KR102487759B1 - High strength hot-rolled steel sheet and hot-rolled plated steel sheet, and manufacturing method for thereof - Google Patents

Abstract

One aspect of the present invention, in weight percent, carbon (C): 0.1 ~ 0.2%, manganese (Mn): 0.8 ~ 2.0%, silicon (Si): 0.01 ~ 0.3%, 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): 0.01 to 0.03%, Boron (B): 0.0005 to 0.005%, Niobium (Nb) ): 0.01% or less, molybdenum (Mo): 0.1% or less, vanadium (V): 0.1% or less, including iron (Fe) and unavoidable impurities, the contents of B and Ti satisfy a specific relationship, and the volume fraction 55 to 90% of bainite and 10 to 45% of ferrite as a microstructure, and among the carbides present in the bainite, the number of carbides per unit area having a long axis of 25 to 500 nm is 3 * 10 ⁶ / mm ² or more, and the average aspect ratio of carbides present in the bainite is 2.0 or less, a hot-rolled galvanized steel sheet using the same, and a manufacturing method thereof are provided.

Description

High-strength hot-rolled steel sheet and hot-rolled plated steel sheet, and manufacturing method for them}

The present invention relates to a high-strength hot-rolled steel sheet, a hot-rolled galvanized steel sheet, and a manufacturing method thereof, and more particularly, to a hot-rolled steel sheet having physical properties suitable for steel building materials, a hot-rolled galvanized steel sheet, and a manufacturing method thereof.

High-strength hot-rolled steel sheets and hot-rolled galvanized steel sheets are mainly used as structural materials for support. In particular, high-strength hot-rolled galvanized steel sheet has excellent deformation resistance and corrosion resistance, which are characteristics that prevent deformation while supporting a load, and has excellent economic feasibility compared to cold-rolled galvanized steel sheet. It is widely used as a material for steel construction materials.

However, although there is an increasing demand for high strength and light weight for such hot-rolled steel sheet or hot-rolled galvanized steel sheet, a realistic solution has not been provided so far to provide a hot-rolled steel sheet or hot-rolled galvanized steel sheet having suitable physical properties as a structural material.

Patent Documents 1 to 4 disclose techniques for securing the strength of a steel sheet by precipitation hardening by adding an alloying element. These are made using a conventional manufacturing method of HSLA steel (High Strength Low Alloy Steel), and alloy elements such as Ti, Nb, V, and Mo must be added in a certain amount or more. Therefore, since a large amount of expensive alloying elements must be added, it is not preferable in terms of manufacturing cost.

Patent Documents 5 to 7 disclose techniques for securing strength by using a dual phase structure of ferrite and martensite or by utilizing a complex structure of ferrite, bainite, martensite, and retained austenite. However, ferrite and retained austenite have advantages in terms of workability but have disadvantages in terms of yield strength, and thus have technical difficulties in securing suitable strength as a structural material.

Therefore, it is necessary to develop low-cost hot-rolled steel sheets and hot-rolled galvanized steel sheets that have high-strength characteristics suitable as structural materials for support, are lightweight and do not add a large amount of expensive alloy elements, but no technology has been developed to meet these demands. did not

Korean Patent Publication No. 10-2005-0113247 (published on December 1, 2005) Japanese Unexamined Patent Publication No. 2002-322542 (published on November 8, 2002) Japanese Unexamined Patent Publication No. 2006-161112 (published on June 22, 2006) Korean Patent Publication No. 10-2006-0033489 (published on April 19, 2006) Japanese Unexamined Patent Publication No. 2005-298967 (published on October 27, 2005) US Patent Publication No. 2005-0155673 (published on July 21, 2005) European Patent Publication No. 1396549 (published on March 10, 2004)

According to one aspect of the present invention, a high-strength hot-rolled steel sheet and a hot-rolled coated steel sheet, and a manufacturing method thereof may be provided.

The object of the present invention is not limited to the above. A person skilled in the art will have no difficulty understanding the further subject matter of the present invention from the general content of this specification.

One aspect of the present invention,

In % by weight, carbon (C): 0.1 to 0.2%, manganese (Mn): 0.8 to 2.0%, silicon (Si): 0.01 to 0.3%, aluminum (Al): 0.05% or less, phosphorus (P): 0.05% Below, sulfur (S): 0.03% or less, nitrogen (N): 0.01% or less, titanium (Ti): 0.01 to 0.03%, boron (B): 0.0005 to 0.005%, niobium (Nb): 0.01% or less, molybdenum (Mo): 0.12% or less, vanadium (V): 0.1% or less, including iron (Fe) and unavoidable impurities, and satisfying the following relational expression 1,

By volume fraction, it contains 55 to 90% of bainite and 10 to 45% of ferrite as a microstructure,

Among the carbides present in the bainite, the number of carbides having a long axis of 25 to 500 nm per unit area is 3*10 ⁶ /mm ² or more,

Provided is a hot-rolled steel sheet in which the average aspect ratio (minor axis/major axis) of carbides present in the bainite is 2.0 or less.

[Relationship 1]

0.11 ≤ [B]/[Ti]

(In the relational expression 1, the [B] represents the average weight% content of boron (B) in the hot-rolled steel sheet, and the [Ti] represents the average weight% content of titanium (Ti) in the hot-rolled steel sheet.)

In addition, another aspect of the present invention,

the hot-rolled steel sheet; and

A plating layer formed on at least one surface of the hot-rolled steel sheet;

The plating layer is made of any one selected from zinc, aluminum, a zinc-based alloy, and an aluminum-based alloy, and provides a hot-rolled coated steel sheet.

