing cooling stage is controlled within the range of 360-400℃ because when the cooling rate is controlled within the range of 30-50℃ / s, the measured values ​​of the Ms point (the start temperature of the martensitic transformation of steel) and Mf point (the termination temperature of the martensitic transformation of steel) of 20CrMnTi steel are 405℃ and 292℃, respectively. Controlling the termination temperature of the water spraying or piercing cooling stage within a reasonable range between the Ms point and the Mf point can obtain the martensitic and austenitic structures required for high strength and high plasticity. During the subsequent natural cooling process in air, carbon distribution can be achieved, thereby obtaining and retaining the residual austenitic structure, achieving a simultaneous improvement in strength and plasticity. Example 1

[0020] In this embodiment, the 20CrMnTi steel building pipe is a seamless circular pipe with a wall thickness of 5mm and a diameter of 42mm. The heat treatment method is described in detail below.

[0021] (1) Induction heating step: The 20CrMnTi steel seamless round tube is heated to 900℃ at a rate of 30℃ / s by induction heating.

[0022] (2) Controlled cooling steps: The seamless round pipe of 20CrMnTi steel after induction heating is cooled to 380℃ at a rate of 50℃ / s by water immersion, and then naturally cooled to room temperature in the air to obtain the finished 20CrMnTi steel building pipe.

[0023] The main mechanical properties obtained by conducting longitudinal tensile tests on the entire pipe are shown in Table 1. It can be seen that the yield strength (R) of the material... p0.2 The strength of the metal is 531 MPa, the tensile strength is 1144 MPa, the elongation after fracture is 14.54%, and the yield strength ratio is 0.46. Example 2

[0024] In this embodiment, the 20CrMnTi steel building pipe is a high-frequency resistance welded round steel pipe with a wall thickness of 3.5mm and a diameter of 25mm. The heat treatment method is as follows.

[0025] (1) Induction heating step: The 20CrMnTi steel high-frequency resistance welded round steel pipe is heated to 860℃ at a rate of 23℃ / s by induction heating.

[0026] (2) Controlled cooling steps: The 20CrMnTi steel high-frequency resistance welded round steel pipe after induction heating is cooled to 400℃ at a rate of 45℃ / s by water immersion, and then naturally cooled to room temperature in the air to obtain the finished 20CrMnTi steel building pipe.

[0027] The main mechanical properties obtained by conducting longitudinal tensile tests on the entire pipe are shown in Table 1. It can be seen that the yield strength (R) of the material... p0.2 The strength of the metal is 523 MPa, the tensile strength is 1127 MPa, the elongation after fracture is 13.16%, and the yield strength ratio is 0.46. Example 3

[0028] In this embodiment, the 20CrMnTi steel building pipe is a high-frequency resistance welded rectangular steel pipe with a wall thickness of 5.5mm and a cross-sectional size of 140mm×80mm. The heat treatment method is as follows.

[0029] (1) Induction heating step: The 20CrMnTi steel high-frequency resistance welded rectangular steel pipe is heated to 880℃ at a rate of 15℃ / s by induction heating.

[0030] (2) Controlled cooling steps: The 20CrMnTi steel high-frequency resistance welded round steel pipe after induction heating is cooled to 360℃ at a rate of 35℃ / s by water spraying, and then naturally cooled to room temperature in the air to obtain the finished 20CrMnTi steel building pipe.

[0031] The main mechanical properties obtained by cutting A50 plate-shaped tensile specimens along the longitudinal direction of the pipe and conducting tensile tests are shown in Table 1. The yield strength (R0) of the material is evident. p0.2 The strength of the metal is 542 MPa, the tensile strength is 1163 MPa, the elongation after fracture is 12.47%, and the yield strength ratio is 0.47. Example 4

[0032] In this embodiment, the 20CrMnTi steel building pipe is a circular seamless steel pipe with a wall thickness of 3.0mm and an outer diameter of 50mm. The heat treatment method is described in detail below.

[0033] (1) Induction heating step: The 20CrMnTi seamless steel pipe is heated to 900℃ at a rate of 20℃ / s by induction heating.

[0034] (2) Controlled cooling steps: The 20CrMnTi steel round seamless steel pipe after induction heating is cooled to 360℃ at a rate of 40℃ / s by water spraying, and then naturally cooled to room temperature in the air to obtain the finished 20CrMnTi steel building pipe.

