US-20260125774-A1 - COLD-ROLLED STEEL SHEET AND METHOD FOR MANUFACTURING SAME

US20260125774A1US 20260125774 A1US20260125774 A1US 20260125774A1US-20260125774-A1

Abstract

The present invention provides a cold-rolled steel sheet including 0.15 to 0.20% by weight of carbon (C), 1.0 to 2.0% by weight of silicon (Si), 1.5 to 3.0% by weight of manganese (Mn), greater than 0% by weight and 0.02% by weight or less of phosphorus (P), greater than 0% by weight and 0.003% by weight or less of sulfur (S), 0.01 to 0.3% by weight of aluminum (Al), greater than 0% by weight and 0.01% by weight or less of nitrogen (N), 48/14·[N] to 0.1% by weight of titanium (Ti) ([N] is a content (% by weight) of nitrogen), remaining iron (Fe), and other unavoidable impurities, wherein a final microstructure of the cold-rolled steel sheet consists of ferrite, needle-shaped residual austenite, a martensite/austenite composite structure, and block-shaped martensite; the area fraction of the ferrite may be 30 to 60%, the area fraction of the needle-shaped residual austenite may be 5 to 12%, the area fraction of the martensite/austenite composite structure may be 25 to 50%, and the area fraction of the block-shaped martensite may be 5 to 12%; and an amount of carbon concentrated in residual austenite may be 1.1% by weight or more.

Inventors

Kyeong Min KIM
Hyun Seong Noh
Han Sol Maeng
Nam Hoon Goo
Seong Kyung Han

Assignees

HYUNDAI STEEL COMPANY

Dates

Publication Date: 20260507
Application Date: 20241101
Priority Date: 20220503

Claims (18)

