High-strength hot-rolled steel sheet, hot-rolled plated steel sheet and methods for producing the same

Abstract

The hot rolled steel sheet according to one aspect of the present invention may contain, by weight%, carbon (C): 0.1-0.25%, manganese (Mn): 0.2-2.0%, silicon (Si): 0.3% or less, aluminum (Al): 0.05% or less, phosphorus (P): 0.05% or less, sulfur (S): 0.03% or less, nitrogen (N): 0.01% or less, titanium (Ti): 0.005-0.05%, boron (B): 0.00 05-0.005%, niobium (Nb): less than 0.01%, molybdenum (Mo): less than 0.1%, vanadium (V): less than 0.1%, the balance of iron (Fe) and inevitable impurities, the microstructure may contain 55-90% bainite and 10-45% ferrite by volume fraction, and among the carbides present in the bainite, the number per unit area of ​​the carbides with a major axis length of 25-500nm may be 3*10 6 Pieces / mm 2 As described above, the average aspect ratio (major axis / minor axis) of the carbides present in the bainite may be 2.0 or less.

Description

Technical Field

[0001] The present invention relates to a high-strength hot-rolled steel plate, a hot-rolled plated steel plate and a method for manufacturing the same, and more particularly to a hot-rolled steel plate, a hot-rolled plated steel plate having physical properties suitable for steel building materials and a method for manufacturing the same. Background Art

[0002] High-strength hot-rolled steel sheets and hot-rolled plated steel sheets are mainly used as supporting structural parts. In particular, high-strength hot-rolled plated steel sheets have excellent deformation resistance and corrosion resistance, and are more economical than cold-rolled plated steel sheets, so they are widely used as steel building materials such as construction scaffolding, plastic shed structural parts, and solar brackets.

[0003] However, despite the increasing demand for high strength and light weight of hot-rolled steel sheets or hot-rolled plated steel sheets as described above, a realistic method for providing hot-rolled steel sheets or hot-rolled plated steel sheets having physical properties suitable as structural members has not been provided so far.

[0004] Patent Documents 1 to 4 disclose a technique for ensuring the strength of a steel plate by adding alloying elements for precipitation strengthening. These utilize conventional methods for manufacturing high-strength low-alloy steel (HSLA steel) and require the addition of alloying elements such as Ti, Nb, V, and Mo. Therefore, expensive alloying elements must be added, which is not preferred in terms of manufacturing cost, and the alloying elements as described above increase the rolling load, so there is a problem that thin materials cannot be manufactured.

[0005] Patent Documents 5 to 7 disclose a technology for ensuring strength by using a dual phase structure of ferrite and martensite or a composite structure of ferrite, bainite, martensite and retained austenite. However, ferrite and retained austenite have advantages in workability but are accompanied by disadvantages in yield strength, and there are technical difficulties in ensuring appropriate strength for structural parts.

[0006] Therefore, there is an urgent need to develop a hot-rolled steel sheet and a hot-rolled plated steel sheet which have high strength characteristics and can be reduced in thickness in terms of weight reduction and are suitable for use as supporting structural members.

[0007] (Prior art literature)

[0008] (Patent Document 1) Korean Patent Publication No. 10-2005-0113247 (published on December 1, 2005)

[0009] (Patent Document 2) Japanese Patent Publication No. 2002-322542 (published on November 8, 2002)

[0010] (Patent Document 3) Japanese Patent Publication No. 2006-161112 (published on June 22, 2006)

[0011] (Patent Document 4) Korean Patent Publication No. 10-2006-0033489 (published on April 19, 2006)

[0012] (Patent Document 5) Japanese Patent Publication No. 2005-298967 (published on October 27, 2005)

[0013] (Patent Document 6) U.S. Patent Publication No. 2005-0155673 (published on July 21, 2005)

[0014] (Patent Document 7) European Patent Publication No. 1396549 (published on March 10, 2004) Summary of the invention

[0015] Technical issues to be solved

[0016] According to one aspect of the present invention, a high-strength thin hot-rolled steel sheet, a hot-rolled plated steel sheet, and methods for manufacturing the same can be provided.

[0017] The technical problems of the present invention are not limited to the above contents. A person skilled in the art can easily understand the additional technical problems of the present invention from the overall content of this specification.

[0018] Technical Solution

[0019] The hot rolled steel sheet according to one aspect of the present invention may contain, by weight%, carbon (C): 0.1-0.25%, manganese (Mn): 0.2-2.0%, silicon (Si): 0.3% or less, aluminum (Al): 0.05% or less, phosphorus (P): 0.05% or less, sulfur (S): 0.03% or less, nitrogen (N): 0.01% or less, titanium (Ti): 0.005-0.05%, boron (B): 0.00 05-0.005%, niobium (Nb): less than 0.01%, molybdenum (Mo): less than 0.1%, vanadium (V): less than 0.1%, the balance of iron (Fe) and inevitable impurities, the microstructure may contain 55-90% bainite and 10-45% ferrite by volume fraction, and among the carbides present in the bainite, the number per unit area of ​​the carbides with a major axis length of 25-500nm may be 3*10 6 Pieces / mm 2As described above, the average aspect ratio (major axis / minor axis) of the carbides present in the bainite may be 2.0 or less.

[0020] The microstructure of the hot-rolled steel sheet may include, by volume fraction, one or more of 10% or less (including 0%) pearlite, 1% or less (including 0%) martensite, and 1% or less (including 0%) retained austenite.

[0021] The average lath bundle size of the bainite may be 50-200% of the average grain size of the ferrite.

[0022] The hot rolled steel plate may satisfy the following Relationship 1.

[0023] [Equation 1]

[0024] 30*[Ti]+100*[Nb]+5*[V]≤1.65

[0025] In the relational formula 1, [Ti], [Nb], and [V] each represent the content (weight %) of titanium (Ti), niobium (Nb), and vanadium (V) contained in the hot-rolled steel sheet.

