KR20190045453A - Hot rolled steel sheet and method of manufacturing the same - Google Patents

Abstract

The present invention provides a hot rolled steel sheet, comprising: 0.17-0.22 wt% of carbon (C); 0.1 wt% or less (excluding 0 wt%) of silicon (Si); 1.2-1.8 wt% of manganese (Mn); 0.005-0.025 wt% of niobium (Nb), wherein the sum of the Nb and vanadium (V) is 0.01-0.05 wt%; 0.005-0.025 wt% of vanadium (V); and the remainder consisting of Fe and unavoidable impurities. Moreover, the final micro structure comprises ferrite and perlite.

Description

Technical Field [0001] The present invention relates to a hot rolled steel sheet,

More particularly, the present invention relates to a high-strength hot-rolled steel sheet and a manufacturing method thereof.

The steel plates for earthquake-resistant structures should have excellent resistance to low-cost and impact, and at the same time they must satisfy high strength. Among the currently known steel grades, for example, a steel sheet exhibiting a high strength of 700 MPa or more in strength and a high impact resistance and impact value is not provided.

As a prior art, there is a Korean Patent Publication No. 10-2001-0038132 (published on May 5, 2001, entitled " Method of producing a steel plate with excellent resistance to vibration and shock resistance).

The present invention provides a hot-rolled steel sheet having a high impact value and a low yield ratio, and a method for manufacturing the same.

In order to achieve the above object, a hot-rolled steel sheet according to an embodiment of the present invention comprises 0.17 to 0.22% carbon (C), 0.1% or less silicon (Si) : 0.001 to 0.025% of niobium (Nb), 0.005 to 0.025% of vanadium (V), and the balance of iron (Fe) and vanadium (V), wherein the sum of niobium and vanadium is 0.01 to 0.05% And the final microstructure is composed of ferrite and pearlite structure.

The hot-rolled steel sheet according to claim 1, wherein the sum of phosphorus (P): 0.03% or less (excluding 0%), sulfur (S): 0.005% (Excluding 0%), but not more than 0.05% (excluding 0%) of copper (Cu) and not more than 0.05% (excluding 0%) of tin (Sn).

The hot-rolled steel sheet has a yield strength of 500 MPa or more, a tensile strength of 700 MPa or more, an elongation of 14% or more, an impact value of 150 J or more, and a yield ratio of 80 or less.

(A) 0.17-0.22% carbon (C), 0.1% silicon (Si) or less (excluding 0%) by weight, carbon (C) (Mn): 1.2 to 1.8%, the sum of niobium (Nb) and vanadium (V) being 0.01 to 0.05%, niobium (Nb): 0.005 to 0.025%, vanadium (V): 0.005 to 0.025% Providing a slab of iron (Fe) and unavoidable impurities; (b) reheating the slab to a temperature of 1200-1250 占 폚; (c) hot-rolling the reheated slab at a finishing rolling temperature of 820 to 880 캜; And (d) winding the hot-rolled steel at 580 to 640 占 폚; .

Wherein the slab contains 0.03% or less phosphorus (excluding 0%), sulfur (S) is 0.005% or less (excluding 0%), copper (Cu) and tin (Excluding 0%) of copper (Cu): 0.05% or less (excluding 0%) and tin (Sn): 0.05% or less (excluding 0%).

In the method for manufacturing a hot-rolled steel sheet, the step (b) may be carried out by holding for more than 2 hours.

The method of manufacturing a hot-rolled steel sheet may further include, between the step (c) and the step (d), cooling the hot-rolled steel at a cooling rate of 20 to 50 ° C / s or more.

According to the embodiment of the present invention, it is possible to realize a hot-rolled steel sheet having a high impact value and a low yield ratio, and a manufacturing method thereof. Of course, the scope of the present invention is not limited by these effects.

1 is a flowchart showing a method of manufacturing a hot-rolled steel sheet according to an embodiment of the present invention.

Hereinafter, a hot-rolled steel sheet according to an embodiment of the present invention and a method of manufacturing the same will be described in detail. The terms used below are appropriately selected terms in consideration of functions in the present invention, and definitions of these terms should be made based on the contents throughout this specification.

