A low-cost 500mpa-grade hot-dip galvanized dual-phase steel plate with high hole expansion performance and a method for manufacturing the same

Abstract

The present application belongs to the technical field of cold-rolled automobile steel, and particularly relates to a low-cost 500MPa-grade hot-dip galvanized dual-phase steel plate with high hole-expanding performance and a preparation method thereof. The alloy composition system of C-Si-Mn-Cr is adopted. C is the most basic strengthening element. The appropriate Si element can 'purify' the ferrite matrix, guarantee the drawing performance, and the appropriate Mn and Cr elements are added to improve the hardenability of the steel, so that appropriate martensite structure is generated. Mo, Nb and other high-priced alloy elements are not added, thereby reducing the manufacturing cost of the product. By adopting LF+RH double refining, low superheat, reasonable hot rolling temperature, annealing temperature and other key parameters, the product organization is uniform, the grain is fine, there is no banded structure and no large-size inclusion segregation, the organization ratio is reasonable, thereby guaranteeing that the product has high hole-expanding performance and the application scene is greatly increased. On the other hand, by controlling the atmosphere and dew point in the furnace during annealing, the substrate has good wettability, the surface of the steel forms an Fe2Al5 inhibition layer during galvanizing, the adhesion of the galvanizing is improved, and good surface quality is obtained.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of cold-rolled automobile steel, and particularly relates to a low-cost 500MPa-grade hot-dip galvanized dual-phase steel plate with high hole expansion performance and a preparation method thereof. BACKGROUND

[0002] With the increasing demand of the state and the market for automobile lightweight, galvanized dual-phase steel is widely used in automobile structure parts due to its high strength, low yield ratio, good formability and excellent corrosion resistance. At present, the most widely used galvanized dual-phase steel in automobile structural parts and reinforcing parts is 600MPa-grade and 800MPa-grade dual-phase steel. The application of 600MPa-grade and below hot-dip galvanized dual-phase steel in vehicle body structural parts is less. The reason is that the price of low-alloy high-strength steel and high-strength IF steel is lower than that of dual-phase steel, and the formability can meet the demand of general vehicle body structural parts, so in the demand scenario of 600MPa and below strength hot-dip galvanized steel, most of them are taken by low-alloy high-strength steel and high-strength IF steel.

[0003] However, in recent years, with the increasing demand of vehicle body structural parts for comprehensive performance of materials, hot-dip galvanized dual-phase steel has gradually occupied the market of 600MPa and below strength hot-dip galvanized steel due to its low yield ratio, continuous yield, high drawability, high tensile strain hardening index and no obvious aging.

[0004] CN 110172640 A discloses a 500MPa-grade high work hardening rate hot-dip galvanized dual-phase steel plate and a preparation method thereof. The chemical composition of the steel plate is as follows in terms of weight percentage: C: 0.02-0.05%, Si: 0.10-0.20%, Mn: 0.80-1.20%, Cr: 0.15-0.30%, Mo: 0.30-0.50%, Nb: 0.025-0.040%, Als: 0.02-0.04%, P≤0.020%, S≤0.0030%, and the balance is Fe and inevitable residual impurity elements. Mo alloy is expensive. According to the molybdenum-iron alloy price in 2022-2023, the cost of molybdenum-iron alloy required for a ton of steel with Mo content of 0.30-0.50% is as high as 1200-2000 yuan, which is difficult to be industrialized and popularized. The invention mentions that the hot-rolling coiling temperature is 700-730℃, which belongs to high-temperature coiling, but does not cooperate with the setting of U-shaped coiling, which easily leads to the fast heat dissipation of the head and tail and low temperature in the hot coiling slow cooling process, and the difference between the structure and performance of the coiled steel and the middle of the coil is large, which is inherited to the finished coil and easily causes the large difference between the performance of the whole coil, affecting the stamping use of customers.

