800MPa-grade high-strength hot-based galvanized steel and short-process production method

Abstract

The invention discloses 800MPa-grade high-strength hot-based galvanized steel and a short-process production method, molten steel obtained through smelting and refining is cast into a plate blank at the pulling speed of 4-6 m / min, and the temperature of the molten steel is smaller than or equal to the sum of the liquidus temperature and 10 DEG C; the plate blank is heated by a soaking pit furnace, subjected to finish rolling and layer cooling control cooling and then coiled to obtain a hot rolled steel strip; and performing cold deformation on the hot rolled steel strip after surface pickling, annealing the deformed steel strip in an annealing furnace, and putting the annealed steel strip into a zinc pot for galvanizing. The prepared hot-rolled strip steel obtains a fine and uniform ferrite and pearlite structure, the ferrite content is 60-80%, and the pearlite content is 15-35%; due to the large ferrite content in the steel, the material has high plasticity and low-temperature toughness, the elongation at break is larger than or equal to 12%, and the material has good cold forming performance; meanwhile, high-content nanoscale TiC is dispersed and separated out, and the high strength that the yield strength of the steel strip is larger than or equal to 700 MPa and the tensile strength is larger than or equal to 800 MPa is guaranteed.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of low alloy steel production, and particularly relates to an 800MPa high-strength hot-base galvanized steel and a short-process production method. BACKGROUND

[0002] In order to improve the corrosion resistance of the material, it has become a common means to be plated with zinc or zinc-aluminum-magnesium on the surface. The conventional way of producing zinc plating or zinc-aluminum-magnesium plating is cold-base galvanizing, that is, after the hot-rolled base material is pickled, it is cold-rolled to the target thickness, annealed, and then immersed in a zinc pot to complete the production of the surface coating. However, for the demand of high-strength galvanized material, there are problems such as great difficulty in cold rolling deformation, excessive load on the rolling mill, etc. Therefore, hot-base galvanizing becomes the choice. Compared with cold-base galvanizing, the main difference is that after the hot-rolled base material is directly pickled to remove the surface iron oxide scale, it is directly annealed and galvanized, thereby the cold rolling process is omitted, and the manufacturing cost is greatly reduced. In order to meet the specification requirements of the finished galvanized material, the hot-rolled base material for hot-base galvanizing also needs to be correspondingly thinned.

[0003] Short-process manufacturing is increasingly attracting attention because of its simple, efficient and energy-saving manufacturing process, high dimensional accuracy, and product characteristics of high performance uniformity. The biggest advantage of short-process production line manufacturing is to produce thin-gauge materials, which can be matched with hot-base galvanizing. In the existing research results, all the disclosed materials are of a strength grade of 600MPa or below, and some high-strength materials are made by relying on high Mn or adding a large amount of expensive alloys such as Nb, Mo and Cr, which has high manufacturing cost. Therefore, how to manufacture an economically applicable high-strength hot-base galvanized material to meet the high-strength lightweight and surface corrosion-resistant requirements of various industries is undoubtedly of great significance. SUMMARY

[0004] The present application provides an 800MPa high-strength hot-base galvanized steel and a short-process production method with high strength, high elongation and low cost, and can meet the processing and use requirements such as rolling and bending forming.

[0005] To achieve the above-mentioned purpose, the present application provides an 800MPa high-strength hot-base galvanized steel short-process production method. The molten steel obtained by smelting and refining is cast into a slab at a casting speed of 4-6m / min, and the temperature of the molten steel is ≤liquidus temperature+10℃, so as to inhibit the formation and growth of columnar crystals, and make the as-cast structure equiaxed crystal rate ≥70%, thereby ensuring that a uniform and fine structure is obtained after rolling, and the segregation of elements such as Mn and P is reduced, so that the element distribution of each part of the slab is relatively uniform. After the slab is heated in a soaking furnace, it is precisely rolled, the cooling is controlled by layer cooling, and then it is coiled to obtain a hot-rolled steel strip. After the surface of the hot-rolled steel strip is pickled to remove the surface iron oxide scale, cold deformation is carried out after the cold rolling unit or the flattening unit, the deformed steel strip is annealed in a continuous annealing furnace, and the annealed steel strip is galvanized in a zinc pot.

[0006] Further, the chemical elements of the slab include C: 0.165-0.25%, Si: 0.10-0.30%, Mn: 0.61-0.99%, P≤0.018%, S≤0.005, Ti: 0.13-0.30%, Als: 0.015-0.10%, N≤0.006, Ca: 0.0010-0.0025 by weight percentage, and satisfy 0.5≤Mn*Ti / C≤1.5, the balance being Fe and unavoidable impurity elements; according to actual requirements, 0.20-0.40% of Cr or ≤0.03% of Nb can be supplemented and added; and satisfy 0.7≤(Cr+Mn)*Ti / C≤1.5.

