EP4610386A1

EP4610386A1 - Steel sheet having excellent bendability and manufacturing method thereof

Info

Publication number: EP4610386A1
Application number: EP23883039.2A
Authority: EP
Inventors: Joo-Hyun Ryu; Ki-Cheol KANG; Do-Yub Kim
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2022-10-24
Filing date: 2023-10-23
Publication date: 2025-09-03
Also published as: KR20240057522A; WO2024090933A1; CN120019172A; JP2025533666A; MX2025004169A

Abstract

The present invention relates to a steel sheet and a manufacturing method thereof and, more specifically, to a steel sheet having excellent bendability and a manufacturing method thereof.

Description

Technical Field

The present disclosure relates to a steel sheet and a method for manufacturing the same, and more specifically, to a steel sheet with excellent bendability and a method for manufacturing the same.

Background Art

In order to secure the safety of passengers in the event of a car collision, safety regulations for automobiles have been strengthened. To this end, automobile steel sheets have to have high strength or must be thick. However, due to environmental issues, automobile manufacturers have continuously demanded a weight reduction of vehicle bodies in order to improve fuel efficiency. Therefore, high strength steel sheets are essential to secure both collision stability and weight reduction of vehicles.
In general, methods for strengthening steel include solid solution strengthening, precipitation strengthening, strengthening by grain refinement, and transformation strengthening. Thereamong, precipitation strengthening-type high-strength steel using precipitation strengthening is a technology of strengthening steel sheets by adding carbide and nitride forming elements, such as Cu, Nb, Ti, and V to precipitate carbide and nitride or securing strength by refining grains through suppression of grain growth by fine precipitates. This technology has the advantage of easily obtaining high strength at low manufacturing costs but has the disadvantage that high-temperature annealing has to be performed to secure ductility by causing sufficient recrystallization because a recrystallization temperature rises rapidly due to fine precipitates. In addition, precipitation-strengthened steel, strengthened by precipitating carbides and nitrides in a ferrite matrix, has the problem that it may be difficult to obtain high-strength steels of 600 MPa or higher.
Meanwhile, various types of transformation-strengthened high-strength steels, such as ferrite-martensite dual-phase steels including hard martensite in a ferrite matrix, transformation induced plasticity (TRIP) steels using transformation-induced plasticity of residual austenite, or complexed phase (CP) steels including ferrite and hard bainite or martensite structures, have been developed.
Recently, steel sheets for automobiles are required to have higher strength to improve fuel efficiency and durability, and high-strength steel sheets with tensile strength of 780 MPa or more have been increasingly used for body structures or as reinforcing materials in terms of collision safety and passenger protection.
However, as steel sheets have had gradually increased strength, cracks and wrinkles may occur during a press forming process of automobile parts, reaching the limit of manufacturing complex parts. In particular, if ductility (El) and bendability may be improved in DP steel, which is the most widely used among transformation-strengthened high-strength steels, processing defects, such as cracks and wrinkles that occur during press forming, may be prevented, thereby expanding the application of high-strength steel to complex parts.
As related art for such high-strength steel sheets, an invention disclosed in patent document 1 may be cited. The above-mentioned related art relates to a cold rolled steel sheet having a composite structure including ferrite, bainite, martensite, and residual austenite and discloses a manufacturing method for securing ductility of the steel sheet by adding Si to the steel and introducing residual austenite into a finally annealed steel sheet through bainite transformation. However, due to the addition of Si, there is a possibility that dents may occur in a furnace during continuous annealing or liquid metal embrittlement may occur during spot welding of the plated steel sheet at a customer company.

[Related art document]

[Patent document]

(Patent document 1) Korean Application Publication No. 2019-0076258

Summary of Invention

Technical Problem

An aspect of the present disclosure is to provide a steel sheet having excellent bendability and a manufacturing method thereof.
The problem of the present disclosure is not limited to the above-mentioned contents. Those skilled in the art will have no difficulty in understanding additional problems of the present disclosure from the overall contents of this specification.

Solution to Problem

According to an aspect of the present disclosure, a steel sheet includes: in wt%, carbon (C): 0.05 to 0.20%, silicon (Si): 0.10% or less, manganese (Mn): 1.0 to 3.0%, aluminum (sol.Al): 1.00% or less, chromium (Cr): 0.1 to 1.0%, niobium (Nb): 0.05% or less, titanium (Ti): 0.05% or less, phosphorus (P): 0.100% or less, sulfur (S): 0.0100% or less, nitrogen (N): 0.010% or less, and a remainder of iron (Fe) and inevitable impurities,

having a T value of 1648 or more defined in Relational Expression 1 below, and
having a microstructure of, in area%, 50 to 80% of ferrite, 5 to 25% of bainite, 10 to 30% of fresh martensite, and 5% or less of residual austenite.

$T = 279 * [C] + 711 * [Mn] + 474 * [Nb] + 177 * [Ti] - 75 * [Cr]$
where [C], [Mn], [Nb], [Ti], and [Cr] are wt% of each element.

The steel sheet may have an RT value of 0.01 or more defined in Relational Expression 2 below.

