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KR-20260064846-A - Steel and method of manufacturing the same

KR20260064846AKR 20260064846 AKR20260064846 AKR 20260064846AKR-20260064846-A

Abstract

The present invention comprises the steps of: hot rolling a steel material comprising, in weight percent, carbon: 0.08%~0.11%, silicon: 0.10%~0.20%, manganese: 1.0%~1.2%, aluminum: 0.015%~0.055%, niobium: 0.005%~0.015%, titanium: 0.012%~0.022%, chromium: 0.45%~0.55%, molybdenum: 0.45%~0.55%, nickel: 1.0%~1.4%, copper: 0.1%~0.2%, boron: 0.001%~0.002% or less, and the remainder being iron, and air cooling to room temperature; and performing a first heat treatment of the steel material at a temperature of 850℃ to 950℃. The present invention provides a method for manufacturing steel, comprising: a step of rapidly cooling the heat-treated steel to room temperature; a step of performing a second heat treatment on the heat-treated steel at 790°C to 830°C; a step of air-cooling the heat-treated steel to room temperature; and a step of tempering the air-cooled steel at a temperature of 550°C to 650°C, wherein the second heat treatment temperature has a relationship with the tempering temperature.

Inventors

  • 유재홍
  • 김영준
  • 이희웅
  • 송준무
  • 조영욱

Assignees

  • 현대제철 주식회사

Dates

Publication Date
20260508
Application Date
20241029

Claims (6)

  1. (a) a step of hot rolling a steel material comprising, in weight percent, carbon (C): 0.08%~0.11%, silicon (Si): 0.10%~0.20%, manganese (Mn): 1.0%~1.2%, aluminum (Al): 0.015%~0.055%, niobium (Nb): 0.005%~0.015%, titanium (Ti): 0.012%~0.022%, chromium (Cr): 0.45%~0.55%, molybdenum (Mo): 0.45%~0.55%, nickel (Ni): 1.0%~1.4%, copper (Cu): 0.1%~0.2%, boron (B): 0.001%~0.002% or less, and the remainder being iron and unavoidable impurities, and air cooling to room temperature; (b) a step of performing a first heat treatment on the steel at a first temperature ( T1 ) within 850℃ to 950℃; (c) a step of rapidly cooling the heat-treated steel to room temperature; (d) a step of performing a second heat treatment on the above-mentioned quenched steel at a second temperature ( T2 ) within 790℃ to 830℃; (e) a step of air-cooling the heat-treated steel to room temperature; and (f) a step of tempering the air-cooled steel at a third temperature ( T3 ) within 550℃ to 650℃; wherein The above third temperature ( T3 ) is characterized by being associated with the above second temperature ( T2 ) and satisfying the following Equation 1 or Equation 2, Method of manufacturing steel. Equation 1: T3 - 0.5 × T2 ≤ 235 (where 790℃ ≤ T2 ≤ 810℃) Equation 2: T3 + 2 × T2 ≤ 2260 (where 810℃ < T2 ≤ 830℃) (Here, T2 is the value of the second temperature (unit: ℃), and T3 is the value of the third temperature (unit: ℃))
  2. In Article 1, The steel material after performing steps (a) to (f) above satisfies a yield strength of 500 MPa or more, a low-temperature toughness of -60°C of 100 J or more, and a yield ratio of 0.8 or less, Method of manufacturing steel.
  3. In Article 1, The microstructure of the steel after performing steps (a) to (f) above consists of ferrite: 60 to 80% and tempered martensite: 20 to 40% in area fraction, Method of manufacturing steel.
  4. In Article 1, The above steel further comprises, in weight percent, one or more of phosphorus (P): greater than 0 and less than or equal to 0.012%, sulfur (S): greater than 0 and less than or equal to 0.003%, and nitrogen (N): greater than 0 and less than or equal to 0.006%, Method of manufacturing steel.
  5. In weight percent, it contains carbon (C): 0.08%–0.11%, silicon (Si): 0.10%–0.20%, manganese (Mn): 1.0%–1.2%, aluminum (Al): 0.015%–0.055%, niobium (Nb): 0.005%–0.015%, titanium (Ti): 0.012%–0.022%, chromium (Cr): 0.45%–0.55%, molybdenum (Mo): 0.45%–0.55%, nickel (Ni): 1.0%–1.4%, copper (Cu): 0.1%–0.2%, boron (B): 0.001%–0.002% or less, and the remainder being iron and unavoidable impurities, Satisfying a yield strength of 500 MPa or more, a low-temperature toughness of -60℃ of 100 J or more, and a yield ratio of 0.8 or less, and The final microstructure consists of ferrite: 60–80% and tempered martensite: 20–40% by area fraction, Steel.
  6. In Article 5, In weight percent, further comprising one or more of Phosphorus (P): greater than 0 and less than or equal to 0.012%, Sulfur (S): greater than 0 and less than or equal to 0.003%, and Nitrogen (N): greater than 0 and less than or equal to 0.006%, Steel.

