High-plating-layer-quality high-aluminum 780MPa-grade continuous hot galvanizing dual-phase steel and production method thereof

Abstract

The invention relates to high-aluminum 780MPa-grade continuous hot galvanizing dual-phase steel with high plating quality and a production method thereof. The dual-phase steel comprises the following chemical components in percentage by mass: 0.07 to 0.09 percent of C, 0 to 0.10 percent of Si, 1.5 to 2.5 percent of Mn, 0.02 to 0.05 percent of Cr, 0.05 to 0.2 percent of Mo, 0.1 to 1.0 percent of Al, 0.013 to 0.028 percent of Nb, 0.010 to 0.025 percent of Ti, Plt and the balance of Fe. 0.015%, Slt; 0.003% of the total weight; in the preparation process, the pre-oxidation process is added, the dew point control range is + 10 DEG C to-40 DEG C, selective oxidation of alloy elements is controlled, the interface layer structure of the hot-dip galvanized dual-phase steel is improved, and the coating quality is improved. The method has the advantages that the cost of steel plate elements is low, selective oxidation is easy to control and the like, and has important significance in the field of advanced hot-dip galvanizing high-strength steel research.

Description

Technical Field

[0001] This invention relates to a high-quality 780MPa hot-dip galvanized duplex steel and its production method, belonging to the technical field of galvanized high-strength duplex steel production. Background Technology

[0002] With increasing demands for lightweight and corrosion-resistant automotive components, hot-dip galvanized duplex steel has gained popularity and is increasingly widely used. During the hot-dip galvanizing process, a suitable amount of Al is typically added to the zinc bath to form a continuous and dense Fe-Al layer between the galvanized layer and the steel substrate, inhibiting the Fe-Zn reaction and resulting in a high-quality galvanized layer. Currently, to improve production efficiency, continuous hot-dip galvanizing processes are commonly used in industry, closely combining continuous annealing heat treatment of steel with hot-dip galvanizing. During the continuous annealing stage, selective oxidation occurs on the surface of the duplex steel, and the resulting oxides adversely affect subsequent galvanizing interface reactions. In particular, film-like Si oxides severely hinder the nucleation and growth of the Fe-Al phase, altering the interface layer structure and deteriorating the hot-dip galvanizing performance of the duplex steel. For example, patent documents CN114480986A and CN110331341A show Si contents of 0.4–0.8% and 0.20–0.50%, respectively. To improve the coatability of duplex steel, the Si content is typically reduced during composition design, while the content of precious metals such as Cr, Mo, and Nb is increased. This significantly increases the production cost of the steel. For example, patent document CN 109852900A discloses a 600MPa grade hot-dip galvanized duplex steel with different yield strength ratios and its production method. Its chemical composition is as follows: C: 0.07–0.12%; Mn: 1.50–1.90%; Si≤0.05%; Al: 0.03–0.07%; Cr: 0.15–0.35%; Mo: 0.13–0.24%, with the balance being Fe. The Cr and Mo alloys added in the composition design of this patent document are costly and will significantly increase the production cost per ton of steel. Summary of the Invention

[0003] The purpose of this invention is to address the shortcomings of current technologies by providing a high-quality, high-alumina 780MPa grade hot-dip galvanized duplex steel and its production method. This hot-dip galvanized duplex steel, through compositional design, reduces the Si content while increasing the Al content, and decreases the content of precious metals such as Cr and Mo in the steel, thus providing a production method for 780MPa grade high-alumina hot-dip galvanized duplex steel. Furthermore, by using an internal oxidation process with a dew point control range of +10 to -40℃, the selective oxidation of alloying elements is controlled, improving the interface layer structure of the hot-dip galvanized duplex steel and enhancing the coating quality. This invention has advantages such as low steel plate element cost and easy control of selective oxidation, and is of great significance in the research field of advanced high-strength hot-dip galvanized steel.

[0004] The technical solution of this invention is as follows:

[0005] A high-coating-quality, high-alumina 780MPa grade hot-dip galvanized duplex steel has the following chemical composition and mass percentages: C: 0.07–0.09%, Si: 0–0.10%, Mn: 1.5–2.5%, Cr: 0.02–0.05%, Mo: 0.05–0.2%, Al: 0.1–1.0%, Nb: 0.013–0.028%, Ti: 0.010–0.025%, P<0.015%, S<0.003%, with the balance being Fe and other unavoidable impurities.