$RT = [Si] + [Nb] + [Ti]$
where [Si], [Nb], and [Ti] are wt% of each element.
The steel sheet may have a tensile strength (TS) of 780 MPa or more and an elongation (El) of 14.0% or more.
The steel sheet, when subjected to a 180° bending test, may have a value of bending angle (°)/thickness (mm) of 50°/mm or more (here, the bending angle (°) may refer to a bending angle at which no cracking occurs in a bent portion during the 180° bending test).
The steel sheet may further include a hot-dip galvanized layer or an alloy hot-dip galvanized layer on a surface.
According to another aspect of the present disclosure, a method of manufacturing a steel sheet includes: reheating a steel slab including, in wt%, carbon (C): 0.05 to 0.20%, silicon (Si): 0.10% or less, manganese (Mn): 1.0 to 3.0%, aluminum (sol.Al): 1.00% or less, chromium (Cr): 0.1 to 1.0%, niobium (Nb): 0.05% or less, titanium (Ti): 0.05% or less, phosphorus (P): 0.100% or less, sulfur (S): 0.010% or less, nitrogen (N): 0.010% or less, and a remainder of iron (Fe) and inevitable impurities, and having a T value of 1648 or more defined in Relational Expression 1 below;

hot-rolling the reheated steel slab;
coiling the hot-rolled steel sheet and then cooling the coiled steel sheet;
cold-rolling the cooled steel sheet;
heating the cold-rolled steel sheet to a T1 temperature of 800 to 850°C, cooling the heated steel sheet to a T2 temperature of 400 to 600°C at an average cooling rate of 20°C/s or less, and then maintaining the steel sheet for 50 seconds or more for continuous annealing; and
cooling the continuously annealed steel sheet to room temperature,
wherein an R value defined in Relational Expression 3 below is 1797 to 1850.

$T = 279 * [C] + 711 * [Mn] + 474 * [Nb] + 177 * [Ti] - 75 * [Cr]$
where [C], [Mn], [Nb], [Ti], and [Cr] are wt% of each element. R = 174*[C] + 680*[Mn] + 370*[Nb] + 177*[Ti] - 86*[Cr] + 0.33*[T1] - 0.05*[T2]
where [C], [Mn], [Nb], [Ti], and [Cr] are wt% of each element, and T1 and T2 are a heating temperature (°C) and a cooling end temperature (°C) during continuous annealing, respectively.
The steel slab may have an RT value of 0.01 or more defined in Relational Expression 2 below.

$RT = [Si] + [Nb] + [Ti]$
where [Si], [Nb], and [Ti] are the wt% of each element.

The reheating may be performed at a temperature within a temperature range of 1100 to 1300°C,

the hot rolling may be performed at a finishing rolling temperature of 800 to 950°C, and
in the cooling after coiling, the steel sheet may be coiled at a temperature within a temperature range of 400 to 700°C and then cooled to room temperature at an average cooling rate of 0.10°C/s or less, and
the cold rolling may be performed at a reduction ratio of 40 to 70%.

The method may further include: pickling the steel sheet before the cold rolling.
The method may further include: after the continuous annealing and before the cooling, hot-dip galvanizing the steel sheet at a temperature within a temperature range of 430 to 490°C.
The method may further include: after the hot-dip galvanizing, performing alloying heat treatment on the steel sheet at a temperature within a temperature range of 460 to 530°C before cooling.

Advantageous Effects of Invention

According to an aspect of the present disclosure, a steel sheet having excellent bendability and a method for manufacturing the same may be provided.
According to an aspect of the present disclosure, a steel sheet which may be used as an automobile structural member, having excellent processability and being used in complex shapes during press forming, and a method for manufacturing the same may be provided.

Brief Description of Drawings

FIG. 1 is a photograph of a microstructure of Inventive Example 13 according to an embodiment of the present disclosure observed using an electron microscope.
FIG. 2 is a photograph of a microstructure of Comparative Example 6 according to an embodiment of the present disclosure observed using an electron microscope. Best Mode for Invention

Hereinafter, embodiments of the present disclosure will be described. The embodiments of the present disclosure may be modified in various forms, and the scope of the present disclosure should not be construed as being limited to the embodiments described below. These embodiments are provided to describe the present disclosure in more detail to those skilled in the art.
According to an embodiment, the present disclosure has completed upon recognizing that, by optimizing an alloy composition by adding a minimum Si or no Si, the occurrence of in-furnace dents and liquid metal embrittlement during spot welding are reduced and excellent bending properties are obtained, while the physical properties of the related art DP steel are satisfied.
Hereinafter, the present disclosure will be described in detail.
Hereinafter, the steel composition of the present disclosure will be described in detail.
Unless otherwise specifically stated in the present disclosure, % indicating the content of each element is based on weight.
According to an embodiment of the present disclosure, a steel sheet may include, in wt%, carbon (C): 0.05 to 0.20%, silicon (Si): 0.10% or less, manganese (Mn): 1.0 to 3.0%, aluminum (sol.Al): 1.00% or less, chromium (Cr): 0.1 to 1.0%, niobium (Nb): 0.05% or less, titanium (Ti): 0.05% or less, phosphorus (P): 0.100% or less, sulfur (S): 0.010% or less, nitrogen (N): 0.010% or less, and a remainder of iron (Fe) and inevitable impurities.

Carbon (C): 0.05 to 0.20%

Carbon (C) is a very important element added to strengthen a transformation structure. Carbon (C) promotes high strengthening and accelerates the formation of martensite in composite structure steel. As the carbon (C) content increases, the amount of martensite in the steel increases. However, if the content of carbon (C) exceeds 0.20%, the strength of martensite may increase, but a difference in strength from ferrite with a low carbon concentration may increase. This difference in strength may lower bendability because fracture may easily occur in an interphase interface when stress is applied. According to an embodiment, carbon (C) may be included in an amount of 0.17% or less. In addition, weldability may be poor, so welding defects may occur when processing customer company parts. Meanwhile, if the carbon (C) content is less than 0.05%, it may be difficult to secure a desired level of strength. According to an embodiment of the present disclosure, carbon (C) may be included in an amount of 0.07% or more.

Silicon (Si): 0.10% or less

Silicon (Si) is a ferrite stabilizing element promoting ferrite transformation and promotes C enrichment into untransformed austenite, thereby contributing to the formation of martensite. In addition, silicon (Si) is an effective element for reducing a hardness difference between phases by increasing the strength of ferrite due to excellent ability for solid solution hardening and is a useful element securing strength without lowering the ductility of the steel sheet. However, if the silicon (Si) content exceeds 0.10%, it may cause surface scale defects, which deteriorates the surface quality of the plating, and may also cause liquid metal embrittlement during spot welding of the plating material. According to an embodiment, silicon (Si) may be included in an amount of 0.05% or less.

