JP2016187813A - Hot forming method and apparatus of galvanized steel material, infrared furnace and hot-formed article of galvanized steel material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent intercrystalline cracks due to zinc heated to a predetermined temperature range as much as possible in the hot forming of a galvanized steel material.SOLUTION: A hot forming method of a galvanized steel material includes: a step preparing the galvanized steel material as a blank; a step heating the first region of the blank to a quenching temperature or higher and heating the second region of the blank to a non-quenching temperature; a step where a transition region being inevitably heated to a predetermined temperature range related to intercrystalline cracks due to zinc is forcedly cooled and the temperature of the transition region is made lower than the predetermined temperature range and higher than the temperature of the second region in the vicinity of a boundary of the first region and the second region; and a step performing the hot forming of the blank having temperature distribution set by heating or forced cooling.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method and apparatus for hot forming a zinc-based plated steel material, an infrared furnace, and a hot-formed product of a zinc-based plated steel material.

  The use of hot-pressed products for vehicle parts is expanding for the purpose of improving collision safety performance and reducing the weight of vehicle bodies. Accordingly, there is a demand for forming both a high strength region and a low strength region in one vehicle component. For example, for the B pillar, strength is required in the region on the upper side of the vehicle body, and elongation and corrosion resistance are required in the region on the lower side of the vehicle body. In accordance with these two requirements, it is suitable to use a plated steel material, particularly a zinc-based plated steel material, as a blank in order to create a plurality of strengths in one steel material and partially improve the elongation and the like.

  Patent documents related to the above background art are introduced below.

Patent Document 1 discloses the following hot press forming method for a plated steel sheet.
“A before performing hot press forming of a flat plated steel plate K that is uniformly heated to a temperature equal to or higher than the ₁ transformation point, a predetermined portion of the plated steel plate K is subjected to a martensitic transformation start temperature (Ms) by a partial quenching stage. Point) After rapidly cooling to below, the rapidly cooled portion of the plated steel sheet (K) is reheated to a temperature higher than the start temperature of the martensitic transformation (Ms point), and the plated steel sheet (K) is reheated and then molded. In the rapid cooling stage, the forming of the plated steel sheet (K) and the rapid cooling to the end temperature (Mf point) of the martensitic transformation are simultaneously performed. "

Patent Document 2 discloses the following hot press forming method for a plated steel sheet.
“By irradiating the first region X of the steel plate 1 with electromagnetic waves and irradiating the second region Y of the steel plate 1 with strong electromagnetic waves, the second region Y is brought to a temperature of 950 ° C. above the transformation point of the steel plate 1. By heating, the 2nd area | region Y is made into the austenite structure state.After irradiation of electromagnetic waves, the steel plate 1 is pressed by the press molding apparatus 10 and rapidly cooled, so that the second area Y is changed from the austenite structure state to the martensite state. It changes to the state of the site structure, whereby the strength of the second region Y is made higher than that of the first region X. "

Patent Document 3 discloses the following uniform heat treatment and hot press forming method for a zinc-based plated steel sheet.
“In manufacturing a hot press-formed product by forming a surface-treated steel sheet having a Zn—Fe-based plating layer formed on the surface of the base steel sheet by a hot press forming method, the surface-treated steel sheet is converted into an Ac ₁ transformation of the base steel sheet. By heating to a temperature of not less than 950 ° C. and cooling the surface-treated steel sheet to a temperature not higher than the freezing point of the plating layer according to the Fe content in the plating layer, and then starting the molding, Producing hot press-molded products with good properties by avoiding peeling of plating layer and intergranular cracking of base metal. "

Patent Document 4 discloses an infrared furnace as described below.
“The infrared furnace can heat the first region and the second region of the workpiece to different temperature ranges, and a plurality of infrared lamps facing the workpiece, and between the workpiece and the plurality of infrared lamps. And an object disposed on a boundary region between the first and second regions.

Patent Document 5 discloses a quenching method for a galvanized steel material as described below.
“A galvanized steel material having a galvanized layer containing 0.15% by mass or more of Al and Si each having a function of delaying alloying and an easily oxidizable function is oxidized with an oxygen content of 0.1% by volume or more. After heating to 800 ° C. or more and 950 ° C. or less in an atmosphere, cooling to 730 ° C. or less and 500 ° C. or more within 60 seconds, quenching is performed, and the surface of the formed steel material after quenching is mainly composed of Zn: Fe: 30% by mass A method for producing a high-strength quenched molded article excellent in corrosion resistance and fatigue resistance, characterized by forming a layer comprising 30 g / m ² or more comprising:

International Publication No. 2010-150683 (see summary and FIG. 1) Japanese Unexamined Patent Publication No. 2011-200086 (Refer to the means section of the abstract) International Publication No. 2013-031984 (see abstract) Japanese Patent Application Laid-Open No. 2014-148726 (refer to the column of means in the abstract) Japanese Patent No. 4733522 (refer to claim 1)

