APPARATUS AND METHOD FOR PREPARING HOT STAMPED PART

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of International Application No. PCT/CN2023/074748 filed on Feb. 7, 2023, which claims priority of Chinese Patent Application No. 2022115604719 entitled “APPARATUS AND METHOD FOR PREPARING HOT STAMPED PART” filed on Dec. 7, 2022 and Chinese Patent Application No. 2022108965356 entitled “APPARATUS AND METHOD FOR PREPARING HOT STAMPED PART” filed on Jul. 28, 2022, which are incorporated herein by reference.

TECHNICAL FIELD

The disclosure relates to the field of hot stamping, in particular to an apparatus and a method for preparing a hot stamped part.

BACKGROUND ART

Currently, in the hot stamping industry, aluminum-silicon coated plates are most widely used, which can effectively prevent oxidation of blanks during austenitizing heating. Therefore, controlling of oxygen potential is not needed for atmosphere in a commonly used heating furnace currently, but only controlling of a water vapor content is performed by controlling a gas dew point. However, in a heating process in continuous mass production, an aluminum-silicon coating is easy to react with residual water vapor in hot air in an ordinary atmosphere furnace to generate hydrogen, which results in hydrogen induced delayed fracture. The hydrogen induced delayed fracture, also known as hydrogen embrittlement, is phenomenon of weakening or embrittlement in property due to dissolution of hydrogen atoms in metal materials. The hydrogen may accelerate crack propagation in metal, causing a fracture surface of the metal to change from a ductile to brittle in characteristics. In order to relieve this phenomenon, a dew point in an austenitizing furnace is generally required to be controlled at −15° C. to −20° C. in actual production, which hinders production of water vapor in the heating furnace to avoid reduction reaction between the water vapor and surface elements such as iron, aluminum and silicon to generate the hydrogen. However, because a roller-hearth heating furnace and a box-type heating furnace commonly used in practical mass production do not have multiple airtight cavities, when the blanks enter and exit the heating furnace, outside air containing much water vapor enters a cavity of the heating furnace, which makes it difficult to control dew point atmosphere in the furnace. In addition, the blanks may stay in the heating furnace for a long time due to device failure, which may cause a large number of hydrogen atoms to be inhaled into the aluminum-silicon coating, thus causing a risk of hydrogen embrittlement. Furthermore, currently, dry air is introduced into the austenitizing heating furnace to control the water vapor in the heating furnace, which can only slow down a problem of hydrogen absorption by the aluminum-silicon coating during heating.

Moreover, in a manufacturing process of the aluminum-silicon coated plate, a steel plate is placed in molten aluminum, and there is also much hydrogen absorption in the molten aluminum, and generally dry high-purity nitrogen gas is introduced into the molten aluminum to remove hydrogen, but there is always some hydrogen remained in the molten aluminum, which cannot be removed by controlling the dew point in the austenitizing furnace. Although vacuum heating is widely used in the heat treatment industry, the commonly used vacuum heating equipment is only suitable for small parts, with a heating cycle in hours, which cannot meet requirements of high frequency (tens of seconds), especially for large-size parts such as automobile body components, and also cannot be used for the continuous mass production.

Furthermore, with continuous improvement of fuel consumption regulations, demand for lightweight automobiles is becoming increasingly urgent, and more and more lightweight materials are applied to various automobile parts. In recent years, Press Harden Steel (PHS) has been favored by many automobile manufacturers because of its high strength, good formability and high dimensional accuracy, and has been widely used in automobile structural parts. Currently, PHS with tensile strength of 1500 MPa has been widely used, and PHS with tensile strength above 1800 MPa can also be mass-produced in some steel mills. But the higher the strength, the greater the risk of hydrogen embrittlement. A risk of hydrogen induced delayed fracture has existed in steel with tensile strength of 1500 MPa, and the risk of hydrogen-induced delayed fracture is greater and more serious for steel with tensile strength of 1800 MPa or even 2000 MPa. Hydrogen embrittlement seriously limits application of aluminum-silicon coating materials for ultra-high strength steel with tensile strength above 1800 MPa. Moreover, for a coated steel plate with tensile strength of 1500 MPa, its elongation needs to be greater than 5%, while for a coated steel plate with tensile strength above 1800 MPa, its elongation is only about 4%, with a bending angle between 35 to 40 degrees. This lack of toughness results in brittle fracture of the coated plate with tensile strength above 1800 MPa during collision, which seriously affects application of the coated plate above 1800 Mpa.

SUMMARY

In order to overcome at least one defect in related art, solve a problem of hydrogen embrittlement of coated steel plates with tensile strength above 1500 Mpa or even above 1800 Mpa, and improve toughness of these coated steel plates, the disclosure aims to provide a method for mass preparing hot stamped parts.

An object of the disclosure can be achieved by following technical schemes.

In an aspect, an apparatus for mass preparing hot stamped parts is provided in the present disclosure, which includes:

• • a heating furnace unit and a hot stamping unit.

