KR101149193B1 - Steel sheet having excellent formability and galvanizing property, and method for producing the same - Google Patents

Abstract

The present invention relates to a ferritic stainless steel comprising 0.08 to 0.15 wt% of C, 0.1 to 0.3 wt% of Si, 2.0 to 2.8 wt% of Mn, more than 0 to 0.006 wt% of S, : 0.2 to 0.5 wt%, Cr: 0.2 to 0.4 wt%, V: 0.01 to 0.10 wt%, Mo: 0.01 to 0.05 wt%, N: more than 0 to 0.006 wt%, and the balance Fe and other unavoidable impurities , And the microstructure is a steel sheet in which the ferrite phase has a fraction of 80 to 85%, and a manufacturing method thereof.

According to the present invention, a cold rolled steel sheet or a hot-dip galvanized steel sheet having a tensile strength of 780 MPa or more and an elongation of 21% or more can be obtained by increasing the content of retained austenite. In addition, it is easier to process a complicated part shape than the conventional two-phase structure steel, and it is expected that the effect of absorbing shock energy in the event of an automobile crash can be expected due to an increased austenite fraction. Further, instead of lowering the content of Si forming the oxide on the surface of the steel sheet, there is an effect that the plating property is improved by adding Mo.

Steel plate, Plating property, Workability

Description

TECHNICAL FIELD The present invention relates to a steel sheet excellent in plating performance and workability and a method of manufacturing the steel sheet having excellent formability and galvanizing property,

The present invention relates to a steel sheet and a manufacturing method thereof, and more particularly, to a galvanized steel sheet for automobile structural members having a plating performance and a workability and a tensile strength of 780 MPa or more and a manufacturing method thereof.

Since most of automobile parts are formed by press working, excellent press workability is required. Especially, in recent years, automobile design has become complicated and consumers' desires are diversified, so a steel having high strength and excellent plating processability is required .

For example, high-strength steel sheets using TRIP (Transformation Induced Plasticity) steel, DP (Dual Phase) steel and CP (Complex Phase) steel are used for structural members such as automobile bodies.

These steels are mainly applied to parts that require high energy absorbing ability in the event of a vehicle collision such as members, pillar, and bumper reinforcement, and because they are processed using roll forming, they require high elongation such as bending workability, Plating is required.

The two-phase structure steel and the transformed organo-plastic steel can satisfy both high strength and moldability simultaneously. These two material properties are obtained by designing steel components to obtain the microstructure of ferrite and martensite or ferrite and retained austenite and by proper annealing heat treatment technique in continuous plating process.

The bimodal steel is quenched in the austenite and ferrite two phase regions to produce martensite in a fraction of about 10 to 30%. It has superior ductility and stretching workability compared to precipitation hardened steel and is mainly applied to members and bumpers because of its high impact energy absorbing ability.

However, in the case of the above-mentioned two-phase structure steel, in order to obtain a strength of 80 kg / mm ² or more, the martensite ratio must be increased, so that the ductility is lowered and the press formability is lower than that of the transformed organo-plastic steel.

On the other hand, the modified organo-plastic steel has a ductility 5 ~ 10% higher than that of the two-phase structure steel and the composite structure steel.

However, in order to improve the strength and formability of modified organic plastic steels, various alloying elements are added to the material, and these added elements are concentrated on the surface layer of the material, which has a great influence on the reaction between Fe, which is a base material, and Zn, .

In particular, the existing developed modified organo-plastic steels increase the Mn, Si, and C contents rapidly to obtain ultra-high strength and add a large amount of alloying elements to form precipitation phase, Based oxide (Mn2SiO4 or the like) is formed and an unplated layer is generated. Therefore, there are many disadvantages in hot-dip coating.

In the case of 1 wt% or more of high-Si and Mn-added steels, Cu and Ni elements are added to steel based on Si and Mn steel using a widely used oxidation-reduction method to change the shape of oxides formed in the surface layer of the steel sheet, There is a report on how to improve.

