JP2019508575A - Hot press-formed product in which fine cracks are suppressed and method for manufacturing the same - Google Patents

Abstract

A base material steel sheet and a hot press-formed product produced by hot-press forming a zinc-based plated steel sheet including a zinc-based plating layer formed on the surface of the base steel sheet, wherein the zinc-based plating layer is Sb, Sn And one or more elements selected from the group consisting of: 0.05 to 2.0% by weight in total, the balance including Zn and unavoidable impurities, and one or more types selected from the group consisting of Sb, Sn, and Bi described above 70% by weight or more of the element is concentrated in a region within 3 μm from the surface of the alloyed zinc-based plating layer of the hot press-formed product formed by alloying the zinc-based plating layer An article and method of making the same are disclosed.

Description

  The present invention relates to a hot press-formed product in which fine cracks are suppressed and a method of manufacturing the same.

  Recently, the use of high strength steels has been increased to reduce the weight of automobiles, but such high strength steels have a problem that they easily wear or break when processed at normal temperature. Moreover, when processing, the phenomenon of a spring back also generate | occur | produces, precise dimension processing is difficult, and shaping | molding of a complicated product is difficult. Thereby, hot press forming (HPF) is applied as a preferable method for processing high strength steel.

  Hot press forming (HPF) is a method of processing a steel sheet into a complex shape at high temperature by utilizing the property of softening and high ductility at high temperature, and more specifically, heating the steel sheet to austenite region or more It is a method which can transform the structure of a steel plate to martensite by performing quenching in the processed state and quenching, and can manufacture a product having a high strength and precise shape.

  However, when heating steel materials to high temperatures, there is a possibility that phenomena such as corrosion and decarburization may occur on the surface of steel materials, but zinc is used as a material for hot press forming to prevent this. A plated steel material having a base or aluminum-based plating layer formed thereon is often used. In particular, a galvanized steel sheet having a zinc-based plating layer is a steel material in which the corrosion resistance is improved by utilizing the self-corrosion corrosion resistance of zinc.

  However, when such a plated steel material is hot press-formed, a crack is generated in the plated layer at the deep drawn portion where the mold and the plated layer are in direct contact and the surface friction is large, and the crack generated in the plated layer There is a problem that fine cracks occur along the surface of the base steel sheet.

  In order to solve such a problem, Patent Document 1 (US Pat. No. 6,296,805) proposes a technique of performing Al-based plating on the surface of a steel plate. As proposed in Patent Document 1 above, Al-based plating is performed, the oxidation reaction on the surface of the steel sheet is suppressed while the plating layer is maintained in the heating furnace, and corrosion resistance is increased by utilizing the formation of the passive film of Al. Al-plated steel sheet has a problem that the corrosion resistance is significantly inferior.

  In order to solve such problems, research on Zn-plated hot-pressed steel sheets has been focused again, but the plated steels are alloyed in a hot working environment with a high-temperature working environment exceeding 900 ° C. There is a problem that micro-cracks are generated to the surface of the base steel sheet due to stress due to friction between the Zn-Fe alloyed layer and the die. Such a micro crack may act as a starting point for the crack to propagate in the base steel plate or may act as a cause of causing a fatigue crack, and there is a problem that the possibility of inhibiting the durability of the part is high. .

U.S. Pat. No. 6,296,805

  One of the objects of the present invention is to provide a hot press-formed product in which fine cracks are suppressed and a method of manufacturing the same.

  According to one embodiment of the present invention, it is a hot press-formed product manufactured by hot-press forming a base steel sheet and a zinc-based plated steel sheet including a zinc-based plated layer formed on the surface of the base steel sheet The zinc-based plating layer contains one or more elements selected from the group consisting of Sb, Sn and Bi: a total of 0.05 to 2.0% by weight, the balance Zn and unavoidable impurities, and the Sb, Sn and Bi 70% by weight or more of one or more elements selected from the group consisting of: 3 μm or less from the surface of the alloyed zinc-based plated layer of the hot press-formed product formed by alloying the zinc-based plated layer A hot-press formed part is provided which concentrates in the area.