[0026] The hot rolled steel sheet may further include 0.2% or less of chromium (Cr) in terms of weight %.

[0027] The bendability (R / t) of the hot-rolled steel sheet may be 1.0 or less.

[0028] The hot-rolled steel sheet may have a yield strength (YS) of 550 MPa or more, and a tensile strength (TS) of 650 MPa or more.

[0029] The thickness of the hot-rolled steel plate may be 3 mm or less (except 0 mm).

[0030] A hot-rolled plated steel sheet according to one aspect of the present invention may include a base steel sheet and a coating formed on at least one side of the base steel sheet, wherein the base steel sheet may be the hot-rolled steel sheet, and the coating may be any one coating selected from zinc, aluminum, zinc-based alloy, and aluminum-based alloy.

[0031] The method for manufacturing a hot-rolled steel plate according to one aspect of the present invention may include: a slab heating step, heating the slab, wherein the slab contains, in terms of weight %, carbon (C): 0.1-0.25%, manganese (Mn): 0.2-2.0%, silicon (Si): 0.3% or less, aluminum (Al): 0.05% or less, phosphorus (P): 0.05% or less, sulfur (S): 0.03% or less, nitrogen (N): 0.01% or less, titanium (Ti): 0.005-0.05%, boron (B): 0.0005-0.005%, niobium (Nb): 0.01% or less, molybdenum (Mo): 0.1% or less, vanadium (V): 0.1% or less, and the remainder is iron (Fe) and unavoidable impurities; and a hot rolling step, heating the heated slab to a rolling termination temperature (T fm ) is hot rolled to provide a steel plate; a first cooling step, cooling the hot rolled steel plate at a first cooling rate to a first cooling termination temperature (T) satisfying the following relational expression 4 1 ); a second cooling step, cooling the first cooled steel plate at a second cooling rate of 50-500°C / second faster than the first cooling rate to a second cooling termination temperature (T 2 ); a third cooling step, cooling the steel plate after the second cooling at a third cooling rate slower than the second cooling rate to a third cooling termination temperature (T 3 ); winding step, at the third cooling termination temperature (T 3 ) under the third cooling hot-rolled steel sheet is coiled; and a heat treatment step, in the temperature range of 450-720 ℃, the coiled steel sheet is heat treated.

[0032] [Equation 3]

[0033] T fm (℃)≥800+3000*[Ti]+10000*[Nb]-20*t^ (1 / 2)

[0034] In the relational expression 3, [Ti] and [Nb] each represent the content (weight %) of titanium (Ti) and niobium (Nb) contained in the slab, and t represents the thickness (mm) of the hot-rolled steel sheet.

[0035] [Equation 4]

[0036] T fm (℃)-80℃≤T 1 (℃)≤T fm (℃)-5℃

[0037] In the above relation 4, T fm represents the rolling end temperature (Tfm ).

[0038] [Equation 5]

[0039] T 2 (℃)≤750-270*[C]-90*[Mn]

[0040] In the relational expression 5, [C] and [Mn] each represent the content (weight %) of carbon (C) and manganese (Mn) contained in the slab.

[0041] [Equation 6]

[0042] 540-423*[C]-30.4*[Mn]≤T 3 (℃) <T 2 (℃)

[0043] In the above relation 6, T 2 represents the second cooling end temperature (T 2 ), [C] and [Mn] each represent the content (weight %) of carbon (C) and manganese (Mn) contained in the slab.

[0044] The first cooling rate may be 5-50° C. / second (s), and the third cooling rate may be 5-50° C. / second.

[0045] The slab may satisfy the following Relationship 1.

[0046] [Equation 1]

[0047] 30*[Ti]+100*[Nb]+5*[V]≤1.65

[0048] In the relational formula 1, [Ti], [Nb], and [V] each represent the content (weight %) of titanium (Ti), niobium (Nb), and vanadium (V) contained in the slab.

[0049] The slab may further include 0.2% or less of chromium (Cr) in terms of wt%.

[0050] The thickness of the hot-rolled steel plate may be 3 mm or less.

[0051] A method for manufacturing a hot-rolled coated steel sheet according to one aspect of the present invention may include: a step of preparing a base steel sheet; and a plating step of forming a coating on at least one side of the base steel sheet by any method selected from a hot-dip plating method, an electroplating method, and a plasma method, wherein the base steel sheet is provided by the method for manufacturing a hot-rolled steel sheet, and the coating is any coating selected from zinc, aluminum, a zinc-based alloy, and an aluminum-based alloy.

[0052] The solutions to the technical problems described do not list all the features of the present invention. The various features of the present invention and their advantages and effects can be understood in more detail with reference to the following specific implementation schemes.

[0053] Beneficial Effects

[0054] According to one aspect of the present invention, a hot-rolled steel sheet, a hot-rolled plated steel sheet, and a method for producing the same which have high strength characteristics suitable for a high-strength supporting structural member and can be reduced in weight and thickness can be provided.

[0055] The effects of the present invention are not limited to the above, and can be construed to include effects that can be derived from the description described below by those skilled in the art. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Figure 1 This is a photograph of the microstructure of the test piece 1 observed using a scanning electron microscope (SEM).

[0057] Best Practices

[0058] The present invention relates to a hot-rolled steel sheet, a hot-rolled plated steel sheet and a method for manufacturing the same. The preferred specific embodiments of the present invention are described below. The specific embodiments of the present invention can be modified into various forms, and it should not be interpreted that the scope of the present invention is limited to the specific embodiments described below. This specific embodiment is provided to explain the present invention in more detail to those skilled in the art.

[0059] Hereinafter, the hot-rolled steel sheet of the present invention will be described in more detail.