The heating furnace unit is provided with a plurality of independent airtight chambers, including a feeding chamber, a heating chamber and a discharging chamber which are communicated in sequence. The hot stamping unit includes a stamping press.

The discharging chamber is communicated with the stamping press.

Optionally, an airtight feeding furnace door is provided at an inlet of the feeding chamber, and a first airtight isolation furnace door is provided between an outlet of the feeding chamber and the inlet of the heating chamber.

Optionally, an airtight discharging furnace door is provided at an outlet of the discharging chamber, and a second airtight isolation furnace door is provided between an inlet of the discharging chamber and an outlet of the heating chamber.

Optionally, the heating chamber is selected from a vacuum chamber or an atmosphere chamber.

Optionally, when the heating chamber is the vacuum chamber, a vacuum in the heating chamber is 1 to 10000 Pa, preferably 100 to 1000 Pa, and temperature in the heating chamber is 880 to 1000° C., preferably 930° C. The heating chamber is configured to heat multiple groups of blanks at the same time.

Optionally, when the heating chamber is the atmosphere chamber, atmosphere in the heating chamber is dry air or other dry gas, and water vapor content of the dry air or other dry gas is less than 1000 ppm (volume fraction), preferably 100 ppm (volume fraction). An air pressure in the heating chamber is an outdoor atmospheric pressure, and temperature in the heating chamber is 880 to 1000° C., preferably 930° C. The heating chamber is configured to heat multiple groups of blanks at the same time.

Optionally, temperature in the discharging chamber is 400 to 800° C., preferably 600 to 700° C.

Optionally, a feeding platform is provided upstream of the feeding chamber, and a discharging platform is provided downstream of the discharging chamber.

In another aspect, a method for mass preparing hot stamped parts is further provided in the disclosure, which functions in preparation by adopting the apparatus described above and includes:

• • feeding a blank into the airtight feeding chamber and then pumping the feeding chamber to a certain vacuum;
  • feeding the blank to the airtight heating chamber for austenitizing heating to obtain a hot blank;
  • feeding the hot blank to the airtight discharging chamber; and transferring the blank conveyed from the discharging chamber to the stamping press for hot stamping forming.

Optionally, the blank is fed into the feeding chamber through the feed furnace door. The blank in the feeding chamber is fed to the heating chamber through the first isolation furnace door. The hot blank in the heating chamber is fed to the discharging chamber through the second isolation furnace door. The blank in the discharging chamber is fed to the stamping press through the discharging furnace door.

Optionally, the blank includes an aluminum-silicon coated blank. Optionally, tensile strength of the aluminum-silicon coated blank ranges from 1300 to 2200 Mpa.

Optionally, when the heating chamber is the vacuum chamber, after the blank enters the feeding chamber through the feeding furnace door, the feeding chamber is pumped to a vacuum higher than 10000 Pa, preferably of 100 to 1000 Pa.

After vacuum in the feeding chamber is close to the vacuum in the heating chamber, the first isolation furnace door between the feeding chamber and the heating chamber is opened, and the blank is fed into the heating chamber for austenitizing heating, and then the first isolation furnace door between the feeding chamber and the heating chamber is closed.

Optionally, when the heating chamber is a vacuum chamber, the discharging chamber is pumped to a vacuum higher than 10000 Pa, preferably 100 to 1000 Pa before heat treatment in the heating chamber is completed and the blank is ready to be discharged.

After the vacuum in the discharging chamber is close to the vacuum in the heating chamber, the second isolation furnace door between the discharging chamber and the heating chamber is opened, and the blank is fed into the discharging chamber, and then the second isolation furnace door between the discharging chamber and the heating chamber is closed.

Before the discharging furnace door of the discharging chamber is opened, dry air or other dry gas is filled into the discharging chamber to reach the outdoor air pressure, and then the discharging furnace door is opened.

Optionally, when the heating chamber is a dry atmosphere chamber, after the blank enters the feeding chamber through the feeding furnace door, the feeding chamber is pumped to a vacuum higher than 10000 Pa, preferably of 100 to 1000 Pa; and then dry air or other dry gas is introduced into the feeding chamber, and water vapor content of the dry air or other dry gas is lower than 1000 ppm, preferably 100 ppm.

After an air pressure in the feeding chamber is close to an air pressure in the heating chamber, the first isolation furnace door between the feeding chamber and the heating chamber is opened, and the blank is fed into the heating chamber for austenitizing heating, and then the first isolation furnace door between the feeding chamber and the heating chamber is closed.

Optionally, when the heating chamber is a vacuum chamber, the discharging chamber is pumped to a vacuum higher than 10000 Pa, preferably of 100 to 1000 Pa before the heat treatment in the heating chamber is completed and the blank is ready to be discharged; then dry air or other dry gas is introduced into the discharging chamber, water vapor content of the dry air or other dry gas is lower than 1000 ppm, preferably 100 ppm.