[0005] CN 105401071 A discloses a 500MPa grade galvanized dual-phase steel for cars and a production method, the components and the weight percentage content of which are as follows: C: 0.04-0.09%, Si: ≤0.01%, Mn: 1.0-2.0%, P: ≤0.015%, S: ≤0.010%, Al: 0.01-0.08%, Mo: 0.01-0.30% or Cr: 0.02-0.9%, Nb: 0.001-0.03%, N: ≤0.005%, and the rest is iron and other inevitable impurities. In order to ensure a good surface of the hot galvanized steel strip, a very low Si content is adopted, and Mo or Cr elements are appropriately added, but the Mo or Cr element alloy cost is high, which is not conducive to industrialized production and popularization. Meanwhile, the invention does not mention the flanging and hole expanding performance of the 500MPa grade galvanized dual-phase steel, but the complex forming performance requirements of flanging and hole expanding of the current high-end passenger cars are increasingly improved. SUMMARY

[0006] Based on the deficiencies of the prior art, the purpose of the present application is to provide a low-cost high-hole-expanding 500MPa grade hot galvanized dual-phase steel plate and a preparation method thereof.

[0007] The technical scheme adopted by the present application to solve its technical problems is as follows: a low-cost high-hole-expanding 500MPa grade hot galvanized dual-phase steel plate, the chemical composition of which is as follows in terms of mass percentage: C: 0.05%-0.08%, Si: 0.20-0.40%, Mn: 1.20%-1.60%, Cr: 0.2%-0.4%, P ≤0.020%, S ≤0.007%, Al: 0.035%-0.065%, N ≤0.0050%, and the rest is Fe and inevitable impurities; and the microstructure is as follows in terms of volume percentage: ferrite 85-90%, martensite 7-12%, and bainite 3-5%.

[0008] The basic principle of the content design of each alloying element in the present application is as follows:

[0009] C: C is the most economical and effective strengthening element, C plays a phase transformation strengthening role in dual-phase steel, and the content of martensite in the final dual-phase steel is controlled to control the strength of the steel strip; in order to ensure good welding performance, the steel strip requires a low carbon content; therefore, the C content is controlled in the range of 0.05%-0.08% in the present application.

[0010] Si: Si is a ferrite-forming element. During heat preservation and slow cooling in the two-phase region, it can effectively promote the diffusion of carbon into austenite, which has a significant purification effect on ferrite, improves the purity of ferrite in duplex steel, and ensures that the strip has good drawing performance. With the improvement of the pre-oxidation and reduction capability of galvanizing production lines, galvanized duplex steel with strength of 600MPa and above is gradually changing the traditional idea of ​​"replacing silicon with aluminum". Adding an appropriate amount of silicon does not affect the surface quality of galvanized products. Therefore, this invention controls the Si content in the range of 0.20% to 0.40%.

[0011] Mn: Mn is a commonly used solid solution strengthening element. In dual-phase steel, it helps to expand the austenite region and delays the transformation of pearlite and bainite during the cooling process of austenite, thereby improving the hardenability of the steel. To ensure good strength, the Mn content in this invention is controlled within the range of 1.20% to 1.60%.

[0012] Cr: Like Mn, Cr can improve the hardenability of steel. When added to steel in combination with Mn, it can greatly improve the hardenability, thereby delaying the transformation of pearlite and bainite. At the same time, the appropriate addition of Cr can reduce the yield strength ratio of duplex steel, improve the distribution of martensite, and thus increase the elongation. However, excessive Cr content will deteriorate the weldability. Therefore, this invention controls the Cr content within the range of 0.20% to 0.40%.

[0013] P, S, and N are impurity elements in steel, which worsen the steel's plasticity and toughness. Generally, they need to be controlled at low levels. Considering the actual product performance requirements, this invention controls P ≤ 0.020%, S ≤ 0.007%, and N ≤ 0.0050%.

[0014] Alt: The main role of aluminum in steel is to refine the grain size and fix the nitrogen content, thereby improving the impact toughness of the steel and reducing its tendency to become brittle and age. Therefore, this invention controls the alt content to be between 0.035% and 0.065%.

[0015] Furthermore, the steel plate has a yield strength of 310–350 MPa, a tensile strength of 500–540 MPa, an elongation after fracture (A80) of 24–28%, a tensile strain hardening index (n0) of 0.15–0.19, and a hole expansion rate (λ) of 70–90%.

