KR20140118312A - Cold-rolled steel sheet and method of manufacturing the same - Google Patents

Abstract

Disclosed are a cold rolled steel sheet capable of ensuring excellent formability by reducing the content of phosphorus (P) and manganese (Mn), and performing heat treatment in a batch annealing furnace by refining a grain and adding niobium (Nb) which is a precipitation hardening element; and a method of manufacturing the same. According to the present invention, the method of manufacturing the cold rolled steel sheet comprises: (a) a step of finish hot rolling a slab comprising 0.05-0.08 wt% of carbon (C), 0.025 wt% or less of silicon (Si), 0.7-0.9 wt% of manganese (Mn), 0.02 wt% or less of phosphorus (P), 0.008 wt% or less of sulfur (S), 0.01-0.05 wt% of aluminum (Al), 0.02-0.03 wt% of niobium (Nb), and the remainder consisting of Fe and inevitable impurities at 800-900°C which is a finishing delivery temperature (FDT); (b) a step of coiling the slab by cooling the hot rolled slab to 600-700°C which is a coiling temperature (CT); (c) a step of cold rolling the slab after pickling the slab by uncoiling the coiled slab; and (d) a step of heat treating the cold rolled slab for 10-15 hours at 680-700°C in a batch annealing furnace.

Description

Technical Field [0001] The present invention relates to a cold-rolled steel sheet and a method of manufacturing the same,

The present invention relates to a cold-rolled steel sheet and a method of manufacturing the same, and more particularly, to a high-strength cold-rolled steel sheet capable of securing excellent workability through control of alloy components and process conditions and a method of manufacturing the same.

Steel plates used in automobiles, household appliances, etc. are increasingly demanding for quality and diversification of quality.

In particular, cold rolled steel sheets, which have excellent surface quality and workability, are mainly applied to steel sheets applied to outer plates of automobiles and household appliances.

Generally, the cold-rolled steel sheet is manufactured by a hot-rolling process, a cooling / coiling process, an acid pickling process, a cold-rolling process and an annealing process .

A related prior art is Korean Patent Laid-Open Publication No. 10-2011-0026751 (Mar. 16, 2011), which discloses an ultra-thin cold-rolled steel sheet having high strength and high-strength formation and a manufacturing method thereof.

An object of the present invention is to provide a cold-rolled steel sheet which can reduce the content of phosphorus (P) and manganese (Mn) and which is capable of ensuring excellent workability by carrying out box annealing heat treatment by addition of niobium (Nb) And a method for producing the same.

Another object of the present invention is to provide a cold rolled steel sheet produced by the above method and having a tensile strength (TS) of 440 to 480 MPa, a yield strength (YP) of 260 to 380 MPa and an elongation (EL) of 30 to 40%.

(A) 0.05 to 0.08% by weight of carbon (C), 0.025% by weight or less of silicon (Si), and 0.7 to 0.9% of manganese (Mn) in accordance with an embodiment of the present invention. (Fe), 0.02 wt% or less of phosphorus (P), 0.008 wt% or less of sulfur (S), 0.01 to 0.05 wt% of aluminum (Al), 0.02 to 0.03 wt% of niobium (Nb) And finishing hot rolling at a finishing delivery temperature (FDT) of 800 to 900 ° C; (b) cooling the hot-rolled plate by cooling to a CT (Coiling Temperature) of 600 to 700 占 폚 and winding; (c) uncoiling and pickling the rolled sheet material, followed by cold rolling; And (d) subjecting the cold-rolled plate to box annealing at 680 to 700 ° C for 10 to 15 hours.

According to another aspect of the present invention, there is provided a cold rolled steel sheet comprising 0.05 to 0.08% by weight of carbon (C), 0.025% by weight or less of silicon (Si), 0.7 to 0.9% by weight of manganese (Mn) (Fe) in an amount of 0.02 wt% or less, sulfur (S) in an amount of 0.008 wt% or less, aluminum (Al) in an amount of 0.01 to 0.05 wt%, niobium (Nb) And has tensile strength (TS) of 440 to 480 MPa, yield strength (YP) of 260 to 380 MPa and elongation (EL) of 30 to 40%.

The present invention reduces the content of phosphorus (P) and manganese (Mn) and performs box annealing by addition of niobium (Nb) which is a crystal grain refinement and precipitation hardening type element, thereby minimizing occurrence of a temper color, A cold rolled steel sheet exhibiting workability can be produced.

