KR101528011B1 - Plated steel sheet for hot-press forming and hot-press formed products, method for manufacturing thereof - Google Patents

Abstract

A steel sheet comprising 0.1 to 0.4% of C, 0.1 to 4.0% of Mn, 2% or less of Si (excluding 0%), B of 0.0001 to 0.01%, Fe, and other unavoidable impurities step; Rolling the base steel sheet at a temperature of 680 to 740 占 폚; Subjecting the hot-rolled steel sheet to pickling and reducing heat treatment; And a step of hot-pressing the steel sheet to form a plated layer, a method of manufacturing a method for manufacturing a hot-plated steel sheet for hot press forming, and a hot press-formed article using the coated steel sheet, and a method of manufacturing the same.
According to the present invention, it is possible to improve the productivity by promoting mutual diffusion between the base steel and the plated layer during the heat treatment, and to reduce the production cost by omitting the cold rolling process.

Description

TECHNICAL FIELD [0001] The present invention relates to a coated steel sheet for hot press forming, a hot press formed product, and a method for manufacturing the same. BACKGROUND ART < RTI ID = 0.0 >

The present invention relates to a coated steel sheet for hot press forming and a hot press formed article, and a method for producing the same.

Due to recent environmental regulations, demand for high-strength steel plates is rapidly increasing as a measure to meet stringent automobile fuel efficiency regulations. As the strength of automotive steel increases, the elongation and r value decrease and spring back occurs during molding, which makes it difficult to mold complex products.

Therefore, in order to solve such a problem, an Al-based plating is applied to the surface of the steel sheet as in Patent Document 1, thereby suppressing the oxidation reaction on the surface of the steel sheet while increasing the corrosion resistance by using the formation of a passive film of Al Products have been developed and commercialized. This metal plating layer is formed by a continuous hot dip coating process in a plating bath composed of about 90% of Al and 10% of Si, and the Fe in the base iron is diffused into the plating layer to form a plating layer having high heat resistance. However, in the case of Al-coated steel sheet, the plating layer itself is resistant to corrosion but does not have a sacrificial property, so that the plating layer is maintained in the corrosive environment, but corrosion begins inside the steel sheet.

In order to solve the problem of corrosion resistance of such an aluminum plating material, development of a hot-dip galvanized steel sheet has been actively carried out all over the world in recent years. The Zn plating material for hot pressing developed by POSCO is characterized by minimizing the deterioration of the Zn plating layer because it suppresses the volatilization of Zn and restrains the growth of the oxide by generating a uniform Al ₂ O ₃ oxide film on the plating surface layer. However, in the case of Zn, the hardness at high temperature is lower than that of Al and the adherence between the plating layer and the substrate iron is excellent. Therefore, the Zn-coated steel sheet produced by a conventional method is not only plated layer in the processing part, Microcracks having a depth of up to several hundreds of micrometers are generated up to the inside of the iron. This phenomenon occurs when the Zn in the liquid state at high temperature contacts with the surface of the base steel to which the tensile stress is applied because the diffusion of Fe in the Zn plating layer is insufficient during the heat treatment and this is called a liquid metal embrittlement. Such cracks in Zn-plated members may deteriorate corrosion resistance or impact performance when exposed to corrosive environments.

U.S. Patent No. 6296805

One aspect of the present invention is to provide a hot press capable of suppressing the lubrication of the liquefied metal generated during hot pressing of the hot-rolled hot-dip coated steel sheet for hot pressing to solve the problem of cracks generated in the processed portion, Plated steel sheet for molding, a hot press molded article, and a manufacturing method thereof.

However, the problems to be solved by the present invention are not limited to the above-mentioned problems, and other problems not mentioned can be clearly understood by those skilled in the art from the following description.

In order to achieve the above object, one aspect of the present invention provides a method of manufacturing a semiconductor device, comprising: 0.1 to 0.4% of C, 0.1 to 4.0% of Mn, 2% or less of Si (excluding 0% 0.01%, the balance Fe, and other unavoidable impurities; Rolling the base steel sheet at a temperature of 680 to 740 占 폚; Subjecting the hot-rolled steel sheet to pickling and reducing heat treatment; And a step of hot-dip coating the steel sheet to form a plated layer.

According to another aspect of the present invention, there is provided a method of manufacturing a coated steel sheet, the method comprising: heat treating a coated steel sheet produced by the manufacturing method at 880 to 950 캜 for 3 to 5 minutes; And a step of cooling the hot press molded product.