After an air pressure in the discharging chamber is close to an air pressure in the heating chamber, the second isolation furnace door between the discharging chamber and the heating chamber is opened, and the blank is fed into the discharging chamber, and then the second isolation furnace door between the discharging chamber and the heating chamber is closed.

Before the discharging furnace door of the discharging chamber is opened, dry air or other dry gas is filled into the discharging chamber to reach the outdoor air pressure, and then the discharging furnace door is opened.

Optionally, a method for hot stamping includes laser welding the discharged blank.

Optionally, blank temperature for the hot stamping is controlled within a range of 500 to 700° C., and a heating rate of the hot stamping is less than 7° C./s.

In another aspect, an aluminum-silicon coated hot stamped part is further provided in the disclosure, which is prepared by the method described above.

Optionally, tensile strength of the hot stamped part is equal to or more than 1500 MPa.

Optionally, a production cycle of the hot stamped parts is between 20 to 40 s.

The disclosure has advantages as follows.

In the disclosure, the heating furnace with an airtight chamber is adopted and gas in the chamber is pumped out, so that the vacuum is higher than 10000 Pa. The water vapor content in the heating furnace is caused to be less than 1000 ppm, and a dew point value corresponding to 1000 ppm is about −15° C. At this time, heating can be carried out in two ways. One is to introduce dry air with a lower dew point value after air in the heating furnace with independent chambers is pumped out, so that air pressures inside and outside the furnace are consistent with each other. The other is to carry out heating with the airtight chamber being maintained with a certain vacuum, that is, to perform vacuum heating. In addition, the vacuum heating facilitates diffusion of hydrogen in a raw material of the blank into vacuum, and reduction of hydrogen content in the raw material. Adoption of three independent airtight chambers cleverly avoids disadvantages of general vacuum furnaces. That is to say, atmosphere of the heating chamber for holding multiple groups of blanks while heating is always kept at a certain vacuum or as a certain dry atmosphere, and there is no need to frequently pump the heating chamber from one bar to a certain vacuum. This not only avoids damage of air pumping at high temperature to vacuum pumping equipment, but also does not result in long time required for air pumping in a large-volume chamber. Only the feeding chamber and the discharging chamber are pumped. The feeding chamber and the discharging chamber only accommodate a group of blanks, which has a small volume and exhibits short pumping time. If a single independent chamber is adopted for feeding, discharging and heating, pumping of gas at high temperature above 900° C. may seriously damage the vacuum pumping equipment. In addition, because mass production needs multiple groups of blanks to be heated at the same time, and if space of the feeding chamber and the discharging chamber is too large, time for vacuum pumping may be too long. The discharging chamber has a certain furnace temperature, which is also one of innovative points of the disclosure. This is because the blank needs to be quickly transferred to the press for press forming after austenitizing heating. If transferring time exceeds 12 seconds, martensite structures may be insufficient in a product subjected to hot pressing quenching and mechanical properties of the product may be unqualified. When the blank comes out of the heating chamber and passes through the discharging chamber, there may be a waiting period in the discharging chamber to wait for an outer furnace door of the discharging chamber to open. This results in too long time for the blank to arrive at the press, which affects the mechanical properties of the product. It is found through applicant's researches that if the discharging chamber is kept at certain temperature, enough temperature may be kept for the blank during the waiting period in the discharging chamber, and then the mechanical properties of the product after being sent to the press are qualified. Although the discharging chamber should also be pumped, the gas temperature is much lower than that of the heating chamber, which has no adverse effect on the vacuum pumping equipment.

In order to make the above and other objects, features and advantages of the disclosure more obvious and understandable, a detailed description is made below for the preferred embodiments with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to explain the embodiments of the present disclosure or the technical scheme in the prior art more clearly, the drawings required in the description of the embodiments or the prior art will be briefly introduced below; obviously, the drawings in the following description are only some embodiments of the present disclosure, and other drawings can be obtained according to these drawings by those of ordinary skilled in the art without paying creative labor.

FIG. 1 is a schematic structural view of an apparatus for mass preparing hot stamped parts according to the present disclosure.

FIG. 2 shows experimental results of spot-welding tests of hot stamped parts prepared in Embodiment 1 of the present disclosure.

FIG. 3 shows experimental results of structural adhesive tests of hot stamped parts prepared in Embodiment 1 of the present disclosure.

FIG. 4 shows experimental results of electrophoretic coating tests of hot stamped parts prepared in Embodiment 1 of the present disclosure.

Reference numbers are illustrated as follows: 1. Blank; 2. Feeding Platform; 3. Feeding Furnace Door; 4. Feeding Chamber; 5. First Isolation Furnace Door; 6. Heating Chamber; 7. Second Isolation Furnace Door; 8. Discharging Chamber; 9. Discharging Furnace Door; 10. Discharging Platform; 11. Stamping Press.