[0006] The preferred chemical composition and mass percentage of the dual-phase steel are: C: 0.07–0.09%, Si: 0–0.10%, Mn: 1.95–2.05%, Cr: 0.02–0.03%, Mo: 0.16–0.18%, Al: 0.55–0.65%, Nb: 0.013–0.028%, Ti: 0.010–0.025%, P<0.015%, S<0.003%, with the balance being Fe and other unavoidable impurities.

[0007] The coating weight of the hot-dip galvanized layer is 50-70 g / m. 2 .

[0008] The method for preparing high-coating-quality high-alumina 780MPa grade hot-dip galvanized duplex steel includes the following steps:

[0009] (1) The composition of continuously cast steel is controlled as follows: C: 0.07-0.09%, Si: 0-0.10%, Mn: 1.5-2.5%, Cr: 0.02-0.05%, Mo: 0.05-0.2%, Al: 0.1-1.0%, Nb: 0.013-0.028%, Ti: 0.010-0.025%, P<0.015%, S<0.003%, with the balance being Fe and other unavoidable impurities;

[0010] (2) The initial casting temperature is 1520~1550℃, and the casting speed is 1.5~5cm / min to obtain the continuous casting billet;

[0011] (3) Hot rolling of the billet: the initial rolling temperature is between 1100 and 1150℃, and the final rolling temperature is between 890 and 910℃, to obtain a hot-rolled plate with a thickness of 2.5 to 5.0 mm.

[0012] (4) The hot-rolled plate is pickled and cold-rolled, with the total reduction rate controlled at 60% to 70%, and the reduction rate of the last cold rolling pass controlled at 8% to 12%, to obtain a cold-rolled strip steel with a thickness of 1.0 to 2.5 mm.

[0013] (5) The cold-rolled strip steel is fed into an annealing furnace for pre-oxidation treatment. The heating rate of the oxidation section is 5-10℃ / s, the heating temperature is 620-640℃, the holding time is 80-130s, and the dew point control range is +10--40℃.

[0014] (6) The cold-rolled strip steel is subjected to continuous annealing. The annealing temperature is controlled between 775 and 805℃, the holding time is 80 to 160s, the strip steel is cooled after holding, the cooling rate is controlled between 20 and 45℃ / s, and the hydrogen content in the furnace is controlled between 2.5 and 3%.

[0015] (7) The steel plate that has undergone continuous annealing is subjected to hot-dip galvanizing surface treatment. The temperature of the strip steel when it enters the zinc pot is 460±5℃, the temperature of the zinc liquid is 460±2℃, and the aluminum content in the zinc liquid is 0.18~0.2%.

[0016] (8) After plating, rapid cooling is performed to obtain high-quality high-alumina 780MPa hot-dip galvanized duplex steel.

[0017] The zinc liquid in step (7) consists of 0.18-0.2% Al, <0.05% Fe, and the remainder is Zn.

[0018] The essential features of this invention are:

[0019] In this invention, the Al content in the steel composition is increased to improve the coating properties of duplex steel, while the Cr and Mo contents are reduced to lower production costs. On the other hand, by adjusting the dew point of the pre-oxidation section, with the dew point control range being +10 to -40°C, the selective oxidation of duplex steel during the heating process can be more effectively controlled, thereby improving the surface condition of the duplex steel and facilitating the formation of a high-quality galvanized layer on the surface of the duplex steel.

