KR101505302B1 - High strength steel sheet and method for manufacturing the same - Google Patents

Abstract

A high strength steel sheet exhibiting high tensile strength of 780 MPa or more and excellent in bending workability and a method for producing the same.
The method for manufacturing a high strength steel sheet according to the present invention comprises the steps of (a) 0.06 to 0.1% of carbon (C), 0.1 to 0.3% of silicon (Si), 2.0 to 2.2% of manganese (Mn) : A steel slab made of a steel slab composed of 0.02% or less of sulfur (S), 0.05% or less of aluminum (Al), 0.03% or less of aluminum (Al), 0.01 to 0.02% of niobium (Nb) ; (b) pickling and cold rolling the hot-rolled steel sheet; (c) annealing the cold-rolled steel sheet at 790 to 860 ° C; And (d) cooling the annealed steel sheet to a temperature in the martensite temperature range.

Description

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

More particularly, the present invention relates to a high strength steel sheet having a high tensile strength of 780 MPa or more and excellent bending workability, and a method of manufacturing the same.

Generally, a DP (dual phase) steel sheet contains ferrite and martensite as the final microstructure. The DP steel sheet is produced by solidifying molten steel by continuous casting to produce a slab, and hot rolling, cold rolling and annealing the slab. Such a DP steel sheet is excellent in strength and moldability, and is used for applications such as automobile exterior materials.

However, in the case of a DP steel plate, there is a problem that cracks occur during bending. During bending of the DP steel sheet, stress is concentrated on the martensite to form and grow voids, which causes cracks to propagate easily.

In order to suppress the formation and growth of such voids, it is necessary to lower the difference in phase hardness between ferrite and martensite.

As a prior art related to the present invention, there is a super high strength manufacturing method disclosed in Korean Patent Laid-Open Publication No. 10-2005-0063981 (published on June 29, 2005), which is excellent in bending workability.

An object of the present invention is to provide a high strength steel sheet having a high strength of 780 MPa or more and a tensile strength of 780 MPa or more through control of process conditions such as an alloy component and annealing, and excellent in bending workability and a manufacturing method thereof.

In order to accomplish the above object, the present invention provides a method of manufacturing a high strength steel sheet, comprising: (a) 0.06 to 0.1% carbon, 0.1 to 0.3% silicon, manganese (Mn) (S): more than 0% to 0.05%, aluminum (Al): more than 0% to 0.03%, niobium (Nb): 0.01 to 2.0% To 0.02% and the balance of iron (Fe) and unavoidable impurities; (b) pickling and cold rolling the hot-rolled steel sheet; (c) annealing the cold-rolled steel sheet at 790 to 860 ° C; And (d) cooling the annealed steel sheet to a temperature in the martensite temperature range.

At this time, the hot-rolled steel sheet manufacturing step may include reheating the steel slab at a temperature of 1200 ° C or lower, hot rolling the steel slab at a finish rolling temperature of 900 ° C or higher, and then winding the steel slab at a temperature of 550 ° C or higher.

In order to achieve the above object, a high strength steel sheet according to an embodiment of the present invention includes 0.06 to 0.1% of carbon (C), 0.1 to 0.3% of silicon (Si), 2.0 to 2.2% of manganese (Mn) (P): more than 0% to 0.02%, sulfur (S): more than 0% to 0.05%, aluminum (Al): more than 0% to 0.03%, niobium (Nb): 0.01 to 0.02% (Fe) and inevitable impurities, and includes ferrite and martensite, wherein the difference in phase hardness between the ferrite and martensite is 4.8 GPa or less.

The high strength steel sheet may have a yield strength (YP) of 495 to 523 MPa, a tensile strength (TS) of 790 to 835, an elongation (El) of 19 to 23% and a bending radius (R) of 1 or less.

According to the method of manufacturing a high-strength steel sheet according to the present invention, it is possible to control an alloy component such as niobium (Nb), and control of process conditions such as annealing and cooling by eliminating the addition of expensive alloying elements such as chromium (Cr) and molybdenum (R) of 1 or less while having a tensile strength of 780 MPa or more.

That is, in the case of the high-strength steel sheet according to the present invention, the niobium (Nb) addition makes the structure finer and the martensite can be uniformly distributed. By increasing the annealing heat treatment temperature after the cold rolling, a large amount of austenite fraction The inter-phase hardness of the ferrite and martensite of the final structure can be reduced. As a result, in the case of the high-strength steel sheet according to the present invention, stress concentration is dispersed in martensite during bending to suppress void formation and growth, thereby improving bendability.