Manganese (Mn): 1.0 to 3.0%

Manganese (Mn) is an element that refines particles without damaging ductility and completely precipitates S in the steel as MnS, thereby preventing hot embrittlement due to the formation of FeS and strengthening the steel. In addition, manganese (Mn) plays a role in lowering a critical cooling rate at which martensite is obtained in composite structure steel, making it easier to form martensite. If the manganese (Mn) content is less than 1.0%, it may be difficult to secure the strength targeted by the present disclosure. According to an embodiment, manganese (Mn) may be included in an amount of 1.6% or more. Meanwhile, if the content exceeds 3.0%, there is a high possibility that problems, such as weldability and hot-rollability, may occur, martensite may be excessively formed, making the material unstable, and Mn-band (band of Mn oxide) is formed in the structure, thereby increasing the risk of processing cracks and strip breakage. In addition, during annealing, Mn oxide may be dissolved on the surface, thereby significantly inhibiting plating properties. According to an embodiment, manganese (Mn) may be included in an amount of 2.5% or less.

Aluminum (sol.Al): 1.00% or less

Aluminum (sol.Al) is an element added for grain refinement and deoxidation of steel and is a ferrite stabilizing element similar to Si. In addition, aluminum (sol.Al) is an effective component for distributing C in ferrite to austenite to improve martensite hardenability and is a useful element that may improve the ductility of the steel sheet by effectively suppressing the precipitation of carbide in bainite when maintained in a bainite region. However, if the content of aluminum (sol.Al) exceeds 1.00%, it is advantageous for increasing strength due to the grain refinement effect, but there may be a problem that the possibility of causing surface defects of the plated steel sheet due to excessive formation of inclusions during steelmaking and continuous casting, as well as increasing manufacturing costs. According to an embodiment of the present disclosure, aluminum (sol.Al) may be included in an amount of 0.50% or less.

Chromium (Cr): 0.1 to 1.0%

Chromium (Cr) is a component that may be added to improve the hardenability of steel and secure high strength. In addition, chromium (Cr) is an element that plays a very important role in the formation of martensite and is also advantageous for the production of composite structure steel with high ductility by minimizing a decrease in elongation compared to an increase in strength. In particular, during a hot rolling process, Cr-based carbides, such as Cr₂₃C₆, are formed, and some of the carbides are dissolved during the annealing process and some remain undissolved, so that the amount of solid solution C in martensite may be controlled to an appropriate level or less after cooling, thereby suppressing the occurrence of yield point elongation (YP-El), making it an advantageous element for the production of composite structure steel with a low yield ratio. Therefore, in the present disclosure, the content of chromium (Cr) may be limited to 0.1% or more. According to an embodiment of the present disclosure, chromium (Cr) may be included in an amount of 0.8% or less. However, if the content of chromium (Cr) exceeds 1.0%, not only will the above-described effect be saturated, but there may also be a problem that cold rolling properties deteriorate due to an excessive increase in hot rolling strength, and since the fraction of Cr-based carbides increases and coarsens, the martensite size after annealing may coarsen, resulting in a decrease in elongation. According to an embodiment, chromium (Cr) may be included in an amount of 0.2% or more.

Niobium (Nb): 0.05% or less

Niobium (Nb) is an element that segregates in austenite grain boundaries, suppresses the coarsening of austenite grains during annealing heat treatment, and forms fine carbides, thereby contributing to increased strength. However, if the niobium (Nb) content exceeds 0.05%, coarse carbides may be precipitated and the strength and elongation may decrease due to a decrease in the amount of carbon in the steel and manufacturing costs may also increase. According to an embodiment of the present disclosure, niobium (Nb) may be included in an amount of 0.04% or less.

Titanium (Ti): 0.05% or less

Titanium (Ti) is a fine carbide-forming element that may contribute to securing yield strength and tensile strength. In addition, titanium (Ti) is a nitride-forming element that precipitates N in the steel as TiN, thereby suppressing AlN precipitation, and thus having the advantage of reducing the risk of cracks occurring during casting. However, if the titanium (Ti) content exceeds 0.05%, coarse carbides may precipitate, the strength and elongation may decrease due to a decrease in the amount of carbon in the steel, and nozzle clogging may occur during casting. In an embodiment of the present disclosure, titanium (Ti) may be included in an amount of 0.03% or less.

Phosphorus (P): 0.100% or less

Phosphorus (P) is a substitutional element with the greatest solid solution strengthening effect and is the most advantageous element for improving in-plane anisotropy and securing strength without significantly reducing formability. However, if phosphorus (P) is added excessively, the possibility of brittle fracture may significantly increase and there is a problem that it acts as an element that may cause slab fracture during hot rolling and impairs the plating surface characteristics, and thus, in the present disclosure, the content of phosphorus (P) may be limited to 0.100% or less. However, considering the level that is inevitably added during the manufacturing process, 0% is excluded.

Sulfur (S): 0.010% or less

Sulfur (S) is an impurity element inevitably added to steel and is an element reducing ductility and weldability, so it is important to manage sulfur (S) as low as possible. In particular, since there is a problem of increasing the possibility of causing red-hot embrittlement, it is desirable to control the content of sulfur (S) to 0.010% or less. However, considering the level that is inevitably added during the manufacturing process, 0% is excluded.