DETAILED DESCRIPTION

In the following, the technical scheme in the embodiment of the disclosure will be described clearly and completely in connection with the drawings; obviously, the described embodiment is intended to be only a part of the embodiment of the disclosure, but not all of them. On a basis of the embodiments in this disclosure, all other embodiments obtained by the ordinary skilled in the art without any creative effort are within the protection scope of this disclosure.

An apparatus for mass preparing hot stamped parts is provided in an embodiment of the present disclosure, which, as shown in FIG. 1, includes:

• • a heating furnace unit and a hot stamping unit. The heating furnace unit is provided with a plurality of independent airtight chambers, including a feeding chamber 4, a heating chamber 6 and a discharging chamber 8 which are communicated in sequence. The hot stamping unit includes a stamping press 11. The discharging chamber 8 is communicated with the stamping press 11.

Optionally, an airtight feeding furnace door 3 is provided at an inlet of the feeding chamber 4, and a first airtight isolation furnace door 5 is provided between an outlet of the feeding chamber 4 and the inlet of the heating chamber 6.

Optionally, an airtight discharging furnace door 9 is provided at an outlet of the discharging chamber 8, and a second airtight isolation furnace door 7 is provided between an inlet of the discharging chamber 8 and an outlet of the heating chamber 6.

Optionally, the heating chamber 6 is selected from a vacuum chamber or an atmosphere chamber.

Optionally, when the heating chamber 6 is the vacuum chamber, a vacuum in the heating chamber 6 is 1 to 10000 Pa, preferably 100 to 1000 Pa, and temperature in the heating chamber is 880 to 1000° C., preferably 930° C. The heating chamber 6 is configured to heat multiple groups of blanks at the same time.

Optionally, when the heating chamber 6 is the atmosphere chamber, atmosphere in the heating chamber 6 is dry air or other dry gas, and water vapor content of the dry air or other dry gas is less than 1000 ppm (volume fraction), preferably 100 ppm (volume fraction). An air pressure in the heating chamber is an outdoor atmospheric pressure, and temperature in the heating chamber is 880 to 1000° C., preferably 930° C. The heating chamber 6 is configured to heat multiple groups of blanks at the same time.

Optionally, temperature in the discharging chamber 8 is 400 to 800° C., preferably 650° C.

Optionally, a feeding platform 2 is provided upstream of the feeding chamber 4, and a discharging platform 10 is provided downstream of the discharging chamber 8.

A method for mass preparing hot stamped parts is further provided in the disclosure, which functions in preparation by adopting the apparatus described above and includes:

• • feeding a blank 1 into the airtight feeding chamber 4 and then pumping the feeding chamber 4 to a certain vacuum;
  • feeding the blank to the airtight heating chamber 6 for austenitizing heating to obtain a hot blank;
  • feeding the hot blank to the airtight discharging chamber 8; and
  • transferring the blank conveyed from the discharging chamber 8 to the stamping press 11 for hot stamping forming.

Optionally, the blank 1 is fed into the feeding chamber 4 through the feed furnace door 3. The blank in the feeding chamber 4 is fed to the heating chamber 6 through the first isolation furnace door 5. The hot blank in the heating chamber 6 is fed to the discharging chamber 8 through the second isolation furnace door 7. The blank in the discharging chamber 8 is fed to the stamping press 11 through the discharging furnace door 9.

Optionally, the blank 1 is selected from aluminum-silicon coated blanks.

Optionally, when the heating chamber 6 is the vacuum chamber, after the blank 1 enters the feeding chamber 4 through the feeding furnace door 3, the feeding chamber 4 is pumped to a vacuum higher than 10000 Pa, preferably of 100 to 1000 Pa.

After vacuum in the feeding chamber 4 is close to the vacuum in the heating chamber 6, the first isolation furnace door 5 between the feeding chamber 4 and the heating chamber 6 is opened, and the blank is fed into the heating chamber 6 for austenitizing heating, and then the first isolation furnace door 5 between the feeding chamber 4 and the heating chamber 6 is closed.

Optionally, when the heating chamber 6 is the vacuum chamber, the discharging chamber 8 is pumped to a vacuum higher than 10000 Pa, preferably 100 to 1000 Pa before heat treatment in the heating chamber 6 is completed and the blank is ready to be discharged.

After the vacuum in the discharging chamber 8 is close to the vacuum in the heating chamber 6, the second isolation furnace door 7 between the discharging chamber 8 and the heating chamber 6 is opened, and the blank is fed into the discharging chamber 8, and then the second isolation furnace door 7 between the discharging chamber 8 and the heating chamber 6 is closed.

Before the discharging furnace door 9 of the discharging chamber 8 is opened, dry air or other dry gas is filled into the discharging chamber 8 to reach the outdoor air pressure, and then the discharging furnace door 9 is opened.

Optionally, when the heating chamber 6 is a dry atmosphere chamber, after the blank 1 enters the feeding chamber 4 through the feeding furnace door 3, the feeding chamber 4 is pumped to a vacuum higher than 10000 Pa, preferably of 100 to 1000 Pa; and then dry air or other dry gas is introduced into the feeding chamber 4, and the water vapor content of the dry air or other dry gas is lower than 1000 ppm, preferably 100 ppm.

