KR101424863B1 - Cold-rolled steel sheet and method of manufacturing the cold-rolled steel sheet - Google Patents

Abstract

A cold-rolled steel sheet with boron added to improve the formability of a hard-bake hardened steel and having dent resistance to the entire outer sheath, and a method for producing the cold-rolled steel sheet.
The method for manufacturing a cold-rolled steel sheet according to the present invention comprises the steps of: (a) providing a steel sheet containing less than 0.005% by weight of carbon, 0.03% by weight or less of silicon, 1.0% by weight or less of manganese, (Fe) and inevitable impurities, which contains aluminum (Al) in an amount of 0.01 to 0.06 wt%, nitrogen (N) in an amount of 0.003 wt% or less and boron (B) in an amount of 0.0005 to 0.002 wt% Reheating the substrate; (b) hot-rolling the reheated plate to a finishing delivery temperature (FDT) of 860 to 900 占 폚; (c) cooling the hot-rolled plate by cooling to a CT (Coiling Temperature) of 640 to 680 캜; (d) pickling the rolled sheet material; (e) a step of cold-rolling and annealing after the pickling treatment; (f) cold rolling and temper rolling the cold-rolled and annealed sheet material; And (g) cooling the cold rolled and quenched rolled plate.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a cold-rolled steel sheet,

The present invention relates to a cold-rolled steel sheet and a method of manufacturing the same, and more particularly, to a cold-rolled steel sheet having improved durability of the entire outer sheath and a method of manufacturing the same.

 Automotive exterior panel parts include hoods, fenders, doors, loops, trunk lids. Side outer and the like. Outer panel parts are a part of the aesthetics of the automobile and must be free from defects and require dent resistance so that they are not easily deformed by external impact.

In order to satisfy such characteristics, the hardened steel used in the preforming and painting is widely used in automobile exterior panels due to its good moldability and dent resistance because it utilizes the phenomenon that the welding element is fixed to the electric potential and increases in yield strength come. However, EDDQ grade Interstitial Free Steel has been limitedly applied because it requires wide width characteristics and deep drawing of the side outer jacket.

Related Prior Art Korean Patent Publication No. 10-2001-0040682 (published May 15, 2001) discloses a high strength cold rolled steel sheet and a manufacturing method thereof.

An object of the present invention is to provide a method for producing a cold-rolled steel sheet having improved durability of the entire outside sheathing by adding boron to improve the moldability of the hardened steel.

Another object of the present invention is to provide a cold-rolled steel sheet produced by the above-mentioned method and having a sintering property (EL) of not less than 42%, a rank pod (r-bar) of not less than 2.2 and a hardening property of not less than 3 Kgf / mm ² .

In order to accomplish the above object, a cold rolled steel sheet manufacturing method according to an embodiment of the present invention comprises the steps of: (a) a carbon (C) content of more than 0 wt% to less than 0.005 wt%, a silicon (Si) content of more than 0 wt% (Al): 0.01 to 0.06 wt%, N (N): 0 wt%, Mn: not less than 0 wt% to not more than 1.0 wt%, P: not less than 0 wt% (B): 0.0005 to 0.002 wt.%, And remainder of the slab plate consisting of the remaining iron (Fe) and unavoidable impurities; (b) hot-rolling the reheated plate to a finishing delivery temperature (FDT) of 860 to 900 占 폚; (c) cooling the hot-rolled plate by cooling to a CT (Coiling Temperature) of 640 to 680 캜; (d) pickling the rolled sheet material; (e) a step of cold-rolling and annealing after the pickling treatment; (f) cold rolling and temper rolling the cold-rolled and annealed sheet material; And (g) cooling the cold rolled and quenched rolled plate.

According to another aspect of the present invention, there is provided a cold-rolled steel sheet comprising a steel sheet having a carbon content of more than 0 wt% to less than 0.005 wt%, a silicon content of more than 0 wt% to 0.03 wt% (P): more than 0 wt% to 0.10 wt%, aluminum (Al): 0.01 to 0.06 wt%, nitrogen (N): more than 0 wt% to 0.003 wt% (EL) of 42% or more and a rank-pod (r-bar) of 2.2 or more, which comprises boron (B) in an amount of 0.0005 to 0.002% by weight and boron (B) in an amount of 0.0005 to 0.002% .

The cold-rolled steel sheet manufacturing method according to the present invention can improve the formability of the hardened steel by adding boron.

Further, the cold-rolled steel sheet produced by the above method can satisfy the hardening of hardening of not less than 42% elongation (EL), rank-pod (r-bar) not less than 2.2 and 3Kgf / mm ² .