[0020] The beneficial effects of this invention are as follows:

[0021] Compared with existing technologies, this invention improves the coatability of duplex steel by substituting Si with Al in the composition design and controlling the pre-oxidation process, thereby enhancing the adhesion between the galvanized layer and the duplex steel. In the duplex steel matrix, Al and Si have similar functions; both can expand the (α+γ) region of the Fe-C phase diagram, improve the flexibility of heat treatment processes, and refine martensite grains. Therefore, replacing Si with Al does not adversely affect the microstructure and properties of the duplex steel matrix. The low-Si content composition design after replacing Si with Al effectively improves the surface condition of hot-dip galvanized duplex steel during continuous annealing heat treatment, significantly reducing surface oxides, especially Si oxides, which are severely detrimental to galvanizing. This generates a more continuous adhesive layer structure, which is beneficial for the bonding between the galvanized layer and the substrate, improving the coatability of the duplex steel. No coating peeling was observed in the 180° bending test. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the hot-dip galvanized interface layer structure of the present invention;

[0023] Figure 2 Electron microscopy observation and elemental distribution diagram of the hot-dip galvanized interface layer structure of a 1mm thick steel plate at a dew point of +10℃ in Example 1;

[0024] Figure 3 Electron microscopy observation and elemental distribution diagram of the hot-dip galvanized interface layer structure of a 1mm thick steel plate at a dew point of +5℃ in Example 2;

[0025] Figure 4 Electron microscopy observation and elemental distribution diagram of the hot-dip galvanized interface layer structure of a 1mm thick steel plate with a dew point of -20℃ in Example 3;

[0026] Figure 5 Electron microscopy observation and elemental distribution diagram of the hot-dip galvanized interface layer structure of a 1mm thick steel plate with a dew point of -40℃ in Example 4;

[0027] Figure 6 This is a photograph of a hot-dip galvanized duplex steel plate with a thickness of 1mm and a dew point of +5℃, as shown in Example 2, after undergoing a 180° bending test.

[0028] Figure 7 This is a photograph of the hot-dip galvanized duplex steel with a thickness of 1mm and a dew point of -20℃, as shown in Example 3, after undergoing a 180° bending test. Detailed Implementation

[0029] This invention provides a high-quality, high-aluminum 780MPa grade continuous hot-dip galvanized duplex steel. The chemical composition of the duplex steel described in Examples 1-12 by weight percentage is shown in Table 1.

[0030] Table 1 Chemical composition control of the examples (weight percentage, %)

[0031]

[0032] The roles and proportions of each element in the alloy of this invention are based on the following:

[0033] C: Carbon is the most effective strengthening element, and its content determines the hardness, percentage of martensite phase, and morphology of duplex steel. However, excessively high C content leads to a decrease in the elongation of the steel, so the C content in duplex steel needs to be strictly controlled. This invention controls the carbon content to 0.07–0.09%.

[0034] Silicon (Si) is a ferrite stabilizing element and a non-carbide forming element, effectively inhibiting carbide precipitation. However, a large amount of silicon-containing oxides inevitably forms on the surface of silicon-containing duplex steel, affecting surface quality and plating applicability. Therefore, this invention minimizes the silicon content in duplex steel by replacing it with aluminum, an element with similar properties. The silicon content is controlled to be 0–0.10%.

[0035] Mn: Manganese significantly improves the hardenability of steel and is relatively inexpensive, making it a major alloying element in duplex steel. Manganese strengthens through solid solution and refines ferrite, while significantly delaying pearlite and bainite transformations. However, excessive manganese content can degrade the weldability of the steel plate. In this invention, the manganese content is 1.5–2.5%, preferably 1.95–2.05%.

[0036] Cr: Chromium can significantly enhance the hardenability of austenite formed during critical zone heating, which can largely delay the pearlite and bainite transformations. Simultaneously, chromium can reduce the solid solubility of interstitial atoms in ferrite, thus decreasing the yield strength of ferrite. The chromium content in this invention is 0.02–0.05%, preferably 0.02–0.03%.

[0037] Mo: Molybdenum is a strengthening element in steel, significantly improving its hardenability. When used in combination with titanium, molybdenum achieves both high strength and high toughness, thus improving the steel's resistance to delayed fracture while maintaining good overall mechanical properties. In this invention, the molybdenum content is 0.05–0.2%, preferably 0.16–0.18%.

[0038] Al: The role of aluminum in steel is similar to that of silicon; it can expand the two-phase region, purify ferrite, and promote the uniform distribution of martensite in the microstructure. Aluminum can also increase the martensite transformation initiation temperature (Ms point) and reduce the amount of residual austenite after quenching. Therefore, aluminum can be considered as a substitute for silicon to reduce the silicon content in dual-phase steel. In this invention, the aluminum content is 0.1–1.0%, preferably 0.55–0.65%.