In general, steel plates for earthquake-resistant structures are required to have low yield ratio and high impact values for use as steel plates used for building structures. Among the currently known steel grades, for example, a steel sheet having a high strength of 700 MPa or more and a high resistance and a high impact resistance value can not be provided, but demand for high strength is gradually increasing. Accordingly, in order to secure the strength of the earthquake-resistant steel sheet, the steel pipe and the steel pipe are formed and then heat-treated to secure the strength. However, such additional heat treatment has a disadvantage of raising the manufacturing cost.

Hereinafter, a hot-rolled steel sheet excellent in impact value and low in yield ratio and high in strength and a method of manufacturing the same are provided. Specifically, the present invention provides, for example, a high strength hot-rolled steel sheet for a seismic structure having a tensile strength of 700 MPa or higher, a yield ratio of 80% or lower, and an impact value of 150 J or higher.

Hot-rolled steel sheet

The hot-rolled steel sheet according to an embodiment of the present invention comprises 0.17 to 0.22% of carbon (C), 0.1% or less (excluding 0%) of silicon (Si), 1.2 to 1.8% of manganese (Mn) (Nb): 0.005 to 0.025%, vanadium (V): 0.005 to 0.025%, and the balance of iron (Fe) and unavoidable impurities, wherein the sum of niobium (Nb) and vanadium (V) is 0.01 to 0.05%. Further, the hot-rolled steel sheet preferably contains 0.03% or less (excluding 0%) of phosphorus (P), 0.005% or less (excluding 0%) of sulfur (S), a sum of copper (Cu) (Exclusive of 0%) of copper (Cu): not more than 0.05% (excluding 0%) and 0.05% or less of tin (exclusive of 0%).

Hereinafter, the role and content of each component included in the hot-rolled steel sheet according to an embodiment of the present invention will be described.

Carbon (C)

Carbon is a basic element that increases the strength of steel as an interstitial solid element, but when it is added in a large amount exceeding 0.22% by weight, it is difficult to secure moldability in the process of steel pipe and section steel, thereby increasing the probability of failure. When the carbon content is less than 0.17% by weight, the desired tensile, yield strength and appropriate impact value can not be secured. Therefore, the content of carbon is preferably 0.17-0.22% by weight.

Silicon (Si)

Silicon acts as a deoxidizer and is effective in improving the toughness and ductility of steel by forming ferrite as a ferrite stabilizing element. However, when added in a large amount exceeding 0.10 wt%, the occurrence of a red scale due to Si may affect the surface quality deterioration and the machine operation. Accordingly, it is preferable that the content is 0.10% by weight or less (excluding 0% by weight).

Manganese (Mn)

Manganese prevents the embrittlement of brittleness due to FeS formation, which is inevitably contained in the steel making process, and enhances the strength of steel by generating a solid solution strengthening effect. The reason why the content of manganese is added in an amount of 1.2 wt% or more is to make a 700 MPa grade high-strength hot-rolled steel sheet, and the problem that the steel sheet is hardened due to a large amount of Mn added to lower the workability can be prevented by regulating the S content to a minimum, Is contained in an amount of more than 1.8% by weight, inclusions such as MnS may be formed to deteriorate impact toughness and internal quality. Therefore, the content of manganese is suitably 1.2 to 1.8 wt%.

Niobium (Nb) and vanadium (V)

Niobium (Nb) and vanadium (V) combine with elements such as carbon and nitrogen in the steel to form carbonitride, and inhibit grain growth during rolling to refine the grain. It is suitable that the total content of niobium (Nb) and vanadium (V) is set to 0.01 to 0.05% by weight in order to improve toughness and precipitate strengthening due to formation of fine precipitates and to greatly improve the strength of steel. If the total content of niobium (Nb) and vanadium (V) is less than 0.01 wt%, the effect may be insignificant. If the total content of niobium (Nb) and vanadium (V) exceeds 0.05 wt% And this can be a major factor that hinders the workability in the manufacture of steel pipes.

When each of niobium (Nb) and vanadium (V) is less than 0.005% by weight, precipitation strengthening effect is insignificant. When each of niobium (Nb) and vanadium (V) exceeds 0.025% by weight, The impact toughness is deteriorated, and the toughness of the steel may be lowered. Therefore, it is preferable that each of the niobium (Nb) and the vanadium (V) is in the range of 0.005 to 0.025% by weight within the range of 0.01 to 0.05% by weight, which is the total content of the niobium (Nb) and the vanadium (V) Do.

