KR102464386B1 - Hot rolled steel having high strength and high deformation and method of manufacturing the same - Google Patents

Abstract

The present invention provides a high strength, high elongation hot-rolled steel material capable of securing a high elongation with high strength, and a method for manufacturing the same. According to an embodiment of the present invention, the high-strength high elongation hot-rolled steel material is, by weight, carbon (C): 0.04% to 0.06%, manganese (Mn): 1.1% to 1.3%, aluminum (Al): 0.015% to 0.06%, titanium (Ti): 0.055% to 0.065%, niobium (Nb): 0.06% to 0.07%, vanadium (V): 0.015% to 0.025%, phosphorus (P): greater than 0% to 0.02%, sulfur (S): more than 0% to 0.003%, and the balance contains iron (Fe) and other unavoidable impurities, tensile strength (TS): 600 MPa to 700 MPa, yield strength (YS): 500 MPa to 620 MPa; and elongation (EL): 20% to 30%.

Description

High-strength high-strength hot-rolled steel and manufacturing method thereof

The technical idea of the present invention relates to a steel material and a method for manufacturing the same, and more particularly, to a high-strength high elongation hot-rolled steel material and a method for manufacturing the same.

Recently, the automobile industry is responding to environmental regulations and promoting weight reduction for the purpose of improving fuel efficiency. In order to reduce the weight of parts, a method of applying a high-strength material and reducing the thickness is common, so a high-strength steel sheet is applied.

Automotive chassis parts are applied to the lower part of the car body to connect and support the car body, powertrain, steering and driving parts, and because they receive repeated shocks and loads, high durability and fatigue properties are required. Since automobile chassis parts are closely related to driver safety, high-strength steel plates are used. Recently, due to the complexity and integration of automobile parts, it is necessary to develop a steel plate having a higher level of formability than before. Conventionally, it is possible to secure a high strength of a tensile strength of 60K class by adding a precipitating element such as niobium (Nb), titanium (Ti), vanadium (V), but there is a problem that the elongation decreases as the strength increases.

Korean Patent Application No. 10-2015-0042766

A technical problem to be achieved by the technical idea of the present invention is to provide a high-strength, high-strength hot-rolled steel material capable of securing a high elongation with high strength and a method for manufacturing the same.

However, these tasks are exemplary, and the technical spirit of the present invention is not limited thereto.

According to one aspect of the present invention, there is provided a high-strength high-strength hot-rolled steel material and a method for manufacturing the same.

According to an embodiment of the present invention, the high-strength high elongation hot-rolled steel material is, by weight, carbon (C): 0.04% to 0.06%, manganese (Mn): 1.1% to 1.3%, aluminum (Al): 0.015% to 0.06%, titanium (Ti): 0.055% to 0.065%, niobium (Nb): 0.06% to 0.07%, vanadium (V): 0.015% to 0.025%, phosphorus (P): greater than 0% to 0.02%, sulfur (S): more than 0% to 0.003%, and the balance contains iron (Fe) and other unavoidable impurities, tensile strength (TS): 600 MPa to 700 MPa, yield strength (YS): 500 MPa to 620 MPa; and elongation (EL): 20% to 30%.

According to an embodiment of the present invention, the high-strength high elongation hot-rolled steel material may have a single-phase ferrite structure.

According to an embodiment of the present invention, the high-strength high elongation hot-rolled steel material may include precipitates including TiC, NbC, or VC in grains and at grain boundaries of the ferrite structure.

According to an embodiment of the present invention, the manufacturing method of the high-strength high elongation hot-rolled steel material is, by weight, carbon (C): 0.04% to 0.06%, manganese (Mn): 1.1% to 1.3%, aluminum (Al) : 0.015% to 0.06%, titanium (Ti): 0.055% to 0.065%, niobium (Nb): 0.06% to 0.07%, vanadium (V): 0.015% to 0.025%, phosphorus (P): more than 0% to 0.02 %, sulfur (S): more than 0% ~ 0.003%, and the remainder is iron (Fe) and other unavoidable impurities comprising the steps of reheating the steel at a temperature of 1,200 ℃ ~ 1,240 ℃; hot-rolling the heated steel material to be terminated at a temperature of 860°C to 900°C; cooling the hot-rolled steel; and winding the cooled steel material.

According to an embodiment of the present invention, the cooling step may be performed at a cooling rate of 50° C./sec to 80° C./sec.

According to an embodiment of the present invention, the winding step may be performed at a winding temperature of 580°C to 620°C.

