Hot-dip galvanized high-strength steel plate with excellent cold bending and broaching performance and production method of hot-dip galvanized high-strength steel plate

Abstract

The invention provides a hot-dip galvanized high-strength steel plate with excellent cold bending and reaming performance and a production method thereof. The hot-dip galvanized high-strength steel plate comprises the following components: 0.07-0.12% of C; 0.30% to 0.50% of Si; mn: 2.0 to 3.0%; 0.3 to 0.6 percent of Cr; 0.04-0.10% of Nb + Ti + V and less than or equal to 0.04% of Ti; 0.03% to 0.06% of Als; less than or equal to 0.015% of P; s: less than or equal to 0.04%; n is less than or equal to 0.005%, B is 0.0001-0.001%, and the balance is Fe and inevitable impurities. Compared with the prior art, proper tissue and mechanical properties are obtained through reasonable components and process design; and a semi-decarburized layer with the thickness of 20-40 microns is formed on the surface of a matrix through dew point control of a galvanizing annealing furnace, the transverse and longitudinal 180-degree cold bending center diameter of a final finished product reaches 0T (T is the plate thickness), and the hole expansion rate is larger than or equal to 40%.

Description

Technical Field

[0001] This invention belongs to the field of cold-rolled hot-dip galvanized automotive steel manufacturing, specifically relating to a hot-dip galvanized high-strength steel sheet with excellent cold bending and hole expansion properties and its production method. Background Technology

[0002] Energy conservation, environmental protection, and safety are the three major challenges facing the automotive industry today. Implementing lightweighting in automobiles can reduce vehicle weight and energy consumption without sacrificing safety performance; it will also improve vehicle handling stability and collision safety to a certain extent. With the advancement of automotive lightweighting and the development of automotive steel, high-strength steel, especially advanced high-strength steel, is increasingly widely used in automobiles. Ferritic / martensitic dual-phase steel with a dual-phase structure is the most representative. Currently, cold-rolled and galvanized dual-phase steel products with tensile strengths below 800MPa are very mature and have achieved large-scale application. Through reasonable composition and process design, measures such as increasing the martensite content and refining the grain size can yield higher-strength dual-phase steel products. Higher-strength grades, such as 1000MPa cold-rolled and galvanized dual-phase steel, have also achieved mass application.

[0003] With the rise of new energy vehicles in my country, steel battery pack trays have provided new application scenarios and market demand for cold-formed ultra-high strength steel. The side beams and reinforcing beams of steel battery pack trays are usually produced by roll forming of 1000MPa grade duplex steel, and roll forming places higher demands on the bending performance of the product.

[0004] The microstructure of duplex steel determines its excellent comprehensive mechanical properties, extremely low yield strength ratio, and good elongation, giving it a certain ability to be drawn and formed. However, when applied to high-strength safety components primarily subjected to cold bending, defects such as cracking and necking at the radius (R-angle) are prone to occur. This is because, to achieve the required strength level, high-strength duplex steel inevitably incorporates alloying elements such as C and Mn. These elements segregate during casting and hot rolling, resulting in uneven microstructure or even banded structures in the finished product, reducing the material's local deformation capacity and severely affecting its cold bending performance. Therefore, for duplex steel products with a strength of 1000 MPa or higher, the main technical challenge is improving the material's local formability, such as cold bending and hole expansion rate.

[0005] The patent document published on October 18, 2022, with publication number CN115198173A and application number 202210681899.2, provides 980MPa grade hot-dip galvanized multiphase steel, its steel matrix, and preparation method. The chemical composition of the steel matrix disclosed therein, in mass fraction, includes: C: 0.08%-0.16%, Si: 0.05%-0.1%, Mn: 1.8%-2.5%, Al: 0.02%-0.06%, P: 0-0. The composition is as follows: Mn: 0.008%, S: 0-0.001%, Cr: 0.05%-0.2%, Mo: 0.2%-0.6%, V: 0.001%-0.08%, Ti: 0.02%-0.12%, with the remainder being Fe and unavoidable impurities. The composite strengthening method using Mn, Cr, Mo, V, and Ti not only ensures a tensile strength of 980 MPa or higher for the multiphase steel but also improves the yield strength and yield-to-tensile ratio, achieving good local forming properties and a hole expansion rate exceeding 66%. However, the presence of large amounts of precious metals such as Mo and V increases manufacturing costs.

