KR20160138772A

KR20160138772A - Hot rolled steel sheet having high workability and method of manufacturing the same

Info

Publication number: KR20160138772A
Application number: KR1020150073053A
Authority: KR
Inventors: 김동용; 박대범; 허성열
Original assignee: 현대제철 주식회사
Priority date: 2015-05-26
Filing date: 2015-05-26
Publication date: 2016-12-06

Abstract

According to an embodiment of the present invention, the hot-rolled steel sheet having a high workability comprises: 0.003-0.01 wt% of carbon (C); over 0 to 0.03 wt% of silicon (Si); over 0 to 1.8 wt% of manganese (Mn); 0.05-0.10 wt% of phosphorus (P); over 0 to 0.01 wt% of sulfur (S); 0.01-0.06 wt% of aluminum (Al); 0.01-0.05 wt% of titanium (Ti); 0.01-0.05 wt% of niobium (Nb); 0.01-0.02 wt% of molybdenum (Mo); over 0 to 0.005 wt% of nitrogen (N); and the remaining iron (Fe) and unavoidable impurities. The present invention aims to provide a hot-rolled steel sheet and a manufacturing method thereof, which control alloy substances and thermal treatment processes to have a high workability.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot rolled steel sheet,

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-rolled steel sheet for forming a high-

In the case of an automotive steel sheet, the formation of a solid is required, and in particular, an excellent deep drawing is required in terms of a processing method. In the case of automobile materials and general structural materials, it is judged whether or not the material is defective due to high deep drawability.

In order to ensure deep drawability, it is basically necessary to secure an excellent plastic deformation ratio (r-value, hereinafter, r value). The high r value can be easily ensured by the cold-rolled annealed steel sheet or the cold-rolled steel sheet. Specifically, in the case of cold-rolled and annealed steel sheets, the hot-rolled steel sheet is subjected to hot rolling at a temperature of Ar3 or higher to form ferrite transformation during cooling, forming a texture close to random textures, (111) texture is developed and r value is improved.

Related Prior Art Korean Patent Publication No. 2005-0068249 (published on Jul. 5, 2005, entitled " Steel sheet having excellent processability and its manufacturing method ") is available.

The present invention provides a hot-rolled steel sheet excellent in moldability by controlling an alloy component control and a heat treatment process and a method for manufacturing the same.

The hot-rolled steel sheet according to one aspect of the present invention comprises 0.003 to 0.01% of carbon (C), more than 0 to 0.03% of silicon (Si), no more than 1.8% of manganese (Mn) (Al), 0.01 to 0.06% of aluminum (Al), 0.01 to 0.05% of titanium (Ti), 0.01 to 0.05% of niobium (Nb), 0.01 to 0.02% of molybdenum (Mo) %, Nitrogen (N) more than 0 and not more than 0.005%, and the balance of iron (Fe) and unavoidable impurities.

In one embodiment, the inevitable element comprises at least one of copper (Cu), tin (Sn), nickel (Ni) and chromium (Cr) Sn) of more than 0 to 0.01% by weight, nickel (Ni) of more than 0 to 0.07% by weight, and chromium (Cr) of more than 0 and 0.07% by weight or less.

A method of manufacturing a hot-formed steel sheet according to another aspect of the present invention is disclosed. In the above manufacturing method, it is preferable that 0.003 to 0.01% of carbon (C), 0.03% or more of silicon (Si), 0.03% or less of manganese (Mn) (S) of more than 0 and not more than 0.01%, aluminum (Al) of 0.01 to 0.06%, titanium (Ti) of 0.01 to 0.05%, niobium (Nb) of 0.01 to 0.05%, molybdenum (Mo) And the residual steel (Fe) and unavoidable impurities is heated to a temperature of 1100 占 폚 or higher. The reheated steel slab is hot-rolled at a temperature of 750 to 800 占 폚 below Ar3. The hot-rolled steel is cooled, maintained at a temperature of 650 to 700 ° C for 5 to 10 seconds, and cooled to a temperature of 200 to 300 ° C. The steel is heated and annealed at a temperature of 750 to 850 캜 for 20 to 30 seconds. The annealed annealed steel is cooled and maintained at a temperature of 400 to 500 ° C for 20 to 30 seconds, followed by cooling and winding.

