KR20180001308A - Partial softening hot stamping method - Google Patents

Abstract

A partial softening hot-stamping method according to an embodiment of the present invention includes: heating a blank to a predetermined first temperature range in a furnace, such that the black is transformed in phase into a full austenitic metal structure; cooling the blank to a predetermined second temperature range by using cooling air, such that the blank is transformed in phase into a metallic structure in which austenite and ferrite metal structures partially coexist; molding the blank overall by using a hot stamping mold; and rapidly cooling the austenitic metal structure of a molded product to a predetermined third temperature range through a cooling means inside the hot stamping mold, such that austenitic metal structure of the molded product is transformed in phase into a martensitic metallic structure.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a hot stamping method,

The present invention relates to a localized hot stamping method for heating a blank as a whole and locally cooling the heated blank to have a high strength portion and a low strength portion together in a single molded product to be molded.

Vehicles with various strengths are used. For example, parts that need to absorb energy when colliding or overturning the vehicle are required to have relatively weak strength, and strong strength is required in parts where shape maintenance is required in order to secure a passenger's living space.

If the portion of the energy that needs to absorb energy at the time of the collision is too high, the impact energy can not be absorbed properly, but it can be delivered to the other portion as it is, so that the impact can be excessively transmitted to the passenger and other parts of the vehicle.

Vehicles are constantly required to be lightweight and cost-effective, and it is therefore necessary for one part to have a partially different dissimilar strength.

Some sections of the component require high strength to protect occupants, but some sections require relatively low strength to absorb impact energy.

These components are typically exemplified by the B-pillar of the passenger car and the body parts associated with the collision member.

The B-pillar of the vehicle refers to a portion connecting the roof and the bottom of the vehicle body between the front door and the rear door of the passenger vehicle.

The B filler requires a relatively low tensile strength at the bottom and a high tensile strength at the top. That is, at the time of a vehicle collision, a portion in which a shape is maintained at a high strength and a portion in which a shock is absorbed while collapsing are simultaneously required.

Although the specific strength required depends on the type and form of the vehicle, a tensile strength of about 1500 MPa is required for the upper portion of the B-pillar, while a tensile strength of about 600 MPa to about 1000 MPa is required for the lower portion of the B-pillar.

Conventionally, a method of attaching a separate reinforcement member to a portion requiring high strength after forming a component using a low strength material has been used. However, when one component is required to have different strengths in a section, And the lower part was made of a material having a low strength by welding two materials to make a blank, and a hot stamping process was used to produce the final product.

However, in the case of welding different types of materials, there is a fear that a manufacturing cost will increase due to the addition of a welding process, and serious problems such as a decrease in strength or breakage in a welded portion after hot stamping remain.

This hot stamping is a step of heating a steel sheet containing boron to austenitizing temperature (900 to 950 ° C) and then molding and cooling it with a press, thereby improving the product strength by 2.5 times (600 → 1500 MPa) have.

In recent years, we have also been manufacturing localized hot stamping parts for the purpose of increasing impact energy absorption. Since the strength improvement of the hot stamping part is due to the martensite transformation, it is partially softened by decreasing the cooling rate by increasing the temperature of the mold or by hot stamping the part where ductility is required when the material is heated .

Partial quenching (partial quenching) by partially heating the mold temperature requires heating the mold temperature to 450 ° C or higher. Therefore, when the softened portion is taken out at 450 ° C. and placed at room temperature, distortion occurs due to the residual stress and thermal deformation due to the temperature difference with the martensitic transformation region, which severely deviates from the required degree.

Partial quenching, on the other hand, has become a hot issue in hot stamping, and efforts are continuing to mass-produce products at the manufacturing site.

In connection with the technique of performing partial quenching by controlling the mold temperature, Patent Application No. 10-2012-0081126 entitled " Local Hot Stamping Forming Apparatus and Hot Stamping Forming Method Using the Same ", Patent Application No. 10-2010-0079222 &Quot; Hot Stamping Forming Apparatus For Controlling Local Strength Patent Laid-Open No. 10-2015-0124793 ", Patent Application No. 10-2009-0119148 "Local Strengthening Hot Stamping Method for Improving Collision Performance of Auto Parts ", Patent Application No. 10-2009-0132258 entitled "Mold system for manufacturing hot press forming articles having locally different strengths, and method for manufacturing formed articles using the same ".

6 is a schematic configuration diagram and a flowchart showing a conventional hot stamping method.

Referring to FIG. 6, the hot stamping method includes a heating step S610, a transfer step S620, a press forming and a partial quenching step S630, and a product unloading step S640.

