Gigapascal cold base galvanized high-strength steel and preparation method thereof

Abstract

The application discloses a gigapascal cold base galvanizing high-strength steel and a preparation method thereof, and belongs to the technical field of metal material processing. The chemical components of the steel are as follows: C: 0.08-0.16%, Mn: 1.0-2.0%, Si: 0.4-0.8%, Al: 0.6-1.0%, Cr: 0.4-0.8%, Mo: 0.06-0.13%, Nb: 0.02-0.04%, Ti: 0.02-0.08%, P: less than or equal to 0.015%, S: less than or equal to 0.006%, and the rest is Fe and inevitable impurities. The preparation method comprises smelting, continuous casting, hot rolling, acid rolling and continuous galvanizing processes. The steel is combined with precipitation strengthening, fine-grain strengthening, solid solution strengthening and controlled rolling and controlled cooling technology to obtain a structure matching of martensite, ferrite, bainite and residual austenite, and the galvanizing high-strength steel has excellent performance and low liquid metal embrittlement sensitivity.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of metal material processing, and particularly relates to a gigapascal-level cold-based galvanized high-strength steel and a preparation method thereof. BACKGROUND

[0002] With the development of automobile lightweight, the traditional dual-phase steel is difficult to meet more high-drawing needs. Although TRIP steel and QP steel have good strength and ductility, their poor welding performance and high production cost limit their wide use. DH steel introduces residual austenite on the basis of the two phases of the traditional dual-phase steel, and the strength and ductility are obviously improved, and has better application prospect. At the same time, in order to improve the material and its corrosion resistance, galvanized dual-phase steel has obvious advantages. Galvanized gigapascal dual-phase steel is generally commonly used in front and rear bumper beams, front and rear longitudinal beams and other safety parts, and needs to be baked and welded during the automobile manufacturing process, thereby improving the safety and service life of the whole vehicle. At the same time, liquid metal embrittlement (LME) phenomenon is easily generated in the welding process of galvanized high-strength steel, thereby reducing the mechanical properties of the galvanized high-strength steel sheet. Therefore, it has great practical application significance to develop a gigapascal high-strength steel product with low liquid metal embrittlement sensitivity and high plasticity.

[0003] Chinese patent CN107058869B discloses a 980MPa-level cold-rolled dual-phase steel with ultra-low yield ratio and a manufacturing method thereof. The dual-phase steel adopts a low-C Al-free basic composition system in the composition design, but the low plasticity easily causes forming problems such as stamping cracking in actual application.

[0004] Chinese patent CN107058895A discloses a 1000MPa-level hot-dip galvanized dual-phase steel and a preparation method thereof. However, the product has poor ductility, and the elongation rate is only about 13%, which is difficult to meet the stamping needs of complex parts in actual application, and the liquid metal embrittlement problem is easily generated.

[0005] Chinese patent CN114480957A discloses a 980MPa-level hot-dip galvanized dual-phase steel with good formability and a manufacturing method thereof. The dual-phase steel contains a high content of Si element, which easily deteriorates the coating property of the coating layer and easily causes the liquid metal embrittlement problem, and it is difficult to ensure that no cracking problem occurs in the welding forming process of the material.

[0006] Chinese patent CN114032457A discloses a continuous hot-dip galvanized high-strength steel sheet and a manufacturing method thereof. The galvanized high-strength steel prepared has low liquid metal embrittlement, but the carbon content is relatively high, which is not conducive to subsequent welding processing. At the same time, after cold rolling, the dew point is too high in the heat treatment process, and the surface layer is easily enriched with elements to cause quality problems such as zinc wettability, zinc bonding force and plating leakage points. SUMMARY

[0007] To solve the above technical problems, the application provides a gigapascal cold-based galvanized high-strength steel and a preparation method thereof, and the technical scheme is as follows:

[0008] The high-strength steel has the following chemical components and mass percentages: C: 0.08-0.16%, Mn: 1.0-2.0%, Si: 0.4-0.8%, Al: 0.6-1.0%, Cr: 0.4-0.8%, Mo: 0.06-0.13%, Nb: 0.02-0.04%, Ti: 0.02-0.08%, P≤0.015%, S≤0.006%, and the rest is Fe and inevitable impurities.