In another embodiment, the process of cooling the hot-rolled steel to a temperature of 200 to 300 캜 may be conducted at a cooling rate of 10 to 30 캜 / s.

According to the embodiment of the present invention, it is possible to produce a hot-rolled steel sheet having excellent moldability such as deep drawability by controlling the content of alloying elements such as Ti, Nb, and Mo in the content of low carbon steel and controlling the hot rolling process. Particularly, the hot-rolled steel sheet has an advantage that excellent workability can be ensured even if the steelmaking process is performed in an environment where tramp elements such as copper, tin, nickel, and chromium are present in an electric furnace using scrap iron as a raw material have.

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

Hereinafter, a hot-formed steel sheet and a method of manufacturing the same according to an embodiment of the present invention 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.

Hot-formed steel sheet

(S) more than 0 and not more than 0.03%, manganese (Mn) more than 0 and not more than 1.8%, phosphorus (P) 0.01 to 0.06% of aluminum (Al), 0.01 to 0.05% of titanium (Ti), 0.01 to 0.05% of niobium (Nb), 0.01 to 0.02% of molybdenum (Mo) (Fe) and inevitable impurities.

The inevitable element includes at least one of copper (Cu), tin (Sn), nickel (Ni), and chromium (Cr) (Ni) of more than 0 to 0.07% by weight, and chromium (Cr) of more than 0 and 0.07% by weight or less.

In particular, the copper, tin, nickel, or chromium may correspond to a tramp element when a steelmaking process is performed in an electric furnace using scrap metal as a raw material.

In the hot-rolled steel sheet according to the embodiment of the present invention, the texture of the (111) orientation is developed and can have excellent moldability. Specifically, it may have an r value of 1.7 or more and an elongation of about 40% or more.

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

Carbon (C)

Carbon (C) is present as a solid element in the steel sheet and can inhibit the formation of (111) aggregate structure favorable to the workability in the process of forming the aggregate structure of the steel sheet during cold rolling and annealing, thereby reducing workability and formability. However, if C is present in the steel, it may cause an aging problem and cause a Strecher strain problem. If the carbon content is more than 0.01% by weight, the addition amount of the carbide forming elements Ti and Nb should be increased. In this case, not only the cost rise of the steel but also the addition of a large amount of Ti and Nb may deteriorate the material and surface physical properties. On the other hand, the lower the carbon content, the more advantageous it is, but there is a limit of steelmaking technology and there is possibility of grain boundary embrittlement phenomenon. Therefore, in consideration of this point, the content of carbon in the steel sheet is determined to be 0.003 to 0.01% by weight.

Silicon (Si)

Silicon (Si) can increase the strength by the solid solution strengthening effect. The higher the content, the greater the strength. However, when the content of silicon is more than 0.03% by weight, an upper limit may be set to lower the plating characteristics in a subsequent step. Taking this into consideration, the content of silicon (Si) in the steel sheet is determined to be more than 0 to 0.03% by weight.

Manganese (Mn)

Manganese (Mn) is an austenite stabilizing element as an element that increases the strength through the effect of improving the solid solution strengthening and ingotability. In addition, MnS can be formed to prevent cracks caused by S. However, when the content of manganese exceeds 1.8% by weight, it interferes with workability and formability due to grain boundary segregation of manganese, and may adversely affect the plating characteristics. Taking this point into consideration, the content of manganese (Mn) in the steel sheet is determined to be 0.45 to 0.75% by weight.

In (P)

Phosphorus (P) can perform the function of increasing the strength of the strength by solid solution strengthening. However, if the content of phosphorus is less than 0.05% by weight, it is difficult to sufficiently exhibit the above-described effect of improving the strength. On the other hand, if it exceeds 0.10% by weight, the elongation and r value can be deteriorated. In consideration of this point, the content of phosphorus (P) in the steel sheet is determined to be 0.05 to 0.10% by weight.

Sulfur (S)

Sulfur is one of the elements that inhibits the processability. However, in the embodiment of the present invention, the content is controlled and added in order to improve descaling of the facility. However, when the content of sulfur exceeds 0.01% by weight, the workability may be adversely affected. In consideration of this point, the content of sulfur (S) in the steel sheet is maintained at 0 to 0.01% by weight or less.

