Method for producing hot-dip galvanized sheet

Abstract

The invention provides a method for producing a hot-dip galvanized sheet. A double-roller cast-rolled thin strip is used as a raw material, the roughness difference in the plate surface edge of the thin strip raw material is improved by designing the roller surface roughness distribution of a texturing roller of a fifth acid rolling rack, and the edge color difference problem after the thin strip raw material is galvanized is solved. The raw material components of each coil of thin strip are combined, and the combination of a zinc layer and a thin strip substrate is effectively improved by formulating an annealing furnace dew point adjustment scheme and performing reasonable production scheduling, so that the problem of coating falling is solved; the increase of the silicon content of the plating solution caused by the high-silicon-component thin strip substrate along with the increase of the working time is fully considered, and the zinc pot components are balanced in time by formulating a zinc pot component adjusting scheme, so that the generation and aggregation of zinc pot slag substances are reduced, and the surface slag defect problem is improved. And finally, stable production of the cast-rolled thin-strip hot-dip galvanized sheet is achieved, and the surface quality requirement of the galvanized sheet of a household appliance is met. The surface quality of a finished product reaches the FB level, the yield strength is 230-270 MPa, the tensile strength is 340-380 MPa, the ductility is larger than or equal to 32%, and the hot-dip galvanized steel plate for the commercial household appliances belongs to a hot-dip galvanized steel plate for the commercial household appliances.

Description

Technical Field

[0001] The invention belongs to the technical field of twin-roll casting and rolling, and particularly relates to a method for producing hot-dip galvanized sheets. Background Art

[0002] With the introduction and gradual implementation of carbon fee policies in recent years, various industries have placed increasingly stringent requirements on carbon emissions during the production of products and raw materials. Taking the 3C and consumer electronics industries as an example, leading companies in the industry, such as Dell and HP, have all proposed their own carbon emission reduction plans, one of which is to use "green steel" (steelmaking raw materials with a scrap ratio of at least 50%) instead of traditional cold-rolled steel strip as the galvanized substrate for home appliances. Given the current technological level and maturity of the steel industry, the technical requirements of "green steel" basically limit its production process to an electric furnace + plate production line. Currently, Shagang is the only company in China equipped with a full-process equipment configuration of electric furnace + ultra-thin strip + cold-rolled galvanizing, meeting the production line and equipment capabilities for the full "green steel" production process.

[0003] Twin-roll thin strip casting technology, also known as ultra-thin strip technology, is a near-net ...

[0004] Patent document CN201910888761.8, "A Method for Producing Martensitic Steel Strip by Twin-Roll Thin Strip Casting," discloses a process for producing martensitic steel strip using a thin strip casting process. Using scrap steel as the raw material, the process controls key parameters such as slag alkalinity and steel inclusion types. Strip steel with a thickness of 1.5-3.0 mm is cast via twin-roll thin strip casting. The strip is then hot-rolled in an enclosed online rolling mill and coiled to obtain a coil. The composition of the martensitic steel strip is: 0.16-0.26 wt% C, 0.1-0.5 wt% Si, 0.4-1.7 wt% Mn, P ≤ 0.02 wt%, S ≤ 0.007 wt%, 0.004-0.010 wt% N, with the remainder being Fe and unavoidable impurities. Its properties include a yield strength ≥ 900 MPa, a tensile strength ≥ 1300 MPa, and an elongation ≤ 12%.

[0005] Patent document CN201710288311.6, "A Twin-Roller Casting Process for Super Austenitic Stainless Steel Strip," discloses a process for directly producing super austenitic stainless steel strip through twin-roller casting. By simplifying the hot rolling process, this method avoids the surface cracking and delamination cracking of super austenitic stainless steel during hot rolling in existing processes, enabling the production of super austenitic stainless steel with a thickness of 1.0-3.5 mm. Furthermore, the entire production process utilizes argon or a mixture of argon and hydrogen for protection, resulting in high-purity molten steel and a thin surface oxide scale. This improves product surface quality, reduces subsequent pickling costs, and increases production efficiency.

[0006] Patent document CN201810545642.8, "Method for Producing High-Strength Low-Alloy Steel by Twin-Roll Thin Strip Casting and Aging Process," discloses a method for producing high-strength low-alloy steel by twin-roll thin strip casting and aging. Compared to existing high-strength low-alloy aged steel strip, this invention combines optimized Mo and Nb microalloying with a rapid solidification process during thin strip casting. The subsequent aging treatment further improves the overall mechanical properties of the low-alloy high-strength steel. The steel composition is: 0.01-0.05wt% C, 0.5-2wt% Mn, 0.2-0.5wt% Si, 0.2-0.6wt% Mo, 0.05-0.5wt% Cr, 0.005-0.3wt% V, 0.01-0.2wt% Nb, P ≤ 0.02wt%, S ≤ 0.02wt%, with the remainder being Fe and unavoidable impurities. Its properties are: yield strength of 800-950 MPa, tensile strength of 850-1000 MPa, and elongation after fracture of 10-20%.

[0007] Patent document CN201110220789.8 "A method for manufacturing non-oriented silicon steel sheets based on twin-roll thin strip continuous casting technology" discloses a process scheme for preparing non-oriented silicon steel using twin-roll thin strip continuous casting technology. By directly performing warm rolling (200-600°C) on the cast strip coil, the cast strip structure is refined, the plasticity, plate shape and surface quality of the cast strip are improved, and convenient conditions are provided for cold rolling; in addition, by regulating the pouring temperature of the molten steel and the speed of the crystallization roller, effective control of the cast strip microstructure is achieved, and a non-oriented silicon steel cast strip with a developed columnar crystal solidification structure and texture is obtained, thereby effectively improving the magnetic induction intensity of the non-oriented silicon steel sheet.

[0008] As can be seen from the above, there is no technical solution for cast and rolled thin strip hot-dip galvanized sheet and its production process at present. The main reason is that the existing technology still has the following defects that have not been solved.

[0009] Defect 1

[0010] Due to the inherent production technology characteristics of thin strip casting and rolling technology, during the casting process, the edges of the casting rolls are in a state of enclosing the molten steel to prevent edge leakage and edge rot and maintain casting continuity. This causes the thickness of the cast strip edge within 40mm to decrease sharply. The impact of this sudden thickness change is further amplified during the hot rolling process. Compared with the middle of the strip, the thinning area at the edge: (1) the roll coverage rate is low, the surface morphology is different, and the surface roughness of the edge is greater than that of the middle, and its morphology is rougher; (2) the hot rolling reduction is low, the recrystallization driving force is small, the proportion of recrystallized structure is low, and the microstructure is different. Specifically, the microstructure of the cast-rolled strip changes from the edges to the center as follows: The microstructure at the very edges of the cast-rolled strip is similar to that of the cast strip, with high strength and low elongation. However, as the strip approaches the center, the hot rolling reduction increases, and the grains of the strip gradually grow, transforming from a highly uneven, irregular, non-equiaxed ferrite mixed crystal structure to a fully recrystallized hot-rolled microstructure. This structural difference causes uneven grain deformation during rolling, leading to fluctuating surface morphology.

[0011] In response to the above-mentioned color difference problem on the edges of cast-rolled thin strip galvanized sheets, there is currently no patent for relevant technical solution design and innovation.

[0012] Defect 2

[0013] Due to the inherent production technology characteristics of thin strip casting and rolling technology, silicon deoxidation must be used to control the composition of the molten steel during the composition system design process. This also means that compared with the conventional cold-rolled raw materials of galvanized steel sheets for household appliances (≤0.03wt% Si), the silicon content of the cast and rolled thin strip is not less than 0.1%. During the annealing and heating process, the silicon and manganese elements in the galvanized substrate are easily oxidized to form oxides that are enriched on the surface of the strip. At the same time, they are not easily reduced during the subsequent soaking and reduction process, which ultimately affects the wettability of the strip surface. In severe cases, it can cause zinc layer leakage and affect product quality. The commonly used technical means at present is to add a pre-oxidation section to the heating section of the annealing furnace to pre-oxidize the galvanized substrate, oxidizing the silicon and manganese elements in the substrate to the inner layer of the substrate to prevent their oxides from enriching on the substrate surface.

