HIGH-STRENGTH AND HIGH-FORMABILITY STEEL SHEET, AND METHOD FOR MANUFACTURING SAME

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

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.

FIELD

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.

BACKGROUND

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.

SUMMARY

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_{C, wt %}+0.066×X_{Si, wt %}+0.043×X_{Mn, wt %}≤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_{C, wt %}+0.066×X_{Si, wt %}+0.043×X_{Mn, wt %}≤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_{C, wt %}+0.066×X_{Si, wt %}+0.043×X_{Mn, wt %}≤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.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart of a method of manufacturing a high-strength and high-formability steel sheet, according to an embodiment of the present disclosure.

FIG. 2 is a microscopic image showing the microstructure of a high-strength and high-formability steel sheet according to an embodiment of the present disclosure.

DETAILED DESCRIPTION

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.

High-Strength and High-Formability Steel Sheet

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.

Carbon (C): 0.1 wt % to 0.3 wt %

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.

Silicon (Si): 1.0 wt % to 2.0 wt %

Si is a ferrite-stabilizing element which inhibits the formation of carbides (e.g., Fe₃C) 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., SiO₂) 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.

Manganese (Mn): 1.5 wt % to 3.0 wt %

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.

Nitrogen (N): More than 0 wt % and Up to 0.006 wt %

N is an element inevitably introduced while producing steel, and an excessive amount of N may lead to the precipitation of nitrides in a large amount and a decrease in ductility. Therefore, the content of N needs to be limited to more than 0 wt % and up to 0.006 wt % of the total weight of the steel sheet.

The remainder of the high-strength and high-formability steel sheet is iron (Fe). However, due to the inevitable introduction of unintended impurities from raw materials or the surrounding environment during the typical steelmaking process, the addition of impurities may not be completely excluded. These impurities are known to anyone of ordinary skill in the art and, therefore, are not particularly mentioned in this specification.

The contents of C, Mn, and Si in the high-strength and high-formability steel sheet meet the relationship of X_{C, wt %}+0.066×X_{Si, wt %}+0.043×X_{Mn, wt %}≤0.4.

The high-strength and high-formability steel sheet manufactured by controlling specific components of the above-described alloy composition and the content ranges thereof, and performing the following steel sheet manufacturing 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. For example, the high-strength and high-formability steel sheet may meet an YS: 500 MPa to 760 MPa, a TS: 980 MPa to 1180 MPa, a T.EL: 23% to 30%, and a product of tensile strength and elongation: 23,000 MPa % to 35,400 MPa %. The uniform elongation/total elongation ratio may be 0.7 or more and less than 1. This ratio refers to a ratio of the total material to a portion where uniform elongation is enhanced through TRIP behavior.

Factors influencing the material properties of the high-strength and high-formability steel sheet include the achievement of strength and elongation through the phase transformation of retained austenite due to the TRIP effect, the achievement of the stability of retained austenite and the enhancement of elongation based on ferrite, the increase in strength based on tempered martensite as the primary matrix, and the increase in strength through grain refinement and precipitation hardening. The high-strength and high-formability steel sheet has a product of tensile strength and elongation of 23,000 MPa % or more, which is generally superior to the values proposed at similar ultra-high strength levels.

When 5% plastic deformation is applied in a direction perpendicular to the rolling direction of the high-strength and high-formability steel sheet, the reduction rate in the area fraction of retained austenite of the high-strength and high-formability steel sheet before and after the application is more than 0% and no more than 50%. When the reduction rate is less than 50%, TRIP behavior does not manifest in the early stage and continues through the mid-deformation stage, thereby contributing to elongation enhancement.

The high-strength and high-formability steel sheet may have a mixed structure of retained austenite, ferrite, and martensite/tempered martensite. The area fraction of ferrite may significantly affect the overall material properties, and may be, for example, 20% to 50%. When the area fraction of ferrite is less than 20%, the high yield ratio may reduce workability and decrease elongation. When the area fraction of ferrite is greater than 50%, the fraction of tempered martensite as the matrix structure may decrease and thus sufficient yield strength and tensile strength may not be easily achieved. The area fraction of retained austenite may be, for example, 5% to 20%. The area fraction of martensite/tempered martensite may be the remaining area fraction, for example, being 30% to 75%. The area fraction refers to an area percentage derived from a microstructural image using an image analyzer.