[0039] Niobium (Nb) exists primarily as NbC particles in dual-phase steel, playing a role in grain refinement and dispersion precipitation strengthening. Furthermore, NbC has a low dissolution temperature, allowing for high solubility in the matrix even at annealing temperatures reaching the two-phase region. The concentration of carbon atoms in the austenite through solid solution enhances stability, and the precipitated carbon atoms after cooling contribute to precipitation strengthening. This invention incorporates 0.013-0.028% niobium.

[0040] Ti: Titanium can reduce free nitrogen through nitrogen fixation, and the formation of TiC can organize the recrystallization of deformed austenite. The titanium content added in this invention is 0.010-0.025%.

[0041] P: Phosphorus can play a significant role in solid solution strengthening. However, excessive phosphorus content can make duplex steel brittle, so it is necessary to strictly control the phosphorus content in duplex steel. This invention controls the phosphorus content to below 0.015%.

[0042] S: Sulfur is an impurity element in duplex steel, and the lower its content, the better. In this invention, the sulfur content is controlled to be below 0.003%.

[0043] This invention also provides a production method for obtaining high-quality high-alumina 780MPa grade continuous hot-dip galvanized duplex steel through dew point control in the pre-oxidation stage. The control of dew point parameters in this invention will be further described in detail below with reference to specific embodiments. The embodiments are only descriptions of the best implementation of this invention and do not limit the scope of this invention in any way.

[0044] Example 1

[0045] The specific preparation steps in this embodiment are as follows:

[0046] (1) The actual smelting composition is shown in Table 2. The chemical composition is smelted according to the mass percentage of the chemical composition in Table 1.

[0047] Table 2 Actual components (weight percentage, %) of Examples 1-12

[0048] C Si Mn Cr Mo Al Nb Ti P S 1 0.076 0.09 2.01 0.025 0.175 0.63 0.017 0.019 0.012 0.001 2 0.071 0.07 1.98 0.024 0.176 0.58 0.016 0.017 0.012 0.002 3 0.077 0.08 1.99 0.026 0.179 0.61 0.014 0.014 0.013 0.001 4 0.078 0.08 2.02 0.023 0.167 0.64 0.019 0.016 0.014 0.001 5 0.084 0.07 1.96 0.022 0.166 0.59 0.014 0.018 0.012 0.002 6 0.079 0.07 1.97 0.024 0.171 0.62 0.020 0.017 0.013 0.001 7 0.082 0.08 2.02 0.024 0.173 0.65 0.014 0.019 0.013 0.002 8 0.076 0.09 2.04 0.025 0.170 0.57 0.018 0.018 0.014 0.001 9 0.081 0.09 1.98 0.022 0.175 0.64 0.017 0.020 0.013 0.001 10 0.083 0.07 1.97 0.027 0.168 0.58 0.019 0.019 0.012 0.002 11 0.074 0.08 2.00 0.025 0.164 0.62 0.023 0.021 0.014 0.001 12 0.078 0.09 1.97 0.023 0.177 0.63 0.017 0.020 0.013 0.001

[0049] (2) The molten steel obtained by smelting is continuously cast. The molten steel is obtained by adding alloy materials to the molten iron, such as medium carbon ferromanganese, medium carbon ferrochrome, ferromolybdenum, ferroniobium, ferrotitanium, etc., and is obtained through refining process. The Si in the molten steel is oxidized by oxygen blowing smelting and forms silicate inclusions that rise to the slag, thereby achieving the required silicon content. The initial casting temperature is 1540℃ and the casting speed is 1.52cm / min to obtain the continuous casting billet. The appearance quality and low magnification inspection are carried out. The appearance inspection is to check whether there are defects such as shock marks, depressions, cracks, and pores on the narrow face of the continuous casting billet. The low magnification inspection rating is to check whether there are porosity, shrinkage cavities, microcracks, inclusions, etc. that are visible under low magnification.

[0050] (3) Hot rolling of the billet: The forging billet is heated to 1250°C and held for 190 min. The initial rolling temperature is between 1250°C and the final rolling temperature is between 890°C. After rolling, the steel plate is rapidly cooled to 630°C by laminar flow cooling and then coiled. The steel plate is then placed in a heat preservation pit and kept at a temperature of at least 48 h. After that, the steel plate is taken out and air-cooled to room temperature and then flattened to obtain a hot-rolled plate with a thickness of between 2.5 mm.