Japan Welding Boundary / Welding Information Center, “JWES Joining / Welding Technology Q & A 1000”, [online], [March 1, 2015 search], Internet <URL: http: //www-it.jwes.or. jp / qa / details.jsp? pg_no = 0050020500>

The following analysis is in accordance with the present invention.
However, when a blank made of galvanized steel material is heated and the blank is hot formed at least partially within a predetermined temperature range (for example, 700 ° C. or more and less than 800 ° C.), this predetermined value is obtained. There is a problem that intergranular cracking is likely to occur in the temperature range. In particular, when the heating temperature distribution having a temperature difference is formed in order to make the strength as described above, a transition portion having this predetermined temperature range is inevitably generated.

  As shown in Non-Patent Literature 1 and the like, it is considered that the grain boundary cracking in the zinc-based plated steel material occurs by the following mechanism. Zinc is heated and melted and penetrates into the grain boundary phase of the steel. When a tensile stress is applied to the grain boundary phase embrittled by zinc at the time of molding, grain boundary cracking develops.

  In Patent Documents 1 and 2, there is no description relating to zinc, a grain boundary crack caused by the zinc, and a predetermined temperature range causing the grain boundary crack. In Patent Document 3, there is a description regarding the grain boundary cracking, but there is no description regarding the temperature range that causes the grain boundary cracking due to zinc, and it is related to forming a high strength portion and a low strength portion in one steel material. There is no description to do. According to the infrared furnace of Patent Document 4, the transition portion having a predetermined temperature range can be narrowed, but it is difficult to make it zero. In Patent Document 5, there is no description regarding how to separate the intensity distribution, so-called baking.

  If there is a grain boundary crack, it is difficult to obtain the required strength or deformation shape (for example, deformation shape at the time of collision). Accordingly, an object of the present invention is to prevent as much as possible grain boundary cracking due to zinc heated to a predetermined temperature range in hot forming of a zinc-based plated steel material.

In a first aspect, a hot forming method is provided that includes the following steps:
Preparing a zinc-based plated steel material as a blank;
Heating the first region of the blank above the quenching temperature and heating the second region of the blank to a temperature below the quenching temperature;
In the vicinity of the boundary between the first region and the second region, a transition region that is inevitably heated to within a predetermined temperature range related to grain boundary cracking due to zinc is forcibly cooled, and the temperature of the transition region is set to the predetermined region. Making the temperature less than the temperature range and higher than the temperature of the second region;
A step of hot forming a blank having a temperature distribution set by the heating or forced cooling.

In a second aspect, a hot forming apparatus (system) having the following elements is provided:
A heating unit that uses a zinc-based plated steel material as a blank, heats the first region of the blank above the quenching temperature, and heats the second region of the blank to a temperature lower than the quenching temperature;
In the vicinity of the boundary between the first region and the second region, a transition region that is inevitably heated to within a predetermined temperature range related to grain boundary cracking due to zinc is forcibly cooled, and the temperature of the transition region is set to the predetermined region. A forced cooling section that is less than the temperature range and higher than the temperature of the second region;
A hot forming section that hot forms a blank having a temperature distribution set by the heating or forced cooling.

In a third aspect, an infrared furnace having the following elements is provided:
One or a plurality of first lamps arranged opposite to the first region of the blank to be heated by infrared rays and capable of outputting a relatively high-intensity first infrared ray;
One or a plurality of second lamps arranged opposite to the second region of the blank and capable of outputting a relatively low-intensity second infrared ray;
A first chamber in which the blank and the first and second lamps extend or stand in a direction opposite to each other between the first and second lamps, and the first and second regions are respectively disposed; A light shielding wall that serves as a barrier between the second chambers and shields the one infrared ray from entering the second chamber and the two infrared rays from entering the first chamber.

In a fourth aspect, a hot formed article having the following elements is provided:
A first region substantially fully quenched;
A partially or incompletely quenched transition region formed in a region adjacent to the first region and gradually decreasing in strength as the distance from the first region increases.

  The above viewpoints contribute to the following effects. In hot forming of zinc-based plated steel materials, the transition region heated within a predetermined temperature range that causes grain boundary cracking due to zinc in the steel material is forcibly cooled, and the temperature of the transition region is set to a temperature outside the predetermined temperature range. Reduce. Thereby, the occurrence of intergranular cracking due to the plated zinc is highly prevented.