Description

Steel and method of manufacturing the same The present invention relates to steel and a method for manufacturing the same, and more specifically, to a high-strength heat-treatable steel having excellent low-temperature toughness and low yield performance, and a method for manufacturing the same. With the tightening of global environmental regulations, interest in the treatment of CO2 gas emitted from industry is increasing. Consequently, the demand for tanks to liquefy, capture, and store CO2 gas is rising, and the demand for steel materials required for their production is also surging. Since CO2 gas has a liquefaction point of approximately -55°C under pressure of 7 to 8 bar, materials suitable for tanks require high strength characteristics capable of withstanding high pressure and low temperature, as well as excellent low-temperature toughness. In addition, stress relief in welded joints is crucial when manufacturing gas tanks. Generally, post-weld heat treatment (PWHT) or mechanical stress relief (MSR) using hydraulic pressure are used for stress relief; however, for large tanks, the use of mechanical stress relief is unavoidable because heat treatment is difficult. Nevertheless, since the use of high hydraulic pressure makes material deformation likely, steel materials used for this purpose require a low yield ratio in addition to high strength and low-temperature toughness. FIG. 1 is a flowchart illustrating a method for manufacturing steel according to one embodiment of the present invention. Figure 2 is a graph showing the temperature over time in a method for manufacturing steel according to one embodiment of the present invention. FIG. 3 is a graph illustrating the correlation between the second temperature (unit: ℃) at which the second heat treatment is performed and the third temperature (unit: ℃) at which the tempering heat treatment is performed in the comparative example and embodiment according to the first experimental example of the present invention. Figure 4 is a photograph showing the microstructure of a steel material after the tempering process according to the first experimental example of the present invention. Figure 5 is a photograph showing the microstructure of a steel material before and after the tempering process according to an embodiment of the second experimental example of the present invention. Figure 6 is a photograph showing the microstructure of a steel material before the tempering process according to a comparative example among the second experimental examples of the present invention. Figure 7 is a graph showing the change in stress according to the elongation rate in steel materials according to the example and comparative example of the second experimental example of the present invention. The present invention will be described in detail below. However, in describing the present invention, if it is determined that a detailed description of related known technologies or configurations may unnecessarily obscure the essence of the present invention, such detailed description will be omitted. Furthermore, the terms described below are defined in consideration of their functions in the present invention; since these may vary depending on the intentions or practices of the user or operator, their definitions should be based on the content throughout this specification describing the present invention. The present invention provides a heat-treatable steel with excellent low-temperature toughness and low yield performance, satisfying a yield strength of 500 MPa or more, a low-temperature toughness of 100 J or more at -60°C, and a yield ratio of 0.8 or less, and having a final microstructure consisting of ferrite: 60 to 80% and tempered martensite: 20 to 40% in area fraction, and a method for manufacturing the same. steel plate A steel sheet according to one embodiment of the present invention comprises, in weight percent, carbon (C): 0.08%~0.11%, silicon (Si): 0.10%~0.20%, manganese (Mn): 1.0%~1.2%, aluminum (Al): 0.015%~0.055%, niobium (Nb): 0.005%~0.015%, titanium (Ti): 0.012%~0.022%, chromium (Cr): 0.45%~0.55%, molybdenum (Mo): 0.45%~0.55%, nickel (Ni): 1.0%~1.4%, copper (Cu): 0.1%~0.2%, boron (B): 0.001%~0.002% or less, and the remainder being iron and unavoidable impurities. In addition, the steel plate may further contain one or more of, in weight percent, phosphorus (P): greater than 0 and less than or equal to 0.012%, sulfur (S): greater than 0 and less than or equal to 0.003%, and nitrogen (N): greater than 0 and less than or equal to 0.006%. Hereinafter, the role and content of each component included in the steel plate according to one embodiment of the present invention are described. The following contents refer to weight percent. Carbon (C): 0.08%~0.11% Carbon (C) is the primary element that forms a martensite constituent (MA), which is susceptible to toughness. While excessive carbon content is detrimental to toughness, the addition of an appropriate am