KR100723157B1 - Steel sheet having ultra-high strength and excellent corrosion resistance after hot press forming and the method for manufacturing thereof - Google Patents

Abstract

Provided are a hot press sheet formed by a hot press method, which is mainly used as a structural member or reinforcement of an automobile body, and a hot forming member using the same, and a method of manufacturing the same.

The steel sheet for hot forming is, in weight%, C: 0.1-0.5%, Si: 0.01-1.0%, Mn: 0.5-4.0%, P: 0.1% or less, S: 0.03% or less, soluble Al: 0.1% or less , N: 0.01 ~ 0.1%, W: 0.1% or less, remaining Fe and other unavoidable impurities, and has a plated layer of Al-Si in the range of 40-80g / m ² on a one-side basis. The hot forming member is a hot forming of the steel sheet, and has an Al-Si-Fe-based film having a martensite structure fraction of 80% or more and a Si content of 4.5-8.4% in the surface layer.

According to the present invention, the hot forming member has an ultra high strength of 1470 MPa or more and a yield strength of 120 MPa or more after coating is improved, and the surface appearance, coating adhesion, and corrosion resistance are excellent.

Hot Press, Automobile Structural Member, Ultra High Strength, N, Al Plating

Description

Steel sheet having ultra-high strength and excellent corrosion resistance after hot press forming and the method for manufacturing technique}

Korean Laid-Open Patent Publication 2005-062194

Korean Unexamined Patent Publication No. 2003-049731

Japanese Laid-Open Patent Publication 2005-126733

Japanese Laid-Open Patent Publication 2003-034854

The present invention relates to a hot forming steel sheet mainly used as a structural member or reinforcement of an automobile body, a hot forming member using the same and a method of manufacturing the same. More specifically, the present invention relates to a hot forming steel sheet having an ultra high strength of 1470 MPa or more after hot forming and a yield strength of 120 MPa or more after coating, and excellent surface appearance, coating adhesion, and corrosion resistance after coating, a hot forming member using the same, and a method of manufacturing the same. .

In recent years, as safety regulations of automobile passengers have spread, studies on lightweighting the vehicle body and high strength steel sheets accordingly have been conducted to improve impact resistance of the vehicle body. However, increasing the strength of automotive steel sheets has a problem of significantly lowering the formability of steel sheets.

In order to solve such a problem, a method of manufacturing a high strength steel sheet having excellent formability is disclosed in Korea Patent Laid-Open Publication No. 2005-062194. The prior art is a TRIP (Transformation Induced Plasticity) steel sheet using the martensite transformation of the retained austenite, it is possible to manufacture a steel sheet excellent in formability of 980MPa class tensile strength. However, in order to secure more strength, elements such as C and Mn must be added, and thus, manufacturing cost problems occur, and shape freezing inferiority due to high strength and mold damage due to high strength when forming ultra high strength steel sheet A manufacturing problem occurs.

In order to solve this problem, techniques using hot forming technology have been proposed. For example, in Korean Laid-Open Patent Publication No. 2003-049731, after performing heat treatment and press molding in an austenitic single-phase zone using low strength and high workability before heat treatment, ultra-high strength cold rolled steel sheet in a final product by fast cooling by a mold A manufacturing method for obtaining is proposed. In addition, Japanese Laid-Open Patent Publication No. 2005-126733 manufactures a hot press steel sheet having excellent high temperature workability by adding Mo and Nb alone or in combination.

The prior art using the hot forming method focuses on improving tensile strength after hot forming, and has a technical limitation on securing a shock characteristic by increasing yield strength after coating.

On the other hand, Japanese Laid-Open Patent Publication No. 2003-034854 proposes a hot forming member having a Fe—Al-based coating containing more than 0.1% of Cr and Mn as a whole. In this Fe-Al coating, the structure change of the coating layer is induced by Mn or Cr to improve corrosion resistance. In order to obtain such a coating layer, the viscosity of the bath surface is increased due to the addition of Cr to the plating bath, and it is difficult to adjust a constant concentration, and thus there is a problem that normal high-speed operation is impossible.