[0006] Therefore, it is essential to provide a duplex steel product that is low in cost and has excellent cold bending and hole expansion properties. Summary of the Invention

[0007] The purpose of this invention is to provide a hot-dip galvanized high-strength steel sheet with excellent cold bending and hole-expanding properties, and its production method. Through reasonable composition and process design, the content and ratio of ferrite and martensite / bainite in the microstructure are ensured, and the dispersion and distribution of martensite / bainite and ferrite are controlled, thereby obtaining a 1000MPa grade hot-dip galvanized high-strength steel product with excellent mechanical properties. This invention not only achieves excellent cold bending and hole-expanding properties, but also does not contain precious metals such as Mo and V, resulting in low cost.

[0008] The specific technical solution of this invention is as follows:

[0009] The present invention provides a hot-dip galvanized high-strength steel sheet with excellent cold bending and hole expansion properties, the matrix of which comprises the following components by mass percentage:

[0010] C: 0.07-0.12%; Si: 0.30-0.50%; Mn: 2.0-3.0%; Cr: 0.3-0.6%; Nb+Ti+V: 0.04-0.10% and Ti≤0.04%; Als: 0.03-0.06%; P: ≤0.015%; S: ≤0.01%; N: ≤0.005%; B: 0.0001-0.001%, with the balance being Fe and unavoidable impurities.

[0011] The hot-dip galvanized high-strength steel sheet with excellent cold bending and hole expansion properties has a ferrite volume content of 30-50% and a hard phase structure of martensite + bainite volume content of 50-70%.

[0012] The hot-dip galvanized high-strength steel sheet with excellent cold bending and hole expansion properties meets the following mechanical properties: yield strength of 590MPa-730MPa, tensile strength ≥980MPa, and elongation A. 80 ≥10%.

[0013] Preferably, the hot-dip galvanized high-strength steel sheet with excellent cold bending and hole expansion properties has the following mechanical properties: tensile strength ≥ 1000 MPa, elongation A 80 ≥14%.

[0014] The hot-dip galvanized high-strength steel sheet with excellent cold bending and hole expansion properties is formed on the substrate surface by controlling the dew point of the galvanizing annealing furnace, with a thickness of 20-40μm. The microhardness of the substrate is 310-360HV, and the surface semi-decarburized layer has a hardness of 200-280HV. The difference between the hardness of the surface semi-decarburized layer and the hardness of the substrate is more than 60HV.

[0015] The finished hot-dip galvanized high-strength steel sheet with excellent cold bending and hole expansion properties has a 180° cold bending mandrel diameter of 0T (T is the plate thickness) and a hole expansion rate of ≥40%.

[0016] The present invention provides a method for producing hot-dip galvanized high-strength steel sheets with excellent cold bending and hole expansion properties, including smelting, hot continuous rolling, pickling and cold rolling, and continuous annealing and galvanizing.

[0017] The smelting process involves smelting according to the mass percentage of each element in the above-mentioned composition system and casting it into a slab.

[0018] The hot continuous rolling process utilizes cast slabs or ingots. Specifically, the cast slab is heated to 1200-1250℃ and rolled to a thickness of 30-40mm using a roughing mill. It is then continuously rolled in 5-7 passes to a thickness of 2.0-6.0mm. The initial rolling temperature is 1000-1150℃, and the final rolling temperature is 870-930℃. A final rolling temperature lower than the Ar3 temperature will result in rolling in the ferrite + austenite two-phase region, leading to excessively high rolling load and uneven material properties and large performance fluctuations in the hot-rolled steel sheet. Subsequently, the steel is coiled into coils at a laminar flow cooling temperature within the range of 540-600℃, with rapid initial cooling. Excessively high coiling temperatures result in uneven microstructure and properties of the hot-rolled sheet, leading to large fluctuations in cold-rolled thickness; excessively low coiling temperatures result in high strength of the hot-rolled coil and increased cold-rolling load.

