Patent Application
20250196213
The invention relates to a hot stamping method for a pre-coated steel plate.
Proposals of “carbon peak” and “carbon neutral” materials for energy conservation and emission reduction for vehicles have become top priorities, and the light weight of a vehicle is an effective way of implementing the same. Among numerous lightweight materials, ultra-high strength steel is an idea material for achieving lightweight while ensuring the safety of the vehicle because of its advantages, such as high strength, low cost, mature technology, etc.
Hot stamping is a process of stamping and forming completely austenitizing steel plates under high temperature, while cooling the same quickly to finally obtain the ultra-high strength steel component. The hot stamping process has the advantages of high precise for formation, good effect of reducing weight, etc., and is especially suitable for forming of the complex and integrated thin-walled components, for example, A pillar, B pillar, back top beam or the like of a car body in white (i.e., the basic skeletal frame and body of an automobile).
In the process of actual hot stamping, in order to avoid the oxidation and the decarburization on the surface of the steel caused by high temperature, pre-coating is applied on the surface of the steel plate. Among them, the most widely used method is described in the patent for anti-high temperature Al—Si coating invented by the company ArcelorMittal in 1999, i.e., DE19942025A1. The commercialization of the hot formed steel plate with the above mentioned Al—Si coating was achieved in 2007. However, in the more than 20 years since the invention of the Al—Si plating, the problem of balancing between the alloying degree of the pre-coating and the fracture performance of matrix of the steel plate remains unsettled.
First, one of the problems is to increase the alloying degree of the pre-coating.
If the alloying degree is not enough, scaling and wear of the pre-coated steel plate occurs at the locations of the furnace rollers, forming die and the like, which is very adverse to practical production. In the meanwhile, insufficient alloying degree causes a relatively low proportion of the ductile phase and a relatively high brittle phase in the intermetallic compound, thus forming plating cracks easily and having impact on the resistance of the costing to corrosion, etc. of the pre-coated steel plate.
During the process of hot stamping of the pre-coated steel plate, in order to ensure the alloying degree, generally a relatively high heating temperature is used along with a relatively long time for the austenitizing step. For example, CN101583486B (hereinafter referred to as the '486 patent) discloses a process of hot stamping of pre-coated steel, which is currently the prevailing producing process for pre-coated steel and in which it is specified that the process of heating and temperature holding for austenitizing (1) when the Al—Si coating has a thickness of 20-33 μm; (2) when the steel plate has a thickness of 0.7-1.5 mm, the temperature and the time are defined within a quadrilateral composed of (3 minutes, 930° C.), (6 minutes, 930° C.), (13 minutes, 880° C.) and (4.5 minutes, 880° C.); and (3) when the steel plate has a thickness of 1.5-3 mm, the temperature and the time are defined within a quadrilateral composed of (4 minutes, 940°° C.), (8 minutes, 940°° C.), (13 minutes, 900° C.) and (6.5 minutes, 900° C.). The '486 patent also specifies the ratio of Fe2Al5 phase to FeAl phase in the pre-coating microstructure after the hot stamping, to ensure the performance of the coating, welding, resistance to corrosion and etc. of the pre-coated steel member after the hot stamping.
The second problem is how to enhance the fracture performance of the pre-coated steel plate matrix.
For enhancing the fracture performance, generally relatively low heating temperature and relatively short time for temperature holding are used to avoid excessive austenite grains growth under high temperature. However, the above requirement and that of increasing the alloying degree of the pre-coating layer conflict with each other. The techniques in prior art can only find compromise between them, but cannot both enhance the alloying degree of the pre-coating layer and the fracture properties of the steel plate matrix.
It is noteworthy that the present hot stamped member of the pre-coated steel still cannot satisfy the criteria for fracture performance for all automobile factories. The above-mentioned fracture performance involves low-temperature fracture, bending fracture, hydrogen induced delay fracture and etc. Non-ideal low-temperature fracture, bending fracture, or hydrogen induced delay fracture properties each may cause the local cracks and failure of the car body members during service or collision of the car, which endangers the safety of passenger(s). Especially, the Al—Si coating may be alloyed under high temperature to form a brittle intermetallic compound, a brittle high alumina high silicon ferrite and a brittle high carbon martensite. These brittle structures further decrease the fracture performance of the hot stamped steel member. In practical industrial manufacture, after hot forming, the fracture performance of the pre-coated steel plate is significantly lower than that of the steel plate without the pre-coating layer.