The present invention is to improve the above-described problems, and after processing into a product of a complex shape at high temperature to perform a rapid cooling to secure a high tensile strength, after the heat treatment of paint to raise a high yield strength, excellent impact characteristics, The present invention provides a hot formed steel sheet having excellent surface appearance, coating adhesion, and corrosion resistance after coating, a hot forming member using the same, and a method of manufacturing the same.

Hot forming steel sheet of the present invention for achieving the above object,

By weight%, C: 0.1-0.5%, Si: 0.01-1.0%, Mn: 0.5-4.0%, P: 0.1% or less, S: 0.03% or less, soluble Al: 0.1% or less, N: 0.01-0.1% , W: 0.1% or less, remaining Fe and other unavoidable impurities, and has a plated layer of Al-Si in the range of 40-80 g / m ² on a one-side basis.

The hot forming member of the present invention is a hot forming of the hot forming steel sheet, an Al-Si-Fe-based film having a martensite structure fraction of 80% or more and a Si content of 4.5-8.4% in the surface layer. To have.

In the present invention, the Si content is preferably 5.9-8.0% in the Al-Si-Fe-based coating surface layer.

In the hot forming steel sheet and the hot forming member of the present invention, at least one of Mo or Cr may be added in an amount of 0.01 to 1.5%. Alternatively, at least one of Ti, Nb, or V may be added in an amount of 0.001 to 0.1%. Alternatively, one or more of Cu: 0.005 to 1.0% and Ni: 0.005 to 2.0% may be added. Or in addition, B: 0.001-0.01% may be added.

The manufacturing method of the hot forming member of this invention,

By weight%, C: 0.1-0.5%, Si: 0.01-1.0%, Mn: 0.5-4.0%, P: 0.1% or less, S: 0.03% or less, soluble Al: 0.1% or less, N: 0.01-0.1% , W: 0.1% or less, the balance of Fe and other unavoidable reheating the steel slab which is the composition as an impurity in the 1100 ~ 1300 ℃ by hot as a condition of finish rolling in a range from 1000 ℃ Ar ₃ transformation point or rolling, and coiling at 500 ~ 750 ℃ then Pickling, cold rolling and

Al-Si plating the cold rolled sheet in a range of 40-80 g / m ² on a single side in a plating bath including Si: 8-10%, remaining Al and other unavoidable impurities,

And heating the Al-Si plated steel sheet at a rate of 1 to 100 ° C./second in a temperature range of 800 to 1000 ° C., maintaining 10 to 1000 seconds, and hot forming and quenching at a speed of 10 to 500 ° C./second.

It has an Al-Si-Fe type | system | group coating whose martensite structure fraction is 80% or more and whose content of Si is 4.5-8.4% in a surface layer.

In the present invention, the cold rolled plate may be continuously annealed at a temperature of 700 ~ 900 ℃ before plating.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

In the present invention, the term 'hot forming member' refers to a molded product hot formed into a steel sheet. Such moldings may be used as automobile structural members or reinforcements, but are not limited thereto.

The present invention is derived from the research process for improving the mechanical and surface properties of the hot formed member.

(1) mechanical properties

The present invention is characterized by actively using W as an element for securing heat treatment hardenability to secure ultra high strength. In addition, the present invention was completed by paying attention to the fact that the design strength of the high-nitrogen in the W-added steel rapidly increases the yield strength after coating, thereby ensuring the impact resistance characteristics.

(2) Surface characteristics (corrosion resistance, film adhesion, surface appearance)

In the present invention, there is a feature to improve the surface characteristics by controlling the content of Si in the surface layer of the Al-Si-Fe-based coating film for the hot forming member of the W-added high nitrogen component system. That is, when the film is formed on the steel sheet using the Al-Si plating bath, the surface properties are improved by the content of Si in the surface layer of the film without addition of an alloying element to the plating bath.

