Hot Bath Forming Process of a Highly Corrosion-Resistant Member That Is Easy to Weld and Thermocompress

Abstract

The present invention provides a hot bath forming process for highly corrosion-resistant components that are easy to weld and hot press, and is related to the field of thin plate hot stamping and plate metal parts manufacturing. The process includes the steps of: S1, heating a plated hot-formed steel sheet material containing any one of GI-type zinc-plated layer, GA-type zinc-plated layer, and Zn-Al-Mg alloy-plated layer in a heating furnace to fully heat to an austenitic state; S2, transferring the plated hot-formed steel sheet material after heating to a boiling water tank and immersing it in boiling water to clean the oxide layer; S3, forming, holding and quenching the plated hot-formed steel sheet material under the combined action of boiling water and upper and lower dies to obtain a part; and S4, removing the part and drying it in a blowing or drying oven to remove moisture in the part's plating layer. The process provided by the present invention involves immersing the sheet material in boiling water, and using bubbles generated between the boiling water and the hot sheet material to uniformly and controllably remove the oxide layer on the surface of the steel sheet, uniformly and accurately control the forming temperature of the sheet material, and simultaneously perform forming and quenching in boiling water, which can improve the production quality of parts, increase the service life of the mold, and save production costs.

Description

Technical Field

[0001] The present invention relates to the fields of thin plate hot stamping and the manufacture of plate metal parts, and specifically to the hot bath forming process of highly corrosion-resistant members that are easy to weld and hot press. This application claims the priority of a Chinese patent application filed with the Chinese Patent Office on September 14, 2021, with application number 202111073382.7 and invention title "Hot Bath Forming Process of Highly Corrosion-Resistant Members that are Easy to Weld and Hot Press", and all of its contents are incorporated into this application by reference.

Background Art

[0002] The main processes in the hot stamping forming process are blank heating - stamping forming and quenching - laser trimming - shot peening. The hot stamping forming technology has been widely applied due to advantages such as small forming force, small springback of parts, and high strength of parts after forming. However, the hot - formed parts of bare plates and Al - Si plated plates lack cathodic protection, so corrosion at the edge - cutting position, especially in underbody parts such as sill beams, occurs during the service of the parts.

[0003] Since the melting point of the zinc plating layer is low (the melting point of pure Zn is only about 400 °C), while the austenitizing temperature of the base material is high (850 - 900 °C). On the other hand, in the direct hot forming process, the lower the temperature of the plating layer, the better (below 650 °C), and the higher the temperature of the substrate, the better (above 750 °C). In the conventional 22MnB5 - type base material, when the forming temperature decreases (above 650 °C), ferrite is generated and the strength becomes insufficient. When the forming temperature of the zinc plating layer material is high (for example, 780 °C), the liquefied phase in the plating layer penetrates into the austenite grain boundaries during tensile stress deformation, causing the substrate to crack, that is, causing the liquefied metal - induced embrittlement (LMIE) phenomenon. Therefore, in zinc - plated hot - formed steel, there is a contradiction between the plating layer and the substrate. Currently, the mainly solved methods are mainly two types of procedures, without tensile stress deformation and reducing the forming temperature.

[0004] There is no tensile stress deformation like the preform process. The main processes are cold stamping forming of parts first - austenitization by zero - part heating - pressure - holding quenching - shot peening. The parts in this process have already been deformed beforehand. After heating, the parts are transferred to the mold and only quenched, without tensile stress deformation, so the phenomenon of liquid metal induced embrittlement (LMIE) does not occur. However, in this process, the parts need to be cold - stamped and formed beforehand, the parts are heated in the furnace, the cost is high, and the automation is complex.

[0005] Lower the forming temperature like early cooling. The main processes are that the sheet after heating is first cooled (using medium gas, dry ice, etc.) - formed - pressure - holding quenching. However, the automation control of this method is difficult, the temperature - lowering process and temperature control of the sheet are difficult, and it is also difficult to uniformly remove the oxide layer on the part surface.