[0014] Taking patent document CN202210991748.7 "A method for controlling surface defects of hot-dip galvanized steel strips" as an example, the patent discloses a method for controlling surface defects of hot-dip galvanized steel strips. By adding a pre-oxidation function to the heating section of the continuous hot-dip galvanizing production line, the silicon and manganese diffused from the inside to the surface of the strip undergo internal oxidation instead of surface oxidation of the strip, thereby improving the wettability of the zinc liquid, solving the surface defects of the hot-dip galvanized steel strips, and improving the surface quality of the strips.

[0015] However, current technical documents fail to consider the impact of a uniform pre-oxidation process on galvanized substrates during large-scale production operations, when the silicon content of the substrate varies significantly. Although surface iron oxides oxidized during the pre-oxidation phase are reduced in the soaking zone, the reduced iron essentially regenerates elemental iron on the strip surface, and the microstructure, properties, and lattice structure of this iron inevitably differ from those of the substrate strip. When the silicon content of the strip is relatively low, the required degree of pre-oxidation is low; however, when the silicon content is high, the required degree of pre-oxidation increases. To ensure effective pre-oxidation in mass production, the oxidation degree standard is benchmarked against the maximum silicon and manganese content in the strip composition. This means that under these production process conditions, there is a risk of over-oxidation during the pre-oxidation process when producing galvanized sheet with relatively low silicon and manganese content. The resulting changes in the strip's microstructure further impact the stamping performance and surface quality of the finished galvanized product, failing to meet the stringent product requirements of brand customers.

[0016] Defect 3

[0017] Due to the inherent production characteristics of thin strip casting and rolling technology, after long hot-dip galvanizing operations, some elements in the high-silicon steel strip gradually diffuse into the zinc pot, increasing the silicon content in the zinc pot and affecting the zinc pot's composition. When the silicon content in the zinc pot is higher than normal during the strip's entry into the zinc pot, silicon oxide particles are easily formed and accumulate around the strip, forming slag defects and affecting the surface quality of the galvanized sheet. Summary of the Invention

[0018] In view of the above defects of the prior art, the present invention provides a cast-rolled thin strip hot-dip galvanized sheet and a production method thereof.

[0019] The present invention controls the entire process from pickling, annealing, and coating to solve the problems of edge color difference, zinc layer shedding, and slag surface defects encountered when producing hot-dip galvanized sheets using short-process cast-rolled thin strips as raw material substrates. The surface quality of the finished product reaches FB grade.

[0020] The cast-rolled thin-strip galvanized steel sheet has the following steel composition: 0.01-0.03 wt% C, 0.3-0.8 wt% Si, 0.3-0.4 wt% Mn, 0-0.0025 wt% S, 0-0.020 wt% P, and 0-0.0055 wt% N. It has a yield strength of 230-270 MPa, a tensile strength of 340-380 MPa, and an elongation of 32% or greater, making it a commercial-grade hot-dip galvanized steel sheet for household appliances.

[0021] Specifically, the core technical means of the present invention are as follows:

[0022] (1) By rationally designing the roughness distribution of the textured roller surface of the fifth stand of pickling mill, the problem of the roughness difference between the edge of the raw material sheet and the middle area is improved. The roughness difference of the hard edge of the thin strip after pickling continuous rolling is reduced to 0.2μm or less, and the color difference problem of the edge of the thin strip after galvanizing is solved.

[0023] (2) During the batch production process, the silicon content of each roll of thin strip raw material is taken into consideration:

[0024] In the range of 0.3-0.8wt% Si, by formulating an annealing furnace dew point adjustment plan and implementing reasonable production scheduling, the bonding between the zinc layer and the thin strip substrate is effectively improved, and the problem of coating shedding is solved;

[0025] (3) Taking into full consideration the increase in silicon content in the plating solution caused by high-silicon thin strip substrates as the working time increases, a zinc pot composition adjustment plan is formulated to timely balance the zinc pot composition, reduce the generation and aggregation of zinc pot slag, and improve the surface slag defect problem. Ultimately, a stable production of ultra-thin strip galvanized sheet is achieved, and the surface quality requirements of household appliance galvanized sheet are met.

[0026] More specifically, the hot-dip galvanized sheet of the present invention uses twin-roll cast thin strip as raw material. By designing the surface roughness distribution of the texturing rolls in the fifth stand of the pickling mill, the surface roughness variation of the raw strip is improved, thus resolving the problem of color difference at the edges of the raw strip after galvanizing. During mass production, a dew point adjustment scheme for the annealing furnace is developed based on the raw strip composition of each roll, and production is rationally scheduled to effectively improve the bonding between the zinc layer and the thin strip substrate, thus resolving the problem of coating shedding. Furthermore, by fully considering the increase in silicon content in the plating bath caused by high-silicon thin strip substrates over time, a zinc pot composition adjustment scheme is developed to timely balance the zinc pot composition, reduce the formation and accumulation of zinc pot slag, and alleviate surface slag defects. Ultimately, this method achieves stable production of cast thin strip hot-dip galvanized sheet that meets the surface quality requirements of galvanized sheet for household appliances. The finished product achieves FB-grade surface quality, with a yield strength of 230-270 MPa, a tensile strength of 340-380 MPa, and an elongation of 32% or greater, making it a commercial-grade hot-dip galvanized steel sheet for household appliances.

[0027] The invention adopts twin-roll cast thin strip with a thickness of 1.4-1.7 mm and a width of 1220 mm as raw material, and carries out acid continuous rolling, continuous annealing, hot-dip galvanizing, leveling, post-treatment passivation and coiling to obtain cast thin strip hot-dip galvanized sheet.

[0028] First, the cast and rolled thin strip raw material of the present invention meets the following characteristics:

[0029] (1) The raw material composition system of the cast thin strip is: 0.01-0.03 wt% C, 0.3-0.8 wt% Si, 0.3-0.4 wt% Mn, 0-0.0025 wt% S, 0-0.020 wt% P, 0-0.0055 wt% N, and the rest is Fe and unavoidable inclusions.

[0030] The chemical composition is calculated by mass percentage: Si: 0.3-0.6%.

[0031] Among them, the chemical composition is calculated by mass percentage: Si: 0.6-0.8%.

[0032] (2) The surface roughness range of the middle area of the cast and rolled thin strip raw material (610 mm from the edge) meets the requirements of 2.0 to 3.2 μm, and the surface roughness range of the area 10 mm from the edge meets the requirements of 4.6 to 6.0 μm.

[0033] Among them, the roughness range of the middle area (610 mm from the edge) meets 2.0 to 2.9 μm.

[0034] Among them, the roughness range of the middle area (610mm from the edge) meets 2.9~3.2μm.

[0035] Among them, the roughness range of the area 10mm away from the edge meets 4.6~4.8μm.

[0036] Among them, the roughness range of the area 10mm away from the edge meets 4.8~6.0μm.

[0037] (3) The mechanical properties of the cast and rolled thin strip raw materials meet the yield strength range of 210 to 250 MPa, the tensile strength range of 320 to 360 MPa, and the elongation ≥ 28%.

[0038] Second, the cast thin strip rolled hard plate meets the following characteristics: the surface roughness difference in the middle area of the rolled hard plate edge is reduced to 0.2μm and below.

[0039] Third, the cast-rolled thin strip galvanized steel sheet meets the following characteristics: the surface quality reaches the FB level of the finished product, the yield strength is 230-270MPa, the tensile strength is 340-380MPa, and the elongation is ≥32%. It is a commercial-grade hot-dip galvanized steel sheet for home appliances.