First, the component of the hot forming member of this invention is demonstrated.

C is preferably 0.1 to 0.5%.

C is an essential element for increasing the strength of the steel sheet. In order to generate hard phases such as austenite and martensite, and to obtain a strength of 1470 MPa or more, it is preferable to add C content of 0.1% or more. If the content is less than 0.1%, it is difficult to secure the target strength even if heat treatment is performed in the austenitic single phase region. Moreover, when it exceeds 0.5%, the possibility of the fall of toughness and weldability becomes high. In addition, the steel sheet may not only be difficult to weld in the pickling and rolling process of the hot rolled steel sheet, but may also cause problems in the manufacturing process, such as significantly increasing the strength of the steel sheet in the annealing and plating process, and degrading the sheet flowability of the steel sheet.

The content of Si is preferably 0.01 to 1.0%.

Si contributes to the strength increase of the steel sheet as a solid solution strengthening element. If the content is less than 0.01%, it is difficult to remove the surface scale of the hot rolled steel sheet. In addition, when the content is more than 1.0% can increase the manufacturing cost. More preferable Si content is 0.051-0.5%.

The content of Mn is preferably 0.5 to 4.0%.

Mn is a solid solution strengthening element and contributes greatly to the strength increase, and plays an important role in delaying the transformation of austenite to ferrite. If the content is less than 0.5%, in order to heat-treat the steel sheet in the austenitic single-phase zone, a high heat treatment temperature is required, which accelerates the oxidation of the steel sheet and affects the corrosion resistance even when the plated steel sheet is used. Moreover, it is difficult to ensure the target 1470 Mpa or more by ferrite and austenite abnormal reverse heat processing. On the other hand, when it exceeds 4.0%, there is a concern that problems such as weldability and hot rolling property may occur. More preferred Mn content is 0.5-2.0%.

The content of P is preferably 0.1% or less.

P exhibits the effect of strengthening the steel, but it is preferable to limit it to 0.1% or less, because workability may be degraded due to excessive P addition.

The content of S is preferably 003% or less.

Since S is a high impurity element in steel, it is highly desirable to limit the ductility and weldability of the steel sheet to 0.03% or less.

The content of soluble Al is preferably 0.1% or less.

Soluble Al is an element added for deoxidation of steel, and is managed to 0.1% or less for this purpose. When the Al content is more than 0.1%, the inclusions such as alumina are excessively formed to form AlN, so that the yield strength is not increased due to the decrease of solid solution N.

The content of N is preferably 0.01 to 0.1%.

N is a very important component in the present invention. N is a solid solution strengthening element and combines with Ti, Nb, Al and the like to form nitride to increase yield strength. In addition, in the present invention, sufficient N is added for the heat treatment characteristics and the yield strength after coating, which increases the yield point by hindering the shift of dislocation after coating while N remains as solid solution N in the grain before coating. It acts as a major factor in ascension. When the content of N is less than 0.01%, it is difficult to expect such an effect. If it exceeds 0.1%, not only the steel sheet may be difficult to dissolve and play, but also may cause problems in manufacturing processes such as workability degradation or blow holes during welding. Preferably the content of N is 0.011% to 0.1%. More preferably, the content of N is made 0.02 to 0.1%.

The content of W is preferably 0.1% or less.

W is an element which improves the heat treatment hardenability of the steel sheet, and W is a very important element in the present invention because the W-containing precipitate acts advantageously to secure the strength. When the content of W exceeds 0.1%, not only this effect is saturated, but there is a problem in that the manufacturing cost increases.

In addition to the steel formed as described above, one or two or more selected from Mo, Cr, Ti, Nb, V, Cu, and Ni may be added. Mo and Cr are hardenability enhancing elements, Ti, Nb and V are precipitation enhancing elements, and Cu and Ni are strength enhancing elements, which will be described in more detail.

