KR20100025928A - High strength hot- dip galvanized steel sheet having excellent formability and galvanizing property, and method for producing the same - Google Patents

Abstract

PURPOSE: A high-strength hot-dip galvanized steel plate with excellent formability and galvanizing property is provided to reduce the total weight of a vehicle and increase fuel efficiency by increasing the content of retained austenite through hot rolling control technology and annealing heat treatment temperature control. CONSTITUTION: The high-strength hot-dip galvanized steel plate with excellent formability and galvanizing property comprises alloy composition and micro-structure. The alloy composition comprises carbon(C) 0.15~0.25 weight%, silicon(Si) 0.10~0.50 weight%, manganese(Mn) 1.50~2.50 weight%, aluminum(Al) 0.50~2.00 weight%, niobium(Nb) 0.10 weight% or less, titanium(Ti) 0.10 weight% or less, vanadium(V) 0.10 weight% or less, nickel(Ni) 0.50 weight% or less, phosphorus(P) 0.10 weight% or less, copper(Cu) 0.50 weight% or less, sulfur(S) 0.02 weight% or less, nitrogen(N) 0.01 weight% or less, rest iron(Fe), and impurities. The micro-structure has three phase structure of ferrite, bainite, and retained austenite.

Description

High strength hot dip galvanized steel sheet having excellent formability and galvanizing property, and method for producing the same

The present invention relates to an ultra-high strength hot-dip galvanized steel sheet and a method of manufacturing the same, and more particularly, to an ultra-high-strength hot-dip galvanized steel sheet for 780 to 1180 MPa class automotive structural members excellent in plating properties and workability.

The steel industry and the automotive industry are concentrating on environmental pollution, high strength and light weight, and are making efforts to increase body rigidity and improve fuel efficiency in order to satisfy the stricter regulations on safety and environmental regulations.

In particular, in recent years, the automobile industry is demanding steel having high strength and excellent plating property and formability as the design of automobiles and the needs of consumers are diversified.

For example, high-strength steel sheet using transformation organic plastic (TRIP) steel, dual phase steel (DP), complex phase steel (CP) is used for structural members such as automobile bodies.

These steels are mainly applied to parts that require high energy absorption in collisions such as members, pillars, bumper reinforcements, etc., and are processed using roll forming, which requires high elongation such as bending workability and molten zinc to prevent body corrosion. Plating is required.

Two-phase tissue steels and composite steels are optimized by adjusting the fractions of the phases such as martensite and bainite, which are obtained during rapid austenite quenching, in addition to ferrite and pearlite, which are basic structures, and trace alloys such as niobium (Nb) and titanium (Ti). Secure the tensile strength and yield strength by using precipitated phase made by adding element.

However, these steels increase the strength of martensite or precipitate to increase the strength, but the ferrite fraction decreases the ductility, so it is difficult to apply to parts requiring complex shapes such as press molding.

Accordingly, after forming austenite in the rolling process, the cooling rate and the cooling end temperature are controlled during the cooling process to partially retain the austenite at room temperature, and the residual austenite is transformed into martensite during plastic deformation. Mainly applied are metamorphic organic plastic steels which increase the ductility by mitigating the stress.

Since TRP steel has 5 ~ 10% higher ductility than two phase steel and composite steel, it is advantageous for machining complex parts and has high impact energy absorption.

However, in the case of strained organic plastic (TRIP) steel, various alloying elements are added to the material in order to improve the strength and formability. These additive elements are concentrated on the surface layer of the material, and the base material iron (Fe) and the plating element zinc (Zn) are added. Will affect the reaction of many.

In particular, the conventionally developed metamorphic organic plastic steels rapidly increase the content of manganese (Mn), silicon (Si), and carbon (C) to secure ultra high strength, and add a large amount of alloying elements to form precipitated phases. As the strength increases, Si-based oxides (Mn ₂ SiO _4, etc.) are formed on the surface of the material, so that an unplated layer is generated, and hot dip plating is absolutely disadvantageous.

 Therefore, there has been an attempt to reduce the content of C, Si, Mn and to satisfy the ultra high strength and formability by using a hot press forming process instead. However, this method also cannot be hot-dip galvanized, so it is difficult to develop a new plating method and to invest in expensive hot press forming equipment.

As described above, the development of ultra high strength steel sheet using metamorphic organic plastic steel involves many factors such as thermodynamic and metallurgical factors. Therefore, in order to satisfy the requirements of automobiles, early research is needed.