It is a figure for demonstrating an example of the flow of the hot forming method which concerns on one Example, and a hot forming apparatus. It is a perspective view which shows an example of the hot forming goods obtained by the hot forming method and apparatus illustrated in FIG. (A)-(B) is a figure for demonstrating an example of an infrared furnace employ | adopted as the hot forming method and apparatus illustrated in FIG. 1, Comprising: (A) is the structure of an infrared furnace. (B) is a plan view showing an example of the temperature distribution of a blank heated by the infrared furnace of (A), and (C) is an example of the temperature distribution of (B). It is a graph to show. (A)-(B) is a figure for demonstrating an example of the forced cooling part employ | adopted as the hot forming method and apparatus illustrated in FIG. 1, (A) is the structure of a forced cooling part. (B) is a plan view showing an example of a temperature distribution of a blank partially cooled by the forced cooling section of (A), and (C) is a temperature of (B). It is a graph which shows an example of distribution. It is a graph which shows the change by the time of the heating temperature distribution of the zinc system plating steel materials heated according to the heating method which concerns on one Example. 6 is a graph showing the hardness (strength) distribution of a hot-worked product obtained by hot forming and quenching the zinc-based plated steel material having the heating temperature distribution after forced cooling shown in FIG.

From the above viewpoints, the following modes are possible.
(Form 1) Form 1 is as in the first viewpoint. The optimum heating temperature varies depending on the composition and the shape of the blank. This is because the temperature and the like of the austenite transformation point change depending on the composition, and the mass effect changes depending on the shape of the blank. Therefore, it is preferable to set various heating temperatures in consideration of the composition and the size of the blank.

(Form 2)
The predetermined temperature range is a range of 700 ° C. or higher and lower than 800 ° C. In the heating step, the first region is heated to a temperature of 800 ° C. or higher, and the second region is heated to less than 700 ° C. In the forced cooling step, the transition region is forcibly cooled to a temperature lower than 700 ° C. but higher than the second region. Thereby, grain boundary cracking due to heating of zinc to a predetermined temperature range can be prevented, and a sudden change in characteristics in the transition region can be mitigated.

(Form 3)
In the heating step, the first region is heated to a temperature of A3 point or higher. In the heating step, the second region is heated to a temperature below the A1 point. In the forced cooling step, the transition region is forcibly cooled so as to be in a temperature range that is less than the lower limit of the predetermined temperature range and exceeds the temperature of the second region. However, the A1 point is a temperature (Ac1 point) at which austenite starts to be generated during heating, and a temperature at which austenite completes transformation to ferrite, ferrite, or cementite during cooling. The A3 point is a temperature at which ferrite completes transformation to austenite during heating (Ac3 point), and a temperature at which ferrite transformation starts at cooling (Ar3 point).

(Form 4)
Due to the forced cooling, the transition region has a temperature distribution in which the temperature gradually decreases from the first region side toward the second region side. In this way, by forming a temperature distribution with a temperature difference also in the transition region itself, extreme stress concentration due to a sudden change in temperature or strength can be prevented.

(Form 5)
During or after the hot forming, the blank is quenched to quench the first region and the transition region is partially quenched to have a strength between the first and second regions. In the case of die quenching, rapid cooling is performed during hot forming. When rapid cooling is performed after hot forming, the formed blank may be immersed in a bath. The partial quenching of the transition region means that the abundance ratio of the quenching structure in the transition region is lower than the abundance ratio of the quenching structure in the first region, and a significant characteristic between the first region and the transition region. It means there is a difference (intensity difference, etc.).

(Form 6) Form 6 is as in the second viewpoint.

(Form 7) Form 7 is as the third viewpoint.

(Form 8)
The light shielding walls are erected on both sides of the blank with the blank interposed therebetween. With this structure, high-intensity infrared light is highly prevented from leaking to the second region side set to low intensity, or low-intensity infrared light is leaked to the first region side set to high and low intensity.

(Form 9)
The light shielding wall has a slit through which the blank can pass in an intermediate portion thereof. This slit facilitates the conveyance of the blank.

(Mode 10) As in the fourth aspect. Preferably, the hot-formed product of the galvanized steel material further has a second region that is not substantially quenched, and the transition region is located between the first and second regions. .

  According to one embodiment, the zinc-based plated steel material is heated to a high temperature in a first region where relatively high strength is desired, and to a low temperature where a second region where relatively low strength is desired (this is Next, the transition region (intergranular cracking temperature generating portion) that has been heated to a predetermined temperature range that causes the occurrence of grain boundary cracking is forcibly partially cooled, and this transition region is called. The galvanized steel material having such a temperature distribution state is hot-formed. During hot forming, the temperature of the cooled zinc is out of the predetermined temperature range that causes intergranular cracking, so intergranular cracking is as much as possible even if tensile stress associated with forming acts on the intergranular boundary. To be prevented.

  By the forced cooling, the temperature of the transition region is quickly lowered to outside the predetermined temperature range. As a result, the amount of heat conduction from the first region to the second region is significantly reduced compared to the case where the temperature is lowered until the temperature in the transition region falls outside the predetermined temperature range, and the temperature drop in the first region and The temperature rise of the second region and the expansion of the width of the transition region (intermediate strength region) are prevented as much as possible. Moreover, when compared with the above-mentioned cooling by forced cooling, the cycle time until a molded product is obtained can be greatly shortened.