[0006] Chinese Patent CN106795578A discloses a "method for intermediate steel sheet cooling". In this method, spraying is performed on the surface using an "airflow containing dry ice, dry snow or dry ice particles", thereby realizing the cleaning of the oxide layer on the surface of the galvanized steel sheet and the temperature reduction of the sheet. The cleaning force of the oxide layer on the steel sheet surface is derived from the impact force of "spraying" with external high pressure. This method has difficulty in uniformly cleaning the surface oxide layer, uniformly controlling the forming temperature of the sheet, is difficult in automation control, and it is necessary to manufacture "particles such as dry ice and dry snow" beforehand, resulting in high production costs.

[0007] Chinese Patent CN101821429A discloses a method and equipment for secondary removal of phosphorus from a metal strip by low-pressure water injection. In this method, it is described that during the hot rolling process of a hot-rolled steel slab, high-pressure water is used to spray onto the surface of the steel slab between the "rough rolling process" and the "finish rolling process" to remove the oxide layer on the surface of the steel slab. The thickness of the steel slab in this process is usually 80 - 200 mm, which is relatively large, and the thickness of the oxide layer is usually 100 μm - 1 mm. Since it is easy to clean the entire plating layer, it is not applicable to an ultra-thin zinc plating oxide layer of about 1 μm. Room temperature water cools the thin plate to room temperature (the cooling rate of a 1.5 mm thick hot steel plate at room temperature is 500 - 1000 °C / s), and it is difficult to control the temperature of the plate material.

[0008] Chinese Patent CN107922988A discloses a method for non-contact cooling of a steel plate and the equipment used in this method. In this method, air cooling is carried out using a matrix tube. It is easy for the cooling temperature of the plate material to become non-uniform, and it is difficult for automated control. Moreover, the oxide layer on the surface of the material after heating cannot be cleaned.

[0009] Chinese Patent CN10717238A discloses a hot stamping forming method for a zinc-based plated steel plate or steel strip. In this method, the forming temperature of the plated plate material is reduced by the edge cutting process of the hot plate material. However, it is difficult for the edge cutting process to ensure uniform cooling of the temperature of the material. The cooling rate at the edge cutting position is low, and the temperature at other positions is high. Automated control is difficult.

[0010] Therefore, it is extremely important to develop a hot forming process with low cost, high corrosion resistance, easy welding, capable of uniformly and controllably removing the oxide layer, and with a uniform and controllable cooling temperature.

Summary of the Invention

Problems to be Solved by the Invention

[0011] The present invention overcomes the deficiencies in the prior art, especially the difficulties in uniformly and controllably cleaning an extremely thin surface oxide layer, and problems such as difficulty in controlling the pre-cooling temperature. It provides a hot bath forming process for highly corrosion-resistant members that are easy to weld and thermocompress, balances the forming temperature of the plating layer and the forming temperature of the substrate, immerses the sheet material in boiling water, and uses the bubbles generated between the boiling water and the hot sheet material to uniformly and controllably remove the oxide layer on the steel sheet surface, with the aim of uniformly and accurately controlling the forming temperature of the sheet material.

Means for Solving the Problems

[0012] The present invention S1. Heating a plated hot-forming steel sheet material containing any one of a GI-type zinc plating layer, a GA-type zinc plating layer, and a Zn-Al-Mg alloy plating layer in a heating furnace to a completely austenitized state; S2. Transferring the heated plated hot-forming steel sheet material to a boiling water tank, immersing it in boiling water, and cleaning the oxide layer; S3. Forming, pressure-holding, and quenching the plated hot-forming steel sheet material under the combined action of boiling water and upper and lower molds to obtain parts; S4. Taking out the parts and drying them in a blow or drying furnace to remove the moisture in the part plating layer, and providing a hot bath forming process for highly corrosion-resistant members that are easy to weld and thermocompress.