Herein, “martensite/tempered martensite” refers to the combination of martensite and tempered martensite, without distinguishing between the two.

In the high-strength and high-formability steel sheet, the formation of cementite and pearlite may be minimized, and thus cementite and pearlite may be excluded from the microstructure.

A method of manufacturing a high-strength and high-formability steel sheet according to the present disclosure will now be described with reference to the attached drawings.

Method of Manufacturing High-Strength and High-Formability Steel Sheet

FIG. 1 is a flowchart of a method of manufacturing a high-strength and high-formability steel sheet, according to an embodiment of the present disclosure.

Referring to FIG. 1, the method includes producing a hot-rolled steel sheet by hot rolling a steel material (S110); producing a cold-rolled steel sheet by cold rolling the hot-rolled steel sheet (S120); annealing the cold-rolled steel sheet (S130); and multi-stage cooling the cold-rolled steel sheet (S140).

The method may further include post-annealing the cold-rolled steel sheet (S150).

Hot-Rolled Steel Sheet Production (S110)

In the hot-rolled steel sheet production step S110, a steel material including C: 0.1 wt % to 0.3 wt %, Si: 1.0 wt % to 2.0 wt %, Mn: 1.5 wt % to 3.0 wt %, Al: more than 0 wt % and up to 0.05 wt %, P: more than 0 wt % and up to 0.02 wt %, S: more than 0 wt % and up to 0.005 wt %, N: more than 0 wt % and up to 0.006 wt %, and the balance of Fe and other unavoidable impurities is prepared.

The steel material may further include a combination of Ti, Nb, and V: more than 0 wt % and up to 0.05 wt %.

In the method according to the present disclosure, the semi-finished product to be hot-rolled may be, for example, a slab. The slab provided as a semi-finished product may be produced by continuously casting molten steel with a certain composition obtained through a steelmaking process.

The steel material is reheated to a temperature of Ac3 or higher, e.g., a slab reheating temperature (SRT) ranging from 1,150° C. to 1,250° C. Through the reheating process, the components segregated during casting and the precipitates may redissolve. When the SRT is lower than 1,150° C., the hot rolling load may rapidly increase. When the SRT is higher than 1,250° C., the slab may not be easily loaded into or unloaded from the furnace due to warping, and the strength of the finally produced steel sheet may not be easily achieved due to the coarsening of initial austenite grains.

Subsequently, the reheated steel material is heated and then hot-rolled to adjust its shape. The hot rolling process may be performed continuously through width rolling, rough rolling, and finishing rolling. Due to the hot rolling process, the steel material may be formed into a hot-rolled steel sheet.

In the hot rolling process, the steel material may undergo finishing rolling at a finishing delivery temperature (FDT) ranging, for example, from 850° C. to 1,000° C. When the FDT is lower than 850° C., although increased strength may be achieved due to grain refinement, edge cracking and occur, and increased rolling load and decreased productivity may be caused. When the FDT is higher than 1,000° C., the quality of the steel sheet may degrade due to surface scaling of the steel sheet.

The finishing rolling process may be performed with a cumulative reduction ratio of 70% or more and 90% or less. In the present disclosure, the microstructure after hot rolling is completed is a mixed structure of pearlite and ferrite. For this, in the present disclosure, the transformation from austenite into pearlite and ferrite is promoted due to plastic deformation by increasing the cumulative reduction ratio during finishing rolling of hot rolling.

When plastic deformation is applied by external force while the steel material with the austenite phase is being cooled, the phase transformation into ferrite and pearlite may occur earlier and thus more ferrite and pearlite may be formed. Therefore, in the present disclosure, a sufficient cumulative reduction ratio of 70% or more is provided during finishing rolling to more easily obtain a mixed structure of pearlite and ferrite in the hot-rolled steel sheet. When the cumulative reduction ratio is less than 70%, the phase transformation into ferrite and pearlite may not occur quickly and thus the desired amounts of ferrite and pearlite may not be easily formed. However, when the cumulative reduction ratio is greater than 90%, an excessive load may be applied to the rolling equipment, leading to an impractical process.

Then, the hot-rolled steel material is cooled to a certain coiling temperature. The cooling process may be performed using air cooling or water cooling at a cooling rate of, for example, 10° C./s to 30° C./s. The cooling process may be performed to a coiling temperature of, for example, 500° C. to 700° C., and more specifically, 550° C. to 650° C. When the cooling rate is less than 10° C./s, the average particle size of precipitates may increase and thus strength may not be easily achieved. On the other hand, when the cooling rate is greater than 30° C./s, the microstructure of the steel material may become harder and thus impact toughness may decrease.