In (P)

Phosphorus (P) enhances the strength of the strength by solid solution strengthening and can function to inhibit the formation of carbide. However, when the content of phosphorus exceeds 0.03% by weight, there is a problem that the corrosion resistance is lowered due to slab center segregation. In addition, phosphorus is highly likely to be segregated in the production of steel, and the segregated steel lowers the material and toughness and causes a delayed fracture which is destroyed after a certain time after molding. Taking this into consideration, it is preferable to control the content of phosphorus in the steel to 0.03 wt% or less.

Sulfur (S)

Sulfur is combined with manganese to form a nonmetallic inclusion such as MnS and is stretched in the rolling direction during rolling. Since these inclusions are impurities which cause defects such as hook cracks during processing, it is important to reduce the content. Therefore, in the present invention, it is preferable to control the S content to 0.005 wt% or less.

Copper (Cu) and tin (Sn)

Copper and tin are metals with low melting point of the alloy, which melts at the crystal grain boundaries on the surface of the steel sheet during hot rolling, making it hard to descale by generating a firm scale. Therefore, the total content of copper and tin is preferably controlled to 0.1 wt% or less.

If the amount of each of copper (Cu) and tin (Sn) exceeds 0.05% by weight, it may become a factor to deteriorate high temperature sticking ability. Therefore, it is preferable that each of copper (Cu) and tin (Sn) is controlled to be 0.05 wt% or less within the range of 0.1 wt% or less, which is the total content of copper (Cu) and tin (Sn).

The final microstructure of the hot-rolled steel sheet having the above-described alloy composition is composed of ferrite and pearlite. The hot-rolled steel sheet has a yield strength of 500 MPa or more, a tensile strength of 700 MPa or more, an elongation of 14% or more, an impact value of 150 J or more, and a yield ratio of 80 or less.

Manufacturing method of hot-rolled steel sheet

A method of manufacturing a hot-rolled steel sheet according to an embodiment of the present invention will be described in detail below.

1 is a flowchart illustrating a method of manufacturing a hot-rolled steel sheet according to an embodiment of the present invention. 1, a method for manufacturing a hot-rolled steel sheet comprises the steps of (a) 0.17 to 0.22% of carbon (C), 0.1% or less (exclusive of 0%) of silicon (Si) (Nb): 0.005 to 0.025%, vanadium (V): 0.005 to 0.025%, and the balance of iron (Fe) and inevitable impurities, the content of the niobium (Nb) (SlOO); (b) reheating the slab to a temperature of 1200 to 1250 캜 (S200); (c) hot rolling the reheated slab at a finishing rolling temperature of 820 to 880 캜 (S300); And (d) winding the hot-rolled steel at 580 to 640 占 (S400); .

Wherein the slab contains 0.03% or less phosphorus (excluding 0%), sulfur (S) is 0.005% or less (excluding 0%), copper (Cu) and tin (Excluding 0%) of copper (Cu): 0.05% or less (excluding 0%) and tin (Sn): 0.05% or less (excluding 0%).

In the method for manufacturing a hot-rolled steel sheet, the step (b) may be carried out by holding for more than 2 hours.

The method of manufacturing a hot-rolled steel sheet may further include, between the step (c) and the step (d), cooling the hot-rolled steel at a cooling rate of 20 to 50 ° C / s or more.

Reheating slabs

In the slab reheating step (S200), the slab is heated at 1200 to 1250 DEG C for at least 2 hours to recycle the segregated components during casting. When the slab temperature is less than 1200 ° C, the added Nb and V are not sufficiently reused, and it is difficult to secure the strength due to coarsening of precipitates. On the other hand, if the temperature exceeds 1250 DEG C, the strength of the steel sheet may be lowered due to the coarsening of crystal grains. When the slab reheating proceeds for less than 2 hours, the precipitation elements can not be sufficiently employed in the slab.

Hot rolling

In the step S300 of hot-rolling the reheated slab, since niobium (Nb) and vanadium (V) are added for high strength, such precipitation elements are precipitated in the finishing rolling temperature range to refine and precipitate the austenite When the finish rolling temperature is lower than 820 ° C, the impact is lowered due to the influence of the non-recrystallized austenite. At a temperature exceeding 880 ° C, recrystallization of the austenite is caused, and rapid grain growth is caused, The hot-rolling temperature was set in the range of 820 to 880 ° C

Coiling

In step S400 of winding the hot-rolled steel material, if the coiling temperature is higher than 640 DEG C, coarse ferrite and pearlite are formed in the microstructure, and precipitates precipitated by niobium (Nb) or vanadium (V) become coarse, If the coiling temperature is lower than 580 占 폚, bainite structure is generated and the strength is increased. However, since the toughness and impact value of the material are greatly lowered, it is preferable to set the coiling temperature to 580 to 640 占 폚. The method may further include cooling the hot-rolled steel material at a cooling rate of 20 to 50 DEG C / s or more between the hot rolling step (S300) and the winding step (S400).