Another aspect of the present invention is a ferritic stainless steel comprising 0.1 to 0.4% of C, 0.1 to 4.0% of Mn, 2% or less of Si (excluding 0%), B of 0.0001 to 0.01% A base steel sheet made of impurities; And a plating layer formed on the base steel sheet, wherein the roughness of the interface between the plating layer and the base steel sheet is Rmax 3 to 10 占 퐉.

Another aspect of the present invention provides a hot press formed product obtained by heat-treating the hot-dip coated steel sheet, hot pressing and cooling.

According to the present invention, it is possible to improve the productivity by promoting mutual diffusion between the base steel and the plated layer during the heat treatment without causing a working crack after the hot pressing step, and the production cost can be reduced by omitting the cold rolling step.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic view showing a process for manufacturing a hot-plated steel sheet for a hot press forming process and a hot press formed product of the present invention.
2 is a photograph of the surface shape of a coated steel sheet produced according to an embodiment of the present invention.
3 is a cross-sectional photograph of a coated steel sheet produced in accordance with an embodiment of the present invention in the thickness direction.
4 is a cross-sectional SEM photograph of a hot press formed article obtained by molding a coated steel sheet produced according to one embodiment (a) and one comparative example (b) of the present invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings, in which: FIG. 1 is a perspective view of a hot-press plated steel sheet according to an embodiment of the present invention; The steps of the method for producing a hot stamping coated steel sheet and a hot press molded article of the present invention are schematically shown in FIG.

The present inventors have found that by forming fine irregularities on the surface of a hot-rolled steel sheet by pickling after hot rolling, the interface between the steel plate and the plating layer is increased by the unevenness when plating is performed in the future, Found the fact and led to the present invention. That is, when the interface between the plating layer and the steel sheet increases, the diffusion between the plating layer metal (zinc) and the steel sheet metal (iron) is facilitated during the subsequent heating for the hot press forming step and the alloying can proceed more rapidly, Can be suppressed.

Therefore, the base steel sheet to be applied to the method for manufacturing a coated steel sheet for hot press forming of the present invention is a hot-rolled steel sheet applied to general hot press forming. If the hot-rolled steel sheet for hot-pressing according to the present invention is further subjected to cold rolling, the irregularities on the surface of the steel sheet are pressed by cold rolling so that the surface of the steel sheet becomes flat, do. Therefore, the interface between the steel sheet and the plated layer after plating is reduced, and as a result, there is no significant difference from the cold-rolled steel sheet.

That is, since the material of the hot-press molded article is determined by heating and cooling during hot pressing rather than by hot-rolling or cold-rolling annealing, quality can be ensured without omitting the cold rolling step. The present invention relates to a method for manufacturing a hot-plated steel sheet for hot press forming which can reduce production costs by omitting cold rolling and prevent cracking of a machining portion caused by insufficient diffusion of Fe in the Zn plating layer during heat treatment to enhance durability of a molded product .

In order to produce the coated steel sheet according to the present invention, the steel sheet comprises 0.1 to 0.4% of C, 0.1 to 4.0% of Mn, 2% or less of Si (excluding 0%), B of 0.0001 to 0.01% And other unavoidable impurities.

The reason for limiting the numerical values of the above components will be described as follows. Hereinafter, it is necessary to pay attention that the content unit of each component is weight% unless otherwise stated.

C: 0.1 to 0.4%

C is a key element for increasing the strength of a steel sheet, which produces a hard phase of austenite and martensite. When the content of C is less than 0.1%, it is difficult to obtain a desired strength even if a hot press is performed in a single phase of austenite. Therefore, the content of C is preferably 0.1% or more. If the content of C exceeds 0.4%, there is a high possibility that the toughness and weldability are lowered, and the strength is excessively increased, which is disadvantageous in the manufacturing process such as inhibiting the ducting in the annealing and plating process. .

Mn : 0.1 to 4.0%

Mn not only contributes greatly to the strength increase as a solid solution strengthening element but also plays an important role in delaying the transformation from austenite to ferrite. When the content of Mn is less than 0.1%, the ferrite transformation temperature (Ae3) is increased in austenite, and a heat treatment temperature as high as that is required for press-working a steel sheet in austenite single phase.

On the other hand, if the content of Mn exceeds 4.0%, the weldability and hot rolling property may be deteriorated, which is not preferable. In this case, it is more preferable that the content of Mn is 0.5% or more in order to sufficiently reduce the ferrite transformation temperature (Ae3) due to Mn and to satisfactorily reduce the incombustibility.