  As the blank (starting material), a hot dip galvanized steel material, an alloyed hot dip galvanized steel material, an electrogalvanized steel material, or the like can be used. For example, zinc plating, zinc iron alloy plating, zinc nickel alloy plating and tin-zinc alloy plating (see JIS D 0210), hot dip galvanized steel sheet and steel strip (see JIS G 3302), hot dip zinc-aluminum alloy plated steel sheet and steel Band (JIS G 3317), hot-dip aluminum-zinc alloy plated steel plate and steel strip (JIS G 3321), hot-dip zinc-aluminum-magnesium alloy plated steel plate and steel strip (JIS G 3323), and electrogalvanized steel plate (JIS G 3313) And the like.

  The first region having high strength may be quenched so as to have strength higher than at least the strength of the base material. The first region may not have a completely quenched structure. For example, the first region is quenched so that its tensile strength (see JIS Z 2201) is 1000 MPa class, 1300 MPa class, 1500 MPa class, or 1600 MPa class. For this reason, the 1st field is heated, for example to A3 point or more. When the set temperature of the first region is indicated by a numerical value, the first region is heated to a range of 800 to 950 ° C., for example.

  The second region to be reduced in strength only needs to have the same strength as that of the base material, but may be higher than the strength of the base material. The second region may partially have a hardened structure. For example, the second region is heated so that its tensile strength is 500 MPa class, 600 MPa class, or 700 MPa class or less. If necessary, the tensile strength of the second region can be 700 MPa or higher. The second region may only be heated to, for example, less than A1 point. The lower limit of the heating temperature in the second region is preferably determined in consideration of workability. This is because processing is generally easier when the heating temperature is higher. When the set temperature of the second region is represented by a numerical value, the second region is heated to a range of, for example, 250 to 650 ° C, 300 to 650 ° C, 350 to 650 ° C, or 300 to 600 ° C.

  The predetermined temperature range that causes the occurrence of intergranular cracking varies depending on the composition of the plating containing the base material and zinc. Typically, the predetermined temperature range is 700 ° C. or higher and lower than 800 ° C. Depending on the composition, this predetermined temperature range fluctuates up and down in the range of approximately several tens to one hundred degrees Celsius.

  The transition region (gradual change region) is forcibly cooled to have a temperature and / or strength between the first region and the second region. The transition region may have a quenched structure. However, the abundance ratio of the quenched structure in the transition region is preferably smaller than the abundance ratio of the quenched structure in the first region and larger than the abundance ratio of the quenched structure in the second region.

  As a heating method of the blank, infrared heating, energization heating, induction heating, and atmosphere heating can be employed. Among them, it is preferable to use infrared heating that can perform highly directional heating and can heat a relatively wide range at a time. In infrared heating, in order to form a heating distribution having a temperature difference, it is preferable to use near infrared rays having a relatively strong directivity, for example, near infrared rays having a wavelength in the range of 0.8 to 2 μm.

  Examples of the forced cooling method of the transition region include spraying of a cooling medium, pressing of a cooling metal, or sandwiching. As the cooling medium, liquid, gas, and a mixture thereof can be used. When spraying the cooling medium, a guide is provided between the transition region and the first and second regions so that the first and second regions are not cooled, and only the transition region is reliably cooled. Is preferred. Alternatively, a cylindrical member having a diameter corresponding to the size of the transition region may be used as the guide. As the cooling metal, a metal or a cooling metal containing a refrigerant can be used. Further, forced cooling may be performed only from one side or the other side of the blank, or from both sides of the one side and the other side.

  Examples of hot forming methods include hot stamping (hot pressing) and die quenching. The rapid cooling for quenching can be performed by, for example, a mold used for molding. A part or all of the mold may be cooled. This quenching is efficient to be performed at the same time as molding, but can be performed after molding in some cases.

  Preferably, the infrared furnace has first and second chambers separated from each other by the furnace wall and the light shielding wall. It is preferable to form a slit for carrying a blank on the light shielding wall. However, it is preferable to form the slit narrowly so that high-intensity infrared rays do not leak from the first chamber into the second chamber through the slit. Further, the light shielding wall and the furnace wall may not be in close contact with each other, but their clearance is preferably narrow. The light shielding wall is preferably formed of a material that has heat resistance and light resistance and does not react with zinc or the base material, and is formed of, for example, stainless steel or ceramics.

  Although it is sufficient to arrange the lamps arranged in the infrared furnace and outputting infrared rays on one side of the blank, they may be arranged on both sides of the blank so as to sandwich the blank. When the lamp is arranged on one side of the blank, it is preferable to arrange a reflecting surface on the other side of the blank in order to increase the heating efficiency. Further, in order to improve the directivity of the lamp, a reflecting material or a shielding material may be partially attached to the lamp or its periphery.

