CN110180957B

CN110180957B - Heat treatment method and hot stamping process of galvanized steel sheet

Info

Publication number: CN110180957B
Application number: CN201810687643.6A
Authority: CN
Inventors: 杨洪林
Original assignee: Rongning Seiko New Material Technology Shanghai Co ltd
Current assignee: Rongning Seiko New Material Technology Shanghai Co ltd
Priority date: 2018-06-28
Filing date: 2018-06-28
Publication date: 2020-11-03
Anticipated expiration: 2038-06-28
Also published as: CN110180957A

Abstract

The invention discloses a heat treatment method and a hot stamping process of a galvanized steel sheet, which comprises the following steps: the galvanized steel sheet with certain specification is subjected to preheating treatment to improve the zinc layer structure and meet the requirements of a direct hot stamping process; and then, austenitizing and stamping the steel plate under certain conditions. The invention has definite limits on the specification of the galvanized steel sheet, the pre-heat treatment process and the hot stamping process, so as to ensure that the finished product has better coating property, corrosion resistance and high temperature resistance, and avoid large stamping cracks in a matrix. By adopting the process method provided by the invention, the zinc oxide layer on the surface of the punched finished product part does not need to be subjected to shot blasting treatment to improve the coating performance, and the method avoids the generation of large cracks of the matrix during punching. In addition, the zinc-plated steel sheet which has been heat-treated in advance can be heated at a faster heating rate during austenitizing without the problem of failure of the zinc layer.

Description

Heat treatment method and hot stamping process of galvanized steel sheet

Technical Field

The invention relates to the technical field of hot stamping processes, in particular to a heat treatment method of a galvanized steel sheet and application of a product of the invention, such as application in coating and stamping, and relates to a stamping forming process of galvanized stamped steel.

Background

In recent years, in order to satisfy the requirements for collision safety and vehicle weight reduction of vehicle bodies, ultrahigh-strength steel sheets are increasingly used for vehicle bodies. Due to high strength, the ultrahigh-strength steel has many problems in forming, such as poor shape in cold working, high load in forming, easy generation of scratch and crack in forming, high resilience and the like. In order to solve this problem, hot forming techniques have been widely used, and steel sheets have excellent formability at higher temperatures and tensile strength of stamped parts produced by hot forming can reach 1500MPa or more.

Because the steel plate needs to be austenitized before the hot stamping process, the surface of the steel plate can be oxidized and decarburized in the heating process, and iron scales can fall off and adhere to a die in the subsequent stamping process, so that the friction coefficient between the steel plate and the die is increased, the cooling capacity of the die is influenced, and the subsequent coating performance is influenced. At present, the shot blasting method is often adopted to remove oxide skin generated on the surface of a steel plate during heating, so that the cost is increased, the condition of the shot blasting environment is severe, and microcracks are generated on the surface of the steel plate after shot blasting. In the prior art, in order to prevent and reduce the oxidation of the surface of the steel plate in the heating process, austenitization is usually performed in a nitrogen environment, which also causes great increase of equipment cost and maintenance cost.

In order to reduce costs and improve working conditions, published patent CN 101583486a by the french anshel company proposes the use of an aluminum-silicon coating to prevent oxidation and decarburization of the steel sheet during heating of the steel sheet. The aluminum-silicon plating layer in the patent has good high temperature resistance and corrosion resistance, but the plating layer after austenitizing treatment does not have sacrificial protection performance, the price is higher than that of a galvanized plate, and the plating layer is easy to be bonded with a ceramic roller in a heating furnace during heating to influence production.

In japanese sumitomo metal publication CN 1575348A, it is proposed to improve the high temperature resistance of the galvanized steel sheet by forming an oxide layer on the galvanized steel sheet in advance by a coating method, so that the high temperature resistance of the coating can reach 1000 ℃, the galvanized steel sheet has better notch corrosion resistance and scratch corrosion resistance, but the process of coating the oxide layer on the zinc layer increases the equipment investment and the production cost.

