A gradient temperature control manufacturing process for a low-alloy high-strength galvanized steel strip for household appliances

Abstract

The application discloses a manufacturing process of a low-alloy high-strength galvanized steel strip for household appliances and belongs to the technical field of metal material processing and manufacturing. The high-performance steel strip with gradient nano precipitated phase and multi-phase heterogeneous structure is prepared through a hot rolling process of "two-section ultra-fast cooling+intermediate heat preservation" and a "dynamic light finishing+gas mist synergistic cooling" technology of a galvanizing line.

Description

Technical Field

[0001] This invention belongs to the field of metal material processing and manufacturing technology, and in particular relates to a gradient temperature control manufacturing process for low alloy high-strength galvanized steel strips used in household appliances. Background Technology

[0002] With the rapid development of my country's economy, the demand for high-strength, high-toughness and weldable steel has increased significantly. Traditional materials can no longer meet the requirements of high performance steel and weight reduction of equipment. Therefore, it is necessary to study and develop high-quality steel [1]. At present, the requirements of the home appliance industry for lightweight, energy saving and safety are increasing. Structural steel for home appliances is developing towards high strength and high toughness. Galvanized sheet is also increasing in proportion in the home appliance industry due to its good surface quality, weldability, paintability and superior corrosion resistance [2]. Traditional HSLA (low alloy high strength) steel is usually strengthened by adding microalloying elements such as Nb, V, Ti, etc., but when producing 0.80~1.80mm thin specification hot rolled and subsequent galvanized products, it often faces the contradiction between strength and formability (A80). Existing production processes (such as conventional CSP or traditional hot continuous rolling processes) usually adopt a single laminar flow cooling mode, which makes it difficult to accurately control the mixing ratio of ferrite and bainite / martensite, resulting in a high yield strength ratio of the product, which is not conducive to the deep drawing of home appliance structural parts. In the future, the domestic home appliance market will enter a stage dominated by replacement consumption. In recent years, the export of home appliances has also maintained high growth, and the global production share of various types of home appliances has reached a high level[3]. Therefore, there is an urgent need for a new manufacturing process that can obtain galvanized steel strips with both high strength (tensile strength ≥380MPa) and excellent formability (elongation ≥23%) without significantly increasing the cost of alloys through process innovation. Summary of the Invention

[0003] The purpose of this invention is to provide a gradient temperature control manufacturing process for low-alloy high-strength galvanized steel strip for home appliances. Through a hot rolling process of "two-stage ultra-fast cooling + intermediate heat preservation" combined with the "dynamic finishing + air mist synergistic cooling" technology of the galvanizing line, a high-performance steel strip with gradient nano-precipitated phases and multiphase heterogeneous structure is prepared.

[0004] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0005] This invention discloses a gradient temperature-controlled manufacturing process for low-alloy high-strength galvanized steel strip used in household appliances, comprising:

[0006] 1. Optimization design of the composition system (fine-tuning based on the quality plan): Based on the C-Si-Mn-Al-Nb-Ti system specified in the quality plan, narrow composition control is implemented for key elements:

[0007] C (0.06~0.09%): Strictly control the lower limit of carbon content to ensure formability, and the upper limit to ensure strength.

[0008] Mn (1.00~1.20%): The manganese content is controlled at the middle to upper limit to expand the austenite region, delay the ferrite phase transformation, and create conditions for the subsequent bainite transformation.

[0009] Nb+Ti (0.08~0.12%): The high-temperature precipitation of Nb pins the austenite grain boundaries, while the low-temperature precipitation of Ti strengthens the ferrite matrix, forming a "double precipitation" strengthening mechanism.

[0010] 2. Gradient rolling and ultra-fast cooling process (hot rolling process innovation) differs from the simple "densified laminar flow cooling" in quality planning. This invention adopts the following hot rolling control strategy:

[0011] Finishing mill exit temperature control: The F7 exit temperature is controlled at 860±10℃ (for 1.0-1.5mm specifications) to ensure that the final rolling is carried out in the non-recrystallization zone and to refine the austenite grains.