After an air pressure in the feeding chamber 4 is close to an air pressure in the heating chamber 6, the first isolation furnace door 5 between the feeding chamber 4 and the heating chamber 6 is opened, and the blank is fed into the heating chamber 6 for austenitizing heating, and then the first isolation furnace door 5 between the feeding chamber 4 and the heating chamber 6 is closed.

Optionally, when the heating chamber 6 is the vacuum chamber, the discharging chamber 8 is pumped to a vacuum higher than 10000 Pa, preferably of 100 to 1000 Pa before the heat treatment in the heating chamber 6 is completed and the blank is ready to be discharged; then dry air or other dry gas is introduced into the discharging chamber 8, water vapor content of the dry air or other dry gas is lower than 1000 ppm, preferably 100 ppm.

After an air pressure in the discharging chamber 8 is close to an air pressure in the heating chamber 6, the second isolation furnace door 7 between the discharging chamber 8 and the heating chamber 6 is opened, and the blank is fed into the discharging chamber 8, and then the second isolation furnace door 7 between the discharging chamber 8 and the heating chamber 6 is closed.

Before the discharging furnace door 9 of the discharging chamber 8 is opened, dry air or other dry gas is filled into the discharging chamber 8 to reach the outdoor air pressure, and then the discharging furnace door 9 is opened.

Optionally, a method for hot stamping in the stamping press 11 includes laser welding the discharged blank.

Optionally, blank temperature for the hot stamping in the stamping press 11 is controlled within a range of 500 to 700° C., and a heating rate of the hot stamping is less than 7° C./s.

An aluminum-silicon coated hot stamped part is further provided in the disclosure, which is prepared by the method described above.

Optionally, tensile strength of the hot stamped part is equal to or more than 1500 MPa.

Optionally, a production cycle of the hot stamped parts is between 20 to 40 s.

The present disclosure will be described in detail below through specific embodiments.

Embodiment 1

Firstly, a 1.8 mm aluminum-silicon coated plate made of hot stamping sheets with tensile strength of 1800 MPa after hot stamping is fed into a vacuum furnace with three airtight chambers, namely, the feeding chamber 4, the heating chamber 6 and the discharging chamber 8. The vacuum of the heating chamber of the vacuum furnace is 10 Pa. A preparing process is as follows: the feeding chamber 4 is filled with air to reach an outdoor air pressure before the blank enters the feeding chamber 4, and then the feed furnace door 3 is opened, the blank is fed in, and the feed furnace door 3 is closed. Air pumping is started for the feeding chamber 4 to a vacuum of 10 to 100 Pa, then the airtight first isolation furnace door 5 between the feeding chamber 4 and the heating chamber 6 is opened, and the blank is fed to the heating chamber 6 for heating. After austenitizing heating, the blank is ready to be fed to the discharging chamber 8. Before the airtight second isolation furnace door 7 between the heating chamber 6 and the discharging chamber 8 is opened, the discharging chamber 8 should be vacuumized to a vacuum of 10 to 100 Pa. Then the second isolation furnace door 7 is opened, and the blank is fed into the discharging chamber 8. Then the second isolation furnace door 7 is closed. Before the discharging furnace door 9 is opened, dry air or other dry gas with a dew point of −45° C. is filled into the discharging chamber 8 to reach the outdoor air pressure, and then the discharging furnace door 9 is opened. Temperature in the heating chamber is 930 degrees, heating time of the heating chamber is 300 s, and temperature in the discharging chamber is 650 degrees. After staying in the discharging chamber for 10 s, the blank is fed from the furnace door to a platform of the stamping press 11 for stamping forming. A period of a whole production line is 30 s.

Embodiment 2

Firstly, a 1.8 mm aluminum-silicon coated plate made of hot stamping sheets with tensile strength of 1800 MPa after hot stamping is fed into a vacuum furnace with three airtight chambers, namely, the feeding chamber 4, the heating chamber 6 and the discharging chamber 8. The vacuum of the heating chamber of the vacuum furnace is 10 Pa. A preparing process is as follows: the feeding chamber 4 is filled with air to reach an outdoor air pressure before the blank enters the feeding chamber 4, and then the feed furnace door 3 is opened, the blank is fed in, and the feed furnace door 3 is closed. Air pumping is started for the feeding chamber 4 to a vacuum of 10 to 100 Pa, then the airtight first isolation furnace door 5 between the feeding chamber 4 and the heating chamber 6 is opened, and the blank is fed to the heating chamber 6 for heating. After austenitizing heating, the blank is ready to be fed to the discharging chamber 8. Before the airtight second isolation furnace door 7 between the heating chamber 6 and the discharging chamber 8 is opened, the discharging chamber 8 should be vacuumized to a vacuum of 10 to 100 Pa. Then the second isolation furnace door 7 is opened, and the blank is fed into the discharging chamber 8. Then the second isolation furnace door 7 is closed. Before the discharging furnace door 9 is opened, dry air or other dry gas with a dew point of −45° C. is filled into the discharging chamber 8 to reach the outdoor air pressure, and then the discharging furnace door 9 is opened. Temperature in the heating chamber is 930 degrees, heating time of the heating chamber is 600 s, and temperature in the discharging chamber is 700 degrees. After staying in the discharging chamber for 9 s, the blank is fed from the furnace door to a platform of the stamping press 11 for stamping forming. A period of a whole production line is 28 s.