  The output ratio of the first lamp on the first area side and the second lamp on the second area side may be basically set according to the ratio of the set temperatures of the first area and the second area. The output intensity of the lamp can be controlled by adjusting the amount of electric power to be input or the amount of current flowing through the cathode line that emits infrared rays. Further, the arrangement density of the lamps may be partially changed, or a shield may be partially provided between the lamp and the blank, or a member for adjusting the light transmittance may be provided.

  In the hot-formed product of the obtained zinc-based plated steel material, the width of the transition region is, for example, 5 to 100 mm. Of course, this width can be set according to the required specifications. Further, by forming an intensity distribution having a difference in intensity in the transition region itself, a sudden change in intensity can be mitigated, and extreme stress concentration can be prevented. Preferably, the transition region is not completely quenched, i.e. partially or incompletely quenched, compared to the first region. In the transition region, it is preferable that the abundance ratio of the hardened structure is smaller than that in the first region, and the abundance ratio of the hardened structure is larger than that in the second region.

  The material of the light shielding wall can be selected from heat resistant metal plates such as stainless steel, ceramics, heat resistant boards, and heat resistant silica.

  The above-described embodiments can be appropriately combined as long as a predetermined effect is achieved.

  An embodiment will be described below with reference to the drawings. In the following description, reference numerals used in the drawings are added for convenience to the elements in the drawings for the sake of understanding, and are not intended to limit the present invention to the illustrated embodiments. .

  FIG. 1 is a diagram schematically illustrating a flow of a hot forming method according to an embodiment and a configuration of a hot forming apparatus. In particular, in the present embodiment, an example in which an infrared furnace is used as a heating unit that performs a heating process will be described.

  Referring to FIG. 1, a hot forming apparatus (system) 1 includes a heating unit (hereinafter referred to as “infrared furnace”) 10 that forms a predetermined temperature distribution by infrared heating a blank W made of galvanized steel. The forced cooling unit 20 forcibly and partially cooling the heated blank W, and the hot forming unit 30 for molding and quenching the partially and forcedly cooled blank W are included. The blank W can be obtained by cutting or blanking, for example, a steel strip made of zinc-based plated steel.

  FIG. 2 is a perspective view showing an example of a hot-formed product that can be obtained by the hot-forming method and apparatus illustrated in FIG. The hot-formed product shown in FIG. 2 is a pillar P that is one of body parts of an automobile. The pillar P has a hat-like cross-sectional shape as a whole in order to reduce the weight and improve the collision safety, and further has a different strength (partially strengthened or partially softened). Specifically, the pillar P includes a first region 66 that is hardened and has a higher strength than the base material, a second region 67 that is not hardened and maintains the same strength as the base material, and a first region 66. And a transition region 68 between the first region 66 and the second region 67 that is partially or incompletely quenched. A plurality of holes 65 are formed in the pillar P. It is also possible to reduce the strength of the portion from which the hole 65 is removed or increase the strength around the hole 65.

  Next, each step of the hot forming method illustrated in FIG. 1 and each element of the hot forming apparatus will be described in detail.

[Heating section, infrared furnace 10]
FIG. 3A is a front view illustrating the structure of an infrared furnace that is suitably employed in the molding method and apparatus illustrated in FIG. FIG. 3B is a plan view showing an example of the temperature distribution of the blank heated by the infrared furnace of FIG. FIG. 3C is a graph showing an example of the temperature distribution in FIG.

Referring to FIG. 3A, the infrared furnace 10 includes the following elements:
A plurality of first lamps 12a arranged opposite to the first region R1 of the blank W to be infrared-heated and capable of outputting a relatively high-strength first infrared ray 13a;
A plurality of second lamps 12b arranged opposite to the second region R2 of the blank W and capable of outputting a relatively low-intensity second infrared ray 13b;
The first chamber 11a and the second chamber 11b in which the first and second regions R1, R2 are respectively placed
A light-shielding wall extending or standing between the first and second lamps 12a and 12b along the lamp arrangement direction and in a direction in which the blank W and the first and second lamps 12a and 12b face each other. 15.

  The first and second lamps 12a and 12b are arranged on one side of the blank W. On the other surface side of the blank W (on the opposite side of the one surface), a reflective surface 14 is disposed to increase the heating efficiency. The reflection surface 14 generates a first reflected light 14a having a relatively high intensity on the first chamber 11a side, and generates a second reflected light 14b having a relatively low intensity on the second chamber 11b side.

  The first and second chambers 11a and 11b are separated from each other by the light shielding wall 15, the reflecting surface 14, and the furnace wall 10a shown in FIG. In particular, the light shielding wall 15 serves as a barrier between the first chamber 11a and the second chamber 11b so that the first infrared ray 13a enters the second chamber 11b and the second infrared ray 13b enters the first chamber 11a. Is preventing. The light shielding walls 15 are erected on both sides of the blank W to increase the degree of light shielding. A slit 15 a is formed in the middle portion of the light shielding wall 15. The conveyed blank W can pass through the slit 15a.