[0013] Preferably, in step S1, the atmospheric oxygen content (volume percentage) in the heating furnace is 5-20%. During heating, the surface of the plating layer oxidizes, and the aluminum element between the plating layer and the substrate diffuses to the surface of the plating layer to form a dense A12O3 layer, suppressing the ZnO thickness. However, if the oxygen content is too low, the surface layer cannot form an oxide layer, most of the zinc volatilizes, and the surface corrosion-resistant layer of the plated hot-forming steel sheet material of the plating layer is lost. If the oxygen content is too high, the ZnO layer is too thick and affects the welding performance.

[0014] The heated plated hot forming steel sheet material may be a laser welded sheet, a patch welded sheet or an unequal thickness rolled sheet. On the premise of ensuring sufficient austenitization of the matrix material, the heating time should be as short as possible to prevent over-diffusion between the plating layer and the matrix, so that corrosion-resistant elements such as Zn in the plating layer are too low, leading to a reduction in the cathodic protection effect. The heating temperature is 850 - 900 °C, and after the sheet material reaches the holding temperature, it is held for 0.5 - 4 minutes.

[0015] Preferably, in step S2, the boiling water temperature is 80 - 100 °C, and the pressure of the boiling water on the surface of the oxide layer is 0 - 0.1 bar.

[0016] Preferably, the depth of the plated hot forming steel sheet material in boiling water is 3 - 1000 mm. When the sheet material is immersed in boiling water, a steam insulation layer is formed on its surface, and the heat transfer rate between the temperature of the sheet material and the water is extremely reduced. The cooling rate of a 1.5 mm thick sheet material in a vertical state is only 30 - 50 °C / s. When the depth is 3 - 1000 mm, the internal pressure of the insulation layer is greater than the hydrostatic pressure received by the position of the steel sheet in the water, forming bubbles, the insulation layer is destroyed, the surface is cleaned during the bubble formation process, and oxides such as ZnO, Al2O3, and MnO on the surface layer are removed. "Insulation layer bubbles" are continuously formed, forming a cleaning effect on the steel sheet surface. The time of the sheet material in boiling water is 2 - 20 s, the cooling rate in boiling water is uniform and controllable, and it is easy to implement automation. It is only necessary to control the time, posture, and position of the sheet material in boiling water.

[0017] Furthermore, in step S2, the boiling water also contains a dissolving agent with a mass fraction of 0 - 10% containing NaOH. Parameters such as the time of the sheet material in boiling water and the boiling water temperature are determined based on the thickness of the oxide layer and the forming performance of the part. If necessary, a dissolving solution such as NaOH that accelerates the dissolution of the oxide layer can be added to the boiling water at a certain concentration, and NaOH must be washed before the drying treatment of the part.

[0018] Preferably, in step S3, the forming temperature is 400 to 650 °C. The lower die of the mold is in a boiling water bath, the material sheet is arranged above the lower die, and when the press descends, it pulls the descent of the upper die, and the electroplated hot forming steel sheet material is formed under the combined action of the boiling water and the upper and lower dies, pressurized, and quenching is realized. During the forming and pressurizing process, the mold destroys the heat insulation layer on the steel sheet surface, the mold and the plate directly contact, and rapid heat exchange between the plate and the mold realizes the quenching of the plate.

[0019] Preferably, after the sheet material has gone through the heating stage and the cleaning stage in the boiling water bath, if the surface state of the plating layer does not meet the requirements of subsequent processes such as welding, the method further includes taking out the parts from the boiling water tank and transferring them to an oxygen-free room temperature water for ultrasonic cleaning before taking out the parts for blowing or drying treatment in a drying furnace.

[0020] Furthermore, the time for ultrasonic cleaning is 0.5 to 5 min.

[0021] Preferably, the raw material components of the electroplated layer hot forming steel sheet material are, by mass percentage, C 0.05 to 0.35 wt%, Si 0.05 to 0.2 wt%, Mn 0.5 to 2.5 wt%, Cr 0 to 0.3 wt%, Mo 0 to 0.25 wt%, Ti 0.02 to 0.04 wt%, Nb 0 to 0.2 wt%, V 0 to 0.2 wt%, B 0.002 to 0.006 wt%, P 0 to 0.020 wt%, S 0 to 0.003 wt%, Al 0.02 to 0.06 wt%, N 0 to 0.006 wt%, and the balance is Fe.