[0019] The pickling and cold rolling process involves pickling hot-rolled strip steel in a hydrochloric acid bath to remove surface iron oxide scale, followed by continuous cold rolling or cold rolling with a cold rolling reduction rate of 40-65% until the target thickness is reached. If the pickling and rolling reduction rate is too high, the rolling mill equipment will be under heavy load. To ensure that the grains are sufficiently refined after annealing, the pickling and rolling reduction rate should not be too low.

[0020] The continuous annealing and galvanizing process involves: cold-rolled strip steel being slowly heated to 180℃ at a heating rate of 3-6.5℃ / s for preheating; the preheated strip steel is then further heated to 760-800℃. During this process, recrystallization of cold-rolled pearlite and ferrite occurs. Simultaneously, to ensure the coating performance of the zinc layer, the water vapor or oxygen content in the annealing furnace from preheating to heating is adjusted, controlling the dew point temperature range between -20℃ and +20℃, resulting in a 20-40μm semi-decarburized layer (oxide layer) on the strip steel surface. Austenite begins to form when the temperature exceeds Ac1. The heated strip steel is then held at 760-800℃ for 60-120s for two-phase annealing. The heat-insulated strip steel is cooled in two stages: first, it is slowly cooled to 670-710℃ at a cooling rate of 1.5-5℃ / s, and then rapidly cooled to 470-500℃ at a cooling rate of 17.5-38℃ / s; the strip steel is hot-dip galvanized at 450-470℃, and the temperature of the zinc bath is usually controlled at 455-465℃; after exiting the zinc bath, it is cooled to 200℃ or lower at a cooling rate of more than 5℃ / s.

[0021] Furthermore, after galvanizing, alloying is carried out. The alloying process is also applicable to zinc-iron alloy coating products. After the strip steel exits the zinc pot, it is heated to 490-530℃ in an alloying furnace for alloying treatment. After exiting the alloying furnace, it is cooled to 200℃ or lower at a cooling rate of 5℃ / s or higher to obtain zinc-iron alloy coating products.

[0022] The design concept of this invention is as follows:

[0023] Carbon (C): One of the most effective strengthening elements, playing a crucial role in martensite formation. The carbon content in steel determines the strength level and martensite properties of duplex steel. Too low a C content makes it difficult to meet strength requirements, while too high a C content results in excessively high strength but is detrimental to weldability and microstructure uniformity. Considering the design requirements for martensitic strength and hardenability, as well as microstructure uniformity, a lower carbon content is more suitable for duplex steels of this strength level. In this invention patent, the C content is 0.07-0.12%.

[0024] Si plays a role in solid solution strengthening. During heat preservation and slow cooling in the two-phase region, it can effectively promote the diffusion of carbon into austenite, significantly purifying ferrite, improving the purity of ferrite in steel, and stabilizing the austenite structure. On the other hand, when the Si content exceeds a certain limit, high-melting-point oxides are easily formed on the surface of the steel plate, thus affecting the coating effect of the finished product. Therefore, the amount of Si added must be reasonably controlled. In this invention patent, the amount of Si added is 0.3-0.5%.

[0025] Mn: Improves the stability of austenite, thus significantly increasing hardenability. Mn also plays a role in solid solution strengthening and refining ferrite grains. On the other hand, Mn expands the γ-region; while a high manganese content delays the onset of pearlite transformation, it also slows down ferrite precipitation. However, too low a manganese content fails to meet hardenability requirements, while too high a content affects material elongation. In this invention patent, the amount of Mn added is 2.0-3.0%.

[0026] Cr: A medium-strong carbide-forming element, Cr, like Mn, improves the hardenability of steel. When added to steel in combination with other alloying elements, it significantly enhances hardenability, thereby delaying the pearlite and bainite transformations and widening the coiling window. Cr is also a solid solution strengthening element, reinforcing the matrix. On the other hand, excessively high Cr content greatly increases the hardenability of steel, thus significantly increasing its strength, but deteriorating its formability and weldability. In this invention patent, the amount of Cr added is 0.30-0.60%.