[0060] The hot rolled steel sheet according to one aspect of the present invention may contain, by weight%, carbon (C): 0.1-0.25%, manganese (Mn): 0.2-2.0%, silicon (Si): 0.3% or less, aluminum (Al): 0.05% or less, phosphorus (P): 0.05% or less, sulfur (S): 0.03% or less, nitrogen (N): 0.01% or less, titanium (Ti): 0.005-0.05%, boron (B): 0.00 05-0.005%, niobium (Nb): less than 0.01%, molybdenum (Mo): less than 0.1%, vanadium (V): less than 0.1%, the balance of iron (Fe) and inevitable impurities, the microstructure may contain 55-90% bainite and 10-45% ferrite by volume fraction, and among the carbides present in the bainite, the number per unit area of ​​the carbides with a major axis length of 25-500nm may be 3*10 6 Pieces / mm 2 As described above, the average aspect ratio (major axis / minor axis) of the carbides present in the bainite may be 2.0 or less.

[0061] The steel composition of the present invention will be described in more detail below. Unless otherwise specified, the percentages indicating the content of each element are based on weight.

[0062] The hot rolled steel sheet according to one aspect of the present invention may contain, by weight%, carbon (C): 0.1-0.25%, manganese (Mn): 0.2-2.0%, silicon (Si): 0.3% or less, aluminum (Al): 0.05% or less, phosphorus (P): 0.05% or less, sulfur (S): 0.03% or less, nitrogen (N): 0.01% or less, titanium (Ti): 0.005-0.05%, boron (B): 0.0005-0.005%, niobium (Nb): 0.01% or less, molybdenum (Mo): 0.1% or less, vanadium (V): 0.1% or less, the balance being iron (Fe) and unavoidable impurities, and may further contain 0.2 weight% or less of chromium (Cr).

[0063] Carbon (C): 0.1-0.25%

[0064] Carbon (C) is an element that helps to effectively improve the strength of the steel plate. In addition, carbon (C) is also an element that needs to be appropriately added to ensure the fine structure to be achieved by the present invention. Therefore, in order to ensure the desired level of strength and to ensure the desired level of bainite fraction during cooling after hot rolling, the present invention can add more than 0.1% carbon (C). The preferred lower limit of the carbon (C) content can be 0.12%, and the more preferred lower limit of the carbon (C) content can be 0.14%. However, when too much carbon (C) is added, a large amount of carbides are formed, resulting in poor formability, or weldability may be reduced when used as steel building materials, so the present invention can limit the carbon (C) content to less than 0.25%. The preferred upper limit of the carbon (C) content can be 0.24%, and the more preferred upper limit of the carbon (C) content can be 0.23%.

[0065] Manganese (Mn): 0.2-2.0%

[0066] Manganese (Mn) is not only an element that improves the strength and hardenability of steel, but also combines with sulfur (S) inevitably contained in the steel manufacturing process to form MnS, and is therefore also an element that helps to effectively inhibit the generation of cracks caused by sulfur (S). Therefore, the present invention may contain more than 0.2% manganese (Mn). The preferred lower limit of the content of manganese (Mn) may be 0.3%, and the more preferred lower limit of the content of manganese (Mn) may be 0.4%. However, when too much manganese (Mn) is added, not only the weldability is reduced, but it is also not preferred in terms of economic efficiency, so the present invention may limit the content of manganese (Mn) to less than 2.0%. The preferred upper limit of the content of manganese (Mn) may be 1.9%, and the more preferred upper limit of the content of manganese (Mn) may be 1.8%.

[0067] Silicon (Si): 0.3% or less

[0068] Silicon (Si) is not only an element that acts as a deoxidizer, but also an element that helps to effectively improve the strength of the steel plate. Therefore, the present invention can add silicon (Si) to achieve the above-mentioned effects. The preferred lower limit of the silicon content can be 0.01%, and the more preferred lower limit of the silicon content can be 0.02%. However, when the silicon (Si) content exceeds a certain level, the oxide scale formed on the surface of the steel plate causes the surface quality to decrease, and the weldability may deteriorate. The present invention can limit the silicon (Si) content to less than 0.3%. The preferred upper limit of the silicon (Si) content can be 0.2%, and the more preferred upper limit of the silicon (Si) content can be 0.1%.

[0069] Aluminum (Al): 0.05% or less

[0070] Aluminum (Al) is an element that acts as a deoxidizer by combining with oxygen in steel. The present invention can add aluminum (Al) for the above-mentioned effect. The lower limit of the preferred aluminum (Al) content can be 0.005%, and the more preferred lower limit of the aluminum (Al) content can be 0.01%. However, when too much aluminum (Al) is added, not only will the inclusions in the steel plate increase, but the workability of the steel plate may also decrease. Therefore, the present invention can limit the content of aluminum (Al) to less than 0.05%. The preferred upper limit of the aluminum (Al) content can be 0.04%.

[0071] Phosphorus (P): 0.05% or less

[0072] Phosphorus (P) is an impurity element inevitably contained in steel, and is an element that segregates on grain boundaries and reduces the toughness of steel. Therefore, it is preferred to control the P content to a low level as much as possible, and theoretically it is advantageous to limit the phosphorus (P) content to 0%. However, considering the content that is inevitably contained in the manufacturing process, the present invention can limit the phosphorus (P) content to less than 0.05%. The preferred upper limit of the phosphorus (P) content can be 0.04%, and the more preferred phosphorus (P) content can be 0.03%.

[0073] Sulfur (S): 0.03% or less

[0074] Sulfur (S) is an impurity inevitably contained in steel. It reacts with manganese (Mn) to form MnS, thereby increasing the content of precipitates, and is an element that makes steel embrittled. Therefore, it is preferred to control the S content to a low level as much as possible. In theory, it is advantageous to limit the sulfur (S) content to 0%. However, considering the content that is inevitably contained in the manufacturing process, the present invention can limit the sulfur (S) content to less than 0.03%. The upper limit of the preferred sulfur (S) content may be 0.02%, and the more preferred upper limit of the sulfur (S) content may be 0.01%.

