High Strength Cold-Rolled Steel Sheet and Manufacturing Method Therefor

This application is a continuation of International Application No. PCT/KR2023/020876 filed on Dec. 18, 2023, which claims under 35 U.S.C. § 119 (a) the benefit of Korean Patent Application No. 10-2022-0189693 filed on Dec. 29, 2022, the entire contents of which applications are incorporated by reference herein.

The present disclosure relates to a cold-rolled steel plate, and more particularly a high-strength cold-rolled steel plate having high strength-elongation balance and excellent flatness and a method of manufacturing the same.

Recently, various environmental and energy use regulations have lead to the demand for high-strength steel plates for improved fuel efficiency and durability. For steel plates for automobiles, it is important to increase strength to ensure user safety and reduce the weight of a car body, and to secure elongation to facilitate processing. Among general ultra-high strength steels currently in use, there are dual-phase steels that secure strength and elongation through two phases of ferrite and martensite, and transformation-induced plasticity steels that simultaneously secure strength and elongation through phase transformation of residual austenite remaining in the structure during plastic deformation. Transformation-induced plasticity steels secure strength by forming a martensite matrix and secures elongation by forming retaining austenite there inside. It has been proposed to produce steel with improved mechanical properties and good moldability by using so-called quenching and partitioning as methods of making high-strength transformation-induced plasticity steel based on martensite. To secure high strength and high moldability, an appropriate fraction of retained austenite and the stability of retained austenite are required. Transformation-induced plasticity steels generally contain bainite, so it is difficult to secure elongation. In addition, to improve stability during operation, it is necessary to control the flatness of a steel plate below a certain height by suppressing the deformation of the steel plate that frequently occurs during quenching.

In one aspect, the present disclosure provides a cold-rolled steel plate having excellent flatness while improving the mechanical properties of a steel plate by improving the stability of retained austenite and a method of manufacturing the same. It will be understood that the technical problems are only provided as examples, and the technical idea of the present disclosure is not limited thereto.

In accordance with an aspect of the present disclosure, the above and other objects can be accomplished by the provision of a high-strength cold-rolled steel plate.

According to an embodiment of the present disclosure, a high-strength cold-rolled steel plate is provided and suitably includes: in % by weight, carbon (C): 0.1% to 0.3%, silicon (Si): 1.0% to 2.0%, manganese (Mn): 1.5% to 3.0%, aluminum (Al): 0.01% to 0.05% or less, phosphorus (P): 0.02% or less, sulfur(S): 0.005% or less, a remainder being iron (Fe) and other inevitable impurities.

According to an embodiment of the present disclosure, the high-strength cold-rolled steel plate has a microstructure comprising, by area ratio, 25 to 35% ferrite, 10 to 18% retained austenite, 5% or less M-A (martensite-austenite composite phase) and a remainder being martensite.

According to an embodiment of the present disclosure, the high-strength cold-rolled steel plate preferably has a flatness (deformation height) of 3.0 mm or less.

According to an embodiment of the present disclosure, the high-strength cold-rolled steel plate suitably may further include one or more of niobium (Nb), titanium (Ti) and vanadium (V), wherein the total content of niobium (Nb), titanium (Ti) and vanadium (V) is 0.05% or less (greater than 0).

According to an embodiment of the present disclosure, the carbon concentration in the retained austenite may be 1.1% to 1.4% in % by weight.

According to an embodiment of the present disclosure, the high-strength cold-rolled steel plate may have a yield strength (YS) of 550 MPa or more, a tensile strength (TS) of 980 MPa or more, an elongation index (EI) of 20% or more and a tensile strength*elongation index of 20,000 MPa % or more.

According to an embodiment of the present disclosure, the martensite may include fresh martensite and tempered martensite, and a value (FM/TM) obtained by dividing the area ratio of the fresh martensite (FM) by the area ratio of the tempered martensite (TM) may range from 0.1 to 0.6.

According to an embodiment of the present disclosure, an area ratio of retained austenite having an aspect ratio of 3 or more, which is obtained by dividing a major axis length of the retained austenite by a minor axis length thereof, may range from 3% to 8%.

According to an embodiment of the present disclosure, a value obtained by dividing the total area ratio of the retained austenite by the area ratio of the retained austenite having an aspect ratio of 3 or more may range from 0.5 to 0.8.

In accordance with another aspect of the present disclosure, there is provided a method of manufacturing a high-strength cold-rolled steel plate.

In one aspect, a method of manufacturing a high-strength cold-rolled steel plate is provided, the method suitably comprising:

In aspects, first cooling the annealed cold-rolled steel plate is at a first cooling rate in a first cooling end temperature of 650° C. to 800° C.