Thereafter, the hot-rolled steel sheet is coiled at a coiling temperature (CT) ranging, for example, from 550° C. to 650° C. When the CT is lower than 500° C., the significant difference between the FDT and CT may degrade the surface quality of the steel material, and the increased strength may increase the rolling load during cold rolling. When the CT is higher than 700° C., carbonitride elements may not remain in a solid solution form, undesired precipitates may be formed, and defects may occur in subsequent processes due to surface oxidation or the like. The coiled steel material may be cooled to room temperature. Specifically, the CT may be 550° C. to 650° C.

The hot-rolled steel sheet may have a mixed structure of ferrite and pearlite. The area fraction of ferrite may range, for example, from 20% to 50%. The area fraction of pearlite may be the remaining area fraction, for example, ranging from 50% to 80%.

Cold-Rolled Steel Sheet Production (S120)

The cold-rolled steel sheet production step S120 is performed to obtain the thickness of the finally produced steel sheet by using the hot-rolled steel sheet. The coiled hot-rolled steel sheet is pickled with acid. Then, a cold-rolled steel sheet is formed by cold rolling the pickled hot-rolled steel sheet with a cold rolling reduction ratio of 40% to 60%. When the cold rolling reduction ratio is less than 40%, because nucleation for recrystallization during subsequent soaking is insufficient, grains may grow excessively during soaking and thus strength may rapidly decrease. When the cold rolling reduction ratio is greater than 60%, because nucleation occurs excessively, grains formed during soaking may become excessively fine, and ductility and formability may decrease.

The microstructure of the cold-rolled steel sheet has an elongated form of the microstructure of the hot-rolled steel sheet, and the microstructure of the finally produced steel sheet is determined during subsequent heat treatment.

Annealing (S130)

In the annealing step S130, the cold-rolled steel sheet may be heat-treated in a typical continuous annealing furnace with a slow cooling period. The annealing process may be performed in the intercritical temperature region of austenite and ferrite. This is done to achieve the target final material properties of the steel sheet based on the final microstructure with an ideal mixture of retained austenite, ferrite, and tempered martensite by obtaining ferrite with an appropriate shape and fraction.

In the annealing process, the cold-rolled steel sheet is heated at a heating rate of, for example, 1° C./s to 10° C./s, and more specifically, 1° C./s to 5° C./s, and then maintained at, for example, the intercritical temperature, and more specifically, a temperature higher than 780° C. and lower than 840° C., for, for example, 40 sec. to 120 sec., and more specifically, 50 sec. to 110 sec.

When the heating rate is low, because austenite remains in the transformation region for a long time, some may be recrystallized, and the initial average austenite grain size may increase. In this case, the material properties are negatively affected. Therefore, a higher heating rate is preferred.

When the annealing temperature is lower than 780° C., austenite may not be easily formed in a sufficient amount and thus the target strength and elongation may not be easily achieved. When the annealing temperature is higher than 840° C., the fraction of ferrite may decrease and thus a sufficient elongation may not be obtained.

Multi-Stage Cooling (S140)

In the multi-stage cooling step S140, the annealed cold-rolled steel sheet is primarily cooled at a cooling rate of, for example, 1° C./s to 10° C./s, and more specifically, 5° C./s to 10° C./s, to, for example, 550° C. to 750° C., and more specifically, 600° C. to 700° C. The primary cooling process may also be called a slow cooling process. The primary cooling process is performed to achieve plasticity by obtaining a certain amount of ferrite in the final microstructure, and to achieve an appropriate fraction of retained austenite depending on the shape and fraction of ferrite formed during heat treatment.

Subsequently, the cold-rolled steel sheet is secondarily cooled at a cooling rate of, for example, 50° C./s or more, for example, 70° C./s or more, and more specifically, 50° C./s to 150° C./s, to, for example, a temperature at or below Ms, and more specifically, a temperature higher than 180° C. and lower than 240° C., and held for, for example, 5 sec. to 20 sec., and more specifically, 10 sec. The secondary cooling process may also be called a rapid cooling process. The secondary cooling process is performed to transform austenite into martensite in the microstructure after slow cooling by controlling the rapid cooling end temperature, and thus to easily achieve the final material properties. The secondary cooling process is performed at a cooling rate of 50° C./s or more to inhibit the phase transformation during rapid cooling.