Additionally, the windows for the temperature holding and the process time of the present hot forming process are narrow, causing a high requirement for practical industrial production.
For example, CN106466697B (hereinafter referred to as the '697 patent) discloses another hot stamping process for an Al—Si pre-coated steel plate, in which it is specified that the process of heating and temperature holding is 900-950° C. and 2.5-10 minutes, preferably the heating furnace temperature is 935-950° C. or 945-950° C., and preferably the holding time is 2.5-5minutes. Although the temperature in the heating and temperature holding process in the '967 patent is only slightly higher than that in the '486 patent, in the practical production, the temperature in the '967 patent may cause austenite grains in the steel plate matrix to become excessively large, the fracture performance non-desirable, and the practicality of applying the process to form car body parts more difficult.
In summary, the current hot forming process is unable to improve the alloying degree of the pre-coating layer and the steel plate matrix fracture performance. Thus, there is an urgent need for improving the fracture performance of the pre-coated steel plate to satisfy the fracture performance criteria in the automobile industry. In addition, expanding the hot forming process windows based on the present hot forming process is also a benefit to practical industrial production.
Therefore, an objective of the present invention is to solve the above problems, which are a technical bottleneck, by providing a hot stamping method which can improve the alloying degree of the pre-coating layer of a hot stamped steel plate, a hot stamping method which can improve fracture performance of a hot stamped steel plate or a hot stamping method which can improve the alloying degree and fracture performance of the plate pre-coating layer of hot stamped steel at the same time. Another objective of the invention is to expand the processing intervals of temperature and time for the process of hot stamping, to enhance the stability of the process of hot stamping.
The present inventor found that performing preliminary heat treatment on a pre-coated steel plate before hot stamping achieves the above objectives effectively. This preliminary heat treatment mainly comprises a step of heating and temperature holding and a step of cooling.
Based on this, the invention provides a hot stamping method for pre-coated steel plate, comprising performing a hot stamping on a pre-coated steel plate using a die at a hot stamping temperature to obtain a hot stamped component, wherein the method further comprises performing a preliminary heat treatment on the pre-coated steel plate before the hot stamping, the preliminary heat treatment comprising:
(1) a step of heating and temperature holding: heating the pre-coated steel plate to 850-920° C. and temperature holding for 7 to 15 minutes, or heating the same to 920-960° C. and temperature holding for 5 to 10 minutes, for austenitizing the pre-coated steel plate matrix and alloying a pre-coating layer;
(2) a step of cooling: cooling the pre-coated steel plate to below 300° C. after the step of heating and temperature holding at a cooling rate not less than 5° C./s; and
(3) optionally, repeating the above step of heating and temperature holding and the step of cooling one or more times.
In one implementation of the invention, in addition to ferrum (iron), the pre-coated steel plate matrix comprises, the following composition represented by weight percentage: carbon 0.2-0.4%; manganese 0.5-1.5%; boron 0-0.005%; one or more alloying element(s) not more than 1%, selected from aluminum, silicon, chromium, molybdenum, niobium, vanadium; and other inevitable impurities.
In another implementation of the invention, in addition to ferrum (iron), the pre-coated steel plate matrix further comprises the following composition represented by weight percentage: carbon 0.3-0.5%; manganese 0.5-2.5%; boron 0-0.005%; one or more alloying element(s) not more than 3%, selected from aluminum, silicon, chromium, molybdenum, niobium, vanadium; and other inevitable impurities.
Increasing the content of carbon, manganese and silicon may increase the hardenability of the steel plate matrix. In the meantime it is easy to obtain martensite after the preliminary heat treatment, in order to achieve a refining of the grain. However, an excessive content of carbon, manganese and silicon may have an adverse impact on the fracture performance of the steel plate matrix. Adding a small amount of boron can increase the hardenability of the steel plate matrix without affecting the fracture performance. Aluminum may be deoxidized in the smelting process, which may protect the effectiveness of boron. Other elements such as chromium and molybdenum, etc. can also increase the hardenability, but adversely, increase the cost of the steel plate significantly. Vanadium and niobium can, on one hand, refine the austenite grains, and on the other hand, cause a precipitation hardening effect, which may enhance the strength of the hot stamped member. As a preferable instance, the pre-coated steel plate matrix may be selected as 22MnB5 or 34MnB5 steel which can be purchased on the market.