According to an embodiment of the present invention, the high-strength high elongation hot-rolled steel material manufactured after performing the winding step has a tensile strength (TS): 600 MPa to 700 MPa, a yield strength (YS): 500 MPa to 620 MPa , and elongation (EL): 20% to 30% may be satisfied.

According to an embodiment of the present invention, the high-strength high-stretch hot-rolled steel material manufactured after performing the winding step may have a single-phase ferrite structure, and TiC, NbC, or VC in the grains and at the grain boundaries of the ferrite structure. It may contain a precipitate containing.

According to the technical idea of the present invention, the high-strength high-elongation hot-rolled steel material provides a method for manufacturing a hot-rolled steel sheet having a high tensile strength of 60K class with a high elongation through control of alloy composition and process conditions. was achieved by forming precipitates on a single ferrite phase to secure a strength of 60K or higher, and also secured high formability by having an elongation of 21% or more. As described above, the high strength and high elongation hot rolled steel can be applied to chassis arms or members of automobiles.

The above-described effects of the present invention have been described by way of example, and the scope of the present invention is not limited by these effects.

1 is a process flowchart schematically illustrating a method for manufacturing a high-strength high-stretch hot-rolled steel material according to an embodiment of the present invention.
2 is a photomicrograph showing the microstructures of Examples and Comparative Examples manufactured by using the method for manufacturing a high-strength, high-stretched hot-rolled steel material according to an embodiment of the present invention.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. The embodiments of the present invention are provided to more completely explain the technical idea of the present invention to those of ordinary skill in the art, and the following examples may be modified in various other forms, The scope of the technical idea is not limited to the following examples. Rather, these embodiments are provided so as to more fully and complete the present disclosure, and to fully convey the technical spirit of the present invention to those skilled in the art. In the present specification, the same reference numerals refer to the same elements throughout. Furthermore, various elements and regions in the drawings are schematically drawn. Accordingly, the technical spirit of the present invention is not limited by the relative size or spacing drawn in the accompanying drawings.

The high-strength, high-elongation hot-rolled steel according to the technical idea of the present invention can secure a tensile strength of 60K or higher and at the same time optimize the carbon content to have a high level of formability, thereby securing elongation compared to the prior art. As such, in order to secure a high elongation, it is necessary to control the carbon content and the cooling rate to form a single ferrite phase. In the present invention, while securing elongation by optimizing the carbon content from a porous system to a low carbon system, a method for supplementing the reduced strength by adding a precipitating element is used. In order to secure high strength in the ferrite single-phase structure, the effect of the precipitation element must be maximized, so the winding temperature was set in the range of 580 ° C.

Hereinafter, a high-strength high-stretch hot-rolled steel material as an aspect of the present invention will be described.

High strength, high elongation hot rolled steel

High-strength, high-elongation hot-rolled steel, which is an aspect of the present invention, by weight, carbon (C): 0.04% to 0.06%, manganese (Mn): 1.1% to 1.3%, aluminum (Al): 0.015% to 0.06%, Titanium (Ti): 0.055% to 0.065%, Niobium (Nb): 0.06% to 0.07%, Vanadium (V): 0.015% to 0.025%, Phosphorus (P): >0% to 0.02%, Sulfur (S): greater than 0% to 0.003%, and the balance contains iron (Fe) and other unavoidable impurities.

Hereinafter, the role and content of each component included in the high-strength high-stretch hot-rolled steel material according to the present invention will be described as follows. In this case, the content of the component elements all mean wt%.

Carbon (C): 0.04% to 0.06%

Carbon is added to improve the strength of steel and secure strength by forming precipitates. When the carbon content is less than 0.04%, it is difficult to secure strength. When the content of carbon exceeds 0.06%, it is difficult to reduce the fraction of two phases during continuous cooling and secure a single phase of ferrite. Accordingly, carbon is preferably added in an amount of 0.04% to 0.06% of the total weight of the steel.

Manganese (Mn): 1.1% to 1.3%

Manganese is an element contributing to increase in strength and improvement in hardenability by solid solution strengthening. When the manganese content is less than 1.1%, the effect of adding manganese is insufficient. When the content of manganese exceeds 1.3%, segregation is likely to occur and the formability may be deteriorated due to the influence of the segregation zone. Therefore, manganese is preferably added in 1.1% to 1.3% of the total weight of the steel.

Aluminum (Al): 0.015% to 0.06%

Aluminum acts as a deoxidizer to remove oxygen from the steel. When the content of aluminum is less than 0.015%, the effect of adding aluminum is insufficient. When the content of aluminum exceeds 0.06%, nitride (AlN) is precipitated by reacting with nitrogen during continuous casting, which may cause corner cracks or surface defects of the slab. Therefore, it is preferable that aluminum is added in an amount of 0.015% to 0.06% of the total weight of the steel.