The cold rolled steel sheet produced by the method according to the present invention can exhibit a tensile strength (TS) of 440 to 480 MPa, a yield strength (YP) of 260 to 380 MPa and an elongation (EL) of 30 to 40% It is possible to cut the width of 50 mm or less from the edges of both sides and use it.

As a result, the cold-rolled steel sheet produced by the method according to the present invention is suitable for use as a structural member for automobiles or a reinforcing material, which requires a large amount of forming work, because deep drawing and high strength can be secured.

FIG. 1 is a process flow chart showing a cold-rolled steel sheet manufacturing method according to an embodiment of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS The advantages and features of the present invention, and the manner of achieving them, will be apparent from and elucidated with reference to the embodiments described hereinafter in conjunction with the accompanying drawings. It should be understood, however, that the invention is not limited to the disclosed embodiments, but is capable of many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, To fully disclose the scope of the invention to those skilled in the art, and the invention is only defined by the scope of the claims. Like reference numerals refer to like elements throughout the specification.

Hereinafter, a cold-rolled steel sheet according to a preferred embodiment of the present invention and a method of manufacturing the same will be described in detail with reference to the accompanying drawings.

Cold rolled steel plate

The cold-rolled steel sheet according to the present invention has a tensile strength (TS) of 440 to 480 MPa, a yield strength (YP) of 260 to 380 MPa and an elongation (EL) of 30 to 40% .

The cold-rolled steel sheet according to the present invention comprises 0.05 to 0.08 wt% of carbon (C), 0.025 wt% or less of silicon (Si), 0.7 to 0.9 wt% of manganese (Mn) 0.008 wt% or less of sulfur (S), 0.01 to 0.05 wt% of aluminum (Al), 0.02 to 0.03 wt% of niobium (Nb), and the balance of iron (Fe) and unavoidable impurities.

Hereinafter, the role and content of each component contained in the cold-rolled steel sheet according to the present invention will be described.

Carbon (C)

Carbon (C) is an intrinsic solid solution strengthening element in steel. It is hardened not only in solid solution strengthening but also in austenite, and contributes to formation of martensite and increase of strength in cold rolling heat treatment.

The carbon (C) is preferably added in an amount of 0.05 to 0.08% by weight based on the total weight of the cold-rolled steel sheet according to the present invention. When the content of carbon (C) is less than 0.05% by weight, the amount of NbC precipitates is reduced and the size of crystal grains becomes large, which causes surface defects such as orange peel during molding. On the contrary, when the content of carbon (C) is more than 0.08% by weight, since the employment of carbon dioxide greatly deteriorates the antioxidant property, the increase in the manufacturing cost due to the addition of a large amount of expensive niobium (Nb) there is a problem.

Silicon (Si)

Silicon (Si) plays a role in suppressing the formation of carbide and enhancing the hardening hardenability by increasing the solid carbon.

However, when the content of silicon (Si) is more than 0.025 wt% in the present invention, elongation at the yield point occurs and the strength is increased but ductility is lowered.

Therefore, silicon (Si) is preferably added at a content ratio of 0.025% by weight or less based on the total weight of the cold-rolled steel sheet according to the present invention.

Manganese (Mn)

Manganese (Mn) is a solid solution strengthening element that prevents MnS precipitates from reacting with sulfur (S) dissolved in a steel sheet to prevent hot shortness due to solid solution sulfur. It stabilizes austenite to obtain a two- And it is easy to generate martensite even at a low critical cooling rate.

The manganese (Mn) is preferably added in an amount of 0.7 to 0.9% by weight based on the total weight of the cold-rolled steel sheet according to the present invention. When the content of manganese (Mn) is less than 0.7% by weight, it may be difficult to secure a sufficient strength. On the other hand, when the content of manganese (Mn) exceeds 0.9% by weight, there is a problem that it is difficult to secure moldability due to a decrease in elongation due to an excessively high strength of the steel sheet.

In (P)

Phosphorus (P) is an element effective in strengthening the steel sheet by solid solution strengthening and suppressing the formation of carbide.

However, if the content of phosphorus (P) exceeds 0.02% by weight in the cold-rolled steel sheet according to the present invention, a large amount of phosphorus (P) may cause a problem of brittleness. Therefore, in the present invention, the content of phosphorus (P) is limited to 0.02% by weight or less based on the total weight of the cold-rolled steel sheet.

