High-Strength and High-Formability Steel Sheet, and Method for Manufacturing Same

This application is a continuation of International Application No. PCT/KR2023/016860 filed on Oct. 27, 2023, which claims under 35 U.S.C. § 119(a) the benefit of Korean Patent Application No. 10-2022-0189702 filed on Dec. 29, 2022, the entire contents of which applications are incorporated by reference herein.

The present disclosure relates to a steel material, and more particularly, to a steel sheet with high strength and high formability, and a method of manufacturing the same.

Automotive steel sheets have been developed with a focus on increasing strength to ensure user safety and lightweight the vehicle body, as well as achieving elongation to facilitate processing. Typical ultra-high-strength steels currently in use include dual-phase steel, which achieves elongation through two phases of ferrite and martensite, and transformation-induced plasticity (TRIP) steel, which achieves both strength and elongation through the phase transformation of retained austenite in the final microstructure during plastic deformation. However, the development based on dual-phase steel, which cannot overcome the limitations of the rule of mixture (ROM), and TRIP steel, which has relatively lower strength due to its bainite matrix, has reached its limits. Therefore, the development of next-generation ultra-high-strength automotive steel sheets capable of achieving ultra-high strength and high formability by improving the microstructure of TRIP steel is attracting attention from steel manufacturers. The prior art document includes Korean Patent Application No. 10-2016-0077463.

In aspects, the present disclosure provides a high-strength and high-formability steel sheet and a method of manufacturing the same.

However, the above description is an example, and the scope of the present disclosure is not limited thereto.

The present disclosure provides a high-strength and high-formability steel sheet and a method of manufacturing the same.

According to an aspect of the present disclosure, there is provided a high-strength and high-formability steel sheet including carbon (C): 0.1 wt % to 0.3 wt %, silicon (Si): 1.0 wt % to 2.0 wt %, manganese (Mn): 1.5 wt % to 3.0 wt %, aluminum (Al): more than 0 wt % and up to 0.05 wt %, phosphorus (P): more than 0 wt % and up to 0.02 wt %, sulfur (S): more than 0 wt % and up to 0.005 wt %, nitrogen (N): more than 0 wt % and up to 0.006 wt %, and a balance of iron (Fe) and other unavoidable impurities, wherein contents of C, Mn, and Si meet a relationship of X+0.066×X+0.043×X≤0.4, and preferably wherein the high-strength and high-formability steel sheet meets a yield strength (YS): 500 MPa or more, a tensile strength (TS): 980 MPa or more, a total elongation (T.EL): 23% or more, and a product of tensile strength and elongation: 23,000 MPa % or more.

In a further aspect, a high-strength and high-formability steel sheet is provided that consists essentially of or consists of carbon (C): 0.1 wt % to 0.3 wt %, silicon (Si): 1.0 wt % to 2.0 wt %, manganese (Mn): 1.5 wt % to 3.0 wt %, aluminum (Al): more than 0 wt % and up to 0.05 wt %, phosphorus (P): more than 0 wt % and up to 0.02 wt %, sulfur (S): more than 0 wt % and up to 0.005 wt %, nitrogen (N): more than 0 wt % and up to 0.006 wt %, and a balance of iron (Fe) and other unavoidable impurities, wherein contents of C, Mn, and Si meet a relationship of X+0.066×X+0.043×X≤0.4, and preferably wherein the high-strength and high-formability steel sheet meets a yield strength (YS): 500 MPa or more, a tensile strength (TS): 980 MPa or more, a total elongation (T.EL): 23% or more, and a product of tensile strength and elongation: 23,000 MPa % or more.

The high-strength and high-formability steel sheet may have a mixed structure of retained austenite, ferrite, and martensite/tempered martensite, an area fraction of ferrite may be 20% to 50%, an area fraction of retained austenite may be 5% to 20%, and an area fraction of martensite/tempered martensite may be a remaining area fraction. Herein, “martensite/tempered martensite” is defined as a combination of fresh martensite and tempered martensite.