Embodiment 3

Firstly, a 1.8 mm aluminum-silicon coated plate made of hot stamping sheets with tensile strength of 1800 MPa after hot stamping is fed into an atmosphere furnace with three airtight chambers, namely, the feeding chamber 4, the heating chamber 6 and the discharging chamber 8. A dew point of atmosphere in the heating chamber 6 of the atmosphere furnace is −45° C., and an air pressure of the heating chamber is one bar. A preparing process is as follows: the feeding chamber 4 is filled with air to reach an outdoor air pressure before the blank enters the feeding chamber 4, and then the feed furnace door 3 is opened, the blank is fed in, and the feed furnace door 3 is closed; air pumping is started for the feeding chamber 4 to a vacuum of 10 to 100 Pa, and then the feeding chamber 4 is filled with dry air with a dew point of −45° C. to reach an air pressure of the heating chamber 6, that is, one bar. Then the airtight first isolation furnace door 5 between the feeding chamber 4 and the heating chamber 6 is opened, and the blank is fed to the heating chamber 6 for heating. After austenitizing heating, the blank is ready to be fed to the discharging chamber 8. Before the airtight second isolation furnace door 7 between the heating chamber 6 and the discharging chamber 8 is opened, the discharging chamber 8 should be vacuumized to a vacuum of 10 to 100 Pa, and filled with dry air with a dew point of −45° C. to reach the air pressure of the heating chamber 6, that is, one bar. Then the second isolation furnace door 7 is opened, and the blank is fed into the discharging chamber 8, and then the second isolation furnace door 7 is closed. Temperature in the heating chamber is 930 degrees, heating time of the heating chamber is 300 s, temperature in the discharging chamber is 700 degrees, and the blank stays in the discharging chamber for 8 s. Then, the discharging furnace door 9 is opened, and the blank is fed from the furnace door to a platform of the stamping press 11 for stamping forming. A period of a whole production line is 30 s.

Embodiment 4

Firstly, a 1.8 mm aluminum-silicon coated plate made of hot stamping sheets with tensile strength of 2000 MPa after hot stamping is fed into a vacuum furnace with three airtight chambers, namely, the feeding chamber 4, the heating chamber 6 and the discharging chamber 8. The vacuum of the heating chamber of the vacuum furnace is 5 Pa. A preparing process is as follows: the feeding chamber 4 is filled with air to reach an outdoor air pressure before the blank enters the feeding chamber 4, and then the feed furnace door 3 is opened, the blank is fed in, and the feed furnace door 3 is closed. Air pumping is started for the feeding chamber 4 to a vacuum of 10 to 100 Pa, then the airtight first isolation furnace door 5 between the feeding chamber 4 and the heating chamber 6 is opened, and the blank is fed to the heating chamber 6 for heating. After austenitizing heating, the blank is ready to be fed to the discharging chamber 8. Before the airtight second isolation furnace door 7 between the heating chamber 6 and the discharging chamber 8 is opened, the discharging chamber 8 should be vacuumized to a vacuum of 10 to 100 Pa. Then the second isolation furnace door 7 is opened, and the blank is fed into the discharging chamber 8. Then the second isolation furnace door 7 is closed. Before the discharging furnace door 9 is opened, dry air or other dry gas with a dew point of −45° C. is filled into the discharging chamber 8 to reach the outdoor air pressure, and then the discharging furnace door 9 is opened. Temperature in the heating chamber is 930 degrees, heating time of the heating chamber is 240 s, and temperature in the discharging chamber is 750 degrees. After staying in the discharging chamber for 8 s, the blank is fed from the furnace door to a platform of the stamping press 11 for stamping forming. A period of a whole production line is 30 s.