To this end, Korean Patent No. 10-2005-0032721 describes a method for manufacturing a general cold rolled steel sheet using a transformed organic plastic steel having a strength of 120 kg that increases the strength of the steel sheet by adding more than 0.6% of silicon and forms a constant ferrite in the final structure. Suggesting. However, the method has a problem that the manufacturing cost is increased compared to the molten plating material because the electroplating (EG: Electro Galvanized) is difficult to surface the hot dip plating.

In addition, Korean Patent No. 10-2005-0032721 proposes a method for manufacturing a general cold rolled steel sheet using a transformed organic plastic steel having a strength of 120 kg that adds 0.75% silicon and 4-7% manganese to secure residual austenite. have. However, this method also has a problem of difficult surface treatment of hot dip plating.

In addition, Japanese Patent No. 10-2005-7013049 controls nickel so as to satisfy operating conditions of Ni ≧ 1/5 × Si (5) + 1/10 × Al (%) for surface treatment of molten plating while containing silicon and aluminum. To improve the strength and plating performance is proposed. However, there is a problem that the increase in the content of expensive nickel leads to an increase in the manufacturing cost, which makes commercial production impossible.

The present invention is to solve the conventional problems as described above, the object of the present invention is to improve the plating properties and workability, while the plating properties to satisfy the tensile strength and high elongation of 780 ~ 1180MPa or more excellent zinc coating properties It is to provide an ultra high strength hot dip galvanized steel sheet excellent in workability and a method of manufacturing the same.

According to a feature of the present invention for achieving the above object, the present invention is a weight% of carbon (C) 0.15 to 0.25%, silicon (Si) 0.10 to 0.50%, manganese (Mn) 1.50 to 2.50%, aluminum (Al ) 0.50 to 2.00%, niobium (Nb) 0.10% or less, titanium (Ti) 0.10% or less, vanadium (V) 0.10% or less, nickel (Ni) 0.50% or less, phosphorus (P) 0.10% or less, copper (Cu) 0.50% or less, sulfur (S) 0.02% or less, nitrogen (N) 0.01% or less, and an alloy composition composed of remaining iron (Fe) and other unavoidable impurities, and the microstructure is a three-phase structure of ferrite, bainite, and retained austenite Has

The ferrite has a grain size of 10 μm or less and has a fraction of 30 to 50%, the residual austenite has a grain size of 5 μm or less and a fraction of 10 to 30%, and the bainite has a grain size of 10 μm. It has a fraction of 20 to 40% or less.

By weight%, carbon (C) 0.15 to 0.25%, silicon (Si) 0.10 to 0.50%, manganese (Mn) 1.50 to 2.50%, aluminum (Al) 0.50 to 2.00%, niobium (Nb) 0.10% or less, titanium (Ti) ) 0.10% or less, vanadium (V) 0.10% or less, nickel (Ni) 0.50% or less, phosphorus (P) 0.10% or less, copper (Cu) 0.50% or less, sulfur (S) 0.02% or less, nitrogen (N) 0.01 A steel slab having an alloy composition of less than% and the remaining iron (Fe) and other unavoidable impurities is homogenized at 1150 to 1250 ° C, hot rolled at Ar3 to Ar3 + 50 ° C, and wound up, and cold reduction rate is 30 to 70%. Cold-rolled between annealing and hot dip galvanizing.

After the hot rolling is quenched to a range of 750 ~ 850 ℃ at a cooling rate of 30 ~ 100 ℃ / sec and then air-cooled for 5 to 10 seconds, and 300 ~ 400 ℃ at a cooling rate of 30 ~ 100 ℃ / sec after the air cooling Quench to the extent and wind up.

After the hot rolling, it is rapidly cooled to 500 to 700 ° C at a cooling rate of 10 to 50 ° C / sec, and wound up.

The annealing is cooled to a temperature range of 400 ~ 500 ℃ at a cooling rate of 10 ~ 100 ℃ / sec after heat treatment of the cold-rolled steel sheet in the ferrite-austenite two-phase zone of Ar1 ~ Ar3.

The present invention is capable of hot-dip galvanizing by lowering the content of silicon forming oxides on the surface of steel sheet, and increasing the content of residual austenite through hot rolling control technology and annealing heat treatment temperature control to obtain tensile strength of 780 ~ 1180MPa grade. Manufacture cold rolled steel sheet or hot dip galvanized steel sheet having an elongation of more than%.