[0009] Further, in the microstructure of the high-strength steel, the ferrite accounts for 25-42% by volume percentage, the martensite accounts for 40-65% by volume percentage, the bainite accounts for 5-10% by volume percentage, and the residual austenite accounts for 2-10% by volume percentage.

[0010] Further, the surface layer of the high-strength steel further comprises a zinc plating layer with a thickness of 5-12 μm, and the chemical components of the zinc plating layer include, by weight content, Al: 0.3-0.8%, Si: 0.5-1.0%, Ce: 0.01-0.1%, La: 0.01-0.05%, Yr: 0.01-0.08%, and 0.5≤La / Ce≤1.2, and the rest is Zn and inevitable impurities.

[0011] Further, the finished steel has a tensile strength≥1000 MPa, a yield strength≥550 MPa, a longitudinal elongation after break A80≥17.0%, and a hole expansion ratio≥60%.

[0012] The component design reasons of the application are as follows:

[0013] Carbon (C): can improve the hardenability of the material, has a solid solution strengthening effect, and improves the material strength. At the same time, it helps to improve the stability of austenite and improve the formability of the material. On the other hand, too high C will cause material embrittlement and deteriorate the weldability of the material. From the above perspective, the C component range is 0.08-0.16%.

[0014] Manganese (Mn): as an element for expanding the austenite phase region, can improve the stability of austenite. At the same time, it can refine the grains and improve the hardenability of the material. Too low Mn content will lead to insufficient material strength and reduced austenite stability. When the Mn content is too high, the segregation phenomenon in the structure is significant, the material plasticity deteriorates, and increasing the Mn content will aggravate the formation of the internal oxidation layer on the surface of the steel and increase the sensitivity of the material LME.

[0015] Si: As ferrite strengthening element, it can inhibit carbide precipitation and cementite formation, and also has solid solution strengthening effect. Too high Si will lead to the decrease of material plasticity and weldability, and also deteriorate the coating property of the plated layer, and increase the LME sensitivity of the galvanized high strength steel. From the above-mentioned perspective, the content of Si is 0.4-0.8%; high Si content will increase the LME sensitivity of the high strength steel.

[0016] Aluminum (Al): As a complementary element of Si, it also has the effect of inhibiting carbide precipitation and cementite formation, and can also improve the platability of the material and improve the quality of the plated layer. Too high Al content will not only lead to difficulties in continuous casting production but also cause welding difficulties. Therefore, the content of Al element in the present application is controlled in the range of 0.6%-1.0%.

[0017] Chromium (Cr): As a hardenability element, it can stabilize austenite, help to realize the adjustment of martensite and austenite structure, and realize the ductility of the material. However, too high Cr will lead to the deterioration of the performance and the quality of the plated layer, and increase the LME sensitivity of the material. From the above-mentioned perspective, the content of Cr is set to 0.4-0.8%.

[0018] Mo: It helps to stabilize the residual austenite and improve the hardenability of the steel, and also helps to form the bainite structure. However, Mo element has high cost, so the content of Mo element in the present application is controlled in the range of 0.06-0.13%.

[0019] Nb and Ti: Both have significant grain refinement strengthening, solid solution strengthening and precipitation strengthening effect, and Nb can also increase the austenite recrystallization temperature of the material, which helps to regulate the proportion of each phase. At the same time, both can form stable carbides to effectively regulate the free carbon and solid solution carbon in the material. From the above-mentioned perspective, the content of Ti is set to 0.02-0.08%, and the content of Nb is set to 0.02-0.04%.

