800MPa-grade hot-based galvanized complex phase steel with low cost and high yield ratio and preparation method of 800MPa-grade hot-based galvanized complex phase steel

Abstract

The invention discloses 800MPa-grade hot-based galvanized complex-phase steel with low cost and high yield ratio and a preparation method thereof, and belongs to the technical field of plate and strip production. The hot-based galvanized complex phase steel comprises the following components in percentage by mass: 0.060%-0.150% of C, 0.10%-0.35% of Si, 1.00%-2.90% of Mn, less than or equal to 0.020% of P, less than or equal to 0.01% of S, 0.01%-0.07% of Als, 0.10%-0.20% of Ti, less than or equal to 0.050% of N and the balance of Fe and impurities. The preparation method adopts an ultralow-temperature annealing galvanizing process, and comprises the following steps: heating to 530-630 DEG C after acid pickling, carrying out soaking and heat preservation, then cooling to 440-500 DEG C, carrying out heat preservation, carrying out hot galvanizing, taking out of a zinc pot, and cooling to room temperature. According to the hot-based galvanized complex-phase steel produced through the method, the yield strength is larger than or equal to 740 Mpa, the tensile strength is larger than or equal to 800 Mpa, the A80 ductility is larger than or equal to 14%, and the hole expansion rate is larger than or equal to 60%.

Description

Technical Field

[0001] The invention belongs to the technical field of plate and strip production, and specifically relates to a low-cost, high-yield-strength ratio 800MPa grade hot-base galvanized complex phase steel and a preparation method thereof. Background Art

[0002] At present, automobile lightweighting and full-process energy conservation and emission reduction are particularly important. Automobiles use high-strength steel and ultra-high-strength steel to achieve lightweight bodies, thereby achieving energy conservation and emission reduction. The chassis of automobiles has been in service for a long time in an environment with high humidity, high temperature, and even high salt and alkali corrosion. In addition to its own high load-bearing capacity and fatigue-resistant structural characteristics, hot-base galvanized complex phase steel can perfectly match the occasional characteristics of automobile chassis parts. Therefore, hot-base galvanized complex phase steel is widely used in automobile chassis structural parts. However, the chassis parts often correspond to a series of processing processes such as low curvature mold corner radius or flange stretching during the forming process, so excellent local forming ability is very important.

[0003] The basic requirement of automobile chassis parts for materials is to have high yield-to-strength ratio. Materials with high yield-to-strength ratio can improve the strength of automobile chassis parts and the safety factor of chassis system, and improve the fatigue life of parts. In addition, the amount of automobile chassis parts is very large, so the production characteristics of low cost and low energy consumption can achieve the goals of economic benefits and energy conservation and emission reduction.

[0004] CN116121641A discloses a 780MPa grade hot-base galvanized complex phase steel, the composition of the 780MPa grade hot-base galvanized complex phase steel includes the following chemical components in mass percentage: C: 0.06-0.12%, Si: 0.10-0.40%, Mn: 1.50-1.90%, P≤0.025%, S≤0.015%, Mo: 0.20-0.33%, Nb: 0.020-0.05%, Ti: 0.030-0.080%, Als: 0.010-0.070%, N≤0.0050%, and the remaining elements are Fe and unavoidable impurities. The hot-base galvanized complex phase steel described in the invention contains a very expensive Mo element and a large content, and the cost of the Nb element is also very high, so the cost of the product is very high, which seriously affects the market competitiveness of the product. In addition, the product yield strength of the invention scheme is low, which is not conducive to the application scenario of hot-base galvanized steel sheet, i.e., automotive chassis parts.