  In the state shown in FIG. 3A, the transition region R3 having a temperature at which grain boundary cracking due to zinc occurs is located on the first chamber 11a side. A part of the transition region R3 may be formed immediately below the light shielding wall 15 (position of the slit 15a) or may be formed on the second chamber 11b side.

[Heating process]
A heating process of the blank W in the infrared furnace 10 will be described with reference to FIGS. In the heating step, for example, infrared heating is performed so that the temperature T1 of the first region R1 is in the range of 800 ° C. or more and 950 ° C. or less, and the temperature T2 of the second region R2 is less than 700 ° C. The temperature of the first region is set, for example, in a range of 810 to 940 ° C or 820 to 930 ° C. The lower limit of the temperature T2 in the second region R2 is preferably set as described above in consideration of at least workability. Depending on the case, you may set temperature T2 in the range of 600-695 degreeC, 625-675 degreeC, or 630-670 degreeC.

  Here, a predetermined temperature range Tr related to or generated in the grain boundary cracking due to zinc is determined to be 700 ° C. or more and less than 800 ° C. Since the transition region R3 is between the first and second regions R1 and R2, it inevitably has a portion included in the predetermined temperature range Tr. Further, the temperature T3 of the transition region R3 gradually decreases from the first region R1 to the second region R2.

The temperatures T1, T2 of the first and second regions R1, R2 may be set as follows:
Quenching temperature <T1, or A3 point ≦ T1;
T2 <temperature lower than quenching temperature or T2 <A1 point.

[Forced cooling section]
A blank W having a temperature distribution as shown in FIG. 3C is carried out from the infrared furnace 10 and conveyed to the forced cooling unit 20. In the forced cooling unit 20, forced cooling of the transition portion R3 is performed. In addition, the forced cooling part 20 can also be installed in the heating part 10 thru | or an infrared furnace.

  FIG. 4A is a front view illustrating the structure of the forced cooling unit 20 that is preferably employed in the hot forming method and the hot forming apparatus 1 illustrated in FIG. 1. Referring to FIG. 4A, the forced cooling unit 20 includes a nozzle 21 that blows the cooling medium F onto the transition region R3, and a guide 22 that limits the range of contact with the cooling medium F to the transition region R3.

[Forced cooling process]
In the forced cooling unit 20, as shown in FIG. 3C, the cooling medium F is sprayed onto the transition region R3 inevitably heated to a predetermined temperature range Tr related to the grain boundary cracking due to zinc. As a result, as shown in FIG. 4C, the temperature T3 of the transition region R3 falls below the predetermined temperature range Tr. However, it is preferable that the temperature T3 of the transition region R3 is higher than the temperature of the second region R2 in accordance with a design requirement regarding strength. The transition region R3 may have a temperature distribution or a temperature gradient in which the temperature gradually decreases from the first region R1 toward the second region R2. This can alleviate sudden changes in intensity.

When the temperatures T1 and T2 of the first and second regions R1 and R2 are set as follows, the temperature T3 of the transition region R3 is less than the predetermined temperature range Tr, and the quenching temperature or the A3 point or less, and It is preferable to set the temperature to be lower than the quenching temperature or higher than the A1 point.
Quenching temperature <T1, or A3 point ≦ T1;
T2 <temperature lower than quenching temperature or T2 <A1 point.

[Hot forming part]
A blank W having a temperature distribution as shown in FIG. 4C is unloaded from the forced cooling unit 20 and conveyed to the hot forming unit 30. In the hot forming part (hot forming and rapid cooling part) 30, rapid cooling can be executed together with hot forming. In addition to the hot forming unit 30, a quenching unit may be provided.

[Hot forming process]
In the hot forming section 30, a blank having a temperature distribution as shown in FIG. 4C is rapidly cooled simultaneously with forming, and a hot formed product having a shape as shown in FIG. 2, for example, is manufactured. The hot forming unit 30 is configured as a die quench device, for example.

  The rapid cooling is performed at a speed at which the first region R1 of the blank W is sufficiently or completely quenched. By this rapid cooling, the transition region R3 is also partially or incompletely quenched. Since the formation of the transition region R3 is started from outside the predetermined temperature range related to the grain boundary cracking, the occurrence of the grain boundary cracking in the transition region R3 is prevented as much as possible during the molding. Further, since the temperature of the second region R2 is set sufficiently low, it is not substantially quenched. As a result, the first region R1 has a relatively high strength, the second region R2 has a relatively low strength, and the transition region R3 has a strength between the first and second regions R1 and R2. Since the transition region R3 has the above-described temperature gradient before rapid cooling, the intensity of the transition region R3 gradually decreases from the first region R1 toward the second region R2. Thus, the hot-formed product obtained from the blank W has a strength distribution basically proportional to the temperature distribution shown in FIG.