[0075] Nitrogen (N): 0.01% or less

[0076] Nitrogen (N) is an impurity inevitably contained in steel, and is an element that forms nitrides during continuous casting and causes cracks in the slab. Therefore, it is preferred to control the N content to be as low as possible, and theoretically it is advantageous to limit the nitrogen (N) content to 0%. However, considering the content that is inevitably contained in the manufacturing process, the present invention can limit the nitrogen (N) content to less than 0.01%. The preferred nitrogen (N) content may be less than 0.008%, and the more preferred nitrogen (N) content may be less than 0.006%.

[0077] Titanium (Ti): 0.005-0.05%

[0078] Titanium (Ti) is an element that combines with carbon (C) or nitrogen (N) to form carbides and nitrides. In the present invention, boron (B) is added to ensure hardenability, but titanium (Ti) combines with nitrogen (N) before boron (B) combines with nitrogen (N), so the effect of adding boron (B) can be improved. The present invention can add more than 0.005% titanium (Ti) to achieve the effect as described above. The lower limit of the preferred titanium (Ti) content can be 0.01%, and the lower limit of the more preferred titanium (Ti) content can be 0.015%. However, when too much titanium (Ti) is added, it becomes a cause of reducing the continuous casting property in the slab manufacturing step, and increasing the rolling load during the hot rolling process may become a cause of reducing the rollability. Therefore, in the present invention, the content of titanium (Ti) can be limited to less than 0.05%. The lower limit of the preferred titanium (Ti) content can be 0.04%, and the lower limit of the more preferred titanium (Ti) content can be 0.03%.

[0079] Boron (B): 0.0005-0.005%

[0080] Boron (B) is an element that plays an important role in improving the hardenability of steel plates, and is an element that inhibits the phase transformation of ferrite or pearlite when cooling after rolling. The present invention can add more than 0.0005% of boron (B) to achieve the above-mentioned effects. The lower limit of the preferred boron (B) content can be 0.0007%, and the more preferred lower limit of the boron (B) content can be 0.001%. However, when the addition amount of boron (B) exceeds a certain level, there is a problem that the excessive addition of boron (B) combines with iron (Fe) to make the grain boundary brittle. The present invention can limit the content of boron (B) to less than 0.005%. The upper limit of the preferred boron (B) content can be 0.004%, and the more preferred upper limit of the boron (B) content can be 0.003%.

[0081] Niobium (Nb): 0.01% or less, Molybdenum (Mo): 0.1% or less, Vanadium (V): 0.1% or less

[0082] Niobium (Nb), molybdenum (Mo) and vanadium (V) are elements that react with carbon (C) or nitrogen (N) to form precipitates such as carbides or nitrides. However, they are expensive elements, and the price rises as the amount added increases. In addition, the rolling load increases during hot rolling, so it may be difficult to produce thin materials. Therefore, the purpose of the present invention is to suppress the addition of niobium (Nb), molybdenum (Mo) and vanadium (V), and even if they are inevitably added, the content of niobium (Nb), molybdenum (Mo) and vanadium (V) can be limited to less than 0.01%, less than 0.1%, and less than 0.01%, respectively. The upper limits of the preferred contents of niobium (Nb) and molybdenum (Mo) can be 0.005% and 0.01%, respectively. The contents of niobium (Nb), molybdenum (Mo) and vanadium (V) can be 0%, respectively, and the lower limits can be 0.0005%, respectively.

[0083] In addition, the contents of titanium (Ti), niobium (Nb), and vanadium (V) which form precipitates and reduce the rolling properties preferably satisfy the following Relational Expression 1.

[0084] [Equation 1]

[0085] 30*[Ti]+100*[Nb]+5*[V]≤1.65

[0086] In the relational formula 1, [Ti], [Nb], and [V] each represent the content (weight %) of titanium (Ti), niobium (Nb), and vanadium (V) contained in the hot-rolled steel sheet.

[0087] The above-mentioned relational expression 1 is a condition for providing a thin material steel sheet that ensures high strength characteristics and high formability. That is, when the contents of titanium (Ti), niobium (Nb) and vanadium (V) do not satisfy relational expression 1, hot rolling at a high temperature is unavoidable in order to manufacture a thin material, and the aspect ratio (major axis / minor axis) of carbides exceeds a desired level, so that the bending workability (R / t) may be deteriorated.

[0088] Chromium (Cr): 0.2 wt% or less

[0089] Chromium (Cr) is an element that helps improve the hardenability of steel, so the present invention can selectively add chromium (Cr) to achieve this effect. However, chromium (Cr) is an expensive element, and it is not economically preferable to add too much chromium (Cr). Adding too much chromium (Cr) may cause a decrease in weldability, so the present invention can limit the content of chromium (Cr) to less than 0.2%. The upper limit of the preferred chromium (Cr) content can be 0.15%, and the more preferred upper limit of the chromium (Cr) content can be 0.1%.

[0090] In addition to the above-mentioned components, the hot-rolled steel sheet of one aspect of the present invention may contain a balance of Fe and other inevitable impurities. However, in the usual manufacturing process, undesirable impurities are inevitably mixed from the raw materials or the surrounding environment, so these impurities cannot be completely excluded. These impurities are well known to those skilled in the art, so all of them are not particularly mentioned in this specification. In addition, the further addition of effective ingredients other than the above-mentioned components is not completely excluded.

[0091] The microstructure of the hot-rolled steel sheet according to one aspect of the present invention may include 55-90 volume % of bainite and 10-45 volume % of ferrite, and may further include one or more of 10 volume % or less (including 0%) of pearlite, 1 volume % or less (including 0%) of martensite, and 1 volume % or less (including 0%) of retained austenite.

[0092] Bainite is an effective microstructure for improving strength, and ferrite is an effective microstructure for ensuring ductility, so in order to achieve both high strength characteristics and excellent formability, it is necessary to appropriately control the fractions of bainite and ferrite. Therefore, the present invention can control the fraction of bainite within the range of 55-90 volume %, and the fraction of ferrite within the range of 10-45%.