At least one selected from Mo and Cr: 0.01 to 1.5% is preferable.

The Mo and Cr not only increases the hardenability but also increases the toughness of the heat-treated steel sheet, so that the effect of Mo and Cr is very high when added to a steel sheet characterized by high impact energy absorption. In addition, since the hardenability is improved, the strength reduction of the portion not directly contacting the mold during high temperature molding processing can be prevented. If the content of Mo or Cr is less than 0.01%, sufficient hardenability cannot be obtained. Even if the amount of the addition is continuously increased, the hardenability does not increase significantly. Therefore, it is preferable to limit the amount to 1.5% because it greatly increases the manufacturing cost required for steel sheet production. Do. More preferred Mo. The content of Cr is each 0.01-0.5%.

1 or more types chosen from Ti, Nb, and V: 0.001 to 0.1% is preferable.

Ti, Nb, and V are elements that improve strength, refinement of grain size, and heat treatment characteristics of a steel sheet. When the content of Ti, Nb and V is less than 0.001%, such an effect cannot be obtained. In addition, if the content exceeds 0.1%, it is difficult to secure the target strength and yield strength due to an increase in manufacturing cost and excessive carbon and nitride production.

At least one selected from Cu: 0.005 to 1.0% or Ni: 0.005 to 2.0% is preferred.

The Cu is an element that produces fine Cu precipitates to improve strength. If the content of Cu is less than 0.005%, sufficient strength cannot be obtained, and if the content of Cu is more than 1.0%, it is accompanied by deterioration of workability, it is preferable to limit the content to 0.005 to 1.0%.

Ni is an element that improves strength and heat treatment characteristics. When the content of Ni is less than 0.005%, the effect is hardly exhibited, and when the content of Ni is more than 2.0%, an increase in manufacturing cost and deterioration of workability may occur.

The content of B is preferably 0.0001 to 0.01%.

B is an element having a very high hardenability, and even if a small amount is added, high strength can be ensured in the heat-treated steel. If the content of B is less than 0.0001%, sufficient hardenability cannot be obtained, and even if the amount thereof is continuously increased, the hardenability does not increase significantly, resulting in deterioration of hot workability.

Steel having a component system as described above may be used as a hot rolled steel sheet, a cold rolled steel sheet, a cold rolled annealed steel sheet, a plated steel sheet, or a hot forming member or a coating treatment hot forming member. In the present invention, the Al-Si plated hot forming steel sheet and the plated layer is formed of Al-Si-Fe film during the hot forming process, and the hot-forming member in which the Si content is controlled in the surface layer of the film.

It is preferable that the steel sheet for hot forming of the present invention has a plated layer of Al-Si in the range of 40-80 g / m ² on one surface basis at the surface layer thereof. The plating amount of Al-Si affects the content of Si in the surface layer of the Al-Si-Fe film in the hot formed member.

The hot formed member of the present invention preferably has a martensite structure fraction of 80% or more. If the martensite fraction is less than 80%, the target high tensile strength cannot be secured. More preferably, the fraction of martensite is 90% or more. Accordingly, the yield strength of the member is increased by 120 MPa or more when the member is baked by baking.

The hot formed member of the present invention has an Al-Si-Fe-based coating having a Si content of 4.5-8.4% in the surface layer. In the present invention, the surface layer means an area within about 5 μm of the surface of the coating. When Al-Si is plated on a steel sheet and hot formed as a member, the film composition has an Al-Si-Fe film by Fe diffusion in the steel sheet. At this time, when the content of Si in the surface layer is 4.5-8.4%, the surface properties are improved. The properties are best when the Si in the surface layer diffuses into the alloy layer so that the Si content in the surface layer is 5.9-8.0%.

Hereinafter, the manufacturing method of the hot forming member of this invention is demonstrated.