In Toyota iron works, published patent CN 1600877A, a galvanized steel sheet is heated in an atmosphere having an oxygen concentration of 0% or more and a dew point of 0 ℃ or more, and then hot-stamped. The patent only describes the galvanized sheet and does not limit the hot stamping process of different heating processes. When the oxygen concentration of the galvanized plate is lower, the zinc layer on the surface of the steel plate is easy to volatilize in the austenitizing process. And an oxide layer formed on the surface of the GA coating layer in the heating process needs to be treated by shot blasting.

It is proposed in korean patent publication No. CN 102781601a that the zinc-plated sheet is maintained at 600 ℃ for 20min or less by the secondary heating method and then heated to the austenitizing temperature at a relatively fast heating rate, which can suppress the volatilization of the zinc layer and the generation of scale, but the too long primary heating time reduces the production efficiency, there is no description about the problem of press cracking, and the too fast secondary heating tends to cause the ablation of the zinc layer.

For galvanized steel sheets, the cost of the GA steel sheet is increased due to the addition of an alloying process in production, and the GA steel sheet is easy to generate a pulverization phenomenon during indirect forming, so that the protective performance of a plating layer at high temperature is influenced. In order to improve the coating property, the hot finished products formed by GA coating generally need to remove the zinc oxide layer on the surface by shot blasting.

In the GI steel sheet, the coating layer has a liquid zinc phase during the direct hot stamping process, and the matrix is likely to generate large cracks to affect the use of the finished product, so the GI steel sheet is generally only suitable for the indirect hot stamping forming. However, in order to prevent the GI coating from having a liquid zinc phase during stamping and affecting the stamping performance, the austenitizing time is often increased, which not only reduces the production efficiency, but also reduces the protective performance of the substrate due to too high iron content in the zinc layer after stamping.

The present invention is made in view of the above circumstances, and provides a heat treatment method and a hot stamping process for a galvanized steel sheet. The steel plate has better coating performance under the condition of not using shot blasting treatment, and can be suitable for direct hot stamping forming.

Disclosure of Invention

The invention provides a heat treatment method and a hot stamping process for a galvanized steel sheet, which can solve the problems of poor coating performance and stamping cracking in the hot stamping process of the galvanized steel sheet.

In order to solve the problems, the invention adopts the following technical scheme:

in a first aspect of the present invention, there is provided a heat treatment method for heating a galvanized steel sheet in advance before hot stamping, the heat treatment method comprising the steps of:

s11: heating the galvanized steel sheet to any temperature within a preset temperature interval, and then carrying out heat preservation on the galvanized steel sheet for heat preservation time corresponding to the preset temperature interval, wherein the preset temperature interval is determined according to the following modes: taking the heat preservation time (seconds) as an abscissa axis and the annealing temperature (DEG C) as an ordinate axis, four straight lines are respectively correspondingly connected with the following coordinate points: between (0, 600) and (200, 600); between (200, 600) and (30, 910); between (30, 910) and (0, 910); the temperature region enclosed by the four straight lines between (0, 910) and (0, 600) is the predetermined temperature interval.

S12: the galvanized steel sheet is then cooled to 550 ℃ or less.

Preferably, the predetermined temperature interval described in S11 is determined as follows: taking the heat preservation time (seconds) as an abscissa axis and the annealing temperature (DEG C) as an ordinate axis, four straight lines are respectively correspondingly connected with the following coordinate points: between (0, 780) and (90, 780); between (90, 780) and (20, 900); between (20, 900) and (0, 900); the temperature region enclosed by the four straight lines between (0, 900) and (0, 780) is the predetermined temperature interval.

Preferably, the galvanized steel sheet is cooled to 420 ℃ or less in S12.

Preferably, in S12, the galvanized steel sheet is cooled by air cooling, or gas spraying.

Preferably, the structure of the coating layer of the galvanized steel sheet after the heat treatment is mainly composed of a phase (ζ) or a-fe (zn) phase(s), and the aluminum element in the coating layer is substantially diffused to the surface of the coating layer.