Japanese Patent Application Laid-Open No. 2001-288550 proposes a method of suppressing the formation of the grains formed on the surface during the cold rolling annealing by preliminary oxidation of the hot rolling coils before and after the cold rolling. However, this method is not effective in the addition of specific elements , There is a problem that the metallurgical behavior of the additive element is unclear and the necessary manufacturing method is not sufficient and the workability is lowered.

It is an object of the present invention to improve the elongation and to control the content of Si, Al and Mo by partially utilizing the retained austenite of the modified organofunctional steel. To provide a hot-dip galvanized steel sheet excellent in plating performance and workability by improving the plating ability, and a method for producing the same.

According to an aspect of the present invention, there is provided a hot-dip galvanized steel sheet comprising 0.08 to 0.15 wt% of C, 0.1 to 0.3 wt% of Si, 2.0 to 2.8 wt% of Mn, S: more than 0 to 0.006 wt%, P: more than 0 to 0.01 wt%, Al: 0.2 to 0.5 wt%, Cr: 0.2 to 0.4 wt%, V: 0.01 to 0.10 wt% N: more than 0 and not more than 0.006 wt%, the balance Fe and other unavoidable impurities, and the microstructure has a fraction of the ferrite phase of 80 to 85%.

The content of Al, Cr, and V can satisfy the formula of 0.4 wt%? Al + Cr + V? 1 wt%.

In addition, the microstructure may have a fraction of 15 to 20% of the phase containing martensite and retained austenite.

According to another aspect of the present invention, there is provided a method of manufacturing a hot-dip galvanized steel sheet, comprising: 0.08 to 0.15 wt% of C, 0.1 to 0.3 wt% of Si, 2.0 to 2.8 wt% of Mn, P: 0 to 0.01 wt%, Al: 0.2 to 0.5 wt%, Cr: 0.2 to 0.4 wt%, V: 0.01 to 0.10 wt%, Mo: 0.01 to 0.05 wt%, N: 0 A steel slab having an alloy composition of not more than 0.006 wt% and the balance of Fe and other unavoidable impurities is homogenized at a temperature of 1200 캜 or higher, rolled at a temperature of 880 to 920 캜 for finishing hot rolling, Rolled, annealed and then subjected to hot-dip galvanizing.

The annealing may be performed by maintaining the cold-rolled steel sheet at a temperature of 785 to 815 캜 for 5 to 120 seconds, and then cooling the steel sheet to a temperature of 480 캜 or less at a cooling rate of 5 to 50 캜 / sec.

The hot-dip galvanizing treatment is a process in which the steel sheet is dipped in a plating bath at a temperature of 450 to 480 캜 and can be cooled to 250 캜 or lower at a cooling rate of 20 to 50 캜 / sec or more after the hot-dip galvanizing treatment.

After the hot-dip galvanizing treatment, the hot-dip galvanized film is re-heated to a temperature of 500 to 520 ° C to cool the hot-dip galvanized film to 250 ° C or less at a cooling rate of 20 to 50 ° C / sec.

The present invention relates to a method of increasing the content of Mn, Si, Cr, Al, and V in order to improve the strength of the steel, and reducing the content of residual austenite by controlling the hot rolling control technique and annealing heat treatment temperature, By increasing the content, a cold-rolled steel sheet or a hot-dip galvanized steel sheet having a tensile strength of 780 MPa or more and an elongation of 21% or more can be obtained.

Therefore, it is possible to replace the conventional steel sheet having a tensile strength of 440 to 590 MPa, and it is possible to reduce the thickness of the steel sheet due to the increase in strength. Therefore, when the steel sheet is molded into an automobile structural member, And the like.

In addition, it is easier to process a complicated part shape than the conventional two-phase structure steel, and it is expected that the effect of absorbing shock energy in the event of an automobile crash can be expected due to an increased austenite fraction.

Further, instead of lowering the content of Si forming the oxide on the surface of the steel sheet, Mo is added to improve the hot dip galvanizing property.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, preferred embodiments of a steel sheet excellent in platability and workability according to the present invention and a method for producing the same will be described in detail.

