JP6152836B2 - Manufacturing method of hot press-formed product - Google Patents

Description

  The present invention relates to a hot press-formed product and a method for manufacturing the same, and in particular, when a surface-treated steel sheet that has been preheated is pressed, a method and a method for manufacturing a hot press-formed product that obtains a predetermined strength by quenching simultaneously with shape formation. It is related to hot press molded products.

In recent years, high strength and thinning of automobile parts have been demanded, and press workability has deteriorated with the increase in strength of steel sheets used, and it has become difficult to process steel sheets into desired part shapes.
In order to solve such problems, a steel plate heated to a high temperature is subjected to hot press molding into a desired shape using a mold, and the heat is extracted and quenched in the mold, and the parts after hot press molding A technique for increasing the strength of the steel is known.
For example, Patent Document 1 discloses that when a blank plate (steel plate) heated to an austenite single phase region around 900 ° C. is hot pressed to produce a part having a predetermined shape, it is quenched in the mold simultaneously with hot press forming. A technique for increasing the strength of parts by performing the above has been proposed.

  However, in the technique proposed in Patent Document 1, when the steel plate is heated to a high temperature of about 900 ° C. before pressing, oxide scale (iron oxide) is generated on the surface of the steel plate, and the oxide scale is formed during hot press forming. There exists a problem of peeling and damaging a metal mold | die or damaging the member surface after hot press molding. In addition, the oxide scale remaining on the surface of the member also causes poor appearance and poor paint adhesion. For this reason, usually, treatment such as pickling or shot blasting is performed to remove the oxidized scale on the surface of the member, but these treatments cause a decrease in productivity. Furthermore, excellent corrosion resistance is also required for automobile underbody members and vehicle body structural members, etc., but the technology proposed in Patent Document 1 does not have a rust preventive film such as a plating layer on the material steel plate, Corrosion resistance of the hot press-formed member becomes insufficient.

  For the above reasons, there is a demand for a hot press molding technique that can suppress the formation of oxide scale during heating before hot press molding and can improve the corrosion resistance of members after hot press molding. In response to such a demand, a surface-treated steel sheet provided with a coating such as a plating layer on the surface and a hot press forming method using the surface-treated steel sheet have been proposed. For example, in Patent Document 2, a steel sheet coated with Zn or a Zn base alloy is heated to 700 to 1200 ° C. and then hot pressed to form a Zn—Fe base compound or a Zn—Fe—Al base on the surface. A technique for forming a hot press-formed member provided with a compound has been proposed. Further, in Patent Document 2, by using a steel sheet coated with Zn or a Zn base alloy, it becomes possible to suppress the oxidation of the steel sheet surface, which becomes a problem during heating before hot press forming, and has excellent corrosion resistance. Further, it is described that a hot press-formed member is obtained.

  According to the technique proposed in Patent Document 2, generation of oxide scale on the surface of a hot press-formed member is suppressed to some extent. However, liquid metal embrittlement cracking due to Zn in the plating layer occurs, and a crack having a depth of about 100 μm may occur in the surface layer portion of the hot press-formed member. When such a crack occurs, various troubles such as deterioration of the fatigue resistance of the hot press-formed member are caused.

  With respect to such a problem, in Patent Document 3, a surface-treated steel sheet in which a Zn-Fe-based plating layer is formed on the surface of the base steel sheet is heated to a temperature not lower than the Ac1 transformation point of the base steel sheet and not higher than 950 ° C. And the method of starting shaping | molding after cooling a surface-treated steel plate to the temperature below the freezing point of a plating layer is proposed. Patent Document 3 describes that liquid metal embrittlement can be suppressed by cooling the surface-treated steel sheet to a temperature not higher than the freezing point of the plating layer and then starting forming.

GB 1490535 Patent No. 3663145 JP 2013-91099 A

According to the technique proposed in Patent Document 3, liquid metal embrittlement cracks occur, that is, occur on the surface of a hot press-formed member, and the depth from the plating layer-base metal interface to the inside of the base metal is about 100 μm. It is considered that cracks in which Zn is detected at the interface of the cracked portion (hereinafter referred to as “macro crack”) can be suppressed. In order to suppress such macro cracks, the present inventors examined the use of Zn—Ni alloy plating containing about 9 to 25% Ni in Zn as a high melting point plating layer. In order to ensure the corrosion resistance of Zn-Ni alloy plating, it is necessary to use Zn-Ni alloy as the γ phase. The γ phase present in the equilibrium diagram of the Zn-Ni alloy has a melting point of 860 ° C or higher and becomes a normal Zn-based plating layer. It is very high compared to the above, and the occurrence of macro cracks can be suppressed even under normal pressing conditions.
However, on the surface of the hot press-formed member, the depth from the plating layer-base metal interface to the inside of the iron core is not more than about 30 μm, not Zn, and Zn is not present at the interface of the cracked part. It is known that microcracks that are not detected occur. This microcrack is called a microcrack and penetrates through the plating layer-base metal interface to the inside of the base metal (base steel plate), which adversely affects various properties (such as fatigue resistance) of hot press-formed members. .
For example, when a hat cross-section member is press-molded, macro cracks occur even in portions where only tensile strain occurs, such as the punch contact side of the die shoulder R, whereas micro cracks do not occur in such portions. However, it occurs when subjected to a tensile strain after (bending) compression (bending back) as on the die contact side of the vertical wall, and it is assumed that the generation mechanism is different between the two.