In addition, another aspect of the present invention,

In % by weight, carbon (C): 0.1 to 0.2%, manganese (Mn): 0.8 to 2.0%, silicon (Si): 0.01 to 0.3%, aluminum (Al): 0.05% or less, phosphorus (P): 0.05% Below, sulfur (S): 0.03% or less, nitrogen (N): 0.01% or less, titanium (Ti): 0.01 to 0.03%, boron (B): 0.0005 to 0.005%, niobium (Nb): 0.01% or less, molybdenum (Mo): 0.12% or less, vanadium (V): 0.1% or less, a slab heating step of heating a slab containing the remaining iron (Fe) and unavoidable impurities and satisfying the relational expression 1;

A hot rolling step of providing a steel sheet by hot rolling the heated slab to satisfy the following relational expression 3;

A first cooling step of cooling the hot-rolled steel sheet at a first cooling rate to a first cooling stop temperature (T ₁ ) so as to satisfy the following relational expression 4;

A second cooling step of cooling the first cooled steel sheet at a second cooling rate of 50 to 500° C./s faster than the first cooling rate to a second cooling stop temperature (T ₂ ) that satisfies the following relational expression 5;

A third cooling step of cooling the second cooled steel sheet at a third cooling rate slower than the second cooling rate to a third cooling stop temperature (T ₃ ) that satisfies the following relational expression 6;

A winding step of winding the third cooled hot-rolled steel sheet at the third cooling stop temperature (T ₃ ); and

It provides a method for manufacturing a hot-rolled steel sheet, including a heat treatment step of heat-treating the rolled steel sheet in a temperature range of 450 to 720 ° C.

[Relationship 3]

T _fm (℃)≥880+1000*[Ti]+3000*[Nb]-20*t^ ^(1/2)

(In the above relational expression 3, [Ti] and [Nb] each independently represent the average weight % content for the corresponding element included in the slab, the T _fm represents the rolling end temperature, and the t is the average of the hot-rolled steel sheet It represents the thickness, and the unit is mm.)

[Relationship 4]

T _fm (℃)-60℃≤T ₁ (℃)≤T _fm (℃)-5℃

(In the above relational expression 4, T _fm is the same as the definition in the above relational expression 3.)

[Relationship 5]

T ₂ (℃)≤560+120*[C]+40*[Mn]

(In Equation 5, [C] and [Mn] each independently represent the average weight% content of the corresponding element included in the slab.)

[Relationship 6]

420+575*[C]+37*[Mn]-56*t^ ^(1/2) ≤T ₃ (℃)<T ₂ (℃)

(In the relational expression 6, [C] and [Mn] are as defined in the relational expression 5.)

Another aspect of the present invention is,

Manufacturing a hot-rolled steel sheet by the above-described hot-rolled steel sheet manufacturing method; and

forming a plating layer on at least one surface of the hot-rolled steel sheet by any one method selected from a hot-dip plating method, an electroplating method, and a plasma method;

including,

The plating layer is made of any one selected from zinc, aluminum, zinc-based alloys, and aluminum-based alloys, and provides a method for manufacturing a hot-rolled coated steel sheet.

The means for solving the above problems do not enumerate all the features of the present invention, and the various features of the present invention and the advantages and effects thereof will be understood in more detail with reference to the specific examples below.

According to one aspect of the present invention, it is possible to provide a hot-rolled steel sheet, a hot-rolled galvanized steel sheet, and a manufacturing method thereof that can be lightweight while having high-strength characteristics suitable as a structural material for high-strength support.

Effects of the present invention are not limited to the above, and may be interpreted as including effects that can be inferred from the description described below by those skilled in the art.

1 is a photograph of the microstructure of specimen 1 of the present application observed with a scanning electron microscope (SEM).

The present invention relates to a hot-rolled steel sheet and a manufacturing method thereof, and hereinafter, preferred embodiments of the present invention will be described. Embodiments of the present invention may be modified in various forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. These embodiments are provided to those skilled in the art to further elaborate the present invention.

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

In the hot-rolled steel sheet according to one aspect of the present invention, in weight%, carbon (C): 0.1 to 0.2%, manganese (Mn): 0.8 to 2.0%, silicon (Si): 0.01 to 0.3%, 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): 0.01 to 0.03%, boron (B): 0.0005 to 0.005% , niobium (Nb): 0.01% or less, molybdenum (Mo): 0.12% or less, vanadium (V): 0.1% or less, the remaining iron (Fe) and unavoidable impurities.

Hereinafter, the steel composition of the present invention will be described in more detail. At this time, unless otherwise indicated, % representing the content of each element is based on weight.

Carbon (C): 0.1~0.2%

Carbon (C) is an element that effectively contributes to improving the strength of a steel sheet. In addition, carbon (C) is also an element that requires proper addition in order to secure the microstructure to be implemented in the present invention. Therefore, in order to secure a desired level of strength and a desired level of bainite fraction during cooling after hot rolling, in the present invention, carbon (C) of 0.1% or more may be added. However, a more preferable lower limit of the carbon (C) content may be 0.12%, and a more preferable lower limit of the carbon (C) content may be 0.14%. On the other hand, if carbon (C) is excessively added, a large amount of carbide is formed, resulting in poor formability or poor weldability when used for steel building materials, so the content of carbon (C) can be limited to 0.20% or less. there is. However, a more preferable upper limit of the carbon (C) content may be 0.19%, and a more preferable upper limit of the carbon (C) content may be 0.18%.

