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KR-20260067420-A - HOT-ROLLED STEEL SHEET AND METHOD OF MANUFACTURING THE SAME, AND STEEL PIPE AND METHOD OF MANUFACTURING THE SAME

KR20260067420AKR 20260067420 AKR20260067420 AKR 20260067420AKR-20260067420-A

Abstract

A hot-rolled steel sheet and a method for manufacturing the same, and a steel part and a method for manufacturing the same are disclosed. The hot-rolled steel sheet according to the present invention comprises, in weight %, C: 0.25~0.42%, Si: 0.05~0.4%, Mn: 0.9~2.0%, Al: 0.001~0.05%, B: 0.005% or less, Cr: 0.5% or less, Ti: 0.05% or less, Nb: 0.04% or less, and the remainder being Fe and unavoidable impurities, and satisfies a steel pipe forming index of 0.022 or more calculated by the following relationship. Relationship) Steel Pipe Forming Index = Uniform Elongation ÷ Yield Strength (In the above relationship, uniform elongation refers to the elongation at which tensile strength is reached.)

Inventors

  • 김선미
  • 박용국
  • 주솔빈
  • 송영우

Assignees

  • 주식회사 포스코

Dates

Publication Date
20260513
Application Date
20241104

Claims (18)

  1. A hot-rolled steel sheet comprising, in weight %, C: 0.25~0.42%, Si: 0.05~0.4%, Mn: 0.9~2.0%, Al: 0.001~0.05%, B: 0.005% or less, Cr: 0.5% or less, Ti: 0.05% or less, Nb: 0.04% or less, and the remainder being Fe and unavoidable impurities, satisfying a steel pipe forming index of 0.022 or more calculated by the following relationship. Relationship) Steel Pipe Forming Index = Uniform Elongation ÷ Yield Strength (In the above relationship, uniform elongation refers to the elongation at which tensile strength is reached.)
  2. In paragraph 1, The above hot-rolled steel sheet comprises, in weight %, C: 0.29~0.4%, Si: 0.05~0.3%, Mn: 1.0~1.6%, Al: 0.001~0.05%, B: 0.005% or less, Cr: 0.5% or less, Ti: 0.05% or less, Nb: 0.1% or less, and the remainder being Fe and unavoidable impurities.
  3. In paragraph 1, The above hot-rolled steel sheet is a hot-rolled steel sheet satisfying a carbon equivalent of 0.022 or more calculated by the following relationship. Relationship) Carbon Equivalent = C + Si/24 + Mn/6 + Cr/5 + Nb/40 (In the above relationship, the element symbol represents the weight % of each element.)
  4. In paragraph 1, The above hot-rolled steel sheet comprises, based on area %, a matrix of at least 30% pearlite and 10–90% ferrite, and less than 20% low-temperature hard phases (bainite, martensite, retained austenite).
  5. In paragraph 1, The above hot-rolled steel sheet has a yield strength of 550 MPa or less, a tensile strength of 610 to 880 MPa, an elongation of at least 18%, and a uniform elongation of 9% or more.
  6. In paragraph 1, The above hot-rolled steel sheet is a hot-rolled steel sheet having a work hardening index of 150 to 400 MPa, defined as the difference between tensile strength and yield strength.
  7. A method for manufacturing hot-rolled steel sheets according to claim 1, A step of reheating a steel material containing, in weight %, C: 0.25~0.42%, Si: 0.05~0.4%, Mn: 0.9~2.0%, Al: 0.001~0.05%, B: 0.005% or less, Cr: 0.5% or less, Ti: 0.05% or less, Nb: 0.04% or less, and the remainder being Fe and unavoidable impurities, at a temperature of 1050~1300℃. Hot rolling stage completed at a finishing hot rolling temperature of Ar3~1000℃; A method for manufacturing a hot-rolled steel sheet comprising the step of coiling the above hot-rolled steel material.
  8. In Paragraph 7, A method for manufacturing a hot-rolled steel sheet comprising, in the above %s: C: 0.29~0.4%, Si: 0.05~0.3%, Mn: 1.0~1.6%, Al: 0.001~0.05%, B: 0.005% or less, Cr: 0.5% or less, Ti: 0.05% or less, Nb: 0.1% or less, and the remainder being Fe and unavoidable impurities.
  9. In Paragraph 7, The above-mentioned coiling step is a method for manufacturing hot-rolled steel sheets by coiling at a temperature of 510 to 720℃,
  10. A steel pipe manufactured by forming the hot-rolled steel sheet of paragraph 1, A full-length steel pipe having a value (t/D) obtained by dividing the thickness (t) of the hot-rolled steel plate by the outer diameter (D) of the steel pipe of 0.07 or more.
  11. In Paragraph 10, The above-mentioned full-length steel pipe has a yield strength of 500 MPa or more.
  12. By a method of manufacturing the full-length steel pipe of Clause 10, A step of reheating a steel material containing, in weight %, C: 0.25~0.42%, Si: 0.05~0.4%, Mn: 0.9~2.0%, Al: 0.001~0.05%, B: 0.005% or less, Cr: 0.5% or less, Ti: 0.05% or less, Nb: 0.04% or less, and the remainder being Fe and unavoidable impurities, at a temperature of 1050~1300℃. Hot rolling stage completed at a finishing hot rolling temperature of Ar3~1000℃; A step of winding the above hot-rolled steel material; A step of manufacturing a hot-rolled steel strip by slitting the above-mentioned coiled hot-rolled steel sheet; A method for manufacturing a steel bar tube comprising the step of forming the above hot-rolled steel plate or hot-rolled steel strip into a tube by electric resistance welding.
  13. A steel pipe manufactured by drawing the full-length steel pipe of Clause 10, Drawn steel pipe with a yield strength of 590 MPa or higher and a tensile strength of 660 MPa or higher.
  14. By a method for manufacturing drawn steel pipes according to Paragraph 13, A step of reheating a steel material containing, in weight %, C: 0.25~0.42%, Si: 0.05~0.4%, Mn: 0.9~2.0%, Al: 0.001~0.05%, B: 0.005% or less, Cr: 0.5% or less, Ti: 0.05% or less, Nb: 0.04% or less, and the remainder being Fe and unavoidable impurities, at a temperature of 1050~1300℃. Hot rolling stage completed at a finishing hot rolling temperature of Ar3~1000℃; A step of winding the above hot-rolled steel material; A step of manufacturing a hot-rolled steel strip by slitting the above-mentioned coiled hot-rolled steel sheet; A step of forming the above hot-rolled steel plate or hot-rolled steel strip into a tube by electric resistance welding; A method for manufacturing a drawn steel pipe comprising the step of drawing the above-mentioned formed steel pipe.
  15. In Paragraph 14, A method for manufacturing a drawn steel pipe, further comprising a normalizing treatment step prior to the drawing step.
  16. In paragraph 15, The above normalizing treatment is a method for manufacturing drawn steel pipes in which the temperature is raised to the austenite single-phase region, specifically to the Ac3~1000℃ range, maintained for 100~10,000 seconds, and then cooled at a rate of 100℃/second or less.
  17. In Paragraph 14, A method for manufacturing a drawn steel pipe, further comprising a medium-temperature heat treatment step after the above-mentioned drawing step.
  18. In Paragraph 17, The above medium-temperature heat treatment step is a method for manufacturing drawn steel pipes by maintaining the drawn steel pipe at 300 to 700°C for 100 to 10,000 seconds.