  According to another embodiment of the present invention, the step of preparing a zinc-based plated steel sheet, and the primary heating of the zinc-based plated steel sheet to a temperature of 640-680 ° C. at a speed of 3.5-4.2 ° C./sec. And secondarily heating the primary heated zinc-based plated steel sheet to a temperature of 900 to 930 ° C. at a rate of 1.1 to 1.6 ° C./sec, and secondarily heating the zinc-based plated steel plate And holding the plated steel sheet at a constant temperature for 1 to 5 minutes, and forming and rapidly cooling the zinc-based plated steel sheet maintained at the constant temperature with a mold, wherein the zinc-based plated steel sheet comprises a base steel plate and the base steel plate. A method of producing a hot press-formed product comprising a zinc-based plating layer formed on the surface and containing 0.05 to 2.0% by weight in total of one or more elements selected from the group consisting of Sb, Sn and Bi Provided.

  As one of various effects of the present invention, the hot press-formed product according to one embodiment of the present invention effectively suppresses the propagation of micro cracks in the plating layer generated in the hot press forming to the base steel plate. And the durability is excellent.

  However, various and beneficial advantages and effects of the present invention are not limited to the above-described contents, and can be more easily understood in the process of describing specific embodiments of the present invention.

The micro crack of Comparative Example 1 is observed and shown. The micro crack of the invention example 1 is observed and shown. The micro crack of the invention example 3 is observed and shown. The micro crack of Comparative Example 4 is observed and shown. The micro crack of the invention example 5 is observed and shown. (A) is GDS data which analyzed content of Al, Mg, and Sn by the depth of the plating layer of the invention example 1, (b) is content of Al, Mg, and Sn by the depth of the plating layer of the invention example 3 And (c) are GDS data obtained by analyzing the contents of Al, Mg and Sn according to the depth of the plating layer of Inventive Example 5.

  As a result of intensive studies conducted by the present inventors to provide a hot press-formed product in which micro cracks are suppressed, the zinc-based plated steel sheet having a zinc-based plated layer containing an appropriate amount of grain boundary segregation elements is hot pressed The microcracks in the plating layer propagate to the base steel plate by using it as a forming material and by appropriately controlling the heating conditions in hot press forming to concentrate the grain boundary segregation element on the surface layer of the plating layer It has been found that it is possible to effectively block things, and the present invention has been completed.

  Hereinafter, a hot press-formed product according to an embodiment of the present invention will be described in detail.

  The hot press-formed product according to an embodiment of the present invention is manufactured by hot press-forming a zinc-based plated steel plate including a base steel plate and a zinc-based plated layer formed on the surface of the base steel plate.

  In the present invention, the type of the base steel sheet is not particularly limited, and may be, for example, a hot-rolled steel sheet or a cold-rolled steel sheet used as a base of a normal zinc-based plated steel sheet. However, in the case of a hot-rolled steel sheet, it has a large amount of oxide scale on its surface, and such an oxide scale has a problem of reducing the plating adhesion and lowering the plating quality. It is more preferable to use the removed hot-rolled steel sheet as a substrate.

  On the other hand, the zinc-based plating layer is formed on one side or both sides of the base steel plate, and the zinc-based plating layer is alloyed in heat treatment for hot press forming to change to an alloyed zinc-based plating layer.

  The zinc-based plating layer preferably contains one or more elements selected from the group consisting of Sb, Sn, and Bi: 0.05 to 2.0% by weight in total, the balance Zn, and unavoidable impurities.

  Sb, Sn and Bi are grain boundary segregation elements, and are elements that play a role of suppressing the formation of internal oxides due to the grain boundary penetration of oxygen in a high temperature working environment. In order to exhibit such effects in the present invention, the sum of the contents of the above elements is preferably 0.05% by weight or more, and more preferably 0.3% by weight or more. However, if the content is too high, the formation of the aluminum oxide film formed on the surface of the plating layer may be hindered and the barrier role of aluminum may be inhibited, and the effect is reduced compared to the increase in the content, which is economical There is a problem that Therefore, the sum of the contents of the above elements is preferably 2.0% by weight or less, more preferably 1.5% by weight or less.

  According to an example, the zinc-based plating layer may further include 0.1 to 5.0% by weight of Mg and 0.1 to 7.5% by weight of Al.

  Mg is an element that plays a role in improving the corrosion resistance of a hot press-formed product. In order to exhibit such an effect in the present invention, the content is preferably 0.1% by weight or more, and more preferably 1% by weight or more. However, when the content is too large, there is a problem that the dross of the plating bath is generated due to the oxidation of Mg in the plating bath. Therefore, the upper limit of the magnesium content is preferably 5.0% by weight, more preferably 4.0% by weight, and still more preferably 3.0% by weight.