[0040] The key process steps of acid continuous rolling, continuous annealing and coating of the present invention are described as follows.

[0041] 1. Acid continuous rolling

[0042] The raw material of the acid continuous rolling of the present invention is a twin-roll cast thin strip, the width of the same batch of cast thin strip raw materials is 1220mm, the thickness is 1.4-1.7mm, the roughness value of the middle part of the thin strip raw material (610mm away from the edge of the strip) is r 中 (2.0μm≤r 中 ≤3.2μm), the surface roughness of the strip at 10mm away from the edge of the strip is r 边10 (4.6μm≤r 边10 ≤6.0μm).

[0043] The acid continuous rolling adopts five-stand continuous rolling. The length of the texture roller of the 5# stand is 1420mm. The edge of the strip is 100mm away from the edge of the texture roller. The roughness of the middle area of the texture roller of the 5# stand (710mm away from the edge of the roller) is R 中 , R 中 The roller surface roughness from the edge to the area 110mm away from the roller edge is R 边10 The roller surface roughness in the area from 110mm to 125mm from the roller edge is R 边25 The roller surface roughness in the area from 125mm to 140mm from the roller edge is R 边40 The roller surface roughness in the area from 140mm to 160mm from the roller edge is R 边60 .

[0044] The cast and rolled thin strip is rolled through five stands to obtain the target thickness range of 0.5 to 0.8 mm galvanized raw material hardened strip steel. The surface morphology of the cast and rolled thin strip changes after acid rolling. After acid rolling, the roughness of the middle part of the thin strip hardened coil is Г 中 , the surface roughness of the strip steel at 10mm away from the edge of the strip steel is Г 边10 .

[0045] The surface roughness Γ of the hardened steel sheet after pickling is approximately the product of the surface roughness of the 5# texture roller during pickling continuous rolling and the overprint coefficient K, i.e., Γ≈R×K. The overprint coefficient K reflects the degree of overprinting of the 5# texture roller's roughness onto the strip surface topography during pickling continuous rolling. Its value is related to the steel grade, rolling force, and overprinting method. The smaller |K-1|, the greater the degree of overprinting of the texture roller's surface roughness. In the mass production of cast-rolled thin hot-dip galvanized steel, the influence of steel grade and rolling force on the degree of overprinting of the strip surface topography is negligible.

[0046] Among them, the degree of roller surface roughness overprinting varies with the overprinting method. That is, when the surface roughness R of the 5# texturing roller is less than the surface roughness r of the raw material plate of the cast and rolled thin strip, that is, when the small roughness overprints the large roughness, the |K-1| value is large, and the greater the difference between the two, the larger the |K-1| value, K≥1; when the surface roughness R of the 5# texturing roller is greater than the surface roughness r of the raw material plate of the cast and rolled thin strip, that is, when the large roughness overprints the small roughness, the |K-1| value is small, and the greater the difference between the two, the larger the |K-1| value, 0<K≤1. The specific scheme is as follows (the roughness of the middle area of the 5# frame texturing roller is R 中 3.6μm):

[0047] The roughness of the middle area (610mm from the edge) meets 2.0μm≤r 中 <2.9μm and the area 10mm away from the edge meets 4.6μm≤r 边10 When ≤4.8μm, the middle area has large roughness covering small roughness, 50%<r 中 / R 中 <80%, K 中 The value is 0.95; the edge area has a small roughness overlaying a large roughness, 75% ≤ R 中 / r 边10 ≤80%, K 边10 The value is 1.05.

[0048] The roughness of the middle area (610mm from the edge) meets 2.0μm≤r 中 <2.9μm and the area 10mm away from the edge meets 4.8μm<r 边10 When ≤6.0μm, the middle area has large roughness overlaying small roughness, 50%<r 中 / R 中 <80%, K 中 The value is 0.95; the edge area has a small roughness overlaying a large roughness, 50% < R 中 / r 边10 <75%, K 边10 The value is 1.2.

[0049] The roughness of the middle area (610 mm from the edge) meets 2.9 μm ≤ r 中 ≤3.2μm and the area 10mm away from the edge meets 4.6μm≤r 边10 When ≤4.8μm, the middle area has large roughness covering small roughness, r 中 / R 中 ≥80%, K 中 The value is 1; the edge area has a small roughness overlaying a large roughness, 75% ≤ R 中 / r 边10 ≤80%, K 边10 The value is 1.05.

[0050] The roughness of the middle area (610 mm from the edge) meets 2.9 μm ≤ r 中 ≤3.2μm and the area 10mm away from the edge meets 4.8μm<r 边10 When ≤6.0μm, the middle area has large roughness covering small roughness, r 中 / R 中 ≥80%, K 中 The value is 1; the edge area has a small roughness overlaying a large roughness, 50% < R 中 / r 边10 <75%, K 边10 The value is 1.2.

[0051] According to the formula Г≈R×K, when the surface roughness of the cast-rolled thin strip raw material in the area 10mm away from the edge is greater than that in the middle area (610mm away from the edge of the strip), in order to ensure the surface quality of the galvanized finished product, it is necessary to ensure that the surface roughness of the galvanized raw material rolled hard plate after pickling is consistent. Therefore, it is necessary to adjust the surface roughness distribution of the pickling 5# texturing roller, that is, reduce the roller surface roughness in the edge area of the texturing roller, and gradually increase it as it approaches the middle of the strip, and finally keep it consistent with the roller surface roughness in the middle area (710mm away from the edge of the texturing roller). The specific solution is:

[0052] R 边10 ≈(R 中 ×K 中 ) / K 边10 ≈(K 中 / K 边10 )×3.6;

[0053] R 边25 =R 边10 +(R 中 -R 边10 )×70%≈(0.7+0.3K 中 / K 边10 )×3.6;

[0054] R 边40 =R 边10 +(R 中 -R 边10 )×90%≈(0.9+0.1K 中 / K 边10 )×3.6;

[0055] R 边60 ≈3.6.

[0056] 2. Continuous annealing

[0057] The annealing raw material of the present invention is the cast-rolled thin strip hard coil produced after the aforementioned acid continuous rolling process. Annealing is performed in a horizontal continuous annealing furnace, and the annealing process is divided into preheating, heating, soaking, and cooling sections. At the end of the heating section, the furnace atmosphere is adjusted to increase the dew point and oxygen content to achieve pre-oxidation of the strip. This section is called the pre-oxidation section. After pre-oxidation, the strip enters the soaking reduction section, where the dew point of the atmosphere is maintained at -50°C to ensure sufficient reduction of iron oxide on the strip surface.

[0058] The soaking reduction stage is heated to 800°C to 850°C. At this temperature, the strip's crystal structure transforms from a deformed, irregular structure to an equiaxed, fully recrystallized structure. Simultaneously, precipitates such as AlN further precipitate within the strip, achieving precipitation strengthening and slightly improving the strip's mechanical properties.

[0059] Among them, the same batch of cast and rolled thin strip raw materials are arranged into hard coils with silicon content m from low to high, and the silicon content range satisfies 0.3wt%≤m≤0.8wt%. As the production process progresses, the silicon content of the strip increases, the required pre-oxidation degree increases, and the required dew point temperature also gradually increases. The dew point of the pre-oxidation stage at this stage is recorded as T dm The specific plan is:

[0060] When the silicon content gradually increases in the range of 0.3wt%≤m<0.6wt%, the dew point of the pre-oxidation section is -22℃≤T dm Increase slowly within -16℃, where the dew point temperature T increases with every 0.1wt% increase in silicon content m. dm Increase by 2℃;

[0061] When the silicon content gradually increases in the range of 0.6wt%≤m≤0.8wt%, the dew point of the pre-oxidation section is -16℃≤T dm Increase slowly within ≤-11℃, where the dew point temperature T increases with every 0.1wt% increase in silicon content m. dm Increase by 2.5℃.