Description

Hot-rolled steel sheet and method of manufacturing the same, and steel pipe and method of manufacturing the same The present invention relates to a technology for manufacturing hot-rolled steel sheets, and more specifically, to a hot-rolled steel sheet for hydraulic cylinder parts and a method for manufacturing the same. In addition, the present invention relates to a steel part (specifically, a steel pipe) using the hot-rolled steel sheet and a method for manufacturing the same. Hydraulic cylinders are devices that convert fluid pressure into mechanical force and are used in various fields, such as industrial machinery, construction equipment, and agricultural machinery. Increasing the hydraulic pressure used in a hydraulic cylinder has the advantage of generating greater mechanical force and, furthermore, reducing energy consumption by improving efficiency. Accordingly, there is a demand for hydraulic systems capable of applying higher hydraulic pressures, and recently, significant efforts are being made in the industrial and construction machinery sectors to develop high-pressure hydraulic cylinders. While general hydraulic cylinders have an operating pressure range of 70 to 210 bar, high-pressure hydraulic cylinders can operate at pressures of 210 bar or higher. When selecting suitable materials for the components (or parts) of a hydraulic cylinder, various factors such as the performance and durability of the hydraulic cylinder must be considered. The cylinder barrel, one of the components of a hydraulic cylinder, is a part shaped like a tube (or pipe) through which hydraulic fluid enters and the piston moves. Therefore, the cylinder barrel is made of a material suitable for withstanding fluid pressure. In particular, materials suitable for cylinder barrels used in high-pressure hydraulic cylinders require materials capable of withstanding higher hydraulic pressures. This is because when the cylinder barrel is exposed to a repetitive load environment where hydraulic pressure changes periodically, the part may be damaged or broken due to the material's lack of durability. Since the thickness of the cylinder barrel is determined based on the maximum hydraulic pressure at which the cylinder operates, thicker material is required for the cylinder barrel to withstand higher operating pressures. In addition, as the diameter of the cylinder barrel increases, the stress exerted by the hydraulic pressure inside the barrel on the barrel wall increases, requiring thicker material. Steel pipes are generally used for cylinder barrels. Generally, since steel pipes are made by processing steel plates, the lower the yield strength of the steel plate before forming, the easier it is to manufacture the pipes both technically and economically. On the other hand, as the steel plate becomes thicker, more energy is consumed to process the steel pipe from it, and as a result, the associated costs increase. In addition, in order to manufacture small-diameter steel pipes using thick steel plates, the elongation of the steel plates must also be high. In contrast, in the case of steel pipes, as the strength of the steel pipe increases, the cylinder barrel made of the steel pipe can withstand higher hydraulic pressure. On the other hand, if excessive residual stress exists in the steel pipe state, there is a problem where durability deteriorates in an environment where hydraulic pressure is repeatedly applied. Accordingly, it is necessary to manufacture steel pipes by applying steel sheets having moderately low yield strength and high elongation characteristics in the normalized state without special heat treatments such as rapid cooling or multi-stage cooling. In summary, a material is suitable that possesses low yield strength and high elongation (especially uniform elongation) when in the form of a steel plate, while simultaneously exhibiting high yield strength and tensile strength after being formed into a steel pipe. The advantages and features of the present invention and the methods for achieving them will become clear by referring to the embodiments described in detail below. However, the present invention is not limited to the embodiments disclosed below but may be implemented in various different forms. These embodiments are provided merely to ensure that the disclosure of the present invention is complete and to fully inform those skilled in the art of the scope of the invention, and the present invention is defined only by the scope of the claims. The following describes in detail the hot-rolled steel sheet for steel pipes and the method of manufacturing the same, as well as the steel component and the method of manufacturing the same, according to a preferred embodiment of the present invention. Hot-rolled steel sheets The hot-rolled steel sheet according to an embodiment of the present invention comprises, in weight %, C: 0.25~0.42%, Si: 0.05~0.4%, Mn: 0.9~2.0%, Al: 0.001~0.05%, B: 0.005% or less, Cr: 0.5% or less,