In an implementation of the invention, the pre-coated steel plate matrix has a thickness of 0.8-2.5 mm, the pre-coating layer has a thickness of 6-36 μm. In some implementations, the pre-coating layer has a thickness of 15-27 μm. In some specified implementations, the pre-coated steel plate matrix has a thickness of 1.0-1.3 mm, which is a typical thickness of the steel plate used for safety components of car body in white.
In some implementation of the invention, the pre-coating layer includes aluminum or aluminum alloy, the pre-coating layer is provided on at least one surface of the pre-coated steel plate, and the initial thickness of the pre-coating layer on each of the surfaces is less than 36 μm.
In some specified implementations, the pre-coating layer is an Al—Si alloy coating, generally comprising, by weight percentage, Si 8-11%, Fe 2-4%, Al 85-90% and inevitable impurities. Al mainly provides stability at high temperatures and resistance to oxidation of the coating, but adversely, Al will be alloyed with the steel plate matrix under high temperature, forming a brittle intermetallic compound. Si may be used to restrain the growth of the intermetallic compound in order to reduce the damage of the brittle intermetallic compound to the fracture performance of the steel plate.
As an example, commercial Al—Si pre-coated 22MnB5 steel was used, wherein the 22MnB5 steel plate matrix has the following content by weight percentage: C 0.20-0.23%, Mn 0.9-1.4%, and Si 0.20-0.28%. Additionally, the pre-coated Al—Si coating has a thickness of around 25 μm.
As a preferable implementation, when the thickness of the pre-coating layer is less than 20 μm, the step of heating and temperature holding is performed at 850-920° C., for 7 to 15 minutes; when the thickness of the pre-coating layer is greater than or equal to 20 μm, the step of heating and temperature holding is performed at 920-960° C., for 5 to 10 minutes. In some implementations, the pre-coating layer is an Al—Si plating, and when the thickness of the Al—Si plating is less than 20 μm, the temperature holding has a range of 870-915° C. and a total time of 2-7 minutes. When the thickness of the Al—Si plating is greater than or equal to 20 μm, the temperature holding has a range of 890-935° C. and a total time of 4-9 minutes is used.
The selection of the process of heating and temperature holding in the preliminary heat treatment is mainly to ensure the alloying of the coating and the obtaining of suitable intermetallic compound structure. When the pre-coating layer is aluminum or aluminum alloy, the intermetallic compound comprises an FeAl phase with its volume fraction not less than 60%. Compared to other intermetallic compounds, FeAl phase has the advantages of high melting point, good fracture toughness etc., which can reduce the loss of the stamping die, and at the same time can reduce the coating cracks, and enhance the fracture performance of the steel plate as well as its resistance to corrosion and the like.
Additionally, during the preliminary heat treatment, the poor effect caused by the growth of the steel plate matrix austenite may be eliminated or remedied by the martensite or bainite structure created after cooling. The martensite or bainite structure mentioned above may provide more austenite nucleation points in the subsequent hot stamping and refine the austenite structure to improve the fracture performance of the steel plate matrix. Thus, the process ranges of the preliminary heat treatment according to the invention are greater than those of the existing hot stamping process, which is beneficial to improve the stability of the process.
The cooling rate in the preliminary heat treatment depends mainly on the hardenability of the steel plate matrix. It is needed to specify that, the cooling rate of around 5° C./s cannot avoid the formation of carbide completely, and cannot obtain a microstructure of full martensite or bainite. However, the inventors found that a small amount of martensite or bainite microstructure (not less than 30%, or the sum of volume fractions of the two not less than 30%) is capable of playing a role of refining the austenite grains during the subsequent hot stamping. In the meanwhile, uniformly distributed carbide may also provide amounts of austenite nucleation locations during heating, playing the effect of refining the austenite grains.