[0016] Furthermore, the steel plate thickness is 0.6–2.5 mm, the hot-dip galvanized steel plate has no missed galvanizing, no bending or zinc peeling, the inhibition layer is continuous and dense, and the surface quality is good.

[0017] A method for preparing low-cost, high-expansion-performance 500MPa grade hot-dip galvanized duplex steel sheet includes the following steps:

[0018] (1) Steelmaking and continuous casting processes:

[0019] 1) LF refining + RH refining are adopted;

[0020] 2) The superheat of the molten steel is 15–30°C;

[0021] 3) Continuously cast billets can be sent to the hot rolling furnace while still hot;

[0022] (2) Hot rolling process:

[0023] The slab homogenization temperature is 1190-1230℃, the hot delivery heating time of the slab is 160-200 min, the cold delivery heating time of the slab is 200-240 min, and the furnace exit temperature is 1180-1220℃; laminar flow cooling adopts front-stage sparse cooling and a coiling temperature of 540-570℃; after hot rolling, it is stacked in the hot rolling silo for slow cooling for 72 hours;

[0024] (3) Pickling and cold rolling process:

[0025] The total cold rolling reduction rate is 52-70%, and the reflectivity of the strip after pickling and cold continuous rolling is ≥70%.

[0026] (4) Continuous hot-dip galvanizing process:

[0027] Continuous annealing soaking temperature: 790~810℃; slow cooling end temperature: 670~690℃; rapid cooling end temperature: 440~460℃; furnace exit temperature: 0.6≤t<1.0mm, 450~470℃; 1.0≤t<1.80mm, 445~465℃; 1.80≤t≤2.50mm, 440~460℃; post-plating cooling rate ≥18℃ / s, belt speed 90~130m / min, pre-oxidation chamber temperature 600~680℃, compressed air injected into the oxidation chamber preheated to above 300℃, oxygen content in the oxidation chamber range 1.0~2.0%, dew point -15~0℃, finishing elongation set at 0.6-1.2%.

[0028] Impact of key manufacturing processes on the product of this invention:

[0029] In the steelmaking process, LF refining + RH refining ensures that impurities such as P, S, and N are controlled at low levels, and that the alloy composition and molten steel temperature are uniformly controlled. When the Mn content is high, continuous casting billets are prone to segregation, which can easily lead to banded structures in the finished strip, deteriorating the strip's expansion and forming properties. The molten steel superheat is 15–30℃; a lower superheat reduces the degree of segregation in the continuous casting billets, ensuring the expansion and forming properties of the strip. Continuous casting billets can be sent to the hot rolling furnace while still hot; however, for energy conservation, environmental protection, and to improve production speed, this is the preferred method.

[0030] In the hot rolling process, the slab homogenization temperature is 1190–1230℃, the hot billet heating time is 160–200 min, the cold billet heating time is 200–240 min, and the furnace exit temperature is 1180–1220℃. The heating time and temperature of the billet must ensure uniform heating of the hot or cold billet. A lower furnace exit temperature of 1180–1220℃ avoids excessively thick iron oxide scale that is difficult to remove and can be inherited by subsequent passes. Laminar flow cooling employs sparse cooling in the front section, using a coiling temperature of 540–570℃. Low-temperature coiling is used to avoid the difficulty in removing the fourth layer of iron oxide scale generated after coiling, which would affect the surface quality of the galvanized product. After hot rolling, the coil is stacked and slowly cooled in the hot rolling silo for 72 hours. This stacking and slow cooling ensures a uniform cooling rate throughout the coil, thus guaranteeing uniform properties and structure and avoiding thickness fluctuations and poor sheet shape in the acid-rolled coil.