Nitrogen (N): 0.010% or less

Nitrogen (N) is an element that effectively stabilizes austenite, but if the content of nitrogen (N) exceeds 0.010%, there may be a problem that the refining cost of the steel may increase rapidly. In addition, since the risk of cracks occurring due to AlN formation during casting may significantly increase, it is desirable to limit the upper limit to 0.010%. However, considering the level that is inevitably added during the manufacturing process, 0% is excluded.
The steel of the present disclosure may include the remainder of iron (Fe) and unavoidable impurities in addition to the compositions described above. Since unavoidable impurities may be unintentionally mixed in during a normal manufacturing process, they cannot be excluded. Since these impurities are known to those skilled in the art of normal steel manufacturing, not all of their contents are specifically mentioned in this specification.
The steel sheet according to an embodiment of the present disclosure may have a T value of 1648 or more defined in Relational Expression 1 below.

$T = 279 * [C] + 711 * [Mn] + 474 * [Nb] + 177 * [Ti] - 75 * [Cr]$
(In Relational Expression 1, [C], [Mn], [Nb], [Ti], and [Cr] are wt% of each element.)
Relational Expression 1 is an expression that quantitatively expresses how the added elements in the steel sheet contribute to the strength and bendability of the steel sheet. In the present disclosure, among the steel components forming the steel sheet, C, Mn, Nb, Ti, and Cr, which are representative component systems, may be limited to be contained so as to satisfy Relational Expression 1.
Specifically, C and Mn have the effect of increasing the strength of the steel sheet due to the solid solution strengthening effect of the steel. However, the contribution of each element to the strength of the steel sheet is different, and a constant value multiplied by each component in the corresponding Relational Expression relatively indicates the contribution of each element to the strength. In addition, Nb and Ti have a precipitation strengthening effect, contributing to the improvement of strength, and are precipitated in a ferrite matrix in DP steel and have a ferrite strengthening effect, reducing a hardness difference in phase between ferrite and martensite, and thus increasing the bendability of the steel sheet, so the multiplied constant value is expressed as a positive value. Meanwhile, Cr has the least solid solution strengthening effect among the elements and significantly increases hardenability, so if Cr is added in large quantities, a large amount of martensite may be generated, which may decrease the bendability, and thus the constant value may have a negative value.
If the T value defined in the Relational Expression 1 is less than 1648, there is a problem that the strength and bendability characteristics of the steel sheet targeted by the present disclosure cannot be secured. According to an embodiment of the present disclosure, the T value defined in Relational Expression 1 may be 1650 or more. Although not specifically limited in the present disclosure, if components are excessively added, there may be a problem that the strength may increase excessively to decrease the elongation below a target level, and thus, considering this, the upper limit of the T value may be effectively limited to 1800 or less.
The steel sheet according to an embodiment of the present disclosure may have an RT value of 0.01 or more, defined in Relational Expression 2.

$RT = [Si] + [Nb] + [Ti]$
(In Relational Expression 2, [Si], [Nb], and [Ti] are the wt% of each element.)
According to an embodiment of the present disclosure, if a large amount of Si is added to the steel sheet, there may be problems, such as dent defects in the steel sheet in an annealing furnace and inferior phosphate treatment properties of cold rolled steel sheet and liquid metal embrittlement and plating properties of the plated steel sheet. Therefore, Si is intended to be minimized. Meanwhile, if the amount of Si added is reduced, there is a possibility that the mechanical properties may be inferior. To overcome this, Nb and Ti may be added to prevent deterioration of the mechanical properties due to carbide precipitation.
If the RT value defined in the Relational Expression 2 is less than 0.01, the properties targeted by the present disclosure cannot be secured. According to an embodiment of the present disclosure, the RT value defined in Relational Expression 2 may be 0.02 or more. In addition, although not specifically limited in the present disclosure, the upper limit of the RT value may be limited to 0.2%, which is the same as the maximum addition of each component restriction range.
Hereinafter, a steel microstructure of the present disclosure will be described in detail.
Unless otherwise specifically stated in the present disclosure, % indicating the fraction of microstructure is based on the area.
The microstructure of the steel sheet according to an embodiment of the present disclosure may include, in area %, 50 to 80% of ferrite, 5 to 25% of bainite, 10 to 30% of fresh martensite, and 5% or less of residual austenite.
The ferrite is a soft structure and may contribute to the ductility of the steel sheet. If an area fraction of the ferrite is less than 50% compared to the entire microstructure included in the steel sheet, it may be difficult to secure the target bendability. Meanwhile, if the area fraction exceeds 80%, it may be difficult to secure the strength at the level targeted by the present disclosure.
The bainite is a phase having an intermediate hardness between ferrite and martensite and may be appropriately included. If the area fraction of the bainite is less than 5%, ferrite and martensite may be dominant, resulting in poor bendability. Meanwhile, if the area fraction exceeds 25%, there may be a problem of reduced strength.
The fresh martensite is a phase contributing to increased strength, and if the area fraction is less than 10%, the target strength cannot be secured. Meanwhile, if the area fraction exceeds 30%, there may be a problem of reduced bendability due to a relative decrease in the area fraction of bainite.
The residual austenite may be generated in small amounts of 5% or less during a final cooling process, and a plated steel sheet with a high area fraction of residual austenite tends to be vulnerable to liquid metal embrittlement during spot welding of automobile parts assembly, so it is desirable to control the residual austenite to 5% or less in the steel sheet.
According to an embodiment of the present disclosure, as for the microstructure fraction, a matrix structure at a 1/4 point of a sheet thickness of a continuously annealed steel sheet may be analyzed, and specifically, the area fraction of the microstructure may be measured using FE-SEM, an image analyzer, and XRD.
Hereinafter, a method of manufacturing a steel sheet of the present disclosure will be described in detail.
The steel sheet according to an embodiment of the present disclosure may be manufactured by reheating, hot rolling, coiling, cooling, cold rolling, continuous annealing, and cooling a steel slab satisfying the alloy composition described above.