According to an embodiment of the present disclosure, the method of manufacturing a high-strength hot-rolled steel plate suitably comprises: manufacturing a hot-rolled steel plate by hot-rolling a steel comprising: in % by weight, carbon (C): 0.1% to 0.3%, silicon (Si): 1.0% to 2.0%, manganese (Mn): 1.5% to 3.0%, aluminum (Al): 0.01% to 0.05% or less, phosphorus (P): 0.02% or less, sulfur(S): 0.005% or less, a remainder being iron (Fe) and other inevitable impurities; manufacturing a cold-rolled steel plate by cold-rolling the hot-rolled steel plate; annealing the cold-rolled steel plate in a dual-phase temperature range of austenite and ferrite; first cooling the annealed cold-rolled steel plate at a first cooling rate in a first cooling end temperature of 650° C. to 800° C.; second cooling the first cooled cold-rolled steel plate at a cooling rate higher than the first cooling rate of the first cooling; partitioning by re-heating the second cooled cold-rolled steel plate, and by third cooling at a cooling rate of 0.07° C./sec to 0.21° C./sec immediately when reaching a target temperature of 350° C. to 460° C.

According to an embodiment of the present disclosure, the second cooling of the above methods suitably may include first rapid cooling and second rapid cooling, wherein the first rapid cooling is performed by cooling up to a first rapid cooling end temperature of Ms (martensite transformation start temperature) or higher at a cooling rate of 70° C./sec to 110° C./sec, and the second rapid cooling is performed by cooling up to a second rapid cooling end temperature being lower than Ms and above Mf (martensite transformation end temperature) at a cooling rate of 30° C./sec or higher and lower than 70° C./sec.

According to an embodiment of the present disclosure, the first rapid cooling end temperature of the above methods suitably may range from 320° C. to 350° C., and the second rapid cooling end temperature may range from 200° C. to 260° C.

According to an embodiment of the present disclosure, the partitioning of the methods suitably may be performed for a time ranging from 30 sec to 600 sec.

According to an embodiment of the present disclosure, the dual-phase temperature range of the methods suitably may be 780° C. to 860° C.

According to an embodiment of the present disclosure, the first cooling end temperature of the methods suitably may range from 680° C. to 800° C.

According to an embodiment of the present disclosure, a third cooling end temperature during the third cooling of the methods suitably may range from 340° C. to 400° C.

In aspects, the stability of retained austenite can be improved by controlling a cooling rate in a partitioning step, thereby providing a cold-rolled steel plate having excellent strength and elongation index characteristics. In addition, the deformation of the steel plate that generally occurs during rapid cooling can be suppressed by controlling a cooling rate by section during a cooling step, so that the flatness of a final steel plate can be controlled to 3.0 mm or less, thereby providing a cold-rolled steel plate with an excellent shape and improving operational stability. The effects of the present disclosure are described as examples, and the scope of the present disclosure is not limited by these effects.

A steel composition microstructure (e.g. a cold-rolled steel plate microstructure) can suitably be determined by microscopy such as using a scanning electron microscope.

As referred to herein, a flatness (deformation height) of a steel plate can be determined by standard techniques including ASTM Standard A568/A568M (for that ASTM Standard A568/A568M, see document: A568/A568M-11B, Standard Specification for Steel, Carbon, Structural, and High-Strength, Low-Alloy, Hot-Rolled and Cold-Rolled, General Requirement for: Section X5. Alternative Methods for Expressing Flatness, X5.1 Introduction and Definitions, page 27P (ASTM International).

Hereinafter, preferred embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. Embodiments of the present disclosure are provided to more completely explain the technical idea of the present disclosure to those skilled in the art, and the following embodiments may be modified in many different forms, but the scope of the technical idea of the present disclosure is not limited to the following embodiments. Rather, the embodiments are provided to make the disclosure thorough and complete and to fully convey the technical idea of the disclosure to those skilled in the art. Like reference numerals in the specification denote like elements. Further, various elements and regions in the drawings are schematically drawn. Therefore, the technical idea of the disclosure is not limited by the relative size or spacing drawn in the accompanying drawings.

In this specification and the accompanying claims, the term “phase fraction” means an area ratio derived from a microstructure photograph using an image analyzer. In addition, the content or concentration of a specific ingredient is based on % by weight unless otherwise specified.

A high-strength cold-rolled steel plate according to an embodiment of the present disclosure includes: in % by weight, carbon (C): 0.1% to 0.3%, silicon (Si): 1.0% to 2.0%, manganese (Mn): 1.5% to 3.0%, aluminum (Al): 0.01% to 0.05% or less, phosphorus (P): 0.02% or less, sulfur(S): 0.005% or less, the remainder being iron (Fe) and other inevitable impurities. Additionally, one or more of niobium (Nb), titanium (Ti) and vanadium (V) may be further included. Here, the total content of niobium (Nb), titanium (Ti) and vanadium (V) may be limited to 0.05% or less (greater than 0).

Hereinafter, the role and content of each of components included in the high-strength cold-rolled steel plate according to the present disclosure are explained as follows. Here, the content of component element means % by weight of the entire steel plate.

Carbon is added to secure the strength of steel, and especially increases the strength of a martensite structure. In addition, it contributes to securing elongation through the TRIP effect by being diffused into retained austenite during a partitioning step to stabilize retained austenite. The content of carbon may range from 0.1% to 0.3% % by weight, for example, 0.18% to 0.22%. If the carbon content is less than 0.1% by weight, it is difficult to obtain a target strength, and if it exceeds 0.3%, it is detrimental to weldability.