[0031] In the pickling and cold rolling process, the total cold rolling reduction is 52-70%. After pickling and cold rolling, the strip reflectivity is ≥70%. A reasonable total reduction can refine the grains and provide recrystallization deformation energy. At the same time, it is necessary to consider controlling the load of the pickling and rolling mill within a reasonable range. Therefore, the total cold rolling reduction is set at 52-70%. After pickling and cold rolling, the strip reflectivity is ≥70%. The cold-rolled coil has a high surface reflectivity, which makes it easier to ensure a high surface cleanliness after cleaning in the galvanizing process and provides good wettability.

[0032] The continuous hot-dip galvanizing process has the following parameters: continuous annealing soaking temperature: 790–810℃; slow cooling ending temperature: 670–690℃; rapid cooling ending temperature: 440–460℃; furnace exit temperature: 0.6≤t<1.0mm, 450–470℃; 1.0≤t<1.80mm, 445–465℃; 1.80≤t≤2.50mm, 440–460℃. Post-galvanizing cooling rate ≥18℃ / s. Strip speed 90–130 m / min. The annealing soaking temperature and strip speed control the degree of austenitization, affecting the ferrite, austenite ratio, and austenite stability. The slow cooling ending temperature, rapid cooling ending temperature, and furnace exit temperature affect the content of newly formed ferrite and pre-galvanized bainite. The post-galvanizing cooling rate, i.e., the cooling speed of the strip after exiting the zinc bath, is controlled above 18℃ / s, mainly to control the martensite content in the finished product microstructure and ensure sufficient strength. The microstructure, by volume percentage, is: ferrite 85-90%, martensite 7-12%, and bainite 3-5%.

[0033] The pre-oxidation chamber temperature is 600–680℃, and the compressed air injected into the oxidation chamber is preheated to above 300℃. The oxygen content in the oxidation chamber ranges from 1.0% to 2.0%, and the dew point is -15℃ to 0℃. Reasonable pre-oxidation and reduction parameters are set to avoid the formation of large amounts of silicon and manganese oxides on the substrate surface, which would affect wettability and ensure the formation of a good inhibition layer on the substrate surface, meeting the requirements of no zinc peeling and no plating defects during bending. The finishing elongation is set to 0.6–1.2% to control the surface roughness of the hot-dip galvanized product and meet the customer's stamping requirements. A large finishing elongation is not set to avoid affecting the drawing performance of the finished product.

[0034] This invention offers the following advantages: It employs a C-Si-Mn-Cr alloy composition system, where C is the most fundamental strengthening element. An appropriate amount of Si can "purify" the ferrite matrix, ensuring drawing performance. Simultaneously, the addition of appropriate amounts of Mn and Cr improves the hardenability of the steel, promoting the formation of a suitable amount of martensite. The absence of high-priced alloying elements such as Mo and Nb reduces manufacturing costs. By employing two refining processes (LF+RH), lower superheat, and appropriate hot rolling and annealing temperatures, key parameters such as uniform microstructure, fine grains, absence of banded structures, and absence of large inclusions and segregation are ensured, resulting in a reasonable microstructure ratio and high porosity, significantly expanding its application scenarios. Furthermore, by controlling the furnace atmosphere and dew point during annealing, the substrate exhibits good wettability, allowing the steel surface to form a Fe2Al5 inhibition layer during galvanizing, thus improving galvanizing adhesion and achieving better surface quality. Attached Figure Description

[0035] Figure 1 This is a diagram of the microstructure of the hot-dip galvanized steel sheet in Example 1.

[0036] Figure 2 This is a surface morphology diagram of the hot-dip galvanized steel strip in Example 1. Detailed Implementation

[0037] The following are specific embodiments of the present invention, which further describe the technical solution of the present invention. However, the scope of protection of the present invention is not limited to these embodiments. All changes or equivalent substitutions that do not depart from the concept of the present invention are included within the scope of protection of the present invention.

[0038] The present invention will be further described in detail below through specific embodiments 1 to 5:

[0039] In the steelmaking process, LF refining + RH refining is adopted; the superheat of molten steel is 15-30℃; the continuously cast billet can be sent to the hot rolling furnace while still red-hot. When the slab quality is qualified, it is preferred to send it to the hot rolling furnace while still red-hot. When the slab needs finishing, it is sent to the hot rolling furnace after slow cooling finishing.