[0020] P and S: As impurity elements, P element exists in ferrite, which is easy to cause secondary cold working brittleness, so the lower the content of P element, the better; S element will interact with Mn element to form MnS, which has negative effects on the hole expansion performance and hot workability of the material, so the lower the content of S element, the better. In the present application, the upper limit of P content is set to 0.015%, and the upper limit of S content is set to 0.006%.

[0021] The preparation method of the above-mentioned gigapascal cold base galvanized high strength steel includes smelting and continuous casting, hot rolling, pickling, and continuous galvanizing processes.

[0022] The smelting and continuous casting process: smelting according to the chemical composition requirements, and then preparing a slab by continuous casting;

[0023] Hot rolling process: finish rolling starting temperature 1050-1095℃, edge temperature compensation 30-60℃, finish rolling temperature 870-910℃;

[0024] Pickling process: cold rolling reduction 50-75%, roughness Ra of F4 rack rolling mill 1.0-1.5μm, roll replacement mileage ≤300km; roughness Ra of F5 rack roughened roll 3.0-3.6μm, roll surface Pc value ≥75 / cm, roll surface chromium plating layer 10-20μm, reduction 1-7%, F5 roll replacement mileage ≤100km;

[0025] Continuous galvanizing process: pre-oxidation section dew point temperature -20--5℃, time 10-30s, oxygen content 6-15*10 3 ppm; soaking temperature 800-850℃, soaking time 100-200s, soaking section dew point temperature -45--20℃; slow cooling temperature 650-700℃; fast cooling temperature 440-470℃, fast cooling cooling rate 20-40℃ / s, entry into zinc pot strip temperature = zinc pot temperature +0-5℃.

[0026] Further, the hot rolling process, under the condition of 1190-1255℃ temperature, holding for 200-300min, then 3+1 rough rolling passes are carried out, and 7 finish rolling passes are carried out. After rolling, the layer cooling section adopts rear water cooling to 500-600℃ for coiling, and then the wind wall is placed for ≥72h.

[0027] Further, the pickling process, pickling temperature 80-90℃, acid concentration 150-200g / L.

[0028] Further, the continuous galvanizing process, zinc pot temperature 440-470℃, furnace nose dew point temperature -30--10℃; post-plating cooling tower temperature 150-200℃, and the strip is cooled to room temperature at a cooling rate of 10-15℃ / s.

[0029] Further, the continuous galvanizing process, finishing elongation 0.3-0.8%.

[0030] Reasons for the process design of the present application:

[0031] 1. By controlling the composition, especially reasonably adjusting the composition of the steel body and the plating layer, the high-temperature adhesion of the plating layer is improved, Ce, La and Yr are added to reduce the high-temperature brittleness of the plating layer, thereby being conducive to reducing the liquid metal brittleness of the material.

[0032] 2. By controlling the roughness and peak density of the roughening roller, the base substrate surface layer can form an ideal decarburized layer in the pre-oxidation stage, thereby inhibiting the formation of surface layer austenite, and also helping to enhance the adhesion of the plated layer, and being beneficial to the improvement of the material formability in the later stage. The strip temperature entering the zinc pot is not lower than the zinc liquid temperature, which is beneficial to the formation of the Fe2Al5 inhibiting layer, the improvement of the plated layer adhesion, and the improvement of the strip surface quality.

[0033] 3. By reasonable control of the temperature and cooling rate, the phase ratio and size distribution are realized; the material strength and ductility are improved through fine-grain strengthening, solid solution strengthening and precipitation strengthening.

[0034] 4. A certain amount of residual austenite and bainite is introduced into the ferrite and martensite, thereby improving the comprehensive performance of the material.