[0093] In addition, the microstructure of the hot-rolled steel sheet of the present invention may include one or more of pearlite, martensite, retained austenite and other precipitates, but in terms of both high strength properties and excellent formability, the fraction thereof may be limited to a certain range or less. When a large amount of pearlite is formed, the formation of a composite structure leads to a decrease in formability or a decrease in the fraction of bainite, so it is difficult to ensure strength. The present invention may limit the fraction of pearlite to less than 10 volume % (including 0%). Martensite is advantageous in ensuring strength, but disadvantageous in ensuring formability, so the present invention may limit the fraction of martensite to less than 1% (including 0%). Retained austenite is advantageous in ensuring formability, but disadvantageous in ensuring yield strength, so the present invention may limit the fraction of retained austenite to less than 1% (including 0%).

[0094] Of the carbides present in the bainite, the number per unit area of ​​carbides having a major axis length of 25 to 500 nm is preferably 3*10 6 Pieces / mm 2 Among the carbides present in bainite, carbides with too small a particle size will increase the rolling load during hot rolling, making it difficult to roll thin materials, and carbides with too large a particle size will have an adverse effect on ensuring strength and ductility. In addition, the amount of carbides present in bainite is preferably a certain amount or more to ensure strength. Therefore, among the carbides present in bainite of the hot-rolled steel sheet of the present invention, the amount per unit area of ​​carbides with a major axis length of 25-500 nm can be controlled to 3*10 6 Pieces / mm 2 above.

[0095] The shape of carbides in bainite is also a factor that greatly affects the formability of the steel sheet. The closer the shape of carbides in bainite is to a sphere, the more advantageous it is for ensuring the formability of the steel sheet. Therefore, in the present invention, the average aspect ratio (major axis / minor axis) of carbides in bainite can be limited to 2.0 or less in terms of ensuring formability.

[0096] In addition, the larger the size difference between bainite and ferrite, the less preferable it is in terms of formability. Therefore, the average lath bundle size of bainite in the hot-rolled steel sheet of the present invention can be controlled to a level of 50-200% of the average grain size of ferrite.

[0097] The hot rolled steel sheet of one aspect of the present invention may be a thin material having a thickness of 3 mm or less (except 0 mm), and may have a yield strength (YS) of 550 MPa or more, a tensile strength (TS) of 650 MPa or more, and a bendability (R / t) of 1.0 or less.

[0098] The yield strength and thickness of the hot-rolled steel sheet of one aspect of the present invention may satisfy the following Relationship 2, thereby achieving the goals of high strength and light weight due to thinning.

[0099] [Equation 2]

[0100] YS-100*t≥350

[0101] In the above relational expression 2, YS represents the yield strength of the hot-rolled steel sheet (MPa), and t represents the thickness of the hot-rolled steel sheet (mm).

[0102] In addition, the hot-rolled steel sheet of one aspect of the present invention may be a hot-rolled plated steel sheet having a coating provided on at least one side. The composition of the coating provided on the hot-rolled plated steel sheet of the present invention is not particularly limited, and as a non-limiting example, it may be any coating selected from zinc, aluminum, a zinc-based alloy, and an aluminum-based alloy. In addition, as a non-limiting example, the zinc-based alloy coating may be a coating comprising one or more selected from aluminum (Al), magnesium (Mg), nickel (Ni), and iron (Fe) and zinc (Zn) as the remainder, and the aluminum-based alloy coating may be a coating comprising one or more selected from silicon (Si), magnesium (Mg), nickel (Ni), and iron (Fe) and aluminum (Al) as the remainder.

[0103] Hereinafter, a method for producing a high-strength hot-rolled steel sheet according to one aspect of the present invention will be described in more detail.

[0104] The method for manufacturing a hot-rolled steel plate according to another aspect of the present invention may include: a slab heating step, heating the slab, wherein the slab contains, in terms of weight %, carbon (C): 0.1-0.25%, manganese (Mn): 0.2-2.0%, silicon (Si): 0.3% or less, aluminum (Al): 0.05% or less, phosphorus (P): 0.05% or less, sulfur (S): 0.03% or less, nitrogen (N): 0.01% or less, titanium (Ti): 0.005-0.05%, boron (B): 0.0005-0.005%, niobium (Nb): 0.01% or less, molybdenum (Mo): 0.1% or less, vanadium (V): 0.1% or less, and the remainder is iron (Fe) and unavoidable impurities; and a hot rolling step, heating the heated slab to a rolling termination temperature (T fm ) is hot rolled to provide a steel plate; a first cooling step, cooling the hot rolled steel plate at a first cooling rate to a first cooling termination temperature (T) satisfying the following relational expression 4 1 ); a second cooling step, cooling the first cooled steel plate at a second cooling rate of 50-500°C / second faster than the first cooling rate to a second cooling termination temperature (T 2); a third cooling step, cooling the steel plate after the second cooling at a third cooling rate slower than the second cooling rate to a third cooling termination temperature (T 3 ); winding step, at the third cooling termination temperature (T 3 ) under the third cooling hot-rolled steel sheet is coiled; and a heat treatment step, in the temperature range of 450-720 ℃, the coiled steel sheet is heat treated.

[0105] [Equation 3]

[0106] T fm (℃)≥800+3000*[Ti]+10000*[Nb]-20*t^ (1 / 2)

[0107] In the relational expression 3, [Ti] and [Nb] each represent the content (weight %) of titanium (Ti) and niobium (Nb) contained in the slab, and t represents the thickness (mm) of the hot-rolled steel sheet.

[0108] [Equation 4]

[0109] T fm (℃)-80℃≤T 1 (℃)≤T fm (℃)-5℃

[0110] In the above relation 4, T fm represents the rolling end temperature (T fm ).

[0111] [Equation 5]

[0112] T 2 (℃)≤750-270*[C]-90*[Mn]

[0113] In the relational expression 5, [C] and [Mn] each represent the content (weight %) of carbon (C) and manganese (Mn) contained in the slab.