The steel sheet of the present invention contains 0.08 to 0.15 wt% of C, 0.1 to 0.3 wt% of Si, 2.0 to 2.8 wt% of Mn, more than 0 to 0.006 wt% of S, Of Al, 0.2 to 0.5 wt% of Cr, 0.2 to 0.4 wt% of Cr, 0.01 to 0.1 wt% of V, 0.01 to 0.05 wt% of Mo, more than 0 to 0.006 wt% of N and an alloy of Fe and other unavoidable impurities And the microstructure has a fraction of 80 to 85% of the ferrite phase.

Specifically, the present invention relates to a method of increasing the content of Mn, Si, Cr, Al, and V in order to improve the strength of the steel, and reducing the content of Si forming the oxide on the steel sheet surface Instead, Mo was added to improve the plating characteristics.

The steel sheet is subjected to hot rolling, cold rolling, hot dip galvanizing and then has a three-phase structure composed of ferrite, retained austenite and magnesite, so that an elongation of 21% or more is secured.

Hereinafter, reasons for limiting the function and content of the alloying elements of the present invention will be described.

C: 0.08 to 0.15 wt%

C is an indispensable element for imparting high strength to the steel sheet, stabilizes the austenite in the bainite transformation temperature range, diffuses and transfers into the austenite inside the ferrite, and 1 to 5w% of retained austenite is present even after cooling to room temperature And transformational organic firing is generated at the time of processing to improve moldability. When the content of C is less than 0.08 wt%, 1 wt% or more of retained austenite is not ensured and elongation characteristics are deteriorated. When the content of C is more than 0.15 wt%, spot weldability is lowered and weldability and toughness are lowered. %.

Si: 0.1 to 0.3 wt%

Si is a useful element that can improve the strength without deteriorating the ductility of the steel sheet. It contributes strength to ferrite stabilizing elements incorporated in the ferrite and promotes the formation of martensite by promoting carbon enrichment to untransformed austenite. However, in the present invention, if it is added in an excess amount, the plating property is lowered, so that it is preferable to minimize the addition amount. In the present invention, the upper limit is set to 0.3 wt%. If the content is less than 0.1 wt%, the strength of the ferrite is decreased and the effect of suppressing the carbide is decreased. Therefore, the content should be 0.1 wt% or more.

Mn: 2.0 to 2.8 wt%

Mn is an austenite stabilizing element as an element that increases the strength through the effect of improving the solid solution strengthening and ingotability. It is found that the excellent spot weldability and the material stability can not be obtained unless the C content is reduced by decreasing the C content, thereby reducing the C content. Considering these points, Mn content is particularly preferably at least 2.0w%, and when added in an excessively large amount, the weldability and platability are lowered, and hydrogen embrittlement is caused by formation of inclusions. In hot rolling, It is limited to a maximum of 2.8wt%.

S: more than 0 and not more than 0.006 wt%

If the content of S exceeds 0.01 wt%, emulsion inclusions are formed and cracks are generated. In particular, in the present invention, since Mn is added in a large amount, the lower the content of S, the better the upper limit is 0.006w% .

P: more than 0 and not more than 0.01 wt%

P is an element useful for securing the strength of a material, but when added in a large amount, not only the workability is deteriorated but also the weldability is also lowered, so that the upper limit is limited to 0.01 wt%.

Al: 0.2 to 0.5 wt%

Al is an element useful as a deoxidizer and is used as an alternative element of Si, an element that inhibits plating, to increase ferrite formation and C concentration in the austenite phase to suppress pearlite formation and promote residual austenite formation. In order to promote carbon diffusion to the austenite phase, it should be added in an amount of 0.2 wt% or more. When added in a large amount, the surface defects are increased during the slab manufacturing process and the Al- Is increased to deteriorate the plating characteristics, the content of Al is limited to the range of 0.2 to 0.5 wt%.