[0062] 3. Coating process

[0063] The strip steel of the present invention is continuously annealed, cooled to a zinc pot entry temperature, and then enters the zinc pot through a furnace nose to complete hot-dip galvanizing.

[0064] The temperature of the strip entering the zinc pot is 480±5°C, and the zinc pot temperature is 460±5°C. At this temperature, the induction heater of the zinc pot is kept at a high power state, thereby promoting the flow of the melt in the zinc pot and preventing slag from depositing at the bottom.

[0065] Among them, as the production of cast and rolled thin strip hot-dip galvanizing continues, the zinc pot composition needs to be sampled and measured every 4 hours of production to facilitate the adjustment of the plating solution composition. The specific plan is:

[0066] When the silicon content of the plating solution exceeds 0.01wt%, pure zinc ingot (Zn99.99) is added to adjust the composition;

[0067] When the silicon content of the plating solution exceeds 0.03wt%, it is necessary to switch to transition coil or other galvanized varieties;

[0068] When the silicon content drops below 0.01wt%, the cast and rolled thin strip galvanized sheet is cut back for production.

[0069] The resulting cast-rolled thin-strip hot-dip galvanized steel sheet meets the surface quality requirements of galvanized steel sheets for home appliances. The finished product achieves FB-grade surface quality, with a yield strength of 230-270 MPa, a tensile strength of 340-380 MPa, and an elongation of 32% or greater, making it a commercial-grade hot-dip galvanized steel sheet for home appliances.

[0070] Beneficial technical effects

[0071] Compared with the prior art, the technical concept and corresponding technical solutions of the present invention can at least achieve the following beneficial technical effects:

[0072] (1) The present invention takes into account the production technology characteristics of thin strip casting and rolling technology, that is, the large differences in organizational structure, strength performance and surface quality in the edge and middle areas of the cast and rolled thin strip raw materials. By designing the roughness distribution of the textured roller surface of the fifth stand of the acid rolling, the problem of the roughness of the edge of the thin strip raw material being higher than that in the middle area is improved, and a hard-rolled plate surface with a uniform morphology is obtained. After acid continuous rolling, the roughness difference in the hard-rolled edge of the thin strip is reduced to 0.2 μm or less, avoiding the color difference problem caused by the surface morphology difference of the finished product edge after subsequent hot-dip galvanizing, thereby improving the surface quality of the ultra-thin galvanized sheet;

[0073] (2) The present invention takes into account the production technology characteristics of the thin strip casting technology, that is, the silicon content of the cast and rolled thin strip caused by silicon deoxidation is not less than 0.1%. Compared with the conventional method of adding a pre-oxidation section in the annealing furnace to oxidize the silicon and manganese elements to the inner layer of the substrate, the present invention further combines the silicon content of each roll of thin strip raw material, formulates the annealing furnace dew point adjustment plan within the range of 0.3~0.8wt% Si, and performs reasonable production scheduling to avoid the risk of excessive oxidation when the silicon content of the raw materials varies greatly in large-scale production, and at the same time effectively improves the bonding between the zinc layer and the thin strip substrate, solving the problem of coating shedding.

[0074] (3) The present invention takes into account the production technology characteristics of thin strip casting and rolling technology, that is, the strip steel in a high silicon composition system undergoes a long-term hot dip plating operation, which causes the composition of the plating solution to change. By formulating a zinc pot composition adjustment plan, the zinc pot composition is balanced in time, the generation and aggregation of zinc pot slag is reduced, and the surface slag defect problem is improved. DETAILED DESCRIPTION

[0075] To make the purpose, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions of the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the described embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0076] Unless otherwise defined, technical or scientific terms used in the present invention shall have the same meaning as commonly understood by one of ordinary skill in the art to which the present invention belongs.

[0077] The following are embodiments of the present invention. The described embodiments are only part of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments that can be obtained by ordinary technicians in this field without making any creative work are within the scope of protection of the present invention.

[0078] As described in the invention content, the present invention controls the entire process from pickling, annealing, and coating key processes to solve the problems of edge color difference, zinc layer shedding, and slag surface defects encountered when producing hot-dip galvanized sheets using short-process cast-rolled thin strips as raw material substrates, and the surface quality of the finished product reaches FB level.

[0079] The cast-rolled thin-strip galvanized steel sheet has the following steel composition: 0.01-0.03 wt% C, 0.3-0.8 wt% Si, 0.3-0.4 wt% Mn, 0-0.0025 wt% S, 0-0.020 wt% P, and 0-0.0055 wt% N. It has a yield strength of 230-270 MPa, a tensile strength of 340-380 MPa, and an elongation of 32% or greater, making it a commercial-grade hot-dip galvanized steel sheet for household appliances.

[0080] Specifically, the core technical means of the present invention are as follows:

[0081] (1) By rationally designing the roughness distribution of the textured roller surface of the fifth stand of pickling mill, the problem of the roughness difference between the edge of the raw material sheet and the middle area is improved. The roughness difference of the hard edge of the thin strip after pickling continuous rolling is reduced to 0.2μm or less, and the color difference problem of the edge of the thin strip after galvanizing is solved.

[0082] (2) During the batch production process, the silicon content of each roll of thin strip raw material is taken into consideration:

[0083] In the range of 0.3-0.8wt% Si, by formulating an annealing furnace dew point adjustment plan and implementing reasonable production scheduling, the bonding between the zinc layer and the thin strip substrate is effectively improved, and the problem of coating shedding is solved;

[0084] (3) Taking into full consideration the increase in silicon content in the plating solution caused by high-silicon thin strip substrates as the working time increases, a zinc pot composition adjustment plan is formulated to timely balance the zinc pot composition, reduce the generation and aggregation of zinc pot slag, and improve the surface slag defect problem. Ultimately, a stable production of ultra-thin strip galvanized sheet is achieved, and the surface quality requirements of household appliance galvanized sheet are met.

[0085] More specifically, the hot-dip galvanized sheet of the present invention uses twin-roll cast thin strip as raw material. By designing the surface roughness distribution of the texturing rolls in the fifth stand of the pickling mill, the surface roughness variation of the raw strip is improved, thus resolving the problem of color difference at the edges of the raw strip after galvanizing. During mass production, a dew point adjustment scheme for the annealing furnace is developed based on the raw strip composition of each roll, and production is rationally scheduled to effectively improve the bonding between the zinc layer and the thin strip substrate, thus resolving the problem of coating shedding. Furthermore, by fully considering the increase in silicon content in the plating bath caused by high-silicon thin strip substrates over time, a zinc pot composition adjustment scheme is developed to timely balance the zinc pot composition, reduce the formation and accumulation of zinc pot slag, and alleviate surface slag defects. Ultimately, this method achieves stable production of cast thin strip hot-dip galvanized sheet that meets the surface quality requirements of galvanized sheet for household appliances. The finished product achieves FB-grade surface quality, with a yield strength of 230-270 MPa, a tensile strength of 340-380 MPa, and an elongation of 32% or greater, making it a commercial-grade hot-dip galvanized steel sheet for household appliances.

[0086] The invention adopts twin-roll cast thin strip with a thickness of 1.4-1.7 mm and a width of 1220 mm as raw material, and carries out acid continuous rolling, continuous annealing, hot-dip galvanizing, leveling, post-treatment passivation and coiling to obtain cast thin strip hot-dip galvanized sheet.

[0087] First, the cast and rolled thin strip raw material of the present invention meets the following characteristics:

[0088] (1) The raw material composition system of the cast thin strip is: 0.01-0.03 wt% C, 0.3-0.8 wt% Si, 0.3-0.4 wt% Mn, 0-0.0025 wt% S, 0-0.020 wt% P, 0-0.0055 wt% N, and the rest is Fe and unavoidable inclusions.

[0089] The chemical composition is calculated by mass percentage: Si: 0.3-0.6%.