1 is a flowchart schematically showing a method of manufacturing a high-strength steel sheet according to an embodiment of the present invention.
Fig. 2 shows the microstructure of the specimen according to Comparative Example 1. Fig.
Fig. 3 shows the microstructure of the specimen according to Example 1. Fig.
Fig. 4 shows the result of 90 ° bending test of the specimen according to Comparative Example 1. Fig.
Fig. 5 shows the result of 90 ° bending test of the specimen according to Example 1. Fig.

BRIEF DESCRIPTION OF THE DRAWINGS The advantages and features of the present invention and the manner of achieving them will become apparent with reference to the embodiments described in detail below with reference to 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.

Hereinafter, a high strength 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.

The high strength steel sheet according to the present invention corresponds to a DP (dual phase) steel sheet having a microstructure including ferrite and martensite.

The high-strength steel sheet according to the present invention comprises 0.06 to 0.1% of carbon (C), 0.1 to 0.3% of silicon (Si), 2.0 to 2.2% of manganese (Mn) (S): more than 0% to 0.05%, aluminum (Al): more than 0% to 0.03%, and niobium (Nb): 0.01 to 0.02%.

The rest of the above components are composed of iron (Fe) and unavoidable impurities.

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

Carbon (C)

Carbon (C) contributes to the improvement of the martensite fraction and hardness in the DP steel sheet.

The carbon is preferably added in an amount of 0.06 to 0.1% by weight based on the total weight of the steel sheet. When the addition amount of carbon is less than 0.06% by weight, it is difficult to secure a tensile strength of 780 MPa or more. On the other hand, when the carbon content exceeds 0.1% by weight, carbide formation in the steel is promoted and the elongation rate is lowered.

Silicon (Si)

Silicon (Si) is a solid solution strengthening element which accelerates the purification of steel and carbon enrichment in austenite. It improves the flowability of molten metal during welding among steels to which manganese (Mn) is added, to be. In addition, silicon improves the strength without inhibiting the balance of the yield ratio and elongation, and slows the diffusion rate of carbon in the ferrite, thereby inhibiting carbide growth and stabilizing ferrite, thereby contributing to enhancement of elongation.

The silicon is preferably added in an amount of 0.1 to 0.3% by weight based on the total weight of the steel sheet. When the addition amount of silicon is less than 0.1% by weight, the effect of the addition is insufficient. On the other hand, when the addition amount of silicon exceeds 0.3% by weight, the performance is deteriorated and the plating property is deteriorated.

Manganese (Mn)

Manganese (Mn) is a solid solution strengthening element which stabilizes austenite and lowers the phase transformation temperature, and martensite is easily produced even at a low cooling rate.

The manganese is preferably added in an amount of 2.0 to 2.2% by weight based on the total weight of the steel sheet. When the content of manganese is less than 2.0% by weight, the effect of addition thereof is insufficient. On the contrary, when the content of manganese exceeds 2.2% by weight, a manganese band develops in the center of the thickness direction of the material, and the elongation rate is lowered.

In (P)

Phosphorus (P) contributes to the improvement of the strength of the steel sheet by solid solution strengthening, and as an element effective in suppressing the formation of carbide, it plays a role of preventing elongation decrease due to formation of carbide during annealing and cooling. It is also effective for obtaining martensite by improving manganese equivalence. However, when phosphorus is added in excess, Fe3P stddite is formed, which causes hot brittleness.

In the present invention, the content of phosphorus is limited to more than 0% by weight and not more than 0.02% by weight of the total weight of the steel sheet.

Sulfur (S)

Sulfur (S) inhibits toughness and weldability, and increases MnS nonmetallic inclusions, which hinders the effect of incombustibility of Mn and causes cracks. Also, if sulfur is contained excessively, the coarse inclusions are increased to deteriorate the fatigue characteristics.

Therefore, in the present invention, the content of sulfur is limited to more than 0 wt% to 0.05 wt% or less of the total weight of the steel sheet.

Aluminum (Al)

Aluminum (Al) is an element mainly used as a deoxidizer. It contributes to stabilize austenite by improving ferrite and improving elongation rate and increasing carbon concentration in austenite. In addition, aluminum acts as a layer between the iron and the zinc plated layer to improve the plating property, and is an effective element for suppressing the formation of manganese bands in the hot-rolled coil.

The aluminum is preferably added in an amount of more than 0 wt% to 0.03 wt% of the total weight of the steel sheet. If the added amount of aluminum exceeds 0.03% by weight, the performance is deteriorated and AlN is formed in the slab, thereby causing hot cracking.

