KR101005392B1 - Method for manufacturing the ultra high strength steel parts with excellent corrosion resistance - Google Patents

Abstract

Provided is a method for producing an ultra high strength steel molded article having excellent corrosion resistance and workability. Ultra high strength steel molded body manufacturing method according to the invention is 100kg / mm ² Manufacturing a plated steel sheet having a Zn-Al-Mg-based or Zn-Al-Mg-Si-based plating layer formed on a surface of a cold rolled steel sheet (steel) having the following tensile strength; Heating the plated steel sheet to a temperature of 850 ° C to 950 ° C; And quenching the heated plated steel sheet into a desired shape in the mold while simultaneously quenching.

Plated Steel Sheet, Zn-Al-Mg, Zn-Al-Mg-Si, Quenching, Martensite

Description

Method for manufacturing the ultra high strength steel parts with excellent corrosion resistance}

The present invention relates to a method of manufacturing a high corrosion resistance ultra-high strength steel, to prepare a super-strength steel molded by using a plated steel sheet formed with a Zn-Mg-Al (ZAM) or Zn-Mg-Al-Si (ZAMS) -based plating layer It is about a method.

Recently, the application of high-strength steel sheet in consideration of collision safety in order to secure the safety of passengers and drivers has tended to increase gradually.

In addition, the application of the surface-treated steel sheet is increasing in order to meet the increase in the anti-rust warranty.

In particular, since the corrosion resistance of automotive steel sheets depends on the plating thickness, the demand for hot-dip galvanized steel sheets which can be plated at a low price is rapidly increasing.

In general, a method using high tensile steel to increase the strength of automotive parts is used, but 100Kg / mm ² The high-strength steel having the above tensile strength is not only difficult to form, but also has a strong springback characteristic to return to its original shape after molding, and thus it is very difficult to be applied to a complex shaped part.

In order to solve this problem, it has a low strength before molding, but after heating to a high temperature, which is easy to mold, high strength can be given in a state in which springback phenomenon is suppressed by rapidly marching in a mold to obtain martensite structure. There is active application of public law.

The materials applied to these methods can be divided into two categories.

The first is a cold rolled steel sheet having no plating layer. In the case of manufacturing ultra-high strength parts by applying cold rolled steel sheet to such a method, a thick surface oxide layer is generated during high temperature heating and molding, so post-processing such as pickling or shot blast is essential to remove this. It becomes indispensable.

This post-process not only deteriorates productivity, but also hardly gives excellent corrosion resistance to the manufactured parts, so it is very difficult to apply to parts requiring corrosion resistance.

In addition, since the thick oxide layer generated during the high temperature heating makes it difficult to transfer heat during cooling in the mold, there is a problem that it becomes difficult to manufacture a component having sufficient strength by slowing down the cooling rate.

Secondly, zinc-based and aluminum-based plated steel sheets may be cited for improving corrosion resistance. When the plated steel sheet is applied to this method to fabricate an ultra high strength component, the plated layer on the surface prevents oxidation of the base iron even after high temperature heating and molding.

In addition, the zinc-based compound is zinc oxide layer and aluminum-based plated steel sheet in the high temperature heating process, the aluminum oxide layer is thinly formed on the surface, thereby suppressing evaporation of the plating layer, thus making it possible to manufacture ultra-high strength parts that preserve the plating layer. Rather, it gives excellent corrosion resistance compared to cold rolled steel.

The plated layer of the plated steel sheet used to impart corrosion resistance includes a Zn-Fe alloy, a Zn-Mn alloy, and a Sn-8% Zn alloy.

Among them, plated steel sheet with Al-5% Si alloy layer is widely applied to mass production line, but in order to lower production cost and secure cost competitiveness, plated steel sheet with relatively low price Zn-Fe alloy layer is applied to mass production line. Efforts are being made to do this.

Parts manufactured using plated steel sheets with Al-5% Si and Zn-Fe alloy layers under application and review in the mass production line are superior to parts without oxide layer after forming using cold rolled steel sheets without plated layers. Corrosion resistance

However, in order to apply to parts used in more severe corrosion conditions, it is indispensable to manufacture parts with improved corrosion resistance.

In addition, considering the manufacturing cost, the application of Zn-based plated steel sheet should be considered first rather than Al-based plated steel sheet.

The present invention is to solve the problems as described above, after forming a steel sheet having a tensile strength of less than 100kg / mm ² (hereinafter referred to as the base iron) made of super-strength steel molding having a tensile strength of 140kg / mm ² or more To provide a way to do it.