Embodiment 5

Firstly, a 1.6 mm aluminum-silicon coated plate made of hot stamping sheets with tensile strength of 1500 MPa after hot stamping is fed into a vacuum furnace with three airtight chambers, namely, the feeding chamber 4, the heating chamber 6 and the discharging chamber 8. The vacuum of the heating chamber of the vacuum furnace is 50 Pa. A preparing process is as follows: the feeding chamber 4 is filled with air to reach an outdoor air pressure before the blank enters the feeding chamber 4, and then the feed furnace door 3 is opened, the blank is fed in, and the feed furnace door 3 is closed. Air pumping is started for the feeding chamber 4 to a vacuum of 10 to 100 Pa, then the airtight first isolation furnace door 5 between the feeding chamber 4 and the heating chamber 6 is opened, and the blank is fed to the heating chamber 6 for heating. After austenitizing heating, the blank is ready to be fed to the discharging chamber 8. Before the airtight second isolation furnace door 7 between the heating chamber 6 and the discharging chamber 8 is opened, the discharging chamber 8 should be vacuumized to a vacuum of 10 to 100 Pa. Then the second isolation furnace door 7 is opened, and the blank is fed into the discharging chamber 8. Then the second isolation furnace door 7 is closed. Before the discharging furnace door 9 is opened, dry air or other dry gas with a dew point of −45° C. is filled into the discharging chamber 8 to reach the outdoor air pressure, and then the discharging furnace door 9 is opened. Temperature in the heating chamber is 930 degrees, heating time of the heating chamber is 220 s, and temperature in the discharging chamber is 650 degrees. After staying in the discharging chamber for 10 s, the blank is fed from the furnace door to a platform of the stamping press 11 for stamping forming. A period of a whole production line is 30 s.

Comparative Embodiment 1

Firstly, a 1.8 mm 22MnB5 aluminum-silicon coated plate made of hot stamping sheets with tensile strength of 1800 MPa is placed in an atmosphere furnace with a dew point of −5° C. for heating, with furnace temperature of 930 degrees and heating time of 300 s. After austenitizing heating, it was placed on a platform of the stamping press for stamping forming. A period of a whole production line is 30 s.

Comparative Embodiment 2

Firstly, a 1.8 mm 22MnB5 aluminum-silicon coated plate made of hot stamping sheets with tensile strength of 1800 MPa is placed in an atmosphere furnace with a dew point of −5° C. for heating, with furnace temperature of 930 degrees and heating time of 600 s. After austenitizing heating, it was placed on a platform of the stamping press for stamping forming. A period of a whole production line is 30 s.

Comparative Embodiment 3

Firstly, a 1.8 mm 22MnB5 aluminum-silicon coated plate made of hot stamping sheets with tensile strength of 1800 MPa is placed in an atmosphere furnace with a dew point of −5° C. for heating, with furnace temperature of 930 degrees and heating time of 300 s. After austenitizing heating, it was placed on a platform of the stamping press for stamping forming. A period of a whole production line is 30 s.

Comparative Embodiment 4

Firstly, a 1.8 mm 22MnB5 aluminum-silicon coated plate made of hot stamping sheets with tensile strength of 1800 MPa is placed in an atmosphere furnace with a dew point of −5° C. for heating, with furnace temperature of 930 degrees and heating time of 600 s. After austenitizing heating, it was placed on a platform of the stamping press for stamping forming. A period of a whole production line is 30 s.

Comparative Embodiment 5

Firstly, a 1.8 mm aluminum-silicon coated plate made of hot stamping sheets with tensile strength of 1800 MPa after hot stamping is fed into a vacuum furnace with three airtight chambers, namely, the feeding chamber 4, the heating chamber 6 and the discharging chamber 8. The vacuum of the heating chamber of the vacuum furnace is 50 Pa. A preparing process is as follows: the feeding chamber 4 is filled with air to reach an outdoor air pressure before the blank enters the feeding chamber 4, and then the feed furnace door 3 is opened, the blank is fed in, and the feed furnace door 3 is closed. Air pumping is started for the feeding chamber 4 to a vacuum of 10 to 100 Pa, then the airtight first isolation furnace door 5 between the feeding chamber 4 and the heating chamber 6 is opened, and the blank is fed to the heating chamber 6 for heating. After austenitizing heating, the blank is ready to be fed to the discharging chamber 8. Before the airtight second isolation furnace door 7 between the heating chamber 6 and the discharging chamber 8 is opened, the discharging chamber 8 should be vacuumized to a vacuum of 10 to 100 Pa. Then the second isolation furnace door 7 is opened, and the blank is fed into the discharging chamber 8. Then the second isolation furnace door 7 is closed. Before the discharging furnace door 9 is opened, dry air or other dry gas with a dew point of −45° C. is filled into the discharging chamber 8 to reach the outdoor air pressure, and then the discharging furnace door 9 is opened. Temperature in the heating chamber is 930 degrees, heating time of the heating chamber is 240 s, and temperature in the discharging chamber is room temperature. After staying in the discharging chamber for 6 s, the blank is fed from the furnace door to a platform of the stamping press 11 for stamping forming. A period of a whole production line is 30 s.