The heat treatment method of the galvanized steel sheet of the invention comprises the following steps:

after a plurality of experiments, the fact that after the galvanized steel sheet is subjected to heating pretreatment is found that aluminum elements in a zinc layer are basically enriched to the surface of the coating, in the subsequent cooling process, the surface aluminum oxide film can be cracked under the action of thermal stress, the surface of the coating can not form the aluminum oxide film but form zinc oxide in the austenitizing heating process, and the zinc layer is prevented from being oxidized. In addition, after the preheating treatment, because aluminum has higher affinity with oxygen and diffuses to the surface of the coating, the reaction of zinc and iron in the subsequent austenitizing process is more violent than the reaction in the preheating process, so that liquid zinc or zinc-iron phases in the coating are reduced, and the generation of large cracks on a matrix during stamping is avoided.

The experimental data show that when the heating temperature of the steel plate is lower than 600 ℃, the aluminum of the zinc layer can be diffused to the surface of the coating layer only by long diffusion time, and the production efficiency is reduced. When the heating temperature is higher than 910 ℃, the zinc volatilization can reduce the corrosion resistance of the coating when the temperature is kept for more than 30s, and the higher iron content in the coating can form a thicker oxide layer in the subsequent austenitizing process to influence the coating performance. In consideration of the above-mentioned factors, the present invention heats a galvanized steel sheet to any temperature within a preheating zone 1, and then keeps it warm for a holding time corresponding to the predetermined temperature interval 1, the preheating zone 1 being determined as follows: taking the heat preservation time (seconds) as an abscissa axis and the annealing temperature (DEG C) as an ordinate axis, four straight lines are respectively correspondingly connected with the following coordinate points: between (0, 600) and (200, 600); between (200, 600) and (30, 910); between (30, 910) and (0, 910); between (0, 910) and (0, 600), the temperature area enclosed by these four straight lines is the preheating area 1, and then the steel plate is cooled to 550 ℃ or below.

When the galvanized steel sheet is heated to any temperature in the preheating zone 2 and then is subjected to heat preservation for a heat preservation time corresponding to the preheating zone 2, a layer of alpha-Fe (Zn) phase is formed at the interface of the coating and the matrix, which is particularly beneficial for preventing the corrosion of the zinc layer to the austenite grain boundary, and is a preferable preheating treatment process zone, wherein the preheating zone 2 is determined according to the following mode: taking the heat preservation time (seconds) as an abscissa axis and the annealing temperature (DEG C) as an ordinate axis, four straight lines are respectively correspondingly connected with the following coordinate points: between (0, 600) and (200, 600); between (200, 600) and (30, 910); between (30, 910) and (0, 910); between (0, 910) and (0, 600), the temperature region enclosed by these four straight lines is the preheating region 2. When the galvanized steel sheet is cooled to below 550 ℃ after being heated in the preheating zone 2, the formation of the solid phase on the surface of the coating can destroy the alumina film on the surface layer, and when the galvanized steel sheet is cooled to below 420 ℃, the liquid zinc or zinc-iron phase on the surface of the coating is solidified, the aluminum oxide film on the surface layer is more completely broken under the action of thermal stress, and an oxide layer mainly comprising zinc oxide can be formed on the surface layer in the subsequent austenitizing process.

Too slow cooling rate after heat treatment affects production efficiency, and too fast deformation of the galvanized steel sheet is too large. Therefore, in the present invention, cooling is performed by air cooling, gas mist cooling, or the like, or by a combination thereof.

The joint of the coating and the steel substrate of the galvanized steel sheet after the heating treatment forms a phase or an alpha-Fe (Zn) phase. It is known from the Fe-Zn phase diagram that the phases and the alpha-Fe (Zn) phase have higher melting points, and the existence of the phases can prevent the corrosion of liquid zinc relative to austenite crystal boundaries during the austenite heating process, thereby preventing the generation of larger cracks during the direct hot stamping process.

In a second aspect of the present invention, there is provided a hot stamping process for a galvanized steel sheet for hot stamping the galvanized steel sheet after the heat treatment method according to any one of claims 1 to 5 is applied, the hot stamping process comprising the steps of:

s21: the galvanized steel sheet is subjected to the preliminary heat treatment by the heat treatment method as set forth in any one of claims 1 to 5, and then the heat-treated galvanized steel sheet is heated to a temperature between 830 ℃ and 920 ℃, the heating rate between 20 ℃ and 830 ℃ is greater than 10 ℃/s, the heating rate between 830 ℃ and 920 ℃ is less than 10 ℃/s, and then the heat preservation time is less than 10min for austenitizing.