[0005] CN202211644418.7 discloses an 800Mpa grade hot-dip galvanized complex phase steel plate and a manufacturing method thereof. In terms of mass fraction, the chemical composition of the complex phase steel plate includes: C: 0.05%-0.10%, Si: ≤0.45%, Mn: 1.4-1.8%, Al: 0.02-0.08%, Cr: ≤0.5%, Ti: 0.05-0.15%, Nb: ≤0.05%, V: ≤0.2%, Fe and other inevitable impurities. The hot-dip galvanized complex phase steel described in the invention contains very expensive Cr and Nb elements, so the product cost is very high and seriously affects the market competitiveness of the product. In addition, the product of the invention scheme has a large ferrite content, and the difference between the soft and hard phases in the organization is obvious. The organizational ratio is not conducive to the hole expansion performance of the hot-dip galvanized steel plate, and the chassis parts forming requires great flanging and hole expansion performance of the material. Therefore, the application performance of the product is poor, which is not conducive to promotion and creation of benefits.

[0006] Therefore, it is urgent to study a hot-dip galvanized composite steel plate with low production cost and comprehensive performance that meets the requirements of automobile chassis parts. Summary of the invention

[0007] The technical problem to be solved by the present invention is that the production cost of the existing hot-dip galvanized complex phase steel plate is relatively high, and the comprehensive performance (such as yield strength ratio and hole expansion performance) cannot meet the application requirements of automobile chassis parts.

[0008] The technical solution adopted by the present invention to solve the technical problem is: low-cost high-yield ratio 800MPa grade hot-dip galvanized complex phase steel, the chemical composition of which is as follows by mass percentage: C 0.060%-0.150%, Si 0.10%-0.35%, Mn1.00%-2.90%, P≤0.020%, S≤0.01%, Als 0.01%-0.07%, Ti 0.10%-0.20%, N≤0.050%, and the remaining elements are Fe and inevitable impurities.

[0009] The microstructure of the above-mentioned low-cost high-yield ratio 800MPa grade hot-dip galvanized complex phase steel consists of 55-65% bainite, 25-35% ferrite, and 5-15% martensite + residual austenite.

[0010] The above-mentioned low-cost high yield strength ratio 800MPa grade hot-dip galvanized complex phase steel has a yield strength ≥740Mpa, a tensile strength ≥800Mpa, an A80 elongation ≥14%, and a hole expansion rate ≥60%.

[0011] The above-mentioned low-cost high yield strength ratio 800MPa grade hot-dip galvanized complex phase steel has a yield strength ratio of ≥0.925.

[0012] The method for preparing the low-cost, high-yield-to-strength ratio 800MPa grade hot-dip galvanized complex phase steel comprises the steps of hot rolling, pickling, annealing galvanizing and leveling.

[0013] Furthermore, in the above preparation method, the hot rolling process includes heating the ingot, dephosphorization, rough rolling, finish rolling, cooling and coiling.

[0014] Furthermore, the above hot rolling process is specifically as follows: the heating temperature of the ingot is 1180℃~1280℃; the hot rolling start temperature is 1000℃~1100℃, and the final rolling temperature is 850℃~930℃; the cooling adopts the front-stage upper and lower concentrated cooling process; the coiling temperature is 450~530℃, and slow cooling is carried out after coiling.

[0015] Furthermore, in the above preparation method, the pickling process is specifically as follows: the straightening elongation is 0.2% to 2.0%; the belt speed in the pickling process section is 50 to 120 m / min; and the liquid temperature in the pickling tank is 60 to 90°C.

[0016] Furthermore, in the above preparation method, the annealing and galvanizing process is specifically as follows: after pickling, the steel coil is heated to 530-630°C at a rate of 0.8-2.0°C / s, and the heat-averaging and heat-keeping time is 80-150s; then the strip is cooled to 440-500°C at a rate of 8-15°C / s, kept warm for 30-70s, and then enters the zinc pot for hot-dip galvanizing; after exiting the zinc pot, it is cooled to room temperature at a rate of 3-8°C / s.

[0017] Furthermore, in the above annealing and galvanizing process, the belt speed of the unit is controlled to be 40-70 m / min.

[0018] Furthermore, in the above preparation method, the leveling process is specifically as follows: controlling the skin-pass reduction of the steel strip after it exits the zinc pot to be 0.3% to 1.0%.