TABLE 1
Performance Results of Embodiments and Comparative Embodiments

				Tensile	Yield		Three-		Hydrogen
		Dew	Heating	Strength/	Strength//	Elongation	point	Hardness/	Content/
Heating Furnace	Vacuum	Point	Time	Mpa	Mpa	%	Bending	HV	ppm

Embodiment 1	10 pa	/	300 s	1874.01	1376.25	6.16	50.98°	619.5	0.03
Embodiment 2	10 pa	/	600 s	1863.41	1372.69	7	52.51°	619.5	0.01
Embodiment 3	10 pa	/	600 s	1863.41	1372.69	7	52.51°	619.5	0.06
Embodiment 4	5 pa	/	240 s	1971.36	1562.38	6.1	51.33°	620.3	0.01
Embodiment 5	50 pa	/	220 s	1530.61	1076.32	7.2	66.51°	490.3	0.09
Comparative	101523 pa	−5° C.	300 s	1633.98	1340.33	1.66	34.29°	615.6	0.17
Embodiment 1
Comparative	101523 pa	−5° C.	600 s	1735.56	1388.7	2.64	36.55°	612.4	0.18
Embodiment 2
Comparative	101523 pa	−15° C.	300 s	1895.69	1388.35	4.5	41.35°	595.8	0.09
Embodiment 3
Comparative	101523 pa	−15° C.	600 s	1819.97	1371.35	4.12	43.07°	590.8	0.1
Embodiment 4
Comparative	10 Pa	/	300 s	1760.13	1358.4	3.64	35.55°	612.4	0.05
Embodiment 5

It is found from data analysis of the Embodiments and Comparative Embodiments that mechanical properties of Comparative Embodiments 1 and 2 with poor controlled dew points of an ordinary atmosphere furnace such as tensile strength, yield strength, elongation and three-point bending angle are unqualified at a similar production period, and their hydrogen content is high. In a case of a well-controlled dew point of the ordinary atmosphere furnace, such as in Comparative Embodiment 3, its tensile strength, yield strength, elongation and three-point bending are all qualified, but its elongation and three-point bending value are low, and its hydrogen content is lower than that of Comparative Embodiments 1 and 2. By heating the aluminum-silicon coating with the vacuum heating furnace, as in Embodiments 1 to 3, it is found that the elongation and three-point bending values are much higher than those of Comparative Embodiments 3 and 4 with the well-controlled dew point of the ordinary atmosphere furnace, for example, the elongations exceed 6% and the bending angles exceed 50°, and hydrogen contents are also lower than that of Comparative Embodiments 3 and 4 for the ordinary atmosphere furnace.

Parts prepared in Embodiments 1 and 2 and Comparative Embodiments 1, 2, 3 and 4 were sprayed with salt spray for 10 minutes every 4 hours under neutral salt spray, and results are shown in Table 2.

TABLE 2
Comparison of four-point bending performance
between Embodiment and Comparative Embodiment

			Four-point
	Heating Furnace	Atmosphere	Bending

	Embodiment 1	10	pa	OK
	Embodiment 2	10	pa	OK
	Embodiment 3	10	pa	OK
	Embodiment 4	5	pa	OK
	Embodiment 5	50	pa	OK
	Comparative Embodiment 1	−5°	C.	OK
	Comparative Embodiment 2	−5°	C.	Crack
	Comparative Embodiment 3	−15°	C.	OK
	Comparative Embodiment 4	−15°	C.	Crack
	Comparative Embodiment 5	10	pa	OK

Spot welding tests are carried out on the parts prepared in Embodiment 1. Experimental results are shown in Table 3 and FIG. 2 below, and it can be seen that welding performance is Ok.

TABLE 3
Experimental results of spot-welding tests

	Condition	Number	Tensile	Weld Nugget
	Embodiment 1	1#	17.71 KN	5.4 mm
		2#	23.14 KN	7.4 mm
		3#	24.77 KN	6.3 mm
		4#	25.41 KN	6.7 mm

Structural adhesive tests are carried out on the parts prepared in Embodiment 1, and results are shown in FIG. 3. It can be seen that adhesive performance is OK.

Electrophoretic coating tests are carried out on the parts prepared in Embodiment 1, and results are shown in FIG. 4. It can be seen that coating performance is OK.

In addition, as can be seen from Table 1 and Table 2, the parts prepared by this method have lower content of diffused hydrogen, higher elongation than those by a method using the ordinary atmosphere furnace, better bending performance and lower risk of hydrogen embrittlement. There is no problem in welding performance, adhesion performance and coating performance. A reason for this is that when the parts are heated in vacuum, fewer H₂O molecules react with the aluminum-silicon coating and less diffused hydrogen is generated. However, when heating is made in the ordinary atmosphere furnace, there are more H₂O molecules in the furnace, which react with the aluminum-silicon coating to generate more hydrogen; and more hydrogen enters heated austenite blanks, and then internal stress and defects of martensite formed by cooling molding increase, which results in increase in elongation, bending angle and risk of hydrogen embrittlement.

In this disclosure, embodiments are intended to explain principle and implementations of the disclosure. Illustration of the embodiments described above are only used to facilitate understanding of methods and core ideas of the disclosure. Meanwhile, changes may be made to the specific implementation and application scope by ordinary skilled in the art according to the ideas of this disclosure. To sum up, contents of this specification should not be construed as limitation to this disclosure.