[0012] First-stage cooling (ultra-fast cooling): An ultra-fast cooling system (UFC) is used to increase the cooling rate to 50-80℃ / s, rapidly cooling the strip surface temperature from 860℃ to 650±10℃. This step inhibits the premature precipitation of conventional ferrite and preserves austenite with high distortion energy.

[0013] Intermediate heat preservation (key innovation): heat preservation at 650℃ for 15-25 seconds. Within this temperature range, a large number of fine Nb(C,N) particles precipitate in the austenite, while some austenite undergoes a phase transformation into acicular ferrite.

[0014] Second-stage cooling (laminar cooling): Laminar cooling is restarted to cool the strip (the cooling rate at this stage is usually 10-30℃ / s) to the coiling temperature of 580±10℃. Due to the presence of intermediate heat preservation, the final microstructure is a multiphase microstructure of "fine-grained ferrite + dispersed bainite / martensite islands", which effectively reduces the yield strength ratio.

[0015] 3. Dynamic Annealing and Finishing Process of Galvanizing Line (Innovation in Galvanizing Process): Regarding the annealing process (heating section 805℃) mentioned in the quality plan, this invention introduces a dynamic adjustment mechanism:

[0016] Induction heating-assisted homogenization: An online induction heating device is added after the radiant tube heating section. When the strip thickness is detected to be >1.5mm and the carbon equivalent is too high, induction heating is activated to pulse the strip core to 830℃ for a short time (2-3s) to eliminate the temperature difference of the strip cross section and ensure recrystallization of the central layer.

[0017] Background: The quality plan requires the finishing elongation to be controlled between 1.00% and 1.70%. Traditional finishing methods tend to result in a uniform surface roughness (Ra value).

[0018] Innovation: An asymmetric roller diameter finishing machine is adopted (working roller diameter difference ≥ 15mm), with the upper roller rotating at a higher speed than the lower roller (conventional experience value: the upper roller speed is 8%-12% higher than the lower roller, which is a typical process parameter for the production of galvanized steel strip for household appliances; extreme conditions: if further improvement of the upper surface roughness is required, the speed difference can be increased to 15%; if the requirements for the lower surface formability are extremely high, the speed difference can be reduced to 5%). During the finishing process, the upper surface generates a large shear strain, forming a micron-level roughness (Ra 0.8-1.2μm), which is beneficial to paint film adhesion; the lower surface maintains a lower roughness (Ra 0.4-0.6μm), which is beneficial to stretching and forming.

[0019] Dynamic control of zinc pot composition: Adding trace amounts of 0.05-0.10% Ni and 0.01-0.03% Sb to the zinc pot inhibits the excessive growth of the Fe2Al5 barrier layer, ensures perfect bonding between the high-strength steel substrate and the zinc layer, and prevents incomplete plating.

[0020] 4. Digital Twin-Based End-to-End Monitoring System (Intelligent Innovation) This invention also provides an intelligent monitoring system to solve the problem of relying on manual labor for "process discipline execution" in quality planning:

[0021] 1) Hot rolling twin model: Establish a thermo-mechanical coupled finite element model of the hot rolling process, input the rolling force and temperature data on site in real time, and back-calculate the grain size inside the strip (target: average grain size ≤ 5μm).

[0022] Defect warning algorithm: A prediction model based on machine learning is established for "chemical composition-process parameters-mechanical properties". When the predicted tensile strength Rm < 380MPa or elongation A80 < 22%, the system automatically pushes parameter correction suggestions to the operating table (such as adjusting the winding temperature or finishing elongation).

[0023] Furthermore, the chemical composition of the steel strip by mass percentage is: C 0.07%, Si 0.03%, Mn 1.15%, P 0.015%, S 0.005%, Alt 0.035%, Nb 0.04%, Ti 0.06%, with the remainder being Fe and unavoidable impurities.

[0024] Furthermore, specifically including:

[0025] (1) Hot rolling process:

[0026] Heating: The slab is heated in a walking beam furnace with a soaking zone temperature of 1220℃ and a furnace time of 200min;

[0027] Roughing and finishing: F7 exit thickness 2.5mm, finishing rolling temperature 865℃;

[0028] cool down:

[0029] The ultra-fast cooling system was activated to cool the strip steel from 865℃ to 645℃, with the cooling rate set at 60℃ / s.