[0090] Among them, the chemical composition is calculated by mass percentage: Si: 0.6-0.8%.

[0091] (2) The surface roughness range of the middle area of the cast and rolled thin strip raw material (610 mm from the edge) meets the requirements of 2.0 to 3.2 μm, and the surface roughness range of the area 10 mm from the edge meets the requirements of 4.6 to 6.0 μm.

[0092] Among them, the roughness range of the middle area (610 mm from the edge) meets 2.0 to 2.9 μm.

[0093] Among them, the roughness range of the middle area (610mm from the edge) meets 2.9~3.2μm.

[0094] Among them, the roughness range of the area 10mm away from the edge meets 4.6~4.8μm.

[0095] Among them, the roughness range of the area 10mm away from the edge meets 4.8~6.0μm.

[0096] (3) The mechanical properties of the cast and rolled thin strip raw materials meet the yield strength range of 210 to 250 MPa, the tensile strength range of 320 to 360 MPa, and the elongation ≥ 28%.

[0097] Second, the cast thin strip rolled hard plate meets the following characteristics: the surface roughness difference in the middle area of the rolled hard plate edge is reduced to 0.2μm and below.

[0098] Third, the cast-rolled thin strip galvanized steel sheet meets the following characteristics: the surface quality reaches the FB level of the finished product, the yield strength is 230-270MPa, the tensile strength is 340-380MPa, and the elongation is ≥32%. It is a commercial-grade hot-dip galvanized steel sheet for home appliances.

[0099] The key process steps of acid continuous rolling, continuous annealing and coating of the present invention are described as follows.

[0100] 1. Acid continuous rolling

[0101] The raw material of the acid continuous rolling of the present invention is a twin-roll cast thin strip, the width of the same batch of cast thin strip raw materials is 1220mm, the thickness is 1.4-1.7mm, the roughness value of the middle part of the thin strip raw material (610mm away from the edge of the strip) is r 中 (2.0μm≤r 中 ≤3.2μm), the surface roughness of the strip at 10mm away from the edge of the strip is r 边10 (4.6μm≤r 边10 ≤6.0μm).

[0102] The acid continuous rolling adopts five-stand continuous rolling. The length of the texture roller of the 5# stand is 1420mm. The edge of the strip is 100mm away from the edge of the texture roller. The roughness of the middle area of the texture roller of the 5# stand (710mm away from the edge of the roller) is R 中 , R 中 The roller surface roughness from the edge to the area 110mm away from the roller edge is R 边10 The roller surface roughness in the area from 110mm to 125mm from the roller edge is R 边25The roller surface roughness in the area from 125mm to 140mm from the roller edge is R 边40 The roller surface roughness in the area from 140mm to 160mm from the roller edge is R 边60 .

[0103] The cast and rolled thin strip is rolled through five stands to obtain the target thickness range of 0.5 to 0.8 mm galvanized raw material hardened strip steel. The surface morphology of the cast and rolled thin strip changes after acid rolling. After acid rolling, the roughness of the middle part of the thin strip hardened coil is Г 中 , the surface roughness of the strip steel at 10mm away from the edge of the strip steel is Г 边10 .

[0104] The surface roughness Γ of the hardened steel sheet after pickling is approximately the product of the surface roughness of the 5# texture roller during pickling continuous rolling and the overprint coefficient K, i.e., Γ≈R×K. The overprint coefficient K reflects the degree of overprinting of the 5# texture roller's roughness onto the strip surface topography during pickling continuous rolling. Its value is related to the steel grade, rolling force, and overprinting method. The smaller |K-1|, the greater the degree of overprinting of the texture roller's surface roughness. In the mass production of cast-rolled thin hot-dip galvanized steel, the influence of steel grade and rolling force on the degree of overprinting of the strip surface topography is negligible.

[0105] Among them, the degree of roller surface roughness overprinting varies with the overprinting method. That is, when the surface roughness R of the 5# texturing roller is less than the surface roughness r of the raw material plate of the cast and rolled thin strip, that is, when the small roughness overprints the large roughness, the |K-1| value is large, and the greater the difference between the two, the larger the |K-1| value, K≥1; when the surface roughness R of the 5# texturing roller is greater than the surface roughness r of the raw material plate of the cast and rolled thin strip, that is, when the large roughness overprints the small roughness, the |K-1| value is small, and the greater the difference between the two, the larger the |K-1| value, 0<K≤1. The specific scheme is as follows (the roughness of the middle area of the 5# frame texturing roller is R 中 3.6μm):

[0106] The roughness of the middle area (610mm from the edge) meets 2.0μm≤r 中 <2.9μm and the area 10mm away from the edge meets 4.6μm≤r 边10 When ≤4.8μm, the middle area has large roughness covering small roughness, 50%<r 中 / R 中 <80%, K 中 The value is 0.95; the edge area has a small roughness overlaying a large roughness, 75% ≤ R 中 / r 边10 ≤80%, K 边10 The value is 1.05.

[0107] The roughness of the middle area (610mm from the edge) meets 2.0μm≤r 中<2.9μm and the area 10mm away from the edge meets 4.8μm<r 边10 When ≤6.0μm, the middle area has large roughness overlaying small roughness, 50%<r 中 / R 中 <80%, K 中 The value is 0.95; the edge area has a small roughness overlaying a large roughness, 50% < R 中 / r 边10 <75%, K 边10 The value is 1.2.

[0108] The roughness of the middle area (610 mm from the edge) meets 2.9 μm ≤ r 中 ≤3.2μm and the area 10mm away from the edge meets 4.6μm≤r 边10 When ≤4.8μm, the middle area has large roughness covering small roughness, r 中 / R 中 ≥80%, K 中 The value is 1; the edge area has a small roughness overlaying a large roughness, 75% ≤ R 中 / r 边10 ≤80%, K 边10 The value is 1.05.

[0109] The roughness of the middle area (610 mm from the edge) meets 2.9 μm ≤ r 中 ≤3.2μm and the area 10mm away from the edge meets 4.8μm<r 边10 When ≤6.0μm, the middle area has large roughness covering small roughness, r 中 / R 中 ≥80%, K 中 The value is 1; the edge area has a small roughness overlaying a large roughness, 50% < R 中 / r 边10 <75%, K 边10 The value is 1.2.

[0110] According to the formula Г≈R×K, when the surface roughness of the cast-rolled thin strip raw material in the area 10mm away from the edge is greater than that in the middle area (610mm away from the edge of the strip), in order to ensure the surface quality of the galvanized finished product, it is necessary to ensure that the surface roughness of the galvanized raw material rolled hard plate after pickling is consistent. Therefore, it is necessary to adjust the surface roughness distribution of the pickling 5# texturing roller, that is, reduce the roller surface roughness in the edge area of the texturing roller, and gradually increase it as it approaches the middle of the strip, and finally keep it consistent with the roller surface roughness in the middle area (710mm away from the edge of the texturing roller). The specific solution is:

[0111] R 边10 ≈(R 中 ×K 中 ) / K 边10≈(K 中 / K 边10 )×3.6;

[0112] R 边25 =R 边10 +(R 中 -R 边10 )×70%≈(0.7+0.3K 中 / K 边10 )×3.6;

[0113] R 边40 =R 边10 +(R 中 -R 边10 )×90%≈(0.9+0.1K 中 / K 边10 )×3.6;

[0114] R 边60 ≈3.6.

[0115] 2. Continuous annealing

[0116] The annealing raw material of the present invention is the cast-rolled thin strip hard coil produced after the aforementioned acid continuous rolling process. Annealing is performed in a horizontal continuous annealing furnace, and the annealing process is divided into preheating, heating, soaking, and cooling sections. At the end of the heating section, the furnace atmosphere is adjusted to increase the dew point and oxygen content to achieve pre-oxidation of the strip. This section is called the pre-oxidation section. After pre-oxidation, the strip enters the soaking reduction section, where the dew point of the atmosphere is maintained at -50°C to ensure sufficient reduction of iron oxide on the strip surface.