Manganese (Mn): 0.8 to 2.0%

Manganese (Mn) is not only an element that improves the strength and hardenability of steel, but also effectively suppresses cracks caused by sulfur (S) by forming MnS by combining with sulfur (S), which is inevitably contained during the steel manufacturing process. element that contributes Therefore, in order to expect the above-mentioned effect in the present invention, 0.8% or more of manganese (Mn) is included. However, a more preferable lower limit of the manganese (Mn) content may be 0.85%, and a more preferable lower limit of the manganese (Mn) content may be 0.9%. On the other hand, when manganese (Mn) is added excessively, weldability is not only deteriorated, but also is not desirable from the viewpoint of economic efficiency, so the manganese (Mn) content is limited to 2.0% or less in the present invention. However, a more preferable upper limit of the manganese (Mn) content may be 1.9%, and a more preferable upper limit of the manganese (Mn) content may be 1.8%.

Silicon (Si): 0.01 to 0.3%

Since silicon (Si) is an element that not only acts as a deoxidizer but also effectively contributes to improving the strength of a steel sheet, the present invention may add silicon (Si) to express such an effect. However, a more preferable lower limit of the silicon content may be 0.015%, and a more preferable lower limit of the silicon content may be 0.02%. On the other hand, if the content of silicon (Si) exceeds a certain level, the surface quality and weldability may be deteriorated due to the scale formed on the surface of the steel sheet, so the present invention limits the silicon (Si) content to 0.30% or less. can However, a more preferable upper limit of the silicon (Si) content may be 0.28%, and a more preferable upper limit of the silicon (Si) content may be 0.24%.

Aluminum (Al): 0.05% or less

Aluminum (Al) is an element that acts as a deoxidizer by combining with oxygen in steel, and in the present invention, aluminum (Al) may be added for this effect. A preferable lower limit of the aluminum (Al) content may be 0.005%, and a more preferable lower limit of the aluminum (Al) content may be 0.01%. However, when aluminum (Al) is excessively added, inclusions of the steel sheet may increase and workability of the steel sheet may be deteriorated. In the present invention, the aluminum (Al) content may be limited to 0.05% or less. The upper limit of the preferable aluminum (Al) content may be 0.04%.

Phosphorus (P): 0.05% or less

Phosphorus (P) is an impurity element that is unavoidably contained in steel, and is an element that is incorporated in grain boundaries to reduce the toughness of steel. Therefore, it is desirable to control the content as low as possible, and it is advantageous to limit the theoretical phosphorus (P) content to 0%. However, in consideration of the content unavoidably contained in the manufacturing process, the present invention may limit the phosphorus (P) content to 0.05% or less. The upper limit of the preferable phosphorus (P) content may be 0.04%, and the more preferable phosphorus (P) content may be 0.03%. On the other hand, since the content of phosphorus (P) as an impurity is more than 0%, the lower limit may not be separately limited, but the lower limit may be 0.0001% in consideration of the case where it is inevitably included in the manufacturing process.

Sulfur (S): 0.03% or less

Sulfur (S) is an impurity inevitably contained in steel, and is an element that reacts with manganese (Mn) to form MnS to increase the content of precipitates and embrittle the steel. Therefore, it is desirable to control the content as low as possible, and theoretically, it is advantageous to limit the content of sulfur (S) to 0%. However, in consideration of the content unavoidably contained in the manufacturing process, the present invention may limit the content of sulfur (S) to 0.03% or less. The upper limit of the preferable sulfur (S) content may be 0.02%, and the upper limit of the more preferable sulfur (S) content may be 0.01%. On the other hand, since the sulfur (S) content is ideal when the content is 0% as an impurity, the lower limit may not be separately limited, but the lower limit may be 0.0001% in consideration of the case where it is inevitably included in the manufacturing process.

Nitrogen (N): 0.01% or less

Nitrogen (N) is an impurity unavoidably contained in steel, and is an element that causes cracks in slabs by forming nitrides during continuous casting. Therefore, it is desirable to control the content as low as possible, and theoretically, it is advantageous to limit the content of nitrogen (N) to 0%. However, in consideration of the content unavoidably contained in the manufacturing process, the present invention may limit the content of nitrogen (N) to 0.01% or less. A preferable nitrogen (N) content may be 0.008% or less, and a more preferable nitrogen (N) content may be 0.006% or less. On the other hand, since nitrogen (N) is an impurity and ideally has a content of 0%d, the lower limit may not be separately limited.

Titanium (Ti): 0.01 to 0.05%

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 secure hardenability. At this time, boron (B) is bonded to nitrogen (N) before titanium (Ti) is combined with nitrogen (N) so that Addition effect can be improved. In the present invention, 0.01% or more of titanium (Ti) may be added to achieve such an effect. However, a more preferable lower limit of the titanium (Ti) content may be 0.011%, and a more preferable lower limit of the titanium (Ti) content may be 0.012%. On the other hand, when titanium (Ti) is excessively added, it may cause deterioration in playability in the slab manufacturing step, and may cause deterioration in rollability by increasing the rolling load during hot rolling. Therefore, the present invention may limit the titanium (Ti) content to 0.05% or less. However, a more preferable lower limit of the titanium (Ti) content may be 0.04%, and a more preferable lower limit of the titanium (Ti) content may be 0.03%.