Niobium (Nb)

Niobium (Nb) improves the strength of a steel sheet produced by forming a niobium-based precipitate, in particular, reduces a manganese band layer by making crystal grains finer and contributes to martensite dispersion.

The niobium is preferably added in an amount of 0.01 to 0.02% by weight based on the total weight of the steel sheet. When the addition amount of niobium is less than 0.01% by weight, the effect of addition thereof is insufficient. On the contrary, when the addition amount of niobium exceeds 0.02 wt%, the workability is lowered.

The high strength steel sheet according to the present invention comprising the above alloy component exhibits a microstructure including ferrite and martensite according to a process described later. Particularly, in the case of the high-strength steel sheet according to the present invention, the difference in phase hardness between ferrite and martensite is 4.8 GPa or less. This is because the difference in phase hardness between ferrite and martensite of a typical DP steel sheet is about 5.1 GPa This corresponds to a low hardness difference. This low interphase hardness difference could be achieved by forming a large amount of austenite at the beginning of annealing after cold rolling.

The high strength steel sheet according to the present invention has a Yield strength (YP) of 495 to 523 MPa, a tensile strength (TS) of 790 to 835, an elongation (El) of 19 to 23% and a bending radius (R) 1 or less.

Hereinafter, a method for manufacturing a high strength steel sheet according to the present invention will be described in detail.

FIG. 1 is a flowchart schematically showing a method for manufacturing a high-strength steel sheet according to an embodiment of the present invention, and more specifically, a method for manufacturing a cold-rolled steel sheet.

Referring to FIG. 1, the high-strength steel sheet manufacturing method includes a hot-rolled steel sheet manufacturing step (S110), a cold rolling step (S120), an annealing step (S130), and a cooling step (S140).

Hot-rolled steel sheet manufacturing

In the hot-rolled steel sheet manufacturing step (S110), the steel slab having the above-mentioned composition is reheated to the slab, hot rolled and cooled to produce a hot-rolled steel sheet. The hot-rolled steel sheet may be prepared by various known process conditions, and more preferably, the following process may be employed.

First, the steel slab is reheated at 1200 DEG C or less, more preferably at 1150 to 1200 DEG C. If the slab reheating temperature is less than 1150 ° C, the segregated components may become difficult to reuse during casting. On the other hand, when the slab reheating temperature exceeds 1200 ° C, the austenite grain size can be coarsened.

Next, the reheated steel slab is hot-rolled to a finish rolling temperature of 910 캜 or more, more preferably 910 to 950 캜. If the finishing rolling temperature is less than 900 占 폚, a problem may arise such as a complicated structure due to an abnormal reverse rolling.

Next, the hot-rolled steel is cooled at a cooling rate of about 50 DEG C / sec or less and wound at 550 DEG C or higher, more preferably at 550 to 650 DEG C. Various methods such as natural cooling and forced cooling can be applied to the cooling. If the cooling termination temperature is less than 550 ° C, ductility and processability may be deteriorated.

Cold rolling

Next, in the cold rolling step (S120), the hot-rolled steel sheet is pickled and then cold-rolled to form the final thickness of the steel sheet. The reduction rate of the cold rolling can be set to about 50 to 70% depending on the thickness of the hot-rolled steel sheet and the final thickness of the steel sheet to be targeted.

Annealing treatment

In the annealing step (S130), the cold-rolled steel sheet is heated and annealed for about 100 to 200 seconds. The austenite phase fraction can be controlled through annealing, and the desired strength and elongation can be secured.

In the present invention, the annealing is carried out at 790 to 860 ° C. As a result of heating the cold-rolled steel sheet to the above-mentioned temperature and adding niobium in the steel sheet, the crystal grains can be made finer and thus the bending workability can be improved. If the annealing treatment temperature is less than 790 캜, it may become difficult to secure sufficient austenite. On the other hand, if the annealing treatment temperature exceeds 860 DEG C, the austenite grain size may greatly increase and the physical properties of the steel sheet may deteriorate.

It is more preferable that annealing is performed at 840 to 860 캜. When the annealing is performed in the temperature range, the difference in phase hardness between ferrite and martensite in the microstructure of the steel sheet to be finally produced can be 4.8 GPa or less, which is more preferable for improving the bending workability.