[0035] The beneficial effects produced by the above technical scheme are as follows: (1) the steel base and plated layer composition and process are reasonably designed, the material strength and ductility are improved through fine-grain strengthening, solid solution strengthening and precipitation strengthening, the microstructure of the high-strength steel of the present application mainly includes ferrite, martensite, bainite and residual austenite, wherein the ferrite accounts for 25-42%, the martensite accounts for 40-65%, the bainite accounts for 5-10%, and the residual austenite accounts for 2-10%. The introduction of residual austenite improves the material formability, and the introduction of bainite improves the material phase synergistic deformation ability and flanging property, thereby solving the problems of poor formability and insufficient ductility of the super-high-strength grade automobile steel; (2) the rolling process is combined with the design of the plating solution composition and the galvanizing process, the surface layer structure is refined, the formation of surface layer austenite is avoided, the plated layer quality is improved, and the material liquid metal brittleness is improved.

[0036] The present application combines precipitation strengthening, fine-grain strengthening, solid solution strengthening and controlled rolling and controlled cooling technologies to obtain a matched microstructure of martensite, ferrite, bainite and residual austenite, the strength and ductility are obviously improved, the gigapascal cold-based galvanized high-strength steel produced has excellent performance, the tensile strength is ≥1000 MPa, the yield strength is ≥550 MPa, the longitudinal elongation after break A80 is ≥17.0%, the hole expansion rate is ≥60%, and the liquid metal embrittlement sensitivity is low. BRIEF DESCRIPTION OF DRAWINGS

[0037] Figure 1 FIG. 1 is a microstructure diagram of the galvanized high-strength steel of Example 1 of the present application;

[0038] Figure 2 FIG. 2 is a cross-sectional structure schematic diagram of the galvanized high-strength steel of Example 1 of the present application;

[0039] Figure 3 FIG. 3 is a cross-sectional structure schematic diagram of the galvanized high-strength steel of Comparative Example 1. IMPLEMENTATION

[0040] The application will be further described in detail below in connection with the embodiments.

[0041] The chemical composition and mass percentage of the gigapascal cold-based galvanized high-strength steel of the application are as follows: C: 0.08-0.16%, Mn: 1.0-2.0%, Si: 0.4-0.8%, Al: 0.6-1.0%, Cr: 0.4-0.8%, Mo: 0.06-0.13%, Nb: 0.02-0.04%, Ti: 0.02-0.08%, P≤0.015%, S≤0.006%, and the rest is Fe and inevitable impurities. The microstructure of the steel, in terms of volume percentage, is as follows: ferrite 25-42%, martensite 40%-65%, bainite 5-10%, and residual austenite 2-10%.

[0042] The thickness of the zinc plating layer of the above galvanized high-strength steel is 5-12 μm, and the chemical composition of the zinc plating layer, in terms of weight percentage, includes Al: 0.3-0.8%, Si: 0.5-1.0%, Ce: 0.01-0.1%, La: 0.01-0.05%, Yr: 0.01-0.08%, and 0.5≤La / Ce≤1.2, and the rest is Zn and inevitable impurities. The Al in the zinc plating layer composition helps to form an Fe2Al5 inhibiting layer between the substrate and the plating layer to improve the plating layer adhesion, the addition of Si helps to reduce the material friction coefficient and improve the strip steel wettability with zinc liquid. The addition of Ce, La and Yr helps to reduce the intergranular corrosion of the plating layer and enhance the intergranular corrosion resistance of the alloy plating layer. Yr also helps to improve the ductility of the plating layer, thereby reducing the liquid metal brittleness and improving the formability of the material.

[0043] The preparation method of the above gigapascal cold-based galvanized high-strength steel includes the steps of smelting, continuous casting, hot rolling, pickling and cold rolling, and continuous galvanizing.

[0044] (1) smelting according to the chemical composition requirements, and then preparing a slab by continuous casting;

[0045] (2) hot rolling process: heating and holding the slab, rough rolling, finish rolling, coiling, and cooling to obtain a hot-rolled strip.

[0046] (3) pickling and cold rolling process: pickling and cold rolling the hot-rolled strip to obtain a cold hard strip with a certain roughness.

[0047] (4) continuous galvanizing process: pre-oxidizing, soaking, slow and fast cooling annealing, post-plating cooling, and finishing the cold hard strip to obtain a gigapascal galvanized high-strength steel.