[0035] The main mechanical properties obtained by conducting longitudinal tensile tests on the entire pipe are shown in Table 1. It can be seen that the yield strength (R) of the material... p0.2 The strength of the metal is 528 MPa, the tensile strength is 1101 MPa, the elongation after fracture is 13.36%, and the yield strength ratio is 0.48. Example 5

[0036] In this embodiment, the 20CrMnTi steel building pipe is a circular high-frequency resistance welded steel pipe with a wall thickness of 4.0mm and an outer diameter of 100mm. The heat treatment method is as follows.

[0037] (1) Induction heating step: The 20CrMnTi steel round high-frequency resistance welded steel pipe is heated to 860℃ at a rate of 25℃ / s by induction heating.

[0038] (2) Controlled cooling steps: The 20CrMnTi steel high-frequency resistance welded round steel pipe after induction heating is cooled to 380℃ at a rate of 30℃ / s by water spraying, and then naturally cooled to room temperature in the air to obtain the finished 20CrMnTi steel building pipe.

[0039] The main mechanical properties obtained by conducting longitudinal tensile tests on the entire pipe are shown in Table 1. It can be seen that the yield strength (R) of the material... p0.2 The strength of the metal is 513 MPa, the tensile strength is 1138 MPa, the elongation after fracture is 12.68%, and the yield strength ratio is 0.45. Example 6

[0040] In this embodiment, the 20CrMnTi steel building pipe is a high-frequency resistance welded rectangular steel pipe with a wall thickness of 4.75mm and a cross-sectional size of 60mm×60mm. The heat treatment method is as follows.

[0041] (1) Induction heating step: The 20CrMnTi steel high-frequency resistance welded rectangular steel pipe is heated to 880℃ at a rate of 28℃ / s by induction heating.

[0042] (2) Controlled cooling steps: The 20CrMnTi steel high-frequency resistance welded round steel pipe after induction heating is cooled to 370℃ at a rate of 35℃ / s by water spraying, and then naturally cooled to room temperature in the air to obtain the finished 20CrMnTi steel building pipe.

[0043] The main mechanical properties obtained by cutting A50 plate-shaped tensile specimens along the longitudinal direction of the pipe and conducting tensile tests are shown in Table 1. The yield strength (R0) of the material is evident. p0.2 The strength of the metal is 546 MPa, the tensile strength is 1162 MPa, the elongation after fracture is 14.33%, and the yield strength ratio is 0.47.

[0044] Table 1 Mechanical properties of 20CrMnTi steel building pipes in Examples 1-5

[0045]

[0046] The above embodiments are only used to illustrate and not limit the technical solutions of the present invention. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the present invention without departing from the spirit and scope of the present invention. Any modifications or partial substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A strengthening and toughening heat treatment process suitable for 20CrMnTi steel building pipes, characterized in that: The heat treatment process includes two steps: heating and controlled cooling. In the heating step, the 20CrMnTi steel building pipe is heated to 860-900℃ at a rate of 15-30℃ / s. After heating, no heat preservation is required, and it directly enters the cooling step. In the controlled cooling step, the cooling method is water spray cooling or water immersion cooling. The 20CrMnTi steel building pipe after induction heating is cooled to 360-400℃ at a rate of 30-50℃ / s, and then naturally cooled to room temperature in the air to obtain the finished building pipe.

2. The strengthening and toughening heat treatment process for 20CrMnTi steel building pipes according to claim 1, characterized in that: The heating step is performed using induction heating.

3. A strengthening and toughening heat treatment process for 20CrMnTi steel building pipes according to claim 1 or 2, characterized in that: The yield strength of the 20CrMnTi steel building pipe obtained by the heat treatment process is 500-550MPa, and the tensile strength is 1100-1180MPa.

4. A strengthening and toughening heat treatment process for 20CrMnTi steel building pipes according to claim 1 or 2, characterized in that: The 20CrMnTi steel building pipes obtained by the heat treatment process have an elongation at break of ≥12% and a yield strength ratio of ≤0.

5.

5. A strengthening and toughening heat treatment process for 20CrMnTi steel building pipes according to claim 1 or 2, characterized in that: The heat treatment process is applicable to 20CrMnTi steel building pipes with a wall thickness of 3.0 to 5.5 mm.

Images