[0051] (4) After pickling the hot-rolled plate, it is cold-rolled and formed. The total cold rolling reduction rate is controlled at 64%, and the cold rolling reduction rate of the last pass is controlled at 10%, so as to obtain a cold-hardened strip steel with a thickness of 1.0 mm.

[0052] (5) Cold-rolled strip steel was subjected to continuous annealing and hot-dip galvanizing according to the process parameters shown in Table 2. First, it was heated to approximately 640℃ at a heating rate of 10℃ / s (Heating 1), with the annealing atmosphere dew point controlled at +10℃, and held for 80s (Holding Time 1). Then, the temperature was further increased to approximately 780℃ (Heating 2), and held for approximately 90s (Holding Time 2). Subsequently, it was cooled to approximately 460℃ (the temperature at which it entered the zinc bath) at a cooling rate of 45℃ / s for galvanizing. The hydrogen content in the furnace was 2.7%, the aluminum content in the zinc bath was 0.19%, and the galvanizing time was 2s (the coating thickness was controlled by an air knife). After exiting the zinc bath, it was rapidly cooled to 180℃, then water-cooled to room temperature, finally obtaining hot-dip galvanized duplex steel. The weight of the galvanized layer on the sample was 50g / m². 2 .

[0053] Example 2-12

[0054] The smelting compositions of Examples 2-4 are shown in Table 2. The other steps are the same as in Example 1, except that the dew point of the annealing atmosphere is controlled to be +5℃, -20℃ and -40℃ respectively instead of +10℃.

[0055] The smelting composition of Examples 5-8 is shown in Table 2. The rolling parameters were changed to obtain a steel plate with a thickness of 1.4 mm. The holding time 1 and the holding time 2 were 95 s and 105 s, respectively. The dew point of the annealing atmosphere was controlled to be +10℃, +5℃, -20℃ and -40℃, respectively. Other parameters were the same as those in Examples 1-4.

[0056] The smelting composition of Examples 9-12 is shown in Table 2. The rolling parameters were changed to obtain a steel plate with a thickness of 1.8 mm. The holding time 1 and the holding time 2 were 115 s and 125 s, respectively. The dew point of the annealing atmosphere was controlled to be +10℃, +5℃, -20℃ and -40℃, respectively. Other parameters were the same as those in Examples 1-4.

[0057] The process parameters for the continuous annealing and hot-dip galvanizing stages in each embodiment are shown in Table 3.

[0058] Table 3 shows the control of key process parameters in the embodiments.

[0059]

[0060] Figure 1 This is a typical structural diagram of the interface layer of hot-dip galvanized duplex steel. The interface layer is formed between the duplex steel substrate and the hot-dip galvanized layer and is composed of two phases: Fe-Al and MnO. Figures 2 to 5These are diagrams showing the interface layer structure and elemental distribution of hot-dip galvanized duplex steel under different annealing atmospheres and dew point conditions, when the dew point is controlled between +5℃ and -20℃ (e.g., ...). Figure 4 and Figure 5 As shown, the Fe-Al inhibition layer consists of alternating Fe-Al and MnO phases, exhibiting good overall continuity. The average thickness is 100–150 nm. Mn is mainly formed by internal oxidation, while the MnO particles formed by external oxidation and the Fe-Al particles have a cross-sectional size of 40–80 nm. This continuously distributed Fe-Al inhibition layer can avoid the formation of brittle Fe-Zn phase, resulting in a hot-dip galvanized interface layer with superior expected quality.

[0061] The coating adhesion of galvanized duplex steel is tested by a 180° bending test. The equipment used is a metal sheet bending forming tester. The specimen must be a longitudinal specimen. The bending mandrel diameter for the coating bending test is 6a. The bending test results must meet the following requirements: there should be no flaking of the coating on the outer surface of the specimen beyond 5mm from the bending edge; coating cracks that do not expose the steel base are allowed, but there should be no peeling marks. Figure 6 and Figure 7 The photos show Case 2 and Case 3. After the hot-dip galvanized duplex steel with dew points of +5℃ and -20℃ respectively was tested in a 180° bending test, no coating peeling occurred on the outer surface of the sample in areas more than 5mm away from the bent edge, indicating good coating adhesion.