FIG. 1 is a process flow chart showing a cold-rolled steel sheet manufacturing method according to an embodiment of the present invention.
FIG. 2 is a graph showing a temperature change according to boron addition to a specimen prepared according to Example 1 and Comparative Example 1 of the present invention. FIG.
3 is a cross-sectional view after boron addition of a specimen prepared according to Example 1 of the present invention.
4 is a graph showing changes in hot rolling of specimens produced according to Examples 1 to 5 of the present invention.
5 is a graph showing changes in elongation for specimens prepared according to Examples 5 to 7 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 aims at satisfying an elongation (EL) of not less than 42%, a rank pod (r-bar) of not less than 2.2 and a hardening property of not less than 3 Kgf / mm ² .

For this, the cold-rolled steel sheet according to the present invention preferably has a carbon content of more than 0 wt% to less than 0.005 wt%, a silicon content of more than 0 wt% to 0.03 wt%, a content of manganese (Mn) of more than 0 wt% Boron (B), aluminum (Al): 0.01 to 0.06 wt%, nitrogen (N): 0 to 0.003 wt% : 0.0005 to 0.002% by weight, and may be composed of the remaining 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 element that causes a hardening of the resin, and it is necessary to control its content in order to control the natural aging. At this time, moldability is lowered or natural aging is accelerated according to the content of carbon (C), so strong carbonitride forming elements such as titanium (Ti) are added to control the solid element. In this case, when the amount of the carbon-containing solid is increased, the moldability is decreased and the yield strength is increased.

In the present invention, when the content of carbon (C) is 0.005% by weight or more of the total weight of the steel sheet, the yield strength increases, the shape crystallinity decreases, and crystal grain growth during recrystallization is inhibited. Therefore, carbon (C) is preferably added at a content ratio of more than 0 wt% to less than 0.005 wt% of the total weight of the steel sheet according to the present invention.

silicon( Si )

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

The silicon (Si) is preferably added in an amount of more than 0 wt% to 0.03 wt% of the total weight of the steel sheet according to the present invention. When the content of silicon (Si) exceeds 0.03% by weight, elongation at the yield point occurs, and the strength is increased but the ductility is deteriorated.

manganese( Mn ), Phosphorus (P)

Manganese (Mn) is an element added to obtain the solid solution strengthening effect. It binds with sulfur in the steel to precipitate MnS to prevent hot brittleness due to the formation of FeS, and to refine the particles without damaging the ductility. Phosphorus (P) is a substitutional alloy element having a large solubility enhancement effect. It plays a role of improving the in-plane anisotropy and enhancing the strength. The phosphorus (P) is added to the carbon (C) So that the hardening property of the filler is increased. In addition, since the grain size is reduced by the addition of phosphorus (P), improvement in hardening of the sintering property and anti-aging property accompanying decrease in grain size can be expected.

The manganese (Mn) is preferably added in an amount of more than 0 wt% to 1.0 wt% of the total weight of the steel sheet according to the present invention. The phosphorus (P) is preferably added in an amount of more than 0 wt% to 0.1 wt% or less of the total weight of the steel sheet according to the present invention.

When the content of manganese (Mn) exceeds 1.0% by weight or the content of phosphorus (P) exceeds 0.1% by weight, surface defects occur during plating and molding, and the elongation is reduced.

Here, manganese (Mn) and phosphorus (P) are formed so as to satisfy 0.9 Mn (weight%) + 10 P (weight%) 1.1.

aluminum( Al )

Aluminum (Al) is used as a deoxidizing agent in steel, and nitrogen (N) and nitride AlN are precipitated during hot rolling to suppress crystal grains.

The aluminum (Al) is preferably added in an amount of 0.01 to 0.06% by weight based on the total weight of the steel sheet according to the present invention. When the content of aluminum (Al) is less than 0.01% by weight, it is difficult to exhibit the deoxidation effect properly, and when the nitrogen dissolved in the steel remains, the yield strength and sintering hardenability are rapidly increased, have. On the contrary, when the content of aluminum (Al) exceeds 0.06% by weight, inclusions are excessively formed during steelmaking and casting operations, defects may occur on the surface of the plating, and the elongation rate may be lowered.

Nitrogen (N)

Nitrogen (N) is an alloying element that causes sintering and aging phenomenon together with carbon (C). It has a better ability to improve sintering hardenability than carbon (C), but deterioration of elongation and formability and aging phenomenon are also rapidly increased, And the alloying element is difficult to apply for the hardening of the sintering.

However, when the content of nitrogen (N) exceeds 0.003 wt% of the total weight of the steel sheet according to the present invention, there is a problem that precipitation increases and grain growth during recrystallization is inhibited. Therefore, in the present invention, the content of nitrogen (N) is limited to more than 0 wt% and 0.003 wt% or less of the total weight of the steel sheet.