Cooling

In the cooling step (S140), the annealed steel sheet is cooled to the martensite region. The cooling may be performed at a cooling rate of approximately 5 to 40 ° C / sec. The cooling method may be a roll quenching method, a gas jet method, or the like. It is also possible to carry out an aggressive treatment at about 420 to 500 DEG C for about 30 to 200 seconds during cooling.

The steel sheet produced by the steps S110 to S140 may have a microstructure including ferrite and martensite. Further, the steel sheet produced by the method according to the present invention can exhibit a tensile strength (TS) of 790 to 835 MPa, a yield strength (YP) of 495 to 523 MPa and an elongation of 19 to 23% The bending radius R, which means a radius that does not occur, may be 1 or less.

This characteristic can be seen from the above-mentioned steel composition containing 0.01 to 0.02% by weight of niobium in the steel sheet and a large amount of austenite at the initial stage of annealing through the high-temperature annealing treatment.

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

Steel slabs 1 to 5 were prepared by hot-rolling, cold-rolling and annealing the steel slab having the alloy composition as shown in Table 1 under the conditions as shown in Table 2.

[Table 1] (unit:% by weight)

[Table 2]

2. Evaluation of mechanical properties

Table 3 shows the results of the tensile test and the bending test of specimens prepared according to the steel specimens 1 to 5.

The bending test was carried out by using a bending tester to judge whether or not a crack occurred in each of the bending radii (R) 2 mm and 1 mm, and the case where no crack occurred was indicated as OK and the case where a crack occurred was indicated as NG.

[Table 3]

Referring to Table 3, the specimen 2 and the specimen 3 satisfying the steel composition and the process conditions proposed in the present invention satisfied all of the desired mechanical properties and bending workability. However, in the case of specimen 1 without niobium, the bending workability was poor. In the case of specimen 4 in which niobium was excessively added, the strength was too high and the elongation was relatively low. In the case of the sample 5 having a relatively low annealing temperature, the bending workability was satisfied, but the mechanical properties were relatively low.

Fig. 2 shows the microstructure of the specimen according to Comparative Example 1, and Fig. 3 shows the microstructure of the specimen according to Example 1. Fig. 4 shows the results of the 90 ° bending test of the test piece according to Comparative Example 1, and Fig. 5 shows the results of the 90 ° bending test of the test piece according to Example 1. Fig.

Table 4 shows the hardness of the final microstructure of Specimen 1 and Specimen 2. The hardness was measured using a nanoindenter.

[Table 4] (Unit: GPa)

Referring to Table 4, in the case of specimen 1 in which niobium is not added and annealing is performed at 810 ° C, the difference in hardness between ferrite and martensite is 5.1 GPa. However, in the case of specimen 2 in which niobium is added and annealing is performed at 850 ° C, it can be seen that the difference in hardness between ferrite and martensite is relatively low at 4.5 GPa.

As shown in Figs. 2 and 3, both the specimen 1 and the specimen 2 exhibit ferrite and pearlite as the hot-rolled structure and ferrite and martensite as the cold-rolled structure. However, as described above, In case of specimen 1, cracks were generated as shown in FIG. 4, but in case of specimen 2, no cracks were generated as shown in FIG. 5.

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.

Claims (5)

delete (a) from 0.06 to 0.1% carbon (C), from 0.1 to 0.3% silicon (Si), from 2.0 to 2.2% manganese (Mn) A steel slab composed of sulfur (S) in excess of 0% to 0.05%, aluminum (Al) in excess of 0 to 0.03%, niobium (Nb) in 0.01 to 0.02% and balance of iron (Fe) Hot rolled at a finish rolling temperature of 900 DEG C or higher, and then rolled at a temperature of 550 DEG C or higher to produce a hot-rolled steel sheet;
(b) pickling and cold rolling the hot-rolled steel sheet;
(c) annealing the cold-rolled steel sheet at 790 to 860 ° C; And
(d) cooling the annealed steel sheet to a temperature in the martensite temperature range.
3. The method of claim 2,
Wherein the annealing is performed at 840 to 860 占 폚.
delete (P): more than 0% to less than 0.02%, sulfur (S): 0.06 to 0.1%, silicon (Si): 0.1 to 0.3%, manganese ): Not less than 0% to not more than 0.05%, aluminum (Al): not less than 0% to not more than 0.03%, niobium (Nb): 0.01 to 0.02%
Wherein the difference in phase hardness between the ferrite and martensite is 4.8 GPa or less,
(YP) of 495 to 523 MPa, a tensile strength (TS) of 790 to 835, an elongation (El) of 19 to 23% and a bending radius (R) of 1 or less.

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