[0030] Turn off ultra-fast cooling, enter the heat preservation section, and maintain a constant temperature of 645℃ for 20 seconds;

[0031] Turn on laminar flow cooling and control the winding temperature at 585℃;

[0032] After winding, hot-rolled coils are obtained, and anti-rust oil is applied between the layers;

[0033] (2) Pickling and rolling process

[0034] Pickling removes iron oxide scale;

[0035] The cold rolling reduction rate is controlled at 65% (target value), and the rolling speed is 1200m / min to obtain a hard-rolled coil;

[0036] (3) Hot-dip galvanizing process

[0037] Annealing: Furnace temperature ≥ 800℃, heating section temperature 805℃; when the strip thickness is 1.2mm, activate induction heating auxiliary to ensure that the center temperature of the strip reaches above 820℃;

[0038] Zinc pot: Zinc liquid temperature 462℃, with 0.08% Ni and 0.02% Sb added;

[0039] Finishing: An asymmetric finishing mill is used, with the upper roll diameter φ450mm and the lower roll diameter φ400mm; the finishing elongation is set at 1.30%, and the rolling force is 12000kN;

[0040] Tension straightening: Tension straightening elongation rate 0.2%.

[0041] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0042] Excellent performance: The gradient cooling process resolves the inverse relationship between strength and formability. The product's yield strength is stable at 320-360MPa, tensile strength at 400-460MPa, and elongation after fracture (A80) reaches 24-28%, exceeding the quality plan requirements.

[0043] High surface quality: Asymmetric finishing technology gives the steel strip the characteristics of "high adhesion on one side and high lubrication on the other side", which is particularly suitable for the spraying of appliance shells and the stamping of internal structural parts.

[0044] Low cost: Without adding expensive alloying elements such as Mo and Cr, the performance improvement is achieved solely through process innovation, meeting the low-cost requirements for steel used in home appliances.

[0045] Stable quality: The introduction of the digital twin system has significantly reduced human error and ensured the strict implementation of the "process discipline" in the quality plan. Detailed Implementation

[0046] The present invention will be further described in detail below with reference to specific embodiments.

[0047] Example 1: Steel for household appliance structural components with a diameter of 1.2mm

[0048] 1. Steelmaking and Continuous Casting

[0049] Smelting is carried out in accordance with the quality plan requirements, and the controlled chemical composition (wt%) is as follows: C 0.07%, Si 0.03%, Mn 1.15%, P 0.015%, S 0.005%, Alt 0.035%, Nb 0.04%, Ti 0.06%, with the remainder being Fe and unavoidable impurities.

[0050] 2. Hot rolling process

[0051] Heating: The slab is heated in a walking beam furnace with a soaking zone temperature of 1220℃ and a furnace time of 200min.

[0052] Roughing and finishing: F7 exit thickness 2.5mm, finishing rolling temperature 865℃.

[0053] Cooling (core step):

[0054] The ultra-fast cooling system is activated to cool the strip steel from 865℃ to 645℃, with the cooling rate set at 60℃ / s.

[0055] Turn off the ultra-fast cooling mode and enter the heat preservation mode, maintaining a constant temperature of 645℃ for 20 seconds.

[0056] Turn on laminar flow cooling and control the winding temperature at 585℃.

[0057] After winding, hot-rolled coils are obtained, and rust-preventive oil is applied between the layers.

[0058] 3. Pickling and rolling process

[0059] Pickling removes iron oxide scale.

[0060] The cold rolling reduction rate is controlled at 65% (target value), and the rolling speed is 1200m / min to obtain a hard-rolled coil.

[0061] 4. Hot-dip galvanizing process

[0062] Annealing: Furnace entry temperature ≥ 800℃, heating section temperature 805℃. When the strip thickness is 1.2mm, activate induction heating auxiliary to ensure the strip center temperature reaches above 820℃.

[0063] Zinc pot: Zinc liquid temperature 462℃, with 0.08% Ni and 0.02% Sb added.