S22: and quickly transferring the austenitized galvanized steel plate into a stamping die for stamping forming, wherein the stamping forming temperature is 650 plus 830 ℃, and the cooling rate of the galvanized steel plate in the die is more than 30 ℃/s, thus obtaining the finished product.

Preferably, the heating in S21 is performed by using electric heating, induction heating or radiation heating.

Preferably, the surface layer of the finished part after hot stamping is zinc oxide with the thickness not more than 5 μm, and the coating diffusion layer of the finished part is composed of alpha-Fe (Zn) phase or alpha-Fe (Zn) phase and trace phase.

The hot stamping process of the galvanized steel sheet comprises the following steps:

since the Fe-Zn phase formed during the heat treatment of the galvanized steel sheet has a high melting point, austenitization can be performed at a fast heating rate. In order to improve the production efficiency, an electric heating mode, an induction heating mode or a radiation heating mode can be adopted, the heating rate between 20 ℃ and 830 ℃ is more than 10 ℃/s, and the good effect is achieved. When the rapid heating is carried out at the temperature of more than 830 ℃, the thickness uniformity of the coating is greatly influenced, and because the liquid phase is easy to gather at high temperature, the uniformity of the coating thickness of the galvanized steel sheet is influenced, and the coating can be ablated, so that the heating is carried out at the temperature of 830-920 ℃ at the temperature of less than 10 ℃/s, which is particularly beneficial to improving the uniformity and the hot stamping performance of the coating and is also beneficial to improving the high temperature resistance of the coating. In addition, the austenitizing temperature range is 830-920 ℃ in consideration of the austenitizing degree and the high temperature resistance of the zinc layer. And the austenitizing time is too long, an excessively thick zinc oxide layer can be generated on the surface layer, and the subsequent coating performance is influenced, so that the heat preservation time is set to be less than 10 min.

In order to obtain a martensitic structure, the press forming temperature at which the galvanized steel sheet is transferred from the inside of the heating furnace to the die cannot be lower than 650 ℃, and below this temperature, transformation of martensite to ferrite, pearlite, or bainite occurs, and the mechanical properties after press forming are affected. The direct hot stamping forming temperature can not be higher than 830 ℃, and the matrix is easy to generate large stamping cracks after the temperature is higher than the temperature. In the experiment, the matrix crack length is found to be smaller when the stamping temperature is between 650-830 ℃. The preferred pressing temperature is 650-800 c, at which conditions the press crack in the matrix is less than 3 μm and the crack tip is blunter. The cooling speed of the galvanized steel sheet in a die can not be lower than 30 ℃, and the transformation from austenite to bainite can occur below the temperature, so that the mechanical property is influenced.

Compared with the prior art, the invention has the following advantages:

(1) the invention has definite limits on the pre-heat treatment process and the hot stamping process so as to ensure that the finished product has better coating property, corrosion resistance and high temperature resistance and avoid generating large stamping cracks in the matrix;

(2) by adopting the process method provided by the invention, the finished product has better coating performance, and the zinc oxide layer on the surface does not need to be treated;

(3) in addition, the zinc-plated steel sheet subjected to the preliminary heat treatment can be heated at a relatively fast austenitizing heating rate without the problem of failure of the zinc layer.

Drawings

FIG. 1 is a schematic view of a heat treatment and hot stamping process of a galvanized steel sheet;

FIG. 2 shows the range of the preheating process parameters of the galvanized steel sheet;

FIG. 3 is a GDS map of the galvanized steel sheet in example 1 after the galvanized steel sheet is heated to 780 ℃ and is kept warm for 20 s;

FIG. 4 is a metallographic structure of a galvanized steel sheet in example 1, which is heated to 780 ℃ and kept warm for 20 s;

FIG. 5 is a cross-sectional metallographic phase of a bent portion after hot stamping in example 1;

FIG. 6 is a photograph of the surface after hot stamping in example 1;

FIG. 7 is a graph showing plating properties of the steel sheet after hot stamping in example 1;

FIG. 8 is a photograph of the surface of the scratch of example 1 after salt spray etching for 60 hours;

FIG. 9 is a GDS map of the galvanized steel sheet in example 2 after the galvanized steel sheet is heated to 600 ℃ and kept warm for 2 min;