In Patent Document 3, it is possible to suppress the occurrence of macro cracks in a surface-treated steel sheet on which a Zn-Fe-based plating layer is formed, but what are the microcracks in a surface-treated steel sheet in which a Zn-Ni plating layer is formed? It has not been taken into consideration, and is not necessarily effective in suppressing the occurrence of microcracks.
In the technique proposed in Patent Document 3, the entire surface-treated steel sheet is press-formed while being cooled to a temperature below the freezing point of the plating layer, and the lower limit value of the temperature at which press forming is started is not shown. The strength of the steel sheet during press forming increases due to the decrease in the forming temperature, and the shape freezing property (the property of maintaining the shape at the bottom dead center of the press with a slight spring back etc.) decreases and the spring back is likely to occur. There is also a problem.

  The present invention has been made in order to solve such a problem. When a hot-pressed member is manufactured by hot-pressing a surface-treated steel sheet on which a Zn-Ni-based plating layer is formed, An object of the present invention is to provide a method for producing a hot press-molded product and a hot press-molded product that suppress the occurrence of microcracks while suppressing a decrease in shape freezing property.

The present inventors have studied a means for suppressing microcracks (microcracks) that are problematic when hot-pressing a Zn-based plated steel sheet.
Although the microcrack formation mechanism is not clear, micro-cracking occurs on the surface of the plated steel sheet by press-forming a Zn-based plated steel sheet at a high temperature below the freezing point of plating, and the same applies to Zn-Ni plating. Occur. This microcrack is a microcrack having a depth of about 30 μm from the plating layer-base steel plate interface and penetrates the plating layer-base steel plate interface to the inside of the base steel plate. As a result of various studies conducted by the present inventors for such problems, it has been clarified that microcracks are suppressed by lowering the temperature during hot press molding. Furthermore, due to the temperature drop during press forming as described above, the effect of greatly reducing the amount of plating adhered to the mold, which is a problem with conventional hot-pressed plated steel sheets, was obtained.

However, when the steel plate temperature during press forming decreases, the strength of the steel plate increases and the shape freezing property decreases, making it impossible to take advantage of the advantages during hot press forming.
Therefore, the present inventors consider that it is sufficient to perform hot press molding after cooling only a portion subjected to processing that generates micro cracks during press molding, and what is the processing in which the micro cracks are generated? Yes, we examined what part is subject to the processing.
First, when examining the processing in which microcracks occur, various effects of processing strain on the occurrence of microcracks were examined. As a result, it was clarified that micro-cracks do not occur only by simple tension, compression deformation and bending deformation, but micro-cracks occur in the portion subjected to bending-bending unbending deformation, where the once bent portion is stretched again.
The processing as described above is limited to a specific part depending on the shape of the molded product, but if the shape of the molded product becomes various, the part subjected to processing that generates micro cracks covers the entire surface of the steel plate. There is also.

  Therefore, we examined a method that can suppress the occurrence of microcracks without being limited to a specific part of the steel plate that is the workpiece, and as a result, a cooling mold that can contact almost the entire surface of the steel plate before press forming. The steel sheet is held at a temperature of 550 ° C or lower and 410 ° C or higher for cooling, and then press forming is started when the steel plate temperature is 550 ° C or lower and 400 ° C or higher. It has been clarified that it is possible to suppress poor shape accuracy while suppressing the occurrence of microcracks.

The reason why the shape accuracy defect is suppressed by cooling with the cooling mold is considered as follows. A typical shape accuracy failure of the hat-shaped member is that the angle formed by the two surfaces sandwiching the bending ridge line becomes larger with respect to the mold angle, and the plane of the vertical wall portion has a curved surface. Wall warping can be mentioned. These are all caused by the difference in stress distribution in the thickness direction, and the higher the flow stress of the steel sheet during processing, the greater the difference in stress distribution and the lower the shape accuracy. That is, in the hot press, the lower the processing temperature, the higher the flow stress during processing of the steel sheet and the lower the shape accuracy. Although it is considered that the steel sheet temperature during processing decreases due to cooling, the shape accuracy is lowered, but the shape is almost reduced by cooling to a holding time where the steel sheet temperature is 410 ° C or higher and then starting press forming at 400 ° C or higher. No decrease in accuracy was observed. This is probably because when the steel plate temperature is 400 ° C. or higher, the structure at the time of pressing is austenite, the martensite transformation after the processing is relaxed, and the stress entered at the time of processing is relieved and the shape accuracy does not decrease.
Conversely, if the holding time is long and the steel sheet temperature is less than 400 ° C, it has already transformed into martensite at the time of press processing, so the increase in steel sheet strength is also added, and it is thought that wall warpage occurs due to the stress entered during processing .
The present invention has been made on the basis of the above-described knowledge, and specifically comprises the following configuration.