Sulfur (S)

Sulfur (S) reacts with manganese (Mn) to form precipitates of fine MnS to improve processability.

However, when the content of sulfur (S) exceeds 0.008% by weight in the cold-rolled steel sheet according to the present invention, the amount of sulfur (S) dissolved therein is too large to significantly reduce ductility and formability, There is a concern. Therefore, in the present invention, the content of sulfur (S) is limited to 0.008% by weight or less based on the total weight of the cold-rolled steel sheet.

Aluminum (Al)

Aluminum (Al) is an element mainly used as a deoxidizer, which improves the elongation by purifying the ferrite grains, and stabilizes the austenite by increasing the carbon concentration in the austenite. In addition, aluminum acts as a layer between the iron and the zinc plated layer to improve the plating ability, and is an effective element for suppressing the formation of manganese bands in the hot-rolled coil.

The aluminum (Al) is preferably added in an amount of 0.01 to 0.05% by weight based on the total weight of the cold-rolled steel sheet according to the present invention. When the content of aluminum (Al) is less than 0.01% by weight, the effect of adding aluminum can be exhibited properly. On the contrary, when the content of aluminum (Al) is over 0.05% by weight, the performance is deteriorated and AlN is formed in the slab to cause hot cracking.

Niobium (Nb)

Niobium (Nb) is a strong carbonitride-forming element. It reacts with carbon (C) and nitrogen (N) present in steel during hot rolling to form fine NbC and NbN precipitates and suppress grain growth. Further, niobium (Nb) has an effect of improving the strength and suppressing the secondary machining brittleness through grain refining effect.

The niobium (Nb) is preferably added in an amount of 0.02 to 0.03% by weight of the total weight of the cold-rolled steel sheet according to the present invention. When the content of niobium (Nb) is less than 0.02% by weight of the total weight of the steel sheet, a certain amount of the solid carbon is excessive and the yield point drawing and aging phenomenon is accelerated. On the contrary, when the content of niobium (Nb) exceeds 0.03% by weight of the total weight of the steel sheet, the amount of solid carbon is reduced and it is difficult to secure the hardening of the hardening, and the phenomenon of increase in yield strength due to grain refinement is accelerated, There is a problem that the toughness is deteriorated.

Cold rolled steel sheet manufacturing method

FIG. 1 is a process flow chart showing a cold-rolled steel sheet manufacturing method according to an embodiment of the present invention.

Referring to FIG. 1, a cold rolled steel sheet manufacturing method according to an embodiment of the present invention includes a slab reheating step S110, a hot rolling step S120, a cooling / winding step S130, a cold rolling step S140, Annealing heat treatment step S150 and cooling step S160. At this time, the slab reheating step (S110) is not necessarily performed, but it is more preferable to carry out the step to derive effects such as reuse of precipitates.

In the cold rolled steel sheet manufacturing method according to the present invention, the semi-finished slab plate to be subjected to the hot rolling process contains 0.05 to 0.08 wt% of carbon (C), 0.025 wt% or less of silicon (Si) (Fe), 0.02 wt% or less of phosphorus (P), 0.008 wt% or less of sulfur (S), 0.01 to 0.05 wt% of aluminum (Al), 0.02 to 0.03 wt% of niobium (Nb) And inevitable impurities.

Reheating slabs

In the slab reheating step S110, the slab plate having the above composition is reheated to a slab reheating temperature (SRT) of 1150 to 1250 ° C. Here, the slab plate can be obtained through a continuous casting process after obtaining a molten steel having a desired composition through a steelmaking process. At this time, in the slab reheating step (S110), the slab plate obtained through the continuous casting process is reheated to reuse the segregated components during casting.

If the slab reheating temperature (SRT) is less than 1150 DEG C in this step, the segregated components in casting may not be sufficiently reused. On the other hand, if the SRT reheating temperature (SRT) exceeds 1250 ° C, the austenite crystal grain size may increase and the strength of the steel sheet may be difficult to secure, and the manufacturing cost of the steel sheet may be increased due to the excessive heating process.

Hot rolling

In the hot rolling step (S120), the slab plate is subjected to finishing hot rolling under the condition of FDT (Finishing Delivery Temperature): 800 to 900 ° C.