Post-Annealing (S150)

In the post-annealing step S150, the cold-rolled steel sheet is heated at a heating rate of, for example, 20° C./s or more, for example, 30° C./s or more, and more specifically, 20° C./s to 50° C./s, to, for example, a temperature of Ms or higher, and more specifically, a temperature higher than 380° C. and lower than 450° C., and held for, for example, 10 sec. or more, and more specifically, 10 sec. to 240 sec. Due to the post-annealing process, C may be enriched in retained austenite, and tempered martensite may be formed thorough martensite tempering, thereby achieving high strength and high elongation and maintaining the final microstructure. The post-annealed cold-rolled steel sheet is cooled to room temperature, for example, 0° C. to 40° C.

The cold-rolled steel sheet manufactured using the above method of the present disclosure may have a mixed structure of retained austenite, ferrite, and martensite/tempered martensite.

The present disclosure is characterized in that the hot-rolled structure is formed as a dual-phase structure consisting of ferrite ranging from 20% to 50% and pearlite being the remainder, and that softening is not performed before cold rolling.

When austenite formation sites are maximized through dislocations formed during cold rolling, austenite may be formed locally and sporadically during subsequent annealing, and thus the initial austenite grain size may be controlled to be relatively small. If the hot-rolled structure is formed as a low-temperature phase structure such as martensite or bainite, softening is essential before cold rolling. Because the dislocations are recovered by the softening process, the dislocation density decreases even after cold rolling, and thus the initial austenite grain size may not be easily reduced.

On the other hand, when the hot-rolled structure is formed as a dual-phase structure of ferrite and pearlite, no softening is performed and thus dislocations are accumulated. Thus, a relatively high dislocation density is maintained after cold rolling, and fine austenite may be formed. In addition, by pre-forming ferrite before post-annealing, austenite-stabilizing elements such as C and Mn may be included in the pearlite region at relatively high contents. As such, retained austenite may be obtained more stably after post-annealing, and thus the target material properties with balanced strength and elongation may be met.

The present disclosure is also characterized in that, when heat treatment is performed, heating is performed fast from Ac1 (the austenite start temperature, approximately 730° C.) to a temperature to be maintained (a temperature below the austenite end temperature, approximately 880° C.) in the austenite phase transformation region. When the heating is performed slow, because austenite remains in the transformation region for a long time, some of austenite may be recrystallized. As such, the initial average austenite grain size may increase, and material degradation may be caused.

TEST EXAMPLES

Test examples will now be described for better understanding of the present disclosure. However, the following test examples are merely to promote understanding of the present disclosure, and the present disclosure is not limited to thereto. The details not described herein may be easily inferred by one of ordinary skill in the art, and therefore, further explanation is omitted.

Table 1 shows the compositions of high-strength and high-formability steel sheets according to embodiments of the present disclosure. In Table 1, the balance consists of Fe and impurities that are inevitably introduced during the steelmaking process or the like. The unit for the content of each component is wt %.

TABLE 1
								Composition
No.		i	n	l				Parameter

Comparative	.18	.70	.30	.03	.014	.002	.003	0.391
Example 1
Comparative	.22	.30	.10	.03	.014	.002	.003	0.396
Example 2
Comparative	.24	.70	.00	.03	.014	.002	.003	0.438
Example 3
Comparative	.26	.80	.80	.03	.014	.002	.003	0.456
Example 4
Embodiment 1	.20	.70	.00	.03	.014	.002	.003	0.398
Embodiment 2	.20	.70	.00	.03	.014	.002	.003	0.398
indicates data missing or illegible when filed

In Table 1, the composition parameter indicates the value of X_{C, wt %}+0.066×X_{Si, wt %}+0.043×X_{Mn, wt %}, which represents the relationship between the C, Mn, and Si contents in the high-strength and high-formability steel sheet. Herein, X_{C, wt %} indicates the C content of the steel sheet expressed in wt %, X_{Si, wt %} indicates the Si content of the steel sheet expressed in _{wt %}, and X_{Mn, wt %} indicates the Mn content of the steel sheet expressed in _{wt %}.

In the embodiments, the value of the composition parameter meets 0.4 or less (X_{C, wt %}+0.066×X_{Si, wt %}+0.043×X_{Mn, wt %}≤0.4). On the other hand, in Comparative Examples 3 and 4, the value of the composition parameter exceeds 0.4.