Cr: 0.2 to 0.4 wt%

Cr is a ferrite-forming element, which has the effect of delaying the transformation of austenite into pearlite and bainite, and has an effect of transforming austenite from martensite at room temperature into an martensite after anomalous reverse annealing and improving strength. If it is added in an amount of 0.2w% or less, it is difficult to obtain a sufficient strength, and when it is added in an amount of 0.4w% or more, the balance between strength and ductility is broken.

V: 0.01 to 0.10 wt%

V is an element that improves strength and serves to help transform austenite into martensite. In addition, it is important to add chromium in order to harmonize with the chromium content because it has synergistic effect when it is added together with chromium and can realize proper strength and elongation. V carbides can be dissolved at relatively low temperatures and are easily dissolved when reheating slabs. If the amount of V added is less than 0.01 wt%, the amount of the finely dispersed complex carbide can not be sufficiently exhibited. If the amount of V is more than 0.1 wt%, the complex carbide becomes coarse and the strength is lowered.

Mo: 0.01 to 0.05 wt%

Mo is an element stabilizing the low-temperature transformation phase similarly to Mn. When Mo is excessively high, it is a cause of deterioration in the formability and is an expensive element, so the amount of use is limited. However, since there is an advantage in terms of plating properties and material stability in addition of Cr, In order to improve the material properties.

N: more than 0 and not more than 0.006 wt%

N is an element which adversely affects ductility, and it is advantageous to keep it as low as possible, and if N is present excessively, the nitride precipitates in a large amount and tends to cause deterioration of ductility, so that the content of N is suppressed to 0.006 wt% .

Nb: 0.01 to 0.10 wt%

The steel sheet of the present invention may further contain a small amount of Nb. Nb combines with C, N to form carbonitride. This suppresses crystal grain growth during rolling and makes crystal grains finer, thereby improving toughness and precipitation strengthening effect after rolling cooling. If the content of Nb is less than 0.01 wt%, the effect is insufficient, while if it exceeds 0.1 wt%, the toughness may be deteriorated. Therefore, the content is set to 0.01 to 0.10 wt%.

On the other hand, it is preferable that the content of Al, Cr and V satisfies the formula of 0.4 wt%? Al + Cr + V? 1 wt%. That is, when the total of the contents of Al, Cr and V is less than 0.4 wt%, the fraction required for the martensite phase can not be satisfied and the strength is lowered. The total content of Al, Cr and V is 1 wt% The martensite phase is excessively formed, and it is difficult to secure a desired elongation.

Specifically, Al promotes ferrite formation and carbon diffusion in the austenite phase to promote formation of the retained austenite phase. When the steel has a composite structure of three phases of a ferrite phase, a bainite phase and a residual austenite phase and the ferrite phase is strengthened by a complex carbide containing Cr and V, the Al addition has a strength and a uniform elongation Thereby improving the balance between the two. V is effective for forming fine complex carbides together with Cr and Mo, and is one of the important elements in the present invention. When V is added, the fine composite carbide particles mainly precipitate in the form of VMoC2, and the fine complex carbide is dispersed and precipitated in a larger amount, which is very effective in increasing the strength of the steel. Further, the carbide of V can be dissolved at a relatively low temperature, so that V easily dissolves during the reheating step of the slab.

The present invention includes the components of the alloy steel, and the remainder is Fe and inevitable elements. It is an element contained according to the conditions of raw materials, materials, manufacturing facilities and the like, and permits fine admixture of unavoidable impurities such as 0.01 wt% .

The slab having the above composition is produced through ingot casting or continuous casting process after obtaining molten steel through a steelmaking process. Here, the slab is produced in the form of a steel plate by hot rolling or cold rolling, and then hot dip galvanized The following process is carried out.

Each process is as follows.

[Heating furnace process]

When the reheating temperature is low, the segregated components can not be reused. If the reheating temperature of the slab is too high, the austenite grain size increases and the ferrite grain size becomes coarse, .