[0048] The giga-level cold base galvanized high-strength steel produced by using the above component design and process has excellent performance, the tensile strength is ≥1000 MPa, the yield strength is ≥550 MPa, the longitudinal elongation after fracture A80 is ≥17.0%, the hole expansion rate is ≥60%, and the liquid metal embrittlement sensitivity is low.

[0049] The chemical components and weight percentages of the giga-level galvanized high-strength steel of the examples and comparative examples are shown in Table 1, the process parameter controls of the hot rolling, pickling, and continuous galvanizing processes are shown in Tables 2-5, and the coating composition and thickness are shown in Table 6.

[0050] Table 1, Chemical component composition (wt%) of the giga-level galvanized high-strength steel of the examples and comparative examples

[0051]

[0052] Table 2, Hot rolling process parameter control of the examples and comparative examples

[0053]

[0054] Table 3, Pickling process parameter control of the examples and comparative examples

[0055]

[0056] Table 4, Continuous galvanizing process parameter control-1 of the examples and comparative examples

[0057]

[0058] Table 5, Continuous galvanizing process parameter control-2 of the examples and comparative examples

[0059]

[0060] Table 6, Coating composition and thickness of the examples and comparative examples

[0061]

[0062] The chemical components of the surface layer of the galvanized high-strength steel substrate of the examples and comparative examples are measured by the glow discharge spectroscopy method, and [C] / C, [Mn] / Mn, [Si] / Si, and [Cr] / Cr represent the mass fraction ratio of the chemical components of the surface layer of the galvanized high-strength steel substrate to the chemical components of the steel plate, as shown in Table 7. The element ratio of the examples is obviously lower than that of the comparative examples, and the surface layer austenitizing temperature of the examples will be obviously higher than that of the substrate and the comparative examples.

[0063] Table 7, Mass fraction ratio of the chemical components of the surface layer of the galvanized high-strength steel substrate to the steel plate

[0064]

[0065] The giga-level cold base galvanized high-strength steel of the examples and the comparative examples was heated to 1000℃ at a speed of 50℃ / s, kept for 30s, the tensile speed was 0.13 / s, the tensile deformation was to 15%, and then was rapidly cooled to room temperature. The cracks on the surface layer of the galvanized high-strength steel were observed and measured, the density of the cracks was measured, and the maximum depth of the cracks was measured. The microstructure, mechanical properties and crack statistics of the examples and the comparative examples are shown in Table 8.

[0066] Table 8, microstructure, mechanical properties and crack statistics of the galvanized high-strength steel

[0067]

[0068] In Table 8, by reasonably designing the steel base composition and process control, the material strength and plasticity of the examples is improved through fine-grain strengthening, solid solution strengthening and precipitation strengthening, the material formability is improved by introducing residual austenite, and the material phase deformation ability and flanging property are improved by introducing bainite, so as to solve the problems of poor formability and insufficient ductility of the super-high strength level automobile steel. As can be seen from Table 8, the crack density and depth of the galvanized high-strength steel of the examples are far less than those of the comparative examples, and the influence on the performance of the steel base is small. The microstructure of the galvanized high-strength steel of Example 1 is shown in Figure 1 , and the cross-sectional structure schematic diagram of the comparative examples is shown in Figure 2 and Figure 3 As can be seen from Figure 2 , 3 , by controlling the chemical composition of the surface layer of the galvanized high-strength steel and the temperature of the zinc liquid into the strip, the austenitizing temperature of the surface layer is significantly increased, which is beneficial to inhibit the formation of the layer and improve the bonding force of the plated layer, thereby improving the liquid metal brittleness of the galvanized high-strength steel, avoiding the occurrence of cracks, and solving the technical problem that deep cracks easily occur on the surface of the galvanized high-strength steel during welding.