The steel slab that satisfies the steel component system according to the present invention is reheated at 1100-1300 ° C. When the reheating temperature is less than 1100 ° C., tissue homogenization and re-use of Ti, Nb, etc. are not sufficient. When the reheating temperature exceeds 1300 ° C., the steel sheet structure is easily coarsened, and manufacturing problems are likely to occur.

The steel slab reheated as described above is hot rolled under the condition of finishing rolling at an Ar ₃ transformation point or more and 1000 ° C. or less. If the finish rolling temperature is below the Ar ₃ transformation point, the hot deformation resistance is likely to increase rapidly, and manufacturing problems may occur. When the finish rolling temperature exceeds 1000 ° C., not only an excessively thick oxidation scale occurs but also a steel sheet is likely to coarsen.

After hot rolling as described above, it is wound up at 500 ~ 750 ℃. When the coiling temperature is less than 500 ℃ excessive martensite or bainite is produced to cause an excessive increase in strength of the hot rolled steel sheet may cause a manufacturing problem such as a shape defect due to the load during cold rolling. On the other hand, when it exceeds 750 ℃ excessive precipitates such as Ti, Nb, Mo may be coarse.

Thereafter, the wound hot rolled sheet is pickled and cold rolled at a reduction ratio of 30 to 80%. When the cold reduction rate is less than 30%, it is difficult to secure a target thickness and shape correction of the steel sheet is difficult. When the cold rolling reduction exceeds 80%, there is a high possibility that cracks in the steel sheet edge part occur, and there is a problem of bringing cold rolling load.

The cold rolled steel sheet may be continuously annealed to form a film. The continuous annealing is preferably 700 to 900 캜. If the annealing temperature is less than 700 ℃ tends to not secure sufficient processability, if the annealing temperature exceeds 900 ℃ is likely to increase the manufacturing cost and surface quality deterioration.

Formation of the coating is not particularly limited, such as hot dip plating, electrolytic plating, vacuum deposition plating, and cladding method, but hot dip coating is preferable in terms of productivity.

The component of the film is preferably Al-Si, and in the case of hot dip plating, the plating composition is Si: 8-10% of the remaining Al and other unavoidable impurities. Fe is an inevitable impurity.

The steel sheet on which the film is formed is hot formed. For example, the temperature of the temperature range of 800 ~ 1000 ℃ at a rate of 1 ~ 100 ℃ / sec, and maintained for 10 to 1000 seconds, then hot forming in a mold, and then quenching at a rate of 10 ~ 500 ℃ / second will be.

 When the heat treatment temperature is less than 800 ℃ sufficient austenite is not produced, so that sufficient martensite is not produced after hot forming, it is difficult to secure the target strength. On the other hand, when it exceeds 1000 degreeC, a manufacturing cost rises and austenite is likely to coarsen. In addition, when the temperature increase rate is less than 1 ° C / sec, the manufacturing efficiency tends to fall, and when it exceeds 100 ° C / sec, excessive manufacturing equipment is required. In addition, when the heat treatment time is less than 10 seconds, the austenite transformation is not sufficient, while when the heat treatment time exceeds 1000 seconds, an increase in manufacturing cost and coarsening of austenite are likely to occur. In addition, when the cooling rate is less than 10 ° C / sec, it is difficult to obtain a structure composed of martensite as a main phase, and thus it is difficult to secure the target strength. On the other hand, when it exceeds 500 ° C / sec, it is preferable to limit the manufacturing cost to 10 to 500 ° C / sec because the manufacturing cost increases due to excessive manufacturing equipment investment and the strength does not increase significantly. The hot formed part has a characteristic that the yield strength change rate increases by 120 MPa or more during baking after coating.

According to the present invention, when a steel sheet on which an Al-Si film is formed is hot formed, Fe of the steel sheet diffuses into the plating layer coating, and Si existing at the base iron / plating layer interface diffuses into the coating layer coating. The content of surface layer Si in such a film is preferably 4.5-8.4%.

Hereinafter, the present invention will be described in more detail with reference to Examples.