Boron (B): 0.0005 to 0.005%

Boron (B) is an element that plays an important role in improving the hardenability of the steel sheet and suppresses the transformation of ferrite or pearlite during cooling after completion of rolling. In the present invention, 0.0005% or more of boron (B) may be added to achieve such an effect. However, a more preferable lower limit of the boron (B) content may be 0.0007%, and a more preferable lower limit of the boron (B) content may be 0.001%. On the other hand, when the added amount of boron (B) exceeds a certain level, there is a problem in that the excessively added boron (B) combines with iron (Fe) to make the grain boundary vulnerable. can be limited to 0.005% or less. The upper limit of a preferable boron (B) content may be 0.0048%, and the upper limit of a more preferable boron (B) content may be 0.0045%.

Niobium (Nb): 0.01% or less, Molybdenum (Mo): 0.12% or less, Vanadium (V): 0.1% or less

Niobium (Nb), molybdenum (Mo), and vanadium (V) are elements that react with carbon (C) or nitrogen (N) to form precipitates such as carbides and nitrides. However, these are expensive elements that not only increase in price as the amount added increases, but also increase the rolling load during hot rolling, making it difficult to manufacture thin materials. Therefore, the present invention is to suppress the addition of niobium (Nb), molybdenum (Mo) and vanadium (V), and even if they are unavoidably added, the contents of niobium (Nb), molybdenum (Mo) and vanadium (V) are each reduced to 0.01%. Below, it can be limited to 0.12% or less, or 0.1% or less. At this time, a more preferable upper limit of niobium (Nb) is 0.005%, a more preferable upper limit of molybdenum (Mo) content is 0.10%, and a more preferable upper limit of vanadium (V) content is 0.09%. On the other hand, since niobium (Nb), molybdenum (Mo), and vanadium (V) are expensive elements, it is advantageous to control the addition amount as low as possible from the viewpoint of preventing cost increase. there is. However, considering the case where it is unavoidably added, the lower limit of the niobium (Nb) content may be 0.0002%, the lower limit of the molybdenum (Mo) content may be 0.001%, and the lower limit of the vanadium (V) content is It may be 0.005%.

Chromium (Cr): 0.2% by weight or less

Since chromium (Cr) is an element that contributes to the improvement of hardenability of steel, the present invention may optionally further add chromium (Cr) to achieve this effect. However, since chromium (Cr) is an expensive element, excessive addition is not desirable from an economic point of view, and excessive addition of chromium (Cr) may cause deterioration in weldability. can be limited to the following. However, a more preferable upper limit of the chromium (Cr) content may be 0.15%, and a more preferable upper limit of the chromium (Cr) content may be 0.1%.

Additionally, the hot-rolled steel sheet may satisfy the following relational expression 1. That is, in the present invention, by controlling the addition ratio of titanium (Ti) and boron (B) to satisfy the following relational expression 1, the bonding of boron (B) with nitrogen (N) is suppressed to prevent boron (B) addition. It is possible to exert the effect of improving the desired hardenability by. In addition, by suppressing the transformation of ferrite or pearlite during cooling after completion of rolling, it is possible to easily secure the desired microstructure in the present invention. On the other hand, more preferably, the [B] / [Ti] value defined in the following relational expression 1 may be in the range of 0.11 to 0.18%.

[Relationship 1]

0.11 ≤ [B]/[Ti]

(In the relational expression 1, the [B] represents the average weight% content of boron (B) in the hot-rolled steel sheet, and the [Ti] represents the average weight% content of titanium (Ti) in the hot-rolled steel sheet.)

In addition, although not particularly limited, according to one aspect of the present invention, the contents of titanium (Ti), niobium (Nb), and vanadium (V) that form precipitates and reduce rollability can satisfy the following relational expression 2 there is.

[Relationship 2]

30*[Ti]+100*[Nb]+5*[V]≤1.65

(In the relational expression 2, the [Ti], [Nb], and [V] each independently represent the average weight% content of the corresponding element (ie, each element in parentheses) included in the hot-rolled steel sheet.)

The aforementioned relational expression 2 is a condition for providing a hot-rolled steel sheet having high strength and high formability. That is, when the contents of titanium (Ti), niobium (Nb), and vanadium (V) do not satisfy relational expression 2, hot rolling must be performed at a high temperature inevitably, and the aspect ratio (short axis / long axis) of carbide is the purpose Bending workability (R / t) may be inferior when the level exceeds the level.

The hot-rolled steel sheet according to one aspect of the present invention may include remaining Fe and other unavoidable impurities in addition to the above components. However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in the normal manufacturing process, it cannot be entirely excluded. Since these impurities can be known to anyone skilled in the art, all of them are not specifically mentioned in the present specification. In addition, additional addition of effective ingredients other than the above-mentioned ingredients is not entirely excluded.

According to one aspect of the present invention, the microstructure of the hot-rolled steel sheet is 55 to 90% of bainite and 10 to 45% of ferrite (more preferably, 55 to 90% of bainite and 10 to 20% by volume fraction). % of ferrite). In the present invention, since bainite is a microstructure effective for increasing strength, and ferrite is a microstructure effective for securing ductility, the bainite and ferrite fractions need to be appropriately controlled in terms of both high strength characteristics and excellent formability. . Therefore, the present invention can control the volume fraction of bainite and ferrite to satisfy the above range.

In addition, according to one aspect of the present invention, although not particularly limited, the bainite can be divided into upper bainite and lower bainite according to the formation temperature, and in the present invention, in terms of simultaneously securing yield strength and workability, In terms of volume fraction with respect to the entire microstructure, lower bainite of 10% or more (or 10 to 80%) may be included as the microstructure. On the other hand, more preferably from the viewpoint of further improving the strength and workability of the hot-rolled steel sheet, the microstructure of the hot-rolled steel sheet is 10.5 to 77.1% of the lower bainite and 1.9 to 62.6% of the upper part in volume fraction with respect to the entire microstructure. Bainite may be included.