Titanium (Ti): 0.055% to 0.065%

Titanium combines with carbon and nitrogen, which have excellent high-temperature stability, to form carbides and nitrides to prevent austenite grain growth, and improve the toughness and strength of hot-rolled products by refining the structure. When the content of titanium is less than 0.055%, the effect of adding titanium is insufficient. If the content of titanium exceeds 0.065%, a decrease in playability may occur during slab manufacturing. Therefore, titanium is preferably added in an amount of 0.055% to 0.065% of the total weight of the steel.

Niobium (Nb): 0.06% to 0.07%

Niobium is a precipitation-strengthening element and makes the grain size fine by delaying the recrystallization rate of austenite during hot rolling. When the content of niobium is less than 0.06%, the effect of adding niobium is insufficient. When the content of niobium exceeds 0.07%, playability, rollability, and elongation of the hot rolling process may be deteriorated due to excessive precipitation. Therefore, niobium is preferably added in an amount of 0.06% to 0.07% of the total weight of the steel.

Vanadium (V): 0.015% to 0.025%

Vanadium, similar to the niobium, may delay recrystallization during hot rolling to achieve grain refinement. Dissolved vanadium during hot rolling is known to delay nucleation and growth of recrystallization, and since such recrystallization delay does not consume defect sites such as dislocations, it promotes nucleation during phase transformation to make grains finer. When the content of vanadium is less than 0.015%, the effect of adding niobium is insignificant. When the content of vanadium exceeds 0.025%, playability, rollability, and elongation of the hot rolling process may be deteriorated due to excessive precipitation. Therefore, vanadium is preferably added in an amount of 0.015% to 0.025% of the total weight of the steel.

Phosphorus (P): >0% to 0.02%

Phosphorus segregates at grain boundaries during the slab reheating process and causes brittleness, so the lower the content, the better. When phosphorus is included in more than 0.02%, weldability and toughness may be deteriorated. Therefore, phosphorus is preferably limited to more than 0% ~ 0.02% of the total weight of the steel.

Sulfur (S): >0% to 0.005%

Sulfur is an element that is unavoidably contained in the manufacture of steel together with phosphorus, and may impair the toughness and weldability of steel. When sulfur is included in excess of 0.005%, it may form emulsion-based inclusions (MnS) to deteriorate the resistance to stress corrosion cracking, thereby generating cracks during the processing of steel, and as a result, the corrosion resistance of the steel may be reduced. . Sulfur is preferably limited to more than 0% ~ 0.003% of the total weight of the steel.

Silicon (Si): 0.04% or less

Although silicon is an element that contributes to increasing the strength of steel, since it is an element that induces surface scale formation, it is desirable to control it as an impurity without intentionally adding it in steel materials. When the content of silicon exceeds 0.40%, since the affinity with oxygen is strong, defects due to surface scale may occur, which may cause deterioration of surface quality. Therefore, silicon is preferably controlled to 0.04% or less of the total weight of the steel, for example, 0% to 0.04% or more than 0% to 0.04%.

The remaining component of the present invention is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in the normal manufacturing process, this cannot be excluded. Since these impurities are known to any person skilled in the art of a conventional manufacturing process, all details thereof are not specifically mentioned in the present specification.

For reference, nitrogen (N) as one of the other unavoidable impurities is preferably controlled to 60 ppm or less, for example, more than 0 ppm to 60 ppm range. Therefore, the high-strength high elongation hot-rolled steel may not contain nitrogen (N), or contain more than 0 ppm ~ 60 ppm.

The high-strength, high-elongation hot-rolled steel manufactured by controlling the specific components of the alloy composition and their content ranges described above, and producing a steel material to be described later, has a tensile strength (TS): 600 MPa to 760 MPa, yield strength (YS) : 550 MPa to 650 MPa, and elongation (EL): 20% to 30% may be satisfied.

The high-strength high elongation hot-rolled steel material may have a single-phase ferrite structure. In addition, precipitates including carbides such as TiC, NbC, or VC may be included in grains and at grain boundaries of the ferrite structure.

Another aspect of the present invention provides a method for manufacturing a high-strength, high-stretch hot-rolled steel material. According to this, the step of reheating the steel made of the above-mentioned alloy composition at a temperature of 1,200 ℃ ~ 1,240 ℃; hot-rolling the heated steel material to be terminated at a temperature of 860°C to 900°C; cooling the hot-rolled steel; and winding the cooled steel material.