(1) The method for producing a hot press-formed product according to the present invention is a method for producing a hot press-formed product by subjecting a surface-treated steel plate having a Zn-Ni plating layer formed on the surface of a base steel plate to a hot press. A method for producing an intermediate press-formed product, comprising: a cooling die having a flat contact surface with the surface-treated steel sheet over the entire surface of the surface-treated steel sheet heated to a temperature range of 1000 ° C. or higher but not lower than the Ac3 transformation point. Using the cooling process to cool the surface-treated steel sheet to a temperature of 800 ° C or lower and 670 ° C or higher at a cooling rate of 100 ° C / s or higher to a temperature of 550 ° C or lower and 410 ° C or higher, and within 5 seconds after cooling A press forming step in which press forming is started within a range where the steel plate temperature is 550 ° C. or lower and 400 ° C. or higher using a press forming die, and the surface treated steel plate is held while being sandwiched between the press forming dies and the surface With a quenching process for quenching the treated steel sheet, Zn-Ni plating layer- The depth from the surface to the inside of the ground iron is about 30 μm or less, and the occurrence of microcracks called microcracks is prevented .

  In the present invention, a hot press-formed product manufacturing method for manufacturing a hot press-formed product by subjecting a surface-treated steel plate having a Zn-Ni plated layer formed on the surface of a base steel plate to hot press, is provided. The entire surface-treated steel sheet heated to a temperature range of not less than the transformation point and not more than 1000 ° C. is 550 at a cooling rate of 100 ° C./s or more using a cooling mold having a flat contact surface with the surface-treated steel sheet. A cooling step of cooling to a temperature of not higher than 410 ° C and a temperature of not lower than 410 ° C, a press forming step of starting press forming in a range where the temperature of the surface-treated steel sheet is not higher than 550 ° C and not lower than 400 ° C within 5 seconds after cooling, and the surface treatment By holding the steel sheet sandwiched between molds and quenching the surface-treated steel sheet, the molded product can be produced without any micro-cracking in any product shape. Has sufficient hardness and a large molding load Increases without, effect that even no problem as shape fixability.

It is explanatory drawing of the manufacturing method of the hot press-formed product which concerns on one embodiment of this invention. It is a schematic diagram which shows the relationship between a metal structure, temperature, and cooling time (the 1). It is a schematic diagram which shows the relationship between a metal structure, temperature, and cooling time (the 2). It is explanatory drawing of the test piece used for the experiment in one embodiment of this invention. It is explanatory drawing of the experimental result in one embodiment of this invention, Comprising: It is a graph which shows the temperature change of a test piece. It is a figure which expands and shows a part of horizontal axis | shaft of FIG. It is a figure which shows the experimental result in one embodiment of this invention, Comprising: It is a SEM image of a vertical wall part. It is a figure which shows the experimental result in one embodiment of this invention, Comprising: It is a figure which shows the relationship between shaping | molding start temperature and a press load. It is a figure which shows the experimental result in one embodiment of this invention, Comprising: It is a figure which shows the relationship between shaping | molding start temperature and opening amount. It is explanatory drawing of the shaping | molding method in one embodiment of this invention. It is explanatory drawing of the press-molded product press-molded in an Example. It is explanatory drawing of the microcrack verified in an Example. It is explanatory drawing of the amount of opening | mouth verification verified in an Example.

The method for manufacturing a hot press-formed product according to an embodiment of the present invention includes a hot press-formed product obtained by hot-pressing a surface-treated steel plate 1 having a Zn-Ni plating layer formed on the surface of a base steel plate. A method of manufacturing a hot press-formed product to be manufactured, as shown in FIG. 1, a surface-treated steel sheet 1 heated to a temperature range of not less than Ac3 transformation point and not more than 1000 ° C. has a contact surface with the surface-treated steel sheet 1. A cooling step (S1) in which a flat cooling die 3 is sandwiched and cooled to a temperature of 550 ° C. or lower and 410 ° C. or higher at a cooling rate of 100 ° C./s or higher, and a press mold within 5 seconds after cooling. 11 and press forming step (S2) in which press forming is started within a range where the steel plate temperature is 500 ° C. or lower and 400 ° C. or higher, and the surface-treated steel plate 1 is held while being sandwiched between the press-molding dies 11. And a quenching step (S3) for quenching 1.
Hereinafter, the raw material of the hot press-formed member, the cooling step (S1), the press-forming step (S2), and the quenching step (S3) will be described in detail.

<Hot press-molded material>
As a raw material for the hot press-formed member, a material in which a Zn—Ni plating layer is provided on the surface of a base steel plate is used. By providing the Zn—Ni plating layer on the surface of the steel sheet, the corrosion resistance of the member after hot press forming can be ensured.
The method for forming the Zn—Ni plating layer on the surface of the base steel plate is not particularly limited, and any method such as hot dipping or electroplating may be used. The amount of plating is preferably 10 g / m ² or more and 90 g / m ² or less per side.

The Ni content in the plating layer is preferably 9% by mass or more and 25% by mass or less. When the Zn-Ni plating layer is formed on the surface of the base steel sheet by electroplating, Ni ₂ Zn ₁₁ , NiZn ₃ , Ni ₅ Zn A γ phase having any one of the ₂₁ crystal structures is formed. Since this γ phase has a high melting point, it is advantageous for suppressing evaporation of the plating layer, which is a concern during heating of the surface-treated steel sheet before hot press forming. It is also advantageous for suppressing liquid metal embrittlement, which is a problem during hot press forming at high temperatures.