Boron (B)

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Boron (B) can minimize the deterioration of materials during processing by the solid-state elements in the steel. Boron (B) can minimize the decrease in elongation due to SPM by removing solute nitrogen. That is, the upper yield point and the yield point elongation phenomenon generated by the solid solution nitrogen are much higher than that of the solid carbon, so that a high SPM reduction is required. Therefore, as the SPM reduction is increased, the elongation of the plate decreases in an inverse proportion, and a proper amount of temper rolling must be applied.

In addition, the solid solution nitrogen deteriorates the elongation rate by suppressing recrystallization after the final cold rolling annealing, thereby deteriorating the final formability.

Further, boron (B) lowers the austenite-ferrite transformation starting temperature to reduce the grain size at the time of transformation, and minimizes the size of the hot-rolled grains through low temperature hot rolling.

Further, boron (B) occupies a position in the grain boundary prior to annealing during hot rolling, thereby suppressing grain boundary segregation of phosphorus and improving secondary workability.

The boron (B) is preferably added in an amount of 0.0005 to 0.002 wt% of the total weight of the steel sheet according to the present invention. If the content of boron (B) is less than 0.0005 wt%, it may be impossible to adjust the finish temperature of the hot rolling end temperature.

On the contrary, when the content of boron (B) exceeds 0.002% by weight, the recrystallization is delayed during the cold rolling annealing and the grain size of the cold rolled steel is decreased.

Cold rolled steel sheet manufacturing method

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

Referring to FIG. 1, the method for manufacturing a cold-rolled steel sheet according to the present invention includes a slab reheating step S110, a hot rolling step S120, a cooling / winding step S130, a pickling step S140, A rolling step (S160) and a cooling step (S170).

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 is composed of more than 0 wt% to less than 0.005 wt% of carbon (C), more than 0 wt% and less than 0.03 wt% of silicon (Si) (Al): 0.01 to 0.06 wt%, N (N): 0 wt%, Mn: not less than 0 wt% to not more than 1.0 wt%, P: not less than 0 wt% (B): 0.0005 to 0.002% by weight, and the balance of iron (Fe) and unavoidable 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 contrary, when the SRT exceeds 1250 DEG C, the austenite grain size increases and the strength is hardly ensured. In addition, the hardening ability and endurance are also reduced. Also, Can only rise.

Hot rolling

In the hot rolling step (S120), the slab plate is hot-rolled to a finishing delivery temperature (FDT) of 860 to 900 占 폚.

If the finishing rolling temperature (FDT) is too low, less than 860 占 폚, there arises a problem that the grain size can not be effectively reduced. On the other hand, when the finish rolling temperature (FDT) is higher than 900 ° C, there is a problem that the austenite grains are coarsened and the hardening ability and the anti-aging property are decreased.

Cooling/ Coiling

In the cooling / winding step (S130), the hot rolled plate is cooled to a CT (Coiling Temperature) of 640 to 680 ° C and wound.

In this step, when the coiling temperature is less than 640 DEG C, the size of the precipitate can be coarsened, but there is a problem of secondary brittleness. On the contrary, when the coiling temperature exceeds 680 DEG C, it is difficult to secure sufficient strength.

Pickling phase

In the pickling step S140, acid pickling is performed to remove the scale of the rolled plate, that is, the hot-rolled coil manufactured through the hot rolling process.

Cold annealing

In the cold-annealing step (S150), the cold-rolled sheet is cold-rolled and annealed at 840 to 860 ° C.

In this step, when the cold-rolling annealing temperature is lower than 840 DEG C, the recrystallization is delayed. On the other hand, when the cold-rolling annealing temperature exceeds 860 DEG C, non-uniform crystal grains are formed due to an increase in grain size, which makes it difficult to secure strength.

Cold rolling

Cold rolling and temper rolling is a process for imparting a movable potential to a sheet material, which creates a potential that is not restrained by the use of carbon and nitrogen. Therefore, it shows continuous yielding behavior and plays a role to prevent deterioration of surface quality due to discontinuous yielding behavior during molding. However, it is preferable to apply the lowest SPM reduction as long as the yield point does not occur because the elongation material decreases as the rough rolling reduction ratio increases.

At this time, in the cold rolling and temper rolling step (S160), it is preferable that the temper reduction rate is 0.8 to 1.2% for the purpose of securing the proper temperature hardenability as well as room temperature endurance. A desired effect can be expected only by subjecting the cold rolling reduction to a temper rolling of 0.8% or more which is capable of correcting the shape of the cold-rolled steel sheet. On the contrary, when the reduction ratio of the roughness exceeds 1.2%, the increase of the friction coefficient causes a reduction in the material flow during molding, which deteriorates the formability. In addition, There is a problem that the elongation at yield point and the effect of improving the natural aging decrease.