Reheating

A steel slab satisfying the alloy composition of the present disclosure may be reheated at a temperature within a temperature range of 1100 to 1300°C.
Reheating may be performed to smoothly perform a subsequent rolling process and sufficiently obtain the target physical properties of the steel sheet. The present disclosure is not particularly limited to these reheating conditions, and any normal reheating condition may be possible. However, a preferable reheating temperature range may be 1100 to 1300°C.
If the reheating temperature is less than 1100°C, there is a concern that re-dissolution of precipitated elements, such as Nb and Ti, may decrease, thereby reducing the effect of adding the corresponding elements. Meanwhile, if the temperature exceeds 1300°C, there may be a problem that the process ratio increases and a large amount of hot-rolled oxides occur, resulting in poor surface quality of the steel sheet.

Hot rolling

The reheated steel slab may be hot-rolled at a finishing rolling temperature of 800 to 950°C.
In the present disclosure, the reheated steel slab may be hot-rolled at a normal hot-rolling temperature. Through hot rolling, a hot-rolled steel sheet in which carbides that become austenite nucleation sites are finely dispersed may be manufactured. By evenly dispersing the fine carbides during the hot rolling process, the austenite generated as the carbides are dissolved during annealing is finely dispersed, and as a result, the martensite generated during cooling after annealing may be finely and uniformly dispersed, which may contribute to improving the strength and elongation of the final steel sheet.
During hot rolling, if the finishing rolling temperature is less than 800°C, there may be a problem that a hot rolling load increases because the hot rolling temperature is low. Meanwhile, if the temperature exceeds 950°C, the grains may become coarser, reducing the strength of the steel sheet, and surface quality of the steel sheet may deteriorate due to an increase in hot-rolled oxides on a surface portion.

Coiling and Cooling

The hot-rolled steel sheet may be coiled at a temperature within a temperature range of 400 to 700°C and then cooled to room temperature at an average cooling rate of 0.10°C/s or less.
If the coiling temperature is less than 400°C, a large amount of low-temperature structures, such as martensite or bainite, may occur, thereby significantly increasing the strength of the hot-rolled steel sheet, which may cause a problem that a rolling load occurs during cold rolling. Meanwhile, if the temperature exceeds 700°C, the hot-rolled microstructure may become coarse, which may reduce the strength of a final annealed steel sheet, and there is a concern that the surface quality and plating properties of the steel sheet may deteriorate due to an increase in oxides on the surface of the steel sheet.
In addition, if the average cooling speed after coiling exceeds 0.10°C/s, the cold-rolling load may increase due to the formation of low-temperature structures and the shape of the hot-rolled steel sheet may deteriorate due to a rapid cooling speed, which may lead to a concern of the occurrence of strip breakage during cold rolling.

Cold rolling

The above-mentioned cooled steel sheet may be cold rolled at a reduction ratio of 40 to 70%.
During cold rolling, if the reduction ratio is less than 40%, it may be difficult to secure the target thickness and it may be difficult to correct the shape of the steel sheet. Meanwhile, if the reduction ratio exceeds 70%, there may be a high possibility of cracks occurring at the edge of the steel sheet and there may be a problem of causing a cold-rolling load. Therefore, in the present disclosure, it is preferable to limit the reduction ratio to 40 to 70%. As an embodiment of the present disclosure, a pickling process for pickling the steel sheet before cold rolling may be further included.

Continuous annealing

The cold rolled steel sheet may be heated to a T1 temperature of 800 to 850°C, cooled to a T2 temperature of 400 to 600°C at an average cooling rate of 20°C/s or less, and then maintained for 50 seconds or more for continuous annealing.
In the present disclosure, continuous annealing may be performed to form ferrite and austenite simultaneously with recrystallization and distribute carbon.
During continuous annealing, if the heating temperature T1 is less than 800°C, sufficient recrystallization may not occur, but it may also be difficult to form sufficient ideal austenite, making it impossible to secure the desired martensite and bainite fractions after annealing. Meanwhile, if the temperature exceeds 850°C, productivity may decrease and excessive austenite may be formed, which may significantly increase the bainite and martensite fractions after cooling, thereby increasing yield strength and decreasing ductility. In addition, surface thickening due to elements that reduce the wettability of hot-dip galvanizing, such as Si, Mn, and B, may become severe, thereby deteriorating plating surface quality. Considering this, in the present disclosure, it is preferable to limit the heating temperature to 800 to 850°C during the continuous annealing. The fractions of ideal austenite and ferrite in the steel sheet are determined in the above temperature range, and the strength of the final steel sheet appears to be different depending on the fractions. In general, as the fraction of ideal austenite increases, the strength of the final annealed steel sheet tends to increase, but a subsequent process may also affect the final microstructure, and thus, the physical properties of the steel sheet may change.
The ideal austenite in the heated steel sheet may transform into ferrite of different fractions depending on the cooling end temperature T2. If the cooling end temperature T2 exceeds 600°C during continuous annealing, a large amount of ferrite transformation may occur during heat treatment, which may cause a problem of reduced strength. Meanwhile, if the temperature is less than 400°C, an excessive bainite fraction may occur during the process of maintaining for 50 or more seconds and martensite formation may decrease, which may cause a problem of reduced strength.
Thereafter, the annealed steel sheet may be cooled to room temperature. When cooling to room temperature, the cooling conditions are not particularly limited, but, for example, air cooling may be performed.
The steel sheet according to an embodiment of the present disclosure may have an R value of 1797 to 1850, defined in Relational Expression 3, during continuous annealing. R = 174*[C] + 680*[Mn] + 370*[Nb] + 177*[Ti] - 86*[Cr] + 0.33*[T1] - 0.05*[T2]
(In Relational Expression 3, [C], [Mn], [Nb], [Ti], and [Cr] are wt% of each element, and T1 and T2 are the heating temperature (°C) and the cooling end temperature (°C) during continuous annealing, respectively.)
In the present disclosure, in order to satisfy both the target strength and bendability of the steel sheet during continuous annealing, the components and annealing conditions are specified, and the contents of the C, Si, Mn, and Al components and the T1 and T2 conditions are optimized.
The T1 temperature may refer to a heating temperature during the continuous annealing process, the fractions of ideal austenite and ferrite in the steel sheet may be determined by the corresponding temperature, and the strength of the final annealed steel sheet may differ depending on the corresponding fractions. In general, the strength of the final annealed steel sheet tends to increase as the fraction of ideal austenite increases, but it may be difficult to describe the effect of the annealing temperature alone because the subsequent process may also affect the final microstructure and change the physical properties of the steel sheet. The ideal austenite may undergo additional transformation into ferrite with different fractions depending on the T2 temperature, which is the cooling end temperature, during the subsequent cooling process, and which is, thus, one of the important factors affecting the physical properties of the steel sheet. In addition, the fractions of bainite, residual austenite, and martensite in the final annealed structure may differ depending on the T2 temperature.
If the T2 temperature is higher than a bainite transformation start temperature and lower than a martensite transformation temperature, bainite cannot be introduced to the steel sheet structure, so the T2 temperature has to be set to a temperature between the bainite transformation start temperature and the martensite transformation start temperature. The above-mentioned T1 and T2 temperatures affect the final annealed steel sheet microstructure together with the steel sheet components and consequently affect the physical properties of the steel sheet. In order to secure the target physical properties, the optimized Relational Expression 3 has to be satisfied. As a result, a high-strength steel sheet with the target strength and excellent bendability may be obtained even while minimizing the added amount of Si.
As described above, the final material of the steel sheet is affected by the components and the temperature and time of each important heat treatment process, and thus, when the conditions of the Relational Expression below are satisfied, a high-tension steel sheet with an optimal combination of physical properties and excellent bendability may be manufactured. Meanwhile, if the R value defined in Relational Expression 3 below is less than 1797, there may be a problem of the strength of the steel sheet being insufficient. Meanwhile, in order to secure the target bendability, the upper limit of the value may be limited to 1850.