  The surface-treated steel sheet 1 is heated to a temperature range from the Ac3 transformation point to 1000 ° C. When the heating temperature of the surface-treated steel sheet 1 is lower than the Ac3 transformation point, an appropriate amount of austenite cannot be obtained during heating, and ferrite exists during press forming, so that sufficient strength can be obtained after hot press forming. It becomes difficult to ensure a good shape freezing property. On the other hand, when the heating temperature of the surface-treated steel sheet 1 exceeds 1000 ° C., the oxidation resistance and the corrosion resistance of the hot press-formed member are deteriorated due to evaporation of the plating layer and excessive generation of oxide at the surface layer portion. Accordingly, the heating temperature is set to the Ac3 transformation point or higher and 1000 ° C or lower. More preferably, it is Ac3 transformation point + 30 ° C. or higher and 950 ° C. or lower. The heating method of the surface-treated steel sheet 1 is not particularly limited, and any method such as heating with an electric furnace, an induction heating furnace, or a direct current heating furnace may be used.

<Cooling process>
The cooling step (S1) is a step in which the heated surface-treated steel sheet 1 is sandwiched between cooling dies 3 and cooled to a temperature of 550 ° C. or lower and 400 ° C. or higher at a cooling rate of 100 ° C./s or higher.
As shown in FIG. 1, the cooling mold 3 includes an upper mold 5 and a lower mold 7 whose contact surfaces with the surface-treated steel sheet 1 are flat, and the lower mold 7 is extended and contracted. A lifter pin 9 of the type is installed. The heated surface-treated steel sheet 1 is placed on the lifter pin 9 and then cooled by being sandwiched between the upper mold 5 and the lower mold 7.
The timing for sandwiching the heated surface-treated steel sheet 1 with the cooling mold 3 is preferably set to 800 ° C. or less without risk of the Zn—Ni plating layer adhering to the mold, and ensuring the strength after hot press forming. From this point, the temperature is preferably 670 ° C. or higher. The cooling mold 3 may be cooled by pressing against one side surface of the surface-treated steel sheet 1.

The reason why the cooling rate is set to 100 ° C./s or more is that a martensite single-phase structure can be obtained without increasing the cost, and high strength can be achieved.
This point will be described in more detail.
FIG. 2 is a schematic diagram showing the relationship between the metal structure, temperature, and cooling time. FIG. 2A shows a case where the molding start temperature is high, and after the molding starts, the mold is rapidly cooled by removing heat into the mold to become a martensite single phase structure.
On the other hand, as shown in FIG. 2B, when the molding start temperature is low, ferrite and bainite are generated before the molding starts, and the strength of the member after press molding is lowered.
In the present invention, since the press molding start temperature is lowered, the configuration shown in FIG. 2 (b) is obtained. By adopting a cooling process that can be rapidly cooled before the press starts, the curve shown by the broken line in FIG. As shown, a martensitic single phase structure can be obtained while lowering the molding start temperature.

  The reason for cooling to 550 ° C. or lower in the cooling step is that if it exceeds 550 ° C., cooling becomes insufficient and microcracks are generated after hot press forming. Further, the lower limit of the cooling temperature is set to 410 ° C., because when the temperature is exceeded and the surface-treated steel sheet 1 is excessively cooled before press forming, the shape freezing property after press forming is lowered. It is.

  Cooling in the mold in the cooling step before pressing was controlled by the time during which the material was held in the cooling mold 3 (see FIG. 1). Regarding the temperature change of the surface-treated steel sheet 1 by sandwiching the surface-treated steel sheet 1 with the cooling mold 3, the temperature of the surface-treated steel sheet 1 was measured by inserting a 0.5mmφ sheath thermocouple 19 into the steel sheet shown in FIG. . FIG. 5 is a graph showing the results, where the vertical axis represents temperature (° C.) and the horizontal axis represents time (s). FIG. 6 is a graph showing an enlarged horizontal axis of a portion surrounded by a broken line in FIG. As shown in FIG. 6, the temperature change due to mold cooling is about 160 ° C./s, and it can be seen that rapid cooling is possible.

  As evaluation items, observe the cross section of the vertical wall of the press-molded product to confirm the presence or absence of microcracks, confirm the hardness of the molded product, confirm the molding load, and the hat opening of the molded product. It is to confirm the shape freezing property by confirming the amount of opening (the difference between the width dimension of the opening part released after molding and the width of the molded product in the mold shape).

  FIG. 7 is an SEM image of the cross section of the steel sheet surface layer on the press molding die 11 side of the vertical wall, and microcracks are not observed when the cooling time in the die is 0.9 s or more (press molding start temperature 550 ° C. or less). I understand that. Moreover, it was confirmed that Hv> 450 under all conditions and there was no deterioration in hardenability.

  FIG. 8 is a graph showing the results of the molding load, in which the vertical axis represents the press load (kN) and the horizontal axis represents the press molding start temperature (° C.). As shown in the graph of FIG. 8, the press load increases with a decrease in the press forming start temperature due to die cooling before pressing, but mild steel (270D, cold It was confirmed that there was no problem with the molding load at the same level as that of (draw molding).

  FIG. 9 is a graph showing the results of the shape freezing property, in which the vertical axis represents the opening amount (mm) of the molded product, and the horizontal axis represents the press molding start temperature (° C.). As shown in the graph of FIG. 9, the amount of opening increases as the molding start temperature decreases due to cooling of the mold before press molding, and the shape freezeability tends to decrease. Until 400 ° C, almost no decrease in shape freezing property is observed.