[0019] The beneficial effects of the present invention are as follows: the present invention provides a low-cost, high-yield ratio 800MPa grade hot-base galvanized complex phase steel, the hot-base galvanized complex phase steel does not add precious alloy elements Mo, Cr, Nb, saves a lot of alloy costs, and has good economic benefits. The present invention also provides a preparation method of a low-cost, high-yield ratio 800MPa grade hot-base galvanized complex phase steel, which adopts an ultra-low temperature annealing process in the annealing process section, can significantly reduce energy consumption to achieve cost savings, and has good economic benefits and carbon reduction effects.

[0020] The yield strength of the hot-base galvanized complex phase steel produced by the method of the present invention is ≥740Mpa, the tensile strength is ≥800Mpa, the A80 elongation is ≥14%, and the hole expansion rate is ≥60%. The hot-base galvanized complex phase steel produced by the present invention has good stamping forming performance and can meet the flanging forming of chassis parts; at the same time, the hot-base galvanized complex phase steel has a high yield strength, which can ensure the strength of chassis parts and improve the safety factor of passenger cars, achieving low cost and high efficiency. The method of the present invention has the value of popularization and application. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 The microstructure of the hot-dip galvanized complex phase steel obtained in Example 1;

[0022] Figure 2 The microstructure of the hot-dip galvanized complex phase steel obtained in Example 2. DETAILED DESCRIPTION

[0023] The technical solution of the present invention can be specifically implemented in the following manner.

[0024] The low-cost, high-yield-to-strength ratio 800MPa grade hot-dip galvanized complex phase steel has the following chemical compositions by mass percentage: C 0.060%-0.150%, Si 0.10%-0.35%, Mn 1.00%-2.90%, P≤0.020%, S≤0.01%, Als 0.01%-0.07%, Ti 0.10%-0.20%, N≤0.050%, and the remaining elements are Fe and unavoidable impurities.

[0025] Among them, C, Si, and Mn elements can improve the strength of the plate. Mn element can improve the hardenability and reduce the critical cooling rate of bainite formation, which is beneficial to the large-scale transformation of bainite during the water cooling process of the hot rolling process. Ti element can inhibit the excessive growth of the original size and form precipitates such as TiC to pin the grain boundaries, refine the grains, and improve the strength and plasticity of the plate.

[0026] The method for preparing low-cost high-yield ratio 800MPa grade hot-dip galvanized complex phase steel includes hot rolling, pickling, annealing galvanizing and leveling processes, as follows:

[0027] 1) Hot rolling process: heating, dephosphorization, rough rolling, finish rolling, cooling and coiling of the ingot;

[0028] ①Heating temperature 1180℃~1280℃;

[0029] ②Hot rolling starting temperature is 1000℃~1100℃, and final rolling temperature is 850℃~930℃;

[0030] ③ Adopt the front section upper and lower centralized cooling process;

[0031] ④The coiling temperature is 450~530℃, and the coiling is followed by slow cooling equipment.

[0032] 2) Pickling process:

[0033] ① The elongation of tension straightening is 0.2% to 2.0%;

[0034] ② The belt speed in the pickling process section is 50-120 m / min;

[0035] ③The liquid temperature of the pickling tank is controlled at 60~90℃.

[0036] 3) Annealing and galvanizing process:

[0037] ① After pickling, the steel coil is heated to 530-630℃ at a rate of 0.8-2.0℃ / s, and the soaking time is 80-150s;

[0038] ②Then the strip is cooled to 440-500℃ at a rate of 8-15℃ / s and kept at this temperature for 30-70s before entering the zinc pot for hot-dip galvanizing;

[0039] ③ After leaving the zinc pot, cool to room temperature at a rate of 3-8℃ / s;

[0040] ④The belt speed of this process section is 40~70m / min.

[0041] 4) Leveling process: After the strip comes out of the zinc pot, the leveling reduction is 0.3% to 1.0%.