  Al plays a role of suppressing Mg oxide dross. If the content is too low, the Mg antioxidation effect in the plating bath is minimal. Therefore, the lower limit of the aluminum content is preferably 0.1% by weight, and more preferably 1.5% by weight. However, if the content is too high, there is a problem that the temperature of the plating bath must be increased. If the temperature of the plating bath is high, it causes erosion of the plating equipment. Therefore, the upper limit of the aluminum content is preferably 7.5% by weight, and more preferably 7.2% by weight.

  According to an example, the Fe alloying degree of the alloyed zinc-based plated layer formed by alloying the zinc-based plated layer is preferably 30 to 85%, more preferably 45 to 78%, and 50 It is further preferable that it is -75%. When the Fe alloying degree satisfies the above range, surface cracks can be effectively prevented in hot pressing, and there is an advantage that the corrosion resistance by sacrificial corrosion protection is excellent. If the Fe alloying degree is less than 30%, a part of the Zn-rich region in the plating layer may exist as a liquid phase, which may lead to liquid phase embrittlement cracking in processing. On the other hand, when the degree of Fe alloying exceeds 85%, the corrosion resistance may be reduced.

  In the hot press-formed product of the present invention, 70% by weight or more of one or more elements selected from the group consisting of Sb, Sn and Bi is concentrated in a region within 3 μm from the surface of the alloyed zinc-based plating layer It is characterized by

  As described above, when Sb, Sn, and Bi are concentrated in large amounts on the surface of the alloyed zinc-based plating layer, Sb, Sn, and Sb are present before oxygen penetrates from the surface of the plating layer and causes grain boundary segregation. By locating Bi on the surface of the plating layer, the formation of internal oxides is suppressed, and the occurrence of intergranular cracks in the plating layer is prevented, whereby the propagation of micro cracks to the base member is interrupted. Can. Furthermore, although micro cracks mainly occur at a site where the friction between the mold and the plating layer is large, the oxides of Sb, Sn and Bi concentrated on the surface reduce the coefficient of friction between the mold and the plating layer The occurrence of micro cracks can be reduced to further improve the durability of the hot press-formed product.

  On the other hand, in the present invention, a specific method of measuring the content of one or more elements selected from the group consisting of Sb, Sn and Bi concentrated in a region within 3 μm from the surface of the alloyed zinc-based plating layer Although not particularly limited, for example, the following method can be used. That is, the hot press-formed product is vertically cut, and distribution of one or more elements selected from the group consisting of Sb, Sn and Bi in the cross section of the plating layer using a glow discharge spectrometer (GDS) By measuring the area and integrating the area in a graph relating to the content of one or more elements selected from the group consisting of Sb, Sn and Bi with respect to the depth from the surface of the plating layer, It is possible to measure the content of one or more elements selected from the group consisting of Sb, Sn and Bi concentrated in a region within 3 μm from the surface of

  The hot press formed article of the present invention described above can be manufactured by various methods, and the manufacturing method is not particularly limited. However, as one embodiment, it can be manufactured by the following method.

  Hereinafter, a method of manufacturing a hot press-formed product excellent in durability according to another embodiment of the present invention will be described in detail.

  First, a zinc-based plated steel sheet having the above-described alloy composition is prepared. In the present invention, a specific method for preparing a zinc-based plated steel sheet is not particularly limited, and it can be produced by a common method for producing a hot-dip galvanized steel sheet, for example, a zinc-based plating bath having the above-mentioned composition. After the base steel sheet is immersed, the galvanized steel sheet can be prepared by cooling it.

However, in order to increase the effect to be achieved in the present invention, it is preferable to carry out bubbling by previously supplying an inert gas into the zinc-based plating bath before immersing the base steel plate in the zinc-based plating bath . At this time, the inert gas may be one or more selected from the group consisting of nitrogen (N ₂ ), argon (Ar) and helium (He).

  When bubbling in a zinc-based plating bath prior to plating in this way, it not only helps to distribute Sb, Sn and Bi more uniformly in the zinc-based plating bath, but is obtained by the plating operation described later It also helps to distribute Sb, Sn and Bi more uniformly in the zinc-based plating layer, and concentrates Sb, Sn and Bi on the surface of the alloyed zinc-based plating layer of the resulting hot-pressed product. It also helps. This is because the surface concentration of Sb, Sn and Bi is easier as the distribution of Sb, Sn and Bi in the plated layer before heating for hot press forming is uniform.