FIG. 10 shows the metallographic structure of the galvanized steel sheet in example 2 after the coating layer was heated to 600 ℃ and kept at the temperature for 2 min;

FIG. 11 is a photograph of the surface after hot stamping in example 2;

FIG. 12 is a cross-sectional metallographic phase of a bent portion after hot stamping in example 2;

FIG. 13 is a graph showing plating properties of the steel sheet after hot stamping in example 2;

FIG. 14 is a photograph of the surface of the scratch after salt spray etching for 60 hours in example 2;

FIG. 15 is a GDS map of the galvanized steel sheet in example 3 after the galvanized steel sheet is heated to 910 ℃ and kept warm for 5 s;

FIG. 16 is a metallographic structure of a galvanized steel sheet in example 3, in which the plating layer is heated to 910 ℃ and kept warm for 5 seconds;

FIG. 17 is a photograph of the surface after hot stamping in example 3;

FIG. 18 is a metallographic cross section of a bent portion after hot stamping in example 3;

FIG. 19 is a graph showing plating properties of the steel sheet after hot stamping in example 3;

FIG. 20 is a photograph of the surface of the scratch after salt spray etching for 60 hours in example 3.

Detailed Description

The present invention is described in detail below with reference to examples:

in the embodiment, the Galvanized (GI) hot stamping steel plate with relatively low production cost and high productivity is selected.

The cold-rolled hot-stamped steel sheet was prepared through conventional hot and cold rolling steps, and the designed composition thereof is shown in table 1, and the others are inevitable impurities. The steel plate is degreased, cleaned and dried, and no residual oil exists on the surface of the steel plate.

TABLE 1 chemical composition and mechanical properties after hot stamping for three steel sheets designed experimentally

And (3) a galvanizing process: and (4) galvanizing according to the traditional hot galvanizing process to obtain the galvanized steel sheet. The aluminum content in the zinc liquid is 0.11-1.0 wt%, and the thickness of the coating is more than 3 μm, and the maximum thickness of the coating after dip plating is 20 μm according to the characteristics of the hot galvanizing process and the control of the cost. And (3) performing a '0T' bending experiment on the zinc layer after dip plating to evaluate the adhesion and the forming performance of the plating layer. Table 2 shows the effect of different bath components on the coating texture and adhesion properties.

TABLE 2 influence of different Al contents on the texture and formability of the Zn layer

Aluminum content in Zinc solution (wt%)	Fe-Zn compound	Fe-Al inhibiting layer	0T bending test
				0.11	Form a	Is not formed	Good adhesion
0.25	Is not formed	Form a	Good adhesion
				0.51	Is not formed	Form a	Good adhesion
1.0	Is not formed	Form a	Good adhesion

Example 1:

heat treatment of galvanized steel sheet: a galvanized steel sheet (coating thickness of 11um) prepared under the condition that the aluminum content of the plating solution during galvanizing is 0.25 wt% is subjected to heat treatment. And (3) putting the galvanized steel sheet into a resistance furnace, heating to 780 ℃ and preserving heat for 20s, wherein the average heating rate is 15 ℃/s, and cooling to 550 ℃ in an air cooling mode after heating. FIG. 3 is a GDS map of the coating after the steel plate is heated to 780 ℃ at the speed of 15 ℃/s and is kept for 20s, and it can be seen from FIG. 3 that the aluminum element in the coating is basically enriched to the surface. FIG. 4 shows a metallographic structure in which a plating layer is mainly composed of a gamma phase and a small amount of a phase as viewed from the structure. The holding time and the heating temperature may be minimally combined as long as the aluminum element in the plating layer is substantially diffused to the surface of the plating layer.

The austenitizing process of the galvanized steel sheet comprises the following steps: the sample cooled to 550 ℃ is heated to 830 ℃ at a heating rate of 50 ℃/s, then heated to 900 ℃ at a heating rate of 6 ℃/s and kept for 90 s.