At this stage, if the finish hot rolling temperature (FDT) is too low, which is lower than 800 占 폚, there is a problem that an uncrosslinked structure due to an abnormal reverse rolling occurs. On the other hand, when the finish hot rolling temperature (FDT) is higher than 900 캜, the austenite grains become coarse, which may make it difficult to secure strength.

Cooling / Winding

In the cooling / winding step (S130), the hot rolled plate is cooled to a CT (Coiling Temperature): 600 to 700 ° C and is wound.

At this stage, when the coiling temperature is less than 600 ° C, there is a problem that the surface quality of the steel sheet is deteriorated due to the rapid difference between the finish rolling temperature and the coiling temperature. On the other hand, when the coiling temperature exceeds 700 캜, the precipitates grow too coarse and the grain refinement effect deteriorates, so that it may be difficult to ensure sufficient strength.

On the other hand, the cooling rate in the cooling / winding step (S130) is preferably 10 to 25 DEG C / sec. In this step, when the cooling rate is less than 10 ° C / sec, the average particle size of the precipitate is more than about 0.2 탆, which makes it difficult to secure the strength. On the other hand, when the cooling rate exceeds 25 DEG C / sec, there is a problem that the structure is weakened and impact toughness is lowered.

Cold rolling

In the cold rolling step (S140), the rolled sheet is uncoiled, pickled, and then cold rolled. At this time, the pickling is carried out for the purpose of removing the scale of the hot-rolled coil manufactured through the hot rolling process.

In this step, the cold rolling reduction rate is preferably 60 to 80%. When the cold rolling reduction rate is less than 60%, there is a problem that the amount of annealed recrystallized nuclei is small, so that crystal grains are excessively grown during box annealing, which will be described later, and the strength is rapidly lowered. On the other hand, when the cold rolling reduction is more than 80%, the amount of nucleation becomes too large, so that the annealing grains are rather too fine to reduce the ductility and deteriorate the formability.

Box annealing heat treatment

In the box annealing heat treatment step S150, the cold-rolled plate material is charged into a batch annealing furnace (BAF), and then subjected to box annealing at 680 to 700 ° C for 10 to 15 hours.

At this time, the heat treatment of the box annealing is one of important process parameters for determining the material of the final product. Particularly, in the present invention, the content of phosphorus (P) and manganese (Mn) is adjusted downward by adjusting the content of niobium to 0.02 to 0.03% by weight, By controlling the temperature down to 680 to 700 ° C, it is possible to secure the same material as that of a steel containing a high manganese content, and additionally adjusting the manganese (Mn) to 0.7-0.9 wt% do.

If the box annealing heat treatment temperature is less than 680 占 폚 or the box annealing heat treatment retention time is less than 10 hours in this stage, recrystallization is not sufficiently completed and it may be difficult to secure the desired ductility. On the other hand, when the box annealing heat treatment temperature exceeds 700 占 폚 or the box annealing heat treatment retention time exceeds 15 hours, there is a problem that strength is lowered due to coarsening of recrystallized grains.

Particularly, in this step, the temperature raising rate of the box annealing heat treatment is preferably 20 to 40 占 폚 / min. When the temperature raising rate of the box annealing heat treatment is less than 20 캜 / min, the precipitates such as NbC and AlN are regrown and toughness is secured, but it may be difficult to secure sufficient strength. On the other hand, when the temperature raising rate of the box annealing heat treatment exceeds 40 캜 / min, precipitates such as NbC and AlN do not occur, but it may be difficult to secure the aimed elongation by atomization of the recrystallized grains.

Cooling

In the cooling step (S160), the recrystallized sheet material is cooled to 550 to 650 deg. C by the box annealing heat treatment process. At this time, it is preferable that the cooling is performed at a temperature of 550 to 650 ° C. and the indirect cooling is performed at a temperature of 550 ° C. to room temperature by using indirect gas jet cooling (GJC).

In this step, the cooling rate is preferably about 10 to 30 DEG C / min. When the cooling rate is less than 10 ° C / min, re-precipitation occurs between the solid carbon and the niobium, so that it may be difficult to ensure sufficient strength. On the other hand, when the cooling rate exceeds 30 캜 / min, it is advantageous in securing the strength but there is a problem that the elongation rate is rapidly lowered.

The cold-rolled steel sheet produced in the above-described processes (S110 to S160) reduces the content of phosphorus (P) and manganese (Mn) and performs annealing of the box annealing by adding grain refinement and precipitation hardening element niobium (Nb) A cold-rolled steel sheet having excellent workability while minimizing occurrence of a temper color can be produced.