In addition, by using a plated steel sheet formed with a Zn-Al-Mg or Zn-Al-Mg-Si-based plating layer on the surface of the base iron to ensure that the finally formed steel molded body has excellent corrosion resistance.

It is also an object of the present invention to provide a method of manufacturing an ultra-high strength steel molded body in which the surface of the plated steel sheet is protected by a plated layer even at high temperature, so that a thick oxide film is not formed, thereby improving cooling performance.

In addition, there are other passive objectives, such as providing ultra-high-strength steel moldings that can reduce the cost of manufacturing without the need for surface oxide removal devices such as pickling equipment or shot blasting equipment when forming ultra-high strength steel moldings. .

Zn-11% Al-3% Mg on the surface of the cold rolled steel sheet (steel) having a tensile strength of 100kg / mm ² or less according to the present invention as a means for solving the technical problem Manufacturing a plated steel sheet having a Zn-11% Al-3% Mg-0.2% Si-based plating layer formed thereon; Heating the plated steel sheet to a temperature of 850 ° C to 950 ° C; And quenching the heated plated steel sheet into a desired shape in the mold while simultaneously quenching.

And the ultra-high strength steel molded body produced by the method is characterized in that the tensile strength of 140Kg / mm ² or more.

In addition, the adhesion amount of the plating layer is characterized in that 30 to 90g / mm ² or more (cross section basis).

According to the ultra-high strength steel molded product manufacturing method according to the present invention by using a plated steel sheet formed with a Zn-Al-Mg or Zn-Al-Mg-Si-based coating layer is finally produced 140kg / mm at the same time excellent steel corrosion resistance There is an effect to improve the high tensile strength of ^two or more.

In addition, after heating to a high temperature of about 850 ~ 950 ℃, the molding is carried out in a high temperature state that is easy to mold, so that not only the complicated shape parts can be easily processed into the desired shape, but also the effect of the precise dimensions of the parts There is.

In addition, the use of such a method does not require surface oxide film removing devices such as pickling equipment or shot blasting equipment when forming ultra-high strength steel molding, thereby reducing the cost of manufacturing.

Hereinafter, with reference to the accompanying drawings will be described for the ultra-high strength steel molded article manufacturing method according to the present invention.

1 and 2 are a flow chart illustrating a method of forming an ultra high strength steel molded body according to the present invention, Figures 3a to 3e is a process chart for forming an ultra high strength steel molded body according to the present invention.

Here, FIGS. 3A to 3E and FIGS. 1 and 2 will be described while matching each other for easy description of the present invention.

As shown in Figure 1 and Figure 3a, to produce a super-high strength steel molding (hereinafter referred to as "steel molding", 10) according to the present invention to manufacture a plated steel sheet 20. (S 110)

In order to explain in detail the process of forming the plated steel sheet 20 will be described with reference to FIG.

Figure 2 is a flow chart illustrating a procedure for forming a plated steel sheet for forming an ultra-high strength steel molding according to the present invention.

The plated steel sheet 20 may provide a hot rolled steel sheet, cold roll the hot rolled steel sheet to form a base iron 210, and form a plated layer 220 by ZAM or ZAMS plating on the surface of the base iron 210. have.

In order to form the base iron 210 by weight, carbon (C) 0.1 to 0.4%, silicon (Si) 0.5% or less, nitrogen (N) 0.1% or less, aluminum (Al) 0.01 to 0.1%, phosphorus (P) ) 0.05% or less, manganese (Mn) 0.8 to 2%, boron (B) 0.002 to 0.01%, molybdenum (Mo) or chromium (Cr) 0.1 to 0.5%, but the B / N element ratio is adjusted to 1 or more And forming a hot rolled steel sheet containing other inevitably added elements. (S 210)

The base steel 210 may be manufactured by cold rolling the hot rolled steel sheet at a reduction ratio of 30% to 80% (thickness before rolling-thickness of the plate after rolling) / thickness of the rolling plate × 100). (S 220)

However, the base iron produced at this time should have a tensile strength of less than 100kg / mm ² when considering the purpose of the present invention.

Next, the plated layer 220 is formed by plating ZAM or ZAMS on the surface of the base iron 210. (S 230)

Wherein ZAM may be specified in the composition of Zn-11% Al-3% Mg, ZAMS is Zn-11% Al-3% Mg-0.2% Si, but the above-described components are representative of one component used Meaning only, if it contains both ZAM or ZAMS components, even if the content of each component is to be considered the same as the plating layer used in the present invention.

As such, the plated steel sheet 20 including the plating layer 220 may be formed through a plating process.