The hot stamping mold 130 includes an upper mold 130a and a lower mold 130b and a cooling means 135 and a heating means 600 are provided at predetermined depths along the molding surfaces of the upper mold 130a and the lower mold 130b. .

The cooling means 135 quenches the molded product, the heating means 600 heats the molded product to a set temperature, and the heated portion is gradually cooled in the air and transformed into the low strength portion 140b. In addition, the portion that is not heated but quenched is deformed into the high strength portion 140b.

On the other hand, in the process of gradually cooling the heated portion in the air, the molded product is distorted, and the cost of forming the heating means 600 and the cooling means 135 in the mold may increase.

The matters described in the background section are intended to enhance the understanding of the background of the invention and may include matters not previously known to those skilled in the art.

Korea Application No. 10-2013-0150246 Korea Application No. 10-2013-0169448

An object of the present invention is to provide a local softening hot stamping method capable of manufacturing parts having different strengths in different sections using a single material in order to solve the above problems.

As described above, the local softening hot stamping method according to the embodiment of the present invention includes the steps of heating the blank to a first set temperature range so as to be converted into a full austenitic metal structure in a heating furnace, Cooling the blank to a second set temperature range such that the blank is partially transformed into a metal structure in which austenite and ferrite metal structure coexist; molding the blank as a whole using a hot stamping mold; And quenching the molten metal to a third predetermined temperature range so that the austenitic metal structure of the molded product through the cooling means is transformed into martensitic metal structure.

In the hot stamping die, the cooling means may be formed as a cooling water channel inside the molding surface.

The molded article can form a softened portion in a portion where the blank is cooled to a second set temperature range.

The blank may be made of a boron steel sheet.

The first set temperature range may be set at 900 degrees Celsius to 950 degrees Celsius.

The second set temperature range may be set to 400 to 700 degrees Celsius.

The third set temperature range may include 150 degrees to 25 degrees Celsius.

A method for local softening hot stamping according to an embodiment of the present invention comprises the steps of heating a blank to a first set temperature range so as to be transformed into a full austenitic metal structure in a heating furnace, Cooling the blank to a second set temperature range so that the blank is transformed into a metal structure in which the ferrite metal structure coexists; molding the blank entirely using a hot stamping mold; And quenching the molten steel to a third predetermined temperature range so that the austenitic metal structure of the molded product formed through the cooling means is transformed into martensitic metal structure; forming a softened portion in the portion cooled to the second predetermined temperature range, And the remainder form reinforcing portions, and the blanks The first set temperature range is set to 900 to 950 degrees Celsius, the second set temperature range is set to be 400 to 700 degrees Celsius, and the third set temperature range is set to be 150 to 25 degrees Celsius . ≪ / RTI >

According to the present invention, a blank is heated to a set temperature as a whole through a heating furnace, a part of the blank is cooled to an air-set temperature, and the blank is entirely formed through a hot stamping device, The high strength portion and the low strength portion can be easily formed.

1 is a schematic configuration diagram and a flowchart of a local softening hot stamping apparatus and a local softening hot stamping method according to an embodiment of the present invention.
2 is a side view of a B-pillar fabricated by a local softening hot stamping method according to an embodiment of the present invention.
3 is a table showing the characteristics of a product manufactured by the local softening hot stamping method according to the embodiment of the present invention.
FIG. 4 is a table showing the characteristics of each step of the local softening hot stamping method according to the embodiment of the present invention.
FIG. 5 is a graph showing tissue that changes with the local softening hot stamping method according to an embodiment of the present invention. FIG.
6 is a schematic configuration diagram and a flow chart showing a hot stamping method associated with the present invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

It is to be understood, however, that the present invention is not limited to the illustrated embodiments, and that various changes and modifications may be made without departing from the spirit and scope of the invention. .

In order to clearly illustrate the embodiments of the present invention, parts not related to the description are omitted, and the same or similar components are denoted by the same reference numerals throughout the entire specification.

In the following description, the names of the components are denoted by the first, second, etc. in order to distinguish them from each other because the names of the components are the same and are not necessarily limited to the order.

1 is a schematic configuration diagram and a flowchart of a local softening hot stamping apparatus and a local softening hot stamping method according to an embodiment of the present invention.

Referring to FIG. 1, the hot stamping method includes a heating step S110, a transfer and local cooling step S120, a press forming and quenching step S130, and an unloading step S140.

The transfer device 110 or the robot sequentially injects the loaded blank 100 into the heating furnace.