Hot-dip galvanizing

According to an embodiment of the present disclosure, the continuously annealed steel sheet may be hot-dip galvanized at a temperature within a temperature range of 430 to 490°C.
Plating may be performed by performing a plating method of immersing the steel sheet manufactured in the present disclosure in a hot-dip galvanizing bath. In the present disclosure, the hot-dip galvanizing conditions are not particularly limited, and the hot-dip galvanizing may be performed under general conditions applicable in the same technical field. Through hot-dip galvanizing, the steel sheet according to an embodiment of the present disclosure may include a hot-dip galvanized layer on the surface. In addition, if necessary, the steel sheet may be alloyed and heat-treated after the hot-dip galvanizing step, and in an embodiment, the hot-dip galvanized steel sheet may be alloyed and heat-treated at a temperature within a temperature range of 460 to 530°C and then cooled to room temperature. Through alloying heat treatment, the steel sheet may include an alloyed hot-dip galvanized layer on the surface.
The steel sheet of the present disclosure manufactured in this manner has a tensile strength (TS) of 780 MPa or more, an elongation (El) of 14.0% or more, and a value of bending angle (°)/thickness (mm) of 50°/mm or more in a 180° bending test (here, the bending angle (°) refers to a bending angle at which no crack occurs in a bent portion in the 180° bending test). Thus, excellent properties of strength and bendability may be secured.

[Mode for invention]

Hereinafter, the present disclosure will be described more specifically through examples. However, it should be noted that the following examples are only intended to illustrate the present disclosure in more detail and are not intended to limit the scope of the rights of the present disclosure.

(Example)

After manufacturing a steel slab having the composition disclosed in Table 1 below, the steel slab was reheated under the conditions of Table 2 below and subjected to final hot rolling. The hot-rolled steel sheets were coiled under the conditions of Table 2 below and cooled to room temperature to manufacture steel sheets. Thereafter, the steel sheets were pickled and cold rolled at a reduction ratio of 50%, and, as disclosed in Table 2 below, heated at a temperature of T1, cooled to a temperature of T2, maintained for 50 or more seconds, hot dipped at a molten plating temperature of 460°C, and then finally cooled to room temperature.

[Table 1]

Stee l grad e	Alloy composition (wt%)										Relatio nal Express ion 1	Relati onal Expres sion2
Stee l grad e	C	Si	Mn	Sol.Al	Nb	Ti	Cr	P	S	N	Relatio nal Express ion 1	Relati onal Expres sion2
A	0.14	0.01	2.3	0.04	0.02	0	0.4	0.008	0.002	0.004	1654	0.03
B	0.08	0.02	2.4	0.04	0.02	0	0.7	0.007	0.002	0.004	1686	0.04
C	0.15	0	2.3	0.04	0	0	0.4	0.006	0.002	0.004	1647	0
D	0.16	0.03	2.3	0.04	0	0	0.3	0.008	0.002	0.004	1657	0.03
E	0.12	0.03	2.3	0.04	0.04	0	0.4	0.008	0.002	0.004	1658	0.07
F	0.12	0	2.3	0.04	0.04	0.02	0.4	0.008	0.002	0.004	1661	0.06

$T = 279 * [C] + 711 * [Mn] + 474 * [Nb] + 177 * [Ti] - 75 * [Cr]$
(In Relational Expression 1, [C], [Mn], [Nb], [Ti], and [Cr] are wt% of each element.)

$RT = [Si] + [Nb] + [Ti]$

(In Relational Expression 2, [Si], [Nb], and [Ti] are wt% of each element.)