[0114] [Equation 6]

[0115] 540-423*[C]-30.4*[Mn]≤T 3 (℃)≤T 2 (℃)

[0116] In the above relation 6, T 2 represents the second cooling end temperature (T 2 ), [C] and [Mn] each represent the content (weight %) of carbon (C) and manganese (Mn) contained in the slab.

[0117] Billet preparation and heating

[0118] A steel slab having a predetermined composition is prepared. The steel slab of the present invention has an alloy composition corresponding to the alloy composition of the hot-rolled steel sheet, and therefore the alloy composition of the steel slab will be replaced by the alloy composition of the hot-rolled steel sheet.

[0119] The prepared steel slab may be heated to a certain temperature range, wherein the heating method and heating temperature of the steel slab are not particularly limited. As an example, the heating temperature of the steel slab may be in the range of 1100-1350° C. This is because, when the heating temperature of the steel slab is lower than 1100° C., hot rolling may be performed in a temperature range below the desired hot finishing rolling temperature range, and when the heating temperature of the steel slab exceeds 1350° C., additional costs may be generated due to excessive energy input or a thick oxide scale may be formed on the surface of the slab.

[0120] Hot Rolling

[0121] After heating the steel slab, hot rolling is performed, so that a hot rolled steel sheet having a thickness of 3 mm or less (except 0 mm) can be provided. If the temperature of the heated steel slab is a temperature at which conventional hot rolling can be performed, hot rolling can be performed directly without special reheating. If the temperature of the heated steel slab is a temperature lower than a temperature at which conventional hot rolling can be performed, hot rolling can be performed after reheating.

[0122] At this time, the hot rolling end temperature (T fm ) preferably satisfies the following relational expression 3. This is because, when the rolling temperature is too low, the rolling load increases, which reduces the rollability, or causes surface roughness due to roll fatigue, and there is a possibility that the aspect ratio of the carbide is not at the desired level due to low-temperature rolling.

[0123] [Equation 3]

[0124] T fm (℃)≥800+3000*[Ti]+10000*[Nb]-20*t^ (1 / 2)

[0125] In the relational expression 3, [Ti] and [Nb] each represent the content (weight %) of titanium (Ti) and niobium (Nb) contained in the slab, and t represents the thickness (mm) of the hot-rolled steel sheet.

[0126] First Cooling

[0127] The hot rolled steel sheet can be cooled at a first cooling rate (V C1 ) is cooled to the first cooling end temperature (T) satisfying the following relation 4 1 ).

[0128] [Equation 4]

[0129] T fm (℃)-80℃≤T 1 (℃)≤T fm (℃)-5℃

[0130] In the above relation 4, T fm represents the rolling end temperature (T fm ).

[0131] When the first cooling end temperature (T 1 ) is below a certain level, a phase transformation to ferrite or pearlite will occur, and the desired fine structure of the present invention cannot be ensured, so the desired level of strength may not be ensured. Therefore, the present invention can reduce the first cooling end temperature (T 1 ) is limited to T fm (℃)-80℃. In addition, when the first cooling end temperature (T 1 ) exceeds a certain level, the austenite grains become coarse and uneven, and there is a possibility that the difference between the average lath bundle size of bainite and the average grain size of ferrite is too large. Therefore, the first cooling end temperature (T 1 ) is limited to T fm (℃)-5℃.

[0132] The first cooling rate (V C1 ) As long as the cooling rate is applied to conventional slow cooling, it can be limited to the range of 5-50°C / second in terms of preventing poor steel plate shape and ensuring the desired fine structure.

[0133] Second cooling

[0134] The first cooling of the steel plate can be carried out at a second cooling rate (V C2 ) is cooled to the second cooling end temperature (T) satisfying the following relational expression 5 2 ).

[0135] [Equation 5]

[0136] T 2 (℃)≤750-270*[C]-90*[Mn]

[0137] In the relational expression 5, [C] and [Mn] each represent the content (weight %) of carbon (C) and manganese (Mn) contained in the slab.

[0138] When the second cooling end temperature (T 2) exceeds a certain level, a phase transformation to ferrite or pearlite will occur, and the desired fine structure of the present invention cannot be ensured, and thus the desired level of strength may not be ensured. Therefore, the present invention can set the second cooling end temperature (T 2 ) is limited to 750-270*[C]-90*[Mn].

[0139] The second cooling is preferably performed at a faster rate than the first cooling. C2 ) can be in the range of 50-500°C / sec. When the second cooling rate (V C2 ) is less than 50°C / sec, the phase transformation to ferrite or pearlite increases, which may cause a decrease in strength. In addition, in order to achieve a cooling rate exceeding 500°C / sec, additional equipment is required, which is not preferred in terms of economic efficiency.

[0140] Third cooling and winding

[0141] The second cooling of the steel plate can be carried out at a third cooling rate (V C3 ) is cooled to the third cooling end temperature (T) satisfying the following relation 6 3 ), and then at the third cooling end temperature (T 3 ) to roll up.

[0142] [Equation 6]

[0143] 540-423*[C]-30.4*[Mn]≤T 3 (℃) <T 2 (℃)

[0144] In the above relation 6, T 2 represents the second cooling end temperature (T 2 ), [C] and [Mn] each represent the content (weight %) of carbon (C) and manganese (Mn) contained in the slab.

[0145] The third cooling step is used to control the coiling temperature and can be performed at a lower cooling rate than the second cooling rate (V C2 ) Slower third cooling rate (V C3 ) is cooled to a temperature lower than the second cooling end temperature (T 2 ) lower third cooling end temperature (T 3 The preferred third cooling rate (V C3 ) can be in the range of 5-50°C / sec.

[0146] However, when the third cooling end temperature (T 3 ) is too low, too much low-temperature structure is formed, so the desired fine structure and formability cannot be ensured. In the present invention, the third cooling termination temperature (T3 ) is limited to 540-423*[C]-30.4*[Mn].