Therefore, it is possible to replace the conventional 440 ~ 590MPa class high strength steel sheet, and because the thickness of the steel sheet can be reduced due to the increase in strength, there is an effect that can contribute to increase the fuel efficiency by reducing the total weight of the vehicle.

In addition, when the molded part is applied to the automotive structural member, the moldability is excellent, the processing of the complicated part shape is easy, and due to the increased austenite fraction it can be expected to improve the impact energy absorbing capacity during the collision.

Hereinafter, a preferred embodiment of the ultra-high strength molten zinc plated steel sheet excellent in plating property and workability according to the present invention and a method of manufacturing the same will be described in detail.

In the present invention, by weight% of carbon (C) 0.15 to 0.25%, silicon (Si) 0.10 to 0.50%, manganese (Mn) 1.50 to 2.50%, aluminum (Al) 0.50 to 2.00%, niobium (Nb) 0.10% or less, Titanium (Ti) 0.10% or less, Vanadium (V) 0.10% or less, nickel (Ni) 0.50% or less, phosphorus (P) 0.10% or less, copper (Cu) 0.50% or less, sulfur (S) 0.02% or less, nitrogen ( N) 0.01% or less and the alloy composition of the remaining iron (Fe).

In more detail, by lowering the content of silicon forming an oxide on the surface of the steel sheet to secure plating properties, the reduced strength increases the content of aluminum to produce more than 10% of the residual austenite in the steel sheet structure to achieve ultra high strength. In addition, a small amount of niobium (Nb), titanium (Ti), vanadium (V), phosphorus (P), nickel (Ni), and copper (Cu), which are fine precipitates or solid solution to increase the strength, are added to the strength. 780 ~ 1180MPa grade super high strength cold rolled steel and hot dip galvanized steel sheet with excellent ductility balance.

The steel sheet has a three-phase structure consisting of ferrite, bainite, and retained austenite after hot rolling, cold rolling, and hot dip galvanizing to ensure elongation of 15% or more.

Three-phase structure is the increase of residual austenite content by hot rolling control technology and annealing heat treatment temperature control. In this case, the ferrite has a grain size of 10 to 30 µm and a fraction of 30 to 50%, and the retained austenite has a grain size of 5 to 5 µm and a fraction of 10 to 30%. In addition, bainite has a grain size of 10 to 40 µm and a fraction of 20 to 40%.

Ferrite is a microstructure associated with securing ductility, and if the fraction is less than 30%, it is difficult to secure ductility of the steel sheet.

Residual austenite is a microstructure related to plastic organic transformation, it is difficult to secure ultra high strength when less than 10%, and the higher the fraction is more than 10%, but less than 30% considering the fraction of ferrite and bainite.

Bainite is a microstructure related to toughness and strength, but less than 20% does not have a strength-contributing effect.

Hereinafter, the function and content of the alloying elements of the present invention will be described in detail.

Carbon (C): 0.15 ~ 0.25wt%

Carbon (C) is an indispensable element for imparting high strength to the steel sheet. The carbon (C) is thickened on the austenite upon annealing in an area of 780 ° C. or higher and stabilizes the austenite in the bainite transformation temperature range. In addition, the carbon (C) is diffused and moved to austenite in the ferrite to ensure residual austenite even after cooling to room temperature, thereby improving the formability by generating metamorphic organic plasticity during processing of the steel sheet.

When a small amount of carbon is added, austenite is transformed into ferrite, making it difficult to secure sufficient retained austenite, which makes it difficult to secure martensite fractions in the final structure, thereby deteriorating strength and elongation characteristics. Therefore, at least 0.15wt% should be added.

On the other hand, when carbon is excessively added, the weldability is lowered and the ductility and stretch-flange properties are reduced as the strength is increased, so the upper limit is limited to 0.25 wt%.

Silicon (Si): 0.10 ~ 0.50wt%

Silicon (Si) contributes to the cleansing of steel as a solid solution strengthening element, promotes carbon concentration of austenite and increases the stability of austenite so that austenite remains at room temperature.

When silicon is added to the steel to which the appropriate manganese (Mn) is added, it improves the flowability of molten metal during welding to reduce the inclusions in the weld as much as possible, and improves the strength without inhibiting the balance between yield ratio and strength and elongation. In addition, silicon slows the diffusion rate of carbon in the ferrite, inhibits the growth of carbides and stabilizes the ferrite to improve the elongation.