[0042] Among them: (1) the heating temperature is 1180℃~1280℃, the hot rolling start temperature is 1000℃~1100℃, and the final rolling temperature is 850℃~930℃, the purpose is to make the steel strip structure in full austenitization during the rolling process; (2) the front-end upper and lower concentrated cooling process and the coiling temperature are 450~530℃, the purpose is to enable the steel strip to undergo bainite phase transformation immediately after rolling and to ensure that the bainite phase transformation is complete; (3) the pickling process is to remove the iron oxide scale; (4) the annealing process temperature is controlled at 530~630℃ to promote the homogenization of the structure grains and ensure the mechanical properties of the steel strip. If the annealing temperature is too high, it will lead to secondary precipitation strengthening of titanium element and excessive strength; (5) the galvanizing process ensures the quality of the steel strip coating.

[0043] The microstructure of the low-cost, high-yield ratio 800MPa grade hot-dip galvanized complex phase steel prepared by the above preparation process consists of 55-65% bainite, 25-35% ferrite, and 5-15% martensite + residual austenite; its yield strength is ≥740Mpa, tensile strength is ≥800Mpa, A80 elongation is ≥14%, and hole expansion rate is ≥60%.

[0044] The technical solution and effects of the present invention are further illustrated below through practical examples.

[0045] Example

[0046] This embodiment provides two groups of embodiments 1-2 and comparative examples 1-3 of hot-dip galvanized complex phase steel prepared by the preparation method of the present invention; the chemical compositions thereof are shown in Table 1, and the remainder is iron and unavoidable impurities.

[0047] Table 1 Chemical composition of hot-dip galvanized steel (in mass percentage, %)

[0048] serial number C Si Mn P S Als Cr Ti Nb Mo N Example 1 0.083 0.18 1.85 0.01 0.002 0.032 — 0.16 — — 0.028 Example 2 0.090 0.25 2.00 0.01 0.005 0.040 — 0.15 — — 0.033 Comparative Example 1 0.090 0.25 2.00 0.01 0.005 0.040 — 0.15 — — 0.033 Comparative Example 2 0.080 0.26 1.80 0.009 0.003 0.043 0.61 0.036 0.032 0.27 0.030 Comparative Example 3 0.100 0.36 1.87 0.011 0.004 0.036 0.62 0.042 0.033 0.23 0.029

[0049] In Examples 1 to 2 and Comparative Examples 1 to 3, ingots were smelted according to the chemical composition in Table 1, and hot-base galvanized complex phase steel was prepared through the process of hot rolling (ingot heating - dephosphorization - rough rolling - finishing rolling - cooling - coiling) - pickling - annealing galvanizing - leveling. The specific process parameters are shown in Table 2.

[0050] Table 2 Preparation process of hot-dip galvanized composite steel

[0051]

[0052]

[0053] The microstructure of the hot-dip galvanized composite steel obtained in Examples 1 and 2 is as follows: Figure 1 , Figure 2 As shown by Figure 1-2 It can be seen that the microstructure of the hot-dip galvanized complex phase steel prepared by the method of the present invention consists of 60% bainite, 30% ferrite, and 10% martensite + residual austenite.

[0054] The mechanical properties of the hot-dip galvanized composite steels prepared in the examples and comparative examples were tested, and the results are shown in Table 3.

[0055] Table 3 Mechanical properties

[0056] serial number Thickness / mm Yield strength / MPa Tensile strength / MPa Elongation A80 / % Yield strength ratio / % Hole expansion rate / % Example 1 2.7 777 820 15.5 94.8 62.5 Example 2 3.0 790 830 15.0 95.2 65.2 Comparative Example 1 3.0 843 893 14.5 94.4 35.7 Comparative Example 2 2.7 648 836 16.5 77.5 63 Comparative Example 3 3.0 647 840 16.0 77.0 69

[0057] It can be seen from Tables 1 to 3 that the hot-dip galvanized complex phase steels prepared by the process of the present invention in Examples 1 to 2 have higher yield strength ratio and hole expansion; although Comparative Example 1 adopts the composition design of the present invention, the final hole expansion rate is poor due to the excessively high soaking temperature; Comparative Documents 2 to 3 use precious metals such as Cr, Nb, and Mo, and simultaneously use high-temperature coiling, high-temperature soaking, and a higher unit speed. Although the steel finally obtained has a higher hole expansion rate, the yield strength ratio is relatively low.