[0040] The chemical composition of the continuous casting billet used in the 500MPa grade hot-dip galvanized duplex steel in specific embodiments 1 to 5 is shown in Table 1, and the process parameters of the steelmaking process are shown in Table 2.

[0041] Table 1 Chemical composition (wt%)

[0042] Example C Si P S N Mn Alt Cr 1 0.065 0.25 0.001 0.007 0.0040 1.42 0.035 0.20 2 0.050 0.27 0.002 0.005 0.0050 1.20 0.035 0.21 3 0.061 0.40 0.001 0.004 0.0040 1.51 0.052 0.31 4 0.080 0.32 0.002 0.007 0.0030 1.60 0.065 0.40 5 0.071 0.20 0.001 0.006 0.0035 1.39 0.046 0.20

[0043] Table 2 Key process parameters for steelmaking

[0044] Example Refining process route Molten steel superheat Continuous casting slab delivery mode 1 LF refining + RH refining 17 Red delivery 2 LF refining + RH refining 20 Red delivery 3 LF refining + RH refining 22 Red delivery 4 LF refining + RH refining 15 Red delivery 5 LF refining + RH refining 30 Red delivery

[0045] In the hot rolling furnace, the slab homogenization temperature is 1190–1230℃, the hot billet heating time is 160–200 min, the cold billet heating time is 200–240 min, and the furnace exit temperature is 1180–1220℃. Laminar flow cooling adopts front-stage sparse cooling with a coiling temperature of 540–570℃. After hot rolling, the slab is stacked in the hot rolling silo for slow cooling for 72 hours. Specific parameters for the hot rolling process are shown in Table 3.

[0046] Table 3 Hot rolling process parameters

[0047] LF refining + RH refining Cold delivery Example Slab type Soaking temperature / °C Heating time / min Discharge temperature / °C 1 Coiling temperature / °C 1220 200 1220 550 72 2 Hot-rolled coil stacking and slow cooling time / h 1220 180 1200 570 72 3 Hot material 1190 190 1180 555 72 4 Hot material 1210 160 1210 560 72 5 Hot material 1230 230 1220 540 72

[0048] The total reduction rate of cold rolling is 52-70%. The reflectivity of the strip after pickling and cold rolling is ≥70%. Hot-rolled coils are transformed into cold-hardened coils through the pickling and cold rolling process. Specific parameters for the pickling and cold rolling process are shown in Table 4.

[0049] Table 4 Process parameters for pickling and cold rolling processes

[0050] Hot material Cold material Example 1 52 70 2 55 73 3 62 72 4 55 71 5 70 73

[0051] The continuous hot-dip galvanizing process has the following parameters: continuous annealing temperature: 790–810℃; slow cooling end temperature: 670–690℃; rapid cooling end temperature: 440–460℃; furnace exit temperature: 0.6≤t<1.0mm, 450–470℃; 1.0≤t<1.80mm, 445–465℃; 1.80≤t≤2.50mm, 440–460℃. The post-galvanizing cooling rate is ≥18℃ / s. The belt speed is 90–130 m / min. The pre-oxidation chamber temperature is 600–680℃, and the compressed air injected into the oxidation chamber is preheated to above 300℃. The oxygen content in the oxidation chamber ranges from 1.0–2.0%, and the dew point is -15–0℃. The finishing elongation is set at 0.6–1.2%. The cold-rolled coils undergo the continuous hot-dip galvanizing process to become finished products. Specific parameters for the continuous hot-dip galvanizing process are shown in Tables 5 and 6.

[0052] Table 5 Process parameters for continuous hot-dip galvanizing process

[0053]

[0054] Table 6 Pre-oxidation-reduction process parameters

[0055] Cold rolling total reduction / % Strip reflectivity Example Pre-oxidation chamber temperature / °C Compressed air preheating temperature / °C 1 650 310 1.0 -15 2 600 340 1.5 -5 3 680 330 2.0 0 4 669 325 1.2 -12 5 640 336 1.3 -10

[0056] The microstructure of hot-dip galvanized steel sheet by volume percentage is as follows: ferrite 85-90%, martensite 7-12%, and bainite 3-5%.