In addition, according to one aspect of the present invention, the hot-rolled steel sheet may further include one or more of pearlite, martensite, retained austenite, and other precipitates as a microstructure. That is, the microstructure of the hot-rolled steel sheet may include 55 to 90% of bainite, 10 to 45% of ferrite, and the rest of the other phases, and the other phases include pearlite, martensite, retained austenite, and other precipitates. can do. However, the fraction of the above-mentioned other phases may be limited to a certain range or less in terms of both high-strength characteristics and excellent moldability. Therefore, according to one aspect of the present invention, the microstructure of the hot-rolled steel sheet includes, in volume fraction, pearlite of 10% or less (including 0%), martensite of 1% or less (including 0%) and 1% or less ( 0%) may further include one or more of retained austenite.

That is, according to one aspect of the present invention, when a large amount of pearlite is formed, formability is lowered due to the formation of a complex structure, or it is difficult to secure strength because the bainite fraction is reduced, so the volume fraction of pearlite in the present invention can be limited to 10% or less (including 0%). Since martensite is advantageous in terms of securing strength but disadvantageous in terms of securing formability, the volume fraction of martensite may be limited to 1% or less (including 0%) in the present invention. Retained austenite is advantageous in terms of securing formability but disadvantageous in securing yield strength, so the volume fraction of retained austenite can be limited to 1% or less (including 0%) in the present invention.

On the other hand, in the hot-rolled steel sheet, the number of carbides per unit area of carbides having a long axis of 25 to 500 nm among carbides present in bainite is preferably 3*10 ⁶ pieces/mm ² or more. Among the carbides present in bainite, carbides having an excessively small particle size increase the rolling load during hot rolling, making it difficult to roll thin materials, and carbides having an excessively large particle size may adversely affect securing strength and ductility. In addition, it is preferable that the number of carbides present in bainite is a certain amount or more in order to secure strength. Accordingly, in the hot-rolled steel sheet of the present invention, the number of carbides having a long axis of 25 to 500 nm among carbides present in bainite per unit area can be controlled to 3*10 ⁶ /mm ² or more.

The shape of the carbide present in bainite is also a factor that greatly affects the formability of the steel sheet, and the closer the shape of the carbide present in bainite is to a spherical shape, the more advantageous it is to secure the formability of the steel sheet. Therefore, the present invention can limit the average aspect ratio (major axis/minor axis) of carbides present in bainite to 2.0 or less in terms of securing formability.

On the other hand, as the size difference between bainite and ferrite increases, it is not desirable in terms of formability. Therefore, in the hot-rolled steel sheet of the present invention, the average packet size of bainite can be controlled to a level of 50 to 200% of the average grain size of ferrite. .

According to one aspect of the present invention, the hot-rolled steel sheet may have a thickness of 3.1 to 6 mm (ie, 3.1 mm or more and 6.0 mm or less). When the thickness of the hot-rolled steel sheet is less than 3.1 mm, the hot-rolling load is high and rollability may be deteriorated, and when the thickness of the hot-rolled steel sheet exceeds 6 mm, it may be difficult to control cooling before winding to a target range.

Further, according to one aspect of the present invention, 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. Although not particularly limited thereto, if the hot-rolled steel sheet according to the present invention does not meet the above-mentioned strength characteristics, it may be difficult to be used suitably for structures requiring high strength, and in particular, if the yield strength is less than 550 MPa, it may cause problems in supporting there is.

In addition, according to one aspect of the present invention, the hot-rolled steel sheet may have an elongation (El) of 12% or more and a bending workability (R/t) of 1.0 or less. As a case where the hot-rolled steel sheet does not satisfy the above-mentioned elongation rate and bending workability, if the elongation rate and formability are low, there may be a problem of cracking during part processing.

On the other hand, another aspect of the present invention, the above-described hot-rolled steel sheet; and a plating layer formed on at least one surface of the hot-rolled steel sheet, and the plating layer may be composed of any one selected from zinc, aluminum, zinc-based alloys, and aluminum-based alloys. That is, the plating layer described above may have a composition of a plating layer commonly used in the art. For example, the zinc-based alloy plating layer may be a plating layer including at least one selected from among aluminum (Al), magnesium (Mg), nickel (Ni), and iron (Fe) and zinc (Zn) as the remainder, and may be an aluminum-based alloy plating layer. The alloy plating layer may be a plating layer including at least one selected from among silicon (Si), magnesium (Mg), nickel (Ni), and iron (Fe) and aluminum (Al) as the remainder.

Hereinafter, a method for manufacturing a hot-rolled steel sheet according to another aspect of the present invention will be described in detail. However, this does not mean that the hot-rolled steel sheet of the present invention must be manufactured by the following manufacturing method.

Preparation and heating of steel slabs

A steel slab having the same composition as the hot-rolled steel sheet described above is prepared. Since the steel slab of the present invention has an alloy composition corresponding to that of the aforementioned hot-rolled steel sheet, the description of the alloy composition of the steel slab is replaced with the description of the alloy composition of the aforementioned hot-rolled steel sheet.