[0007] The reasons for limiting the chemical composition of the application are as follows: C is an effective strengthening element in steel, in addition to solid solution strengthening, it can also form nanoscale second phase precipitated particles with Ti, Nb and other micro-alloy elements, and play the role of precipitate strengthening and refining the structure. Considering the influence of peritectic reaction on the surface quality of the continuous casting billet in the high-speed continuous casting process, the C content is designed to be 0.165-0.25% in the application.

[0008] Si is a commonly used deoxidizing element in steel, which also has a solid solution strengthening effect on steel, and can affect the FeZn reaction in the galvanizing process. Conventional galvanizing materials will choose a low-silicon design of <0.04%, but the smelting cost is too high, so the Si content is designed to be 0.10-0.30%; Mn is an important strengthening element in steel, which has a solid solution strengthening effect, can also reduce the undercooling austenite transformation temperature and ferrite phase transformation temperature, is beneficial to the refinement of the structure and the improvement of the strength and toughness of the material. However, too much Mn content will inhibit the ferrite transformation, the structure will be transformed into bainite, and the plasticity and cold forming performance of the material will be reduced. Therefore, the Mn content is designed to be 0.61%-0.99% in the application. P is a harmful impurity element in steel, and too high content will easily segregate in the center of the slab during continuous casting, reducing the forming performance of the material. The application requires that the P content be ≤0.018%; S is a common harmful impurity element in steel, which has adverse effects on the low-temperature toughness, welding performance, cold forming performance and other aspects of the material, so the S content is required to be ≤0.005%.

[0009] Ti is a strong carbonitride forming element that can precipitate as extremely fine TiC or Ti(C,N) second-phase particles, significantly improving the strength of materials. Ti is relatively inexpensive compared to microalloying elements such as Nb, and therefore is added as an important strengthening element in this patent. Simultaneously, the precipitation of TiC significantly reduces the formation of larger carbides or pearlite from free carbon, resulting in a uniform and fine microstructure. However, excessive addition of Ti gradually weakens the precipitation strengthening effect and begins to significantly affect the low-temperature toughness of the steel. Therefore, the Ti content is designed to be 0.13-0.30%.

[0010] Nitrogen (N) is an impurity element in steel. When N combines with Ti in molten steel, it can form large-sized TiN inclusions. This reduces the effective content of Ti and also significantly impairs the toughness of the steel. Therefore, the content of N should be controlled as low as possible. This invention requires that the N content be ≤0.006%.

[0011] Ca can form spherically dispersed CaS with S, thereby improving the distribution of sulfide inclusions in steel, which can improve the uniform corrosion of materials and increase the toughness of materials. The Ca content is generally limited to 0.0010-0.0025%.

[0012] Cr can improve the hardenability of materials. It also has a certain solid solution strengthening effect. Furthermore, Cr can react with carbon to produce Cr3C metallic compounds, which have high strength and hardness. When distributed in particulate form, they can greatly improve the strength, toughness, and wear resistance of materials. In this invention, 0.20-0.40% Cr can be selectively added.

[0013] Nb is also a strong carbonitride forming element. Its metal compounds can increase the recrystallization temperature of materials and obtain a more uniform and fine structure. At the same time, it has a certain precipitation strengthening effect. The addition of Nb can improve the strength and toughness of materials, but the cost of alloys is relatively high. This invention considers adding ≤0.03%.

[0014] There are certain interactive relationships among alloying elements. Solid solution strengthening elements such as Mn and Cr can increase the strength of the ferrite matrix and inhibit TiC precipitation, thereby reducing the TiC precipitation strengthening effect. Relatively speaking, the effect of solid solution strengthening on yield strength is significantly lower than that of precipitation strengthening. This invention designs a material with high yield strength. In order to better ensure the TiC precipitation strengthening effect, based on research, it is determined that 0.5≤Mn*Ti / C≤1.5 and 0.7≤(Cr+Mn)*Ti / C≤1.5.

[0015] Furthermore, the temperature of the slab entering the soaking furnace is 900-1000℃ and the temperature of exiting the furnace is 1220-1250℃. High-temperature heating promotes the solid solution of titanium.