  On the other hand, in order to obtain the above effects, the supply of the inert gas is preferably maintained for 1 hour or more, more preferably 3 hours or more. On the other hand, the upper limit is not particularly limited because it is advantageous to uniformly distribute the components in the plating bath as the supply time of the inert gas increases.

  Next, primary heating is performed to process the plated zinc-based plated steel sheet into a formed product. This step is carried out to increase the melting point by alloying with the base iron before the zinc of the plating layer oxidizes in the atmosphere, and to sufficiently provide the zinc content of the plating layer in the subsequent heating step. It is.

  The average heating rate of the primary heating is preferably 3.5 to 4.2 ° C./sec. If the temperature is less than 3.5 ° C./sec, the rise time becomes long, the effect of raising the melting point due to alloying is delayed, and excessive oxidation of zinc may occur. On the other hand, if it exceeds 4.2 ° C./sec, zinc on the surface melts earlier than alloying the material, and there is a possibility that the surface oxidation of the plating layer will be remarkable.

  It is preferable that the primary heating completion temperature of primary heating is 640-680 degreeC. If the temperature is lower than 650 ° C., the diffusion coefficient in the plating layer may be low due to the low temperature, and uniform plating of the plating layer may not be achieved. On the other hand, if it exceeds 680 ° C., the plating layer may be liquid phased over the melting point of the zinc Delta phase, and the zinc may be vaporized to cause loss of the plating layer.

  Next, the primarily heated zinc-based plated steel sheet is secondarily heated. At this stage, the internal oxidation inhibitor that is added while stably changing the plating layer that has sufficiently changed to the Delta phase to the Fe-alpha phase first segregates to the grain boundaries to prevent the grain boundary oxidation by oxygen, and micro cracks Is a stage performed to curb

  The average heating rate of the secondary heating is preferably 1.1 to 1.6 ° C./sec. If the temperature is less than 1.1 ° C./sec, the alloying time to the Fe-alpha phase becomes long, and there is a fear of the intergranular oxidation by oxygen rather than the intergranular segregation element. On the other hand, when the temperature exceeds 1.6 ° C./sec, partial plating layer liquefaction occurs on the surface of the high temperature plating layer, and there is a risk of quality deterioration due to the non-uniform surface.

  It is preferable that the secondary heating completion temperature of secondary heating is 900-930 degreeC. If the temperature is less than 900 ° C., sufficient austenite transformation of the material is not performed, and it is difficult to secure the strength of the final product. If the temperature exceeds 930 ° C., all the plating layer is liquid phased and the grain boundary oxidation added The micro crack suppression effect by an element may fall.

  Next, the secondary heated zinc-based plated steel sheet is maintained at a constant temperature for 1 to 5 minutes at the secondary heating end temperature. If the maintenance time is less than one minute, the lack of the total heating time may make it difficult to secure a sufficient time for austenite transformation of the material. On the other hand, if it exceeds 5 minutes, the occurrence of excessive alloying of the plating layer may lead to a reduction in corrosion resistance due to a reduction in the zinc content in the plating layer.

  Next, the secondary-heated zinc-based plated steel sheet is formed by a mold and rapidly cooled. Under the present circumstances, since shaping | molding and quenching by a metal mold | die are sufficient according to the normal hot press-forming method, in this invention, this does not specifically limit.

  Hereinafter, the present invention will be more specifically described by way of examples. However, the following examples are only for illustrating the practice of the present invention, and the present invention is not limited by the following examples. The scope of the present invention is determined by the matters described in the claims and the matters reasonably deduced therefrom.

A low-carbon cold-rolled steel plate with a thickness of 0.8 mm, a width of 100 mm, and a length of 200 mm is prepared as a substrate steel plate as a test specimen for plating, and then the substrate steel plate is immersed in acetone and subjected to ultrasonic cleaning, and rolled on the surface. Removed foreign matter such as oil. Thereafter, heat treatment at a temperature of 750 ° C. in a reducing atmosphere was conducted to secure the mechanical properties of the steel plate at a general hot-dip galvanizing site, and then immersed in a zinc-based plating bath having the composition of Table 1 below to produce a plated steel material. Thereafter, each of the manufactured plated steel materials was gas wiped, the plating adhesion amount was adjusted to 70 g / m ² per one side, and cooled at a rate of 12 ° C./sec.