The prepared steel slab can be heated to a certain temperature range, and at this time, 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 range from 1100 to 1350 °C. If the heating temperature of the steel slab is less than 1100 ° C, there is a possibility of hot rolling in the temperature range below the desired finish hot rolling temperature range, and if the heating temperature of the steel slab exceeds 1350 ° C, additional costs are incurred due to excessive input of energy. This is because there is a possibility that this may occur or that scale may be formed thickly on the surface layer of the slab.

hot rolled

After heating the above-described steel slab, hot rolling is performed to provide a hot-rolled steel sheet having a thickness of 3.1 to 6 mm (ie, 3.1 mm or more and 6.0 mm or less). If the temperature of the heated steel slab is a temperature at which normal hot rolling can be performed, hot rolling can be performed as it is without performing special reheating, and the temperature of the heated steel slab is a temperature at which normal hot rolling can be performed. If the temperature is lower, hot rolling may be performed after reheating.

At this time, the hot rolling is preferably performed to satisfy the following relational expression 3. This means that when the hot rolling temperature is excessively low, the rolling load increases and the rollability deteriorates, the surface roughness is caused by rolling roll fatigue, and the aspect ratio of carbides may deviate from the desired level due to low temperature rolling. because it does

[Relationship 3]

T _fm (℃)≥880+1000*[Ti]+3000*[Nb]-20*t^ ^(1/2)

(In the above relational expression 3, [Ti] and [Nb] each independently represent the average weight % content for the corresponding element included in the slab, the T _fm represents the rolling end temperature, and the t is the average of the hot-rolled steel sheet It represents the thickness, and the unit is mm.)

1st cooling

The hot-rolled steel sheet may be cooled at a first cooling rate (V _C1 ) to a first cooling stop temperature (T ₁ ) so as to satisfy the following relational expression 4.

[Relationship 4]

T _fm (℃)-60℃≤T ₁ (℃)≤T _fm (℃)-5℃

(In the above relational expression 4, T _fm is the same as the definition in the above relational expression 3.)

When the first cooling stop temperature (T ₁ ) is below a certain level, transformation into ferrite or pearlite occurs, so that the desired microstructure of the present invention cannot be secured, and accordingly, there is a concern that the desired level of strength cannot be secured there is. Accordingly, the present invention may limit the lower limit of the first cooling stop temperature (T ₁ ) to T _fm (°C)-60°C. In addition, when the first cooling stop temperature (T ₁ ) exceeds a certain level, there is a possibility that the difference between the average packet size of bainite and the average grain size of ferrite becomes excessively large because the austenite grains become coarse and non-uniform, In the present invention, the upper limit of the first cooling stop temperature (T ₁ ) may be limited to T _fm (°C)-5°C.

At this time, it is not particularly limited, but the above-described first cooling rate (V _C1 ) may be a cooling rate applied to normal slow cooling, but 5 to 50 ° C / s in terms of preventing steel plate shape defects and securing the desired microstructure can be limited to the range of

2nd cooling

The first cooled steel sheet may be cooled at a second cooling rate of 50 to 500° C./s faster than the first cooling rate until a second cooling stop temperature (T ₂ ) that satisfies the following relational expression 5.

[Relationship 5]

T ₂ (℃)≤560+120*[C]+40*[Mn]

(In Equation 5, [C] and [Mn] each independently represent the average weight% content of the corresponding element included in the slab.)

When the second cooling stop temperature (T ₂ ) exceeds a certain level, transformation into ferrite or pearlite occurs, so that the desired microstructure of the present invention cannot be secured, and thus the desired level of strength cannot be secured. There are concerns. Therefore, the present invention may limit the upper limit of the second cooling stop temperature (T ₂ ) to 560+120*[C]+40*[Mn].

The second cooling is preferably carried out at a faster rate than the first cooling, and a more preferable second cooling rate (V _C2 ) may be in the range of 50 to 500 °C/s. When the second cooling rate (V _C2 ) is less than 50° C./s, transformation into ferrite or pearlite increases, which may cause a decrease in strength. In addition, since additional equipment is required to implement a cooling rate of more than 500 ° C / s, it is not preferable in terms of economics.

Third cooling and winding

The second cooled steel sheet is cooled at a third cooling rate (V _C3 ) to a third cooling stop temperature (T ₃ ) that satisfies the following relational expression 6, and then may be wound at the third cooling stop temperature (T ₃ ). .

[Relationship 6]

420+575*[C]+37*[Mn]-56*t^ ^(1/2) ≤T ₃ (℃)<T ₂ (℃)

(In the relational expression 6, [C] and [Mn] are as defined in the relational expression 5.)

The third cooling is a cooling step for controlling the winding temperature, and the third cooling stop is lower than the second cooling stop temperature (T ₂ ) at a third cooling rate (V _C3 ) slower than the second cooling rate (V _C2 ). Cooling can be performed to the temperature (T ₃ ). A preferred third cooling rate (V _C3 ) may be in the range of 5 to 50 °C/s.

However, when the temperature of the third cooling furnace (T ₃ ) is excessively low, the low-temperature structure is excessively formed and the desired microstructure and moldability cannot be secured. Therefore, the present invention provides the lower limit of the temperature of the third cooling furnace (T ₃ ) can be limited to 420+575*[C]+37*[Mn]-56*t^ ^(1/2) .

heat treatment

Heat treatment may be performed by raising the temperature of the rolled steel sheet and then maintaining the temperature. Heat treatment may be performed in a temperature range of 450 to 720° C. in order to secure the temperature of the steel sheet and secure stable carbides in the subsequent plating process.

On the other hand, in the present specification, in the case of the above-described relational expressions 3 to 6, since they are values empirically obtained through experiments, units may not be separately limited in each relational expression. In other words, the units of variables included in each relational expression It is sufficient if it is satisfied (for example, in the case of the relational expression 3 above, it is sufficient to satisfy only ° C, which is the unit of T _fm , weight%, which is the unit of [Ti] and [Nb], and mm, which is the unit of t).