In order to manufacture the ultra-high strength steel molding of the present invention, as shown in Fig. 1 and 3b, the plated steel sheet 20 is put into a heating furnace 320 and heated. (S120)

Here, the heating temperature provided by the heating furnace 320 may be 800 ° C to 1000 ° C, preferably 850 ° C to 950 ° C.

When the plated steel sheet is heated, a barrier film is formed on the plated layer 220 on the surface of the plated steel sheet.

Generally, Zn, Al, etc., which constitute the plating layer, have a melting point at a temperature of 700 ° C. or lower, and thus have a property of easily evaporating when the temperature becomes higher than that.

Therefore, in the heating process, a barrier film made of oxides of Zn, Al, Mg, and Si, that is, the components of the plating layer is formed on the surface of the plating layer at an initial stage. It is a substance with a point.

Due to the formation of such a barrier film, even if the plating layer is heated to a temperature of 850 ° C to 950 ° C, the plating layer does not evaporate and remains on the surface of the base iron .

The heating rate of heating the plated steel sheet in the furnace is not limited, but considering the production rate of at least 5 ℃ / sec. It is preferable to heat the plated steel sheet at the above speed.

At this time, the plating layer 220 formed on the surface of the plated steel sheet 20 may evaporate due to the high temperature heat provided by the heating furnace 320, but the barrier layer 230 formed on the plating layer 220 may be the plating layer 220. By suppressing evaporation of the plating layer 220 may be present on the surface of the base iron (210).

As such, since the plated layer 220 is present on the surface of the base iron 210, the steel formed product 10 finally manufactured by the manufacturing method according to the present invention has excellent corrosion resistance.

Next, as shown in FIG. 3C, the plated steel sheet 20 passing through the heating furnace 320 is loaded into the stamping apparatus 330, and then molded and quenched. (S 130)

The plated steel sheet 20 is in a high temperature state because it passes through the heating furnace 320. Here, the plated steel sheet 20 is formed by a stamping method. That is, the plated steel sheet 20 heated by using the stamping device 330 may be molded to have a predetermined shape.

In order to implement the stamping method, a stamping apparatus 330 should be provided. In this case, it is preferable that the stamping apparatus 330 uses a metal having excellent thermal conductivity so that the plated steel sheet can be cooled at the fastest speed.

At this time, the principle of the quenching at the same time as the molding, the flow path of the cooling water is formed inside the stamping device, the cooling water continues to flow through the flow path to cool the stamping device. However, in this case, the cooling water flow path is generally formed to penetrate the stamping device, and in the present invention, the scope of rights is not limited by the flow path design in the stamping device.

Therefore, during the stamping process, the stamping device can be kept cooled continuously, and since the stamping device is made of a metal having excellent thermal conductivity, stamping the heated plated steel sheet forms a plated steel sheet and rapidly cools the sheet. Can be done.

At this time, the rapid cooling rate of the plated steel sheet to be formed is 20 ℃ / sec. It is desirable to maintain the above reason, because the plated steel sheet used in the present invention has an austenite structure when heated to a high temperature, 20 ℃ / sec. If the cooling at the above rapid quench rate is due to the phase transformation to martensite (martensite) tissue.

That is, the plated steel sheet of the present invention heated to a high temperature is 20 ℃ / sec. If the cooling is less than the cooling rate, the structure, pealite or banite, will have a structure that can not have sufficient strength, maintaining the rapid cooling rate as described above to convert the base steel portion of the plated steel to full martensite structure (full martensite) It allows for phase transformation.

As such, when the forming and quenching step is performed, the plated steel sheet 20 forms a martensite structure, and thus the tensile strength of the steel formed product 10 finally manufactured may be greatly improved.

In this manner, the heated plated steel sheet may be rapidly cooled at the same time as the molding to improve the tensile strength of the steel formed body 10.

Finally, as shown in Figure 3d, to finish the steel molded body 10 formed by quenching and molding (S140).

In the finishing treatment, the plated steel sheet 20 may be formed in an undesired shape such as ductility in a molding process such as stamping. The above shapes are arranged through the finishing treatment.

In FIG. 3D, the die trimming apparatus 340 is illustrated and described. The die trimming device 340 is a device for forming the steel molded body 10 having a desired shape by cutting out unnecessary portions formed in the molding process.

In another embodiment of the finishing process may be finished through a laser or the like.

In this way, the steel molded body manufacturing method to protect the plated layer to maintain the corrosion resistance, it is possible to form an ultra-high strength steel molded body with improved tensile strength by quenching.