[0022] Preferably, the thickness of the plating layer is 5 to 30 μm.

Advantages of the Invention

[0023] The technical solution of the present invention has the following advantages: 1. The process provided by the present invention can simultaneously and uniformly control the oxide layer removal process and the plate temperature cooling process. The automation control of the overall control process can be easily implemented, and it is only necessary to control the flow pressure (flow rate) of the boiling water in the tank, the position, posture, and time of the steel plate in the water.

[0024] 2. The force for removing the oxide layer on the surface of the steel plate in the process provided by the present invention is derived from the heat release between the steel plate and the boiling water. The boiling water on the surface of the steel plate vaporizes and ruptures to form bubbles, generating an impact force. The flow of the surrounding water moves rapidly, quickly carrying away the oxides. This cleaning force is weak and is very suitable for the surface oxide layer of about 1 μm. If the surface pressure is too high, the entire plating layer will be removed, and the oxide layer will be removed unevenly. The present invention can achieve the removal of the oxide layer by the "long time" and "low cleaning force bubble method".

[0025] 3. The process provided by the present invention is convenient for actual production. Just place the mold in boiling water and delay the mold clamping time. During the mass production process, the temperature of the plate is continuously transmitted into the water, and the energy consumption of the boiling water bath in the continuous production process is significantly reduced. The mold is placed in boiling water, and there is no need to open the cooling water channel for the lower mold, which greatly reduces the processing and manufacturing cost of the mold. Moreover, the temperature of the mold is constant, reducing the thermal fatigue of the mold, reducing mold damage, and when forming, the plating layer solidifies and does not stick to the mold, reducing the heat absorption of the mold, which is beneficial to increasing the service life of the mold. It can realize functions such as cleaning and cooling of the plate before forming. During the pressure holding process of the mold, it can realize a reduction in the quenching and cooling rate of the plate, improve the tissue performance of the plate, and effectively improve the problem of cracking during the forming of the surface oxide layer of the low melting point corrosion-resistant coating.

[0026] 4. The mold in this process does not rust in boiling water because the oxygen content in the boiling water is 0, so the Fe element in the mold material cannot contact oxygen and does not cause rust on the mold.

[0027] 5. The stamping of this process is completed in a boiling water bath, blocking the contact between the plate and oxygen and avoiding oxidation of the plate during the transfer and forming processes.

Brief Description of the Drawings

[0028] To more clearly explain the specific embodiments of the present invention or the technical aspects in the prior art, the drawings necessary for use in the description of the specific embodiments or the prior art will be briefly described below. Obviously, the drawings in the following description are some embodiments of the present invention, and those skilled in the art can obtain other drawings based on these drawings without creative labor.

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Embodiments for Carrying out the Invention

[0029]

Examples

[0030] Example 1 As shown in Figure 1, it is a hot bath forming process of a highly corrosion-resistant member that is easy to weld and hot press, S1. Transfer a galvanized hot-formed steel sheet material with a thickness of 1.5 mm (the base composition is C 0.18 - 0.21 wt%, Si 0.05 - 0.2 wt%, Mn 1.5 - 2.2 wt%, Cr 0 - 0.3 wt%, Mo 0 - 0.25 wt%, Ti 0.02 - 0.04 wt%, Nb 0 - 0.1 wt%, B 0.002 - 0.006 wt%, P 0 - 0.020 wt%, S 0 - 0.003 wt%, Al 0.02 - 0.06 wt%, N 0 - 0.006 wt%, double GI-type surface galvanized 150 g / m 2 , with a single-sided thickness of 11 μm) to an 890 °C box-type heating furnace, hold for 5 minutes to complete austenitization. The oxygen content (volume ratio) in the atmosphere in the heating furnace is 20%. The step is as follows: S2. After the heating is completed, transfer the sheet material to a boiling water tank, immerse it in boiling water for uniform cleaning, lower the temperature. The depth is 3 - 1000 mm, the boiling water temperature is 100 °C, and the residence time of the steel sheet in the boiling water is 6 s. The step is as follows: S3. Since the mold is in the boiling water bath, only control a 6 s delay in the mold clamping time of the hydraulic press, and perform forming with the mold clamping of the hydraulic press (the required mold clamping time for pressing is 3 s), and perform pressure holding quenching. Here, since the total time of the sheet material in the boiling water before forming is about 9 s, the temperature of the sheet material before forming is 520 - 560 °C (see Figure 2), the pressure holding time is 10 s, and the mold clamping pressure holding is 100 T (the projected area pressure of the part is 20 MPa). The step is as follows: S4. After the part is taken out of the water, perform blow drying to remove the water on the surface of the part. The step includes the step of obtaining the part.