Silicon (Si) does not have an effect when the content is less than 0.10wt%, and when excessively added, it causes surface defects due to plating property and red scale, and causes problems such as unplating and plating peeling by degrading plating adhesion. Therefore, the upper limit thereof is limited to 0.50 wt%.

Manganese (Mn): 1.50-2.50 wt%

Manganese (Mn) is a solid solution strengthening element, which stabilizes austenite to lower the two-phase annealing temperature and prevents austenite from being decomposed into a pearlite phase (ferrite + cementite) even at a low critical cooling rate, thereby retaining austenite and martensite. Makes it easy to create

Manganese deteriorates the shape of the steel sheet due to the thermal stress generated due to the rapid cooling rate to obtain martensite when a small amount is added, and therefore more than 0.8 wt% is required.

 However, in order to obtain tensile strength of more than 780MPa, at least 1.50wt% must be added to obtain desired elongation and weldability. On the other hand, when added in excess of 2.50wt% manganese (Mn) band structure is formed in the center of the thickness during slab casting and segregation increases sharply to inhibit the workability and weldability of the steel. Therefore, the content of manganese (Mn) is preferably designed in the range of 1.50 ~ 2.50wt%.

Aluminum (Al): 0.50 ~ 2.00wt%

Aluminum (Al) is mainly used as a deoxidizer, but in the present invention, it is used as an alternative element of silicon (Si), which is an element that inhibits plating properties. Aluminum (Al), like silicon, inhibits pearlite formation and promotes ferrite formation and carbon (C) concentration (increase in concentration) in the austenite phase to promote residual austenite formation.

In addition, in addition to the deoxidation in the steelmaking process, aluminum (Al) combines with nitrogen (N) in the steel to precipitate AlN to refine crystal grains, thereby improving the strength of the steel sheet. In addition, it develops the aggregate structure to increase the formability of the steel sheet and keep the dissolved oxygen content in the steel sufficiently low to prevent cracking during slab manufacture.

When a small amount of aluminum is added, the oxygen content in the steel increases, leading to a decrease in ductility. Therefore, an amount of 0.01 wt% or more is required. However, addition of 0.50 wt% or more is essential for securing strength and residual austenite phase. On the other hand, when the content exceeds 2.00 wt%, the plating performance is inhibited like silicon, so the aluminum content is preferably regulated in the range of 0.50 to 2.00 wt%.

Niobium (Nb), Titanium (Ti), Vanadium (V): 0.10 wt% or less

Niobium (Nb), titanium (Ti) and vanadium (V) elements combine with carbon (C) or nitrogen (N) to precipitate in the form of NbC, NbN, TiC, TiN, VC, VN, or solid solution in iron (Fe). Elements that improve the strength of the steel sheet through reinforcement. When added below 0.10wt%, the elements can contribute to the improvement of strength without impairing the gist of the present invention. Therefore, in some cases, one or more of niobium, titanium, and vanadium may be further contained in the range of 0.10 wt% or less.

Nickel (Ni): 0.50 wt% or less

Nickel (Ni) is added as an element for preventing red brittleness generated when copper (Cu) is added to increase strength and improve corrosion resistance. Usually, copper (Cu): nickel (Ni) = 1: 1 to 2 is the best effect when added. When it is difficult to secure quality such as redness and brittleness by adjusting process variables when adding copper (Cu), it is added within the range of 0.5 wt% or less according to the content of copper (Cu).

Phosphorus (P): 0.10wt% or less

Phosphorus (P), like aluminum (Al), is added to inhibit the formation of cementite and to increase strength. If phosphorus content exceeds 0.10wt%, weldability is greatly deteriorated and there is a problem of material deviation caused by slab center segregation. Therefore, it is regulated to 0.10wt% or less.

Copper (Cu): 0.50 wt% or less

Copper (Cu), together with aluminum (Al), suppresses the precipitation of carbon in the bainite transformation zone and improves corrosion resistance along with the role of generating residual austenite. In addition, it has the effect of miniaturizing the ferrite grains has the function of increasing the strength. Copper has an elongation to be reduced below 0.5wt%, so it is regulated below 0.5wt%.

Sulfur (S): 0.02 wt% or less

Sulfur (S) is an element that is inevitably contained in the production of steel, inhibits toughness and weldability, and increases the sulfide-based (MnS) non-metallic inclusions, causing cracking and the like. In particular, sulfur is regulated in the range of 0.02wt% or less because it increases the coarse inclusions to deteriorate the fatigue characteristics when over-added.