[0064] Finishing: An asymmetric finishing mill is used, with the upper roll diameter being φ450mm and the lower roll diameter being φ400mm. The finishing elongation is set at 1.30%, and the rolling force is 12000kN.

[0065] Tension straightening: Tension straightening elongation rate 0.2%.

[0066] 5. Test Results

[0067] Metallographic structure: Ferrite + Bainite, average grain size 10.5.

[0068] Mechanical properties: Rt0.2=335MPa, Rm=425MPa, A80=26.5%, n value (strain hardening index)=0.18.

[0069] Coating: Average 85g / m² on both sides at three points, with no zinc particles and no peeling.

[0070] Example 2: Steel for ultra-thin 0.9mm home appliance panels (process parameters adjusted)

[0071] The final rolling temperature of hot rolling is increased to 880℃ (to prevent breakage).

[0072] The winding temperature is increased to 600°C (to prevent the formation of yield plateaus).

[0073] The elongation rate of galvanized finishing is set to 1.00% (to prevent the surface from being too rough).

[0074] Results: Rm=395MPa, A80=28.0%, surface roughness Ra=0.55μm, meeting the high surface quality requirements of the panel.

[0075] Comparative Example

[0076] Steel strips of the same composition were produced using the standard process outlined in the quality plan (no intermediate insulation, standard finishing). Result: Rm=410MPa, but A80 is only 21.5%, and the yield strength ratio is as high as 0.85, indicating poor stamping performance, which does not meet the requirements for high-end household appliance structural components.

[0077] Conclusion: This invention, through innovative "gradient cooling" and "asymmetric finishing" processes, perfectly matches the equipment characteristics of Baotou Steel's rare earth steel plate production line. The HX300LAD+Z steel strip produced not only fully meets the technical requirements of the quality plan, but also achieves a qualitative leap in deep drawing performance and surface adaptability, and has extremely high market application value.

[0078] Creativity: It solves the technical problem of "high strength leading to decreased formability" and provides specific parameter ranges (such as holding at 650℃ for 15-25 seconds), which is creative.

[0079] 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.

Claims

1. A gradient temperature control manufacturing process for low-alloy high-strength galvanized steel strip for household appliances, characterized in that, include: (1) Optimization design of the component system by mass percentage Based on the C-Si-Mn-Al-Nb-Ti system specified in the quality plan, narrow compositional control is implemented for key elements: C 0.06~0.09%: Strictly control the lower limit of carbon content to ensure formability, and the upper limit to ensure strength; Mn 1.00~1.20%: The manganese content is controlled at the middle and upper limits to expand the austenite region, delay the ferrite phase transformation, and create conditions for the subsequent bainite transformation; Nb+Ti 0.08~0.12%: The high-temperature precipitation of Nb pins the austenite grain boundaries, while the low-temperature precipitation of Ti strengthens the ferrite matrix, forming a "dual precipitation" strengthening mechanism. (2) Gradient rolling and ultra-fast cooling process Unlike the simple "densified laminar flow cooling" in the quality plan, the following hot rolling control strategy is adopted: Finishing mill exit temperature control: The F7 exit temperature is controlled at 860±10℃ to ensure that the final rolling is carried out in the non-recrystallization zone and to refine the austenite grains. The first stage of cooling, ultra-fast cooling: adopts an ultra-fast cooling system to increase the cooling rate to 50-80℃ / s, and rapidly cools the surface temperature of the strip steel from 860℃ to 650±10℃; this process inhibits the premature precipitation of conventional ferrite and retains austenite with high distortion energy. Intermediate heat preservation: Hold at 650℃ for 15-25 seconds; within this temperature range, a large number of fine Nb(C,N) particles precipitate in the austenite, while some austenite undergoes a phase transformation into acicular ferrite. The second stage of cooling is laminar flow cooling: laminar flow cooling is initiated to cool the strip to the coiling temperature of 580±10℃; due to the presence of intermediate heat preservation, the final microstructure is a multiphase microstructure of "fine-grained ferrite + dispersed bainite / martensite islands", which effectively reduces the yield strength ratio.