The high-strength and high-formability steel sheet may have a uniform elongation/total elongation ratio of 0.7 or more and less than 1.

When 5% plastic deformation is applied in a direction perpendicular to a rolling direction of the high-strength and high-formability steel sheet, a reduction rate in an area fraction of retained austenite of the high-strength and high-formability steel sheet before and after the application may be more than 0% and no more than 50%.

The high-strength and high-formability steel sheet may further include a combination of titanium (Ti), niobium (Nb), and vanadium (V): more than 0 wt % and up to 0.05 wt %.

According to another aspect of the present disclosure, there is provided a method of manufacturing a high-strength and high-formability steel sheet, the method including producing a hot-rolled steel sheet by hot rolling a steel material including carbon (C): 0.1 wt % to 0.3 wt %, silicon (Si): 1.0 wt % to 2.0 wt %, manganese (Mn): 1.5 wt % to 3.0 wt %, aluminum (Al): more than 0 wt % and up to 0.05 wt %, phosphorus (P): more than 0 wt % and up to 0.02 wt %, sulfur (S): more than 0 wt % and up to 0.005 wt %, nitrogen (N): more than 0 wt % and up to 0.006 wt %, and a balance of iron (Fe) and other unavoidable impurities; producing a cold-rolled steel sheet by cold rolling the hot-rolled steel sheet; performing annealing by heating the cold-rolled steel sheet at a heating rate of 1° C./s to 10° C./s to a temperature higher than 780° C. and lower than 840° C., and holding for 50 sec. to 110 sec.; multi-stage cooling the cold-rolled steel sheet; and performing post-annealing by heating the cold-rolled steel sheet at a heating rate of 20° C./s or more to a temperature higher than 380° C. and lower than 450° C., and holding for 10 sec. to 240 sec., wherein contents of C, Mn, and Si meet a relationship of X+0.066×X+0.043×X≤0.4.

The producing of the hot-rolled steel sheet may include reheating a steel material with the alloy composition at 1,150° C. to 1,250° C.; producing a hot-rolled steel sheet by hot rolling the reheated steel material at a finishing delivery temperature (FDT) of 850° C. to 1,000° C. with a cumulative reduction ratio of 70% or more and 90% or less; cooling the hot-rolled steel sheet at a cooling rate of 10° C./s to 30° C./s to 500° C. to 700° C.; and coiling the hot-rolled steel sheet at 500° C. to 700° C., and the hot-rolled steel sheet may have a mixed structure of ferrite and pearlite, with an area fraction of ferrite being 20% to 50%, and an area fraction of pearlite being a remaining area fraction.

The multi-stage cooling may include primarily cooling the cold-rolled steel sheet at a cooling rate of 1° C./s to 10° C./s to 550° C. to 750° C.; and secondarily cooling the cold-rolled steel sheet at a cooling rate of 50° C./s or more to a temperature higher than 180° C. and lower than 240° C., and holding for 5 sec. to 20 sec.

The high-strength and high-formability steel sheet manufactured by performing the method may meet a yield strength (YS): 500 MPa or more, a tensile strength (TS): 980 MPa or more, a total elongation (T.EL): 23% or more, and a product of tensile strength and elongation: 23,000 MPa % or more; have a mixed structure of retained austenite, ferrite, and martensite/tempered martensite, with an area fraction of ferrite being 20% to 50%, an area fraction of retained austenite being 5% to 20%, and an area fraction of martensite/tempered martensite being a remaining area fraction; and have a uniform elongation/total elongation ratio of 0.7 or more and less than 1 and, when 5% plastic deformation is applied in a direction perpendicular to a rolling direction of the high-strength and high-formability steel sheet, a reduction rate in the area fraction of retained austenite of the high-strength and high-formability steel sheet before and after the application may be more than 0% and no more than 50%.

As referred to herein, yield strength (YP) and tensile stress (TS) and elongation (EL) can be measured using a commercially available tensile tester and according to the ASTM standard ASTM E8/E8M.