  As described above, by starting press molding at a temperature of 550 ° C or lower and 400 ° C or higher, microcracks are not generated, the hardness of the molded product is sufficient, the molding load does not increase, and the shape freezing property It was confirmed that there was no problem.

<Press molding process>
The press forming step (S2) is a step of press forming the surface-treated steel sheet 1 into a product shape. The press molding process starts within 5 seconds after the cooling process and is performed using the press molding die 11. As shown in FIG. 1, the press molding die 11 includes a die 13 and a punch 17, and performs molding by sandwiching the surface-treated steel sheet 1 between the die 13 and the punch 17.

While the temperature of the surface-treated steel sheet 1 in the cooling process is between 550 ° C. and 410 ° C., the surface-treated steel sheet 1 is extracted from the cooling mold 3 and processed.
The press molding process is started within 5 seconds after the cooling process. If the time until the molding starts after cooling exceeds 5 seconds, ferrite, bainite, etc. are generated before the start of press molding, and martensite This is because a site single-phase structure cannot be obtained and the hardness of the press-formed product becomes insufficient. The time from the cooling step to the start of the press molding step is preferably within 3 seconds.

  The press molding method is not particularly limited. As shown in FIG. 10 (a), draw forming is performed with the die 13 and the blank holder 15 sandwiching the surface-treated steel sheet 1, or is the blank holder 15 lowered as shown in FIG. 10 (b)? Alternatively, it is possible to perform foam molding that performs molding without using the blank holder 15. From the viewpoint of suppressing microcracks, foam molding is preferable because the degree of processing of the vertical wall is small.

<Hardening process>
The quenching step is a step of quenching the surface-treated steel sheet 1 while holding the surface-treated steel sheet 1 while being sandwiched between the press molds 11. In order to quench the surface-treated steel sheet 1 with a die after press molding, it is preferable to stop the slide at the bottom dead center after press molding. The stop time varies depending on the amount of heat removed by the mold, but is preferably 3 seconds or more.

  In order to make the base steel sheet into a quenched structure by holding it in the mold for a predetermined time, for example, in mass%, C: 0.15% to 0.50%, Si: 0.05% to 2.00%, Mn: 0.50% or more 3.00% or less, P: 0.10% or less, S: 0.050% or less, Al: 0.10% or less, N: 0.010% or less, the hot-rolled steel sheet or the cold-rolled steel having a composition composed of Fe and inevitable impurities as the balance A steel plate can be used. The reason for limitation of each component is demonstrated below. Here, “%” indicating the content of a component means “% by mass” unless otherwise specified.

《C: 0.15% to 0.50%》
C is an element that improves the strength of the steel, and the amount is preferably 0.15% or more in order to increase the strength of the hot press-formed member. On the other hand, when the amount of C exceeds 0.50%, the weldability of the hot press-formed member and the blanking property of the material (base steel plate) are significantly reduced. Therefore, the C content is preferably 0.15% or more and 0.50% or less, and more preferably 0.20% or more and 0.40% or less.

<< Si: 0.05% or more and 2.00% or less >>
Si, like C, is an element that improves the strength of steel, and the amount is preferably 0.05% or more in order to increase the strength of the hot press-formed member. On the other hand, when the Si content exceeds 2.00%, the occurrence of surface defects called red scale during hot rolling significantly increases during the production of the base steel sheet. Therefore, the Si content is preferably 0.05% or more and 2.00% or less, and more preferably 0.10% or more and 1.50% or less.

《Mn: 0.50% to 3.00%》
Mn is an element that enhances the hardenability of the steel, and is an effective element for improving the hardenability by suppressing the ferrite transformation of the base steel sheet during the cooling process after hot press forming. Further, Mn is an element effective for lowering the heating temperature of the surface-treated steel sheet 1 before hot press forming because it has an action of lowering the Ac3 transformation point. In order to exhibit such an effect, the Mn content is preferably 0.50% or more. On the other hand, when the amount of Mn exceeds 3.00%, Mn is segregated and the uniformity of the properties of the base steel sheet and the hot press-formed member decreases. Therefore, the Mn content is preferably 0.50% or more and 3.00% or less, and more preferably 0.75% or more and 2.50% or less.

<< P: 0.10% or less >>
When the P content exceeds 0.10%, P is segregated at the grain boundaries, and the low temperature toughness of the base steel sheet and the hot press-formed member decreases. Therefore, the P content is preferably 0.10% or less, and more preferably 0.01% or less.

<< S: 0.050% or less >>
S is an element that combines with Mn to form coarse sulfides and causes a reduction in the ductility of the steel. Therefore, it is preferable to reduce the S content as much as possible, but it is acceptable up to 0.050%. Therefore, the S content is preferably 0.050% or less, and more preferably 0.010% or less.

<Al: 0.10% or less>
If the Al content exceeds 0.10%, the oxide inclusions increase and the ductility of the steel decreases. Therefore, the Al content is preferably 0.10% or less, and more preferably 0.07% or less. However, Al has an action as a deoxidizing material, and from the viewpoint of improving the cleanliness of the steel, its content is preferably 0.01% or more.