[0027] Nb+V+Ti: One or three of the three microalloying elements Nb, V, and Ti can be selectively added. The purpose is to refine the grains and improve toughness by precipitating finely dispersed precipitates during rolling and annealing. However, the amount of Ti added should not be excessive to prevent large TiN particles from affecting the material's toughness. In this invention, Nb+V+Ti is 0.04-0.10%, and Ti ≤ 0.04%.

[0028] Al: Al is an element with deoxidizing properties. Its role in steel is similar to that of Si. Al can also form AlN precipitates, which helps refine the grain size. The presence of a small amount of Al can improve the ductility of duplex steel while maintaining strength. However, excessive Al content can easily clog casting nozzles and cause cracks in the cast billet. In this invention patent, the Al content is 0.03-0.06%.

[0029] P, S, N: To reduce the adverse effects of harmful impurities in steel on its stamping performance, the content of P, S, and N in steel must be strictly controlled.

[0030] B: Adding B at levels above 0.0001% is effective for grain boundary strengthening and significantly improves the hardenability of steel. However, when the content is too high, the manufacturability during hot rolling decreases. Therefore, the amount of B added should be controlled between 0.0001% and 0.001%.

[0031] Compared with existing technologies, this method achieves superior mechanical properties by optimizing the composition and process design. Specifically, it reduces the hardness difference between ferrite and martensite through a low-carbon composition; controls the dispersion and distribution of martensite / bainite and ferrite in the finished product through rapid cooling after hot rolling, avoiding banded structures; and ensures the content and ratio of ferrite and martensite / bainite in the microstructure through a rational hot-dip galvanizing annealing process. This results in a 1000MPa grade hot-dip galvanized high-strength steel product with excellent mechanical properties. The ferrite content in the matrix is ​​30-50%, and the hard phase microstructure (martensite + bainite) accounts for 50-70%. The product's mechanical properties meet the following requirements: yield strength of 590MPa-730MPa, tensile strength ≥980MPa, and elongation A... 80 ≥10%. Simultaneously, through dew point control in the galvanizing annealing furnace, a 20-40μm thick semi-decarburized layer is formed on the substrate surface: the substrate microhardness is 310-360HV, and the surface semi-decarburized layer has a hardness of 200-280HV, with the hardness difference between the surface decarburized layer and the substrate exceeding 60HV. During bending or hole-expanding forming, the deformation degree of the outer surface of the material is much higher than that of the core and inner surface. The low surface hardness gives the material better deformation capacity during local forming such as bending or hole expansion. The final product achieves a 180° cold-bending mandrel diameter of 0T (T is the plate thickness) and a hole expansion rate of ≥40%. Attached Figure Description

[0032] Figure 1 The image shows a typical microstructure of Example 1. The microstructure is uniform and fine, with an average grain size of 1.89 μm, a ferrite volume fraction of 43%, and a martensite volume fraction of 57%.

[0033] Figure 2 The typical microstructure photograph for Comparative Example 1 shows poor microstructure uniformity, with banded microstructure in the core. Scanning electron microscopy shows that the martensite is bright white, indicating a high C content. The volume fraction of ferrite is 41%, and the volume fraction of martensite is 59%. Detailed Implementation

[0034] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0035] Examples 1-3

[0036] A hot-dip galvanized high-strength steel sheet with excellent cold bending and hole expansion properties, the matrix of which includes the following mass percentage composition as shown in Table 1, the balance of which is not shown in Table 1 is Fe and unavoidable impurities.

[0037] Comparative Examples 1-3

[0038] A hot-dip galvanized high-strength steel sheet, the matrix of which comprises the following mass percentage components as shown in Table 1, wherein the balance not shown in Table 1 is Fe and unavoidable impurities.

[0039] Table 1. Matrix steel composition (wt%) of high-strength steel plates in each embodiment and comparative example

[0040]

[0041] Example 1 and Comparative Example 1 have the same components, Example 2 and Comparative Example 2 have the same components, and Example 3 and Comparative Example 3 have the same components.