Total elongation of steel is the percentage increase in the gauge length of a steel specimen, measured from the start of a tensile test until fracture, according to standardized test methods (e.g., ASTM E8/E8M). It represents the total amount of stretching the material undergoes, including both uniform and localized deformation.

According to the present disclosure, a high-strength and high-formability steel sheet capable of obtaining a microstructure consisting of retained austenite, ferrite, and martensite/tempered martensite through the control of the composition system and process conditions, and of meeting a yield strength (YS): 500 MPa or more, a tensile strength (TS): 980 MPa or more, a total elongation (T.EL): 23% or more, and a product of tensile strength and elongation: 23,000 MPa % or more may be manufactured. The effect of the present disclosure is to provide an ultra-high-strength and high-formability steel sheet and process condition design capable of enabling the maintenance of the final microstructure compared to cold-rolled steel sheets.

The above-described effects of the present disclosure are examples, and the scope of the present disclosure is not limited thereto.

Hereinafter, the present disclosure will be described in detail by explaining embodiments of the disclosure with reference to the attached drawings. The disclosure may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of the disclosure to one of ordinary skill in the art. Like reference numerals refer to like elements throughout. Further, various elements and regions in the drawings are schematically illustrated. Therefore, the scope of the present disclosure is not limited by the relative sizes or distances shown in the attached drawings.

To overcome the limitations of the mechanical properties of existing transformation-induced plasticity (TRIP) steel, the development of next-generation ultra-high-strength automotive steel sheets capable of achieving high strength and appropriate elongation by replacing the main matrix from bainite to martensite is attracting attention from various steel manufacturers. According to a conventional method, when a composite structure consisting of ferrite, annealed martensite, and retained austenite is formed, high strength and high elongation are achieved, but a high yield ratio caused by a low fraction of ferrite reduces workability. According to another conventional method, although the volume fraction of ferrite is increased to achieve workability, the requirements such as a tensile strength of 1000 MPa or more, an elongation of 20% or more, and a product of tensile strength and elongation of 20,000 MPa % or more are not met. In another conventional method, although high strength, appropriately high formability, and workability are achieved, high carbon content reduces weldability. According to another conventional method, although cold-rolled steel sheets with excellent burring workability and high strength are produced with a composite structure of ferrite, annealed martensite, retained austenite, and bainite, due to the limitations of heat treatment conditions, they may not be easily produced with a general continuous galvanized line (CGL). For example, a longer overaging time is required compared to the general CGL.

When automotive body parts are formed conventionally, the fracture of parts that occurs during a forming process with an ultra-high-strength material may be explained according to the evaluation criteria such as drawability and biaxial stretchability, which may be checked on a general forming limit diagram, and a hole expansion ratio, which may not be checked on the forming limit diagram. Normally, steel sheets for producing automotive body parts require excellent formability evaluation results for application to complexly formed structures. These formability indices serve as crucial elements in forming automotive body parts, which are mostly processed by press working. In general, ultra-high-strength materials tend to have reduced elongation as their strength increases. To form such ultra-high-strength materials, steel materials are being developed by applying special forming processes or modified mechanisms capable of enhancing formability.

When conventional ultra-high-strength steel with a dual-phase structure consisting of ferrite and martensite undergoes plastic deformation, dislocations form and move in the structure. Plastic deformation occurs according to a basic deformation mechanism for generating fracture due to the formation and growth of defects caused by the movement of the dislocations. Under the influence of this deformation mechanism, to secure strength, hard phases such as martensite and bainite are formed to achieve strength. However, the increase in the fraction of hard phases reduces elongation. To compensate for the reduction in elongation, a soft phase such as ferrite is formed in the microstructure. In ultra-high-strength steel with the above-described final microstructure, strength and elongation follow the rule of mixture (ROM), and thus the enhancement in properties beyond the ROM is not easily achieved.