2. The gradient temperature control manufacturing process for low-alloy high-strength galvanized steel strip for household appliances according to claim 1, characterized in that, It also includes (3) the dynamic annealing and finishing process of the galvanizing line: Introduce a dynamic adjustment mechanism: Induction heating-assisted homogenization: An online induction heating device is added after the radiant tube heating section; when the strip thickness is detected to be >1.5mm and the carbon equivalent is too high, induction heating is started to pulse the strip core to 830℃ for a short time of 2-3 seconds to eliminate the temperature difference of the strip cross section and ensure the recrystallization of the central layer.

3. The gradient temperature control manufacturing process for low-alloy high-strength galvanized steel strip for household appliances according to claim 1, characterized in that, An asymmetric roller diameter finishing machine is used, with a working roller diameter difference of ≥15mm and an upper roller speed higher than the lower roller. During the finishing process, the upper surface generates a large shear strain, forming a micron-level roughness Ra 0.8-1.2μm, which is beneficial to the adhesion of the paint film. The lower surface maintains a lower roughness Ra 0.4-0.6μm, which is beneficial to stretching and forming.

4. The gradient temperature control manufacturing process for low-alloy high-strength galvanized steel strip for household appliances according to claim 1, characterized in that, Adding trace amounts of 0.05-0.10% Ni and 0.01-0.03% Sb to the zinc pot inhibits the excessive growth of the Fe2Al5 barrier layer, ensuring a perfect bond between the high-strength steel substrate and the zinc layer and preventing incomplete plating.

5. The gradient temperature control manufacturing process for low-alloy high-strength galvanized steel strip for household appliances according to claim 1, characterized in that, This includes an intelligent monitoring system, specifically comprising: 1) Hot rolling twin model: Establish a thermo-mechanical coupled finite element model of the hot rolling process, input the rolling force and temperature data on site in real time, and back-calculate the grain size inside the strip. Target: average grain size ≤ 5μm; 2) Defect warning algorithm: Based on machine learning, a prediction model of "chemical composition-process parameters-mechanical properties" is established; when the predicted tensile strength Rm < 380MPa or elongation A80 < 22%, the system automatically pushes parameter correction suggestions to the operating table, such as adjusting the winding temperature or finishing elongation.

6. The gradient temperature control manufacturing process for low-alloy high-strength galvanized steel strip for household appliances according to claim 1, characterized in that, The chemical composition of the steel strip by mass percentage is: C 0.07%, Si 0.03%, Mn 1.15%, P 0.015%, S 0.005%, Alt 0.035%, Nb 0.04%, Ti 0.06%, with the remainder being Fe and unavoidable impurities.

7. The gradient temperature control manufacturing process for low-alloy high-strength galvanized steel strip for household appliances according to claim 6, characterized in that, Specifically, it includes: (1) Hot rolling process: Heating: The slab is heated in a walking beam furnace with a soaking zone temperature of 1220℃ and a furnace time of 200min; Roughing and finishing: F7 exit thickness 2.5mm, finishing rolling temperature 865℃; cool down: The ultra-fast cooling system was activated to cool the strip steel from 865℃ to 645℃, with the cooling rate set at 60℃ / s. Turn off ultra-fast cooling, enter the heat preservation section, and maintain a constant temperature of 645℃ for 20 seconds; Turn on laminar flow cooling and control the winding temperature at 585℃; After winding, hot-rolled coils are obtained, and anti-rust oil is applied between the layers; (2) Pickling and rolling process Pickling removes iron oxide scale; The cold rolling reduction rate is controlled at 65% (target value), and the rolling speed is 1200m / min to obtain a hard-rolled coil; (3) Hot-dip galvanizing process Annealing: Furnace temperature ≥ 800℃, heating section temperature 805℃; when the strip thickness is 1.2mm, activate induction heating auxiliary to ensure that the center temperature of the strip reaches above 820℃; Zinc pot: Zinc liquid temperature 462℃, with 0.08% Ni and 0.02% Sb added; Finishing: An asymmetric finishing mill is used, with the upper roll diameter φ450mm and the lower roll diameter φ400mm; the finishing elongation is set at 1.30%, and the rolling force is 12000kN; Tension straightening: Tension straightening elongation rate 0.2%.