1 . A cold-rolled steel sheet comprising: 0.15 to 0.20% by weight of carbon (C), 1.0 to 2.0% by weight of silicon (Si), 1.5 to 3.0% by weight of manganese (Mn), greater than 0% by weight and 0.02% by weight or less of phosphorus (P), greater than 0% by weight and 0.003% by weight or less of sulfur (S), 0.01 to 0.3% by weight of aluminum (Al), greater than 0% by weight and 0.01% by weight or less of nitrogen (N), 48/14·[N] to 0.1% by weight of titanium (Ti) ([N] is a content (% by weight) of nitrogen), and with balance being iron (Fe), wherein a final microstructure of the cold-rolled steel sheet consists of ferrite, needle-shaped residual austenite, a martensite/austenite composite structure, and block-shaped martensite; wherein an area fraction of the ferrite is 30 to 60%, an area fraction of the needle-shaped residual austenite is 5 to 12%, an area fraction of the martensite/austenite composite structure is 25 to 50%, and an area fraction of the block-shaped martensite is 5 to 12%; and wherein an amount of carbon concentrated in residual austenite is 1.1% by weight or more.
2 . The cold-rolled steel sheet according to claim 1 , wherein the ferrite is composed of polygonal ferrite and needle-shaped ferrite, and an area fraction of the needle-shaped ferrite in the ferrite is 40% or more.
3 . The cold-rolled steel sheet according to claim 1 , wherein the cold-rolled steel sheet has a tensile strength (TS) of 980 MPa or more and an elongation (El) of 23% or more.
4 . The cold-rolled steel sheet according to claim 3 , wherein the cold-rolled steel sheet has a tensile strength (TS) of 980 to 1180 MPa and an elongation (El) of 23 to 25%.
5 . A method of manufacturing a cold-rolled steel sheet, comprising: a step (a) of reheating steel comprising 0.15 to 0.20% by weight of carbon (C), 1.0 to 2.0% by weight of silicon (Si), 1.5 to 3.0% by weight of manganese (Mn), greater than 0% by weight and 0.02% by weight or less of phosphorus (P), greater than 0% by weight and 0.003% by weight or less of sulfur (S), 0.01 to 0.3% by weight of aluminum (Al), greater than 0% by weight and 0.01% by weight or less of nitrogen (N), 48/14·[N] to 0.1% by weight of titanium (Ti) ([N] is a content (% by weight) of nitrogen), and with balance being iron (Fe); a step (b) of hot-rolling the reheated steel; a step (c) of cold-rolling the hot-rolled steel; a first annealing step (d) of maintaining the cold-rolled steel at a first annealing temperature of (Ac1+30° C.) or more and (Ac3−30° C.) or less and cooling the cold-rolled steel to a cooling end point temperature of 340° C. or less; and a second annealing step (e) of maintaining the steel at a second annealing temperature of Ac or more and (Ac3−30° C.) or less, cooling the steel to a cooling end point temperature of a martensite transformation onset temperature (Ms) or more and (bainite transformation onset temperature (Bs)−15° C.) or less, and performing over-aging, wherein the second annealing temperature is lower than the first annealing temperature.
6 . The method according to claim 5 , wherein the step (a) comprises a step of reheating the steel at 1180 to 1300° C., wherein the step (b) comprises a step of performing hot-rolling at a finishing rolling temperature of 850 to 950° C. and a coiling temperature of 450 to 650° C., and wherein the step (c) comprises a step of performing cold-rolling at a reduction ratio of 40 to 70%.
7 . The method according to claim 5 , wherein the first annealing step (d) comprises a process of maintaining the cold-rolled steel at the first annealing temperature for 30 to 120 seconds and cooling the cold-rolled steel to a cooling end point temperature of 340° C. or less at a cooling rate of 15° C./s or more.
8 . The method according to claim 7 , wherein, after performing the first annealing step (d), an area fraction of ferrite in a microstructure of the steel is 30 to 50%.
9 . The method according to claim 5 , wherein the second annealing step (e) comprises a process of maintaining the steel at the second annealing temperature for 30 to 120 seconds, cooling the steel to a cooling end point temperature of a martensite transformation onset temperature (Ms) or more and (bainite transformation onset temperature (Bs)−15° C.) or less at a cooling rate of 15° C./s or more, and performing over-aging for 30 to 300 seconds.
10 . The method according to claim 9 , wherein, after performing the second annealing step (e), a microstructure of the steel is composed of ferrite, needle-shaped residual austenite, a martensite/austenite composite structure, and block-shaped martensite; wherein an area fraction of the ferrite is 30 to 60%, an area fraction of the needle-shaped residual austenite is 5 to 12%; and wherein an area fraction of the martensite/austenite composite structure is 25 to 50%, and an area fraction of the block-shaped martensite is 5 to 12%.
11 . The method according to claim 5 , wherein the cold-rolled steel sheet has a tensile strength (TS) of 980 MPa or more and an elongation (El) of 23% or more.
12 . The method according to claim 5 , wherein the cold-rolled steel sheet has a tensile strength (TS) of 980 to 1180 MPa and an elongation (El) of 23 to 25%.
13 . A cold-rolled steel sheet comprising: 0.15 to 0.20% by weight of carbon (C), 1.0 to 2.0% by weight of silicon (Si), 1.5 to 3.0% by weight of manganese (Mn), greater than 0% by weight and 0.02% by weight or less of phosphorus (P), greater than 0% by weight and 0.003% by weight or less of sulfur (S), 0.01 to 0.3% by weight of aluminum (Al), greater than 0% by weight and 0.01% by weight or less of nitrogen (N), 48/14·[N] to 0.1% by weight of titanium (Ti) ([N] is a content (% by weight) of nitrogen), with the balance being iron (Fe), wherein a final microstructure of the cold-rolled steel sheet consists of ferrite, needle-shaped residual austenite, a martensite/austenite composite structure, and block-shaped martensite.
14 . The cold-rolled steel sheet according to claim 13 , wherein an area fraction of the ferrite is 30 to 60%, an area fraction of the needle-shaped residual austenite is 5 to 12%, an area fraction of the martensite/austenite composite structure is 25 to 50%, and an area fraction of the block-shaped martensite is 5 to 12%.
15 . The cold-rolled steel sheet according to claim 13 , wherein the ferrite is composed of polygonal ferrite and needle-shaped ferrite, and an area fraction of the needle-shaped ferrite in the ferrite is 40% or more.
16 . The cold-rolled steel sheet according to claim 13 , wherein an amount of carbon concentrated in residual austenite is 1.1% by weight or more.
17 . The cold-rolled steel sheet according to claim 13 , wherein the cold-rolled steel sheet has a tensile strength (TS) of 980 or more and an elongation (El) of 23% or more
18 . The cold-rolled steel sheet according to claim 17 , wherein the cold-rolled steel sheet has a tensile strength (TS) of 980 to 1180 MPa and an elongation (El) of 23 to 25%.