[0042] The production methods of hot-dip galvanized high-strength steel sheets in the various embodiments and comparative examples include smelting, hot continuous rolling, pickling and cold rolling, continuous annealing and galvanizing, and alloying:

[0043] The smelting process is applicable to converter, electric furnace and induction furnace smelting; continuous casting is used to produce billets, and dynamic light reduction and electromagnetic stirring systems can be used to ensure the quality of the billets.

[0044] The hot continuous rolling process involves heating the cast billet to (1200-1250)℃ and holding it at that temperature for 2-3 hours. The billet is then rolled in 5-7 passes on a roughing mill to a thickness of 30mm-40mm. A subsequent hot continuous rolling mill is used to roll the billet in 5-7 passes to a thickness of 2.0-6.0mm. The initial rolling temperature is 1000-1150℃, and the final rolling temperature is 870-930℃. After laminar cooling to the target thickness, the billet is coiled within the (540-600)℃ range. The laminar cooling mode is rapid cooling at the front end.

[0045] The pickling and cold rolling process involves pickling the strip steel in a hydrochloric acid bath to remove surface iron oxide scale, followed by continuous cold rolling or cold rolling with a cold rolling reduction rate of 40-65% until the target thickness is reached.

[0046] The continuous annealing hot-dip galvanizing process involves first slowly heating the pickled and rolled strip to 180°C at a heating rate of 3-6.5°C / s for preheating. The preheated strip is then further heated to 760-800°C. During this process, cold-rolled pearlite and ferrite recrystallize. Simultaneously, to ensure the coating performance of the zinc layer, the water vapor or oxygen content in the annealing furnace from preheating to heating is adjusted, controlling the dew point temperature range between -20°C and +20°C, resulting in a 20-40 μm semi-decarburized layer (oxide layer) on the strip surface. Austenite begins to form when the temperature exceeds Ac1. The heated strip is then held at 760-800°C for 60-120 seconds for two-phase annealing. The heat-insulated strip steel is cooled in two stages: first, it is slowly cooled to 670-710℃ at a rate of 1.5-5℃ / s, and then rapidly cooled to 470-500℃ at a rate of 17.5-38℃ / s. The strip steel is then hot-dip galvanized at 450-470℃, with the zinc bath temperature typically controlled at 455-465℃. After exiting the zinc bath, it is cooled again to 200℃ or lower at a rate of at least 5℃ / s. The continuous annealing and galvanizing processes for each embodiment and comparative example are shown in Table 2.

[0047] Table 2. Key production parameters for each embodiment and comparative example.

[0048]

[0049] The performance of each embodiment and comparative example product is shown in Table 3. The bending test standard is GB / T 15825.5 "Test Methods for Forming Properties of Sheet Metals - Part 5", and the hole expansion test standard is GB / T 15825.4 "Test Methods for Forming Properties of Sheet Metals - Part 4: Hole Expansion Test".

[0050] Tensile standard: GB / T 228.1 Metallic materials, tensile testing - Part 1: Test method at room temperature.

[0051] Table 3 Performance of each embodiment and comparative example product

[0052]

[0053] Comparative Examples 1-3 show that the dew point temperature inside the heating furnace does not meet the requirement of (-20)℃-(+20)℃, resulting in no decarburization layer on the substrate surface, high surface hardness, and failure to meet the requirements for cold bending and pore expansion rate.

[0054] This invention does not add precious metals, resulting in low cost. Based on the current market situation, the profit per ton of 1000MPa grade hot-dip galvanized high-strength steel is approximately 1000 yuan. With current annual sales of 5000 tons, this could generate an additional annual profit of 5 million yuan.

[0055] The above description of the embodiments is intended to enable those skilled in the art to understand and use the invention. It will be apparent to those skilled in the art that various modifications can be made to these embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the invention should be within the protection scope of the present invention.