  Thereafter, each of the cooled plated steel materials was heated under the conditions of Table 2 below, and then hot press-formed to obtain a hot press-formed product.

  Thereafter, each hot press-formed product was vertically cut, and the distribution of grain boundary segregated elements in the plating layer was measured by GDS analysis, and the results are shown in Table 2 below. The specific measurement method is as described above.

  Then, the maximum depth of the micro crack in the largest part of the tension at the time of molding and surface friction was measured, and the result is shown together in the following Table 2.

  Referring to Table 2, it can be confirmed that in the case of the invention examples 1 to 10 satisfying all the conditions of the present invention, the maximum depth of the micro cracks is suppressed to 10 μm or less.

  On the other hand, FIG. 1 observes and shows the micro crack of Comparative Example 1, FIG. 2 shows and observes the micro crack of Inventive Example 1, and FIG. 3 shows the micro crack of Inventive Example 3 It observes and shows, FIG. 4 observes and shows the micro crack of Comparative Example 4, and FIG. 5 observes and shows the micro crack of the invention example 5. As shown in FIG. 1 to 5, in the case of the invention example, it can be confirmed that the micro cracks in the plating layer effectively block the propagation to the base steel plate.

  On the other hand, FIG. 6 (a) is GDS data obtained by analyzing the contents of Al, Mg and Sn according to the depth of the plating layer of the invention example 1, and FIG. 6 (b) is the depth of the plating layer of the invention example 3. The GDS data which analyzed the content of Al, Mg, and Sn by A, and (c) of FIG. 6 are the GDS data which analyzed the content of Al, Mg, and Sn by the depth of the plating layer of the invention example 5.

It is preferable that the primary heating completion temperature of primary heating is 640-680 degreeC. If the temperature is lower than 640 ° C., the diffusion coefficient in the plating layer may be low due to the low temperature, and uniform plating of the plating layer may not be achieved. On the other hand, if it exceeds 680 ° C., the plating layer may be liquid phased over the melting point of the zinc Delta phase, and the zinc may be vaporized to cause loss of the plating layer.

Claims (7)

A hot press-formed product manufactured by hot-press forming a base steel sheet and a zinc-based plated steel sheet including a zinc-based plated layer formed on the surface of the base steel sheet,
The zinc-based plating layer contains one or more elements selected from the group consisting of Sb, Sn and Bi: 0.05 to 2.0% by weight in total, the balance Zn and unavoidable impurities,
At least 70% by weight of one or more elements selected from the group consisting of Sb, Sn and Bi is the alloyed zinc-based plated layer of the hot press-formed product formed by alloying the zinc-based plated layer A hot press-formed product that concentrates in an area within 3 μm from the surface of   The hot press-formed product according to claim 1, wherein the zinc-based plating layer contains 0.3 to 1.5% by weight in total of one or more elements selected from the group consisting of Sb, Sn and Bi.   The hot press-formed product according to claim 1, wherein the zinc-based plating layer further comprises, in weight%, Al: 0.1-5.0% and Mg: 0.1-5.0%.   The hot press-formed product according to claim 1, wherein the Fe alloying degree of the alloyed zinc-based plated layer is 30 to 85%. Preparing a galvanized steel sheet,
Primary heating the zinc-based plated steel sheet to a temperature of 640-680 ° C. at a rate of 3.5-4.2 ° C./sec;
Secondarily heating the primary-heated galvanized steel sheet to a temperature of 900 to 930 ° C. at a rate of 1.1 to 1.6 ° C./sec;
Maintaining the temperature of the secondarily heated galvanized steel sheet for 1 to 5 minutes;
Forming and rapidly cooling the zinc-based plated steel sheet maintained at a constant temperature with a mold;
Including
The zinc-based plated steel sheet is formed on the surface of the base steel plate and the base steel plate, and contains 0.05 to 2.0% by weight in total of one or more elements selected from the group consisting of Sb, Sn and Bi. The manufacturing method of a hot press-formed article containing a system plating layer.   The production of a hot press-formed product according to claim 1, wherein the zinc-based plating layer contains 0.3 to 1.5% by weight in total of one or more elements selected from the group consisting of Sb, Sn and Bi. Method.   The method for producing a hot press-formed product according to claim 1, wherein the zinc-based plating layer further comprises, in weight%, Al: 0.1-5.0% and Mg: 0.1-5.0%.

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