Nitrogen (N): 0.01 wt% or less

Nitrogen is refined by the formation of AlN, but is limited to 0.01 wt% or less since it is supersaturated during cooling in the alloying process of the zinc plating layer during hot dip galvanizing to lower the elongation.

The present invention contains the components of the steel sheet, the remainder is substantially iron (Fe) and unavoidable elements, and the incorporation of fine amounts of inevitable impurities as the elements contained in accordance with the situation of raw materials, materials, manufacturing facilities, etc. is also allowed.

The slabs having the composition as described above are obtained by ingot or continuous casting process after obtaining molten steel through the steelmaking process, in this case hot-rolled, cold-rolled and manufactured in the form of steel sheet, hot-dip galvanized on the surface of the steel sheet The process is as follows.

Each process is as follows.

[Heating process]

The process of reheating the slab is to reclaim segregated components during casting. Reheat is heated to a temperature range of 1200 ± 50 ° C. This is because the low reheating temperature prevents segregation of the segregated components, while excessively high recrystallization increases the austenite grain size and decreases the strength as the grain size of the ferrite is coarsened.

[Hot Rolling Process]

The slab reheated in the furnace process finishes hot rolling in the Ar3 to Ar3 + 50 ° C temperature range. Hot rolling finish temperature is less than Ar3, the rolling load is increased, the productivity is reduced, and if the production cost is increased if it exceeds Ar3 + 50 ℃, it is carried out in the Ar3 ~ Ar3 + 50 ℃ temperature range.

After hot rolling, two methods of multistage cooling or general cooling can be performed up to the winding temperature.

In the case of multi-stage cooling, it is quenched to the range of 750 ~ 850 ℃ at the cooling rate of 30 ~ 100 ℃ / sec after hot rolling, and then air cooled for 5 ~ 10 seconds, and 300 ~ at the cooling rate of 30 ~ 100 ℃ / sec after air cooling. It adopts the method of winding rapidly by cooling to 400 ℃ range. Then, a part of the austenite is transformed into ferrite in the air cooling process, so that the final structure of the final hot rolled steel sheet is made of a two-phase structure of ferrite-martensite.

 Hot rolled steel sheet having a two-phase structure of ferrite-martensite has a high recrystallization rate during annealing, and austenite phase formation occurs uniformly in ferrite grain boundaries and near martensite phases. Make it even more uniform. Therefore, it is possible to distribute a uniform retained austenite phase than the general cooling in the final structure.

However, the residual austenite phase of 10% or more can be obtained even by general cooling, and may be cooled by any of the above two methods. In the case of general cooling, the method of winding rapidly after hot rolling to 500 ~ 700 ℃ at the cooling rate of 10 ~ 50 ℃ / sec is adopted.

[Cold rolling process]

Cold rolling is performed to obtain the final desired thickness of the steel sheet and to obtain the desired material. Cold rolling is performed at a reduction ratio of 30 to 70% at room temperature.

[Annealing process]

Hot-rolled Cold-rolled steel sheet is subjected to annealing heat treatment in the ferrite-austenite two-phase zone of Ar1 to Ar3 and cooled. Cooling is carried out in the temperature range of 400 ~ 500 ℃ at a cooling rate of 10 ~ 100 ℃ / sec. At this time, if the cooling rate is too slow, austenite is transformed into ferrite ferrite (cementite), bainite in the cooling process, and when the cooling rate is too fast, a problem of material unevenness occurs.

[Alloyed hot dip galvanizing or hot dip galvanizing process]

The annealed steel sheet is alloy heat treated at 460 ~ 580 ° C and hot-dip galvanized. If the alloying heat treatment temperature is higher than 580 ° C., a problem of steel sheet material degradation occurs, and if it is lower than 460 ° C., it is difficult to ensure stable growth of the alloying degree and the plating layer.

Hereinafter, the above-described ultra high strength molten zinc plated steel sheet and a method of manufacturing the same will be described in comparison with the invention examples and other comparative examples.