<N: 0.010% or less>
When the N content exceeds 0.010%, a nitride such as AlN is formed in the base steel sheet, resulting in a decrease in formability during hot press forming. Therefore, the N content is preferably 0.010% or less, and more preferably 0.005% or less.

Although the above is a preferable basic component of the base steel plate in this invention, this base steel plate may contain the following elements further as needed.
Cr: 0.01% or more and 0.50% or less, V: 0.01% or more and 0.50% or less, Mo: 0.01% or more and 0.50% or less, Ni: 0.01 or more and 0.50% or less.
Cr, V, Mo, and Ni are all effective elements for improving the hardenability of steel. This effect can be obtained by setting the content to 0.01% or more for any element. However, if the Cr, V, Mo, and Ni content exceeds 0.50%, the above effect is saturated, which increases the cost. Therefore, when containing one or more of Cr, V, Mo, Ni, the content is preferably 0.01% or more and 0.50% or less, more preferably 0.10% or more and 0.40% or less .

Ti: 0.01% or more and 0.20% or less
Ti is effective for strengthening steel. The effect of increasing the strength by Ti is obtained by setting its content to 0.01% or more. If it is within the range specified in the present invention, it can be used for strengthening steel. However, if the content exceeds 0.20%, the effect is saturated, which increases the cost. Therefore, when Ti is contained, it is preferably 0.01% or more and 0.20% or less, and more preferably 0.01% or more and 0.05% or less.

Nb: 0.01% or more and 0.10% or less
Nb is also effective for strengthening steel. The effect of increasing the strength by Nb can be obtained by setting its content to 0.01% or more, and can be used for strengthening steel as long as it is within the range specified in the present invention. However, if the content exceeds 0.10%, the effect is saturated, which increases the cost. Therefore, when Nb is contained, it is preferably 0.01% or more and 0.10% or less, and more preferably 0.01% or more and 0.05% or less.

B: 0.0002% or more and 0.0050% or less
B is an element that enhances the hardenability of the steel, and is an element effective for obtaining a quenched structure by suppressing the formation of ferrite from the austenite grain boundaries when the base steel sheet is cooled after hot press forming. The effect can be obtained when the B content is 0.0002% or more. However, if the B content exceeds 0.0050%, the effect is saturated, resulting in a cost increase. Therefore, when B is contained, the content is preferably 0.0002% or more and 0.0050% or less. More preferably, it is 0.0005% or more and 0.0030% or less.

Sb: 0.003% to 0.030%
Sb has an effect of suppressing a decarburized layer generated in the surface layer portion of the base steel sheet after the steel sheet is heated before hot press forming and before the steel plate is cooled by a series of processes of hot press forming. In order to exhibit such an effect, the Sb content is preferably 0.003% or more. However, if the Sb content exceeds 0.030%, the rolling load increases during the production of the base steel sheet, and there is a concern that the productivity may be reduced. Therefore, when Sb is contained, the content is preferably 0.003% or more and 0.030% or less, and more preferably 0.005% or more and 0.010% or less.

  Components other than the above components (remainder) are Fe and inevitable impurities.

In the present invention, the surface-treated steel sheet 1 used as a raw material of the hot press-formed member is not particularly limited in its production conditions. The production conditions of the base steel sheet are not particularly limited. For example, a hot-rolled steel sheet (pickled steel sheet) having a predetermined composition and a cold-rolled steel sheet obtained by cold rolling a hot-rolled steel sheet may be used as the base steel sheet.
The conditions for forming the Zn-Ni plating layer on the surface of the base steel sheet to form the surface-treated steel sheet 1 are not particularly limited. When a hot-rolled steel plate (pickled steel plate) is used as the base steel plate, the surface-treated steel plate 1 can be obtained by subjecting the hot-rolled steel plate (pickled steel plate) to a Zn—Ni plating treatment.

  On the other hand, when a cold-rolled steel sheet is used as the base steel sheet, the surface-treated steel sheet 1 can be obtained by performing a Zn-Ni plating process as it is after cold rolling or after annealing.

  When forming a Zn-Ni plating layer on the base steel plate surface, for example, after degreasing and pickling the base steel plate, nickel sulfate hexahydrate of 100 g / L to 400 g / L, 10 g / L to 400 g / L Perform electroplating at a current density of 10A / dm2 or more and 150A / dm2 or less in a plating bath containing the following zinc sulfate heptahydrate, pH 1.0 to 3.0 and bath temperature 30 ° C to 70 ° C. Thus, a Zn—Ni plating layer can be formed. In addition, when using a cold-rolled steel plate as a base steel plate, you may anneal a cold-rolled steel plate prior to the said degreasing | defatting and pickling. The Ni content in the plating layer can be adjusted to the desired Ni content (for example, 9% by mass to 25% by mass) by appropriately adjusting the concentration and current density of zinc sulfate heptahydrate within the above ranges. can do. Moreover, the adhesion amount of the Zn—Ni plating layer can be set to a desired adhesion amount (for example, 10 g / m 2 or more and 90 g / m 2 or less per side) by adjusting the energization time.

Since an experiment for confirming the effect of the method for producing a hot press-formed product according to the present invention was conducted, this will be described below.
As a slab by melting steel having the components shown in Table 1, the slab is heated to 1200 ° C, subjected to hot rolling at a finish rolling finish temperature of 870 ° C, and then wound up at 600 ° C and heated. A rolled steel sheet was used.