[Experiment 1: Heat test]
Using an infrared furnace as shown in FIG. 3 (A), the zinc-based plated steel material is heated so that the heating temperature distribution as shown in FIGS. 3 (A) to (C) is once formed, Partial forced cooling was performed so that the heating temperature distribution as shown in FIGS. 4A to 4C was finally formed, and the change of the heating temperature distribution with time was measured. The experimental conditions are as follows.

Experimental conditions:
Infrared furnace:
Lamp type: Straight tube with a reflective film to improve directivity Output: Near infrared light shielding wall:
Installation position: boundary between the first region R1 and the second region R2 Dimensions: height and thickness (thickness in the left-right direction in FIG. 3 (A)) substantially the same as the furnace chamber height is 5 mm
Material: Stainless steel (SUS304)
blank:
Material: Zinc-based plated steel Dimensions: (Length l) 500 x (Width w) 300 x (Thickness t) 1 mm
Temperature setting Temperature setting in the first region R1: 860 ° C.
Set temperature of second region R2: 330 ° C.
Predetermined temperature range in which intergranular cracking occurs due to zinc: 700 ° C or higher and lower than 800 ° C Forced cooling: Air sprayed for 5 seconds after 50 seconds of heating

  FIG. 5 shows the change over time in the heating temperature distribution of the zinc-based plated steel material that has been heated as described above and further partially forcibly cooled. In FIG. 5, the temperature T1 of the first region R1 set at a high temperature is indicated by a dotted line, the temperature T2 of the second region R2 set at a low temperature is indicated by a two-dot chain line, and the temperature T3 of the transition region R3 is indicated by a one-dot chain line. In FIG. 5, the temperature T4 of the transition region R3 when the transition region R3 is not forcibly cooled (comparative example) is indicated by a solid line.

  Referring to FIG. 5, at the end of heating (at the time of 50 seconds), the temperature T3 of the transition region R3 was within a predetermined temperature range Tr (see FIG. 4C) where there is a possibility of grain boundary cracking due to zinc. . By forcibly cooling the transition region R3, the temperature T3 decreased to outside the predetermined temperature range. At this time, the temperature T1 of the first region R1 was maintained at a temperature higher than the quenching temperature. The temperature T2 in the second region R2 was hardly affected by forced cooling.

  On the other hand, when the forced cooling of the transition region R3 was not performed, the temperature T4 of the transition region R3 was within the predetermined temperature range Tr for ten and several seconds after the end of heating. Therefore, if hot working is performed in such a state, there is a possibility that a grain boundary crack may occur in the transition region R3.

[Experiment 2: Hardness test as a substitute for tensile strength test]
A blank having a temperature distribution (see T1, T2, and T3 shown in FIG. 5) after partial forced cooling according to Experiment 1 was rapidly cooled while hot forming by die quenching.

  FIG. 6 shows the hardness (strength) distribution of a hot-formed product of the zinc-based plated steel material die-quenched as described above. In a hot-formed product of a steel material having a martensite structure by quenching, the hardness is substantially proportional to the strength. Referring to FIG. 6, the width of the transition region R <b> 3 that should be partially forcedly cooled by the installation of the light shielding wall having a width of 5 mm is about 30 mm or less. In FIG. 6, the left side of about −30 mm corresponds to the first region R1, and the right side of about 5 mm corresponds to the second region R2. The first region R1 was sufficiently quenched and had a hardness corresponding to a tensile strength of 1300 MPa or more, and the second region R2 was not quenched and had a hardness corresponding to a tensile strength of about 500 MPa corresponding to the base material. The transition region R3 had a hardness or tensile strength between the first region R1 and the second region R2. The hardness or tensile strength of the transition region R3 gradually decreased from the first region R1 toward the second region R2, and the sudden change in characteristics in the transition region R3 was alleviated. Further, during hot forming and rapid cooling, no intergranular cracking occurred in the transition region R3.

  As mentioned above, although embodiment and the Example of this invention were described, this invention is not limited to above-described embodiment etc., and is a further deformation | transformation in the range which does not deviate from the fundamental technical idea of this invention. Substitutions or adjustments can be made.

  Each disclosure of the above-mentioned patent document and non-patent document is incorporated herein by reference. Within the scope of the entire disclosure (including claims) of the present invention, the embodiments and examples can be changed and adjusted based on the basic technical concept. Also, various combinations or selections of various disclosed elements (including each element of each claim, each element of each embodiment or example, each element of each drawing, etc.) within the scope of the entire disclosure of the present invention. Is possible. That is, the present invention of course includes various variations and modifications that could be made by those skilled in the art according to the entire disclosure including the claims and the technical idea. Further, the numerical range and the upper limit or lower limit numerical value described in the present application are not limited to the numerical value or numerical range described, and any small range or arbitrary intermediate value is described, unless otherwise specified. Shall be considered.