Si : Less than 2% (excluding 0%)

Si is added for the purpose of deoxidation. When the content of Si is more than 2%, it is difficult to pick up the hot-rolled sheet to cause scaly surface defects due to hot-rolled steel sheet ash and poorly- SiO ₂ oxide is generated on the surface of the steel, which may cause plating, so the upper limit of Si is preferably limited to 2%. More preferably, it is more effective to maximize deoxidation by adding more than 0.3%.

B: 0.0001 to 0.01%

B is an element for retarding ferrite transformation in austenite. When the content is less than 0.0001%, it is difficult to sufficiently attain the effect. When the content of B exceeds 0.01%, the effect is saturated and the hot workability is degraded. It is preferable to limit the upper limit to 0.01%.

The remainder of the present invention is iron (Fe). However, in the ordinary manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably incorporated, so that it can not be excluded. These impurities are not specifically mentioned in this specification, as they are known to any person skilled in the art of manufacturing.

The base steel sheet may contain 0.001 to 0.02% of N, 0.001 to 0.1% of Ti, 0.001 to 0.1% of Nb, 0.001 to 0.1% of V, 0.001 to 1.0% of Cr, 0.001 to 1.0% of Mo, %, Sb: 0.001 to 0.1%, and W: 0.001 to 0.3%.

N: 0.001 to 0.02%

When N is less than 0.001%, the manufacturing cost for controlling N in the steelmaking process may increase significantly, so the lower limit is 0.001%. If the content of N exceeds 0.02%, it is difficult to dissolve and play the steel sheet at the manufacturing process, resulting in an increase in production cost, and slab cracking due to AlN tends to occur, so that the upper limit is set to 0.02%.

Ti , Nb , V: 0.001 to 0.1%

Ti, Nb, and V are effective elements for improving the strength of steel sheet, grain size, and heat treatment. When the content is less than 0.001%, the above effect can not be sufficiently obtained. When the content is more than 0.1%, the effect of increasing the desired strength and yield strength due to an increase in production cost and excessive carbonitride formation can not be expected. desirable.

Cr , Mo : 0.001 to 1.0%

Since Cr and Mo not only increase the hardenability but also increase the toughness of the heat treated steel sheet, the effect is more remarkable when it is added to a steel sheet requiring high impact energy characteristics. When the content is less than 0.001%, the above effect If it exceeds 1.0%, the effect is saturated and the production cost rises. Therefore, it is preferable to limit the upper limit to 1.0%.

Sb : 0.001 to 0.1%

Sb is an element that plays a role of suppressing the selective oxidation of grain boundaries during hot rolling, thereby making the scale uniform and improving the pickling resistance of the hot rolled material. If the Sb content is less than 0.001%, the effect is difficult to achieve, and when the Sb content exceeds 0.1%, the effect is saturated and the production cost is increased and brittleness may occur during hot working. desirable.

W: 0.001 to 0.3%

W is an element which improves the heat-curing ability of the steel sheet and at the same time, the W-containing precipitate is an element which acts to favor strength. When the content is less than 0.001%, the above effect can not be sufficiently obtained and the content is 0.3% The effect is saturated and the manufacturing cost is increased. Therefore, the content is preferably limited to 0.001 to 0.3%.

The prepared steel sheet is hot-rolled and rolled at 680 to 740 ° C.

The coiling temperature influences the mechanical properties as well as the oxidation of the steel sheet. When the base steel sheet is hot-rolled and rolled at 680 to 740 ° C, an internal oxide containing at most 5 g / m 2 of at least one of Fe, Si and Mn at a maximum depth of 20 μm in the depthwise direction is formed . At this time, the oxide is mainly composed of Si, Mn, or a composite oxide, and has the form of SiO ₂ , MnO, Mn ₂ SiO ₄ , MnSiO _3, and the like. When the oxide thus formed is subjected to a pickling process, it is reacted and removed before Fe, which is a base material, and a hole is formed in the base iron as shown in Fig. When the pickled steel sheet is plated, the thus removed space is filled with the molten plated layer as shown in FIG. The thus-formed plated layer accelerates the interdiffusion of Zn and Fe upon heating in the hot press heating furnace, thereby making it possible to stabilize the plating layer in the same heat treatment time and to produce a plated steel sheet for hot press which can be excluded from the conditions of liquefied metal brittleness .