Next, the hot-rolled steel sheet was pickled and cold-rolled at a reduction rate of 50% to obtain a cold-rolled steel sheet having a thickness of 1.6 mm. The Ac3 transformation point shown in Table 1 was calculated from the following formula (1) (William C. Leslie, translated by Kouda Shigeyasu, Kumai Hiroshi, Noda Tatsuhiko, Leslie Steel Materials Science, Maruzen Corporation, 1985 , P.273).
Ac3 (℃) = 910-203√ [C] + 44.7 × [Si] -30 × [Mn] + 700 × [P] + 400 × [Al] (1)
In the formula (1), [C], [Si], [Mn], [P], and [Al] are the content% (mass%) of each element (C, Si, Mn, P, Al). is there.
The cold-rolled steel sheet obtained as described above is used as a base steel sheet, and a surface-treated steel sheet 1 is formed by forming each of the pure Zn plating layer, Zn-Fe plating layer, and Zn-Ni plating layer on the surface of the base steel plate. It was. Each plating layer was formed under the following conditions.

<Pure Zn plating layer>
The cold-rolled steel sheet is passed through a continuous hot-dip galvanizing line, heated to a temperature range of 800 ° C. or higher and 900 ° C. or lower at a temperature increase rate of 10 ° C./s. The Zn plating layer was formed by cooling to a temperature range of 460 ° C. or more and 500 ° C. or less at a cooling rate of ° C./s and dipping in a 450 ° C. zinc plating bath. The adhesion amount of the Zn plating layer was adjusted to a predetermined adhesion amount by a gas wiping method.

<Zn-Fe plating layer>
The cold-rolled steel sheet is passed through a continuous hot-dip galvanizing line, heated to a temperature range of 800 ° C. or higher and 900 ° C. or lower at a temperature increase rate of 10 ° C./s. The Zn plating layer was formed by cooling to a temperature range of 460 ° C. or more and 500 ° C. or less at a cooling rate of ° C./s and dipping in a 450 ° C. zinc plating bath. The adhesion amount of the Zn plating layer was adjusted to a predetermined adhesion amount by a gas wiping method. After adjusting to a predetermined adhesion amount by a gas wiping method, the Zn—Fe plating layer was formed by immediately heating to 500 to 550 ° C. in an alloying furnace and holding for 5 to 60 s. The Fe content in the plating layer was set to a predetermined content by changing the heating temperature in the alloying furnace and the residence time at the heating temperature within the above range.

<Zn-Ni plating layer>
The cold-rolled steel sheet is passed through a continuous annealing line, heated to a temperature range of 800 ° C to 900 ° C at a rate of 10 ° C / s, and retained in the temperature range for 10s to 120s, then 15 ° C / s It cooled to the temperature range below 500 degreeC with the cooling rate of s. Next, after degreasing and pickling, in a plating bath containing 200 g / L nickel sulfate hexahydrate, 10 to 300 g / L zinc sulfate heptahydrate pH 1.3, bath temperature 50 ° C., 30 to A Zn-Ni plating layer was formed by performing an electroplating process in which a current of 10 to 100 s was applied at a current density of 100 A / dm2. The Ni content in the plating layer was set to a predetermined content by appropriately adjusting the concentration and current density of zinc sulfate heptahydrate within the above ranges. Moreover, the adhesion amount of the Zn—Ni plating layer was set to a predetermined adhesion amount by appropriately adjusting the energization time within the above range.

  From the surface-treated steel plate obtained as described above, a blank plate of 200 mm × 400 mm is punched, and after the blank plate is heated by an electric furnace in an air atmosphere, the blank plate is placed in a mold (material: SKD61), Thereafter, cooling with a mold and press molding were performed. And after quenching in a metal mold | die, the hat-shaped press molding member shown in FIG. 11 was manufactured by releasing. The molds were punch punch R: 6 mm and die shoulder R: 6 mm, and the punch-die clearance was 1.6 mm. The surface-treated steel sheet before press forming was cooled by contact with a cooling mold. The press molding was performed by a draw molding in which a 98 kN wrinkle pressing force was applied and a foam molding in which no wrinkle pressing was performed.

  Table 2 shows the heating temperature of the blank plate, the type of the base steel plate, the type of the plating layer, the heating condition, the cooling condition before pressing, the pressing condition, and the state of the sample after pressing.

Samples were taken from the vertical wall portion of the obtained hat-shaped press-molded member, and the cross-section of the surface was observed with 10 fields of view for each sample at a magnification of 1000 using a scanning electron microscope (SEM). The presence or absence of microcracks that occurred on the sample surface and penetrated the interface between the plating layer and the base steel sheet and reached the inside of the base steel sheet, and the average depth of the microcracks were examined. The average depth of microcracks was determined as the average value of the depth of microcracks for 20 arbitrary microcracks. As used herein, “microcrack depth” refers to the length of the crack in the center direction of the thickness of the microcrack 21 measured from the interface between the plating layer 23 and the base steel plate 25 (see FIG. 12). 12, the length of h). When the number of observed microcracks was less than 20, the average depth of all the observed microcracks was taken.
Further, regarding the shape accuracy of the obtained press-molded member, the difference (W−W ₀ ) between the molded product width W after release of the hat member shown in FIG. 13 and the molded product width W ₀ in the mold shape is shown in FIG. As evaluated. The results are also shown in Table 2.