In addition, another aspect of the present invention, manufacturing a hot-rolled steel sheet by the method for manufacturing a hot-rolled steel sheet described above; and forming a plating layer on at least one surface of the hot-rolled steel sheet by any one method selected from a hot-dip plating method, an electroplating method, and a plasma method, wherein the plating layer is zinc, aluminum, zinc-based alloy, or aluminum-based alloy. It provides a method for manufacturing a hot-rolled coated steel sheet made of any one selected from among.

At this time, the description of the hot-rolled steel sheet and the plating layer can equally apply the above-mentioned information.

Meanwhile, in relation to the step of forming the plating layer, after the heat treatment step described above, the plating layer may be formed on at least one surface of the heat-treated steel sheet. The composition and formation method of the plating layer of the present invention are not particularly limited, and may be interpreted as a concept including the composition and formation method of a plating layer commonly provided on a hot-rolled coated steel sheet. As an example, the plating layer may be formed by any one method selected from a hot-dip plating method, an electroplating method, and a plasma method, and the plating layer is any one plating layer selected from zinc, aluminum, zinc-based alloy, and aluminum-based alloy. can be

Hereinafter, the high-strength hot-rolled steel sheet of the present invention and its manufacturing method will be described in more detail through specific examples. It should be noted that the following examples are only for understanding of the present invention, and are not intended to specify the scope of the present invention. The scope of the present invention is determined by the matters described in the claims and the matters reasonably inferred therefrom.

(Example)

A hot-rolled steel sheet was manufactured by applying the process conditions of Table 2 to the steel slab having the alloy composition of Table 1 below. In Table 1, the rest is iron (Fe) and unavoidable impurities, and the steel slab heating condition of 1200 ° C. was commonly applied.

[Table 1]

[Table 2]

Afterwards, the microstructure of each specimen was observed and the mechanical properties were measured and listed in Tables 3 and 4.

After each specimen was cut in a direction parallel to the rolling direction, specimens for microstructure observation were taken from the cut surface at the point of 1/4 of the plate thickness. After polishing the collected samples and corroding them with a nital solution, the microstructure of each specimen was observed using an optical microscope and a scanning electron microscope (SEM). The microstructure fraction was measured through image analysis. The average size of bainite packets and the average size of ferrite grains were measured using electron backscatter diffraction (EBSD) at a point of 1/4 of the plate thickness. The average size of the bainite packet was measured by defining the position exceeding 15 ° of the orientation difference as the grain boundary, and the average size of the ferrite grain was measured using the Linear Intercept Method. In Table 3, F denotes ferrite, P denotes pearlite, B denotes bainite, UB denotes upper bainite, LB denotes lower bainite, M denotes martensite, and R-γ denotes retained austenite. At least 5 electron microscope images of bainite present in different areas of each sample were obtained by increasing the magnification of the scanning electron microscope (SEM) to 5,000 times. was calculated to measure the size, number density and average aspect ratio of carbides. In Table 3, the carbide number density means the number density of carbides having a long axis of 25 to 500 nm.

Tensile strength and yield strength were measured by performing a tensile test in the L direction using the DIN standard for each specimen. In addition, the elongation rate was measured through the deformed length until the test was stopped when the tensile strength fell by 30% or more. Bending workability (R/t) was measured by applying a 90° V-bending bending test. After bending the specimens by 90° with a V-shaped jig having different curvatures (R), cracks in the bent portion were checked and measured.

[Table 3]

[Table 4]

As shown in Tables 1 to 4, specimens satisfying both the alloy composition and process conditions of the present invention have a yield strength (YS) of 550 MPa or more, a tensile strength (TS) of 650 MPa or more, an elongation (El) of 12% or more, and an elongation of 1.0 or less. While satisfying bending workability (R/t), specimens that do not satisfy any one or more of the alloy composition or process conditions of the present invention have a yield strength (YS) of 550 MPa or more, a tensile strength (TS) of 650 MPa or more, and an elongation of 12% or more. (El) and bending workability (R/t) of 1.0 or less are not satisfied at the same time.

On the other hand, the microstructure of specimen 1 was observed with a scanning electron microscope (SEM) and shown in FIG. 1, and as can be seen in FIG. . In addition, in the case of the hot-rolled steel sheet obtained from specimen 15, it was confirmed that the microstructure mainly contained ferrite.

Although the present invention has been described in detail through examples above, other types of embodiments are also possible. Therefore, the spirit and scope of the claims set forth below are not limited to the embodiments.

Claims (11)