[0147] Heat Treatment

[0148] The heat treatment can be performed by heating the coiled steel sheet and then maintaining the temperature. In order to ensure the temperature of the steel sheet in the subsequent plating process and stably ensure carbides, the heat treatment can be performed in the temperature range of 450-720°C.

[0149] Plating

[0150] After the heat treatment, a coating may be formed on at least one side of the heat-treated steel sheet. The composition and formation method of the coating of the present invention are not particularly limited, and can be interpreted as a concept including the composition and formation method of the coating generally provided to the hot-rolled plated steel sheet. As an example, the coating may be formed by any method selected from a hot-dip coating method, an electroplating method, and a plasma method, and the coating may be any coating selected from zinc, aluminum, a zinc-based alloy, and an aluminum-based alloy.

[0151] In the hot-rolled steel sheet provided by the above manufacturing method, the microstructure may include 55-90 volume % of bainite and 10-45 volume % of ferrite, and may further include 10 volume % or less (including 0%) of pearlite, 1 volume % or less (including 0%) of martensite, and 1 volume % or less (including 0%) of retained austenite. Among the carbides present in the bainite, the number per unit area of ​​carbides with a major axis length of 25-500 nm may be 3*10 6 Pieces / mm 2 The average aspect ratio (major axis / minor axis) of carbides present in the bainite may be 2.0 or less. In addition, the average lath bundle size of bainite in the hot-rolled steel sheet provided by the above manufacturing method may be 50-200% of the average grain size of ferrite.

[0152] The hot-rolled steel sheet provided by the above manufacturing method has a thickness of 3 mm or less (except 0 mm), and can satisfy a yield strength (YS) of 550 MPa or more, a tensile strength (TS) of 650 MPa or more, and a bending workability (R / t) of 1.0 or less. In addition, the yield strength and thickness of the hot-rolled steel sheet provided by the above manufacturing method can satisfy the following relational expression 2.

[0153] [Equation 2]

[0154] YS-100*t≥350

[0155] In the above relational expression 2, YS represents the yield strength of the hot-rolled steel sheet (MPa), and t represents the thickness of the hot-rolled steel sheet (mm).

[0156] Specific implementation methods

[0157] The high-strength hot-rolled steel sheet and the method for manufacturing the same of the present invention are described in more detail below through specific implementation schemes. It should be noted that the following implementation schemes are only used to understand the present invention and are not used to specify the scope of rights of the present invention. The scope of rights of the present invention is determined by the contents recorded in the claims and the contents reasonably derived therefrom.

[0158] (Example)

[0159] For the steel slabs having the alloy composition of the following Table 1, the process conditions of the following Table 2 are applied to manufacture hot rolled steel sheets. The remainder in Table 1 is iron (Fe) and unavoidable impurities, and the steel slab heating condition of 1200° C. is generally applied.

[0160] [Table 1]

[0161]

[0162] [Table 2]

[0163]

[0164] Afterwards, the microstructure of each test piece was observed and the mechanical properties were measured and recorded in Table 3.

[0165] Each test piece was cut in a direction parallel to the rolling direction, and then a test piece for microstructure observation was collected at a cross section at a position of 1 / 4 of the plate thickness. The samples collected in this way were ground and corroded with a nitric acid etching solution, and then the microstructure of each test piece was observed using an optical microscope and a scanning electron microscope (SEM). The fraction of the microstructure was measured by image analysis. The average size of the bainite lath bundle and the average size of the ferrite grains were measured at a position of 1 / 4 of the plate thickness by using electron backscatter diffraction (EBSD). The position where the orientation difference exceeded 15° was defined as the grain boundary of the grain and the average size of the bainite lath bundle was measured, and the average size of the ferrite grain was measured using the linear intercept method. In Table 3, F represents ferrite, P represents pearlite, B represents bainite, M represents martensite, and R-γ represents retained austenite. The scanning electron microscope (SEM) was set to 5000 times magnification, and the electron microscope images of bainite present in different regions of each sample were ensured to be at least 5 or more, and square areas of 20 μm in the horizontal direction and 20 μm in the vertical direction were calculated in the images obtained in this way, thereby measuring the size, number density and average aspect ratio of the carbides. The number density of carbides in Table 3 refers to the number density of carbides with a major axis length of 25-500 nm.

[0166] The tensile strength and yield strength were measured by performing a tensile test on each test piece in the L direction according to the DIN standard. The bending workability (R / t) was measured by applying a bending test using a 90° V-bend (V-type bend). The test piece was bent 90° using V-shaped fixtures having different curvatures (R) and then the measurement was performed by confirming whether cracks occurred in the bent portion.

[0167] [Table 3]

[0168]

[0169] As shown in Tables 1 to 3, it can be confirmed that all test pieces satisfying the alloy composition and process conditions of the present invention satisfy the yield strength (YS) of 550 MPa or more, the tensile strength (TS) of 650 MPa or more, and the bending workability (R / t) of 1.0 or less, and the test pieces not satisfying any one or more of the alloy composition or process conditions of the present invention do not simultaneously satisfy the yield strength (YS) of 550 MPa or more, the tensile strength (TS) of 650 MPa or more, and the bending workability (R / t) of 1.0 or less.

[0170] The present invention has been described in detail above through the embodiments, but other forms of embodiments may also be included. Therefore, the technical concept and scope of the claims are not limited to the embodiments.

Claims

1. A hot-rolled steel sheet, comprising, by weight%, carbon (C): 0.1-0.25%, manganese (Mn): 0.2-2.0%, silicon (Si): 0.3% or less, aluminum (Al): 0.05% or less, phosphorus (P): 0.05% or less, sulfur (S): 0.03% or less, nitrogen (N): 0.01% or less, titanium (Ti): 0.005-0.05%, boron (B): 0.0005-0.005%, niobium (Nb): 0.01% or less, molybdenum (Mo): 0.1% or less, vanadium (V): 0.1% or less, and the balance being iron (Fe) and unavoidable impurities, In terms of volume fraction, the microstructure contains 55-90% bainite and 10-45% ferrite. Among the carbides present in the bainite, the number per unit area of ​​carbides having a major axis length of 25-500 nm is 3*10 6 Pieces / mm 2 above, The average aspect ratio of carbides present in the bainite, that is, major axis / minor axis, is 2.0 or less.