In the heating step S110, the heating furnace 105 heats the charged blank 100 to a set temperature (for example, a first set temperature range: 900 to 950 degrees Celsius). The blank 100 heated by the heating furnace 105 is conveyed and discharged. Here, the full austenite temperature of the boron steel is 810 degrees, but the heat loss of the material between the actual hotstay transfer is taken into consideration.

In the conveying and local cooling step S120, the heated air 100 is conveyed and the air jet nozzle 120 blows air to a part (portion to be softened) of the blank 100 to be cooled. Therefore, the heated hot part 100a (about 800 degrees Celsius) and the air-cooled low temperature part 100b (for example, the second set temperature range: 400 to 700 degrees Celsius) are formed in the blank 100. Since the martensite starting temperature (Ms) during cooling is 400 ° C., it must be at least 400 ° C., 700 ° C. is the temperature at which austenite and ferrite phase are formed, and the ferrite ratio is increased when the temperature is lowered to 700 ° C. The strength may be lowered, and the lower the air-cooling temperature, the higher the ratio of ferrite and the lower the strength.

In the press forming and quenching step S130, the hot stamping mold 130 includes the upper mold 130a and the lower mold 130b, the blank 100 to be injected therebetween is press-molded, and the cooling means 135 And is quenched simultaneously with molding.

A local softening hot stamping apparatus according to an embodiment of the present invention includes a heating furnace 105, a transfer device 110, a hot stamping mold 130, and an air injection nozzle 120, (105).

The blank 100 charged into the heating furnace 105 is heated to a first set temperature range and the blank 100 heated by the heating furnace 105 is heated by the air injected by the air injection nozzle 120 To a second set temperature range.

Accordingly, the blank 100 is formed with the low temperature portion 100b cooled by air and the high temperature portion 100a which is not cooled.

The hot stamping mold 130 includes an upper mold 130a and a lower mold 130b and the blank 100 inserted between the upper mold 130a and the lower mold 130b passes through the upper mold 130a and the lower mold 130b, And is quenched by the cooling means 135 formed at a predetermined depth along the molding surfaces of the upper mold 130a and the lower mold 130b at the time of molding. Here, the cooling means 135 may be a cooling water channel through which the cooling water flows.

In the embodiment of the present invention, the molded product 140 has the high strength portion 140a and the low strength portion 140b. The low strength portion 140b is a portion that is quenched in a second set temperature range by the air injected from the air injection nozzle 120. The high strength portion 140a is a portion that is cooled in a first set temperature range, (150 DEG C to 25 DEG C (room temperature)).

Here, when the product is cooled down to the temperature (Mf) at which the transformation of the martensite is completed at the time of quenching, the product must be taken out from the mold to obtain a desired strength.

FIG. 2 is a side view of a B-pillar manufactured by a local softening hot stamping method according to an embodiment of the present invention, and FIG. 3 is a table showing characteristics of a product manufactured by the local softening hot stamping method according to an embodiment of the present invention. to be.

2, the molded product includes a high strength portion 140a, a low strength portion 140b, and a transition section 140c formed at a boundary between the high strength portion 140a and the low strength portion 140b, And protects the passenger when the vehicle collides or overturns.

The low strength portion 140b is a portion bonded to the lower portion of the vehicle body and absorbs an impact when the vehicle collides or overturns. The transition portion 140c is formed by the high strength portion 140a and the low strength portion 140b. The strength is changed in the heat treatment process.

The tensile strength of the high strength portion 140a is 1500 MPa or more, the tensile strength of the low strength portion 140b is 860 to 1010 MPa and the tensile strength of the transition portion 140c is 10000 MPa or more.

Referring to FIG. 3, the yield strength of the high-strength portion is 1000 MPa or more and the elongation is 6% or more.

FIG. 4 is a table showing the characteristics of each step of the local softening hot stamping method according to the embodiment of the present invention.

Referring to Fig. 4, a typical hot stamping process includes a material heating step, a transfer step, a molding and cooling step, and an extraction step.

In the step of heating the material, the blank is charged into the furnace and heated to a first set temperature range (about 900 to 950 degrees Celsius) to phase-transform the metal structure into austenite and place the heated blank in a mold In the process, the heated blank reaches about 800 degrees centigrade, and the blank is quenched at the same time to complete the molded product having a high-strength portion as a whole.

The local softening hot stamping method with local air cooling according to an embodiment of the present invention heats the blank 100 to about 900 to 950 degrees Fahrenheit altogether, thereby converting the metal structure into a full austenitic phase.