[0016] Furthermore, the slab thickness entering the finishing mill is 63-70mm, the thickness exiting is 1.5-3mm, and the cumulative deformation rate during the finishing rolling stage is ≥95%. During the rolling process, the descaling water between the mills is shut off to ensure a temperature difference of ≤30℃ between the edge and center of the strip, and the final rolling temperature is 880-930℃ (preferably 910-930℃). After entering laminar flow cooling, a staged controlled cooling process is adopted; the first stage of cooling rapidly cools to 630-680℃ at a rate of 90-100℃ / s, followed by air cooling for 8-15s, and then the second stage of cooling proceeds at a rate of 15-20℃ / s, with a cooling termination temperature of 560-600℃. After coiling, the strip is sent to a 600℃ insulated pit for slow cooling for 12-24 hours to obtain hot-rolled strip. This process omits the deformation process in the roughing stage, utilizing a hot continuous rolling mill to achieve rapid, large deformation processing, fully breaking down the as-cast microstructure. To ensure the safety and efficiency of the rolling process, the finishing rolling temperature is designed to be 880-930℃, with an optimal temperature of 910-930℃. Furthermore, the high finishing rolling temperature increases the temperature gradient in the laminar cooling process, facilitating high-rate cooling. Laminar cooling employs a two-stage controlled cooling process. The first stage uses a high cooling rate to increase the undercooling of the deformed austenite, promoting the transformation of austenite into ferrite phase nuclei and achieving the formation of a fine and uniform ferrite microstructure. The first stage final cooling at 630-680℃ avoids the growth of the formed ferrite grains. Simultaneously, air cooling for ≥8s allows for some recovery of the dislocation structure within the ferrite, reducing structural stress, and improving elemental distribution through carbon diffusion. The second stage cooling reaches a temperature range of 560-600℃, followed by slow cooling and holding for ≥12 hours, achieving fine and dispersed precipitation of TiC, NbC, etc., resulting in high material strength.

[0017] Furthermore, after the hot-rolled substrate is pickled to remove iron oxide scale, it undergoes 6-18% cold deformation in a cold rolling mill or leveling mill, with the optimal deformation degree being 9-13.5%. Its main functions are, on the one hand, to eliminate the influence of surface defects and improve the surface quality and flatness of the steel strip; on the other hand, through a certain degree of cold deformation, without flattening the original rolled structure, a large number of substructures are formed inside the structure by utilizing the precipitation pinning effect, thereby improving the distortion energy.

[0018] Furthermore, the specific process of annealing the deformed steel strip in the annealing furnace is as follows: the heating temperature in the direct heating section is 750-830℃, and the temperature in the annealing section is 620-720℃. The annealed steel strip enters the zinc pot for galvanizing / zinc-aluminum-magnesium plating at a temperature of 450±20℃, and the speed of the steel strip is ≥150*T, where T is the thickness of the steel strip. Through annealing, the cold-deformed substructure becomes a new nucleation point, resulting in a phase transformation of the microstructure, further refining the microstructure and improving the material strength. If the annealing temperature is too high (≥720℃), it is easy to cause grain growth and coarsening, which reduces the strength; while if the temperature is too low, the microstructure will be in an incomplete annealing state, leaving residual high-stress microstructure from cold deformation, which can easily become an internal crack source during subsequent actual use, affecting the product's forming performance.

[0019] Also provided is an 800MPa grade short-process high-strength hot-dip galvanized steel, prepared by the production method described above. The hot-dip galvanized steel, obtained using fewer alloying elements, exhibits high strength and formability: yield strength ≥700MPa, tensile strength ≥800MPa, elongation at break ≥12%, and cold bending performance that meets the requirement of d=1t, 180° bending qualification (D is the bending diameter, t is the steel plate thickness).

[0020] Compared with the prior art, the beneficial effects of the present invention are as follows: the hot-rolled strip steel prepared by the production method of the present invention obtains a fine and uniform ferrite + pearlite (or carbide) microstructure, with a ferrite content of 60%-80%, a pearlite content of 15%-35%, and the balance being carbides; and the large ferrite content in the steel gives the material high plasticity and low-temperature toughness, with a fracture elongation of ≥12%, giving the material excellent cold forming performance; at the same time, the high content of nano-sized TiC dispersed precipitation ensures high strength of the steel strip with a yield strength of ≥700MPa and a tensile strength of ≥800MPa. Attached Figure Description

[0021] Figure 1 A typical microstructure diagram of a 13% cold-deformation annealed finished product; Figure 2 This is a typical microstructure diagram of a finished product after 18% cold deformation annealing. Detailed Implementation

[0022] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0023] Based on the above design requirements and process scheme, the invention steel was specifically implemented to further illustrate the technical solution of the present invention.