Also, it is necessary to adjust the reheating temperature holding time according to the slab thickness. It is necessary to keep the reheating time longer as the thickness becomes thicker, and to shorten the holding time as the thickness becomes thinner. It is desirable to maintain the proper holding time for about 2 to 3 hours. If the holding time is longer than this, economical damage can be seen. If the holding time is too short, the quality of the material may be deteriorated because the material is homogenized. In the present embodiment, the homogenization treatment is performed by heating at a temperature of 1200 DEG C or higher for about 2 to 3 hours.

[Hot Rolling and Cold Rolling Process]

The slabs reheated in the heating furnace process are subjected to hot finish rolling in the temperature range of 880 to 920 ° C. When the hot rolling finishing temperature is less than 880 DEG C, an excessive dislocation is introduced into the ferrite and grows during cooling or winding to form coarse grains on the surface. In the case of finishing rolling at 920 占 폚 or more, the ferrite grain size increases and the strength decreases. Therefore, in the present embodiment, the temperature is in the range of 880 to 920 占 폚.

After hot rolling, the steel sheet may be cooled to a winding temperature by a multi-stage cooling method or a general cooling method, and then wound. The winding process is preferably performed at a temperature range of 560 to 600 ° C. If the coiling temperature is too high, Mn, Si and Cr contained in large amounts may cause segregation and it is difficult to obtain desired strength and workability. After pickling treatment after hot rolling, cold rolling is performed at a reduction ratio of 55% or more.

[Continuous Annealing Process]

The most important feature of the present invention is to control the fraction of ferrite and martensite by balancing strength and ductility. Therefore, since the strength and elongation of the cold-rolled sheet must be secured at the same time, the strength is preferably controlled by the fraction of martensite, and the elongation is preferably controlled mainly by the ferrite fraction.

For this purpose, the temperature is maintained at 785 to 815 ° C for 5 to 120 seconds and then cooled to a temperature of 480 ° C or less at a cooling rate of 5 to 50 ° C / sec.

During this process it is important to cool the austenite phase produced in the ideal zone to a cooling rate sufficient to prevent transformation into pearlite or bainate phase. When the temperature is maintained in the range of 785 to 815 占 폚 for 5 seconds or more, the austenite phase is not sufficiently formed during heating. Therefore, a proper amount of martensite can not be obtained.

[Hot-dip galvanizing or alloyed hot-dip galvanizing]

As described above, the steel sheet is maintained at a temperature of 785 to 815 캜 for 5 to 120 seconds, cooled to 480 캜 or lower at a cooling rate of 5 to 50 캜 / sec, and then the steel sheet is plated at 450 to 480 캜, And is then immersed in a bath to carry out hot dip galvanizing treatment.

After the hot dip galvanizing treatment, the hot dip galvanized film is reheated to a temperature of 500 to 520 ° C to alloy the hot dip galvanized film and cooled to 250 ° C or less at a cooling rate of 20 to 50 ° C / sec.

It is better to increase the cooling rate for efficient plating and alloying. However, when the cooling rate exceeds 50 DEG C / sec, there is a manufacturing difficulty in that a separate facility is required.

The microstructure of the steel sheet produced by the production method of the present invention is composed of 80 to 85% of ferrite having a grain size of 3 to 5 탆 and 15 to 20% of a phase containing martensite and retained austenite .

 When the ferrite phase is less than 80%, ductility and pressability are lowered. When the ferrite phase is more than 85%, it is difficult to secure strength, so that the ferrite phase fraction is limited to 80 to 85%.

In view of the above, the present invention provides a method of increasing the content of Mn, Si, Cr, Al and V in order to improve the strength, Instead of lowering the content, Mo was added to improve the plating characteristics.

Further, by increasing the content of retained austenite through the hot rolling control technique and the annealing heat treatment temperature control, it is possible to obtain a cold rolled steel sheet or a hot-dip galvanized steel sheet having a tensile strength of 780 MPa or more and an elongation of 21% or more and excellent in press- .

Hereinafter, the above-mentioned hot-dip galvanized steel sheet and its manufacturing method will be described in comparison with comparative examples different from those of the present invention.