Claims

1. A gigapascal-grade cold-dip galvanized high-strength steel, characterized in that, The chemical composition and mass percentage of the high-strength steel are as follows: C: 0.08–0.16%, Mn: 1.0–2.0%, Si: 0.59–0.8%, Al: 0.65–1.0%, Cr: 0.64–0.8%, Mo: 0.06–0.13%, Nb: 0.02–0.04%, Ti: 0.02–0.08%, P≤0.015%, S≤0.006%, with the remainder being Fe and unavoidable impurities. The microstructure of the high-strength steel, by volume percentage, consists of 25-42% ferrite, 40-65% martensite, 5-10% bainite, and 2-10% retained austenite. The surface of the high-strength steel also includes a zinc coating with a thickness of 5-12 μm. The chemical composition of the zinc coating, by weight, includes Al: 0.3-0.8%, Si: 0.5-1.0%, Ce: 0.01-0.1%, La: 0.01-0.05%, Yr: 0.01-0.08%, and 0.5≤La / Ce≤1.2, with the remainder being Zn and unavoidable impurities.

2. The gigapascal-grade cold-dip galvanized high-strength steel according to claim 1, characterized in that, The finished steel has a tensile strength ≥1000MPa, a yield strength ≥550MPa, a longitudinal elongation after fracture A80 ≥17.0%, and a hole expansion rate ≥60%.

3. The method for preparing gigapascal-grade cold-dip galvanized high-strength steel according to claim 1 or 2, characterized in that, It includes smelting and continuous casting, hot rolling, pickling and rolling, and continuous galvanizing processes; Smelting and continuous casting process: Smelting is carried out according to the chemical composition requirements, and then continuous casting is performed to prepare slabs; Hot rolling process: finishing rolling start temperature 1050~1095℃, edge temperature compensation 30~60℃, final rolling temperature 870~910℃; Pickling and rolling process: cold rolling reduction rate 50-75%, roll roughness Ra of F4 stand is 1.0-1.5μm, roll change mileage ≤300km; F5 stand texturized roll roughness Ra is 3.0-3.6μm, roll surface Pc value ≥75 particles / cm, roll surface chromium plating layer 10-20μm, reduction rate 1-7%, F5 roll change mileage ≤100km; Continuous galvanizing process: Dew point temperature of the pre-oxidation section is -20 to -5℃, time is 10 to 30 seconds, and its oxygen content is 6 to 15 × 10⁻⁶. 3 ppm; heating temperature 800~850℃, heating time 100~200s, dew point temperature of heating section -45~-20℃; slow cooling temperature 650~700℃; rapid cooling temperature 440~470℃, rapid cooling rate 20~40℃ / s, temperature of the strip entering the zinc bath = temperature of the zinc pot + 0~5℃.

4. The method for preparing gigapascal grade cold-dip galvanized high-strength steel according to claim 3, characterized in that, The hot rolling process involves holding the temperature at 1190–1255℃ for 200–300 minutes, followed by 3+1 passes of rough rolling and 7 passes of finish rolling.

5. The method for preparing gigapascal grade cold-dip galvanized high-strength steel according to claim 4, characterized in that, In the hot rolling process, after rolling, the cooling section is cooled to 500-600°C by water cooling in the later stage and then coiled, and then placed against the windbreak wall for ≥72 hours.

6. The method for preparing gigapascal grade cold-dip galvanized high-strength steel according to claim 5, characterized in that, In the pickling and rolling process, the pickling temperature is 80-90℃ and the acid concentration is 150-200g / L.

7. The method for preparing gigapascal grade cold-dip galvanized high-strength steel according to claim 6, characterized in that, In the continuous galvanizing process, the zinc pot temperature is 440-470℃, the furnace nose dew point temperature is -30--10℃, the post-galvanizing cooling tower temperature is 150-200℃, and the strip is cooled to room temperature at a cooling rate of 10-15℃ / s.

8. The method for preparing gigapascal grade cold-dip galvanized high-strength steel according to any one of claims 3-7, characterized in that, The continuous galvanizing process has a finishing elongation of 0.3 to 0.8%.

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