  INDUSTRIAL APPLICABILITY The present invention is suitably used for manufacturing body parts, for example, various pillars, side members, or impact bars that are components of doors.

1 Hot forming equipment (system)
DESCRIPTION OF SYMBOLS 10 Heating part, infrared furnace 10a Furnace wall 11a First chamber 11b Second chamber 12a First lamp 12b Second lamp 13a High intensity first infrared ray 13b Low intensity second infrared ray 14 Reflecting surface 14a High intensity first reflection Light 14b Low intensity second reflected light 15 Light shielding wall 15a Slit 20 Forced cooling part 21 Nozzle 22 Guide, cylindrical member 30 Hot forming part F Cooling medium P Pillar 65 Hole 66 First area 67 Second area 68 Transition area W Blank R1 first region (high temperature region, high strength region) made of galvanized steel
R2 2nd region (low temperature region, low strength region)
R3 transition region (medium temperature region, gradual change region), grain boundary cracking temperature generation part T1 first region temperature T2 second region temperature T3 transition region temperature T4 temperature change when the transition region is not forcibly cooled (comparison) Example)
Tr Predetermined temperature range where grain boundary cracking occurs due to zinc

Claims (10)

Preparing a zinc-based plated steel material as a blank;
Heating the first region of the blank above the quenching temperature and heating the second region of the blank to a temperature lower than the quenching temperature;
In the vicinity of the boundary between the first region and the second region, a transition region that is inevitably heated to within a predetermined temperature range related to grain boundary cracking due to zinc is forcibly cooled, and the temperature of the transition region is set to the predetermined region. A forced cooling step of making the temperature lower than the temperature range and higher than the temperature of the second region;
Hot forming a blank having a temperature distribution set by the heating or forced cooling; and
A hot forming method for a zinc-based plated steel material, comprising: The predetermined temperature range is a range of 700 ° C. or higher and lower than 800 ° C.,
In the heating step, the first region is heated to a temperature of 800 ° C. or higher, and the second region is heated to less than 700 ° C.,
In the forced cooling step, the transition region is forcibly cooled to a temperature lower than 700 ° C. but higher than the second region,
The hot forming method for a zinc-based plated steel material according to claim 1. In the heating step,
The first region is heated to a temperature above the A3 point, and the second region is heated to a temperature below the A1 point,
In the forced cooling step,
The transition region is forcibly cooled to be in a temperature range that exceeds the temperature of the second region from less than the lower limit of the predetermined temperature range.
The hot forming method of a zinc-based plated steel material according to claim 1 or 2.   The said transition area | region has the temperature distribution from which the temperature falls gradually toward the said 2nd area | region from the said 1st area | region by the said forced cooling, The Claim 1 characterized by the above-mentioned. Hot forming method for galvanized steel.   During or after the hot forming, the blank is quenched to quench the first region, and the transition region is partially quenched to have a strength between the first and second regions, The hot forming method for a zinc-based plated steel material according to any one of claims 1 to 4. Using a zinc-based plated steel material as a blank, heating the first region of the blank above the quenching temperature, and heating the second region of the blank to a temperature lower than the quenching temperature;
In the vicinity of the boundary between the first region and the second region, a transition region that is inevitably heated to within a predetermined temperature range related to grain boundary cracking due to zinc is forcibly cooled, and the temperature of the transition region is set to the predetermined region. A forced cooling section that is lower than the temperature range and higher than the temperature of the second region;
A hot forming section for hot forming a blank having a temperature distribution set by the heating or forced cooling;
An apparatus for hot forming a zinc-based plated steel material, comprising: One or a plurality of first lamps arranged opposite to the first region of the blank to be heated by infrared rays and capable of outputting a relatively high-intensity first infrared ray;
One or a plurality of second lamps arranged opposite to the second region of the blank and capable of outputting a relatively low-intensity second infrared ray;
A first chamber in which the blank and the first and second lamps extend or stand in a direction opposite to each other between the first and second lamps, and the first and second regions are respectively disposed; A light shielding wall that serves as a barrier between the second chambers and shields the first infrared rays from entering the second chamber and the second infrared rays from entering the first chamber;
An infrared furnace characterized by comprising:   The infrared furnace according to claim 7, wherein the light shielding walls are erected on both sides of the blank with the blank interposed therebetween.   The infrared furnace according to claim 7 or 8, further comprising a slit formed in an intermediate portion of the light shielding wall and through which the blank can pass. A first region that is substantially fully quenched;
A partially or incompletely quenched transition region formed in a region adjacent to the first region and gradually decreasing in strength as the distance from the first region increases;
A hot-formed product of a zinc-based plated steel material, characterized by comprising:

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