Preferably, the cooling rate is not less than 10° C./s, because when that happens the martensite volume fraction after cooling can reach above 50%, pearlite would not be formed, and the size of carbide particles would not be greater than 0.1 μm. More preferably, the cooling rate is not less than 25° C./s, because in that situation the martensite volume fraction after cooling approaches 100%. However, when the cooling rate is increased to 50° C./s, it is not significant that the prior austenite grains are further refined, and the fracture performance after the subsequent hot stamping has little improvement. Therefore, most preferably, the cooling rate is 25° C./s to 50° C./s.
During practical production, it takes a very long time to cool the above-mentioned steel plate after the preliminary heat treatment to a room temperature, and the cooling rate will slow down especially when the temperature is below 200°° C. In order to increase the production efficiency, as a preferable solution, when the steel plate is cooled to a temperature below 300° C. and above 200°° C. after the step of preliminary heat treatment, the steel plate is transferred to a heating furnace, beginning with reheating. In this situation, the martensite transformation during the cooling has been completely or almost completely finished, which is sufficient for refining the prior austenite grains and improving the final fracture performance of the steel plate.
Preferably, the pre-coated steel plate may be deformed using a stamping die during the cooling process of the preliminary heat treatment. On one hand, the amount of deformation during the subsequent hot stamping may be reduced, the requirement for temperature in the subsequent hot stamping may be reduced, and the process windows of heating and temperature holding in the subsequent hot stamping are enlarged. On the other hand, the heating and temperature holding in the subsequent hot stamping can fill the plating cracks caused by the pre-deformation, and at the same time, the amount of deformation required by the subsequent hot forming process is reduced. Both the quantity and depth of the plating cracks caused will be reduced, and thus the resistance to corrosion of the pre-coating layer can be improved, decreasing the adverse impact on the fracture performance by the plating cracks.
Preferably, several preliminary heat treatments are applied to the steel plate, and the same or different processing parameters are used between the several preliminary heat treatments. Through the transformation back and forth between high temperature austenite and room temperature martensite, the refining extent of the grains can be increased. In the meanwhile, using different processing parameters between the several preliminary heat treatments may provide more possibilities for the design of the Fe—Al diffusion layer and the intermetallic compound layer in the pre-coating layer (such as Al—Si plating). In view of this, the chemical component and the microstructure of the steel plate matrix subjected to the preliminary heat treatments several times, the grains can be refined better, and at the same time the alloying degree of the pre-coating layer is higher.
Additionally, during the preliminary heat treatment, the poor effect caused by the growth of the austenite of the steel matrix may be eliminated or remedied by the martensite or bainite structure resulting from the cooling. The martensite or bainite structure mentioned above may provide more austenite nucleation points in the subsequent hot stamping, refining the austenite structure to improve the fracture performance of the steel plate matrix. Therefore, the process ranges of the preliminary heat treatment of the invention are larger than those of the existing hot stamping process, causing an improvement in the stability of the process.
According to the hot stamping method used for pre-coated steel plate provided by the invention, the pre-coated steel plate matrix after the preliminary heat treatment has a microstructure with the following features:
(a) volume fraction of martensite or of bainite or of a sum of both is not less than 30%;
(b) there is no spherulitic carbide grains with grain diameter greater than 0.5 μm, and there is no laminar pearlite colony with grain diameter greater than 1 μm.
In the method of the present invention, the microstructure of the pre-coating layer after preliminary heat treatment is composed of one or more intermetallic compound(s) and ferrite, wherein, the FeAl phase has a volume fraction of not less than 60% among all intermetallic compounds.
In some implementations of the invention, the hot stamping method further comprises: after the preliminary heat treatment, reheating the pre-coated steel plate to 780-940° C. and holding the temperature for 1 to 7 minutes, to complete the austenitizing of the pre-coated steel plate matrix; then keeping the temperature above 500° C. and transferring the pre-coated steel plate to the die for hot stamping.
Preferably, the hot stamping method additionally comprises: after the preliminary heat treatment, reheating the pre-coated steel plate to 830-900° C. and holding the temperature for 1 to 4 minutes, to complete the austenitizing of the pre-coated steel plate matrix; then, keeping the temperature above 600° C. and transferring the pre-coated steel plate to the die for hot stamping.