[0057] The hot-dip galvanized steel sheet has a yield strength of 310–350 MPa, a tensile strength of 500–540 MPa, an elongation after fracture (A80) of 24–28%, a tensile strain hardening index (n0) of 0.15–0.19, and a hole expansion rate (λ) of 70–90%. The mechanical properties of the hot-dip galvanized steel sheets in Examples 1–5 are shown in Table 7.

[0058] Table 7 Mechanical Properties of Hot-Dip Galvanized Steel Sheets

[0059] Oxygen content in the oxidation chamber / % Dew point / °C Example Yield strength / MPa Tensile strength / MPa Post-break elongation A80 / % Tensile strain hardening exponent n0 Hole expansion ratio λ / % 1 312 510 28 0.19 75 2 345 538 26 0.18 79 3 337 529 27 0.18 70 4 328 526 26 0.17 85 5 339 533 27 0.18 86

[0060] This invention is not limited to the above-described embodiments. Anyone should know that any structural changes made under the guidance of this invention, and any technical solutions that are the same as or similar to this invention, fall within the protection scope of this invention.

[0061] The technologies, shapes, and structures not described in detail in this invention are all known technologies.

Claims

1. A method for preparing a low-cost, high-expansion-performance 500MPa grade hot-dip galvanized duplex steel sheet, characterized in that, Includes the following steps: (1) Steelmaking and continuous casting processes: 1) LF refining + RH refining are adopted; 2) The superheat of the molten steel is 15–30°C; 3) The continuously cast billet is sent to the hot rolling furnace; (2) Hot rolling process: The slab is heated to a uniform temperature of 1190-1230℃, the hot delivery heating time of the slab is 160-200 min, and the furnace exit temperature is 1180-1220℃. Laminar flow cooling adopts front-stage sparse cooling and a coiling temperature of 540-570℃. After hot rolling, the slab is stacked in the hot rolling silo for slow cooling for 72 hours. (3) Pickling and cold rolling process: The total cold rolling reduction rate is 52-70%, and the reflectivity of the strip after pickling and cold continuous rolling is ≥70%. (4) Continuous hot-dip galvanizing process: Continuous annealing soaking temperature: 790~810℃; slow cooling end temperature: 670~690℃; rapid cooling end temperature: 440~460℃; furnace exit temperature: 0.6≤t<1.0mm, 450~470℃; 1.0≤t<1.80mm, 445~465℃; 1.80≤t≤2.50mm, 440~460℃; post-plating cooling rate ≥18℃ / s, belt speed 90~130m / min, pre-oxidation chamber temperature 600~680℃, compressed air injected into the oxidation chamber preheated to above 300℃, oxygen content in the oxidation chamber range 1.0~2.0%, dew point -15~0℃, finishing elongation set at 0.6-1.2%; The chemical composition of the steel plate by mass percentage is as follows: C: 0.05%~0.08%, Si: 0.20~0.40%, Mn: 1.20%~1.60%, Cr: 0.2%~0.4%, P≤0.020%, S≤0.007%, Alt: 0.035%~0.065%, N≤0.0050%, with the remaining elements being Fe and unavoidable impurities; the microstructure by volume percentage is as follows: ferrite 85~90%, martensite 7~12%, bainite 3~5%.

2. The method for preparing low-cost, high-expansion-performance 500MPa-grade hot-dip galvanized duplex steel sheet as described in claim 1, characterized in that, The steel plate has a yield strength of 310~350MPa, a tensile strength of 500~540MPa, an elongation after fracture A80 of 24~28%, a tensile strain hardening index n0 of 0.15~0.19, and a hole expansion rate λ of 70~90%.

3. The method for preparing low-cost, high-expansion-performance 500MPa grade hot-dip galvanized duplex steel sheet as described in claim 1, characterized in that, The steel plate thickness is 0.6~2.5mm. The hot-dip galvanized steel plate has no missing galvanizing, no bending or zinc peeling, the inhibition layer is continuous and dense, and the surface quality is good.

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