A slab with the composition of each of the comparative examples and embodiments was reheated to 1200° C. and rolled to produce a hot-rolled steel sheet. At this time, the cumulative reduction ratio was controlled to 80% during finishing rolling. The hot-rolled steel sheet finish-rolled at 900° C. was cooled to a coiling temperature, loaded into a coiler furnace at 600° C. and held for 2 hours, and then coiled and cooled in the furnace. The produced hot-rolled steel sheet was rolled under typical cold rolling conditions to produce a cold-rolled steel sheet. The produced cold-rolled steel sheet was annealed at an intercritical temperature of 800° C. for 60 sec., primarily cooled to 680° C. at a cooling rate of 3° C./s, and then secondarily cooled to a temperature below Ms at a cooling rate of 50° C./s or more. The secondarily cooled cold-rolled steel sheet was loaded into a heat treatment furnace, heated, and post-annealed at 400° C. for 60 sec.

Table 2 shows the mechanical properties of high-strength and high-formability steel sheets according to embodiments of the present disclosure.

TABLE 2
						Tensile
	Yield	Tensile				Strength ×
	Strength	Strength	T.EL	U.EL	U.EL/	Elongation
No.	(MPa)	(MPa)	(%)	(%)	T.EL	(MPa %)

Comparative	636	1074	21.6	14.5	67.1%	23198
Example 1
Comparative	590	1058	22.4	160.	71.4%	23699
Example 2
Comparative	573	1081	24.4	17.4	71.3%	26376
Example 3
Comparative	547	1092	24.7	18.4	74.5%	26972
Example 4
Embodiment 1	552	1026	24.5	17.2	70.2%	25137
Embodiment 2	555	1003	24.7	18.1	73.3%	24774

Table 3 shows the microstructures of high-strength and high-formability steel sheets according to embodiments of the present disclosure.

	TABLE 3
	Martensite/	Retained Austenite (%)

		Tempered		After 5%
	Ferrite	Martensite	Before	Plastic	Reduction
No.	(%)	(%)	Deformation	Deformation	Rate

Comparative	33.3	53.5	13.2	6.1	53.8%
Example 1
Comparative	30.0	60.8	9.2	7.0	23.9%
Example 2
Comparative	23.9	65.7	10.4	8.2	21.2%
Example 3
Comparative	26.7	64.3	9.0	7.5	16.7%
Example 4
Embodiment 1	43.7	48.0	8.3	6.9	16.9%
Embodiment 2	34.2	57.4	8.4	6.4	23.8%

Referring to Tables 2 and 3, in the embodiments, the values of the yield strength, tensile strength, elongation, and tensile strength×elongation meet the ranges proposed in the present disclosure. Additionally, in the embodiments, the reduction rate in the area fraction of retained austenite after 5% plastic deformation is 50% or less, which meets the range proposed in the present disclosure.

In Comparative Example 1, the total elongation does not meet the range of 23% or more proposed in the present disclosure, and the uniform elongation ratio (U.EL/T.EL) is not met. Additionally, the reduction rate in the area fraction of retained austenite does not meet the range proposed in the present disclosure.

In Comparative Example 2, the total elongation does not meet the range of 23% or more proposed in the present disclosure.

In Comparative Examples 3 and 4, the composition parameter is not met, and a deterioration in spot weldability is anticipated due to the increase in C equivalent resulting from the rise in C content.

According to the results of the comparative examples, it is shown that an increase in C content leads to an increase in elongation. Therefore, by controlling the C redistribution behavior into austenite while reducing strength, elongation may be improved. As such, in the embodiments of the present disclosure, the Si content is increased compared to Comparative Example 2, and the C and Mn contents are adjusted to reduce strength.

FIG. 2 is a microscopic image showing the microstructure of a high-strength and high-formability steel sheet according to Embodiment 1.

Referring to FIG. 2, it is shown that the high-strength and high-formability steel sheet has a mixed structure of ferrite, retained austenite, and martensite/tempered martensite. The area fraction of retained austenite was measured at approximately 8%, ferrite at 44%, and martensite/tempered martensite at 48%.

While the present disclosure has been particularly shown and described with reference to embodiments thereof, it will be understood by one of ordinary skill in the art that various changes in form and details may be made therein without departing from the scope of the present disclosure as defined by the following claims.