[0117] The soaking reduction stage is heated to 800°C to 850°C. At this temperature, the strip's crystal structure transforms from a deformed, irregular structure to an equiaxed, fully recrystallized structure. Simultaneously, precipitates such as AlN further precipitate within the strip, achieving precipitation strengthening and slightly improving the strip's mechanical properties.

[0118] Among them, the same batch of cast and rolled thin strip raw materials are arranged into hard coils with silicon content m from low to high, and the silicon content range satisfies 0.3wt%≤m≤0.8wt%. As the production process progresses, the silicon content of the strip increases, the required pre-oxidation degree increases, and the required dew point temperature also gradually increases. The dew point of the pre-oxidation stage at this stage is recorded as T dm The specific plan is:

[0119] When the silicon content gradually increases in the range of 0.3wt%≤m<0.6wt%, the dew point of the pre-oxidation section is -22℃≤T dm Increase slowly within -16℃, where the dew point temperature T increases with every 0.1wt% increase in silicon content m. dm Increase by 2℃;

[0120] When the silicon content gradually increases in the range of 0.6wt%≤m≤0.8wt%, the dew point of the pre-oxidation section is -16℃≤T dm Increase slowly within ≤-11℃, where the dew point temperature T increases with every 0.1wt% increase in silicon content m. dm Increase by 2.5℃.

[0121] 3. Coating process

[0122] The strip steel of the present invention is continuously annealed, cooled to a zinc pot entry temperature, and then enters the zinc pot through a furnace nose to complete hot-dip galvanizing.

[0123] The temperature of the strip entering the zinc pot is 480±5°C, and the zinc pot temperature is 460±5°C. At this temperature, the induction heater of the zinc pot is kept at a high power state, thereby promoting the flow of the melt in the zinc pot and preventing slag from depositing at the bottom.

[0124] Among them, as the production of cast and rolled thin strip hot-dip galvanizing continues, the zinc pot composition needs to be sampled and measured every 4 hours of production to facilitate the adjustment of the plating solution composition. The specific plan is:

[0125] When the silicon content of the plating solution exceeds 0.01wt%, pure zinc ingot (Zn99.99) is added to adjust the composition;

[0126] When the silicon content of the plating solution exceeds 0.03wt%, it is necessary to switch to transition coil or other galvanized varieties;

[0127] When the silicon content drops below 0.01wt%, the cast and rolled thin strip galvanized sheet is cut back for production.

[0128] The resulting cast-rolled thin-strip hot-dip galvanized steel sheet meets the surface quality requirements of galvanized steel sheets for home appliances. The finished product achieves FB-grade surface quality, with a yield strength of 230-270 MPa, a tensile strength of 340-380 MPa, and an elongation of 32% or greater, making it a commercial-grade hot-dip galvanized steel sheet for home appliances.

[0129] The specific embodiments are described as follows (the roughness units in the table below are all μm):

[0130] Table 1 Surface roughness control of cast thin strip and hard coil

[0131]

[0132] Table 2 Dew point control in pre-oxidation section of cast and rolled thin strip galvanized sheet

[0133] sample Silicon content wt% <![CDATA[Dew point T in the pre-oxidation section d > Whether there is leakage plating Sample 1 0.3 -22 no Sample 2 0.4 -20 no Sample 3 0.6 -16 no Sample 4 0.7 -13 no Sample 5 0.8 -11 no Sample 6 0.5 -30 yes

[0134] In summary, the present invention proposes a hot-dip galvanized sheet and its production method. Using twin-roll cast thin strip as raw material, the roughness distribution of the textured rollers in the fifth pickling mill stand is designed to improve roughness variations along the strip's edges, addressing color variations along the strip's edges after galvanizing. During mass production, a dew point adjustment scheme for the annealing furnace, tailored to the composition of each roll of thin strip, is developed and rationally scheduled to effectively improve the bonding between the zinc layer and the strip substrate, addressing coating shedding. Taking into account the increase in silicon content in the plating solution caused by high-silicon thin strip substrates over extended operating time, a zinc pot composition adjustment scheme is developed to timely balance the zinc pot's composition, reduce the formation and accumulation of zinc pot slag, and address surface slag defects. Ultimately, stable production of cast thin strip hot-dip galvanized sheet is achieved, meeting the surface quality requirements for galvanized sheet for household appliances. The finished product achieves FB-grade surface quality, with a yield strength of 230-270 MPa, a tensile strength of 340-380 MPa, and an elongation of 32% or greater, making it a commercial-grade hot-dip galvanized steel sheet for household appliances.

[0135] The invention adopts twin-roll cast thin strip with a thickness of 1.4-1.7 mm and a width of 1220 mm as raw material, and carries out acid continuous rolling, continuous annealing, hot-dip galvanizing, leveling, post-treatment passivation and coiling to obtain cast thin strip hot-dip galvanized sheet.

[0136] First, the cast and rolled thin strip raw material of the present invention meets the following characteristics:

[0137] (1) The raw material composition system of the cast thin strip is: 0.01-0.03 wt% C, 0.3-0.8 wt% Si, 0.3-0.4 wt% Mn, 0-0.0025 wt% S, 0-0.020 wt% P, 0-0.0055 wt% N, and the rest is Fe and unavoidable inclusions.

[0138] The chemical composition is calculated by mass percentage: Si: 0.3-0.6%.

[0139] Among them, the chemical composition is calculated by mass percentage: Si: 0.6-0.8%.

[0140] (2) The surface roughness range of the middle area of the cast and rolled thin strip raw material (610 mm from the edge) meets the requirements of 2.0 to 3.2 μm, and the surface roughness range of the area 10 mm from the edge meets the requirements of 4.6 to 6.0 μm.

[0141] Among them, the roughness range of the middle area (610mm from the edge) meets 2.0~2.9μm.

[0142] Among them, the roughness range of the middle area (610mm from the edge) meets 2.9~3.2μm.

[0143] Among them, the roughness range of the area 10mm away from the edge meets 4.6~4.8μm.

[0144] Among them, the roughness range of the area 10mm away from the edge meets 4.8~6.0μm.

[0145] (3) The mechanical properties of the cast and rolled thin strip raw materials meet the yield strength range of 210 to 250 MPa, the tensile strength range of 320 to 360 MPa, and the elongation ≥ 28%.

[0146] Second, the cast thin strip rolled hard plate meets the following characteristics: the surface roughness difference in the middle area of the rolled hard plate edge is reduced to 0.2μm and below.

[0147] Third, the cast-rolled thin strip galvanized steel sheet meets the following characteristics: the surface quality reaches the FB level of the finished product, the yield strength is 230-270MPa, the tensile strength is 340-380MPa, and the elongation is ≥32%. It is a commercial-grade hot-dip galvanized steel sheet for home appliances.

[0148] The key process steps of acid continuous rolling, continuous annealing and coating of the present invention are described as follows.

[0149] 1. Acid continuous rolling

[0150] The raw material of the acid continuous rolling of the present invention is a twin-roll cast thin strip, the width of the same batch of cast thin strip raw materials is 1220mm, the thickness is 1.4-1.7mm, the roughness value of the middle part of the thin strip raw material (610mm away from the edge of the strip) is r 中 (2.0μm≤r 中 ≤3.2μm), the surface roughness of the strip at 10mm away from the edge of the strip is r 边10 (4.6μm≤r 边10 ≤6.0μm).

[0151] The acid continuous rolling adopts five-stand continuous rolling. The length of the texture roller of the 5# stand is 1420mm. The edge of the strip is 100mm away from the edge of the texture roller. The roughness of the middle area of the texture roller of the 5# stand (710mm away from the edge of the roller) is R 中 , R 中 The roller surface roughness from the edge to the area 110mm away from the roller edge is R 边10 The roller surface roughness in the area from 110mm to 125mm from the roller edge is R 边25 The roller surface roughness in the area from 125mm to 140mm from the roller edge is R 边40 The roller surface roughness in the area from 140mm to 160mm from the roller edge is R 边60 .