Preferably, in the step of reheating, the temperature holding has a range of 830-900° C., and the total time for heating and temperature holding is 1-4 minutes.
It is more important that, with the original austenite grains in the hot stamped steel refined, the martensite lath bundle, the lath block and the lath are all refined, finally improving the fracture performance of the hot stamped member.
In some implementations of the invention, a microstructure of the pre-coated steel plate matrix after hot stamping is a full martensite structure.
In some other implementations of the invention, a microstructure of the pre-coated steel plate matrix after hot stamping is a mixed structure composed of two or more phases of ferrite structure, bainite structure, martensite structure, austenite structure. In this situation, the strength of the hot stamped member is decreased and the fracture performance is improved.
Further, a microstructure of the pre-coated steel plate matrix after hot stamping has an original austenite grain size not greater than 18 μm. As a preferable solution, the size of original austenite grains in the microstructure of the hot stamped member is not greater than 10 μm. Here, since the refining extent of the grains is higher, the fracture performance of the hot stamped steel is better.
The microstructure of the above-mentioned hot stamped steel plate matrix is generally a full martensite structure, and in this situation, the strength of the hot stamped steel may reach 1.5 GPa.
In some implementations of the invention, the steel plate is transferred to the heating furnace for reheating and for hot stamping after the preliminary heat treatment. As a preferable solution, when the steel plate is cooled to a temperature below 300° C. and above 200° C. after the step of preliminary heat treatment, the steel plate is transferred to the heating furnace, beginning with the reheating. In this situation, the martensite transformation during the cooling has been completely or almost completely finished, which is sufficient for refining the austenite grains and improving the final fracture performance of the steel plate. Transferring the steel plate to the heating furnace at a temperature above 200°° C. may increase the production efficiency and save energy at the same time.
As another preferable solution, when the steel plate is cooled to a temperature below 200° C. and above 100° C. after the step of preliminary heat treatment, the steel plate is transferred to the heating furnace, beginning with the reheating. Compared to the previous solution, the present solution is more suitable for the situation in which the alloy content of the pre-coated steel plate matrix is relatively high and the temperature for martensite transformation is relatively low.
The hot stamping method of the invention can refine the original austenite grains because of the following factors: (A) during the course of heating and temperature holding of the preliminary heat treatment, the pre-coating layer has been alloyed, forming an intermetallic compound layer and a diffusion layer, and during the hot stamping, the alloying of the pre-coating layer is no longer a main factor in consideration, and thus low temperature for heating and temperature holding and a short time for temperature heating and holding can be used, restraining the growth of the austenite grains; and (B) during the cooling of the preliminary heat treatment, the steel plate matrix forms full or partial martensite or bainite structures, those structures have characteristics of tiny grains, high defect concentration, etc., and in the steel plate matrix the carbide is distributed in a more tiny and uniform manner, providing more austenite nucleation sites during the reheating of the hot stamping and finally refining the prior austenite grains.
The technique of the Invention improves the fracture performance because of the following reasons: The techniques of the hot stamping of the invention refines the original austenite grains, the martensite lath bundle, the martensite lath block and the martensite lath in the microstructure of the hot stamped component. On one hand, the refining of the grains mentioned above decreases the enrichment of impurity elements such as phosphorus, sulfur, etc, as well as hydrogen at grain boundaries, remedies the adverse impact of the impurity elements and hydrogen on grain boundary fracture, so as to restrain the brittle intergranular fracture. On the other hand, with the refining of grains, the cracks need to pass through more high angle grain boundaries while expanding, including the original austenite grain boundaries, the martensite lath bundle boundaries, martensite lath block boundaries, etc., and thus the expanding of the cracks needs more energy and has more difficulty. Therefore, it finally improves the fracture performance of the hot stamped member.
According to the hot stamping method provided by the invention, the preliminary heat treatment can increase the alloying degree of the pre-coating layer, reduce the fraction of carbide structure in the steel plate matrix, obtain the martensite and/or bainite matrix structure so as to reduce the loss of the stamping die caused by the pre-coating layer during stamping and deforming under a high temperature, refine the grains, and improve the fracture performance of the finished hot stamped member.