Accordingly, the cold-rolled steel sheet produced by the method according to the present invention can exhibit a tensile strength (TS) of 440 to 480 MPa, a yield strength (YP) of 260 to 380 MPa and an elongation (EL) of 30 to 40% It is possible to cut the width of 50 mm or less from the edges of both sides by minimizing the occurrence of the color.

As a result, the cold-rolled steel sheet produced by the method according to the present invention is suitable for use as a structural member for automobiles or a reinforcing material, which requires a large amount of forming work, because deep drawing and high strength can be secured.

Example

Hereinafter, the configuration and operation of the present invention will be described in more detail with reference to preferred embodiments of the present invention. It is to be understood, however, that the same is by way of illustration and example only and is not to be construed in a limiting sense.

The contents not described here are sufficiently technically inferior to those skilled in the art, and a description thereof will be omitted.

1. Preparation of specimens

Specimens according to Examples 1 to 4 and Comparative Examples 1 and 2 were prepared with the composition shown in Table 1 and the process conditions shown in Table 2.

[Table 1] (unit:% by weight)

[Table 2]

2. Evaluation of mechanical properties

Table 3 shows the evaluation results of the mechanical properties of the specimens according to Examples 1 to 4 and Comparative Examples 1 and 2.

[Table 3]

Referring to Tables 1 to 3, the tensile strength (TS) corresponding to the target values of 440 to 480 MPa, the yield strength (YP) of the specimens according to Comparative Examples 1 to 2 and the specimens prepared according to Examples 1 to 4 ): 260 to 380 MPa and elongation (EL): 30 to 40%.

Compared with Comparative Example 1, however, the test pieces according to Comparative Examples 1 and 2 were found not only to have no strength compensation effect despite the addition of manganese in a large amount, but also to cause a temper color at both sides.

As can be seen from the above experimental results, in the case of the specimens prepared according to Examples 1 to 4, the contents of phosphorus (P) and manganese (Mn) were reduced, and grain refinement and precipitation hardening elements such as niobium (Nb) It was confirmed that the generation of the temper color at both side edge portions of the coil can be minimized and excellent workability can be ensured by performing the box annealing heat treatment under the optimum conditions by adding.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments. Such changes and modifications are intended to fall within the scope of the present invention unless they depart from the scope of the present invention. Accordingly, the scope of the present invention should be determined by the following claims.

S110: Slab reheating step
S120: Hot rolling step
S130: cooling / winding step
S140: Cold rolling step
S150: Box annealing heat treatment step
S160: cooling step

Claims (6)

(Si): 0.025 wt% or less, manganese (Mn): 0.7 to 0.9 wt%, phosphorus (P): 0.02 wt% or less, sulfur (S) 0.008 wt% or less, aluminum (Al): 0.01 to 0.05 wt%, niobium (Nb): 0.02 to 0.03 wt%, and the balance of iron (Fe) and unavoidable impurities is subjected to Finishing Delivery Temperature (FDT) Lt; RTI ID = 0.0 > C < / RTI >
(b) cooling the hot-rolled plate by cooling to a CT (Coiling Temperature) of 600 to 700 占 폚 and winding;
(c) uncoiling and pickling the rolled sheet material, followed by cold rolling; And
(d) subjecting the cold-rolled sheet to box annealing at 680 to 700 ° C for 10 to 15 hours.
The method according to claim 1,
In the step (c)
The cooling
At a rate of 10 to 25 占 폚 / sec.
The method according to claim 1,
In the step (d)
The temperature raising rate of the box annealing heat treatment is
20 to 40 占 폚 / min.
The method according to claim 1,
After the step (d)
(e) cooling the box-annealed heat-treated plate to 550 to 650 ° C at a rate of 10 to 30 ° C / min.
(Si): 0.025 wt% or less, Mn: 0.7 to 0.9 wt%, P: 0.02 wt% or less, S: 0.008 wt% or less, (Al): 0.01 to 0.05 wt%, niobium (Nb): 0.02 to 0.03 wt%, and the balance of iron (Fe) and unavoidable impurities,
, A tensile strength (TS) of 440 to 480 MPa, a yield strength (YP) of 260 to 380 MPa and an elongation (EL) of 30 to 40%.
6. The method of claim 5,
The steel sheet
And a width of 50 mm or less is cut from both side edges of the cold-rolled steel sheet.

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