Claims

1. A hot-dip galvanized high-strength steel sheet with excellent cold bending and hole-expanding properties, characterized in that, The matrix of the hot-dip galvanized high-strength steel sheet with excellent cold bending and hole expansion properties comprises the following components by weight percentage: C: 0.07-0.12%; Si: 0.30-0.50%; Mn: 2.0-3.0%; Cr: 0.3-0.6%; Nb+Ti+V: 0.04-0.10% and Ti≤0.04%; Als: 0.03-0.06%; P ：≤0.015%； S：≤0.01%； N: ≤0.005%, B: 0.0001-0.001%, balance is Fe and unavoidable impurities.

2. The hot-dip galvanized high-strength steel sheet with excellent cold bending and hole expansion properties according to claim 1, characterized in that, The hot-dip galvanized high-strength steel sheet with excellent cold bending and hole expansion properties has a ferrite volume content of 30-50% and a hard phase structure of martensite + bainite volume content of 50-70%.

3. The hot-dip galvanized high-strength steel sheet with excellent cold bending and hole expansion properties according to claim 1 or 2, characterized in that, The hot-dip galvanized high-strength steel sheet with excellent cold bending and hole expansion properties meets the following mechanical properties: yield strength of 590MPa-730MPa, tensile strength ≥980MPa, and elongation A. 80 ≥10%.

4. The hot-dip galvanized high-strength steel sheet with excellent cold bending and hole expansion properties according to claim 1, characterized in that, The hot-dip galvanized high-strength steel sheet with excellent cold bending and hole expansion properties is subjected to dew point control in a galvanizing annealing furnace to form a semi-decarburized layer with a thickness of 20-40μm on the substrate surface. The microhardness of the substrate is 310-360HV, and the surface semi-decarburized layer has a hardness of 200-280HV. The hardness difference between the surface semi-decarburized layer and the substrate is greater than 60HV.

5. The hot-dip galvanized high-strength steel sheet with excellent cold bending and hole expansion properties according to claim 1 or 4, characterized in that, The finished hot-dip galvanized high-strength steel sheet with excellent cold bending and hole expansion properties has a 180° cold bending mandrel diameter of 0T (T is the plate thickness) and a hole expansion rate of ≥40%.

6. A method for producing a hot-dip galvanized high-strength steel sheet with excellent cold bending and hole-expanding properties as described in any one of claims 1-5, characterized in that, The production method includes smelting, hot rolling, pickling and cold rolling, and continuous annealing and galvanizing.

7. The production method according to claim 6, characterized in that, The hot continuous rolling process involves heating the cast billet to 1200-1250℃, rolling it to a thickness of 30-40mm using a roughing mill, and then continuously rolling it to 2.0-6.0mm in 5-7 passes, with an initial rolling temperature of 1000-1150℃ and a final rolling temperature of 870-930℃. The billet is then cooled by laminar flow and coiled at a temperature of 540-600℃.

8. The production method according to claim 6, characterized in that, The pickling and cold rolling process involves continuous cold rolling or cold rolling with a cold rolling reduction rate of 40-65% until the target thickness is reached.

9. The production method according to claim 6, characterized in that, The continuous annealing and galvanizing process involves: firstly, preheating the cold-rolled strip to 180℃ at a slow rate of 3-6.5℃ / s; then further heating the preheated strip to 760-800℃, controlling the dew point temperature range between -20℃ and +20℃, to form a 20-40μm semi-decarburized layer on the strip surface; then annealing the heated strip at 760-800℃ for 60-120s in a two-phase region; finally, cooling the strip in two stages: firstly, slowly cooling it to 670-710℃ at a rate of 1.5-5℃ / s, then rapidly cooling it to 470-500℃ at a rate of 17.5-38℃ / s; then hot-dip galvanizing the strip at 450-470℃, with the zinc bath temperature controlled at 455-465℃; and finally cooling it to 200℃ or lower at a rate of 5℃ / s or higher after exiting the zinc bath.

10. The production method according to claim 9, characterized in that, The production method also includes alloying. After the steel comes out of the zinc pot, it is heated to 490-530°C in an alloying furnace for alloying treatment. After exiting the alloying furnace, it is cooled to below 200°C at a cooling rate of more than 5°C / s to obtain a zinc-iron alloy coating product.

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