The reason why the lower limit of the coiling temperature is set at 680 캜 is that the internal oxidation is not sufficiently generated at a temperature lower than 680 캜, so that the Zn layer is not sufficiently diffused into the inner iron during plating after the pickling, It does not affect the diffusion rate of the water. The reason why the upper limit of the coiling temperature is set to 740 占 폚 is that the coiling temperature exceeding 740 占 폚 can not be reached without cooling with water.

The steel sheet is pickled and subjected to reduction heat treatment.

Through the pickling and reduction heat treatment, oxides are removed on the surface and inside of the base steel sheet, and oxide formation is suppressed. The reduction heat treatment is preferably carried out at a temperature of 480 to 550 ° C for 40 to 80 seconds while controlling the dew point temperature to -60 to 0 ° C. When the dew point temperature is less than -60 ° C, it is difficult to control in an actual field line. When the dew point temperature exceeds 0 ° C, it is difficult to reduce the Fe oxide present on the surface during the reduction heat treatment process due to partial pressure of oxygen and water vapor. Unplating occurs.

The equipment for the reduction heat treatment process is not particularly limited, and can be carried out in a normal continuous annealing line. That is, the type of equipment and the operating conditions are irrelevant if there is no problem in ensuring the mechanical properties of the steel sheet by continuous heat treatment and forming a nitrogen atmosphere.

Thereafter, the base steel sheet after the reduction heat treatment process is immersed in a plating bath to form a plating layer, and the method of forming the plating layer is not particularly limited.

For example, when the plating process is a hot dip galvanizing process, hot dip galvanizing may be performed by immersing in a hot dip galvanizing bath containing 0.12 to 0.25 wt% of Al and the remainder of Zn at 460 to 480 캜 for 3 to 5 seconds have. Further, the plating layer formed after plating may be a hot-dip galvanized layer, a hot-dip galvanized layer, a hot-rolled aluminum layer or a hot-rolled aluminum alloy plated layer.

The plated steel sheet for hot press formed with the plating layer through the above process is filled with a molten plated layer in the form of perforated iron, and the plated layer accelerates the interdiffusion of Zn and Fe upon heating in the hot press furnace, thereby stabilizing the plated layer And it is possible to escape from the conditions of liquefied metal brittleness. As described above, it is preferable that the roughness of the interface between the plating layer and the base steel sheet is Rmax of 3 to 10 占 퐉 or the hole of the base metal is distributed within 10 占 퐉 in the direction of the base steel sheet from the interface.

Next, a method of manufacturing a hot press molded article of the present invention will be described.

The method for manufacturing a hot press molded article of the present invention comprises the step of heating the plated steel sheet under a predetermined condition in a heating furnace, followed by hot press forming and cooling.

The plated steel sheet on which the plated layer is formed is heated in a heating furnace to perform hot press forming. At this time, it is preferable to heat the steel sheet at 880 to 950 DEG C for 3 to 5 minutes depending on the thickness of the steel sheet. If the heating condition is less than 880 ° C If the heating temperature is less than 3 minutes, uniform heat treatment of the steel sheet does not occur and the Fe in the plating layer is not sufficiently diffused to cause material unevenness. On the other hand, when the heating condition is more than 950 ° C or the heating time is more than 15 minutes, There is a concern.

Generally, in order to achieve sufficient alloying through diffusion of Fe and Zn during hot press forming, a heat treatment time of more than 5 minutes is required. However, in the present invention, a hole is formed in the base steel, and the melted plating layer is filled in the space in which the oxide is removed when the pickled steel sheet is plated, as shown in FIG. The formed coating layer accelerates the interdiffusion of Zn and Fe during heating in the hot press furnace, and the alloying can be sufficiently performed even by heat treatment for a short time of 3 to 5 minutes, and there is an advantage that cracks in the machined part do not occur.

The heated steel sheet is subjected to hot press forming and cooling. In this case, the forming and cooling conditions are based on a conventional hot press forming method, but the invention is not limited thereto.

As shown in Fig. 4, in the hot-press molded article of the present invention, the base iron and the plating layer are mutually diffused at a high temperature under the general heating condition, and there is no crack in the processed portion after the final molding. That is, when a conventional hot-press hot-dip galvanized steel material is subjected to hot press forming, cracks are generated in the processed portion, but when the developed material of the present invention is subjected to hot press forming under the same heating conditions, cracks do not occur in the processed portion.