The hot stamping process of the galvanized steel sheet comprises the following steps: and (3) quickly transferring the heated galvanized steel plate to a stamping die, controlling the temperature during stamping through a thermocouple welded on the steel plate, quenching the steel plate by using the die with a cooling system, and cooling to room temperature at a cooling rate of 50 ℃/s to obtain full martensite. The temperature at the time of pressing the sample was about 800 ℃, and a metallographic photograph of the bent portion is shown in fig. 5. The plating layer is mainly alpha-Fe (Zn) phase from the metallographic structure, and the length of the punch crack is less than 3 um. From the surface photograph of fig. 6 after stamping, a uniform oxide layer was formed on the surface of the plating layer and no peeling of the plating layer occurred.

And (3) evaluating the coating performance of the hot stamped steel plate: and (3) after phosphorization and electrophoresis treatment are carried out on the prepared sample, the adhesion performance of the coating is evaluated by adopting a grid drawing method, and the side length of the grid is 1 mm. The photograph of the grid after tape bonding is shown in figure 7. From FIG. 7, it can be seen that the grid is complete and no peeling occurs, indicating that the plating performance is better.

Evaluation of corrosion resistance of steel sheet after hot stamping: the coated coupon was scribed so that the scratch penetrated the coating and zinc layer to the substrate. The carved sample is subjected to a salt spray corrosion experiment, and fig. 8 is a photo after 60h corrosion, so that the zinc-based coating can be seen to have better scratch resistance.

Example 2

Heat treatment of galvanized steel sheet: the method comprises the following steps of carrying out pre-heat treatment on a galvanized steel sheet (the thickness of a coating is 15 mu m) prepared under the condition that the aluminum content of the coating solution is 0.11 wt% in the galvanizing process, heating the galvanized steel sheet in a resistance furnace to 600 ℃, keeping the temperature for 2min, wherein the average heating rate is 15 ℃/s, and cooling to 420 ℃ in an air injection mode after heating. FIG. 9 is a GDS spectrum of the plating layer after being heated to 600 ℃ and kept for 2min, and it can be seen that aluminum elements in the plating layer are basically enriched to the surface. Fig. 10 shows a metallographic structure, and the plating layer is mainly composed of a γ phase and a phase when viewed from the structure.

The sample cooled to 420 ℃ is heated to 830 ℃ at the heating rate of 100 ℃/s, and then heated to 900 ℃ at the heating rate of 6 ℃/s, and the temperature is kept for 2 min. The heated sample was subjected to hot press forming and cooling, and cooled to room temperature at a cooling rate of 50 ℃/s, and the temperature during pressing was about 750 ℃. Fig. 11 is a photograph of the surface after punching, and fig. 12 is an interfacial metallographic phase at a bend. It can be seen from FIG. 11 that the surface of the coating is relatively complete and free from the skinning phenomenon, and that the coating is mainly in the α -Fe (Zn) phase as seen from the metallographic structure of FIG. 12, and no significant cracks are generated in the matrix.

FIG. 13 shows the results of evaluation of plating properties of the samples after press working. The grid is complete and no shedding occurs, which shows that the coating performance is better. FIG. 14 is a photograph of the surface of the scratch after 60h of salt spray corrosion, and it can be seen that the plating layer has better scratch resistance.

Example 3

Heat treatment of galvanized steel sheet: the galvanized steel sheet (coating thickness of 5um) prepared under the condition that the aluminum content of the plating solution in the galvanizing process is 1.0 wt% is subjected to preheating treatment. And (3) heating the galvanized steel plate in a resistance furnace to 910 ℃, preserving heat for 5s, and cooling to 200 ℃ in an air injection mode after heating. FIG. 15 is a GDS map of the plated layer after the preheating treatment, from which it can be seen that the aluminum element in the plated layer has been substantially concentrated at the surface. FIG. 16 shows a metallographic structure in which the plating layer is mainly composed of an α -Fe (Zn) phase and a γ phase when viewed from the structure.

The sample cooled to 200 ℃ is heated to 830 ℃ at the heating rate of 200 ℃/s, and then heated to 880 ℃ at the heating rate of 3 ℃/s and is kept for 4 min. FIG. 17 is a photograph of the surface of the plated layer after heating, and it can be seen that the surface of the plated layer is relatively complete and has no peeling phenomenon. From the metallographic structure at the bend in FIG. 18, the plating layer was mainly in the α -Fe (Zn) phase, and no cracks were evident.