  In Invention Examples 1 to 7 and 8 to 9, the type of the plating layer (Zn—Ni plating layer), the cooling method (mold cooling), the cooling time (0.6 s to 1.7 s), the cooling rate (invention range: 100 ° C. / s) or more, and the cooling stop temperature (invention range: 410 ° C. to 550 ° C.), the time until the start of press molding after cooling (invention range: 5 seconds), the press molding start temperature (invention range 400 ° C. to 550 ° C.), All are within the scope of the present invention. The sample after pressing had no microcracks and the opening amount was 0 mm. As a result, it was demonstrated that the press molding method within the scope of the present invention can suppress the generation of microcracks while ensuring good shape freezing property.

  In Comparative Example 1, the type of the plating layer (Zn—Ni plating layer) is the same as that of the present invention, but is formed without cooling with a cooling mold. In Comparative Examples 2 to 4, the type of the plating layer is a Zn—Ni plating layer, but the cooling stop temperature (invention range: 410 ° C. to 550 ° C.) is outside the range of the present invention. Specifically, the cooling stop temperature of Comparative Example 2 is 600 ° C., and the cooling stop temperatures of Comparative Examples 3 and 4 are 340 ° C. and 290 ° C.

When the samples after press molding of Comparative Examples 1 and 2 are viewed, the opening amount is 0 mm, but microcracks are generated. As a result, it was proved that microcracks occur when the forming start temperature of the steel sheet is higher than 550 ° C.
In Comparative Examples 3 and 4, no microcracks are generated, but the amount of opening is 8 mm to 9 mm. As a result, if the cooling time is too long and the cooling stop temperature of the steel sheet is less than 410 ° C, the press forming start temperature is less than 400 ° C, so the strength of the steel sheet increases and the shape freezeability decreases. Has been demonstrated to occur.

In Comparative Examples 5 to 7, the type of the plating layer (Zn—Ni plating layer) is the same as that of the present invention, but the cooling method is gas cooling, so that the cooling rate is within the range of the present invention (100 ° C./s or more). ) And cannot be rapidly cooled. Therefore, in Comparative Examples 5 and 6, the steel sheet cooling stop temperature and press forming start temperature are outside the range of the invention (above 550 ° C.), and microcracks are generated. Further, in Comparative Example 7, the cooling stop temperature is 510 ° C., which is within the range of the present invention, but the shape openability is reduced to 3 mm and the shape freezeability is reduced. This is considered to be because the cooling time was slow due to gas cooling, the cooling rate was slow, and the angle change occurred.
Furthermore, in Comparative Examples 6 and 7, since the quenching was performed after the gas was cooled to a certain degree and then cooled and pressed, the hardness after press molding was lowered.

  In Comparative Examples 8 and 9, cooling method (mold cooling), cooling rate (180 ° C / s, 153 ° C / s), cooling stop temperature (530 ° C, 440 ° C), press molding start temperature (505 ° C, 420 ° C) ) Is within the scope of the present invention, but the time until the start of press molding is 10 seconds and 8 seconds, respectively, which is longer than 5 seconds of the present invention. Therefore, looking at the sample after press molding, the opening amount is 2 mm, and the hardness is reduced (365Hv, 382Hv).

  In Comparative Examples 10 and 11, the cooling method (mold cooling), the cooling rate (167 ° C./s, 190 ° C./s), the cooling stop temperature (500 ° C., 530 ° C.), the time until the start of press molding (2S), The press molding start temperature (495 ° C., 525 ° C.) is within the scope of the present invention, but the type of the plating layer is different. Comparative Example 10 is only Zn, and Comparative Example 11 is a Zn—Fe plating layer. For this reason, microcracks are generated in the sample after pressing.

DESCRIPTION OF SYMBOLS 1 Surface treatment steel plate 3 Cooling die 5 Upper die 7 Lower die 9 Lifter pin 11 Press molding die 13 Die 15 Blank holder 17 Punch 19 Thermocouple 21 Micro crack 23 Plating layer 25 Base steel plate

Claims (1)

A hot press-formed product manufacturing method for manufacturing a hot press-formed product by hot-pressing a surface-treated steel sheet having a Zn-Ni plating layer formed on the surface of a base steel plate,
The entire surface of the surface-treated steel sheet heated to a temperature range of 1000 ° C. or more above the Ac3 transformation point, using a cooling mold having a flat contact surface with the surface-treated steel sheet, the surface-treated steel sheet is 800 ° C. or less. at a timing that is a 670 ° C. or higher, a cooling step of cooling at 100 ° C. / s or more cooling rate until a temperature of 550 ° C. or less 410 ° C. or higher,
A press forming process in which press forming is started within a range of a steel sheet temperature of 550 ° C. or lower and 400 ° C. or higher using a press mold within 5 seconds after cooling;
Holding the surface-treated steel sheet sandwiched between the press-molding dies and quenching the surface-treated steel sheet,
A method for producing a hot press-formed product, characterized in that the depth from the Zn-Ni plating layer-base iron interface to the inside of the base iron is about 30 μm or less and the occurrence of microcracks called microcracks is prevented .