Table 1 shows the component ratios of the invention examples and other comparative examples, Table 2 shows the heat treatment and rolling conditions of the invention examples and comparative examples of Table 1, Table 3 is prepared by the conditions of Table 1 and Table 2 The mechanical properties of the specimens are measured.

division Chemical composition (wt%, balance Fe) Mneq C Si Mn P S Al Cu Nb Ti Ni N Comparative Example 1 0.10 0.55 2.48 0.011 0.002 - - 0.025 - - 0.0045 2.7 Comparative Example 2 0.14 1.61 2.51 0.016 0.002 - 0.64 - - 0.72 0.0024 3.0 Comparative Example 3 0.21 1.50 2.10 0.010 0.003 - - 0.024 0.053 - 0.0041 2.5 Inventive Example 1 0.20 0.51 2.14 0.010 0.002 1.07 - 0.025 - - 0.0042 2.3 Inventive Example 2 0.20 0.50 2.05 0.011 0.002 1.01 - 0.024 0.053 - 0.0042 2.2 Inventive Example 3 0.25 0.50 2.02 0.011 0.002 0.99 - 0.027 0.052 - 0.0049 2.2 Inventive Example 4 0.24 0.48 2.54 0.010 0.002 0.99 - 0.026 - - 0.0035 2.7 Inventive Example 5 0.20 0.50 2.03 0.064 0.001 0.99 - - - 0.11 0.0012 2.4 Inventive Example 6 0.20 0.50 2.09 0.011 0.002 0.98 0.21 - - 0.24 0.0044 2.3

[Mneq: Manganese equivalents (if manganese equivalents are similar alloys, see similar materials)]

division Hot Rolling Condition Cold rolling condition Annealing Heat Treatment Condition Heating temperature (℃) Rolling end temperature (℃) Cooling medium temperature (℃) Cooling temperature (℃) Rolling reduction (%) Annealing Temperature (℃) Overage temperature (℃) Plating temperature (℃) Comparative Example 1-1 1250 910 790 350 53 780 460 520 Comparative Example 1-2 1230 910 - 650 53 780 460 520 Comparative Example 2-1 1230 915 780 330 53 780 460 520 Comparative Example 2-2 1225 920 - 670 53 780 460 520 Comparative Example 3-1 1230 910 790 340 53 780 460 520 Comparative Example 3-2 1230 910 - 650 53 780 460 520 Inventive Example 1-1 1230 910 790 340 53 780 460 520 Inventive Example 1-2 1230 900 - 650 53 780 460 520 Inventive Example 2-1 1230 910 780 350 53 780 460 520 Inventive Example 2-2 1225 910 - 660 53 780 460 520 Inventive Example 3-1 1235 910 790 350 53 780 460 520 Inventive Example 3-2 1220 910 - 650 53 780 460 520 Inventive Example 4-1 1230 915 790 340 53 780 460 520 Inventive Example 4-2 1220 910 - 660 53 780 460 520 Inventive Example 5-1 1220 915 790 340 53 780 460 520 Inventive Example 5-2 1220 910 - 650 53 780 460 520 Inventive Example 6-1 1235 900 780 360 53 780 460 520 Inventive Example 6-2 1210 905 - 660 53 780 460 520

division Mechanical properties Yield strength (MPa) Tensile Strength (MPa) Elongation (%) Strength-ductility balance (TS * EL) Plating Comparative Example 1-1 630 962 14 13468 Good Comparative Example 1-2 550 893 14 12502 Good Comparative Example 2-1 683 1050 23 24150 Bad Comparative Example 2-2 700 996 24 23900 Bad Comparative Example 3-1 705 1090 21 22890 Bad Comparative Example 3-2 685 986 22 21691 Bad Inventive Example 1-1 720 1100 22 24200 Good Inventive Example 1-2 653 985 23 22664 Good Inventive Example 2-1 709 1090 23 25070 Good Inventive Example 2-2 665 984 24 23615 Good Inventive Example 3-1 790 1185 21 24885 Good Inventive Example 3-2 680 1001 22 22020 Good Inventive Example 4-1 782 1203 21 25263 Good Inventive Example 4-2 685 1057 23 24304 Good Inventive Example 5-1 720 1185 22 26070 Good Inventive Example 5-2 613 995 23 22880 Good Inventive Example 6-1 806 1201 22 26422 Good Inventive Example 6-2 643 1014 23 23330 Good

Table 3 is a slab prepared as shown in Table 1 and reheated at 1250 ℃ for 2 hours as shown in Table 2 and hot-rolled at 900 ℃ or more after performing the cooling in two ways to prepare a steel sheet through annealing and cold rolling The specimens were then collected and tested for strength, elongation and plating potential. (For reference, vanadium is a trace element selectively added as necessary.