Aluminum (Al)

Aluminum (Al) is a component serving as a deoxidizing agent, and the amount of dissolved oxygen in the steel can be kept sufficiently low. However, when the content of aluminum is less than 0.01% by weight, it is difficult to sufficiently exert the above-mentioned effects. Conversely, when the content of aluminum exceeds 0.06% by weight, problems may occur during performance and workability may be deteriorated. Taking this point into consideration, the content of aluminum (Al) in the steel sheet is maintained at 0 to 0.01% by weight.

Titanium (Ti)

Titanium (Ti) increases the r value and exhibits the non-aging property as the addition amount increases. By precipitating carbon and nitrogen present in the steel as solid elements in the form of precipitates such as TiN and TiC, the dissolved elements in the steel are removed to improve the r value. Also, when the elemental carbon remains in the steel, it moves with lapse of time to constrain the potential and exhibit an aging phenomenon. The amount of titanium added should be considered to be sufficient to remove carbon. However, if the content of titanium is less than 0.01% by weight, precipitation amount of TiC is too small, and carbon and nitrogen which are the elements of the solid are present in the steel, and the r value and workability may be lowered. On the other hand, when the content of titanium exceeds 0.05% by weight, surface defects may occur during clogging of nozzles and cold-rolling plating. Taking this into consideration, the content of titanium (Ti) in the steel sheet is determined to be 0.01 to 0.05% by weight.

Niobium (Nb)

Niobium (Nb) precipitates carbon and nitrogen present in the steel as solid elements in the form of precipitates of NbC and NbN to remove dissolved elements in the steel to improve the r value, remove carbon from the steel, and exhibit non-aging properties. Characteristics. However, if the niobium content is less than 0.01% by weight, the r value and the workability may be deteriorated due to presence of carbon and nitrogen which are employed as the elements in the steel. Conversely, if the content of niobium exceeds 0.05 wt%, the r value may be lowered, and the manufacturing cost may be increased due to the cost increase. In consideration of this point, the content of niobium (Nb) in the steel sheet is determined to be 0.01 to 0.05% by weight.

Molybdenum (Mo)

Molybdenum (Mo) can improve the workability of steel. Further, the plating ability and the secondary machining brittleness can be suppressed. However, when the content of molybdenum is less than 0.01% by weight, the workability and plating ability deteriorate. On the other hand, when the content of molybdenum exceeds 0.02% by weight, the function of upgrading the workability in the form of precipitates may be deteriorated. In consideration of this point, the content of molybdenum (Mo) in the steel sheet is determined to be 0.01 to 0.05% by weight.

Nitrogen (N)

Nitrogen (N) exists as a solid element in the steel to lower the elongation and reduce the workability and formability of the steel sheet. However, if the content of nitrogen is less than 0.001% by weight, it is advantageous in moldability, but the cost may increase. On the contrary, when the content of nitrogen exceeds 0.005% by weight, workability and moldability may be deteriorated. Taking this point into consideration, the content of nitrogen (N) in the steel sheet is determined to be 0.001 to 0.005% by weight.

Copper (Cu)

Copper (Cu) exists as a solid element in the steel to increase the strength of the steel, but in contrast, it also serves to lower the elongation, r value and surface quality. In addition, it can exist as an impurity which can not be removed in a steelmaking process using scrap as a raw material, that is, as a residual element. When the content of copper exceeds 0.08% by weight, it may be concentrated on the surface to generate a hot brittleness, which may cause surface cracking of the cast steel. Taking this point into consideration, the content of copper (Cu) in the steel sheet is determined to be 0 to 0.08% by weight or less.

Tin (Sn)

Tin (Sn), like copper (Cu), can exist as an impurity, that is, a residual element that can not be removed in a steelmaking process using scrap as a raw material. If the content of tin exceeds 0.01% by weight, the strength of the steel may be rapidly increased and the elongation and r value may be lowered to deteriorate the formability. Taking this point into consideration, the content of tin (Sn) in the steel sheet is determined to be 0 to 0.01 wt% or less.

Nickel (Ni), chromium (Cr)

Nickel (Ni) and chrome (Cr) are residual elements, which can cause the mechanical properties of steel to deteriorate. When the contents of nickel (Ni) and chromium (Cr) are more than 0.07 wt%, the formability may deteriorate with increasing strength. Taking this into consideration, the content of nickel (Ni) and chromium (Cr) in the steel sheet is determined to be 0 to 0.07% by weight or less.