By performing the above-described process, a hot-rolled steel sheet according to an embodiment of the present invention can be manufactured. According to the embodiment of the present invention, it is possible to realize a hot-rolled steel sheet having excellent impact value and low yield ratio and high strength and a manufacturing method thereof. For example, when tensile strength is 700 MPa or higher, yield ratio is 80% A high-strength hot-rolled steel sheet for an earthquake-proof structure having a strength of 150 J or more and a method of manufacturing the same.

Hereinafter, preferred examples of the present invention will be described in order to facilitate understanding of the present invention. It should be understood, however, that the following examples are intended to aid in the understanding of the present invention and are not intended to limit the scope of the present invention.

Table 1 shows the composition, processing conditions, and mechanical properties of the specimen according to the experimental examples of the present invention.

(C): 0.09%, silicon (Si): 0.025%, manganese (Mn): 1.4%, niobium (Nb): 0.02%, and balance iron The slabs were reheated at a temperature of 1180 ° C and the reheated slabs were hot rolled at a finish rolling temperature of 860 ° C and then rolled at 580 ° C to measure the mechanical properties of the hot rolled steel sheets. As a result, the yield strength was 400 MPa, The tensile strength was 550 MPa, the elongation was 23%, the yield ratio was 72, and the impact value was 200J. Comparative Example 1 is different from Examples of the present invention in that vanadium (V) was not contained in the slab composition and the slab reheating temperature was out of the range of 1200 to 1250 DEG C, and the yield strength of the hot-rolled steel sheet was less than 500 MPa , And the tensile strength was measured to be less than 700 MPa.

(C): 0.09%, silicon (Si): 0.025%, manganese (Mn): 1.4%, niobium (Nb): 0.02%, and balance iron The slabs were reheated to a temperature of 1230 ° C and the reheated slabs were hot rolled at a finish rolling temperature of 860 ° C and then rolled at 580 ° C to measure the mechanical properties of the hot rolled steel sheets. As a result, the yield strength was 440.5 MPa , A tensile strength of 570.3 MPa, an elongation of 21%, a yield ratio of 77, and an impact value of 198 J. In Comparative Example 2, unlike the examples of the present invention, the slab composition did not contain vanadium (V), and the yield strength of the hot-rolled steel sheet was measured to be less than 500 MPa and tensile strength less than 700 MPa.

(C): 0.09%, silicon (Si): 0.025%, manganese (Mn): 1.4%, niobium (Nb): 0.04%, vanadium (V): 0.02% And the remainder of iron (Fe) were reheated at a temperature of 1180 占 폚, and the reheated slab was hot-rolled at a finish rolling temperature of 860 占 폚 and then rolled at 580 占 폚 to measure the mechanical properties of the hot- As a result, the yield strength was 549.1 MPa, the tensile strength was 680 MPa, the elongation was 17%, yield ratio was 80, and impact value was 118 J. In Comparative Example 3, the sum of niobium (Nb) and vanadium (V) was out of the range of 0.01 to 0.05% in the slab composition, the composition of niobium was out of the range of 0.005 to 0.025% When the slab reheating temperature was out of the range of 1200 to 1250 占 폚, the tensile strength of the hot-rolled steel sheet was measured to be less than 700 MPa and the impact value was less than 150J.

(C): 0.09%, silicon (Si): 0.025%, manganese (Mn): 1.4%, niobium (Nb): 0.04%, vanadium (V): 0.02% And the remaining iron (Fe) were reheated at a temperature of 1,230 DEG C, and the reheated slab was hot-rolled at a finishing rolling temperature of 860 DEG C, and then the mechanical properties of the hot-rolled steel sheet were measured at 580 DEG C As a result, the yield strength was 560 MPa, the tensile strength was 710 MPa, the elongation was 10%, the yield ratio was 78, and the impact value was 100 J. In Comparative Example 4, when the sum of niobium (Nb) and vanadium (V) is out of the range of 0.01 to 0.05% in the slab composition and the composition of niobium is out of the range of 0.005 to 0.025% , The elongation of the hot-rolled steel sheet was less than 14%, and the impact value was measured to be less than 150J.