Hereinafter will be described with reference to specific examples that the molded article produced by the ultra-high strength steel molded article manufacturing method according to an embodiment of the present invention is excellent in paint adhesion, corrosion resistance, and tensile strength.

Details not described herein are omitted because they can be sufficiently inferred by those skilled in the art.

One. Example  And Comparative example

< Example  1>

The thickness of the steel plate was 1.2 mm, and a plated steel sheet plated on both sides was prepared such that the Zn-11% Al-3% Mg (ZAM) layer had an adhesion amount of 50 g / m ² based on the cross section.

Thereafter, the plated steel sheet was heated at a temperature of 900 ° C. for 5 minutes in a heating furnace in an air atmosphere, and then removed. At this time, the temperature of the plated steel sheet 20 reached 900 ℃ after about 2 minutes, the retention time after reaching to 3 minutes.

Thereafter, a molded body was formed by using a stamping apparatus on the plated steel sheet which became hot. At this time, the stamping conditions were the drawing height 25mm, shoulder radius R 5mm, blank diameter 90mm, punch diameter 50mm, die diameter 53mm.

< Example  2>

A specimen was prepared in the same manner as in Example 1 except that the components of the plating layer formed on the surface of Example 1 were Zn-11% Al-3% Mg-0.2% Si (ZAMS).

< Comparative example  1>

A specimen was prepared in the same manner as in Example 1 except that no plating layer was formed on the surface.

< Comparative example  2>

A specimen was prepared in the same manner as in Example 1 except that the component of the plating layer formed on the surface of Example 1 was Zn-11% Fe (ZF).

< Comparative example  3>

A specimen was prepared in the same manner as in Example 1 except that the components of the plating layer formed on the surface of Example 1 were Al-5% Si (AS).

2. Property Measurement

(1) appearance evaluation after heating

In the above Examples and Comparative Examples was evaluated by visual observation of the appearance state of the specimen after heating for 5 minutes at a temperature of 900 ℃.

(2) Evaluation of moldability (plating adhesion)

In general, formability means how well the molding is performed according to a desired shape by a stamping process. However, in the present invention, a plated steel sheet is used as a molding member. Therefore, it is important that the surface of the coating layer be peeled off after stamping. Therefore, moldability was determined based on this.

Moldability (plating adhesion) evaluation was performed by visually observing whether the plating layer peeled off after stamping.

(3) Paint adhesion evaluation

After the zinc phosphate treatment was carried out on the molded articles prepared in Examples 1 to 3 and Comparative Examples 1 and 2 by PBL-3080 manufactured by Nippon Parkarizing Co., Ltd. under normal image processing conditions, electrodeposition by Kansai Paint was applied. The coating material GT-10 was electrodeposition-coated by the slope electric current of the voltage of 200V, and it sintered for 20 minutes at 150 degreeC of sintering, and was coat | painted. At this time, the thickness of the coating film was about 20 micrometers.

Each specimen was immersed in 50 ° C. ion-exchanged water and taken out after 240 hours. Using a cutter knife, a scratch was placed so that the bottom layer of the coating film was exposed in a checkerboard grid shape having a width × length of 1 mm × 1 mm. 3M and ^TM polyester tape 396 / transparency) When peeling and peeling off, the peeling test of the coating film was done by counting the number of squares of the coating film which remain | survives.

In this case, the total number of squares was 100, and the evaluation criteria indicated that the number of the remaining squares is 90 to 100 as good (O), and the case of 0 to 89 was denoted as bad (X).

(4) corrosion resistance evaluation

Specimens were prepared in the same manner as in the coating adhesion evaluation, and a salt spray test was conducted for 480 hours in the manner specified in JIS Z2371.

Corrosion resistance evaluation was made here by measuring the coating film blister width or rust width from a scratch.

Evaluation criteria were described as very good (◎) 0mm to less than 1mm based on the larger value of the rust width, coating film blister width, (1) to less than 2mm is good (○), 2mm or more and less than 4mm normal ( (Triangle | delta)) and 4 mm or more were described as defect (X).

3. Property measurement result and analysis

Table 1 shows the physical property measurement results for the molded product specimens prepared by the methods of Examples 1 to 3 and Comparative Examples 1 and 2.

Referring to Table 1, in the case of using a non-plated cold rolled steel sheet as in Comparative Example 1, a thick oxide film was formed on the surface, and after heating to 900 ° C., the surface turned black, and the oxide was peeled off during hot stamping molding. .

In addition, in the case of Comparative Example 1, it was found that coating adhesion and corrosion resistance were poor .