[0152] The cast and rolled thin strip is rolled through five stands to obtain the target thickness range of 0.5 to 0.8 mm galvanized raw material hardened strip steel. The surface morphology of the cast and rolled thin strip changes after acid rolling. After acid rolling, the roughness of the middle part of the thin strip hardened coil is Г 中 , the surface roughness of the strip steel at 10mm away from the edge of the strip steel is Г 边10 .

[0153] The surface roughness Γ of the hardened steel sheet after pickling is approximately the product of the surface roughness of the 5# texture roller during pickling continuous rolling and the overprint coefficient K, i.e., Γ≈R×K. The overprint coefficient K reflects the degree of overprinting of the 5# texture roller's roughness onto the strip surface topography during pickling continuous rolling. Its value is related to the steel grade, rolling force, and overprinting method. The smaller |K-1|, the greater the degree of overprinting of the texture roller's surface roughness. In the mass production of cast-rolled thin hot-dip galvanized steel, the influence of steel grade and rolling force on the degree of overprinting of the strip surface topography is negligible.

[0154] Among them, the degree of roller surface roughness overprinting varies with the overprinting method. That is, when the surface roughness R of the 5# texturing roller is less than the surface roughness r of the raw material plate of the cast and rolled thin strip, that is, when the small roughness overprints the large roughness, the |K-1| value is large, and the greater the difference between the two, the larger the |K-1| value, K≥1; when the surface roughness R of the 5# texturing roller is greater than the surface roughness r of the raw material plate of the cast and rolled thin strip, that is, when the large roughness overprints the small roughness, the |K-1| value is small, and the greater the difference between the two, the larger the |K-1| value, 0<K≤1. The specific scheme is as follows (the roughness of the middle area of the 5# frame texturing roller is R 中 3.6μm):

[0155] The roughness of the middle area (610mm from the edge) meets 2.0μm≤r 中 <2.9μm and the area 10mm away from the edge meets 4.6μm≤r 边10 When ≤4.8μm, the middle area has large roughness covering small roughness, 50%<r 中 / R 中 <80%, K 中 The value is 0.95; the edge area has a small roughness overlaying a large roughness, 75% ≤ R 中 / r 边10 ≤80%, K 边10 The value is 1.05.

[0156] The roughness of the middle area (610mm from the edge) meets 2.0μm≤r 中 <2.9μm and the area 10mm away from the edge meets 4.8μm<r 边10 When ≤6.0μm, the middle area has large roughness overlaying small roughness, 50%<r 中 / R 中 <80%, K 中The value is 0.95; the edge area has a small roughness overlaying a large roughness, 50% < R 中 / r 边10 <75%, K 边10 The value is 1.2.

[0157] The roughness of the middle area (610 mm from the edge) meets 2.9 μm ≤ r 中 ≤3.2μm and the area 10mm away from the edge meets 4.6μm≤r 边10 When ≤4.8μm, the middle area has large roughness covering small roughness, r 中 / R 中 ≥80%, K 中 The value is 1; the edge area has a small roughness overlaying a large roughness, 75% ≤ R 中 / r 边10 ≤80%, K 边10 The value is 1.05.

[0158] The roughness of the middle area (610 mm from the edge) meets 2.9 μm ≤ r 中 ≤3.2μm and the area 10mm away from the edge meets 4.8μm<r 边10 When ≤6.0μm, the middle area has large roughness covering small roughness, r 中 / R 中 ≥80%, K 中 The value is 1; the edge area has a small roughness overlaying a large roughness, 50% < R 中 / r 边10 <75%, K 边10 The value is 1.2.

[0159] According to the formula Г≈R×K, when the surface roughness of the cast-rolled thin strip raw material in the area 10mm away from the edge is greater than that in the middle area (610mm away from the edge of the strip), in order to ensure the surface quality of the galvanized finished product, it is necessary to ensure that the surface roughness of the galvanized raw material rolled hard plate after pickling is consistent. Therefore, it is necessary to adjust the surface roughness distribution of the pickling 5# texturing roller, that is, reduce the roller surface roughness in the edge area of the texturing roller, and gradually increase it as it approaches the middle of the strip, and finally keep it consistent with the roller surface roughness in the middle area (710mm away from the edge of the texturing roller). The specific solution is:

[0160] R 边10 ≈(R 中 ×K 中 ) / K 边10 ≈(K 中 / K 边10 )×3.6;

[0161] R 边25 =R 边10 +(R 中 -R边10 )×70%≈(0.7+0.3K 中 / K 边10 )×3.6;

[0162] R 边40 =R 边10 +(R 中 -R 边10 )×90%≈(0.9+0.1K 中 / K 边10 )×3.6;

[0163] R 边60 ≈3.6.

[0164] 2. Continuous annealing

[0165] The annealing raw material of the present invention is the cast-rolled thin strip hard coil produced after the aforementioned acid continuous rolling process. Annealing is performed in a horizontal continuous annealing furnace, and the annealing process is divided into preheating, heating, soaking, and cooling sections. At the end of the heating section, the furnace atmosphere is adjusted to increase the dew point and oxygen content to achieve pre-oxidation of the strip. This section is called the pre-oxidation section. After pre-oxidation, the strip enters the soaking reduction section, where the dew point of the atmosphere is maintained at -50°C to ensure sufficient reduction of iron oxide on the strip surface.

[0166] The soaking reduction stage is heated to 800°C to 850°C. At this temperature, the strip's crystal structure transforms from a deformed, irregular structure to an equiaxed, fully recrystallized structure. Simultaneously, precipitates such as AlN further precipitate within the strip, achieving precipitation strengthening and slightly improving the strip's mechanical properties.

[0167] Among them, the same batch of cast and rolled thin strip raw materials are arranged into hard coils with silicon content m from low to high, and the silicon content range satisfies 0.3wt%≤m≤0.8wt%. As the production process progresses, the silicon content of the strip increases, the required pre-oxidation degree increases, and the required dew point temperature also gradually increases. The dew point of the pre-oxidation stage at this stage is recorded as T dm The specific plan is:

[0168] When the silicon content gradually increases in the range of 0.3wt%≤m<0.6wt%, the dew point of the pre-oxidation section is -22℃≤T dm Increase slowly within -16℃, where the dew point temperature T increases with every 0.1wt% increase in silicon content m. dm Increase by 2℃;

[0169] When the silicon content gradually increases in the range of 0.6wt%≤m≤0.8wt%, the dew point of the pre-oxidation section is -16℃≤T dm Increase slowly within ≤-11℃, where the dew point temperature T increases with every 0.1wt% increase in silicon content m. dm Increase by 2.5℃.

[0170] 3. Coating process

[0171] The strip steel of the present invention is continuously annealed, cooled to a zinc pot entry temperature, and then enters the zinc pot through a furnace nose to complete hot-dip galvanizing.

[0172] The temperature of the strip entering the zinc pot is 480±5°C, and the zinc pot temperature is 460±5°C. At this temperature, the induction heater of the zinc pot is kept at a high power state, thereby promoting the flow of the melt in the zinc pot and preventing slag from depositing at the bottom.

[0173] Among them, as the production of cast and rolled thin strip hot-dip galvanizing continues, the zinc pot composition needs to be sampled and measured every 4 hours of production to facilitate the adjustment of the plating solution composition. The specific plan is:

[0174] When the silicon content of the plating solution exceeds 0.01wt%, pure zinc ingot (Zn99.99) is added to adjust the composition;

[0175] When the silicon content of the plating solution exceeds 0.03wt%, it is necessary to switch to transition coil or other galvanized varieties;

[0176] When the silicon content drops below 0.01wt%, the cast and rolled thin strip galvanized sheet is cut back for production.

