US-12624411-B2 - High-strength steel sheet having excellent formability and method for manufacturing thereof

US12624411B2US 12624411 B2US12624411 B2US 12624411B2US-12624411-B2

Abstract

Provided is a high-strength steel sheet suitable for automobile structural members, etc., and a method for manufacturing same, wherein the high-strength steel sheet has a low yield ratio and high strength and has excellent formability through improvement of ductility that may prevent processing defects such as cracks, wrinkles, or the like, during press forming.

Inventors

Sung-Kyu Kim
Jun-ho Park
Chang-Hyo SEO
Eul-Yong Choi
Sang-Ho Han

Assignees

POSCO CO., LTD

Dates

Publication Date: 20260512
Application Date: 20210616
Priority Date: 20200617

Claims (4)

1 . A steel sheet comprising, by weight %: 0.05 to 0.15% of carbon (C), 0.5% or less (excluding 0%) of silicon (Si), 2.0 to 3.0% of manganese (Mn), 0.2% or less (excluding 0%) of titanium (Ti), 0.1% or less (excluding 0%) of niobium (Nb), 0.2% or less (excluding 0%) of vanadium (V), 0.5% or less (excluding 0%) of molybdenum (Mo), 0.1% or less of phosphorus (P), 0.01% or less of sulfur (S), and a remainder of Fe and unavoidable impurities, wherein a microstructure is composed of ferrite with an area fraction of 20 to 45%, and a remainder of martensite and bainite, and a fraction of non-recrystallized ferrite, among the ferrite is 25 area % or less, and an average aspect ratio thereof (major axis:minor axis) is 1.1 to 2:1.
2 . The steel sheet of claim 1 , wherein the martensite is contained in an area fraction of 10% or less (excluding 0%).
3 . The steel sheet of claim 1 , wherein the steel sheet has a tensile strength of 980 MPa or more, a yield strength of 680 MPa or less, and an elongation of 13% or more.
4 . The steel sheet of claim 1 , wherein the steel sheet has a yield ratio of 0.8 or less.

Description

TECHNICAL FIELD The present disclosure relates to steel suitable as a material for automobiles, and more particularly, to a high-strength steel sheet having excellent formability and a method for manufacturing the same. BACKGROUND ART Recently, the use of high-strength steel is required to improve fuel efficiency and durability due to various environmental regulations and energy use regulations. In particular, as impact stability regulations of automobiles expand, high-strength steel having excellent strength is employed as a material for structural members such as members, seat rails, pillars, and the like, to improve impact resistance of vehicle bodies. These automobile parts have a complex shape according to safety and design, and are mainly manufactured by molding with a pressing mold, so high strength and high formability are required. However, the higher the strength of steel, the more advantageous it is to absorb impact energy, but in general, when the strength increases, elongation decreases, so that there is a problem in that formability may be deteriorated. In addition, when yield strength is excessively high, there is a problem in that an inflow of the material from a mold is reduced during forming, so that formability may be deteriorated. Meanwhile, high-strength steels used as automotive materials are typically dual phase steel (DP steel), transformation induced plasticity steel (TRIP steel), complex phase steel (CP steel), ferrite-bainite steel (FB steel), and the like. DP steel, ultra-high tensile steel, has a low yield ratio of about 0.5 to 0.6, so there is an advantage in that it is easy to process and has the highest elongation after TRIP steel. Accordingly, it is mainly applied to door outers, seat rails, seat belts, suspensions, arms, wheel disks, and the like. TRIP steel has excellent formability (high ductility) as it has a yield ratio in a range of 0.57 to 0.67, and is suitable for parts requiring high formability such as members, roofs, seat belts, bumper rails, and the like. CP steel is applied to side panels and underbody reinforcing materials, or the like due to high elongation and bending workability as well as a low yield ratio, and FB steel is mainly applied to suspension lower arms, wheel disks, or the like due excellent hole expandability thereof. Thereamong, DP steel is mainly composed of ferrite having excellent ductility and martensitic two-phase structure having high strength, and a trace amount of retained austenite may exist. DP steel has excellent characteristics such as low yield strength, high tensile strength, low yield ratio (YR), high work hardening rate, high ductility, continuous yield behavior, aging resistance at room temperature, and bake hardenability. However, in order to secure ultra-high strength of 980 MPa or more of tensile strength, it is necessary to increase a fraction of a hard phase such as a martensite phase, which is advantageous for strength improvement. In this case, there is a problem in that the yield strength increases so that defects such as cracks occur during press forming. In general, DP steel for automobiles manufactures slabs through steelmaking and casting processes, then [heating-rough rolling-finishing hot rolling] on the slabs to obtain hot-rolled coils and then annealing to prepare final products. Here, the annealing process is a process of being mainly performed during manufacturing cold-rolled steel sheets. The cold-rolled steel sheets are manufactured by pickling the hot-rolled coil to remove surface scales, cold rolling the same at room temperature at a constant reduction rate, and then performing an annealing process and an additional temper rolling process if necessary. The cold-rolled steel sheet (cold-rolled material) obtained by cold rolling itself is in a very hardened state and is not suitable for manufacturing parts requiring workability, so that the cold-rolled steel sheet may be softened through heat treatment in a continuous annealing furnace as s subsequent process, to improve workability. For example, in the annealing process, a steel sheet (cold rolled material) is heated to approximately 650 to 850° C. in a heating furnace and maintained for a certain period of time, thereby reducing hardness and improving workability through recrystallization and phase transformation. A steel sheet that has not been subjected to an annealing process has high hardness, particularly high surface hardness and poor workability, whereas a steel sheet subjected to an annealing process has a recrystallized structure and thus hardness, a yield point, and tensile strength are lowered, so that workability may be improved. Meanwhile, as a representative method for lowering the yield strength of DP steel, it is advantageous to make a size of ferrite coarse during continuous annealing and to form austenite with a small size and uniformity. As illustrated in FIG. 1, the continuous annealing process is performed through [heating section-soaki