Hereinafter, the present invention will be described in detail with reference to Examples. However, the following examples are only for illustrating the present invention in more detail and do not limit the scope of the present invention.

[ Example ]

Hot rolled steel sheets having a thickness of 1.6 mm were used as base steel sheets, and the coiling temperature was changed to between 620 and 700 캜 and hot-dip coating was performed in the continuous plating process. The steel sheet thus prepared was subjected to hot pressing under the conditions shown in Table 1 below and the presence of machining cracks formed in the inner side of the iron core of the machined portion was determined.

The machined cracks were observed by using electron microscope and the occurrence of the cracks was judged.

Steel grade Coiling temperature (캜) Plating amount (one side basis, g / m ² ) Heating furnace temperature (캜) Heating time (min) Whether or not there is a crack Remarks One 22MnB5 620 52 880 3 O Comparative Example 1 2 22MnB5 620 49 900 4 O Comparative Example 2 3 22MnB5 620 51 900 4 O Comparative Example 3 4 22MnB5 620 54 930 5 O Comparative Example 4 5 22MnB5 650 52 880 3 O Comparative Example 5 6 22MnB5 650 51 900 4 O Comparative Example 6 7 22MnB5 680 55 900 3 X Inventory 1 8 22MnB5 680 47 900 4 X Inventory 2 9 22MnB5 680 48 930 5 X Inventory 3 10 22MnB5 680 49 930 4 X Honorable 4 11 22MnB5 700 52 900 3 X Inventory 5 12 22MnB5 700 48 900 5 X Inventory 6 13 22MnB5 700 53 930 3 X Honorable 7 14 22MnB5 700 55 930 4 X Honors 8 15 22MnB5 700 53 930 5 X Proposition 9

As shown in Table 1, when the heat treatment time was not sufficient as a result of the analysis of the hot press formed articles according to the comparative example of the conventional method, some cracks occurred in some portions. As mentioned above, it is considered that the diffusion of Fe in the plating layer is caused by the liquefaction metal embrittlement (LME) caused by the Zn present in the liquid state in the plating layer during processing.

On the other hand, in the case of the hot-press molded product manufactured according to the present invention in accordance with the present invention, no processing crack occurred regardless of the heat treatment time and temperature. This is because the plating layer inside the steel after plating increases the interdiffusion rate with the iron during the heat treatment.

As shown in Fig. 4, the inventive example 7 (a) produced according to the present invention has no processed part crack after the final molding. On the other hand, in Comparative Example 1 (b) produced by the conventional method, cracks were generated in the processed portion.

Claims (11)

A steel sheet comprising 0.1 to 0.4% of C, 0.1 to 4.0% of Mn, 2% or less of Si (excluding 0%), B of 0.0001 to 0.01%, Fe, and other unavoidable impurities step;
Rolling the base steel sheet at a temperature of 680 to 740 占 폚;
Subjecting the hot-rolled steel sheet to pickling and reducing heat treatment; And
And forming a plated layer by hot-dipping the steel sheet. The method according to claim 1,
Wherein said base steel sheet comprises 0.001 to 0.02% of N, 0.001 to 0.1% of Ti, 0.001 to 0.1% of Nb, 0.001 to 0.1% of V, 0.001 to 1.0% of Cr, 0.001 to 1.0% of Mo, 0.001 to 0.1% of Sb, and 0.001 to 0.3% of W. The method for producing a coated steel sheet for hot press forming according to claim 1, The method according to claim 1,
And forming an oxide layer containing at least one of Fe, Si and Mn in a total amount of 5 g / m 2 or less within 20 탆 in the depth direction of the base steel sheet through the hot rolling and winding. The method according to claim 1,
Wherein the reduction heat treatment is carried out at a temperature of 480 to 550 占 폚 for 40 to 80 seconds while a dew point temperature is maintained at a temperature of -60 to 0 占 폚. The method according to claim 1,
Wherein the oxide is removed on the surface and inside of the steel sheet through the pickling and reduction heat treatment, and oxide formation is suppressed. Treating the coated steel sheet produced by the method of any one of claims 1 to 5 at a temperature of 880 to 950 캜 for 3 to 5 minutes and subjecting the pressed steel sheet to hot press forming; And
And cooling the hot press molded product. The method according to claim 6,
Wherein the zinc in the plating layer and the Fe of the base steel sheet are mutually diffused through the heat treatment. delete delete delete delete

Images