To improve the properties of the ultra-high-strength steel with a dual-phase structure consisting of ferrite and martensite, TRIP steel is developed. The TRIP steel may obtain retained austenite in the final microstructure and achieve strength and elongation through the phase transformation of retained austenite during plastic deformation. However, in the TRIP steel, the area fraction of retained austenite contained in the final microstructure is small, and thus the improvement in formability may not be significant.

The present disclosure arms to improve the formability of ultra-high-strength steel by obtaining retained austenite in the final microstructure. As such, using a three-phase microstructure consisting of ferrite, retained austenite, and martensite/tempered martensite, improved formability compared to conventional ultra-high-strength steels may be achieved. Additionally, the present disclosure also aims to develop a cold-rolled steel sheet capable of meeting the material requirements while maintaining the microstructure of ferrite, retained austenite, and martensite/tempered martensite through composition system and heat treatment control. The composition system and heat treatment ranges are proposed based on simulation results.

Retained austenite is a structure that is advantageous in achieving the strength, elongation, and formability of the steel sheet through the TRIP mechanism. However, when it is excessively included, large amounts of alloying elements may be required to stabilize the TRIP mechanism, and the hydrogen embrittlement resistance may decrease. Therefore, the fraction of retained austenite may be 5% to 20%, the fraction of ferrite may be 20% to 50%, and the remaining fraction may consist of a combination of tempered martensite and martensite.

Therefore, the method for achieving yield strength, tensile strength, elongation, and hole expansion ratio proposed in the present disclosure through the formation of the above microstructures may be summarized as follows.

{circle around (1)} Steelmaking, continuous casting, hot rolling, and cold rolling are performed using a composition system in which silicon is controlled under optimal conditions to adjust carbon redistribution behavior by inhibiting the formation of carbides and austenite-stabilizing elements such as carbon and manganese to obtain retained austenite in the final microstructure after annealing.

{circle around (2)} The microstructure proposed in the present disclosure is obtained through intercritical annealing-rapid cooling-reheating control by using the obtained cold-rolled coil.

With regard to the design direction {circle around (1)}, strength that may be lacking when achieving elongation and a hole expansion ratio is achieved using TRIP of tempered martensite and retained austenite.

With regard to the design direction {circle around (2)}, elongation is achieved by obtaining a soft phase (e.g., ferrite) in the final microstructure of existing ultra-high-strength steel, and enhanced by obtaining more retained austenite in the final microstructure, as used in TRIP steels.

The present disclosure provides a next-generation ultra-high-strength automotive steel sheet capable of achieving high strength, high elongation, and excellent formability by replacing the main matrix of TRIP steel from bainite to martensite, to overcome the limitations of the mechanical properties of existing TRIP steel.

A high-strength and high-formability steel sheet according to an embodiment of the present disclosure will now be described.

The high-strength and high-formability steel sheet according to an embodiment of the present disclosure may stably achieve both high tensile strength and high elongation by controlling the final microstructure based on process conditions suitable for mass production.

A high-strength and high-formability steel sheet according to an embodiment of the present disclosure includes carbon (C): 0.1 wt % to 0.3 wt %, silicon (Si): 1.0 wt % to 2.0 wt %, manganese (Mn): 1.5 wt % to 3.0 wt %, aluminum (Al): more than 0 wt % and up to 0.05 wt %, phosphorus (P): more than 0 wt % and up to 0.02 wt %, sulfur (S): more than 0 wt % and up to 0.005 wt %, nitrogen (N): more than 0 wt % and up to 0.006 wt %, and the balance of iron (Fe) and other unavoidable impurities.

The high-strength and high-formability steel sheet may further include a combination of titanium (Ti), niobium (Nb), and vanadium (V): more than 0 wt % and up to 0.05 wt %.

The functions and contents of the components included in the high-strength and high-formability steel sheet according to the present disclosure will now be described. In this case, the unit for the content of each constituent element is wt % relative to the total weight of the steel sheet.