[0177] The resulting cast-rolled thin-strip hot-dip galvanized steel sheet meets the surface quality requirements of galvanized steel sheets for home appliances. The finished product achieves FB-grade surface quality, with a yield strength of 230-270 MPa, a tensile strength of 340-380 MPa, and an elongation of 32% or greater, making it a commercial-grade hot-dip galvanized steel sheet for home appliances.

[0178] The above description is only a specific embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, they can make several improvements and modifications without departing from the principles of the present invention. These improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for producing hot-dip galvanized sheet, characterized in that: The method comprises the following steps: (1) Using thin strips with a thickness of 1.4-1.7 mm and a width of 1220 mm produced by twin-roller thin strip continuous casting as raw materials, hardened strip steel was obtained by five-stand acid continuous rolling, wherein the surface roughness of the textured roll of the fifth stand was gradient distributed; (2) Continuously annealing the hardened strip steel, dynamically adjusting the dew point temperature of the pre-oxidation section according to the silicon content of the strip steel during the annealing process; (3) Immerse the annealed strip into a zinc pot for hot-dip galvanizing, and regularly test the silicon content of the zinc liquid to adjust the composition of the zinc pot.

2. The method according to claim 1, characterized in that The cast and rolled thin strip raw material meets the following characteristics: The composition system of the cast and rolled thin strip raw materials is calculated in mass percentage as follows: C: 0.01~0.03%, Si: 0.3~0.8%, Mn: 0.3~0.4%, S: 0~0.0025%, P: 0-0.020%, N: 0-0.0055%, the rest is Fe and inevitable inclusions.

3. The method according to claim 2, characterized in that The cast and rolled thin strip raw material meets the following characteristics: In terms of mass percentage in the chemical composition, Si is 0.3 to 0.6%.

4. The method according to claim 2, characterized in that The cast and rolled thin strip raw material meets the following characteristics: In terms of mass percentage in the chemical composition, Si is 0.6 to 0.8%.

5. The method according to claim 2, characterized in that The surface roughness of the middle area of the cast and rolled thin strip material at a distance of 610 mm from the edge is in the range of 2.0 to 3.2 μm; The surface roughness range of the area 10mm away from the edge is 4.6~6.0μm.

6. The method according to claim 5, characterized in that The roughness range of the middle area at 610 mm from the edge is 2.0 to 2.9 μm.

7. The method according to claim 5, characterized in that The roughness range of the middle area at 610 mm from the edge is 2.9 to 3.2 μm.

8. The method according to claim 5, characterized in that The roughness range of the middle area at 610 mm from the edge is 4.6 to 4.8 μm.

9. The method according to claim 5, characterized in that The roughness range of the middle area 610 mm away from the edge is 4.8 to 6.0 μm.

10. The method according to any one of claims 1 to 9, characterized in that The surface roughness difference in the middle area of the hard-rolled plate edge is reduced to 0.2μm and below.

11. The method according to any one of claims 1 to 9, characterized in that Cast and rolled thin strip galvanized sheet meets the following characteristics: surface The quality reaches the finished product surface quality reaching FB grade, with yield strength of 230-270MPa, tensile strength of 340-380MPa, and elongation ≥32%.

12. The method according to claim 1, wherein: The five-stand acid continuous rolling reduces the roughness of the roller surface in the edge area of the texturing roller and gradually increases it toward the middle of the strip, eventually maintaining consistency with the roughness of the roller surface in the middle area: R 边10 ≈(R 中 ×K 中 ) / K 边10 ≈(K 中 / K 边10 )×3.6; in: R 边10 R is the roller surface roughness from the edge to the area 110 mm from the roller edge. 中 K is the roughness of the middle area of the 5# stand texturing roller at a distance of 710 mm from the roller edge. 中 is the middle overprint coefficient, K 边10 It is the overprint coefficient of the 10mm edge area.

13. The method according to claim 12, characterized in that The roller surface roughness is further controlled as follows: R 边25 =R 边10 +(R 中 -R 边10 )×70%≈(0.7+0.3K 中 / K 边10 )×3.6; in: R 边25 It refers to the roller surface roughness in the area from 110mm to 125mm from the roller edge.

14. The method according to claim 13, characterized in that The roller surface roughness is further controlled as follows: R 边40 =R 边10 +(R 中 -R 边10 )×90%≈(0.9+0.1K 中 / K 边10 )×3.6; in: R 边40 It refers to the roller surface roughness in the area from 125mm to 140mm from the roller edge.

15. The method according to claim 14, characterized in that The roller surface roughness is further controlled as follows: R 边60 ≈3.6； in: R 边60 It refers to the roller surface roughness in the area from 140mm to 160mm from the roller edge.

16. The method according to any one of claims 1 to 9, characterized in that The annealing raw material is the cast-rolled thin strip hard coil produced after the acid continuous rolling process. The annealing adopts a horizontal continuous annealing furnace. The annealing process is divided into preheating section, heating section, soaking section and cooling section.

17. The method according to claim 16, characterized in that At the end of the heating section, the furnace atmosphere is adjusted to increase the dew point and oxygen content to achieve pre-oxidation of the strip. This furnace section is defined as the pre-oxidation section.

18. The method according to claim 17, characterized in that After pre-oxidation, the strip enters the soaking reduction section. The dew point temperature of the atmosphere in this furnace section is -50℃, which ensures the full reduction of the iron oxide on the strip surface.

19. The method according to any one of claims 1 to 9, characterized in that The same batch of cast and rolled thin strip raw materials are arranged into hard coils according to the silicon content m from low to high, and the silicon content range is 0.3wt%≤m≤0.8wt%, As the production process progresses, the silicon content of the strip increases, the required pre-oxidation degree increases, and the required dew point temperature gradually increases. The dew point of the pre-oxidation section at this stage is recorded as T dm . .

20. The method according to claim 19, characterized in that When the silicon content gradually increases in the range of 0.3wt%≤m<0.6wt%, the dew point of the pre-oxidation section is -22℃≤T dm Increase slowly within -16℃, where the dew point temperature T increases with every 0.1wt% increase in silicon content m. dm Increase by 2℃; When the silicon content gradually increases in the range of 0.6wt%≤m≤0.8wt%, the dew point of the pre-oxidation section is -16℃≤T dm Increase slowly within ≤-11℃, where the dew point temperature T increases with every 0.1wt% increase in silicon content m. dm Increase by 2.5℃.

21. The method according to any one of claims 1 to 9, characterized in that After continuous annealing, the strip is cooled to the zinc pot temperature and then enters the zinc pot through the furnace nose to complete hot-dip galvanizing.

22. The method according to claim 21, characterized in that The temperature of the strip entering the zinc pot is 480±5℃, and the zinc pot temperature is 460±5℃. At this temperature, the induction heater of the zinc pot is kept at a high power state, thereby promoting the flow of the melt in the zinc pot and preventing slag from depositing at the bottom.

23. The method according to claim 22, characterized in that As the production of hot-dip galvanizing of cast and rolled thin strip continues, the composition of the zinc pot needs to be sampled and measured every 4 hours of production to facilitate the adjustment of the plating solution composition.

24. The method according to claim 23, wherein When the silicon content of the plating solution exceeds 0.01wt%, pure zinc ingot (Zn99.99) is added to adjust the composition; When the silicon content of the plating solution exceeds 0.03wt%, it is necessary to switch to transition coil or other galvanized varieties; When the silicon content drops below 0.01wt%, the cast and rolled thin strip galvanized sheet is cut back for production.

25. A cast and rolled thin strip hot-dip galvanized sheet, characterized in that: The cast-rolled thin strip hot-dip galvanized sheet is produced by the method according to any one of claims 1 to 24, and the surface quality of the finished product reaches FB grade, the yield strength is 230 to 270 MPa, the tensile strength is 340 to 380 MPa, and the elongation is ≥32%.

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