Table 1 shows the composition ratios of inventive steels and other comparative steels of the present invention. The inventive steels and comparative steels of Table 1 were heated at 1,250 占 폚 for 2 hours, finishing hot-rolled at 900 占 폚, and then rolled at 580 占 폚, held for about 1 hour, After the pickling treatment, cold rolling was carried out at a reduction ratio of 60%. Annealing temperature was annealed at 770 to 830 占 폚, quenched to 460 占 폚, immersed in a plating bath, and alloyed at 500 to 520 占 폚 to measure mechanical properties .

Tables 2 to 4 show yield strength (YS), tensile strength (TS) and elongation (EI) of the inventive steel and comparative steel shown in Table 1, respectively, when the annealing temperatures were 770 캜, 800 캜 and 830 캜, ) And the plating property were measured.

(The remainder Fe, unit: wt%), division C Si Mn P S Al Cr Mo Nb V N Comparative River 1 0.06 0.25 2.20 0.01 0.003 0.02 - 0.1 0.02 - 0.006 Comparative River 2 0.10 0.25 2.20 0.01 0.003 0.02 - 0.1 0.02 - 0.006 Comparative Steel 3 0.12 0.25 2.10 0.01 0.003 0.02 0.40 - - 0.04 0.006 Comparative Steel 4 0.10 0.27 2.23 0.01 0.003 0.1 0.35 - - 0.04 0.006 Comparative Steel 5 0.08 0.01 2.3 0.01 0.003 0.45 0.15 0.02 - 0.05 0.006 Comparative Steel 6 0.11 0.10 2.4 0.01 0.003 0.04 - 0.15 0.02 - 0.006 Comparative Steel 7 0.1 0.01 2.1 0.01 0.003 0.04 0.45 0.11 0.02 - 0.006 Comparative Steel 8 0.16 0.30 2.0 0.01 0.003 0.05 0.35 - 0.02 - 0.006 Comparative Steel 9 0.08 0.01 2.4 0.01 0.003 0.1 0.25 0.11 - 0.02 0.006 Inventive Steel 1 0.09 0.25 2.20 0.01 0.003 0.40 0.25 0.02 - 0.04 0.006 Invention river 2 0.10 0.3 2.22 0.01 0.003 0.48 0.35 0.02 - 0.04 0.006 Invention steel 3 0.12 0.12 2.1 0.01 0.003 0.45 0.38 0.03 - 0.05 0.006

(When the annealing temperature is 770 DEG C) division Yield strength (MPa) Tensile Strength (MPa) Elongation (%) Plating property Comparative River 1 499 821 17.1 Good Comparative River 2 532 845 17.9 Good Comparative Steel 3 501 811 18.9 usually Comparative Steel 4 502 815 19.0 usually Comparative Steel 5 513 813 17.1 Good Comparative Steel 6 541 863 16.3 Good Comparative Steel 7 477 798 18.3 Good Comparative Steel 8 511 799 16.1 Bad Comparative Steel 9 552 887 15.1 Good Inventive Steel 1 524 834 19.5 Good Invention river 2 499 849 19.8 Good Invention steel 3 535 821 19.8 Good

(When the annealing temperature is 800 DEG C) division Yield strength (MPa) Tensile Strength (MPa) Elongation (%) Plating property Comparative River 1 484 790 19.5 Good Comparative River 2 482 807 20.2 Good Comparative Steel 3 493 802 20.1 usually Comparative Steel 4 486 823 20.3 usually Comparative Steel 5 477 783 20.7 Good Comparative Steel 6 517 824 18.9 Good Comparative Steel 7 447 768 20.0 usually Comparative Steel 8 503 793 17.0 Bad Comparative Steel 9 538 826 17.3 Good Inventive Steel 1 508 811 22.4 Good Invention river 2 482 802 22.6 Good Invention steel 3 516 789 22.4 Good

(When the annealing temperature is 830 DEG C) division Yield strength (MPa) Tensile Strength (MPa) Elongation (%) Plating property Comparative River 1 457 761 20.2 Good Comparative River 2 432 722 23.8 Good Comparative Steel 3 462 762 20.8 usually Comparative Steel 4 451 767 21.0 usually Comparative Steel 5 455 750 21.0 Good Comparative Steel 6 494 804 19.4 Good Comparative Steel 7 411 734 23.5 usually Comparative Steel 8 498 766 18.5 Bad Comparative Steel 9 519 799 19.9 Good Inventive Steel 1 496 823 21.9 usually Invention river 2 469 776 23.1 Good Invention steel 3 489 765 23.1 Good

1 is a graph showing the yield strength (YS), the tensile strength (TS) and the tensile modulus (EI) according to the annealing temperature of Inventive Steel 2 shown in Tables 2 to 4 above.