Silicon is a ferrite-stabilizing element that delays the formation of carbides in ferrite and has a solid solution-strengthening effect. The content of silicon may range from 1.0% to 2.0%, for example, 1.5% to 1.9%. When the content of silicon is less than 1.0%, the effect is minimal, and when it is more than 2.0%, oxides such as MnSiOare formed during a manufacturing process, which hinders the plating properties and increases the carbon equivalent, thereby lowering weldability.

Manganese has a solid solution-strengthening effect, and contributes to the improvement of strength by increasing hardenability. The content of manganese may range from 1.5% to 3.0%, for example, 1.8% to 2.2%. When the manganese content is less than 1.5%, retained austenite is not secured sufficiently and the transformation-induced plasticity effect is not sufficient, making it difficult to secure strength. When it exceeds 3.0%, the processability is reduced and the delayed fracture resistance is reduced due to the formation or segregation of inclusions such as MnS, and the carbon equivalent is increased, which may reduce weldability.

Aluminum is used as a deoxidizer and, similar to silicon, can help suppress carbide formation. The content of aluminum ranges from 0.01% to 0.05%. When the aluminum content is less than 0.01%, the deoxidation effect may be insufficient, and when it exceeds 0.05%, AlN may be formed during slab manufacturing, which may cause cracks during casting or hot rolling.

Phosphorus (P): Greater than 0% and 0.02% or Less

Phosphorus is an impurity included in a steel manufacturing process, and it is desirable to limit it to 0.02% or less. When adding phosphorus, it can help improve strength through solidification improvement, but when added in excess of 0.02%, it may cause low-temperature brittleness.

Sulfur(S): Greater than 0% and 0.005% or Less

Sulfur is an impurity included in a steel manufacturing process and it is desirable to limit it to 0.005% or less. Sulfur is limited to 0.005% or less because it forms non-metallic inclusions such as FeS and MnS, which reduces toughness and weldability.

Total Content of Niobium (Nb), Titanium (Ti) and Vanadium (V): Greater than 0% and 0.05% or Less

Optionally, one or more elements of titanium, niobium and vanadium may be additionally added. Titanium, niobium and vanadium may be precipitated as carbides or nitrides in steel, causing precipitation hardening and contributing to grain refinement. However, precipitation hardening by precipitates is not the purpose of the present disclosure. When added in large amounts, there are disadvantages such as reduced elongation and increased manufacturing cost, so the total amount of titanium, niobium, and vanadium is limited to 0.05% or less.

The remaining component of the high-strength cold-rolled steel plate is iron (Fe). However, since unintended impurities from raw materials or a surrounding environment may inevitably be mixed during a normal steelmaking process, this cannot be ruled out. These impurities are known to anyone skilled in the art of manufacturing and therefore are not specifically mentioned in this specification.

The cold-rolled steel plate according to one embodiment of the present disclosure includes, by area ratio, 25 to 35% ferrite, 10 to 18% retained austenite, 5% or less M-A (martensite-austenite composite phase) and the remainder being martensite. The martensite includes at least one of fresh martensite (FM) and tempered martensite (TM).

The area ratio of fresh martensite (FM) divided by the area ratio of tempered martensite (which can be expressed as FM/TM) ranges from 0.1 to 0.6.

In the case of retained austenite, the total area ratio of retained austenite may be expressed as RA, and the area ratio of retained austenite with an aspect ratio (a major axis length divided by a minor axis length) of 3 or more may be expressed as RA. The RAof cold-rolled steel plate according to one embodiment of the present disclosure may be 3% or more, for example, 3% to 8%, and RAdivided by RA(which may be expressed as RA/RA) ranges from 0.5 to 0.8.

The cold-rolled steel plate according to one embodiment of the present disclosure can secure stability by increasing the carbon concentration in retained austenite. The carbon concentration in the retained austenite may range from 1.1 to 1.4% in % by weight.

The cold-rolled steel plate of the present disclosure satisfies the following values simultaneously: yield strength (YS) of 550 MPa or more, tensile strength (TS) of 980 MPa or more, elongation index (EI) of 20% or more and tensile strength*elongation index of 20,000 MPa % or more. For example, the yield strength may range from 550 to 700 MPa, the tensile strength may range from 980 to 1200 MPa, the elongation index may range from 20 to 25%, and the tensile strength*elongation index may range from 20,000 to 25,000 MPa %. In addition, the flatness (deformation height) of the steel plate is controlled to 3.0 mm or less.

Hereinafter, a method of manufacturing a high-strength cold-rolled steel plate according to an embodiment of the present disclosure, which has the microstructure and physical properties described above in the composition range described above, is described with reference to the accompanying drawings.

In the manufacturing method according to the present disclosure, a semi-finished product to be subject to a hot-rolling process may be, for example, a slab. Slabs in a semi-finished product state can be obtained through a continuous casting process after obtaining molten steel with a certain composition through a steelmaking process.

is a flowchart illustrating a step-by-step method of manufacturing the high-strength cold-rolled steel plate according to an embodiment of the present disclosure.