[0062] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

[0063] Matters not covered in this invention are common knowledge.

Claims

1. A high-quality, high-alumina 780MPa hot-dip galvanized duplex steel, characterized in that... The chemical composition and mass percentage of the duplex steel are as follows: C: 0.07-0.09%, Si: 0-0.10%, Mn: 1.5-2.5%, Cr: 0.02-0.05%, Mo: 0.05-0.2%, Al: 0.1-1.0%, Nb: 0.013-0.028%, Ti: 0.010-0.025%, P<0.015%, S<0.003%, with the balance being Fe and other unavoidable impurities.

2. The high-coating-quality, high-alumina 780MPa grade hot-dip galvanized duplex steel as described in claim 1, characterized in that: The chemical composition and mass percentage of the duplex steel are as follows: C: 0.07–0.09%, Si: 0–0.10%, Mn: 1.95–2.05%, Cr: 0.02–0.03%, Mo: 0.16–0.18%, Al: 0.55–0.65%, Nb: 0.013–0.028%, Ti: 0.010–0.025%, P<0.015%, S<0.003%, with the balance being Fe and other unavoidable impurities.

3. The high-coating-quality high-alumina 780MPa grade hot-dip galvanized duplex steel as described in claim 1, characterized in that the amount of the hot-dip galvanized layer is 50-70 g / m². 2 .

4. The method for preparing high-coating-quality high-alumina 780MPa grade hot-dip galvanized duplex steel as described in claim 1, characterized in that: Includes the following steps: (1) The composition of continuously cast steel is controlled as follows: C: 0.07-0.09%, Si: 0-0.10%, Mn: 1.5-2.5%, Cr: 0.02-0.05%, Mo: 0.05-0.2%, Al: 0.1-1.0%, Nb: 0.013-0.028%, Ti: 0.010-0.025%, P<0.015%, S<0.003%, with the balance being Fe and other unavoidable impurities; (2) The initial casting temperature is 1520~1550℃, and the casting speed is 1.5~5cm / min to obtain the continuous casting billet; (3) Hot rolling of the billet: the initial rolling temperature is between 1100 and 1150°C, and the final rolling temperature is between 890 and 910°C to obtain a hot-rolled plate; (4) The hot-rolled plate is pickled and cold-rolled, with the total reduction rate controlled at 60% to 70%, and the reduction rate of the last cold rolling pass controlled at 8% to 12% to obtain cold-hardened strip steel. (5) The cold-rolled strip steel is fed into an annealing furnace for pre-oxidation treatment. The heating rate of the oxidation section is 5-10℃ / s, the heating temperature is 620-640℃, the holding time is 80-130s, and the dew point control range is +10--40℃. (6) The cold-rolled strip steel is subjected to continuous annealing. The annealing temperature is controlled between 775 and 805℃, the holding time is 80 to 160s, the strip steel is cooled after holding, the cooling rate is controlled between 20 and 45℃ / s, and the hydrogen content in the furnace is controlled between 2.5 and 3%. (7) The steel plate that has undergone continuous annealing is subjected to hot-dip galvanizing surface treatment. The temperature of the strip steel when it enters the zinc pot is 460±5℃, the temperature of the zinc liquid is 460±2℃, and the aluminum content in the zinc liquid is 0.18~0.2%. (8) After plating, rapid cooling is performed to obtain high-quality high-alumina 780MPa hot-dip galvanized duplex steel.

5. The method for preparing high-coating-quality high-alumina 780MPa grade hot-dip galvanized duplex steel as described in claim 4, characterized in that: The zinc liquid in step (7) consists of 0.18-0.2% Al, <0.05% Fe, and the remainder is Zn.

6. The method for preparing high-coating-quality high-alumina 780MPa grade hot-dip galvanized duplex steel as described in claim 4, characterized in that: In step (3), the thickness of the hot-rolled plate obtained is 2.5 to 5.0 mm; The thickness of the cold-hardened strip obtained in step (4) is 1.0 to 2.5 mm.

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