Cooling

In the cooling step (S170), the plate material past the annealing section is cooled by a method such as gas jet cooling (GJC). At this time, the cooling can be cooled to 600 to 750 ° C, considering that the subsequent process is an alloying heat treatment.

At this time, it is preferable to perform the cooling at a rate of 5 ° C / sec or more, because if the cooling rate is less than 5 ° C / sec, the curing amount BH hardly decreases.

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

[Table 1] (unit:% by weight)

2. Evaluation of mechanical properties

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

[Table 2]

Referring to Tables 1 to 2, in the case of the specimens prepared according to Examples 1 to 3, elongation (EL) of 42% or more and r-bar value of 2.2 or more corresponding to the target value were all satisfied .

On the other hand, it can be seen that the elongation (El) and rank-pod (r-bar) are below the target value although the yield point (YP) satisfies the target value as compared with Example 1.

FIG. 2 is a graph showing a temperature change according to boron addition to a specimen prepared according to Example 1 and Comparative Example 1 of the present invention. FIG. 3 is a cross-sectional view after boron addition of a specimen prepared according to Example 1 of the present invention.

For the specimens prepared according to Comparative Example 1 and Example 1 of Table 1, referring to Figs. 2 to 3, in the case of specimens to which boron was added, the austenite-ferrite transformation temperature was lowered by cooling during hot rolling . Further, when the transformation temperature is lowered, nucleation of ferrite becomes dominant over nucleation, and the size of crystal grains can be reduced.

4 is a graph showing changes in hot rolling of specimens produced according to Examples 1 to 5 of the present invention.

Referring to FIG. 4, for specimens manufactured according to Examples 1 to 5, the grain size decreases drastically from a diameter of 18 μm to a diameter of 8 μm as the milling temperature is reduced from 900 ° C. to 860 ° C. However, the grain size does not decrease at temperatures below 850 ° C.

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5 is a graph showing changes in elongation for specimens prepared according to Examples 5 to 7 of the present invention.

Referring to FIG. 5, it can be seen that the elongation rate of the plate material decreases in inverse proportion as the SPM reduction is increased. Therefore, when boron is added to remove the dissolved nitrogen, the decrease in elongation due to SPM can be minimized.

Based on the above experimental results, the specimens prepared according to Examples 1 to 3 were prepared by adding 0.0005 to 0.002% by weight of boron (B) to elongation (EL) of 42% or more and rank-pod (r-bar) It can be seen that it satisfies.

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: Reheating of the slab
S120: Hot rolling
S130: Cooling / Winding
S140: pickles
S150: cold rolling annealing
S160: Cold rolling and temper rolling
S170: Cooling

Claims (5)

(Si): more than 0 wt% to 0.03 wt%, manganese (Mn): more than 0 wt% to 1.0 wt%, and phosphorus (C) P): more than 0 wt% to 0.10 wt%, aluminum (Al): 0.01 to 0.06 wt%, nitrogen (N): more than 0 wt% to 0.003 wt%, and boron (B) And reheating the slab plate comprising the remaining iron (Fe) and unavoidable impurities;
(b) hot-rolling the reheated plate to a finishing delivery temperature (FDT) of 860 to 900 占 폚;
(c) cooling the hot-rolled plate by cooling to a CT (Coiling Temperature) of 640 to 680 캜;
(d) pickling the rolled sheet material;
(e) a step of cold-rolling and annealing after the pickling treatment;
(f) cold rolling and temper rolling the cold-rolled and annealed sheet material; And
(g) cooling the cold rolled and quenched rolled plate,
Wherein the steel sheet produced through the steps (a) to (g) has elongation (EL) of 42% or more and rank-pod (r-bar) value of 2.2 or more.
The method according to claim 1,
In the step (a)
Wherein the slab reheating temperature (SRT) is 1150 to 1250 ° C.
The method according to claim 1,
In the step (e)
Wherein the annealing temperature for cold rolling and annealing is 840 to 860 占 폚.
(C): more than 0 wt% to less than 0.005 wt%, Si: more than 0 wt% to 0.03 wt%, manganese (Mn): more than 0 wt% to 1.0 wt% (Al): 0.01 to 0.06 wt%, nitrogen (N): 0 wt% to 0.003 wt%, and boron (B): 0.0005 to 0.002 wt% (Fe) and inevitable impurities,
Elongation (EL) of 42% or more and rank-pod (r-bar) value of 2.2 or more.
5. The method of claim 4,
The manganese (Mn) and phosphorus (P)
0.9? Mn (weight%) + 10P (weight%)? 1.1.

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