In % by weight, carbon (C): 0.1 to 0.2%, manganese (Mn): 0.8 to 2.0%, silicon (Si): 0.01 to 0.3%, aluminum (Al): 0.05% or less, phosphorus (P): 0.05% Below, sulfur (S): 0.03% or less, nitrogen (N): 0.01% or less, titanium (Ti): 0.01 to 0.03%, boron (B): 0.0005 to 0.005%, niobium (Nb): 0.01% or less, molybdenum (Mo): 0.1% or less, vanadium (V): 0.1% or less, including iron (Fe) and unavoidable impurities, and satisfying the following relational expression 1,
As a microstructure, in volume fraction,
55-90% bainite;
10-45% ferrite; and
At least one of pearlite, martensite, retained austenite, and precipitates as the remainder of the other phase;
Among the carbides present in the bainite, the number of carbides having a long axis of 25 to 500 nm per unit area is 3*10 ⁶ /mm ² or more,
The hot-rolled steel sheet, wherein the average aspect ratio (minor/major axis) of carbides present in the bainite is 2.0 or less.
[Relationship 1]
0.11 ≤ [B]/[Ti]
(In the relational expression 1, the [B] represents the average weight% content of boron (B) in the hot-rolled steel sheet, and the [Ti] represents the average weight% content of titanium (Ti) in the hot-rolled steel sheet.)
According to claim 1,
The hot-rolled steel sheet, as a microstructure, contains 1 of 10% or less (including 0%) of pearlite, 1% or less (including 0%) of martensite, and 1% or less (including 0%) of retained austenite in volume fraction Hot-rolled steel sheet, further comprising more than one species.
According to claim 1,
The hot-rolled steel sheet, as a microstructure, contains, in volume fraction, lower bainite of 10% or more.
According to claim 1,
The average packet size of the bainite is 50 to 200% of the average grain size of the ferrite, hot-rolled steel sheet.
According to claim 1,
The hot-rolled steel sheet satisfies the following relational expression 2, the hot-rolled steel sheet.
[Relationship 2]
30*[Ti]+100*[Nb]+5*[V]≤1.65
(In the relational expression 2, the [Ti], [Nb], and [V] each independently represent the average weight% content of the corresponding element included in the hot-rolled steel sheet.)
According to claim 1,
The yield strength of the hot-rolled steel sheet is 550 MPa or more,
Tensile strength is more than 650MPa,
Elongation is 12% or more, hot-rolled steel sheet.
According to claim 1,
The thickness of the hot-rolled steel sheet is 3.1 ~ 6.0mm, hot-rolled steel sheet.
The hot-rolled steel sheet according to any one of claims 1 to 7; and
A plating layer formed on at least one surface of the hot-rolled steel sheet;
The plating layer is made of any one selected from zinc, aluminum, zinc-based alloys, and aluminum-based alloys, hot-rolled coated steel sheet.
In % by weight, carbon (C): 0.1 to 0.2%, manganese (Mn): 0.8 to 2.0%, silicon (Si): 0.01 to 0.3%, aluminum (Al): 0.05% or less, phosphorus (P): 0.05% Below, sulfur (S): 0.03% or less, nitrogen (N): 0.01% or less, titanium (Ti): 0.01 to 0.03%, boron (B): 0.0005 to 0.005%, niobium (Nb): 0.01% or less, molybdenum (Mo): 0.1% or less, vanadium (V): 0.1% or less, a slab heating step of heating a slab containing iron (Fe) and unavoidable impurities and satisfying the following relational expression 1;
A hot rolling step of providing a steel sheet by hot rolling the heated slab to satisfy the following relational expression 3;
A first cooling step of cooling the hot-rolled steel sheet at a first cooling rate to a first cooling stop temperature (T ₁ ) so as to satisfy the following relational expression 4;
A second cooling step of cooling the first cooled steel sheet at a second cooling rate of 50 to 500° C./s faster than the first cooling rate to a second cooling stop temperature (T ₂ ) that satisfies the following relational expression 5;
A third cooling step of cooling the second cooled steel sheet at a third cooling rate slower than the second cooling rate to a third cooling stop temperature (T ₃ ) that satisfies the following relational expression 6;
A winding step of winding the third cooled hot-rolled steel sheet at the third cooling stop temperature (T ₃ ); and
Method for manufacturing a hot-rolled steel sheet comprising a heat treatment step of heat-treating the rolled steel sheet in a temperature range of 450 ~ 720 ℃.
[Relationship 1]
0.11 ≤ [B]/[Ti]
(In the relational expression 1, the [B] represents the average weight% content of boron (B) in the hot-rolled steel sheet, and the [Ti] represents the average weight% content of titanium (Ti) in the hot-rolled steel sheet.)
[Relationship 3]
T _fm (℃)≥880+1000*[Ti]+3000*[Nb]-20*t^ ^(1/2)
(In the above relational expression 3, [Ti] and [Nb] each independently represent the average weight % content for the corresponding element included in the slab, the T _fm represents the rolling end temperature, and the t is the average of the hot-rolled steel sheet It represents the thickness, and the unit is mm.)
[Relationship 4]
T _fm (℃)-60℃≤T ₁ (℃)≤T _fm (℃)-5℃
(In the above relational expression 4, T _fm is the same as the definition in the above relational expression 3.)
[Relationship 5]
T ₂ (℃)≤560+120*[C]+40*[Mn]
(In Equation 5, [C] and [Mn] each independently represent the average weight% content of the corresponding element included in the slab.)
[Relationship 6]
420+575*[C]+37*[Mn]-56*t^ ^(1/2) ≤T ₃ (℃)<T ₂ (℃)
(In the relational expression 6, [C] and [Mn] are as defined in the relational expression 5.)
According to claim 9,
The first cooling rate is 5 ~ 50 ℃ / s,
The third cooling rate is 5 ~ 50 ℃ / s, a method of manufacturing a hot-rolled steel sheet.
Manufacturing a hot-rolled steel sheet by the method of manufacturing a hot-rolled steel sheet according to claim 9 or 10; and
Forming a plating layer on at least one surface of the hot-rolled steel sheet by any one method selected from a hot-dip plating method, an electroplating method, and a plasma method; including,
The plating layer is made of any one selected from zinc, aluminum, zinc-based alloys, and aluminum-based alloys, a method for manufacturing a hot-rolled coated steel sheet.

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