EXAMPLE

The steel slab formed as shown in Table 1 was vacuum-dissolved, heated in a reheating temperature of 1150 ~ 1250 ° C for 1 hour in a heating furnace and wound up after hot rolling. At this time, the hot rolling was finished hot rolling in the temperature range of 850 ~ 950 ℃, the winding temperature was set to 650 ℃. Pickling was performed using a hot rolled steel sheet and cold rolling was performed at a cold rolling reduction of 50%. The cold rolled steel sheet was annealed at 800 ° C. and then subjected to continuous annealing with an overage temperature of 400 ° C.

In addition, Al plating was produced as follows. The degreased cold rolled steel sheet was preheated to 700 ° C. in an anoxic furnace, and then heat treated in a nitrogen-hydrogen reducing atmosphere, and the maximum temperature was 820 ° C. After the heat treatment, the cooled steel plate was deposited and plated in a plating bath maintained at 680 ° C. At this time, the composition of the plating bath was Al-8.5% Si-2.4% Fe, and gas wiping treatment was performed to secure the adhesion amount of 25 ~ 30 microns per one side, and then the spangle adjusting device was operated to make the surface of Al plated steel sheet. The sequins were zeroed.

In order to examine the material after the high-temperature processing heat treatment, the cold rolled and plated steel sheets manufactured as described above were heated at a rate of 10 ° C./sec to 850 to 950 ° C. and heated for 5 minutes, and then transferred to a mold to process the heated steel sheets. Processing was carried out in the state. The steel sheet was quenched at a rate of -80 ° C / sec by the mold at the same time as processing at high temperature. And after the end of the processing was cut to the portion that can be cut tensile specimens to prepare a JIS No. 5 tensile test piece. In order to simulate the material after painting in the structural member for automobiles, the tensile specimens prepared as described above were boiled in oil at 170 ° C. for 20 minutes and then subjected to a tensile test. Tensile tests were carried out using a universal tensile tester.

In addition, the Al-coated steel sheet prepared as described above was heated at 800 to 900 ° C. for 5 minutes in order to investigate corrosion resistance after coating on specimens subjected to high temperature heat treatment, and then water cooled. After phosphate treatment and automotive cation type electrodeposition coating, the steel sheet was cut into cross cuts to a certain depth. Corrosion test was performed with salt spray test and cyclic corrosion test. After 150 cycles with 1 cycle of 8 hours, the blister width of the coating was measured to determine the corrosion resistance.

division Chemical composition (% by weight) Remarks C Si Mn P S Al N W Etc A1 0.24 0.28 1.07 0.012 0.002 0.036 0.03 0.051 B: 0.0018 Ti: 0.023 Invention steel A2 0.23 0.26 1.02 0.012 0.002 0.021 0.03 0.050 B: 0.0020, Mo: 0.05 Invention steel A3 0.24 0.28 1.07 0.012 0.002 0.036 0.0172 0.051 B: 0.0018 Invention steel A4 0.23 0.26 1.02 0.012 0.002 0.021 0.0136 0.050 B: 0.0020 Cr: 0.06, Mo: 0.05 Invention steel A5 0.22 0.27 2.37 0.012 0.002 0.042 0.0160 0.050 Ti: 0.023, V: 0.03 Invention steel A6 0.31 0.62 0.72 0.013 0.002 0.044 0.0140 0.023 B: 0.0011 Cr: 1.19, Mo: 0.67, Ni: 0.06 Invention steel A7 0.23 0.22 1.23 0.011 0.003 0.050 0.0154 0.185 B: 0.0018 Nb: 0.02, Cu: 0.05 Invention steel A8 0.27 0.15 1.36 0.015 0.006 0.067 0.0115 0.035 - Invention steel A9 0.062 0.112 2.519 0.011 0.002 0.036 0.004 0.021 Cr: 1.01 Comparative steel A10 0.065 0.125 3.015 0.012 0.002 0.0254 0.004 - B: 0.0005 Cr: 0.97 Comparative steel A11 0.150 0.229 2.504 0.011 0.003 0.040 0.004 - B: 0.0018 Cr: 0.2, Ti: 0.02 Comparative steel