C is the most crucial alloying element in steelmaking, and is primarily intended for basic strengthening and austenite stabilization. A high concentration of C in austenite may increase the stability of austenite and thus appropriate austenite for enhancing the material properties may be easily obtained. When the content of C is less than 0.1 wt %, the desired yield strength and elongation may not be easily achieved. When the content of C is greater than 0.3 wt %, the increase in C equivalent may decrease weldability. Therefore, the content of C may be 0.1 wt % to 0.3 wt % of the total weight of the steel sheet.

Si is a ferrite-stabilizing element which inhibits the formation of carbides (e.g., FeC) in ferrite, and accelerates the diffusion rate of austenite by increasing the activity of C. As a ferrite-stabilizing element, Si is well-known as an element for improving ductility by increasing the fraction of ferrite during cooling. When the content of Si is less than 1.0 wt %, the Si addition effect is insufficient. When the content of Si is greater than 2.0 wt %, the formation of oxides (e.g., SiO) on the surface of the steel sheet during the process may deteriorate coatability due to the decrease in wettability thereon. Therefore, the content of Si may be 1.0 wt % to 2.0 wt % of the total weight of the steel sheet.

Mn is an austenite-stabilizing element. When Mn is added, the martensite start temperature, Ms, may gradually decrease and thus the fraction of retained austenite may increase during continuous annealing. When the content of Mn is less than 1.5 wt %, the Mn addition effect is insufficient. When the content of Mn is greater than 3.0 wt %, the increase in C equivalent may decrease weldability, and the formation of oxides (e.g., MnO) on the surface of the steel sheet during the process may deteriorate coatability due to the decrease in wettability thereon. Therefore, the content of Mn may be 1.5 wt % to 3.0 wt % of the total weight of the steel sheet.

Aluminum (Al): More than 0 wt % and Up to 0.05 wt %

Al is used as a deoxidizer, stabilizes both ferrite and retained austenite in combination with Si, and primarily serves to strengthen the solid solution and inhibit the formation of carbides. When the content of Al is greater than 0.05 wt %, the formation of AlN during slab production may cause cracks during casting or hot rolling. Therefore, the content of Al may be more than 0 wt % and up to 0.05 wt % of the total weight of the steel sheet.

Combination of Titanium (Ti), Niobium (Nb), and Vanadium (V): More than 0 wt % and Up to 0.05 wt %

Ti, V, and Nb are major elements precipitated in the form of carbides inside steel. Ti, V, and Nb are added to achieve the stability of retained austenite and enhance strength by refining initial austenite grains through the formation of precipitates, and to enable precipitation hardening through the refinement of ferrite grains and the presence of precipitates in ferrite. When the total content of Ti, V, and Nb is greater than 0.05 wt %, a degradation in material properties and an increase in production costs may be caused. Therefore, when optionally included, the total content of Ti, Nb, and V may be more than 0 wt % and up to 0.05 wt % of the total weight of the steel sheet.

The steel sheet may optionally include at least one of Ti, Nb, and V. As such, the content of Ti may be 0 wt % to 0.05 wt % of the total weight of the steel sheet, the content of Nb may be 0 wt % to 0.05 wt % of the total weight of the steel sheet, and the content of V may be 0 wt % to 0.05 wt % of the total weight of the steel sheet.

Phosphorus (P): More than 0 wt % and Up to 0.02 wt %

P is an impurity introduced while producing steel, and may contribute to strength enhancement based on solid solution strengthening. However, an excessive amount of P may cause low-temperature brittleness. Therefore, the content of P needs to be limited to more than 0 wt % and up to 0.02 wt % of the total weight of the steel sheet.

Sulfur (S): More than 0 wt % and Up to 0.005 wt %

S is an impurity introduced while producing steel, and may decrease toughness and weldability by forming non-metallic inclusions such as FeS and MnS. Therefore, the content of S needs to be limited to more than 0 wt % and up to 0.005 wt % of the total weight of the steel sheet.