Manufacturing method of hot-formed steel sheet

Hereinafter, a method of manufacturing a hot-formed hot-rolled steel sheet through a multi-stage heat treatment control after hot rolling will be described.

1 is a flowchart schematically showing a method of manufacturing a hot-formed hot-rolled steel sheet according to an embodiment of the present invention. Referring to FIG. 1, in step S110, a slab of a predetermined alloy composition is prepared, and the slab is hot-rolled after reheating.

The slab may contain 0.003 to 0.01% of carbon (C), 0.03% or less of silicon (Si), 1.8% or less of manganese (Mn) (Al), 0.01 to 0.06% of aluminum (Al), 0.01 to 0.05% of titanium (Ti), 0.01 to 0.05% of niobium (Nb), 0.01 to 0.02% of molybdenum (Mo) , Nitrogen (N) in an amount of 0.001 to 0.005%, and the balance of iron (Fe) and unavoidable impurities. The inevitable element includes at least one of copper (Cu), tin (Sn), nickel (Ni), and chromium (Cr) (Ni) of more than 0 to 0.07% by weight, and chromium (Cr) of more than 0 and 0.07% by weight or less.

The slab is reheated to a temperature of 1100 占 폚 or higher. When the reheating temperature is less than 1100 ° C, the precipitation-type elements can not be sufficiently employed in the slab.

Subsequently, the reheated steel slab is hot-rolled at a temperature of 750 to 800 ° C. below Ar 3. When the hot rolling temperature is lower than 750 DEG C, there is a disadvantage in that the rolling load is increased during rolling. When the hot rolling temperature exceeds 800 DEG C, the aggregate structure other than the (111) .

In step S120, the hot-rolled steel sheet is cooled and maintained at a first temperature.

In this step, first, the hot-rolled steel is cooled to a temperature of 650 to 700 ° C. Then, it is maintained at a temperature of 650 to 700 ° C for 5 to 10 seconds. In this step, formation and growth of precipitates are achieved. If the temperature is lower than 650 ° C, the strength of the steel sheet may increase. If it exceeds 700 ° C, the size of the precipitate may form a coarse texture. And then cooled to a temperature of 200 to 300 캜. The cooling rate is maintained at 10 to 30 DEG C / s. When the cooling rate exceeds 30 DEG C / s, the strength is increased due to the effect of microstructure, and the workability can be lowered. When the cooling rate is 10 DEG C / s or lower,

In step S130, the steel is annealed. Specifically, in this step, the steel is heated and annealed at a temperature of 750 to 850 캜 for 20 to 30 seconds. Annealing heat treatment may have the effect of removing the internal stress after rolling and developing the (111) orientation to increase the r value. When the temperature is less than 750 DEG C, the (111) orientation is difficult to develop, and if it exceeds 850 DEG C, the cost burden is increased.

In step S140, the steel is cooled to a second temperature and maintained. In this step, the annealed annealed steel is first cooled to a temperature of 400 to 500 ° C. and maintained at a temperature of 400 to 500 ° C. for 20 to 30 seconds. If the temperature is lower than 400 ° C. or higher than 500 ° C., the effect of overcoming the effect of overcoming can not be observed, and the strength and elongation can be lowered due to the later aging phenomenon. The cooling rate to the temperature of 400 to 500 占 폚 can be maintained at 10 to 30 占 폚 / s.

In step S150, the steel is cooled to a final cooling temperature and wound. The final cooling temperature may be 300 ° C or less. The cooling rate can be maintained at 10 to 30 DEG C / s.

Example

Best Mode for Carrying Out the Invention Hereinafter, a preferred embodiment of the present invention will be described in more detail. It should be understood, however, that this is a preferred embodiment of the present invention and that the spirit of the present invention is not limited to the following embodiments.

1. Preparation of specimens

As shown in Table 1, specimens satisfying the composition ranges of Comparative Examples and Examples of the present invention were prepared according to the process conditions of Table 2. In the case of the comparative example, a metal alloy such as molybdenum, copper, tin, nickel, and chromium is not contained. In contrast, in the case of the embodiment, the metal is contained within the content range of the present invention.

