KR-20260065227-A - Method for heat treating billets for Cr-based seamless tubes

Abstract

A method for heat-treating a billet for a Cr-based seamless tube is disclosed. The method for heat-treating a billet for a Cr-based seamless tube comprises the steps of: loading the billet for a Cr-based seamless tube into a furnace; creating an inert gas atmosphere inside the furnace; and heating the billet to 950°C to 1150°C by raising the temperature at a preset rate of temperature increase. The present invention can increase the density of the billet structure and improve the surface roughness during the heat treatment of a billet for a Cr-based seamless tube.

Inventors

• 이흥해
• 백운학
• 김상헌

Assignees

• 주식회사 율촌

Dates

Publication Date

20260508

Application Date

20241101

Claims (10)

1. In a method for heat-treating billets for Cr-based seamless tubes, A step of loading the above-mentioned billet for a Cr-based seamless tube into a heating furnace; Step of creating an inert gas atmosphere inside the furnace cabinet; and A method for heat treating a billet for a Cr-based seamless tube, comprising the step of heating the billet to 950°C to 1150°C by raising the temperature at a preset rate of temperature increase.
2. In paragraph 1, A method for heat treating a Cr-based seamless tube billet, characterized in that the material of the above Cr-based seamless tube billet is SCr420H.
3. In paragraph 1, A method for heat-treating a billet for a Cr-based seamless tube, characterized in that the inert gas is N₂ .
4. In paragraph 1, A method for heat treating a billet for a Cr-based seamless tube, characterized in that the above-mentioned preset temperature rise rate is 10°C per minute.
5. In paragraph 1, A method for heat treating a billet for a Cr-based seamless tube, characterized in that the above billet has a heating temperature of 1000°C.
6. In paragraph 1, A method for heat treating a billet for a Cr-based seamless tube, characterized in that the outlet temperature of the above-mentioned heating furnace is 1150°C.
7. In paragraph 1, The heating of the above billet is performed by induction heating using high frequency, but, A method for heat treating a billet for a Cr-based seamless tube, characterized in that the frequency of the induction coil for the above-mentioned induction heating is 1 to 1.3 KHz.
8. In paragraph 1, A method for heat treating a billet for a Cr-based seamless tube, characterized by further including the step of introducing a decarburization prevention gas into the above-mentioned heating furnace.
9. In paragraph 8, A method for heat-treating a billet for a Cr-based seamless tube, characterized in that the above-mentioned decarburization prevention gas is an RX gas using CO gas or hydrocarbon hydrogen gas.
10. In paragraph 1, A method for heat treating a billet for a Cr-based seamless tube, characterized in that the microstructure of the billet undergoes a phase transition from ferrite to austenite at 760°C to 765°C.

Description

Method for heat treating billets for Cr-based seamless tubes The present invention relates to a method for heat-treating a billet for a Cr-based seamless tube before drilling, and more specifically, to a method for heat-treating a billet for a Cr-based seamless tube that can increase the density of the billet's microstructure and improve the surface roughness during the heat treatment of the billet. A seamless tube refers to a steel pipe produced by heating a round billet to a specific temperature, drilling a hole in the center using a drilling machine to create a hollow material, and then undergoing a hot rolling mill process. It is subsequently transformed into a precision tube through cold rolling (plastic deformation). Seamless tubes are manufactured using a non-welding method, eliminating the risk of cracking caused by welds. They feature uniform material properties, structure, and hardness throughout, and offer the advantage of a wide range of material selection options. Furthermore, seamless tubes are essential for special piping applications—such as high pressure, high temperature, low temperature, and corrosion resistance—as well as for mechanical structures and heat exchangers, where welded steel pipes cannot be used. They are widely utilized in various industrial machinery, chemical plants, nuclear power plants, aircraft, shipbuilding, refrigeration condensers, boilers, and automobiles. The manufacturing process of seamless tubes consists of a billet heating process for heating the cut billet, a piercing process for piercing the billet, a Plug Mill process for extending the length, and an SRM (Stretch Reducing Mill) process for controlling the size of the tube. The billet heating process begins by loading the billet into a furnace, and generally, the inside of the furnace is created with an air atmosphere. When heating Cr-based billets, high temperatures can cause changes in the billet's structure or defects. Specifically, failure to control the appropriate heating temperature and gas atmosphere can lead to problems such as deterioration of the billet's surface roughness, the formation of a thick oxide scale due to oxidation reactions resulting in surface defects during piercing, and decarburization where surface carbon (C) is converted into CO or CO2 . FIG. 1 is a flowchart illustrating a method for heat-treating a billet for a Cr-based seamless tube according to an embodiment of the present invention. FIG. 2 is a drawing showing the CS analyzer used in the experimental example of the present invention. FIG. 3 is a diagram showing the CS analysis results of an SCr420H specimen according to an embodiment of the present invention. FIG. 4 is a diagram showing the simulation results of the investigation of the solid-liquid coexistence region of SCr420H according to an embodiment of the present invention. FIG. 5 is a drawing showing a CCT curve according to an embodiment of the present invention. FIG. 6 is a drawing showing the TTT curve of an SCr420H specimen according to an embodiment of the present invention. FIG. 7 is a drawing showing the CCT curve of an SCr420H specimen according to an embodiment of the present invention. FIG. 8 is a diagram showing the computational analysis results for the yield strength (0.2 Prof Stress) of SCr420H according to temperature in accordance with an embodiment of the present invention. FIG. 9 is a diagram showing true stress-true strain curves according to deformation temperature at a strain rate of 0.001/s of SCr420H according to one embodiment of the present invention. FIG. 10 is an exemplary drawing of a billet heating device having eight high-frequency induction coils and an inert gas supply chamber according to one embodiment of the present invention. FIG. 11 is a drawing showing the surface shape of an SCr420H specimen according to factors (temperature, injection gas) in accordance with an embodiment of the present invention. FIG. 12 is a drawing showing the results of surface roughness analysis of an SCr420H specimen before heat treatment according to an embodiment of the present invention. FIG. 13 is a drawing showing the results of surface roughness analysis of an SCr420H specimen subjected to a heat treatment process at 1000°C in an N2 atmosphere according to an embodiment of the present invention. FIG. 14 is a drawing showing the results of surface roughness analysis of an SCr420H specimen subjected to a heat treatment process at 1000°C in an air atmosphere according to an embodiment of the present invention. FIG. 15 is a drawing of an SCr420H specimen according to factors (temperature, injection gas) in accordance with an embodiment of the present invention. FIG. 16 is a drawing showing the results of cross-sectional SEM/EDX mapping analysis of an SCr420H specimen before heat treatment according to an embodiment of the present invention. FIG. 17 is a drawing showing the results of cross-sectional SEM/EDX mapping analysis of an SCr420H specimen subject