[Table 2]

No. of spec imen	Stee l grad e	Reheating	Hot rolling	Coiling	Cooling	Continuous annealing		Relati onal Expres sion3
No. of spec imen	Stee l grad e	Temperat ure (°C)	Finishing temperature (°C)	Temperat ure (°C)	Rate (°C/s)	T1 (°C)	T2 (°C)	Relati onal Expres sion3
1	A	1210	905	610	0.05	770	520	1789
2	A	1210	905	610	0.05	790	520	1796
3	A	1210	905	610	0.05	810	520	1803
4	A	1210	905	610	0.05	830	520	1809
5	B	1198	899	605	0.03	770	520	1821
6	B	1198	899	605	0.03	800	560	1829
7	B	1198	899	605	0.03	810	560	1832
8	B	1198	899	605	0.03	830	560	1839
9	B	1198	899	605	0.03	850	450	1851
10	C	1201	887	608	0.04	810	560	1795
11	C	1201	887	608	0.04	830	560	1802
12	D	1201	887	608	0.04	810	520	1799
13	D	1201	887	608	0.04	830	520	1805
14	D	1201	887	608	0.04	810	560	1805
15	D	1201	887	608	0.04	830	560	1812
16	E	1188	911	599	0.03	810	520	1807
17	E	1188	911	599	0.03	830	520	1813
18	F	1202	888	615	0.04	770	560	1795
19	F	1202	888	615	0.04	790	560	1802
20	F	1202	888	615	0.04	810	560	1808
21	F	1202	888	615	0.04	830	560	1815
22	F	1202	888	615	0.04	830	390	1823

R = 174*[C] + 680*[Mn] + 370*[Nb] + 177*[Ti] - 86*[Cr] + 0.33*[T1] - 0.05*[T2]
(In Relational Expression 3, [C], [Mn], [Nb], [Ti], and [Cr] are wt% of each element, and T1 and T2 are the heating temperature (°C) and the cooling end temperature (°C) during continuous annealing, respectively.)

The results of measuring the mechanical physical properties for each steel sheet manufactured above are shown in Table 3 below. At this time, a tensile test for each test piece was performed in an L direction using the ASTM standard to evaluate the tensile properties at room temperature, and in particular, the bendability was measured as a value obtained by dividing a bending radius at which no cracks occurred in a bent portion by the thickness (mm) of the test piece by performing the 180° bending test. Here, the bent portion may refer to a portion of the steel sheet in which a bending angle is applied and may refer to a portion in which bending is usually applied. For the microstructure fraction, a matrix at a 1/4 point of the plate thickness of the continuously annealed steel sheet was analyzed and the results were used. Specifically, the fractions of ferrite (F), bainite (B), fresh martensite (M), and residual austenite (A) were measured using FE-SEM, an image analyzer, and XRD.

[Table 3]

No. of spec imen	Steel grade	Microstructure (area%)				Physical properties				Classifi cation
No. of spec imen	Steel grade	F	B	M	A	Yield strength (MPa)	Tensile strength (MPa)	Elongati on (%)	Bendabil ity (degree/ mm)	Classifi cation
1	A	83	0	16	1	437	855	11.5	36	Comparat ive Example 1
2	A	81	0	18	1	418	821	13.4	43	Comparat ive Example 2
3	A	68	11	19	2	435	816	16.1	55	Inventiv e Example 1
4	A	65	12	22	1	439	811	16.2	55	Inventiv e Example 2
5	B	81	0	19	1	429	845	13.6	59	Comparat ive Example 3
6	B	69	10	19	2	425	830	15.4	86	Inventiv e Example 3
7	B	67	11	21	1	438	833	17.3	82	Inventiv e Example 4
8	B	66	15	18	1	434	811	17.8	88	Inventiv e Example 5
9	B	20	55	25	0	512	905	12.5	41	Comparat ive Example 4
10	C	56	11	32	1	442	889	15.3	43	Comparat ive Example 5
11	C	52	14	33	1	471	886	15.2	45	Comparat ive Example 6
12	D	60	16	22	2	484	866	14.6	63	Inventiv e Example 6
13	D	67	11	20	2	488	878	14.7	58	Inventiv e Example 7
14	D	67	9	23	1	437	788	16.1	58	Inventiv e Example 8
15	D	72	10	17	1	434	813	15.6	55	Inventiv e Example 9
16	E	59	11	28	2	443	862	15.8	61	Inventiv e Example 10
17	E	69	8	22	1	449	864	16.2	71	Inventiv e Example 11
18	F	81	2	17	0	510	956	8.8	42	Comparat ive Example 7
19	F	81	0	19	0	430	858	13.7	54	Comparat ive Example 8
20	F	69	9	21	1	419	848	16.8	66	Inventiv e Example 12
21	F	72	8	19	1	424	833	17.4	68	Inventiv e Example 13
22	F	63	31	5	1	399	752	18.7	75	Comparat ive Example 9