division Hot Formed Member Tissue Mechanical properties Remarks Martensite fraction Other organizations YS (MPa) TS (MPa) El (%) △ YS (MPs) A1 99 Bainite 982 1492 7.9 186 Invention steel A2 99 Bainite 1035 1521 7.4 175 Invention steel A3 99 Bainite 982 1492 7.9 186 Invention steel A4 99 Bainite 1035 1521 7.4 175 Invention steel A5 99 Bainite 1125 1598 5.3 145 Invention steel A6 100 none 1215 1798 6.5 125 Invention steel A7 99 Bainite 1105 1511 6.5 144 Invention steel A8 98 Bainite 1026 1512 7.2 106 Invention steel A9 35 Ferrite, bainite 925 1178 7.4 41 Comparative steel A10 32 Ferrite, bainite 915 1145 6.2 36 Comparative steel A11 73 Bainite 1036 1518 3.4 35 Comparative steel  DYS: Yield strength change before and after painting

As shown in Table 2, the inventive steels A1 to A8 satisfying the component range of the present invention exhibit ultra high strength of 1470 MPa or more in tensile strength in the hot formed member. In addition, since the steel sheet is processed in a heated state at a high temperature, a product having a complicated shape can be processed. In addition, the yield strength after the simulation for 20 minutes at 170 ℃ can be used as a structural member and reinforcement of the vehicle body with excellent impact characteristics as all the strength increases to 120MPa or more.

On the contrary, comparative steel A9 and comparative steel A10 have low carbon content, so they do not obtain sufficiently strong martensite structure after heat treatment and mold processing, so that not only high tensile strength can be obtained in hot formed members but also yield strength after coating This is lacking. In addition, comparative steel A11 can secure sufficient tensile strength, but lack of yield strength increase.

Steel grade Deposition amount (g / m2) Surface appearance Coating Film Adhesion Corrosion resistance after painting Surface layer composition (%) Si Fe A1 20 ○ ○ △ 8.5 42.9 A1 40 ◎ ◎ ◎ 7.2 50.1 A1 50 ◎ ◎ ◎ 7.0 50.8 A1 60 ◎ ◎ ◎ 6.4 40.4 A1 80 ◎ ◎ ◎ 6.6 40.2 A1 100 ◎ △ △ 4.1 30.3 A2 20 ○ △ △ 8.8 38.7 A2 40 ◎ ◎ ◎ 8.0 54.4 A2 50 ◎ ◎ ◎ 7.8 52.2 A2 60 ◎ ◎ ◎ 6.9 48.3 A2 80 ◎ ◎ ◎ 5.9 44.1 A2 100 ○ △ △ 4.4 29.9 A3 50 ◎ ◎ ◎ 7.0 47.8 A4 40 ◎ ◎ ◎ 7.2 50.1 A5 50 ◎ ◎ ◎ 7.0 50.8 A6 40 ◎ ◎ ◎ 8.0 54.4 A7 60 ◎ ◎ ◎ 6.4 40.4 A8 50 ◎ ◎ ◎ 7.0 52.4

As shown in Table 3, the invention steel (A1-A8) can be seen that when the plating deposition amount is 40 g / m ² or less, the crystal after the phosphate treatment is not clear and the Si content of the surface layer portion is high, so the corrosion resistance is deteriorated. In addition, it was found that the surface appearance is good at 80 g / m ² or more, but because the Si content of the surface layer is small, the coating film adhesion is poor and the corrosion resistance after coating is deteriorated.

As described above, according to the present invention, it is possible to secure ultra high strength of 1470MPa or more, excellent heat resistance after heat treatment to secure the surface appearance without oxidizing scale, excellent corrosion resistance after coating, and higher yield strength of 120MPa or more after coating Due to the excellent impact characteristics, when applied as a structural member or reinforcement of the vehicle has the advantage that can greatly improve the weight reduction and collision characteristics of the vehicle body.