The step of cooling the steel may be performed at a cooling rate of 50 °C / sec to 80 °C / sec.

The winding of the cooled steel material may be performed at a winding temperature of 580°C to 620°C.

Hereinafter, a method for manufacturing a high-strength high-stretch hot-rolled steel material according to the present invention will be described with reference to the accompanying drawings.

Steel manufacturing method

1 is a process flowchart schematically illustrating a method for manufacturing a high-strength high-stretch hot-rolled steel material according to an embodiment of the present invention.

In the method for manufacturing a steel material according to the present invention, the semi-finished product to be subjected to the hot rolling process may be, for example, a slab. The semi-finished slab can be obtained through the continuous casting process after obtaining molten steel of a predetermined composition through the steelmaking process.

The steel is, by weight, carbon (C): 0.04% to 0.06%, manganese (Mn): 1.1% to 1.3%, aluminum (Al): 0.015% to 0.06%, titanium (Ti): 0.055% to 0.065 %, niobium (Nb): 0.06% to 0.07%, vanadium (V): 0.015% to 0.025%, phosphorus (P): greater than 0% to 0.02%, sulfur (S): greater than 0% to 0.003%, and glass Wealth contains iron (Fe) and other unavoidable impurities.

Referring to FIG. 1 , the method for manufacturing a low yield ratio hot-rolled steel material according to an embodiment of the present invention includes a reheating step (S110), a hot rolling step (S120), a cooling step (S130), and a winding step (S140). .

Reheating step (S110)

In the reheating step (S110), the steel having the above composition, for example, a slab plate, is reheated at a reheating temperature (Slab Reheating Temperature, SRT) of 1,200 °C to 1,240 °C. Through such reheating, re-dissolution of segregated components during casting and re-dissolution of precipitates may occur. When the reheating temperature is less than 1,200° C., the maximum re-dissolution of niobium and titanium contained in the steel may not occur, and segregated components may not be sufficiently evenly distributed during casting. When the reheating temperature exceeds 1,240° C., the austenite grains are coarsened, which may cause a decrease in strength. In addition, as the reheating temperature increases, there is a problem in that the heating cost and the additional extraction time required to match the hot rolling temperature increase the manufacturing cost and decrease the productivity.

Hot rolling step (S120)

The heated steel is first subjected to hot rolling after heating to adjust its shape. The hot rolling may be continuously performed by wide rolling, rough rolling, and finishing rolling. By the hot rolling step, the steel may form a steel plate.

The hot rolling, that is, the finishing rolling may be finished at a finish rolling temperature (FRT) of 860° C. to 900° C. When the rolling end temperature is less than 860° C., as the hot rolling is finished in a low temperature region, crystal grains are mixed rapidly and have non-uniform deformability, which may result in a decrease in rollability. When the rolling end temperature exceeds 900 ℃, the strength of the final steel material may decrease due to the growth of precipitates and grains.

Cooling step (S130)

The hot-rolled steel is cooled to 580° C. to 620° C. at a cooling rate of 50° C./sec to 80° C./sec. The cooling may be performed by an air cooling method or a water cooling method. When cooling within the cooling rate range, it is possible to sufficiently secure a single-phase ferrite structure. Cooling after hot rolling is better as soon as possible, for example, a cooling rate of 50°C/sec to 80/sec may be appropriate.

Winding step (S140)

After the cooling is finished, the steel is wound at a coiling temperature (CT) of 580°C to 620°C. When the coiling temperature is less than 580° C., the strength effect through the formation of fine precipitates of niobium, vanadium, and titanium may be reduced. When the coiling temperature exceeds 620 ℃, the precipitate may be coarsened due to heat during winding, resulting in a decrease in strength.

Experimental example

Hereinafter, preferred experimental examples are presented to help the understanding of the present invention. However, the following experimental examples are only for helping understanding of the present invention, and the present invention is not limited by the following experimental examples. Content not described here will be omitted because it can be technically inferred sufficiently by those skilled in the art.