2. The hot rolled steel sheet according to claim 1, wherein: The microstructure of the hot-rolled steel sheet includes, by volume fraction, one or more of pearlite of 10% or less and including 0%, martensite of 1% or less and including 0%, and retained austenite of 1% or less and including 0%.

3. The hot rolled steel sheet according to claim 1, wherein: The average lath bundle size of the bainite is 50-200% of the average grain size of the ferrite.

4. The hot rolled steel sheet according to claim 1, wherein: The hot-rolled steel plate satisfies the following relational expression 1: [Equation 1] 30*[Ti]+100*[Nb]+5*[V]≤1.65 In the relational formula 1, [Ti], [Nb], and [V] each represent the content of titanium (Ti), niobium (Nb), and vanadium (V) contained in the hot-rolled steel sheet, wherein the unit of the content is weight %.

5. The hot rolled steel sheet according to claim 1, wherein: The hot-rolled steel sheet further includes 0.2% or less of chromium (Cr) in terms of weight %.

6. The hot rolled steel sheet according to claim 1, wherein: The hot-rolled steel sheet has a bending workability R / t of 1.0 or less.

7. The hot rolled steel sheet according to claim 1, wherein: The hot-rolled steel sheet has a yield strength YS of 550 MPa or more and a tensile strength TS of 650 MPa or more.

8. The hot rolled steel sheet according to claim 1, wherein: The thickness of the hot-rolled steel sheet is 3 mm or less, excluding 0 mm.

9. A hot-rolled plated steel sheet comprising a base steel sheet and a plated layer formed on at least one side of the base steel sheet, wherein the base steel sheet is the hot-rolled steel sheet according to any one of claims 1 to 8, and the plated layer is any one selected from zinc, aluminum, zinc alloy, and aluminum alloy.

10. A method for manufacturing a hot-rolled steel plate, comprising: a slab heating step, heating the slab, wherein the slab comprises, in terms of weight percent, carbon (C): 0.1-0.25%, manganese (Mn): 0.2-2.0%, silicon (Si): 0.3% or less, aluminum (Al): 0.05% or less, phosphorus (P): 0.05% or less, sulfur (S): 0.03% or less, nitrogen (N): 0.01% or less, titanium (Ti): 0.005-0.05%, boron (B): 0.0005-0.005%, niobium (Nb): 0.01% or less, molybdenum (Mo): 0.1% or less, vanadium (V): 0.1% or less, and the remainder is iron (Fe) and unavoidable impurities; In the hot rolling step, the heated slab is subjected to a rolling end temperature T that satisfies the following relation 3: fm hot rolling to provide steel plates; A first cooling step, cooling the hot-rolled steel plate at a first cooling rate to a first cooling end temperature T1 satisfying the following relational expression 4; A second cooling step, cooling the first cooled steel plate at a second cooling rate of 50-500° C. / s faster than the first cooling rate to a second cooling termination temperature T2 satisfying the following relational expression 5; a third cooling step of cooling the steel plate after the second cooling at a third cooling rate slower than the second cooling rate to a third cooling termination temperature T3 satisfying the following relational expression 6; a coiling step, coiling the third cooled hot-rolled steel sheet at the third cooling end temperature T3; and The heat treatment step is to heat treat the coiled steel plate at a temperature range of 450-720°C. [Equation 3] T fm (℃)≥800+3000*[Ti]+10000*[Nb]-20*t^ (1 / 2) In the relational expression 3, [Ti] and [Nb] respectively represent the contents of titanium (Ti) and niobium (Nb) contained in the slab, wherein the unit of the contents is weight %, t represents the thickness of the hot-rolled steel sheet, wherein the unit of the thickness is mm, [Equation 4] T fm (℃)-80℃≤T1(℃)≤T fm (℃)-5℃ In the above relation 4, T fm represents the rolling end temperature T of the hot rolling step fm , [Equation 5] T2(℃)≤750-270*[C]-90*[Mn] In the above relational expression 5, [C] and [Mn] represent the contents of carbon (C) and manganese (Mn) contained in the slab, respectively, wherein the units of the contents are wt %. [Equation 6] 540-423*[C]-30.4*[Mn]≤T3(℃) <T2(℃) In the relational expression 6, T2 represents the second cooling termination temperature T2 of the second cooling step, and [C] and [Mn] respectively represent the contents of carbon (C) and manganese (Mn) contained in the slab, wherein the unit of the content is weight %.

11. The method for manufacturing a hot-rolled steel sheet according to claim 10, wherein: The first cooling rate is 5-50°C / second, and the third cooling rate is 5-50°C / second.

12. The method for manufacturing a hot-rolled steel sheet according to claim 10, wherein: The slab satisfies the following relation 1, [Equation 1] 30*[Ti]+100*[Nb]+5*[V]≤1.65 In the relational formula 1, [Ti], [Nb], and [V] each represent the content of titanium (Ti), niobium (Nb), and vanadium (V) contained in the slab, wherein the unit of the content is weight %.

13. The method for manufacturing a hot rolled steel sheet according to claim 10, wherein: The slab further includes 0.2% or less of chromium (Cr) in terms of weight %.

14. The method for manufacturing a hot-rolled steel sheet according to claim 10, wherein: The thickness of the hot-rolled steel sheet is 3 mm or less.

15. A method for manufacturing a hot-rolled coated steel sheet, comprising: Steps to prepare the base steel plate; as well as a plating step of forming a plating layer on at least one side of the base steel sheet by any one method selected from a hot dip plating method, an electroplating method, and a plasma method, The base steel sheet is provided by the manufacturing method according to any one of claims 10 to 14, and the coating is any one coating selected from zinc, aluminum, zinc alloy, and aluminum alloy.

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