Looking at Tables 1 to 3, as confirmed in Comparative Example 2 and Comparative Example 3, when the Si content is high, tensile strength, yield strength and elongation are secured to some extent, but it can be seen that the plating properties are poor.

In Comparative Example 1, the amount of Si was decreased to secure the plating property, and the content of Mn was increased to secure the strength lowered by the decrease of the content of Si. .

On the contrary, when looking at the invention example 6 in the invention example 1, only the Si content of Comparative Example 2 and Comparative Example 3 in the case of the present invention steel sheet reduced the content of Si and added Al and trace amounts of Nb, Ti, P, Ni, Cu Tensile strength is much better (1100MPa) and yield strength is superior to the increase. In addition, it can be confirmed that the plating property is secured by decreasing the amount of Si added.

In particular, in the case of the steel sheet of the present invention, when the multi-stage cooling is performed after hot rolling, the tensile strength and the yield strength are further improved. And the present plating property and elongation also satisfied the reference range.

The tensile strength condition of ultra high strength (780 ~ 1180MPa) by controlling the cooling method after the hot rolling and the addition of the trace element which increases the amount of Al, which is a substitute for Si, and produces a fine precipitated phase through the invention examples It can be seen that the steel sheet with improved plating property and formability while satisfying.

Within the scope of the basic technical idea of the present invention, many other modifications are possible to those skilled in the art, and the scope of the present invention should be interpreted based on the appended claims. will be.

Claims (6)

By weight%, carbon (C) 0.15 to 0.25%, silicon (Si) 0.10 to 0.50%, manganese (Mn) 1.50 to 2.50%, aluminum (Al) 0.50 to 2.00%, niobium (Nb) 0.10% or less, titanium (Ti) ) 0.10% or less, vanadium (V) 0.10% or less, nickel (Ni) 0.50% or less, phosphorus (P) 0.10% or less, copper (Cu) 0.50% or less, sulfur (S) 0.02% or less, nitrogen (N) 0.01 Has an alloy composition of less than% and the remaining iron (Fe) and other unavoidable impurities, Ultra-high strength molten zinc plated steel sheet, characterized in that the microstructure has a three-phase structure of ferrite, bainite, residual austenite. The method according to claim 1, The ferrite has a grain size of 10 μm or less and has a fraction of 30 to 50%, the residual austenite has a grain size of 5 μm or less and a fraction of 10 to 30%, and the bainite has a grain size of 10 μm. Ultra-high strength molten zinc plated steel sheet having a fraction of 20 to 40% or less. By weight%, carbon (C) 0.15 to 0.25%, silicon (Si) 0.10 to 0.50%, manganese (Mn) 1.50 to 2.50%, aluminum (Al) 0.50 to 2.00%, niobium (Nb) 0.10% or less, titanium (Ti) ) 0.10% or less, vanadium (V) 0.10% or less, nickel (Ni) 0.50% or less, phosphorus (P) 0.10% or less, copper (Cu) 0.50% or less, sulfur (S) 0.02% or less, nitrogen (N) 0.01 Steel slab with alloy composition of less than% and remaining iron (Fe) and other unavoidable impurities Homogenizing treatment at 1150 ~ 1250 ℃, hot rolling finish at Ar3 ~ Ar3 + 50 ℃, cold rolling between cold rolling rate of 30 ~ 70% and annealing for hot dip galvanizing Method for manufacturing molten zinc plated steel sheet. The method according to claim 3, After the hot rolling is quenched to a range of 750 ~ 850 ℃ at a cooling rate of 30 ~ 100 ℃ / sec and then air-cooled for 5 to 10 seconds, and 300 ~ 400 ℃ at a cooling rate of 30 ~ 100 ℃ / sec after the air cooling Method for producing a super high strength molten zinc galvanized steel sheet characterized by quenching to a range. The method according to claim 3, After the hot rolling, the method of manufacturing ultra-high strength molten zinc plated steel sheet, characterized in that the rapid cooling to 500 ~ 700 ℃ at a cooling rate of 10 ~ 50 ℃ / sec. The method according to any one of claims 3 to 5, The annealing is the ultra-high strength molten zinc, characterized in that the cold-rolled steel sheet is cooled in a temperature range of 400 ~ 500 ℃ at a cooling rate of 10 ~ 100 ℃ / sec after heat treatment in the ferritic austenite two-phase zone of Ar1 ~ Ar3 Method of manufacturing plated steel sheet.