Air is injected into the portion corresponding to the low strength portion 140b in the process of transferring the blank 100 to the mold and cooled to a second set temperature range (400 deg. C to 700 deg. C). Here, a structure having two phases of austenite and ferrite is formed in the air-cooled portion.

The blank 100 is rapidly cooled to a third set temperature range entirely at the same time as the blank 100 is formed by using the hot stamping mold 130. Then, the portion not cooled by air is changed from austenite to martensite, and its tensile strength is 15000 MPa or more.

Then, the air-cooled part has two phases of austenite and ferrite structure, martensite and ferrite, and its tensile strength is included in the range of 800 to 10000 MPa.

FIG. 5 is a graph showing tissue that changes with the local softening hot stamping method according to an embodiment of the present invention. FIG.

5 (a), the horizontal axis represents time and the vertical axis represents temperature.

The blank 100, which is a molding material, is heated to about 900 to 950 degrees Celsius for up to 3600 seconds, and the texture is phase-transformed to full austenite.

A portion of the blank is cooled to reach a temperature of about 400 to 700 degrees Celsius by 3608 seconds to form a low temperature portion 100b having austenite and ferrite and the remaining portion is maintained at about 800 degrees Celsius, (100a).

Hot stamping molding is performed after 3608 seconds and quenched at about 25 degrees Celsius so that the high temperature portion 100a has a martensite structure and the low temperature portion 100b has a martensite and ferrite structure. Accordingly, the high temperature portion 100a is deformed into the high strength portion 140a of 15000 MPa or more, and the low temperature portion 100b is deformed into the low strength portion 140b of 800 to 1010 MPa.

Referring to FIG. 5 (b), the horizontal axis represents time and the vertical axis represents temperature.

The abscissa represents the temperature of the material, and the ordinate represents the volume of the source sinteredite, martensite, and ferrite in the volume of the material.

The solid line represents the deformation characteristic of the existing material (high temperature portion 100a), and the dotted line represents the deformation characteristic of the material having the low temperature portion 100b by injecting air into the material.

The deformation characteristics of the high temperature part 100a are as follows. As the temperature is lowered, the austenite structure is transformed into a martensite structure. The deformation characteristics of the low temperature part 100b are as follows. And a ferrite structure together.

While the present invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, And all changes to the scope that are deemed to be valid.

100: blank 100a: high temperature part
100b: low temperature section 105: heating furnace
110: Feeding device 120: Air jet nozzle
130: hot stamping mold 130a: upper mold
130b: Lower mold 135: Cooling means
140: Molded product 140a: High strength part
140b: low strength portion 140c: transition section

Claims (8)

Heating the blank to a first set temperature range such that the furnace is phase-transformed to full austenitic metal structure in a furnace;
Cooling the blank to a second set temperature range such that the blank is partially transformed into a metal structure in which austenite and ferrite metal structure coexist using cooling air;
Molding the blank as a whole using a hot stamping mold; And
Quenching the molten metal to a third predetermined temperature range so that the austenitic metal structure of the molded product formed through the cooling means inside the hot stamping mold is transformed into the martensitic metal structure;
≪ / RTI > The method according to claim 1,
In the hot stamping mold,
Wherein the cooling means is formed as a cooling water channel inside the molding surface. The method according to claim 1,
The molded article
Wherein the softened portion is formed at a portion where the blank is cooled to a second set temperature range. The method according to claim 1,
Wherein the blank is a boron steel plate. The method according to claim 1,
Wherein the first set temperature range is set between 900 degrees and 950 degrees Centigrade. The method according to claim 1,
Wherein the second set temperature range is set between 400 and 700 degrees Celsius. The method according to claim 1,
And wherein the third set temperature range comprises between 150 and 25 degrees Celsius. Heating the blank to a first set temperature range such that the furnace is phase-transformed to full austenitic metal structure in a furnace;
Cooling the blank to a second set temperature range such that the blank is partially transformed into a metal structure in which austenite and ferrite metal structure coexist using cooling air;
Molding the blank as a whole using a hot stamping mold; And
Quenching the molten metal to a third predetermined temperature range such that the austenitic metal structure of the molded product is transformed into martensitic metal structure through a cooling means formed of a cooling water channel inside the hot stamping mold; Lt; / RTI >
Wherein the softened portion is formed in a portion cooled to the second predetermined temperature range and the reinforcing portion is formed with a transition interval therebetween, the blank is a boron steel plate, and the first set temperature range is set to 900 to 950 degrees centigrade Wherein the second set temperature range is set to 400 degrees to 700 degrees Celsius and the third set temperature range includes 150 degrees to 25 degrees Celsius.

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