[0031] The mechanical properties of the formed part (test standard: GB / T228.1 - 2010 "Metallic materials - Tensile testing - Part 1: Method of test at room temperature"): Tensile strength 1420 - 1600 MPa, elongation at break 5 - 9%. The surface state of the part is shown in Figure 3. The surface state after the boiling water bath treatment is granular, very uniform, and there is almost no continuous oxide layer on a large area. The current window of the welding process of the final part is 1.1 - 1.4 KA, which fully meets the requirements of the current welding process. Also, there is no liquation cracking phenomenon in the plating layer (Figure 4). The Zn content in the galvanized layer after forming quenching is 32 - 55%, which has a relatively good cathodic protection effect.

[0032] Example 2 A hot bath forming process for a highly corrosion-resistant member that is easy to weld and hot press, S1. Transfer a galvanized hot-formed steel sheet material with a thickness of 1.5 mm (matrix components: C 0.05 - 0.35 wt%, Si 0.05 - 0.2 wt%, Mn 0.5 - 2.5 wt%, Cr 0 - 0.3 wt%, Mo 0 - 0.25 wt%, Ti 0.02 - 0.04 wt%, Nb 0 - 0.2 wt%, V 0 - 0.2 wt%, B 0.002 - 0.006 wt%, P 0 - 0.020 wt%, S 0 - 0.003 wt%, Al 0.02 - 0.06 wt%, N 0 - 0.006 wt%, the balance being Fe, double GA-type surface galvanized 150 g / m 2 , one-sided thickness 11 μm) to a 900 °C box-type heating furnace, hold for 5 minutes to complete austenitization, and the oxygen content (volume ratio) in the atmosphere in the heating furnace is 20%. S2. After the heating is completed, transfer the sheet material to a boiling water tank, immerse it in boiling water for uniform cleaning, lower the temperature, the depth of the sheet material is 3 - 1000 mm, the temperature of the boiling water is 80 °C, and the residence time of the steel sheet in the boiling water is 6 s. S3. Since the mold is in the boiling water bath, only control a 6 s delay in the mold clamping time of the hydraulic press, perform mold clamping forming and pressure-holding quenching of the hydraulic press. Here, since the total time of the sheet material in the boiling water before forming is about 9 s, the temperature of the sheet material before forming is 500 - 600 °C, the pressure-holding time is 10 s, and the mold clamping pressure-holding is 100 T. S4. After the parts are taken out of the water, transfer them to warm water in an oxygen-free chamber for ultrasonic cleaning. Perform cleaning on the oxide layer on the surface of the parts by ultrasonic vibration. The cleaning time is 0.5 - 5 min, take them out and blow-dry to remove the water on the surface of the parts.

[0033] The mechanical properties, welding performance, and liquation cracking effect of the plating layer of the formed parts were the same as those in Example 1.

[0034] Comparative Example 1 Produce parts using traditional air-cooling technology (the process specifically refers to CN107922988A). As shown in Figure 5, the surface state is non-uniform and there is a large-area continuous oxide layer.