Claims (11)

In a steel sheet and a steel sheet for hot forming member having an Al-Si plated layer on the steel sheet, The steel sheet is in weight%, C: 0.1-0.5%, Si: 0.01-1.0%, Mn: 0.5-4.0%, P: 0.1% or less, S: 0.03% or less, soluble Al: 0.1% or less, N: 0.01 ~ 0.1%, W: 0.1% or less, the remaining Fe and other unavoidable impurities, the plated layer is ultra-high strength hot-formed steel sheet for excellent corrosion resistance after coating to be in the range of 40-80g / m ² on a single side basis. In a hot forming member having a member and an Al-Si-Fe-based coating on the member, The member is in weight%, C: 0.1-0.5%, Si: 0.01-1.0%, Mn: 0.5-4.0%, P: 0.1% or less, S: 0.03% or less, soluble Al: 0.1% or less, N: 0.01 ~ 0.1%, W: 0.1% or less, consisting of the remaining Fe and other unavoidable impurities, the martensite structure fraction is 80% or more, The coating is a super high strength hot forming member excellent in corrosion resistance after coating, the content of Si in the surface layer is 4.5-8.4%. The ultra-high strength hot forming member having excellent corrosion resistance after coating, further comprising 0.01 to 1.5% of Mo or Cr. The ultra-high strength hot forming member having excellent corrosion resistance after coating, according to claim 2, wherein at least one of Ti, Nb and V is added in an amount of 0.001 to 0.1%. The ultra-high strength hot forming member having excellent corrosion resistance after coating, according to claim 2, wherein at least one of Cu: 0.005 to 1.0% and Ni: 0.005 to 2.0% is added. The ultra-high strength hot forming member excellent in corrosion resistance after coating according to claim 2, wherein B: 0.001% to 0.01% is added. By weight%, C: 0.1-0.5%, Si: 0.01-1.0%, Mn: 0.5-4.0%, P: 0.1% or less, S: 0.03% or less, soluble Al: 0.1% or less, N: 0.01-0.1% , W: 0.1% or less, the balance of Fe and other unavoidable reheating the steel slab which is the composition as an impurity in the 1100 ~ 1300 ℃ by hot as a condition of finish rolling in a range from 1000 ℃ Ar ₃ transformation point or rolling, and coiling at 500 ~ 750 ℃ then Pickling, cold rolling and Continuously annealing the cold-rolled plate at a temperature of 700 ~ 900 ℃, Al-Si plating in the range of 40-80g / m ² on a one-side basis in a plating bath consisting of Si: 8-10%, the remaining Al and other unavoidable impurities. , And heating the Al-Si plated steel sheet at a rate of 1 to 100 ° C./second in a temperature range of 800 to 1000 ° C., maintaining 10 to 1000 seconds, and hot forming and quenching at a speed of 10 to 500 ° C./second. A method for producing an ultra high strength hot formed member having excellent corrosion resistance after coating having an Al-Si-Fe-based coating having a martensite structure fraction of 80% or more and a Si content of 4.5-8.4% in the surface layer. 8. The method for producing an ultra high strength hot formed member having excellent corrosion resistance after coating according to claim 7, wherein at least one of Mo or Cr is added at 0.01 to 1.5%. 8. The method for producing an ultra high strength hot formed member having excellent corrosion resistance after coating according to claim 7, wherein at least one of Ti, Nb or V is added in an amount of 0.001 to 0.1%. 8. The method for producing an ultra high strength hot formed member having excellent corrosion resistance after coating according to claim 7, wherein at least one of Cu: 0.005 to 1.0% and Ni: 0.005 to 2.0% is added. 8. The method for producing an ultra high strength hot formed member having excellent corrosion resistance after coating according to claim 7, wherein B: 0.001% to 0.01% is added.