(C): 0.09%, silicon (Si): 0.025%, manganese (Mn): 1.4%, niobium (Nb): 0.02%, vanadium (V): 0.02% And the remainder of iron (Fe) were reheated at a temperature of 1180 占 폚, and the reheated slab was hot-rolled at a finish rolling temperature of 860 占 폚 and then rolled at 580 占 폚 to measure the mechanical properties of the hot- As a result, the yield strength was 527.3MPa, the tensile strength was 670.4MPa, the elongation was 16%, the yield ratio was 78, and the impact value was 189J. In Comparative Example 5, unlike the examples of the present invention, the tensile strength of the hot-rolled steel sheet was measured to be less than 700 MPa when the slab reheating temperature was out of the range of 1200 to 1250 ° C.

(C): 0.09%, silicon (Si): 0.025%, manganese (Mn): 1.4%, niobium (Nb): 0.02%, vanadium (V): 0.02% And the remaining iron (Fe) were reheated at a temperature of 1,230 DEG C, and the reheated slab was hot-rolled at a finishing rolling temperature of 860 DEG C, and then the mechanical properties of the hot-rolled steel sheet were measured at 580 DEG C As a result, the yield strength was more than 500 MPa, the tensile strength was more than 700 MPa, the elongation was more than 14%, the impact value was more than 150 J, and the yield ratio was 80 or less.

It is to be understood that the invention includes various modifications and equivalent embodiments that can be derived from the disclosed embodiments as well as those of ordinary skill in the art to which the present invention pertains. Accordingly, the technical scope of the present invention should be defined by the following claims.

Claims (7)

(Si): 0.1% or less (excluding 0%), manganese (Mn): 1.2 to 1.8%, and the sum of niobium (Nb) and vanadium (V) (Nb): 0.005 to 0.025%, vanadium (V): 0.005 to 0.025%, and the balance of iron (Fe) and unavoidable impurities,
The final microstructure consists of ferrite and pearlite structure,
Hot rolled steel sheet. The method according to claim 1,
The steel sheet preferably contains 0.1% or less of phosphorus (P): 0.03% or less (excluding 0%), sulfur (S): 0.005% or less (excluding 0% (Exclusive of 0%) of copper (Cu): 0.05% or less (excluding 0%) and 0.05% or less of tin (exclusive of 0%).
Hot rolled steel sheet. The method according to claim 1,
Wherein the steel sheet has a yield strength of 500 MPa or more, a tensile strength of 700 MPa or more, an elongation of 14% or more, an impact value of 150 J or more, and a yield ratio of 80 or less.
Hot rolled steel sheet. (A) 0.1 to 0.2% of carbon (C), 0.1 to 0.1% (exclusive of 0%) of silicon (Si), 1.2 to 1.8% of manganese (Mn), niobium (Nb) and vanadium (V) Providing a slab made of 0.01 to 0.05% of niobium (Nb), 0.005 to 0.025% of vanadium (V), 0.005 to 0.025% of vanadium (V), and balance iron (Fe) and unavoidable impurities;
(b) reheating the slab to a temperature of 1200-1250 占 폚;
(c) hot-rolling the reheated slab at a finishing rolling temperature of 820 to 880 캜; And
(d) winding the hot-rolled steel at 580 to 640 占 폚; / RTI >
A method of manufacturing a hot - rolled steel sheet. 5. The method of claim 4,
The slab may contain 0.1% or less of phosphorus (P): 0.03% or less (excluding 0%), 0.005% or less of sulfur (excluding S), 0.1% or less of copper (Cu) (Exclusive of 0%) of copper (Cu): 0.05% or less (excluding 0%) and 0.05% or less of tin (exclusive of 0%).
A method of manufacturing a hot - rolled steel sheet. 5. The method of claim 4,
Wherein the step (b) is carried out by maintaining at least 2 hours.
A method of manufacturing a hot - rolled steel sheet. 5. The method of claim 4,
Cooling the hot rolled steel material at a cooling rate of 20 to 50 DEG C / s or more between the step (c) and the step (d).
A method of manufacturing a hot - rolled steel sheet.

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