Description

BACKGROUND Technical Field The present invention relates to a cold-rolled steel sheet and a manufacturing method thereof, and more particularly to a cold-rolled ultra-high strength low-carbon steel sheet having excellent formability and a manufacturing method thereof. More particularly, it relates to a cold-rolled ultra-high strength low-carbon steel sheet optimized for demanding applications requiring both high mechanical performance and superior formability. The invention is specifically designed to meet the rigorous performance criteria of modern industries, such as the automotive sector, where the combination of high tensile strength, enhanced elongation, and precise microstructural control, including area fractions of ferrite, residual austenite, and martensite, is critical. This invention also addresses the need for steel compositions that provide excellent balance between strength and formability while ensuring efficient manufacturing processes through precise control of annealing temperatures, cooling rates, and microstructure evolution. The disclosed manufacturing method ensures the production of a steel sheet with tensile strengths of 980 MPa or more, coupled with elongation properties of 23% or higher, making it suitable for advanced forming techniques used in producing complex automotive components. Background Ultra-high strength steel for automobile steel sheets is being developed to address two key factors: reducing vehicle weight in response to environmental regulatory issues and improving crash safety standards due to increasingly stringent safety regulations. However, since strength and elongation typically have a trade-off relationship, the problem of decreased formability as strength increases has emerged. As a result, several studies have been conducted to enhance the formability of high strength steel. TRIP (transformation-induced plasticity)-aided steel, which utilizes the TRIP phenomenon that transforms residual austenite into martensite during the transformation of residual austenite within a microstructure, is being developed as a 3rd generation steel sheet that can secure both high strength and high elongation. The physical properties of these TRIP-aided steels are largely determined by the phase stability and fraction of residual austenite that causes the TRIP phenomenon. Therefore, ensuring the stability of residual austenite within the microstructure is important in manufacturing the steel. SUMMARY Therefore, the present invention has been made in view of the above problems, and it is one object of the present invention to provide a cold-rolled ultra-high strength low-carbon steel sheet having excellent formability and a manufacturing method thereof. In accordance with one aspect of the present invention, provided is a cold-rolled steel sheet including 0.15 to 0.20% by weight of carbon (C), 1.0 to 2.0% by weight of silicon (Si), 1.5 to 3.0% by weight of manganese (Mn), greater than 0% by weight and 0.02% by weight or less of phosphorus (P), greater than 0% by weight and 0.003% by weight or less of sulfur (S), 0.01 to 0.3% by weight of aluminum (Al), greater than 0% by weight and 0.01% by weight or less of nitrogen (N), 48/14·[N] to 0.1% by weight of titanium (Ti) ([N] is a content (% by weight) of nitrogen), with the balance being iron (Fe), wherein a final microstructure of the cold-rolled steel sheet consists of ferrite, needle-shaped residual austenite, a martensite/austenite composite structure, and block-shaped martensite; an area fraction of the ferrite is 30 to 60%, an area fraction of the needle-shaped residual austenite is 5 to 12%, an area fraction of the martensite/austenite composite structure is 25 to 50%, and an area fraction of the block-shaped martensite is 5 to 12%; and an amount of carbon concentrated in residual austenite is 1.1% by weight or more. In the cold-rolled steel sheet, the ferrite may be composed of polygonal ferrite and needle-shaped ferrite, and an area fraction of the needle-shaped ferrite in the ferrite may be 40% or more. The cold-rolled steel sheet may have a tensile strength (TS) of 980 MPa or more and an elongation (El) of 23% or more. More particularly, the cold-rolled steel sheet may have a tensile strength (TS) of 980 to 1180 MPa and an elongation (El) of 23 to 25%. In accordance with one aspect of the present invention, provided is a cold-rolled steel sheet including 0.15 to 0.20% by weight of carbon (C), 1.0 to 2.0% by weight of silicon (Si), 1.5 to 3.0% by weight of manganese (Mn), greater than 0% by weight and 0.02% by weight or less of phosphorus (P), greater than 0% by weight and 0.003% by weight or less of sulfur (S), 0.01 to 0.3% by weight of aluminum (Al), greater than 0% by weight and 0.01% by weight or less of nitrogen (N), 48/14·[N] to 0.1% by weight of titanium (Ti) ([N] is a content (% by weight) of nitrogen), with the balance being iron (Fe), wherein a final microstructure of the cold-rolled steel sheet consists of ferrite,