As shown in Table 3, in the case of Inventive Examples satisfying the alloy composition and manufacturing conditions of the present disclosure, the microstructure characteristics proposed in the present disclosure were satisfied, and the physical properties targeted by the present disclosure were also secured. FIG. 1 is a photograph of the microstructure of Inventive Example 13 according to an embodiment of the present disclosure observed by an electron microscope.
Meanwhile, Comparative Examples 1 and 2 are examples in which the T1 temperature does not satisfy the conditions of the present disclosure during continuous annealing, and Relational Expression 3 is also not satisfied. As a result, the elongation and bendability did not reach the target.
Comparative Example 3 satisfied Relational Expression 3, but the T1 temperature did not satisfy the conditions of the present disclosure, so the elongation was inferior.
Comparative Example 4 satisfied the T1 and T2 temperatures during continuous annealing, but the Relational Expression 3 condition proposed by the present disclosure was not satisfied, so bainite was formed excessively compared to the area fraction targeted by the present disclosure, and thus, the target elongation and bendability characteristics were not secured.
Comparative Examples 5 and 6 are examples in which Relational Expressions 1 and 2 do not satisfy the conditions of the present disclosure, in which martensite was formed excessively compared to the area fraction targeted by the present disclosure, and thus, the bendability was inferior. FIG. 2 is a photograph of the microstructure of Comparative Example 6 according to an embodiment of the present disclosure observed by an electron microscope, and it can be seen that martensite was formed excessively.
Comparative Examples 7 and 8 have alloy composition conditions that satisfy the conditions of the present disclosure, but the T1 temperature was below the conditions of the present disclosure, so that ferrite was formed excessively compared to the area fraction targeted by the present disclosure, and bainite was insufficient, resulting in a decrease in elongation.
Comparative Example 9 had T2 below the conditions of the present disclosure, so that bainite was formed excessively compared to the proposed level and martensite was reduced, failing to secure the desired strength.
Although the present disclosure has been described in detail through examples above, other forms of examples are also possible. Therefore, the technical spirit and scope of the claims described below are not limited to the embodiments.

Claims

A steel sheet comprising:
in wt%, carbon (C): 0.05 to 0.20%, silicon (Si): 0.10% or less, manganese (Mn): 1.0 to 3.0%, aluminum (sol.Al): 1.00% or less, chromium (Cr): 0.1 to 1.0%, niobium (Nb): 0.05% or less, titanium (Ti): 0.05% or less, phosphorus (P): 0.100% or less, sulfur (S): 0.0100% or less, nitrogen (N): 0.010% or less, and a remainder of iron (Fe) and inevitable impurities,

having a T value of 1648 or more defined in Relational Expression 1 below, and

having a microstructure of, in area%, 50 to 80% of ferrite, 5 to 25% of bainite, 10 to 30% of fresh martensite, and 5% or less of residual austenite, $T = 279 * [C] + 711 * [Mn] + 474 * [Nb] + 177 * [Ti] - 75 * [Cr]$

where [C], [Mn], [Nb], [Ti], and [Cr] are wt% of each element.
The steel sheet of claim 1, wherein
the steel sheet has an RT value of 0.01 or more defined in Relational Expression 2 below, $RT = [Si] + [Nb] + [Ti]$
where [Si], [Nb], and [Ti] are wt% of each element.
The steel sheet of claim 1, wherein the steel sheet has a tensile strength (TS) of 780 MPa or more and an elongation (El) of 14.0% or more.
The steel sheet of claim 1, wherein the steel sheet, when subjected to a 180° bending test, has a value of bending angle (°)/thickness (mm) of 50°/mm or more where the bending angle (°) refers to a bending angle at which no crack occurs in a bent portion during the 180° bending test.
The steel sheet of claim 1, wherein the steel sheet further includes a hot-dip galvanized layer or an alloy hot-dip galvanized layer on a surface.
A method of manufacturing a steel sheet, the method comprising:
reheating a steel slab including, in wt%, carbon (C): 0.05 to 0.20%, silicon (Si): 0.10% or less, manganese (Mn): 1.0 to 3.0%, aluminum (sol.Al): 1.00% or less, chromium (Cr): 0.1 to 1.0%, niobium (Nb): 0.05% or less, titanium (Ti): 0.05% or less, phosphorus (P): 0.100% or less, sulfur (S): 0.010% or less, nitrogen (N): 0.010% or less, and a remainder of iron (Fe) and inevitable impurities, and having a T value of 1648 or more defined in Relational Expression 1 below;

hot-rolling the reheated steel slab;

coiling the hot-rolled steel sheet and then cooling the coiled steel sheet;

cold-rolling the cooled steel sheet;

heating the cold-rolled steel sheet to a T1 temperature of 800 to 850°C, cooling the heated steel sheet to a T2 temperature of 400 to 600°C at an average cooling rate of 20°C/s or less, and then maintaining the steel sheet for 50 seconds or more for continuous annealing; and

cooling the continuously annealed steel sheet to room temperature,

wherein an R value defined in Relational Expression 3 below is 1797 to 1850. $T = 279 * [C] + 711 * [Mn] + 474 * [Nb] + 177 * [Ti] - 75 * [Cr]$

where [C], [Mn], [Nb], [Ti], and [Cr] are wt% of each element, R = 174*[C] + 680*[Mn] + 370* [Nb] + 177*[Ti] - 86* [Cr] + 0.33*[T1] - 0.05*[T2]

where [C], [Mn], [Nb], [Ti], and [Cr] are wt% of each element, and T1 and T2 are a heating temperature (°C) and a cooling end temperature (°C) during continuous annealing, respectively.
The method of claim 6, wherein the steel slab has an RT value of 0.01 or more defined in Relational Expression 2 below. $RT = [Si] + [Nb] + [Ti]$ where [Si], [Nb], and [Ti] are the wt% of each element.
The method of claim 6, wherein
the reheating is performed at a temperature within a temperature range of 1100 to 1300°C,

the hot rolling is performed at a finishing rolling temperature of 800 to 950°C, and

in the cooling after coiling, the steel sheet is coiled at a temperature within a temperature range of 400 to 700°C and then cooled to room temperature at an average cooling rate of 0.10°C/s or less, and

the cold rolling is performed at a reduction ratio of 40 to 70%.
The method of claim 6, further comprising pickling the steel sheet before the cold rolling.
The method of claim 6, further comprising, after the continuous annealing and before the cooling, hot-dip galvanizing the steel sheet at a temperature within a temperature range of 430 to 490°C.
The method of claim 10, further comprising, after the hot-dip galvanizing,
performing alloying heat treatment on the steel sheet at a temperature within a temperature range of 460 to 530°C before cooling.