Claims

1. Low-cost, high-yield-strength ratio 800MPa grade hot-dip galvanized composite steel, characterized in that: Its chemical composition by mass percentage is: C 0.060%~0.150%, Si 0.10%~0.35%, Mn 1.00%~2.90%, P≤0.020%, S≤0.01%, Als 0.01%~0.07%, Ti 0.10%~0.20%, N≤0.050%, and the remaining elements are Fe and unavoidable impurities.

2. The low-cost, high-yield-strength ratio 800MPa grade hot-dip galvanized composite steel according to claim 1, characterized in that: Its microstructure consists of 55-65% bainite, 25-35% ferrite, and 5-15% martensite + retained austenite.

3. The low-cost, high-yield-strength ratio 800MPa grade hot-dip galvanized composite steel according to claim 1, characterized in that: Its yield strength is ≥740Mpa, tensile strength is ≥800Mpa, A80 elongation is ≥14%, and hole expansion rate is ≥60%.

4. The low-cost, high-yield-strength ratio 800MPa grade hot-dip galvanized composite steel according to claim 1, characterized in that: Its yield strength ratio is ≥0.

925.

5. The method for preparing the low-cost, high-yield-to-strength ratio 800MPa grade hot-dip galvanized composite steel according to any one of claims 1 to 4, characterized in that: It includes hot rolling, pickling, annealing, galvanizing and smoothing processes; Among them, the annealing and galvanizing process is specifically as follows: after pickling, the steel coil is heated to 530-630°C at a speed of 0.8-2.0°C / s, and the heat-averaging and heat-keeping time is 80-150s; then the strip is cooled to 440-500°C at a speed of 8-15°C / s, kept warm for 30-70s, and then enters the zinc pot for hot-dip galvanizing; after leaving the zinc pot, it is cooled to room temperature at a speed of 3-8°C / s.

6. The method for preparing low-cost, high-yield ratio 800MPa grade hot-dip galvanized composite steel according to claim 5, characterized in that: The hot rolling process includes slab heating, dephosphorization, rough rolling, finish rolling, cooling and coiling.

7. The method for preparing low-cost, high-yield-ratio 800MPa grade hot-dip galvanized composite steel according to claim 6, characterized in that: The specific hot rolling process is as follows: the heating temperature of the ingot is 1180℃~1280℃; the hot rolling start temperature is 1000℃~1100℃, and the final rolling temperature is 850℃~930℃; the cooling adopts the front-stage upper and lower concentrated cooling process; the coiling temperature is 450~530℃, and slow cooling is carried out after coiling.

8. The method for preparing low-cost, high-yield-ratio 800MPa grade hot-dip galvanized composite steel according to claim 5, characterized in that: The specific pickling process is as follows: the straightening elongation is 0.2% to 2.0%; the belt speed in the pickling process section is 50 to 120 m / min; and the liquid temperature in the pickling tank is 60 to 90°C.

9. The method for preparing low-cost, high-yield-ratio 800MPa grade hot-dip galvanized composite steel according to claim 5, characterized in that: In the annealing and galvanizing process, the belt speed of the unit is controlled to be 40-70m / min.

10. The method for preparing low-cost, high-yield ratio 800MPa grade hot-dip galvanized composite steel according to claim 5, characterized in that: The specific leveling process is: controlling the skin-clearing reduction of the strip after it leaves the zinc pot to be 0.3% to 1.0%.

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