FIG. 19 shows the results of evaluation of plating properties of the samples after press working. The grid is complete and the coating performance is better. FIG. 20 is a photograph of the surface of the coating after 60 hours of salt spray corrosion, and it can be seen that the coating has better scratch resistance.

In conclusion, the galvanized product of the hot stamping steel has the advantages of better welding, coating and the like. However, the galvanized steel sheet is easy to crack in the direct hot stamping process, and only indirect hot stamping forming can be carried out at present. After the galvanized steel sheet is subjected to heating treatment, the hot-stamping hot. The heating treatment process has short heating time and low heating temperature, does not influence the production efficiency, and the heating furnace adopts the conventional heating technology, so the equipment investment cost is low. Therefore, the invention has good popularization and application prospect.

The present invention has been described in terms of specific examples, which are provided to aid understanding of the invention and are not intended to be limiting. For a person skilled in the art to which the invention pertains, several simple deductions, modifications or substitutions may be made according to the idea of the invention.

Claims

1. A heat treatment method for a galvanized steel sheet, which is a heat treatment method for preheating the galvanized steel sheet before hot stamping, characterized in that the aluminum content in a plating solution used in the production process of the galvanized steel sheet is 0.11-1.0 wt%, comprising the steps of:

s11: heating the galvanized steel sheet to any temperature within a preset temperature interval, and then carrying out heat preservation on the galvanized steel sheet for heat preservation time corresponding to the preset temperature interval, wherein the preset temperature interval is determined according to the following modes: taking the heat preservation time (seconds) as an abscissa axis and the annealing temperature (DEG C) as an ordinate axis, four straight lines are respectively correspondingly connected with the following coordinate points: between (0, 600) and (200, 600); between (200, 600) and (30, 910); between (30, 910) and (0, 910); between (0, 910) and (0, 600), the temperature area enclosed by these four straight lines is the predetermined temperature interval;

s12: the galvanized steel sheet is then cooled to 550 ℃ or less.

2. The heat treatment method for a galvanized steel sheet according to claim 1, characterized in that the predetermined temperature interval described in S11 is determined in the following manner: taking the heat preservation time (seconds) as an abscissa axis and the annealing temperature (DEG C) as an ordinate axis, four straight lines are respectively correspondingly connected with the following coordinate points: between (0, 780) and (90, 780); between (90, 780) and (20, 900); between (20, 900) and (0, 900); the temperature region enclosed by the four straight lines between (0, 900) and (0, 780) is the predetermined temperature interval.

3. The heat treatment method for a galvanized steel sheet according to claim 1 or 2, characterized in that the galvanized steel sheet is cooled to 420 ℃ or less in S12.

4. The method of claim 1, wherein the galvanized steel sheet is cooled by air cooling, or gas spraying in S12.

5. The heat treatment method for a galvanized steel sheet according to claim 1, wherein the structure in the coating layer of the galvanized steel sheet after the heat treatment consists essentially of phases (. zeta.) or. alpha. -Fe (Zn) phases, (. zeta.) and the aluminum element in the coating layer diffuses substantially to the surface of the coating layer.

6. A hot stamping process for a galvanized steel sheet for hot stamping the galvanized steel sheet after the heat treatment method according to any one of claims 1 to 5, the hot stamping process comprising the steps of:

s21: carrying out pre-heat treatment on the galvanized steel sheet by adopting the heat treatment method as defined in any one of claims 1 to 5, then heating the heat-treated galvanized steel sheet to between 830 ℃ and 920 ℃, wherein the heating rate between 20 ℃ and 830 ℃ is more than 10 ℃/s, the heating rate between 830 ℃ and 920 ℃ is less than 10 ℃/s, and then keeping the temperature for less than 10min for austenitizing;

7. The hot stamping process for a galvanized steel sheet according to claim 6, wherein the heating in S21 is performed by electric heating, induction heating, or radiation heating.

8. The hot-stamping process for a galvanized steel sheet according to claim 6, wherein the surface layer of the finished product is an oxide of zinc having a thickness of not more than 5 μm, and the coating diffusion layer of the finished product is composed of an α -Fe (Zn) phase or an α -Fe (Zn) phase and a trace amount of phase.