Table 1 shows the compositions of high-strength high-strength hot-rolled steels of Comparative Examples and Examples. In Table 1, the remainder consists of iron (Fe) and impurities unavoidably contained in the steelmaking process. The content unit of each component is % by weight.

division C Mn Al Ti Nb V P S comparative example 0.08 1.3 0.04 0.06 0.050 0.02 0.005 0.001 Example 0.05 1.2 0.04 0.06 0.065 0.02 0.005 0.001

Referring to Table 1, compared to Comparative Examples, Examples have a low carbon content, a low manganese content, and a high niobium content. In addition, as an unavoidable impurity in Comparative Examples and Examples, nitrogen (N) was controlled to be less than 60 ppm. Table 2 shows the values of the process conditions for forming the high-strength, high-strength hot-rolled steels of Comparative Examples and Examples.

division reheat temperature
(℃) End rolling temperature
(℃) cooling rate
(℃/sec) winding temperature
(℃) comparative example 1200 900 50 or more 600 Example 1200 880 50 or more 600

Referring to Table 2, the process conditions of Comparative Examples and Examples were almost similar. Table 3 shows the tensile strength (TS), yield strength (YS), elongation (EL), and fineness of the high-strength, high-elongation hot-rolled steel material prepared above. represents the organization.

division The tensile strength
(MPa) yield strength
(MPa) elongation
(%) microstructure comparative example 587 630 18 Ferrite + Perlite + Precipitate Example 585 635 24 Ferrite single phase + precipitate

Referring to Table 3, both Comparative Examples and Examples are included in the yield strength of 500 MPa ~ 620 MPa and the tensile strength of 600 MPa ~ 700 MPa proposed in the present invention. On the other hand, the elongation was higher in the example than in the comparative example. This is analyzed because the carbon content is reduced compared to the comparative example, and the optimized carbon content to secure the desired elongation is controlled.

2 is a photomicrograph showing the microstructures of Examples and Comparative Examples manufactured by using the method for manufacturing a high-strength, high-stretched hot-rolled steel material according to an embodiment of the present invention.

Referring to FIG. 2 , in the comparative example, a microstructure in which pearlite is formed on a ferrite matrix is shown. On the other hand, in the embodiment, a microstructure of a single phase of ferrite appears. The elongation of the embodiment may be increased by such a ferrite single phase.

The technical spirit of the present invention described above is not limited to the above-described embodiments and the accompanying drawings, and it is the technical spirit of the present invention that various substitutions, modifications and changes are possible without departing from the technical spirit of the present invention. It will be apparent to those of ordinary skill in the art to which this belongs.

Claims (8)

By weight%, carbon (C): 0.04% to 0.06%, manganese (Mn): 1.1% to 1.3%, aluminum (Al): 0.015% to 0.06%, titanium (Ti): 0.055% to 0.065%, niobium ( Nb): 0.06% to 0.07%, vanadium (V): 0.015% to 0.025%, phosphorus (P): more than 0% to 0.02%, sulfur (S): more than 0% to 0.003%, and the balance is iron (Fe) ) and other unavoidable impurities,
Silicon is not intentionally added so as to prevent surface scale formation, and the content is controlled to 0.04 wt% or less as the unavoidable impurity,
After hot rolling, after cooling to 580°C to 620°C at a cooling rate of 50°C/sec to 80°C/sec, winding has a single-phase ferrite structure, and TiC, NbC, or VC in the grains and boundaries of the ferrite structure Containing a precipitate comprising
Tensile strength (TS): 600 MPa ~ 700 MPa, yield strength (YS): 500 MPa ~ 620 MPa, and elongation (EL): 20% ~ 30%,
High strength, high elongation hot rolled steel. delete delete By weight%, carbon (C): 0.04% to 0.06%, manganese (Mn): 1.1% to 1.3%, aluminum (Al): 0.015% to 0.06%, titanium (Ti): 0.055% to 0.065%, niobium ( Nb): 0.06% to 0.07%, vanadium (V): 0.015% to 0.025%, phosphorus (P): more than 0% to 0.02%, sulfur (S): more than 0% to 0.003%, and the balance is iron (Fe) ) and reheating the steel containing other unavoidable impurities at a temperature of 1,200 ° C. to 1,240 ° C.;
hot-rolling the heated steel material to be terminated at a temperature of 860°C to 900°C;
cooling the hot-rolled steel; and
Winding the cooled steel at a winding temperature of 580 ° C to 620 ° C.
The high-strength high-stretched hot-rolled steel manufactured by using the method for manufacturing the high-strength high-stretched hot-rolled steel is,
Silicon is not intentionally added so as to prevent surface scale formation, and the content is controlled to 0.04 wt% or less as the unavoidable impurity,
In the cooling step, by winding after cooling to 580 °C to 620 °C at a cooling rate of 50 °C/sec to 80 °C/sec, it has a single-phase ferrite structure, and TiC, NbC, TiC, NbC, or a precipitate comprising VC,
Tensile strength (TS): 600 MPa ~ 700 MPa, yield strength (YS): 500 MPa ~ 620 MPa, and elongation (EL): 20% ~ 30%,
A method for manufacturing high-strength, high-strength, hot-rolled steel. delete delete delete delete

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