The hot stamping method of the invention can be used for the safety structure parts of a car, e.g., reinforced structure parts, wheel members, and car structure parts with high hardness, including but not limited to A pillar, B pillar, back top beam, etc. of a car body in white.
In the following, the implementations of the invention will be described in detail in combination with drawings, in which:
Further detailed description of the invention will be given combined with implementations in the following, and the embodiments are merely for the purpose of explaining the invention but not for limiting the scope of the invention.
To explaining the hot stamping method of the invention specifically,
A cross section of the pre-coating Al—Si layer of a 22MnB5 steel plate used in an embodiment of the invention and the original microstructure of the steel plate matrix are shown in
The following preliminary heat treatments are applied to the pre-coating 22MnB5 steel plate:
(1) heating three of the pre-coated steel plates to 930° C., and temperature holding for 1 minutes (embodiment 1), 5 minutes (embodiment 2) and 10 minutes (embodiment 3) respectively, for austenitizing the pre-coated steel plate matrix and alloying the pre-coating layer;
(2) cooling the steel plate to a room temperature at cooling rate of 300° C./s after completing the step of heating and temperature holding.
After the preliminary heat treatment, the Al—Si plating in the above steel plate is alloyed, the microstructure of which is shown in
In the embodiment 2, the microstructure of the steel plate matrix after preliminary heat treatment is shown in
Numerous parallel tests demonstrate that: although the cooling rate and the temperature of the present embodiment do not fall with the optimum range of conditions of the invention, it will not impact the microstructure and conclusion in this embodiment. That is to say, the microstructure and conclusion based on the optimum cooling rate range and temperature range of the invention are almost the same as those achieved in the above embodiment. However, those optimum ranges of conditions are more beneficial for industrial production.
Re-heating and hot stamping the steel plate obtained in the embodiment 2:
(3) reheating the steel plate to 850° C. and temperature holding for a total time of 1.5 minutes, to achieve complete austenitizing of the steel plate;
(4) transferring the heated steel plate into a die, while ensuring that the temperature of the heated steel plate is above 500° C. when it arrives at the die;
(5) hot stamping the heated steel plate in the die, and obtaining a finished hot stamped steel member.
As a comparison,
In
It can be seen from the above embodiments that the method of the invention can improve the alloying degree of the pre-coating layer and the fracture performance of the steel plate matrix at the same time. The reasons for the improvement in the fracture performance of the steel plate matrix are as follows: the hot stamping technique described in the invention can distribute carbon elements in the matrix more uniformly and can refine the original austenite grains, the martensite lath bundle, the martensite lath block and the martensite lath in the microstructure in the hot stamped member. On the one hand, refining the grains mentioned above may reduce the enrichment of impurity elements such as phosphorus, sulfur, etc. and hydrogen at the grain boundary, alleviate the adverse impact of the impurity elements and hydrogen on the fracture of the grain boundary, and thus suppress the brittle intergranular fracture. On the other hand, by refining the grains, the cracks need to pass through a higher angle grain boundary during expansion, including the original austenite grain boundaries, the martensite lath bundle boundaries, martensite lath block boundaries, etc., and thus the expansion of the cracks requires more energy and has more difficulty in finally improving the fracture performance of the hot stamped member.
The explanation of embodiments above are given for the purpose of illustration and description. Those skilled in the art will understand that the invention is not limited to these embodiments or testing data. The data and parameters recorded in the embodiments are only exemplary, and do not constitute any limitation on the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 202111226071.X | Oct 2021 | CN | national |
This application is a U.S. National Stage Application under 35 U.S.C. § 371 of International Patent Application No. PCT/CN2022/124636, filed Oct. 11, 2022, and claims the benefit of priority under 35 U.S.C. Section 119 (a) of Chinese patent application 202111226071.X filed on Oct. 21, 2021, all of which are incorporated herein by reference in their entireties. The International Application was published in Chinese on Apr. 27, 2023 as International Publication No. WO 2023/066087 A1.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/CN2022/124636 | 10/11/2022 | WO |