As shown in Fig. 1, the region A is a region having a tensile strength (TS) of 780 MPa or more and an elongation (EI) of 21% or more, and the region A is an annealing temperature of 785 to 815 캜 . Therefore, in order to balance the tensile strength TS and the elongation EI appropriately, it is preferable that the annealing temperature is in the range of 785 to 815 캜.

Therefore, in Table 2 to Table 4, the mechanical properties and plating properties of the inventive steel and the comparative steel, particularly in the case of the annealing temperature in the temperature range of 785 to 815 캜, are shown in Table 3, 0.08 wt% or less, and the retained austenite of 1 wt% or more is not ensured, and the elongation characteristics are deteriorated.

In Comparative steels 1 to 4 and Comparative steels 6 to 9, since the content of Al is small, the formation of retained austenite is not promoted and the elongation rate is poor. Since the production of nitrite is promoted, it can be seen that the elongation is excellent.

In addition, in the comparative steel 5, the content of Al is appropriate, but the content of Si and Cr is low, so that the yield strength and tensile strength are remarkably lower than those of the invention steel.

It can be seen from Tables 2 to 4 and FIG. 1 that in the inventive steel, the content of Al, Cr and V satisfies the formula of 0.4 wt%? Al + Cr + V? 1 wt% Can be secured.

Therefore, even if the amount of C added is reduced through inventive steels, the desired tensile strength and elongation can be obtained by controlling the contents of Al, Cr and V, and it can be confirmed that the addition of a small amount of Mo also secures the plating property.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. The scope of the technical protection shall be determined by the technical idea of the appended claims.

FIG. 1 is a graph showing yield strength (YS), tensile strength (TS) and elongation percentage (EI) according to annealing temperature of invention steel 2 according to an embodiment of the present invention.

Claims (7)

delete delete delete 0.1 to 0.3 wt% of Si, 2.0 to 2.8 wt% of Mn, more than 0 to 0.006 wt% of S, 0 to 0.01 wt% or less of P, and 0.2 to 0.5 wt% of Al, a steel slab having an alloy composition of at least 0.1 wt%, Cr at 0.2 to 0.4 wt%, V at 0.01 to 0.10 wt%, Mo at 0.01 to 0.05 wt%, N at more than 0 and at most 0.006 wt%, balance Fe and other unavoidable impurities A homogenizing treatment is performed at a temperature of 1200 占 폚 or higher, hot rolling is performed at a temperature of 880 to 920 占 폚, rolled up, cold rolled at a reduction ratio of 55% or more, annealed, and then hot dip galvanized. The method of claim 4, Wherein the annealing is performed by maintaining the cold-rolled steel sheet at a temperature of 785 to 815 캜 for 5 to 120 seconds and then cooling the steel sheet at a cooling rate of 5 to 50 캜 / sec to a temperature of 480 캜 or less . The method of claim 4, Wherein the hot dip galvanizing treatment is a process of immersing the steel sheet in a plating bath at a temperature of 450 to 480 DEG C and cooling the steel sheet to 250 DEG C or less at a cooling rate of 20 to 50 DEG C / sec or more after the hot dip galvanizing treatment. ≪ / RTI > The method of claim 6, After the hot-dip galvanizing treatment, the hot-dip galvanized steel sheet is reheated to a temperature of 500 to 520 ° C to alloy the hot-dip galvanized film and cooled to 250 ° C or less at a cooling rate of 20 to 50 ° C / sec. Way.

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