[0024] The steel billet was obtained through converter smelting, refining, and continuous casting, and its chemical composition is shown in Table 1. The billet was then heated in a furnace, hot-rolled, controlled-cooled, coiled, pickled, cold-deformed, annealed, and galvanized / zinc-aluminum-magnesium coated to obtain the finished product. The corresponding production process is shown in Table 2. The finished product underwent performance testing, and the relevant properties are shown in Table 3. The metallographic structure of the invented steel was prepared, and typical microstructures are shown in Table 3. Figure 1 , Figure 2 , Figure 1 , Figure 2 The microstructures of the finished products, after annealing following 13% and 18% cold deformation, respectively, both exhibit a uniform and fine ferrite + pearlite + carbide microstructure. The uniform microstructure dominated by ferrite ensures the material's plasticity and machinability. Through small deformation followed by annealing, the structure of the hot-rolled microstructure is optimized. The high C and Ti alloy content leads to high-concentration TiC precipitation, pinning dislocations and forming numerous substructures, further refining the microstructure and enhancing the material's strength.

[0025] Table 1 Chemical composition of the products produced in the embodiments of the present invention Table 2 Process parameters for production in the embodiments of the present invention Table 3. Strip steel production performance according to embodiments of the present invention

Claims

1. A short-process production method for 800MPa grade high-strength hot-dip galvanized steel, characterized in that: Molten steel obtained through smelting and refining is cast into slabs at a casting speed of 4~6m / min, and the temperature of the molten steel is ≤ liquidus temperature +10℃. After the slabs are heated in a soaking furnace, they are finished rolled, cooled in a controlled layer, and then coiled to obtain hot-rolled steel strips. The hot-rolled steel strips are cold-deformed after pickling, and the deformed steel strips are annealed in an annealing furnace. The annealed steel strips are then galvanized in a zinc pot.

2. The short-process production method for 800MPa grade high-strength hot-dip galvanized steel according to claim 1, characterized in that: The chemical elements of the slab, by weight percentage, include C: 0.165-0.25%, Si: 0.10-0.30%, Mn: 0.61-0.99%, P≤0.018%, S≤0.005%, Ti: 0.13-0.30%, Als: 0.015-0.10%, N≤0.006%, and Ca: 0.0010-0.0025, and satisfy the following conditions: The balance consists of Fe and unavoidable impurity elements.

3. The short-process production method for 800MPa grade high-strength hot-dip galvanized steel according to claim 2, characterized in that: The slab also contains 0.20-0.40% Cr or ≤0.03% Nb; and satisfies the following conditions: .

4. The short-process production method for 800MPa grade high-strength hot-dip galvanized steel according to claim 1, characterized in that: The temperature of the slab entering the homogenization furnace is 900-1000℃, and the temperature exiting the furnace is 1220-1250℃.

5. The short-process production method for 800MPa grade high-strength hot-dip galvanized steel according to claim 1, characterized in that: The slab thickness entering the finishing mill is 63-70mm and the thickness exiting the mill is 1.5-3mm. The cumulative deformation rate during the finishing rolling stage is ≥95%. During the rolling process, the dephosphorization water in the mill is shut off to ensure that the temperature difference between the edge and the middle of the steel strip is ≤30℃. The final rolling temperature is 880-930℃.

6. The short-process production method for 800MPa grade high-strength hot-dip galvanized steel according to claim 1, characterized in that: The specific process of coiling after laminar flow cooling is as follows: after entering the laminar flow cooling, a staged controlled cooling process is adopted; the first stage of cooling is rapidly cooled to 630-680℃ at a cooling rate of 90-100℃ / s, followed by air cooling for 8-15s, and then the second stage of cooling is carried out at a cooling rate of 15-20℃ / s, with a cooling termination temperature of 560-600℃. After the steel strip is coiled, it is sent to a heat preservation pit with an environment of 600℃ for slow cooling for 12-24 hours to obtain hot-rolled steel strip.

7. The short-process production method for 800MPa grade high-strength hot-dip galvanized steel according to claim 1, characterized in that: The specific process of annealing the deformed steel strip in the annealing furnace is as follows: the heating temperature of the direct heating section is 750-830℃, and the temperature of the annealing section is 620-720℃.

8. The short-process production method for 800MPa grade high-strength hot-dip galvanized steel according to claim 1, characterized in that: The annealed steel strip enters the zinc bath for galvanizing / zinc-aluminum-magnesium plating at a temperature of 450±20℃, and the speed of the steel strip... T represents the thickness of the steel strip.

9. The short-process production method for 800MPa grade high-strength hot-dip galvanized steel according to claim 1, characterized in that: The amount of cold deformation is 6-18%.

10. A high-strength hot-dip galvanized steel of 800MPa grade, characterized in that: The 800MPa grade high-strength hot-dip galvanized steel is prepared by the short-process production method described in claim 1.

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