(Unit: wt%) C Ti Nb Mo N Cu Sn Ni Cr Comparative Example 0.002 0.03 0.01 - 0.002 - - - - Example 1 0.002 0.02 0.01 0.015 0.002 0.06 0.0008 0.06 0.06 Example 2 0.002 0.02 0.01 0.015 0.002 0.06 0.0008 0.06 0.06 Example 3 0.002 0.02 0.01 0.015 0.002 0.06 0.0008 0.06 0.06 Example 4 0.002 0.02 0.01 0.015 0.002 0.06 0.0008 0.06 0.06 Example 5 0.002 0.02 0.01 0.015 0.002 0.06 0.0008 0.06 0.06 Example 6 0.002 0.02 0.01 0.015 0.002 0.06 0.0008 0.06 0.06

Hot rolling temperature
(° C) Holding temperature (℃) Cooling rate (° C / s) Holding temperature
(° C) Annealing temperature (캜) Cooling rate (° C / s) Holding time (s) Cooling rate (° C / s) Comparative Example 900 - - - - - - - Example 1 900 700 20 400 850 20 400 20 Example 2 870 700 20 400 850 20 400 20 Example 3 870 600 20 400 850 20 400 20 Example 4 870 700 40 400 850 20 400 20 Example 5 870 700 20 400 700 20 400 20 Example 6 870 700 20 400 850 40 400 20

2. Property evaluation

The tensile strength (TS), the elongation (EL) and the R value of the specimens of the above Comparative Examples and Examples 1 to 6 were evaluated by a tensile test, and the results are summarized in Table 3 below.

TS (MPa) EL (%) R value Comparative Example 300 43 1.01 Example 1 304 42 1.32 Example 2 288 47 1.81 Example 3 289 45 1.70 Example 4 292 46 1.66 Example 5 304 43 1.48 Example 6 282 47 1.72

Referring to Table 3, in comparison with the conventional comparative examples, Examples 1 to 6 show similar tensile strength and elongation results. However, in the case of the R value, the results of Examples 1 to 6 are superior to those of Comparative Example.

As described above, according to the embodiment of the present invention, a hot-rolled steel sheet excellent in formability such as deep drawability can be manufactured by controlling the content of an alloy element such as Ti, Nb, Mo, etc. in a low- . Particularly, the hot-rolled steel sheet has an advantage that excellent workability can be ensured even if the steelmaking process is performed in an environment where tramp elements such as copper, tin, nickel, and chromium are present in an electric furnace using scrap iron as a raw material have.

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.

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Claims

The method according to claim 1,
The inevitable element includes at least one of copper (Cu), tin (Sn), nickel (Ni), and chromium (Cr)
(Cu) is more than 0 to 0.08% by weight, tin (Sn) is more than 0 and 0.01% by weight or less, nickel (Ni) is more than 0 and 0.07%
Hot-rolled steel sheet for high-grade formation.

(Si) of more than 0 to 0.03%, manganese (Mn) of more than 0 and not more than 1.8%, phosphorus (P) of 0.05 to 0.10% or less, sulfur (S) (N) 0.01 to 0.02%, nitrogen (N) more than 0 to 0.005%, aluminum (Al) 0.01 to 0.06%, titanium (Ti) 0.01 to 0.05%, niobium 0.01 to 0.05%, molybdenum And reheating the steel slab consisting of the remaining iron (Fe) and unavoidable impurities to a temperature of 1100 캜 or higher;
(b) hot-rolling the reheated steel slab at a temperature of 750-800 DEG C below Ar3;
(c) cooling the hot-rolled steel at a temperature of 650 to 700 ° C for 5 to 10 seconds and then cooling to a temperature of 200 to 300 ° C;
(d) annealing the steel at a temperature of 750 to 850 캜 for 20 to 30 seconds by heating the steel;
(f) cooling the annealed annealed steel at a temperature of 400 to 500 ° C for 20 to 30 seconds, cooling and winding the annealed steel
A method for manufacturing a hot - rolled steel sheet for forming a solid.

The method of claim 3,
The inevitable element includes at least one of copper (Cu), tin (Sn), nickel (Ni), and chromium (Cr)
(Cu) is more than 0 to 0.08% by weight, tin (Sn) is more than 0 and 0.01% by weight or less, nickel (Ni) is more than 0 and 0.07%
A method for manufacturing a hot - rolled steel sheet for forming a solid.

The method of claim 3,
In step (c), the step of cooling the hot-rolled steel to a temperature of 200 to 300 ° C is performed at a cooling rate of 10 to 30 ° C / s
A method for manufacturing a hot - rolled steel sheet for forming a solid.