And, in the case of a plated steel sheet having a Zn-11% Fe plating layer as in Comparative Example 2 and a plated steel sheet having an Al-5% Si plating layer as in Comparative Example 3, the appearance state and formability (plating adhesion) after coating Although adhesiveness had the outstanding property, in the evaluation of corrosion resistance, the average (△) result was shown.

On the other hand, in Example 1 and Example 2 in which the ZAM and ZAMS layers were formed as the plating layers, it was found that the appearance, formability (plating adhesion), coating adhesion, and corrosion resistance after heating had excellent properties.

Hereinafter, in a plated steel sheet in which a ZAM and ZAMS layer is plated on a surface which is shown to have excellent properties in the physical property measurement and results through further embodiments, it is possible to change the plating amount and heating temperature to a condition that can have better physical properties. Let's learn about.

< Example  3 ~ 22>

The rest of the conditions were the same as in Example 1 (ZAM plated steel sheet), and the coating weight (g / m ² ) was changed to (10, 30, 50, 70, 90) g / m ^{2 based} on one side, and each plating Appearance evaluation, moldability (plating adhesion), tensile strength, coating adhesion and corrosion resistance were evaluated after changing the heating temperature to (800, 850, 900, 950) ° C., respectively. Shown in

At this time, the appearance evaluation, formability, coating adhesion and corrosion resistance evaluation were made in the same manner as described above, and further measured tensile strength was carried out in JIS Z 2241 standard using a JIS No. 5 size sample.

< Example  23-42>

The rest of the conditions are the same as in Example 2 (ZAMS plated steel sheet), and the coating weight (g / m ² ) is changed to (10, 30, 50, 70, 90) g / m ^{2 based} on one side, and each plating Appearance evaluation, moldability (plating adhesion), tensile strength, paint adhesion and corrosion resistance were evaluated after changing the heating temperature to (800, 850, 900, 950) ° C., respectively. ~ 17) and the results are shown in Table 3.

Referring to Table 2 and Table 3, it can be seen that the appearance after heating is good for all further examples, except that the plating is based on one side in order to have moldability, tensile strength of 140 kg / mm ² or more, excellent paint adhesion and corrosion resistance. It was found that the deposition amount should be 30 g / m ² or more and the heating temperature should be 850 ° C. or more.

In particular, in order to have better corrosion resistance and tensile strength, it was found that the plating deposition amount should be at least 50 g / m ² or more, and the heating temperature should be 850 ° C. or more.

However, when the deposition amount exceeds 90g / m ² it is not preferable in terms of economics as described above, so it was not set in the conditions of the embodiment, when the heating temperature is above 950 ℃ plating layer evaporation due to barrier film loss Since it is likely to occur, no setting was made under the conditions of the examples.

1 is a flow chart illustrating a method of forming an ultra-high strength steel molding according to the present invention.

Figure 2 is a flow chart illustrating a procedure for forming a plated steel sheet for forming an ultra-high strength steel molding according to the present invention.

Figure 3a to 3e is a process chart for forming an ultra-high strength steel molding according to the present invention.

Claims (12)

By weight%, carbon (C) 0.1-0.4%, silicon (Si) 0.5% or less, nitrogen (N) 0.1% or less, aluminum (Al) 0.01-0.1%, phosphorus (P) 0.05% or less, manganese (Mn) 0.8 To 2%, 0.002 to 0.01% of boron (B), and 0.1 to 0.5% of molybdenum (Mo) or chromium (Cr), but adjusts the B / N element ratio to 1 or more and includes other inevitable elements. Zn-11% Al-3% Mg-based or Zn-11% Al-3% Mg-0.2% Si-based plating layer was formed on the surface of cold rolled steel sheet (steel) having a tensile strength of 100kg / mm ² or less. It is formed to have an adhesion amount of 30 ~ 90g / m ² to form a plated steel sheet, the plated steel sheet 5 ℃ / sec. Heating to a temperature of 850 ° C. to 950 ° C. at an elevated temperature rate or higher; And The plated steel sheet heated by the stamping method was molded into a desired shape in a mold while simultaneously 20 ° C / sec. Including the step of quenching at a cooling rate of more than 140kg / mm ² or more to form a high corrosion resistance ultra-high strength steel molded body; High corrosion resistance candle is formed on the surface of the plating layer to form a barrier film made of oxides of the components (Zn, Al, Mg, Si) constituting the plating layer to prevent evaporation of the plating layer in the heating step before molding High strength steel molding production method. delete delete delete delete delete delete delete delete delete delete delete

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