[0035] Comparative Example 2 Using the direct hot forming technology, refer to "Strength, Plasticity and Fracture Strain of Hot Stamping Forming Steel by Yichi Liang, Zhiyuan Chang, Long Cai, etc. [J]. Acta Metallurgica Sinica, 2020, v.56(04):51-65.", the specific process: the part slab is first heated to about 930 °C in a heating furnace to form a uniform fully austenitic structure, and then transferred to a press by a robot hand. The temperature during die clamping stamping is 700 - 800 °C, in the fully austenitic state during stamping forming, with a tensile strength of about 200 MPa and an elongation rate exceeding 40%. The cooling water system in the mold maintains the mold surface temperature at 50 - 100 °C, and a martensite structure is formed by mold heat conduction quenching simultaneously with stamping forming. After the final assembly of the parts is completed, the white body is subjected to painting and baking, kept warm at 150 - 180 °C for 10 - 20 min. In the plating layer obtained by this process, the liquefied zinc penetrated more than 40 μm into the substrate, and as shown in Figure 6, the service performance, especially fatigue, could not be satisfied.

[0036] Obviously, the above-mentioned embodiments are not intended to limit the embodiments, but are for clearly explaining the examples. In addition to the above description, those of ordinary skill in the art can also make other different forms of changes and variations. Here, it is not necessary to exhaust all embodiments. The obvious changes or variations derived therefrom are within the protection scope of the present invention.

Claims

1. A hot bath forming process for a highly corrosion-resistant member that is easy to weld and hot press, comprising the steps of: heating a plated hot forming steel sheet material containing any one of an S1, GI type zinc plating layer, GA type zinc plating layer, and Zn—Al—Mg alloy plating layer in a heating furnace to a completely austenitized state; S2, transferring the heated plated hot forming steel sheet material to a boiling water tank, immersing it in boiling water, and washing the oxide layer; S3, forming, pressure-holding, and quenching the plated hot forming steel sheet material under the combined action of boiling water and upper and lower molds to obtain a component, characterized by comprising a hot bath forming process for a highly corrosion-resistant member that is easy to weld and hot press.

2. The hot bath forming process for a highly corrosion-resistant member that is easy to weld and hot press according to claim 1, wherein in step S1, the oxygen content in the atmosphere in the heating furnace is 5-20%.

3. The hot bath forming process for a highly corrosion-resistant member that is easy to weld and hot press according to claim 1, wherein in step S1, the heating temperature is 850-900°C, and after the sheet material reaches the holding temperature, it is held for 0.5-4 minutes.

4. The hot bath forming process for a highly corrosion-resistant member that is easy to weld and hot press according to claim 1, wherein in step S2, the boiling water temperature is 80-100°C, and the water pressure of the boiling water on the surface of the oxide layer is 0-0.1 bar.

5. The hot bath forming process for a highly corrosion-resistant member that is easy to weld and hot press according to claim 4, wherein the depth of the plated hot forming steel sheet material in the boiling water is 3-1000 mm.

6. The hot bath forming process for a highly corrosion-resistant member that is easy to weld and hot press according to claim 1, 4 or 5, wherein the water pressure of the boiling water on the surface of the oxide layer of the plated hot forming steel sheet material is 0-0.1 bar.

7. The hot bath forming process of a highly corrosion-resistant member that is easy to weld and hot press according to claim 1, 4 or 5, characterized in that the residence time of the plated hot forming steel sheet material in boiling water is 2 to 20 s.

8. The hot bath forming process of a highly corrosion-resistant member that is easy to weld and hot press according to claim 4, characterized in that the boiling water also contains a dissolving agent with a mass fraction of 0 to 10% containing NaOH.

9. In step S3, the hot bath forming process of a highly corrosion-resistant member that is easy to weld and hot press according to claim 1, characterized in that the temperature of the forming is 400 to 650 °C.

10. In step S3, the hot bath forming process of a highly corrosion-resistant member that is easy to weld and hot press according to claim 1, characterized in that the lower die is in a boiling water bath and the plated hot forming steel sheet material is arranged above the lower die.

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